RAJALAKSHMI RAJALAKSHMI ENGINEERING COLLEGE Thandalam, Chennai – Chennai – 602 602 105
Department of Automobile Engineering
AT6007 NEW GENERATION AND HYBRID VEHICLES UNIT – UNIT – I I INTRODUCTION 1.Electric vehicles: Electric vehicles (EVs) use an electric motor for traction, and chemical batteries, fuel cells, ultracapacitors, and/or flywheels for their corresponding energy sources. The electric vehicle has many advantages over the conventional internal combustion engine vehicle (ICEV), (ICEV), such as an absence of emissions, high efficiency, independence from petroleum, and quiet and smooth operation. Some difference between ICEVs and EVs, such as the use of gasoline tanks vs. batteries, ICE vs. electric motor, and different transmission requirements. This chapter will focus on the methodology of power train design and will investigate the key components including traction motor and energy storages. Configurations of Electric Vehicles Previously, the EV was mainly converted from the existing ICEV by replacing the internal combustion engine and fuel tank with an electric motor drive and battery pack while retaining all the other components, as shown in fig..1.1 Drawbacks such as its heavy weight, lower flexibility, and performance degradation have caused the use of this type of EV to fade out. In its place, the modern EV is built based on original body and frame designs. This satisfies the structure requirements unique to EVs and makes use of the greater flexibility of electric propulsion
Figure 1.1 primary vehicle train. 1
A modern electric drive train is conceptually illustrated illustrated in Figure 1.2 The drive train consists consists of three major subsystems: electric motor propulsion, energy source, and auxiliary. The electric propulsion subsystem is comprised of a vehicle controller, power electronic converter, electric motor, mechanical transmission, and driving wheels.
Fig 1.2 Conceptual illustration of general EV configuration The energy source subsystem involves the energy source, the energy management unit, and the energy refueling unit. The auxiliary subsystem consists of the power steering unit, the hotel climate control unit, and the auxiliary supply unit. Based on the control inputs from the accelerator and brake pedals, the vehicle controller provides proper control signals to the electronic power converter, which functions to regulate the power flow between the electric motor and energy source. The backward power flow is due to the regenerative braking of the EV and this regenerated energy can be restored to the energy source, provided the energy source is receptive. Most EV batteries as well as ultracapacitors and flywheels readily possess the ability to accept regenerated energy. The energy management unit cooperates with the vehicle controller to control the regenerative braking and its energy recovery. It also works with the energy refueling unit to control the refueling unit, and to monitor the usability of the energy source. The auxiliary power supply provides the necessary power at different voltage levels for all the EV auxiliaries, especially the hotel climate control a nd power steering units. There are a variety of possible possible EV configurations due to the the variations in Figure 1.3 (a) shows the configuration of the first alternative, in which an electric propulsion replaces the IC engine of a conventional vehicle drive train. It consists of an electric motor, a clutch, a gearbox, and a differential. The clutch and gearbox may be replaced by automatic transmission. The clutch is used to connect or disconnect the power of the electric motor from the driven wheels. The gearbox provides a set of gear ratios to modify the speed-power The differential is a mechanical device (usually a set of planetary gears), which enables the wheels of both sides to be driven at different speeds when the vehicle runs along a curved path. (b) With an electric motor that has constant power in a long speed range a fixed gearing can replace the multispeed gearbox and reduce the need for a clutch. This configuration not only 2
A modern electric drive train is conceptually illustrated illustrated in Figure 1.2 The drive train consists consists of three major subsystems: electric motor propulsion, energy source, and auxiliary. The electric propulsion subsystem is comprised of a vehicle controller, power electronic converter, electric motor, mechanical transmission, and driving wheels.
Fig 1.2 Conceptual illustration of general EV configuration The energy source subsystem involves the energy source, the energy management unit, and the energy refueling unit. The auxiliary subsystem consists of the power steering unit, the hotel climate control unit, and the auxiliary supply unit. Based on the control inputs from the accelerator and brake pedals, the vehicle controller provides proper control signals to the electronic power converter, which functions to regulate the power flow between the electric motor and energy source. The backward power flow is due to the regenerative braking of the EV and this regenerated energy can be restored to the energy source, provided the energy source is receptive. Most EV batteries as well as ultracapacitors and flywheels readily possess the ability to accept regenerated energy. The energy management unit cooperates with the vehicle controller to control the regenerative braking and its energy recovery. It also works with the energy refueling unit to control the refueling unit, and to monitor the usability of the energy source. The auxiliary power supply provides the necessary power at different voltage levels for all the EV auxiliaries, especially the hotel climate control a nd power steering units. There are a variety of possible possible EV configurations due to the the variations in Figure 1.3 (a) shows the configuration of the first alternative, in which an electric propulsion replaces the IC engine of a conventional vehicle drive train. It consists of an electric motor, a clutch, a gearbox, and a differential. The clutch and gearbox may be replaced by automatic transmission. The clutch is used to connect or disconnect the power of the electric motor from the driven wheels. The gearbox provides a set of gear ratios to modify the speed-power The differential is a mechanical device (usually a set of planetary gears), which enables the wheels of both sides to be driven at different speeds when the vehicle runs along a curved path. (b) With an electric motor that has constant power in a long speed range a fixed gearing can replace the multispeed gearbox and reduce the need for a clutch. This configuration not only 2
reduces the size and weight of the mechanical transmission, but also simplifies the drive train control because gear shifting is not needed. (c) Similar to the drive train in (b), the electric motor, the fixed gearing, and the differential can be further integrated into a single assembly while both axles point at both driving wheels. The whole drive train is further simplified and compacted. (d) In Figure 4.3(d), the mechanical differential is replaced by using two traction motors. Each of them drives one side wheel and operates at a different speed when the vehicle is running along a curved path. (e) In order to further simplify the drive train, the traction motor can be placed inside a wheel. This arrangement is the so-called inwheel drive. A thin planetary gear set may be used to reduce the motor speed speed and enhance the motor torque. The thin planetary profile to match the load requirement electric propulsion characteristics and energy sources, as shown in Figure Figure 1.3.
Fig 1.3 Possible EV configuration gear set offers the advantage of a high-speed reduction ratio as well as an inline arrangement of the input and output shaft. (f) By fully abandoning any mechanical gearing between the electric motor and the driving wheel, the out-rotor of a low-speed electric motor in the in-wheel drive can be directly connected to the driving wheel. The speed control of the electric motor is equivalent to the control of the wheel speed and hence the vehicle speed. However, this arrangement requires the electric motor to have a higher torque to start and accelerate the vehicle. 3
1.1 Traction Motor Characteristics Variable-speed electric motor drives usually have the characteristics shown in Figure 1.4. At the low-speed region (less than the base speed as marked in Figure 1.4), the motor has a constant torque. In the high-speed region (higher than the base speed), the motor has a constant power. This characteristic is usually represented by a speed ratio x, defined as the ratio of its maximum speed to its base speed. In low-speed operations, voltage supply to the motor increases with the increase of the speed through the electronic converter while the flux is kept constant. At the point of base speed, the voltage of the motor reaches the source voltage. After the base speed the motor voltage is kept constant and the flux is weakened, dropping hyperbolically with increasing speed. Hence, its torque also drops hyperbolically with increasing speed
Fig 1.4 Typical variable-speed electric motor characteristics Fig 1.5 shows the torque – speed profiles of a 60 kW motor with different speed ratios x (x_2, 4, and 6). It is clear that with a long constant power region, the maximum torque of the motor can be significantly increased, and hence vehicle acceleration and gradeability performance can be improved and the transmission can be simplified. However, each type of motor inherently has its limited maximum speed ratio. For example, a permanent magnet motor has a small x (<2) because of the difficulty of field weakening due to the presence of the permanent magnet. Switched reluctance motors may achieve x>6 and induction motors about x=4. 2. Hybrid Vehicles: Conventional vehicles with internal combustion engines (ICE) provide good performance and long operating range by utilizing the high energy-density advantages of petroleum fuels. However, conventional ICE vehicles bear the disadvantages of poor fuel economy and environmental pollution. The main reasons for their poor fuel economy are (1) engine fuel efficiency character istics are mismatched with the real operation requirements (2) 4
dissipation of vehicle kinetic energy during braking especially while operating in urban areas, and (3) low efficiency of hydraulic transmission in current automobiles in stop-and-go driving patterns Battery-powered electric vehicles (EV), on the hand, possess some advantages over conventional ICE vehicles, such as high energy efficiency and zero environmental pollution. However, the performance, especially the operation range per battery charge, is far less competitive than ICE vehicles, due to the lower energy content of the batteries vs. the energy content of gasoline. Hybrid electric vehicles (HEV), which use two power sources — a primary power source and a secondary power source — have the advantages of both ICE vehicles and EV and overcome their disadvantages 2.1 Concept of Hybrid Electric Drive Trains Basically, any vehicle power train is required to (1) develop sufficient power to meet the emands of vehicle performance, (2) carry sufficient energy onboard to support vehicle driving in the given range, (3) demonstrate high efficiency, and (4) emit few environmental pollutants. roadly, a vehicle may have more than one energy source and energy converter (power source), such as a gasoline (or diesel) heat engine system, hydrogen – fuel cell – electric motor system, chemical battery – electric motor system, etc. Avehicle that has two or more energy sources and energy converters is called a hybrid vehicle. A hybrid vehicle with an electrical power train (energy source energy converters) is called an HEV. A hybrid vehicle drive train usually consists of no more than two power trains. More than two power train configurations will complicate the system. For the purpose of recapturing part of the braking energy8 that is dissipated in the form of heat in conventional ICE vehicles, a hybrid drive train usually has a bidirectional energy source and converter. The other one is either bidirectional or unidirectional. Figure 1.5 shows the concept of a hybrid drive train and the possible different power flow routes.
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Fig 1.5 Conceptual illustration of a hybrid electric drive train
Hybrid drive trains supply the required power by an adapted power train. There are many available patterns of combining the power flows to meet load requirements as described below: 1. Power train 1 alone delivers power to the load 2. Power train 2 alone delivers power to the load 3. Both power train 1 and 2 deliver power to load at the same time 4. Power train 2 obtains power from load (regenerative braking) 5
5. Power train 2 obtains power from power train 1 6. Power train 2 obtains power from power train 1 and load at the same time 7. Power train 1 delivers power to load and to power train 2 at the same time 8. Power train 1 delivers power to power train 2, and power train 2 delivers power to load 9. Power train 1 delivers power to load, and load delivers power to power train 2. In the case of hybridization with a liquid fuel-IC engine (power train 1) and a batteryelectric machine (power train 2), pattern (1) is the engine-alone propelling mode. This may be used when the batteries are almost completely depleted and the engine has no remaining power to charge the batteries, or when the batteries have been fully charged and the engine is able to supply sufficient power to meet the power demands of the vehicle. Pattern (2) is the pure electric propelling mode, in which the engine is shut off. This pattern may be used in situations where the engine cannot operate effectively, such as very low speed, or in areas where emissions are strictly prohibited. Pattern (3) is the hybrid traction mode and may be used when a large amount of power is needed, such as during sharp acceleration or steep hill climbing. Pattern (4) is the regenerative braking mode, by which the kinetic or potential energy of the vehicle is recovered through the electric motor functioning as a generator. The recovered energy is stored in the batteries and reused later on. Pattern (5) is the mode in which the engine charges the batteries while the vehicle is at a standstill, coasting, or descending a slight grade, in which no power goes into or comes from the load. Pattern (6) is the mode in which both regenerative braking and the IC engine charge the batteries simultaneously. Pattern (7) is the mode in which the engine propels the vehicle and charges the batteries simultaneously. Pattern (8) is the mode in which the engine charges the batteries, and the batteries supply power to the load. Pattern (9) is the mode in which the power flows into the batteries from the heat engine through the vehicle mass. The typical configuration of this mode is two power trains separately mounted on the front and the rear axle of the vehicle 2.3 Architectures of Hybrid Electric Drive Trains The architecture of a hybrid vehicle is loosely defined as the connection between the components that define the energy flow routes and control ports. Traditionally, HEVs were classified into two basic types: series and parallel. It is interesting to note that, in 2000, some newly introduced EVs could not be classified into these kinds Therefore, HEVs are now classified into four kinds: series hybrid, parallel hybrid, series – parallel hybrid, and complex hybrid, which are functionally shown in Figure 1.6. In Figure 1.6, a fuel tank-IC engine and a battery-electric motor are taken, respectively, as examples of the primary power source (steady power source) and secondary power source (dynamic power source). Of course, the IC engine can be replaced by other types of power sources, such as fuel cells. Similarly, the batteries can be replaced by ultracapacitors or by flywheels and their combinations, which will be discussed in detail in the following chapters.
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Fig 1.6 Classification of hybrid electric vehicles 2.3.1 Series Hybrid Electric Drive Trains A series hybrid drive train is a drive train where two power sources feed a single powerplant (electric motor) that propels the vehicle. The most commonly found series hybrid drive train is the series hybrid electric drive train shown in Figure 1.7. The unidirectional energy source is a fuel tank and the unidirectional energy converter is an engine coupled to an electric generator. The output of the electric generator is connected to an electric power bus through an electronic converter (rectifier). The bidirectional energy source is an electrochemical battery pack, connected to the bus by means of a power electronics converter (DC/DC converter). The electric power bus is also connected to the controller of the electric traction motor. The traction motor can be controlled either as a motor or a generator, and in forward or reverse motion. This drive train may need a battery charger to charge the batteries by a wall plug-in from the power network. Series hybrid electric drive trains potentially have the following operation modes: 1. Pure electric mode: The engine is turned off and the vehicle is propelled only by the batteries. 2. Pure engine mode: The vehicle traction power only comes from the engine-generator, while the batteries neither supply nor draw any power from the drive train. The electric machines serve as an electric transmission from the engine to the driven wheels. 3. Hybrid mode: The traction power is drawn from both the engine generator and the batteries. 4. Engine traction and battery charging mode: The engine-generator supplies power to charge the batteries and to propel the vehicle. 5. Regenerative braking mode: The engine-generator is turned off and the traction motor is operated as a generator. The power generated is used to charge the batteries. 7
6. Battery charging mode: The traction motor receives no power and the engine-generator charges the batteries. 7. Hybrid battery charging mode: Both the engine-generator and the traction motor operate as generators to charge the batteries. Series hybrid drive trains offer advantages: The engine is fully mechanical when decoupled from the driven wheels. Therefore, it can be operated at any point on its speed – torque torque characteristic map, and can potentially be operated solely within its maximum efficiency region as shown in Figure 1.7 The efficiency and emissions of the engine can be further improved by optimal design and control in this narrow region. A narrow region allows greater improvements than an optimization across the entire range. Furthermore, the mechanical decoupling of the engine from the driven wheels allows the use of a high-speed engine. This makes it difficult to power the wheels directly through a mechanical link, such as gas turbines or powerplants, with slow dynamics like the Stirling engine.
Fig 1.7 Configuration of a series hybrid electric drive train 2.3.2 Parallel Hybrid Electric Drive Trains power mechanically to the wheels like in a conventional ICE-powered vehicle. It is assisted by an electric motor that is mechanically coupled to the transmission. The powers of the engine and electric motor are coupled solely within its maximum efficiency region as shown in Figure 1.7. A parallel hybrid drive train is a drive train in which the engine supplies its together by mechanical coupling, as shown in Figure 1.8. The mechanical combination of the engine and electric motor power leaves room for several different configurations, detailed h ereafter.
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Fig 1.8 Configuration of a parallel hybrid electric drive train There are a variety of configurations in to rque coupling hybrid drive trains.
Fig 1.9 two axle configuration
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Fig 1.10 two shaft configuration
Fig 1.11 pretransmission singghle shaft torque combination parallel hybrid drive train
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Fig 1.12 posttransmission single shaft torque combination parallel hybrid electric drive train
Fig 1.13 separated axle torque combination parallel hybrid electric drive train 3. Flexible Fuel Vehicles (FFV) A flexible-fuel vehicle (FFV) or dual-fuel vehicle (flex-fuel vehicle) is an alternative fuel vehicle with an internal combustion engine designed to run on more than one fuel, usually gasoline blended with either ethanol or methanol fuel, and both fuels are stored in the same common tank. Flexible fuel vehicles (FFVs) are designed to run on gasoline or gasolineethanol blends of up to 85% ethanol (E85).Except for a few engine and fuel system modifications, they are identical to gasoline-only models. FFVs experience no loss in performance when operating on E85, and some generate more torque and horsepower than when operating on gasoline. Since ethanol contains less energy per volume than gasoline, FFVs typically get about 15% – 30% fewer miles per gallon when fuelled with E85. Modern flex-fuel engines are capable of burning any proportion of the resulting blend in the combustion 11
chamber as fuel injection and spark timing are adjusted automatically according to the actual blend detected by a fuel composition sensor. Flex-fuel vehicles are distinguished from bi-fuel vehicles, where two fuels are stored in separate tanks and the engine runs on one fuel at a time, for example, compressed natural gas (CNG), liquefied petroleum gas (LPG), or hydrogen FFVs have been produced since the 1990s, and more than one hundred models are currently available. Since FFVs look just like gasoline-only models, you may be driving an FFV and not even know it. The most common commercially available FFV in the world market is the ethanol flexible-fuel vehicle, with about 39 million automobiles, motorcycles and light duty trucks manufactured and sold worldwide through October 2013, and concentrated in four markets. Brazil (23.0 million), United States (10 million), Canada (more than 600,000), Europe and Sweden (229,400).
Consumers have had the option to buy FFVs in the United States since 1995. But Brazil is actually the world's biggest flexible-fuel vehicle market. More than 90 percent of vehicles sold today in that country have flex engines FFV Identification Most manufacturers started putting yellow gasoline caps on FFVs as of model year 2008 (2006 for General Motors). Some FFVs have labels on the fuel door indicating fuel type E85 Some capless fuel fillers have a yellow ring around where you insert the fuel nozzle. The owner's manual usually specifies which fuels can be used in your vehicle. If you don't have a manual, it may be available on the manufacturer's website You can't pump E85 into a standard gasoline vehicle's tank without nasty side effects, such as chugging, rough performance and possible engine damage. Instead, you need an FFV designed specifically to work with the chemical corrosiveness and higher octane of ethanol .
Even people who don't know much about FFVs understand that oil consumption is a hot topic. That's where FFVs frequently enter the conversation. Because we can produce ethanol from renewable sources, such as corn and sugar cane, FFVs are often described as more eco-friendly than straight gasoline vehicles. The supposed environmental friendliness of FFVs, however, is a point of hot contention between various factions in manufacturing, politics and science (more on that later). Flex-fuel engines are specially designed to withstand the corrosiveness of ethanol. Run ethanol regularly in a regular gasoline engine and it will rust and break down Flex engines aren't wildly different from gasoline engines, nor are they space -age technology. The Ford Model T car, for instance, was a flex engine that burned either gas or ethanol. FFVs don't cost more to build, and they aren't more expensive in terms of sticker price, either. If you really want to, you can even buy a conversion kit that lets you turn your gasoline motor into one that will happily drink E85. Effects of Ethanol in FFV From a chemical perspective, ethanol is different from gas in two major ways. It's more corrosive and conductive . It also burns hotter, so it has a higher octane rating. A flex engine must be built to handle those technical challenges. In flexible-fuel vehicles, the fuel tank and in12
tank components are built to withstand ethanol's corrosiveness. Similarly, the fuel lines, gaskets, seals and rubber fuel hoses all must be corrosion-resistant, too, as must the fuel injectors. The same goes for internal engine components. Valve seats, piston rings, valves and other parts are all made from materials that won't easily corrode. These parts are also built to diminish the possibility that damage will result due to ethanol's tendency to break down and clean away engine lubricants. Conductivity is another concern. All electrical parts of the fuel system, including wiring, must be insulated against ethanol's higher conductivity. In addition, the fuel filler parts have anti-spark features that reduce the chance of stray sparks or static electricity causing a dangerous flare-up. For starters, your car has to know what kind of fuel you're pumping. An onboard computer analyzes fuel composition to optimize engine ignition, adhere to emissions standards and regulate combustion. No matter how much the ethanol-to-gas ratio fluctuates, your flex-fuel car can adjust on the fly. Electrical worries aside, ethanol has a lower energy density than gasoline, so the engine needs more fuel to achieve high performance. Thus, fuel injectors are created to achieve higher fuel flow rates --otherwise, engine would see a drastic reduction in overall power. The key differences are energy density and CO2 (carbon dioxide) emissions. Lower energy density just means that there's less energy in a gallon of ethanol than there is in regular gasoline, so you have to burn more to travel the same distance that you would with gas. This, in turn, means you have to burn more fuel to get moving, and your vehicle actually releases more CO2 than it does with gas. However, because that CO2 was derived from a plant source instead of petroleum, you're (in theory) simply releasing it back into the atmosphere, where it would have gone anyway as the plant naturally decayed. Still, the eco-friendliness of FFVs is, at best, unclear. Fuel efficiency is another thing altogether. As with all things related to mileage, there's a lot of variability here, but mileage per tank drops between 15 percent and 30 percent when you switch to ethanol. And because ethanol fuels are generally about the same price as gasoline, you might actually spend significantly more money -- perhaps hundreds of dollars per year -- to get around on just ethanol. 4. Solar Powered Vehicles A solar vehicle is an electric vehicle powered completely or significantly by direct solar energy and is a step in saving non renewable sources of energy which are available in limited quantity. The term "solar vehicle" usually implies that solar energy is used to power all or part of a vehicle's propulsion. Solar power may be also used to provide power for communications or controls or other auxiliary functions. Photovoltaic (PV) cells contained in solar panels convert the sun's energy directly into electric energy.The electrical energy prodced by PV cells are stored in the battery. The basic principle of solar vehicle is to use energy that is stored in a battery during and after charging it from a solar panel. The charged batteries are used to drive the motor which serves here as an engine and moves the vehicle in reverse or forward direction. The electrical tapping rheostat is provided so as to control the motor speed. This avoids excess flow of current when the vehicle is supposed to be stopped suddenly as it is in normal cars with regards to fuel. 13
Solar vehicles are not sold as practical day-to-day transportation devices at present, but are primarily demonstration vehicles and engineering exercises, often sponsored by government agencies. However, indirectly solar-charged vehicles are widespread and solar boats are available commercially All recent electric vehicles present drive on AC power supplied motor. The setup requires an inverter set connected to battery through which DC power is converted to AC power. During this conversion many losses take place and hence the net output is very less and lasts for shorter duration of time. Although this is cheaper the setup and maintenance required is much more in AC drive than DC drive. The vehicle designed is controlled by ELECTRICAL means and not by ELECTRONIC means
The above diagram gives an overview of the working of solar vehicle. Sun is the main source of energy for the vehicle. Energy from Sun is captured by the solar panels and is converted to electrical energy. The electrical energy thus formed is being fed to the batteries that get charged and is used to run 24 V DC high torques DC series motor. The shaft of the motor is connected to the rear wheel of the vehicle through chain sprocket. The batteries are initially fully charged and thereafter they are charged by panels. This helps in completing the charging-discharging cycle of the batteries, which is very important for proper working of batteries.
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There are limits to using photovoltaic (PV) cells for vehicles: Power density: Power from a solar array is limited by the size of the vehicle and area that can be exposed to sunlight. While energy can be accumulated in batteries to lower peak demand on the array and provide operation in sunless conditions, the battery adds weight and cost to the vehicle. The power limit can be mitigated by use of conventional electric cars supplied by solar (or other) power, recharging from the electrical grid. Cost:
While sunlight is free, the creation of PV cells to capture that sunlight is expensive. Costs for solar panels are steadily declining Design considerations: Even though sunlight has no lifespan, PV cells do. The lifetime of a solar module is approximately 20 years. Mobile applications are unlikely to require lifetimes as long as building integrated PV and solar parks. Current PV panels are mostly designed for stationary installations. However, to be successful in mobile applications, PV panels need to be designed to withstand vibrations. Also, solar panels, especially those incorporating glass have significant weight. To be useful, the energy harvested by a panel must exceed the added fuel consumption caused by the added weight. Applications Land Solar car. Single-track vehicles (solar bicycle or tricycle, Applications (golf carts), Auxiliary power, Personal Rapid Transit Rail Space Solar powered spacecraft, Solar propelled spacecraft, Planetary exploration. Plug-in hybrid and solar vehicles Compone nts used
Batteries( heavy inverter
Range
Quantity
24V 190Ah
2*12V
Solar module 140W Watt Peak Connecting Motor Cables connection:25Sq.m m high voltage cables. Solar module to charge Motor
High
torque
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1 10 meters 1 meter 1 meter
1
Advantages:
The solar vehicles are the future of the automobile industry. They are highly feasible and can be manufactured with ease. The main advantages of a solar vehicle are that they are pollution less and are very economical. Since they cause no pollution they are very eco-friendly and are the only answer to the increasing pollution levels from automobiles in the present scenario. By harvesting the renewable sources of energy like the solar energy we are helping in preserving the non-renewable sources of energy. The other main advantages of the solar vehicle are that they require less maintenance as compared to the conventional automotives and are very user friendly. 5. Magnetic Track Vehicles Magnetic levitated train is the train which will not move over the rail rather its floats above the rails. The magnetic levitation is brought about by enormous repulsion between two highly power full magnetic fields, one produced by the superconducting magnet inside the vehicle and the other one by the electric current in the guide way.
Fig 1.14 magnetic levitation Power Supply: Batteries on the train power the system, and therefore it still functions without propulsion. The batteries can levitate the train for 30 minutes without any additional energy. Linear generators in the magnets on board the train use the motion of the train to recharge the batteries. Levitation system uses less power than the train‘s air conditioning.
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The Maglev Track: The magnetized coil running along the track, called a guideway, repels the large magnets on the train's undercarriage, allowing the train to levitate between 0.39 and 3.93 inches (1 to 10 cm) above the guideway. Once the train is levitated, power is supplied to the coils within the guideway walls to create a unique system of magnetic fields that pull and push the train along the guideway. The electric current supplied to the coils in the guideway walls is constantly alternating to change the polarity of the magnetized coils. This change in polarity causes the magnetic field in front of the train to pull the vehicle forward, while the magnetic field behind the train adds more forward thrust.
Fig 1.16 Meglev track Types of magnets used: This train uses superconducting electric magnets in the vehicle to levitate and propel the train. The super conducting state is the state in which a material has virtually zero electrical resistance. This means that once electrified these magnets do not require additional energy and Because of diamagnetic nature of the super conducting materials strongly repels external magnets. It leads to a levitation effect. 17
Levitation: The passing of the superconducting magnets by figure eight levitation coils on the side of the track induces a current in the coils and creates a magnetic field. This pushes the train upward so that it can levitate 10 cm above the track. The train does not levitate until it reaches 50 mph, so it is equipped with retractable wheels. Propulsion: An alternating current is ran through electromagnet coils on the guide walls of the guide way. This creates a magnetic field that attracts and repels the superconducting magnets on the train and propels the train forward. Braking is accomplished by sending an alternating current in the reverse direction so that it is slowed by attractive and repulsive forces. Different speeds are achieved by varying the intensity of the current. Only the section of track where the train is traveling is electrified. Lateral Guidance: When one side of the train nears the side of the guide way, the super conducting magnet on the train induces a repulsive force from the levitation coils on the side closer to the train and an attractive force from the coils on the farther side. This keeps the train in the center. Advantages: 1. Maintenance There is very little maintenance because there is no contact between the parts. 2. Comfort The ride is smooth while not accelerating. 3.Speed Since there is no friction these trains can reach high speeds. The train can travel at about 300 mph .For trips of distances up to 500 miles its total travel time is equal to a planes (including check in time and travel to airport.). 4. Environment It uses less energy than existing transportation systems. For every seat on a 300 km trip with 3 stops, the gasoline used per 100 miles varies with the speed. At 200 km/h it is 1 liter, at 300 km/h it is 1.5 liters and at 400 km/h it is 2 liters. This is 1/3 the energy used by cars and 1/5 the energy used by jets per mile. 5. Noise Pollution The train makes little noise because it does not touch the track and it has no motor. Therefore, all noise comes from moving air. This sound is equivalent to the noise produced by city traffic. 6. Economic Efficiency The initial investment is similar to other high speed rail roads.Operating expenses are half of that of other railroads. 7. Safety The trains are virtually impossible to derail because the train is wrapped around the track. Collisions between trains are unlikely because computers are controlling the trains movements. 8. Maintenance There is very little maintenance because there is no contact between the parts. Disadvantage: The only disadvantage of superconducting magnets is their need for refrigeration. However, the power for the refrigerator is small compared to the power to overcome air drag on the vehicle.
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6. Fuel Cells Vehicles A fuel cell is an electrochemical device that produces electricity by means of a chemical eaction, much like a battery. The major difference between batteries and fuel cells is that the latter can produce electricity as long as fuel is supplied, while batteries produce electricity from stored chemical energy and, hence, require frequent recharging. The basic structure of a fuel cell (Figure 1.17) consists of an anode and a cathode, similar to a battery. The fuel supplied to the cell is hydrogen and oxygen. The concept of fuel cell is the opposite of electrolysis of water, where hydrogen and oxygen are combined to form electricity and water. The hydrogen fuel supplied to the fuel cell consists of two hydrogen atoms per molecule chemically bonded together in the form H2. This molecule includes two separate nuclei, each containing one proton, while sharing two electrons. The fuel cell breaks apart these hydrogen molecules to produce electricity. The exact nature of accomplishing the task depends on the fuel cell type, although what remains the same- for all fuel cells is that this reaction takes place at the anode. The hydrogen molecule breaks into four parts at the anode due to the chemical reaction, releasing hydrogen ions and electrons. A catalyst speeds the reaction, and an electrolyte allows the two hydrogen ions, which essentially are two single protons, to move to the cathode through the electrolyte placed between the two electrodes. The flow of electrons from the anode to the cathode through the external circuit is what produces electricity. For the overall cell reaction to complete, oxygen or air must be passed over the cathode. The cathode reaction takes place in two stages. First, the bond between the two oxygen atoms in the molecule breaks and then each ionized oxygen atom grabs two electrons coming from the anode through the external circuit to become negatively charged. The negatively charged
Fig 1.17 Basic Fuel Cell Structure.
oxygen atoms are balanced by the positively charged hydrogen atoms at the cathode, and the combination produces H2O commonly known as water. The chemical reaction taking place in a fuel cell is as follows:
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The fuel cell was first developed for space applications as an alternative power source. The source was first used in a moon buggy and is still used in NASA‘s space shuttles. There has been tremendous interest in fuel cells in recent years for applications in other areas, such as EVs and stationary power systems. The research sponsored by several U.S. research agencies and corporations has attempted to improve cell performance with two primary goals: a desire for higher power cells, which can be achieved through higher rates of reaction, and the desire for fuel cells that can internally reform hydrocarbons and are more tolerant of contaminants in the reactant streams. For this reason, the searches have concentrated on finding new materials for electrodes and electrolytes. There are several different types of fuel cells, each with strengths and weaknesses. Low operating temperature is desirable for vehicle applications, despite the fact that higher temperatures result in higher reaction rates. Rapid operation and cogeneration capabilities are desirable for stationary applications. Cogeneration refers to the capability to utilize the waste heat of a fuel cell to generate electricity using conventional means. FUEL CELL TYPES The six major types of fuel cells are as follows: alkaline, proton exchange membrane, direct ethanol, phosphoric acid, molten carbonate, and solid oxide. A short description of the relevant characteristics of each type in the context of vehicular and stationary applications is given below Alkaline Fuel Cell (AFC) In an alkaline fuel cell (AFC), an aqueous solution of potassium hydroxide (KOH) is used as the electrolyte. Compared to some other fuel cells where acidic electrolytes are used, the performance of the alkaline electrolyte is as good as the acid electrolytes, while being significantly less corrosive toward the electrodes. Alkaline fuel cells have been in actual use for a long time, delivering electrical efficiencies of up to 60%. They require pure hydrogen as fuel and operate at low temperatures (at 80°C); therefore, they are suitable for vehicle applications. Residual heat can be used for heating, but the cell temperature is not sufficiently high to generate steam that can be used for cogeneration. Proton Exchange Membrane (PEM) The proton exchange membrane (PEM) fuel cells use solid electrolytes and operate at low temperatures (around 80°C). Nafion is an example of solid polymer electrolyte. These fuel cells are also known as solid polymer membrane fuel cells. The electrical efficiency of PEM fuel cells is lower than that of the alkaline cells (about 40%). However, a rugged and simple construction makes these types of fuel cells suitable for vehicle applications. The PEM fuel cell and the AFC are currently being considered for vehicle applications. The advantage of PEM cells is that they can tolerate impurity in the fuel, as compared to pure hydrogen which is needed in alkaline fuel cells
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Direct Methanol Fuel Cell (DMFC) The direct methanol fuel cell (DMFC) is a result of research on using methanol as the fuel that can be carried on-board a vehicle and reformed to supply hydrogen to the fuel cell. A DMFC works on the same principle as the PEM, except that the temperature is increased to the range of 90 to 120°C such that internal reformation of methanol into hydrogen is possible. The electrical efficiency of DMFC is quite low at about 30%. This type of fuel cell is still in the design stages, because the search for a good electrocatalyst to reform the methanol efficiently and to reduce oxygen in the presence of methanol is ongoing. Phosphoric Acid Fuel Cell (PAFC) Phosphoric acid fuel cells (PAFC) are the oldest type with an origin that extends back to the creation of the fuel cell concept. The electrolyte used is phosphoric acid, and the cell operating temperature is about 200°C, which makes some cogeneration possible. The electrical efficiency of this cell is reasonable at about 40%. These types of fuel cells are considered too bulky for transportation applications, while higher efficiency designs exist for stationary applications Molten Carbonate Fuel Cell (MCFC) Molten carbonate fuel cells, originally developed to operate directly from coal, operate at 600°C and require CO or CO2 on the cathode side and hydrogen on the anode. The cells use carbonate as the electrolyte. The electrical efficiency of these fuel cells is high at about 50%, but the excess heat can be used for cogeneration for improved efficiency. The high temperatures required make these fuel cells not particularly suitable for vehicular applications, but they can be used for stationary power generation. Solid Oxide Fuel Cell (SOFC, ITSOFC) Solid oxide fuel cells (SOFCs) use a solid ionic conductor as the electrolyte rather than a solution or a polymer, which reduces corrosion problems. However, to achieve adequate ionic onductivity in such a ceramic, the system must operate at very high temperatures. The original designs, using yttria-stabilized zirconia as the electrolyte, required temperatures as high as 1000°C to operate, but the search for materials capable of serving as the electrolyte at lower temperatures resulted in the ―intermediate temperature solid oxide fuel cell‖ (ITSOFC). This fuel cell has high electrical efficiency of 50 to 60%, and residual heat can also be used for cogeneration. Although not a good choice for vehicle applications, it is at present the best option for stationary power generation.
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UNIT II POWER SYSTRM AND NEW GENERATION VEHICLES Hybrid Vehicle Engines The devices that convert heat transfer to work cyclically are known as heat engines. Each cycle in the heat engine consists of several different processes or strokes (such as constant volume, constant pressure, and constant entropy processes) to convert thermal energy into useful work. Different types of cycles are available to design practical heat engines. The practical cycles have evolved due to the limitations of an ideal thermodynamics cycle (such as the Carnot cycle) that operates between the same heat addition and rejection temperatures, and also because of the difference in characteristics of the choices available for the energy source, working fluid, and hardware materials. It must be mentioned that although the heat engines described in the following undergo mechanical cycles, the working fluids do not execute a thermodynamic cycle because matter is introduced with one composition and is later discharged at a different composition The heat engines of interest for EV and HEV applications, primarily the ICE and the gas turbine, will be discussed in this section. An ICE is a heat engine that utilizes fuel as a working fluid. The ICE uses heat cycles that gain its energy from the combustion of fuel within the engine. ICEs can be reciprocating reciprocating motion of a piston is converted to rotary motion through a crank mechanism. ICEs used in automobiles, trucks, and buses are of the reciprocating type, where the processes occur within a reciprocating piston-cylinder arrangements. The gas turbines used in power plants are also ICEs, where the processes occur in an interconnected series of different components. The Brayton cycle gas turbine engine has been adapted to the automotive propulsion engine and has the advantage of burning fuel that requires little refining and fuel that burns completely. Gas turbines have fewer moving parts, because there is no need to convert the rotary motion of the turbine. The disadvantages of gas turbines are complex construction and lower efficiency. Nevertheless, gas turbines are being considered for HEVs, and prototype vehicles have been developed. The performance of a heat engine is measured by the efficiency of the heat engine cycle, defined as the ratio of the net work output per cycle Wnet to the heat transfer into the engine per cycle. Another way of defining the performance of heat engines is to use the mean effective pressure Pme. The Pme is the theoretical constant gage pressure that, if exerted on the piston during the expansion stroke between the largest specific volume and the smallest volume, would produce the same net work as actually produced by the heat engine. Mathematically stated, P me=
The heat engine cycle performance analysis is carried out from the information available at certain convenient state points in the cycle. The parameters needed at the state points are pressure, temperature, volume, and entropy. If two parameters are known at two state points, then the unknown parameters are usually obtained from the process that the working fluid undergoes between the two state points (such as constant pressure, isentropic, etc.) and the laws of thermodynamics. Discussions of the laws of thermodynamics and efficiency analysis of heat engine cycles are beyond the scope of this book. Only a general introduction to the heat engine cycles of interest will be given. Prior to continuing the discussion on ICEs, a few words about entropy are in order. Entropy is a property that is specified for every equilibrium state of a substance. Like energy, entropy is an abstract concept that is extensively used in thermodynamic analysis. Entropy represents the microscopic disorder or uncertainty of a system. Because entropy is a property, the change in entropy in going from one state to another is the same for all
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processes. The SI unit for specific entropy is J/K. The reciprocating engines are considered first in the following section to be followed by a discussion on gas turbines. Stratified Charge Engines
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Learn Burn Engines Principle
A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired for acceleration, but must operate well below that point in normal steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal driving (below the maximum torque capability of the engine) can be higher. The engines designed for lean burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional petrol engines. Ultra lean mixtures with very high air-fuel ratios can only be achieved by direct injection engines. The main drawback of lean burning is that a complex catalytic converter system is required to reduce NOx emissions. Lean burn engines do not work well with modern 3-way catalytic converter — which require a pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions — so most modern engines run at or near the stoichiometric point. Alternatively, ultra-lean ratios can reduce NOx emissions. Heavy-duty gas engines
Lean burn concepts are often used for the design of heavy-duty natural gas, biogas, and liquefied petroleum gas (LPG) fuelled engines. These engines can either be full-time lean burn, where the engine runs with a weak air-fuel mixture regardless of load and engine speed, or part-time lean burn (also known as "lean mix" or "mixed lean"), where the engine runs lean only during low load and at high engine speeds, reverting to a stoichiometric air-fuel mixture in other cases. Heavy-duty lean burn gas engines admit as much as 75% more air than theoretically needed for complete combustion into the combustion chambers. The extremely weak air-fuel mixtures lead to lower combustion temperatures and therefore lower NOx formation. While lean-burn gas engines offer higher theoretical thermal efficiencies, transient response and performance may be compromised in certain situations. Lean burn gas engines are almost always turbocharged, resulting high power and torque figures not achieveable with stoichiometric engines due to high combustion temperatures Heavy duty gas engines may employ precombustion chambers in the cylinder head. A lean gas and air mixture is first highly compressed in the main chamber by the piston. A much richer, though much lesser volume gas/air mixture is introduced to the precombustion chamber and ignited by spark plug. The flame front spreads to the lean gas air mixture in the cylinder This two stage lean burn combustion produces low NOx and no particulate emissions. Thermal efficiency 28
is better as higher compression ratios are achieved. Manufacturers of heavy-duty lean burn gas engines include GE Jenbacher, MAN Diesel & Turbo, Wärtsilä, Mitsubishi Heavy Industries and Rolls-Royce plc. Honda lean burn systems
One of the newest lean-burn technologies available in automobiles currently in production uses very precise control of fuel injection, a strong air-fuel swirl created in the combustion chamber, a new linear air-fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce the resulting NOx emissions that increase under "lean-burn" conditions and meet NOx emissions requirements. This stratified-charge approach to lean-burn combustion means that the air-fuel ratio isn't equal throughout the cylinder. Instead, precise control over fuel injection and intake flow dynamics allows a greater concentration of fuel closer to the spark plug tip (richer), which is required for successful ignition and flame spread for complete combustion. The remainder of the cylinders' intake charge is progressively leaner with an overall average air:fuel ratio falling into the lean burn category of up to 22:1. The older Honda engines that used lean burn (not all did) accomplished this by having a parallel fuel and intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This burning mixture was then opened to the main chamber where a much larger and leaner mix then ignited to provide sufficient power. During the time this design was in production this system (CVCC, Compound Vortex Controlled Combustion) primarily allowed lower emissions without the need for a catalytic converter. These were carbureted engines and the relative "imprecise" nature of such limited the MPG abilities of the concept that now under MPI (Multi-Port fuel Injection) allows for higher MPG too. The newer Honda stratified charge (lean burn engines) operate on air-fuel ratios as high as 22:1. The amount of fuel drawn into the engine is much lower than a typical gasoline engine, which operates at 14.7:1 — the chemical stoichiometric ideal for complete combustion when averaging gasoline to the petrochemical industries' accepted standard of C6H8. This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced combustion must be "complete" by the time the exhaust valve open Low Heat Rejection Engines It is well known fact that about 30% of the energy supplied is lost through the coolant and the 30% is wasted through friction and other losses, thus leaving onl y 30% of energy utilization for useful purposes. In view of the above, the major thrust in engine research during the last one or two decades has been on development of LHR engines Several methods adopted for achieving LHR to the coolant are i) using ceramic coatings on piston, liner and cylinder head ii) creating air gap in the piston and other components with low thermal conductivity materials like superni, cast iron and mild steel etc. Improvement in thermal efficiency with ceramic coated component with diesel as fuel. Reduction in BSFC with an air gap insulated piston at part loads. Creating an air gap in the piston involved the complications of joining two different metals. Though Parker et al. observed effective insulation provided by an air gap, the bolted design employed by them could not provide complete sealing of air in the air gap. attempt of screwing the crown made of low thermal conductivity material, nimonic (an alloy of nickel) to the body of the piston, by keeping a gasket, made of nimonic, in between these two parts has been successfully done. The concept of LHR engine is to reduce heat loss to coolant and that too 29
specifically from the piston top to the body of the piston. It should be expected that the thickness of the air gap play an important role on the insulation effect in LHR engines. In order to increase the degree of the insulation, air gap is not only created in the piston but also in the liner. Conducted experiments on LHR engine which consisted of air gap insulated piston with superni crown and air gap insulated liner with superni insert with advanced injection timings and increased injection pressure with different alternate fuels like alcohols and nonedible vegetable oil and reported improved performance with LHR engine. Some research improved performance with retarded injection timing in LHR engine with diesel as fuel. The complexities involved like heterogeneous combustion of diesel fuel and air in minute fraction of time, various sizes of fuel droplets and their p enetration and evaporation etc., make diesel combustion usually not amenable for effective modelling. Two zone combustion model and a computer simulation model was used by for predicting the performance of the ceramic coated direct injection (DI) diesel engines. zero dimensional multi zone combustion models for predicting the performance of the LHR engine and validated the theoretical results with experimental data and found 57% deviation between these two. LHR diesel engine contains a twopart piston; the top crown made of low thermal conductivity material, superni90 screwed to aluminum body of the piston, providing a 3mmair gap in between the crown and the body of the piston. The optimum thickness of air gap in the air gap piston is found to be 3mm for better performance of the engine with superni inserts with diesel as fuel. A superni90 insert is screwed to the top portion of the liner in such a manner that an air gap of 3mm is maintained between the insert and the liner body. At 500 o C the thermal conductivity of superni90 and air are 20.92 and 0.057 W/mK respectively. Partially stabilized zirconium (PSZ) of thickness 500 microns is coated by means of plasma coating technique. CE has an aluminum alloy piston with a bore of 80mm and a stroke of 110mm. The rated output of the engine is 3.68 kW at a rate speed of 1500 rpm. The compression ratio is 16:1 and manufacturer‘s recommended injection timing and injection pressures are 27 ob TDC and 190 bar respectively. The fuel injector has 3 holes of size 0.25mm. The combustion chamber consists of a direct injection type with no special arrangement for swirling motion of air. The engine is connected to electric dynamometer for measuring brake power of the engine. Burette method is used for finding fuel consumption of the engine. Air consumption of the engine is measured by air box method. The naturally aspirated engine is provided with water cooling system in which inlet temperature of water is maintained at 60 o C by adjusting the water flow rate. The engine oil is provided with a pressure feed system and no temperature control is incorporated, for measuring the lube oil temperature. Copper shims of suitable size are provided in between the pump body and the engine frame, to vary the injection timing and its effect on the performance of the engine is studied, along with the change of injection pressures from 190 bar to 270 bar (in steps of 40 bar) using nozzle testing device. The maximum injection pressure is restricted to 270 bar due to practical difficulties involved. Exhaust gas temperature (EGT) is measured with thermocouples made of iron and iron constantan. Pollution levels of smoke and NO x are recorded by AVL smoke meter and Netel Chromatograph NOx analyzer respectively at the peak load operation of the engine. Piezo electric transducer, fitted on the cylinder head to measure pressure in the combustion chamber is connected to a console, which in turn is connected to Pentium personal computer. TDC encoder provided at the extended shaft of the dynamometer is connected to the console to measure the crank angle of the engine. A special P q software package evaluates the combustion characteristics such as peak pressure (PP), time of occurrence of peak pressure (TOPP), maximum rate of pressure rise (MRPR) and time of occurrence of maximum rate of pressure rise (TOMRPR) from the signals of
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pressure and crank angle at the peak load operation of the engine. Pressure crank angle diagram is obtained on the screen of the personal computer
Hydrogen Engines In general, getting an internal combustion engine to run on hydrogen is not difficult. Getting an internal combustion engine to run well, however, is more of a challenge. This section points out the key components and techniques required to make the difference between a hydrogen engine that just runs and one that runs well. The earliest attempt at developing a hydrogen engine was reported by Reverend W. Cecil in 1820. Cecil presented his work before the Cambridge 31
Philosophical Society in a paper entitled ―On the Application of Hydrogen Gas to Produce Moving Power in Machinery.‖ The engine itself operated on the vacuum principle, in which atmospheric pressure drives a piston back against a vacuum to produce power. The vac-uum is created by burning a hydrogen-air mixture, allowing it to expand and then cool. Although the engine ran satisfac-torily, vacuum engines never became practical Combustive Properties of Hydrogen
wide range of flammability low ignition energy small quenching distance high autoignition temperature high flame speed at stoichiometric ratios high diffusivity very low density
Wide Range of Flammability Hydrogen has a wide flammability range in comparison with all other fuels. As a result, hydrogen can be combusted in an internal combustion engine over a wide range of fuel-air mixtures. A significant advantage of this is that hydrogen can run on a lean mixture. A lean mixture is one in which the amount of fuel is less than the theoretical, stoichiometric or chemically ideal amount needed for combustion with a given amount of air. This is why it is fairly easy to get an engine to start on hydrogen. Generally, fuel economy is greater and the combustion reac-tion is more complete when a vehicle is run on a lean mix-ture. Additionally, the final combustion temperature is generally lower, reducing the amount of pollutants, such as nitrogen oxides, emitted in the exhaust. There is a limit to how lean the engine can be run, as lean operation can sig-nificantly reduce the power output due to a reduction in the volumetric heating value of the air/fuel mixture. Low Ignition Energy Hydrogen has very low ignition energy. The amount of energy needed to ignite hydrogen is about one order of magnitude less than that required for gasoline. This enables hydrogen engines to ignite lean mixtures and ensures prompt ignition. Unfortunately, the low ignition energy means that hot gases and hot spots on the cylinder can serve as sources of igni-tion, creating problems of premature ignition and flashback. Preventing this is one of the challenges associated with run-ning an engine on hydrogen. The wide flammability range of hydrogen means that almost any mixture can be ignited by a hot spot. Small Quenching Distance Hydrogen has a small quenching distance, smaller than gasoline. Consequently, hydrogen flames travel closer to the cylinder wall than other fuels before they extinguish. Thus, it is more difficult to quench a hydrogen flame than a gasoline flame. The smaller quenching distance can also increase the tendency for backfire since the flame from a hydrogen-air mixture more readily passes a nearly closed intake valve, than a hydrocarbon-air flame. High Autoignition Temperature Hydrogen has a relatively high autoignition temperature. This has important implications when a hydrogen-air mix-ture is compressed. In fact, the autoignition temperature is an important factor 32
in determining what compression ratio an engine can use, since the temperature rise during com pression is related to the compression ratio. The temperature may not exceed hydrogen‘s autoignition temperature without causing premature ignition. Thus, the absolute final temperature limits the compression ratio. The high autoignition temperature of hydrogen allows larger compression ratios to be used in a hydrogen engine than in a hydrocarbon engine. High Flame Speed Hydrogen has high flame speed at stoichiometric ratios. Un-der these conditions, the hydrogen flame speed is nearly an order of magnitude higher (faster) than that of gasoline. This means that hydrogen engines can more closely approach the thermodynamically ideal engine cycle. At leaner mixtures, however, the flame velocity decreases significantly High Diffusivity Hydrogen has very high diffusivity. This ability to disperse in air is considerably greater than gasoline and is advanta-geous for two main reasons. Firstly, it facilitates the forma-tion of a uniform mixture of fuel and air. Secondly, if a hydrogen leak develops, the hydrogen disperses rapidly. Thus, unsafe conditions can either be avoided or minimized. Low Density Hydrogen has very low density. This results in two problems when used in an internal combustion engine. Firstly, a very large volume is necessary to store enough hydrogen to give a vehicle an adequate driving range. Secondly, the energy den-sity of a hydrogen-air mixture, and hence the power output, is reduced. Air/Fuel Ratio = 2H2O The theoretical or stoichiometric combustion of hydrogen and oxygen is given as: 2H2 + O2 Moles of H2 for complete combustion
= 2 moles
Moles of O2 for complete combustion
= 1 mole
Because air is used as the oxidizer instead oxygen, the nitro-gen in the air needs to be included in the calculation:
Moles of N2 in air
= Moles of O2 x (79% N2 in air / 21% O2 in air) = 1 mole of O2 x (79% N2 in air / 21% O2 in air) = 3.762 moles N2
Number of moles of air
= Moles of O2 + moles of N2 = 1 + 3.762 = 4.762 moles of air
Weight of O2
= 1 mole of O2 x 32 g/mole = 32 g
Weight of N2
= 3.762 moles of N2 x 28 g/mole 33
= 105.33 g Weight of air
= weight of O2 + weight of N (1) = 32g + 105.33 g = 137.33 g
Weight of H2
= 2 moles of H2 x 2 g/mole =4g
Stoichiometric air/fuel (A/F) ratio for hydrogen and air is: A/F based on mass:
= mass of air/mass of fuel = 137.33 g / 4 g = 34.33:1
A/F based on volume:
= volume (moles) of air/volume (moles) of fuel = 4.762 / 2
= 2.4:1 The percent of the combustion chamber occupied by hydro-gen for a stoichiometric mixture: % H2
= volume (moles) of H2/total volume (2) = volume H2/(volume air + volume of H2) = 2 / (4.762 + 2)
= 29.6% As these calculations show, the stoichiometric or chemically correct A/F ratio for the complete combustion of hydrogen in air is about 34:1 by mass. This means that for complete combustion, 34 pounds of air are required for every pound of hydrogen. This is much higher than the 14.7:1 A/F ratio re-quired for gasoline. Since hydrogen is a gaseous fuel at ambient conditions it displaces more of the combustion chamber than a liquid fuel. Consequently less of the combustion chamber can be occupied by air. At stoichiometric conditions, hydrogen dis-places about 30% of the combustion chamber, compared to about 1 to 2% for gasoline. Figure 3-3 compares combustion chamber volumes and energy content for gasoline and hy-drogen fueled
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engines.
Depending the method used to meter the hydrogen to the engine, the power output compared to a gasoline engine can be anywhere from 85% (intake manifold injection) to 120% (high pressure injection). Because of hydrogen‘s wide range of flammability, hydrogen engines can run on A/F ratios of anywhere from 34:1 (stoichiometric) to 180:1. The A/F ratio can also be ex-pressed in terms of equivalence ratio, denoted by phi (Φ). Phi is equal to the stoichiometric A/F ratio divided by the actual A/F ratio. For a stoichiometric mixture, the actual A/F ratio is equal to the stoichiometric A/F ratio and thus the phi equals unity (one). For lean A/F ratios, phi will be a value less than one. For example, a phi of 0.5 means that there is only enough fuel available in the mixture to oxidize with half of the air available. Another way of saying this is that there is twice as much air available for combustion than is theo-retically required. Pre-Ignition Problems and Solutions The primary problem that has been encountered in the de-velopment of operational hydrogen engines is premature ig-nition. Premature ignition is a much greater problem in hydrogen fueled engines than in other IC engines, because of hydrogen‘s lower ignition energy, wider flammability range and shorter quenching distance. Premature ignition occurs when the fuel mixture in the com-bustion chamber becomes ignited before ignition by the spark plug, and results in an inefficient, rough running en-gine. Backfire conditions can also develop if the premature ignition occurs near the fuel intake valve and the resultant flame travels back into the induction system. A number of studies have been aimed at determining the cause of pre-ignition in hydrogen engines. Some of the re-sults suggest that pre-ignition are caused by hot spots in the combustion chamber, such as on a spark plug or exhaust valve, or on carbon deposits. Other research has shown that backfire can occur when there is overlap between the open-ing of the intake and exhaust valves. It is also believed that the pyrolysis (chemical decomposition brought about by heat) of oil suspended in the combustion chamber or in the crevices just above the top piston ring can contribute to pre-ignition. This pyrolysed oil can enter the combustion chamber 35
through blow-by from the crankcase (i.e. past the piston rings), through seepage past the valve guide seals and/or from the positive crankcase ventilation system (i.e. through the intake manifold). Fuel Delivery Systems Adapting or re-designing the fuel delivery system can be ef-fective in reducing or eliminating pre-ignition. Hydrogen fuel delivery system can be broken down into three main types: central injection (or ―carbureted‖), port injection and direct injection. Central and port fuel delivery systems injection form the fuel-air mixture during the intake stroke. In the case of cen-tral injection or a carburetor, the injection is at the inlet of the air intake manifold. In the case of port injection, it is in-jected at the inlet port. Direct cylinder injection is more technologically sophisti-cated and involves forming the fuel-air mixture inside the combustion cylinder after the air intake valve has closed. Central Injection or Carbureted Systems The simplest method of delivering fuel to a hydrogen engine is by way of a carburetor or central injection system. This system has advantages for a hydrogen engine. Firstly, cen-tral injection does not require the hydrogen supply pressure to be as high as for other methods. Secondly, central injec-tion or carburetors are used on gasoline engines, making it easy to convert a standard gasoline engine to a hydrogen or a gasoline/hydrogen engine. The disadvantage of central injection is that it is more sus-ceptible to irregular combustion due to pre-ignition and backfire. The greater amount of hydrogen/air mixture within the intake manifold compounds the effects of pre-ignition. Port Injection Systems The port injection fuel delivery system injects fuel directly into the intake manifold at each intake port, rather than drawing fuel in at a central point. Typically, the hydrogen is injected into the manifold after the beginning of the intake stroke. At this point conditions are much less severe and the probability for premature ignition is reduced. In port injection, the air is injected separately at the begin-ning of the intake stroke to dilute the hot residual gases and cool any hot spots. Since less gas (hydrogen or air) is in the manifold at any one time, any pre-ignition is less severe. The inlet supply pressure for port injection tends to be higher than for carbureted or central injection systems, but less than for direct injection systems. The constant volume injection (CVI) system uses a mechani-cal cam-operated device to time the injection of the hydrogen to each cylinder. The CVI block is shown on the far right of the photo with four fuel lines exiting on left side of the block (one fuel line for each cylinder).The electronic fuel injection (EFI) system meters the hydro-gen to each cylinder. This system uses individual electronic fuel injectors (solenoid valves) for each cylinder and are plumbed to a common fuel rail located down the center of the intake manifold. Whereas the CVI system uses constant injection timing and variable fuel rail pressure, the EFI sys-tem uses variable injection timing and constant fuel rail pressure. Direct Injection Systems More sophisticated hydrogen engines use direct injection into the combustion cylinder during the compression stroke. In direct injection, the intake valve is closed when the fuel is injected, completely avoiding premature ignition during the intake stroke. Consequently the engine cannot backfire into the intake manifold. The power output of a direct injected hydrogen engine is 20% more than for a gasoline engine and 42% more than a hydrogen engine using a carburetor. While 36
direct injection solves the problem of pre-ignition in the intake manifold, it does not necessarily prevent pre-ignition within the combustion chamber. In addition, due to the reduced mixing time of the air and fuel in a direct injec-tion engine, the air/fuel mixture can be non-homogenous. Studies have suggested this can lead to higher NOx emis-sions than the non-direct injection systems. Direct injection systems require a higher fuel rail pressure than the other methods. Ignition Systems Due to hydrogen‘s low ignition energy limit, igniting hydro-gen is easy and gasoline ignition systems can be used. At very lean air/fuel ratios (130:1 to 180:1) the flame velocity is reduced considerably and the use of a dual spark plug sys-tem is preferred. Ignition systems that use a waste spark system should not be used for hydrogen engines. These systems energize the spark each time the piston is at top dead center whether or not the piston is on the compression stroke or on its exhaust stroke. For gasoline engines, waste spark systems work well and are less expensive than other systems. For hydrogen en-gines, the waste sparks are a source of preignition. Spark plugs for a hydrogen engine should have a cold rating and have non-platinum tips. A cold-rated plug is one that transfers heat from the plug tip to the cylinder head quicker than a hot-rated spark plug. This means the chances of the spark plug tip igniting the air/fuel charge is reduced. Hot-rated spark plugs are designed to maintain a certain amount of heat so that carbon deposits do not accumulate. Since hydrogen does not contain carbon, hot-rated spark plugs do not serve a useful function. Platinum-tip spark plugs should also be avoided since platinum is a catalyst, causing hydrogen to oxidize with air. HCCI Engine Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases chemical energy into a sensible form that can be transformed in an engine into work and heat. Introduction HCCI has characteristics of the two most popular forms of combustion used in SI engines: homogeneous charge spark ignition (gasoline engines) and CI engines: stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, the density and temperature of the mixture are raised by compression until the entire mixture reacts spontaneously. Stratified charge compression ignition also relies on temperature and density increase resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion. The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without an aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations. Recent research has shown that the use of two fuels with different reactivities (such as gasoline and diesel) can help solve some of the difficulties of controlling HCCI ignition and burn rates. 37
RCCI or Reactivity Controlled Compression Ignition has been demonstrated to provide highly efficient, low emissions operation over wide load and speed ranges HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for combustion to occur. Another example is the "diesel" model aircraft engine. Operation Methods A mixture of fuel and air will ignite when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased by several different ways: High compression ratio Pre-heating of induction gases Forced induction Retained or re-inducted exhaust gases Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures. Advantages HCCI provides up to a 30-percent fuel savings, while meeting current emissions standards. Since HCCI engines are fuel-lean, they can operate at a Diesel-like compression ratios (>15), thus achieving higher efficiencies than conventional spark-ignited gasoline engines. Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Actually, because peak temperatures are significantly lower than in typical spark ignited engines, NOx levels are almost negligible. Additionally, the premixed lean mixture does not produce soot. HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. In regards to gasoline engines, the omission of throttle losses improves HCCI efficiency. Disadvantages High in-cylinder peak pressures may cause damage to the engine. High heat release and pressure rise rates contribute to engine wear. The autoignition event is difficult to control, unlike the ignition event in spark ignition (SI) and diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively. HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions. Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low in-cylinder temperatures) and trapped crevice gases, respectively. Control Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult to control than other popular modern combustion engines, such as Spark Ignition (SI)
38
and Diesel. In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel engines, combustion begins when the fuel is injected into compressed air. In both cases, the timing of combustion is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever the appropriate conditions are reached. This means that there is no well-defined combustion initiator that can be directly controlled. Engines can be designed so that the ignition conditions occur at a desirable timing. To achieve dynamic operation in an HCCI engine, the control system must change the conditions that induce combustion. Thus, the engine must control either the compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or reinducted exhaust. Several control approaches are discussed below. Variable compression ratio There are several methods for modulating both the geometric and effective compression ratio. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This is the system used in "diesel" model aircraft engines. The effective compression ratio can be reduced from the geometric ratio by closing the intake valve either very late or very early with some form of variable valve actuation (i.e. variable valve timing permitting Miller cycle). Both of the approaches mentioned above require some amounts of energy to achieve fast responses. Additionally, implementation is expensive. Control of an HCCI engine using variable compression ratio strategies has been shown effective. The effect of compression ratio on HCCI combustion has also been studied extensively. Variable induction temperature In HCCI engines, the autoignition event is highly sensitive to temperature. Various methods have been developed which use temperature to control combustion timing. The simplest method uses resistance heaters to vary the inlet temperature, but this approach is slow (cannot change on a cycle-to-cycle basis). Another technique is known as fast thermal management (FTM). It is accomplished by rapidly varying the cycle to cycle intake charge temperature by rapidly mixing hot and cold air streams. It is also expensive to implement and has limited bandwidth associated with actuator energy Variable exhaust gas percentage Exhaust gas can be very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine work. Hot combustion products conversely will increase the temperature of the gases in the cylinder and advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally. Variable valve actuation Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving finer control over the temperature-pressure-time history within the combustion chamber. VVA can achieve this via two distinct methods: Controlling the effective compression ratio: A variable duration VVA system on intake can control the point at which the intake valve closes. If this is retarded past bottom dead center (BDC), then the compression ratio will change, altering the in-cylinder pressuretime history prior to combustion.
f hot exhaust gas retained in the combustion chamber: A VVA system can be used to control the amount of hot internal exhaust gas recirculation 39
(EGR) within the combustion chamber. This can be achieved with several methods, including valve re-opening and changes in valve overlap. By balancing the percentage of cooled external EGR with the hot internal EGR generated by a VVA system, it may be possible to control the in-cylinder temperature. While electro-hydraulic and camless VVA systems can be used to give a great deal of control over the valve event, the componentry for such systems is currently complicated and expensive. Mechanical variable lift and duration systems, however, although still being more complex than a standard valvetrain, are far cheaper and less complicated. If the desired VVA characteristic is known, then it is relatively simple to configure such systems to achieve the necessary control over the valve lift curve. Also see variable valve timing. Variable fuel ignition quality Another means to extend the operating range is to control the onset of ignition and the heat release rate is by manipulating fuel itself. This is usually carried out by adopting multiple fuels and blending them "on the fly" for the same engine . Examples could be blending of commercial gasoline and diesel fuels , adopting natural gas or ethanol ". This can be achieved in a number of ways; Blending fuels upstream of the engine: Two fuels are mixed in the liquid phase, one with low resistance to ignition (such as diesel fuel) and a second with a greater resistance (gasoline), the timing of ignition is controlled by varying the compositional ratio of these fuels. Fuel is then delivered using either a port or direct injection event. Having two fuel circuits: Fuel A can be injected in the intake duct (port injection) and Fuel B using a direct injection (in-cylinder) event, the proportion of these fuels can be used to control ignition, heat release rate as well as exhaust gas emissions Direct Injection: PCCI or PPCI Combustion Compression Ignition Direct Injection (CIDI) combustion is a well established means of controlling ignition timing and heat release rate and is adopted in Diesel engines combustion. Partially Pre-mixed Charge Compression Ignition (PPCI) also known as Premixed Charge Compression Ignition (PCCI) is a compromise between achieving the control of CIDI combustion but with the exhaust gas emissions of HCCI, specifically soot . On a fundamental level, this means that the heat release rate is controlled preparing the combustible mixture in such a way that combustion occurs over a longer time duration and is less prone to knocking. This is done by timing the injection event such that the combustible mixture has a wider range of air/fuel ratios at the point of ignition, thus ignition occurs in different regions of the combustion chamber at different times - slowing the heat release rate. Furthermore this mixture is prepared such that when combustion occurs there are fewer rich pockets thus reducing the tendency for soot formation . The adoption of high EGR and adoption of diesel fuels with a greater resistance to ignition (more "gasoline like") enables longer mixing times prior to ignition and thus fewer rich pockets thus resulting in the possibility of both lower soot emissions and NOx. High peak pressures and heat release rates In a typical gasoline or diesel engine, combustion occurs via a flame. Hence at any point in time, only a fraction of the total fuel is burning. This results in low peak pressures and low energy release rates. In HCCI, however, the entire fuel/air mixture ignites and burns nearly simultaneously resulting in high peak pressures and high energy release rates. To withstand the higher pressures, the engine has to be structurally stronger and therefore heavier. Several strategies have been proposed to lower the rate of combustion. Two different fuels, with different autoignition properties, can be used to lower the combustion speed. However, this requires
40
significant infrastructure to implement. Another approach uses dilution (i.e. with exhaust gases) to reduce the pressure and combustion rates at the cost of work production. Power In both a spark ignition engine and diesel engine, power can be increased by introducing more fuel into the combustion chamber. These engines can withstand a boost in power because the heat release rate in these engines is slow. However, in HCCI engines the entire mixture burns nearly simultaneously. Increasing the fuel/air ratio will result in even higher peak pressures and heat release rates. In addition, many of the viable control strategies for HCCI require thermal preheating of the charge which reduces the density and hence the mass of the air/fuel charge in the combustion chamber, reducing power. These factors make increasing the power in HCCI engines challenging. One way to increase power is to use fuels with different autoignition properties. This will lower the heat release rate and peak pressures and will make it possible to increase the equivalence ratio. Another way is to thermally stratify the charge so that different points in the compressed charge will have different temperatures and will burn at different times lowering the heat release rate making it possible to increase power. A third way is to run the engine in HCCI mode only at part load conditions and run it as a diesel or spark ignition engine at full or near full load conditions. Since much more research is required to successfully implement thermal stratification in the compressed charge, the last approach is being studied more intensively. Emissions Because HCCI operates on lean mixtures, the peak temperatures are lower in comparison to spark ignition (SI) and Diesel engines. The low peak temperatures prevent the formation of NOx. This leads to NOx emissions at levels far less than those found in traditional engines. However, the low peak temperatures also lead to incomplete burning of fuel, especially near the walls of the combustion chamber. This leads to high carbon monoxide and hydrocarbon emissions. An oxidizing catalyst would be effective at removing the regulated species because the exhaust is still oxygen rich. Difference from Knock Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in a spark ignited engine spontaneously ignite. The unburnt gas ahead of the flame is compressed as the flame propagates and the pressure in the combustion chamber rises. The high pressure and corresponding high temperature of unburnt reactants can cause them to spontaneously ignite. This causes a shock wave to traverse from the end gas region and an expansion wave to traverse into the end gas region. The two waves reflect off the boundaries of the combustion chamber and interact to produce high amplitude standing standing waves. A similar ignition process process occurs in HCCI. HCCI. However, rather than part of the reactant mixture being ignited by compression ahead of a flame front, ignition in HCCI engines occurs due to piston compression. In HCCI, the entire reactant mixture ignites (nearly) simultaneously. Since there are very little or no pressure differences between the different regions of the gas, there is no shock wave propagation and hence no knocking. However at high loads (i.e. high fuel/air ratios), knocking is a possibility even in HCCI Simulation of HCCI Engines The development of computational models for simulating combustion and heat release rates of HCCI engines has required the advancement of detailed chemistry models. This is largely because ignition is most sensitive to chemical kinetics rather than turbulence/spray or spark processes as are typical in direct injection compression ignition or spark ignition engines. 41
Computational models have demonstrated the importance of accounting for the fact that the incylinder mixture is actually in-homogeneous, particularly in terms of temperature. This inhomogeneity is driven by turbulent mixing and heat transfer from the combustion chamber walls, the amount of temperature stratification dictates the rate of heat release and thus tendency to knock . This limits the applicability of considering the in-cylinder mixture as a single zone resulting in the uptake of 3D computational fluid dynamics and faster solving probability density function modelling codes. Other Applications of HCCI Research To date there have only been few prototype engines running in HCCI mode however the research efforts invested into HCCI research have disseminated into/resulted in direct advancements in fuel and engine development. Examples are; PCCI/PPCI combustion - A hybrid of HCCI and conventional diesel combustion offing more control over ignition and heat release rates with lower soot and NOx emissions. Advancements in fuel modelling - HCCI combustion is driven mainly by chemical kinetics rather than turbulent mixing or injection, this reduces the complexity of simulating the chemistry which results in fuel oxidation and emissions formation. This has led to increasing interest and development of chemical kinetics which describe hydrocarbon oxidation. Fuel blending applications- Due to the advancements in fuel modelling, it is now possible to carry out detailed simulations of hydrocarbon fuel oxidation, enabling simulations of practical fuels such as gasoline/diesel and ethanol . Engineers can now blend fuels virtually and determine how they will perform in an engine context
VCR Engine Variable Compression Ratio (VCR) engines Because cylinder bore diameter, piston stroke length and combustion chamber volume are almost always constant, the compression ratio for a given engine is almost always constant, until engine wear takes its toll. One exception is the experimental Saab Variable Compression engine (SVC). This engine, designed by Saab Automobile, uses a technique that dynamically alters the volume of the combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR). The Atkinson cycle engine was one of the the first attempts at variable compression. Since the compression ratio is the ratio between dynamic and static volumes of the combustion chamber the Atkinson cycle's method of increasing the length of the power stroke compared to the intake stroke ultimately altered the compression ratio at different stages of the cycle . Dynamic compression ratio The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume. However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected" compression ratio is commonly called the "dynamic compression ratio". This ratio is higher with with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio. The actual position of the piston can be determined by trigonometry, using the 42
stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by b y compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine en gine with lower compression but earlier intake valve closure Additionally, the cylinder pressure developed when an engine is running will be higher than that shown in a compression test for several reasons. The much higher velocity of a piston when an engine is running versus cranking allows less time for pressure to bleed past the piston rings into the crankcase. a running engine is coating the cylinder walls with much more oil than an engine that is being cranked at low RPM, which helps the seal. the higher temperature of the cylinder will create higher pressures when running vs. a static test, even a test performed with the engine near n ear operating temperature. A running engine does not stop taking air & fuel into the cylinder when the piston reaches BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum and continues briefly after the vacuum ceases (in the same respect that rapidly opening a door will create a draft that continues after movement of the door ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and exhaust tuning determine how effectively an engine en gine scavenges. Compression ratio versus overall pressure ratio Compression ratio and overall pressure ratio are interrelated as follows:
n o i s s e r p o i m o t a C r
2:1
3:1
5:1
10:1
15:1
20:1
25:1
35:1
e r u s o s i e t r a P r
2.64:1
4.66:1
9.52:1
25.12:1
44.31: 1
66.2 9:1
90.60: 1
145.11 :1
The reason for this difference is that compression ratio is defined via t he volume reduction:
while pressure ratio is defined as the pressure increase:
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In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas. With those two assumptions we can define the relationship between change of volume and change of pressure as follows:
where γ is the ratio of specific heats for air (approximately 1.4). The values in the table above are derived using this formula. Note that in reality the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur. Surface Ignition Engines Relative surface ignition resistance of fuels in engines could be predicted from data obtained in the laboratory: heat of combustion of the fuel, heat capacities of the fuel and its products, radiant energy of flames, ignition energies at the temperature and pressure existing in an engine at the time surface ignition occurs, extent and nature of preflame reactions at these temperatures and pressures, effect of fuel-air ratio on energy required for ignition at the temperature and pressure existing in an engine, and the relative activity of the igniting surface under the conditions existing in an engine. General experience has been that there are no short cuts. It is simpler and more satisfactory to measure surfaceignition resistance in an engine operating under the conditions of interest. VVTI Engines VVT-i, or Variable Valve Timing with intelligence , is an automobile variable valve timing technology developed by Toyota, similar in performance to the BMW's VANOS. The Toyota VVT-i system replaces the Toyota VVT offered starting in 24 December 1991 on the 5-valve per cylinder 4A-GE engine. The VVT system is a 2-stage hydraulically controlled cam phasing system. The Toyota motors CEO has been reported to have said, "VVT is the heart of every modern Toyota! VVT-i, introduced in 1996, varies the timing of the intake valves by adjusting the relationship between the camshaft drive (belt, scissor-gear or chain) and intake camshaft. Engine oil pressure is applied to an actuator to adjust the camshaft position. Adjustments in the overlap time between the exhaust valve closing and intake valve opening result in improved engine efficiency. Variants of the system, including VVTL-i, Dual VVT-i, Triple VVT-iE, and Valvematic, have followed. VVTL-i VVTL-i (Variable Valve Timing and Lift intelligent system) is a version that can alter valve lift (and duration) as well as valve timing. In the case of the 16 valve 2ZZ-GE, the engine has 2 camshafts, one operating intake valves and one operating exhaust valves. Each camshaft has two lobes per cylinder, one low rpm lobe and one high rpm, high lift, long duration lobe. Each cylinder has two intake valves and two exhaust valves. Each set of two valves are controlled by one rocker arm, which is operated by the camshaft. Each rocker arm has a slipper follower mounted to the rocker arm with a spring, allowing the slipper follower to move up and down with the high lobe without affecting the rocker arm. When the engine is operating below 60007000 rpm (dependent on year, car, and ECU installed), the low lobe is operating the rocker arm 44
and thus the valves. When the engine is operating above the lift engagement point, the ECU activates an oil pressure switch which pushes a sliding pin under the slipper follower on each rocker arm. This in effect, switches to the high lobe causing high lift and longer duration. In 1998, Dual VVT-i which adjusts timing on both intake and exhaust camshafts was first introduced on the RS200 Altezza's 3S-GE engine. Dual VVT-i is also found in Toyota's new generation V6 engine, the 3.5-liter 2GR-FE first appearing on the 2005 Avalon. This engine can now be found on numerous Toyota and Lexus models. By adjusting the valve timing, engine start and stop occurs almost unnoticeably at minimum compression. In addition fast heating of the catalytic converter to its light-off temperature is possible thereby reducing hydrocarbon emissions considerably. Toyota's UR engine V8 also uses this technology. Dual VVT-i was later introduced to Toyota's latest small 4-cylinder ZR engines found in compact vehicles such as the new Toyota Corolla and Scion xD and in larger 4-cylinder AR engines found in the Camry and RAV4. VVT-iE VVT-iE (Variable Valve Timing - intelligent by Electric motor) is a version of Dual VVT-i that uses an electrically operated actuator to adjust and maintain intake camshaft timing. The exhaust camshaft timing is still controlled using a hydraulic actuator. This form of variable valve timing technology was developed initially for Lexus vehicles. This system was first introduced on the 2007MY Lexus LS 460 as 1UR engine. The electric motor in the actuator spins together with the intake camshaft as the engine runs. To maintain camshaft timing, the actuator motor will operate at the same speed as the camshaft. To advance the camshaft timing, the actuator motor will rotate slightly faster than the camshaft speed. To retard camshaft timing, the actuator motor will rotate slightly slower than camshaft speed. The speed difference between the actuator motor and camshaft timing is used to operate a mechanism that varies the camshaft timing. The benefit of the electric actuation is enhanced response and accuracy at low engine speeds and at lower temperatures. Furthermore, it ensures precise positioning of the camshaft for and immediately after engine starting, as well as a greater total range of adjustment. The combination of these factors allows more precise control, resulting in an improvement of both fuel economy, engine output and emissions performance. Valvematic It offers continuous adjustment to lift volume and timing. Valvematic made its first appearance in 2007 in the Noah and later in early-2009 in the ZR High Energy And Power Density Batteries A basic requirement for electric vehicles (EVs) is a portable source of electrical energy, which is converted to mechanical energy in the electric motor for vehicle propulsion. Electrical energy is typically obtained through conversion of chemical energy stored in devices such as batteries and fuel cells. A flywheel is an alternative portable source in which energy is stored in mechanical form to be converted into electrical energy on demand for vehicle propulsion. The portable electrical energy source presents the biggest obstacle in commercialization of EVs. A near-term solution for minimizing the environmental pollution problem due to the absence of a suitable, high-energy-density energy source for EVs is perceived in the hybrid electric vehicles (HEVs) that combine propulsion efforts from gasoline engines and electric motors. Battery technology has been undergoing extensive research and development efforts over the past 30 years, yet there is currently no battery that can deliver an acceptable combination of power, energy, and life cycle for high-volume production vehicles. The small number of EVs and HEVs that were 45
introduced in the market used batteries that were too expensive and have short calendar life, making the batteries the biggest impediment in commercializing EVs and HEVs
The components of the battery cell are described as follows: • Positive electrode: The positive electrode is an oxide or sulfide or some other compound that is capable of being reduced during cell discharge. This electrode consumes electrons from the external circuit during cell discharge. Examples of positive electrodes are lead oxide (PbO2) and nickel oxy hydroxide (NiOOH). The electrode materials are in the solid state. • Negative electrode: The negative electrode is a metal or an alloy that is capable of being oxidized during cell discharge. This electrode generates electrons in the external circuit during cell discharge. Examples of negative electodes are lead (Pb) and cadmium (Cd). Negative electrode materials are also in the solid state within the battery cell. • Electrolyte : The electrolyte is the medium that permits ionic conduction between positive and negative electrodes of a cell. The electrolyte must have high and selective conductivity for the ions that take part in electrode reactions, but it must be a nonconductor for electrons in order to avoid self-discharge of batteries. The electrolyte may be liquid, gel, or solid material. Also, the electrolyte can be acidic or alkaline, depending on the type of battery. Traditional batteries such as lead-acid and nickelcadmium use liquid electrolytes. In lead-acid batteries, the electrolyte is the aqueous solution of sulfuric acid [H2SO4(aq)]. Advanced batteries currently under development for EVs, such as sealed lead-acid, nickel-metal-hydride (NiMH), and lithium-ion batteries use an electrolyte that is gel, paste, or resin. Lithium-polymer batteries use a solid electrolyte. • Separator: The separator is the electrically insulating layer of material that physically separates electrodes of opposite polarity. Separators must be permeable to the ions of the electrolyte and may also have the function of storing or immobilizing the electrolyte. Present day separators are made from synthetic polymers. 46
The major types of rechargeable batteries considered for EV and HEV applications are: • Lead-acid (Pb-acid) • Nickel-cadmium (NiCd) • Nickel-metal-hydride (NiMH) • Lithium-ion (Li-ion) • Lithium-polymer (Li-poly) • Sodium-sulfur (NaS) • Zinc-air (Zn-Air) Battery Energy The energy of a battery is measured in terms of capacity and discharge voltage. To calculate energy, the capacity of the battery must be expressed in coulombs. A measurement of 1 Ah is equivalent to 3600 C, while 1 V refers to 1 J (J for joule) of work required to move 1 C charge from the negative to positive electrode. Therefore, the stored electrical potential energy in a 12 V, 100 Ah battery is (12)(3.6×105)J=4.32 MJ. In general, the theoretical stored energy is:
where Vbat is the nominal no load terminal voltage, and QT is the theoretical capacity in C. Therefore, using Equation 3.1, we have
The practical available energy is:
where t0 is the time at which the battery is fully charged, tcut is the time at which battery terminal voltage is at Vcut, v is the battery terminal voltage, and/is the battery discharge current. EP is dependent on the manner in which the battery is discharged. Specific Energy The specific energy of a battery is given by
The unit for specific energy is Wh/kg. The theoretical specific energy of a battery is
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If the mass of the battery MB is proportional to the mass of the limiting reactant of the battery mR, then SET is independent of mass. The specific energy of a lead-acid battery is 35 to 50 Wh/kg at Q/3 rate. Because EP varies with discharge rate, the practical specific energy SEP is also variable. The term energy density is also used in the literature to quantify the quality of a battery or other energy source. Energy density refers to the energy per unit volume of a battery. The unit for energy density is Wh/liter. POWER DENSITY APPROACH Given a battery terminal power profile p(t), the specific power SP(t) profile can be obtained by dividing the power profile p(t) by the total vehicle mass Mv. The battery is assumed to be fully charged at t=0. Let fy(t) be equal to the fraction of available energy provided by the battery from 0 to t, where fr(0)=0, because SoD(0)=0. Now, consider the time interval dt over which a fraction of available energy dfr is provided by the battery:
If dE is the energy provided by the battery to the electrical circuit over dt, and Eavail is the total available energy, Now Eavail is a function of instantaneous power, and we know that Therefore,
We will use Peukert‘s equation to relate specific power and specific energy as follows:
Therefore Integrating,
,
At t100%, 100%, all of the available energy has been used by the system.
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UNIT III VEHICLE OPERATION AND CONTROL Computer Control For Pollution And Noise Control Noise mitigation is a set of strategies to reduce noise pollution. The main areas of noise mitigation or abatement are: transportation noise control, architectural design, and occupational noise control. Roadway noise and aircraft noise are the most pervasive sources of environmental noise worldwide, and little change has been effected in source control in these areas since the start of the problem,[citation needed ] a possible exception being the development of hybrid and electric vehicles. Multiple techniques have been developed to address interior sound levels, many of which are encouraged by local building codes; in the best case of project designs, planners are encouraged to work with design engineers to examine trade-offs of roadway design and architectural design. These techniques include design of exterior walls, party walls and floor and ceiling assemblies; moreover, there are a host of specialized means for dampening reverberation from special purpose rooms such as auditoria, concert halls, dining areas, audio recording rooms, and meeting rooms. Many of these techniques rely upon materials science applications of constructing sound baffles or using sound-absorbing liners for interior spaces. Industrial noise control is really a subset of interior architectural control of noise, with emphasis upon specific methods of sound isolation from industrial machinery and for protection of workers at their task stations. Sound masking is the active addition of noise to reduce the annoyance of certain sounds; the opposite of soundproofing. Computer Control for fuel economy Each year, cars seem to get more and more complicated. Cars today might have as many as 50 microprocessors on them. Although these microprocessors make it more difficult for you to work on your own car, some of them actually make your car easier to service. Some of the reasons for this increase in the number of microprocessors are: The need for sophisticated engine controls to meet emissions and fuel-economy standards Advanced diagnostics Simplification of the manufacture and design of cars Reduction of the amount of wiring in cars New safety features New comfort and convenience features Transducers In general terms, the transduction process involves the transformation of one form of energy into another form. This process consists of sensing with specificity the input energy from the measurand by means of a "sensing element" and then transforming it into another form by a "transduction element." The sensor-transduction element combination shown in figure below will henceforth be referred to as the "transducer". Measurand relates to the
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quantity, property, or state
that the transducer seeks to translate into an electrical output
Transducers may be classified as self-generating or externally powered. Self-generating transducers develop their own voltage or current and in the process absorb all the energy needed from the measurand. Externally powered transducers, as the name implies, must have power supplied from an external source, though they may absorb some energy from the measurand
TRANSDUCTION MECHANISMS Capacitive Inductive and electromagnetic Resistive Resistive and thermo resistive Piezo resistive effect Hall effect Lateral effect Extrinsic, inter ferometric and evanesquenching cent effects in optical fiber Magneto resistive effect Tunneling effect
Thermoelectric effects (Seebeck and Peltier) Ionization effects Photoelectric effect Photo resistive effect Photovoltaic effect Acous tooptic effect Fluorescence and fluorescence Field effect Doppler effect
MEASURANDS Displacement Position Velocity Acceleration Force and load Strain Rotation and encoding Vibrations Flow Temperature Pressure Vacuum
Atomic and surface profiles Gas concentration and pH pH and partial pressure of O2 and CO2 in blood Infrared radiation Torque Magnetic field Acoustic fields Medical imaging Non-destructive testing Audio fields and noise Rotation and guidance
SELECTION OF TRANSDUCERS When selecting a transducer, in addition to the question of cost, careful attention must be given to the following Sensitivity
Output impedance 50
Range Physical properties Loading effect and distortion Frequency response Electrical output format
Power requirements Noise Error or accuracy Calibration Environment
MECHANOELECTRIC TRANSDUCERS Displacement transducers Displacement transducers can measure either linear displacement (translation) or angular displacement (rotation). They can also be classified according to the principle of transduction on which they are based. We can distinguish between, for instance, resistive, capacitive, inductive and optical translation or rotation transducers. These mechanical transducers are also referred to as gauges or sensors. Velocity transducers There are two transducers used to measure velocity: translational and angular transducers. The measure of velocity is often converted to a frequency measurement. The conversion is performed by a strip or disc on which a large number of marks (detection elements) have been put at equal distances x. The velocity can be calculated from v=Dxn/t = Dxf, in which n is the number of detection element which passes the detector in t seconds and f is the frequency of output signal. The detection can be performed optically, mechanically, inductively or pacitively. Acceleration transducers Transducers for measuring acceleration rely on the measurement of the force F required to give a known mass (the seismic mass m) the same acceleration, a, as the measurement object. From the force and the mass, the acceleration is determined: a=F/m. The extra mass has to be kept to a minimum, especially when the measurement object is highly elastic or has a low mass (extra ass influences the measure acceleration). Actuators An actuator is the device that brings about the mechanical movements required for any physical process in the factory. Internally, actuators can be broken down into two separate modules: the signal amplifier and the transducer. The amplifier converts the (low power) control signal into a high power signal that is fed into the transducer; the transducer converts the energy of the amplified control signal into work; this process usually involves converting from one form of energy into another, e.g. electrical motors convert electrical energy into kinetic energy
Our objective here is not to study the details of any type of actuators – this is out of the scope of any single course. However, we shall look at the basic principles behind the working of several common actuators. The goal is to get an understanding of what type of actuators are used in different applications and why, and to some extent, understand their benefits and limitations. Everyone is familiar with electric motors – there are probably over 50 motors in any average home (e.g. inside a fan, hair-dryer, electric tooth-brush, vibrating phones, quartz watch, toys, microwave oven, fridge, computer, printer, etc.) We first look at basics of how motors work; next, we look at some simple motor types: DC, AC, Stepper and Servo-motors. Next we look at non-electrical actuators, including Hydraulic systems and Pneumatic actuators 51
Electric Motors: All electric motors use electromagnetic induction to generate a force on a rotational element called the rotor. The torque required to rotate the rotor is created due to the interaction of magnetic fields generated by the rotor, and the part surrounding it, which is fixed, and called the stator. This is easier to see if we first think in terms of magnets. Figure 3.2shows a typical DC motor. Let‘s see how this motor works. A high-strength permanent magnet (field magnet) creates a magnetic field in the space occupied by the rotor. In this DC motor, the rotor is made of an axle with three radial arms fixed at equal angles around it. The axle is supported to the stator by a bearing, so it can rotate freely with respect to the stator. On each of the three arms, a conducting wire is wound in a coil, as shown; the direction of this winding is important. The coils are connected to three separate electrical contacts as shown. You can think of these contacts as a short tube of copper that has been cut to separate it into three pieces; the split ring made by these contacts is called the commutator. Hydraulic Actuators: Hydraulic systems are often used for driving high-power machine tools and industrial robots. They can deliver high power and forces. They also suffer from maintenance problems (e.g. leakage of the hydraulic fluid, dirt/contamination of fluid.) Hydraulic actuators may be linear, or rotary.
Figure 3.7 shows the control system for a typical hydraulic rotary drive. The hydraulic power supply actually comprises of a pump, usually driven by a 3-phase electric motor. The pump may be a gear pump, or radial/axial displacement type. The functioning of the hydraulic motor itself is the opposite of the hydraulic pump. The servo control is exerted by applying a control voltage to a solenoid driven valve; the flow rate of oil through the valve is proportional to the voltage applied (itself proportional to the valve opening). Pneumatic actuators Pneumatic actuators work, in principle, similar to hydraulic actuators. The most common pneumatic controls are linear actuators, which are basically a piston-cylinder assembly connected to a supply tube of compressed air. Since air is highly compressible, pneumatic drives are frequently not used for high force transmission, nor are much good for accurate position control. Typically, they are used for fixed motion of small objects that are very common on assembly and transfer lines. 52
Discrete control of pneumatic systems: Many operations on automation lines require steps like feeding of a part into a fixture, pushing the part on/off a conveyor (or between parallel conveyors, to change direction of travel), or transporting a part between several closely placed assembly or testing stations. Discrete pneumatic controls are ideally suited for such tasks. Figure 3.8 shows an example configuration for testing a part. For example, if a factory is producing high-voltage switches; each switch must be tested by applying high voltage and checking that there are no sparks. When the overhead testing machine is testing a part, all three pistons are retracted (P1, P2, and P3 are OFF). As soon as testing is over, the testing machine sends two signals to the pneumatic relay circuit: the first one indicates that the testing is complete, and the second one indicates if part is good. The first signal is used to open the relay valve for P3, which unloads the part to the out-chute. Full extension of P3 also triggers a limit switch (L1) on the chute. If the limit switch as well as the "part-is-good" signal are ON, P2 is activated, pushing the part into the collection bin. P2 then retracts. Full retraction of P3 triggers another limit switch, L2. Whenever a part is waiting on the in-chute, limit switch L3 is activated. On activation of L3 and L2, the relay valve for P1 is opened, pushing the new part onto the testing fixture.
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Information Technology For Receiving Proper Information And Operation Of The Vehicle Like Optimum Speed And Direction Excessive or inappropriate speed is a significant factor in serious road accidents. Road safety authorities around the world devote considerable resources to addressing the speeding problem particularly compliance with speed limits. One countermeasure that is gaining increasing ttention is the use in-vehicle technology to assist drivers keep to speed limits or even prevent the vehicle from exceeding speed limits on all roads at all times. This is known as Intelligent Speed Adaptation (ISA). CONTRIBUTION OF SPEEDING TO ACCIDENTS The driver was charged with a speeding offence (normally exceeding the speed limit) or • The Police report stated that the vehicle was speeding or • The vehicle movement indicated inappropriate speed (not necessarily in excess of the speed limit). For example loss of control or skidding while negotiating a curve when there were no other factors to explain the incident. It can be seen that there are two aspects to speeding: "Excessive speed" where at least one vehicle was exceeding the speed limit and "Inappropriate speed" where a vehicle was obeying the speed limit but was travelling too fast for the road conditions. INTELLIGENT SPEED ADAPTATION In essence ISA systems constantly monitor the local speed limit and the vehicle speed and take action when the vehicle is found to be exceeding the speed limit. This action can be advisory or "passive", where the driver is warned, or "active" where there is some degree of automated control of vehicle speed. To achieve this ISA systems need to know when the vehicle has entered a new speed zone and when variable speed zones are in force (e.g. school zones). Additional ISA features might be the ability to detect temporary speed zones (such as at accident scenes or near roadworks) and knowledge of advisory speeds such as sharp curves and stop signs (in effect, a stop sign signifies a speed limit of zero). Speed and Location Technology To function, the ISA system needs to know the location of the vehicle, accurate to a few metres. This location information must be linked to a detailed digital map (or its equivalent) containing information such as local speed limits, and the location of known variable speed zones (e.g. schools). Advanced ISA has the capacity for real time updating to include information on areas where speed limits should be reduced due to weather conditions (rain, snow, ice, fog) or around accident scenes and road works. There are three main types of technology currently available for determining location (and, in turn local speed limits). Some of these also determine the speed of the vehicle independent of the vehicle‘s own speedometer (which can be out by as much as 10%). These technologies are: GPS Radio Beacons Dead Reckoning Global Positioning System (GPS) - GPS is based on a network of satellites that constantly transmit radio signals. GPS radio receivers pick up these transmissions and, by comparing the signals from several satellites, are able to pinpoint the receiver‘s location, usually to within a few meters (for advanced receivers). There are currently 24 satellites making up the GPS network and their orbits are configured so that a minimum of five satellites are generally available at any one time for terrestrial users. In theory four satellites is the minimum number of satellites
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required to determine a precise three dimensional position (latitude, longitude and altitude). Despite its popularity, GPS is subject to a number of fundamental problems related to the accuracy of the determined position. The receiver still gets the signal from the satellites, but due to satellites ephemeris uncertainties, propagation errors, timing errors, multiple signal propagation paths (eg reflected signals) and receiver noise, the position given is not always accurate (Kao 1991). Usually these inaccuracies are insignificant for car navigation purposes but sometimes they can be up to hundreds of metres. Furthermore, because GPS relies upon a signal transmitted from a satellite in orbit it does not function when the receiver is underground or in a tunnel and the signal can become weak if tall buildings, trees or heavy clouds come between the receiver and the satellites. Current improvements being made to the GPS satellite network and receivers will help to increase GPS reliability and accuracy but are unlikely to overcome some of the fundamental hortcomings of GPS Radio beacons - Roadside radio beacons work by transmitting data to a receiver in the car. The beacons constantly transmit data, which the car mounted receiver picks up as it passes each beacon. This data could include local speed limits, school zones, variable speed limits or traffic warnings (roadworks, weather, etc). Beacons could be placed near (or on) speed limit signs or other roadside furniture or in the road itself. Mobile beacons could be deployed (that would override fixed beacons or GPS) for use around accident scenes, during poor weather or during special events. A problem with beacon technology is that the vehicle needs to be in the proximity of a beacon in order to determine the speed limit. There would need to be some redundancy in the system to allow for broken beacons and transmission errors. Also, to work properly, every intersection where the roads had different speed limits would need a set of beacons (eg side roads joining arterial roads). Dead reckoning - Dead reckoning (DR) uses a mechanical system linked to the vehicle‘s driving assembly, to predict the path taken by the vehicle. By measuring items such as the rotation of the road wheels and the angle of the steering wheel a reasonably accurate estimation of the vehicle‘s speed and location can be made. More accurate systems rely on specialised sensors (accelerometers, flux gate compass, gyroscope). However, dead reckoning requires the vehicle to begin at a known, fixed point near the start of the journey. Inaccuracies result from a variety of sources, including changes to tyre diameter as the tyres warm up. Errors gradually accumulate and become unworkable unless there is periodic correction with a new reference point. For this reason dead-reckoning needs to work in conjunction with another system, such as GPS. Some top-end GPS based navigation systems use dead reckoning as a backup system in case GPS signal is lost (such as in tunnels). Map matching - Once the vehicle location has been determined, the accuracy can be checked by digital map matching. Under this scheme, the assumed location is compared with known roads (such as those available from a navigation map) and the system snaps to the most likely location on a known road. A check is also made against the last known position to determine if the new location is physically reasonable. The most accurate ISA systems possible today use a combination of GPS, dead reckoning and map matching Optical recognition systems - So far optical recognition technology has been focussed on recognising speed signs only, however other roadside objects, such as the reflective ‗cats eyes‘ that divide lanes could possibly be used. This system requires the vehicle to pass a speed sign (or similar indicator) for data. As the system recognises a sign the speed limit data is obtained and compared to the vehicle‘s speed. The system would use the speed limit from the last sign passed until it recognises a speed sign with a different limit. As with beacons, if speed signs are not 55
present or are obscured the system does not function. This is a particular problem when exiting a side road onto a main road (the vehicle may not pass speed sign for some distance). The accuracy, reliability and effectiveness of optical recognition technology remains unproven in ISA and it appears that the other technologies are more suitable. EVALUATIONS OF ISA The table in the Appendix summarises ISA trials conducted in several countries. There have been atleast 25 trials conducted in a total of 14 countries, with the notable exception of the USA. on the whole, these trials have exceeded the organisations expectations. However, Carsten notes that some trials have issed an opportunity for collecting pertinent data. Accuracy and Reliability of ISA A warning resulting from an incorrect speed limit would be a nuisance for an advisory system. The same errors in an active system could have serious road safety disadvantages, depending on the level of control of the system and the ability of the driver to over-ride the system. A typical example of a safety hazard would be a car travelling at 110km/h on a motorway where the ISA system unexpectedly reduces the vehicle speed to 50km/h, due to a GPS or mapping error. It is therefore important that sources of possible errors be identified and minimised. The overall system comprises three major components (Figure 8): DRIVER/VEHICLE ISA SYSTEM SPEED LIMIT DATABASE The ISA system includes any components that determine the vehicle‘s position or velocity, and compares it with a map of speed limits to determine whether the vehicle is speeding. For passive systems it also includes the way in which the driver is warned of excess speed (flashing lights, sounds, voice prompts, etc). For active systems it includes the controls that are used to automatically prevent speeding. The speed limit database contains all information related to any digital map or other information system that is used to determine local speed limits, some of which may be temporal (such as school zones). The list below shows the potential faults for each of the three main areas of ISA Driver/vehicle The driver may ignore, misunderstand or not notice a speed warning The driver may perform an unintended action in reaction to a speed warning (e.g. become distracted) For active systems, the driver might over-ride the system, or might not be aware that it is not operative (over-ridden by a previous driver). ISA System The speed warning may be a false alarm (e.g. wrong speed limit or incorrect vehicle speed) The speed warning may fail to activate when the vehicle is actually speeding, The vehicle speed or position may be inaccurate or inoperative (GPS ‗drop out‘, beacon failure) Time or date incorrect (time dependant variable speed zones affected) System may be out of operational range (e.g. no beacon close enough or boundaries of digital map reached) With active ISA, the speed controlling mechanism fails to activate or sets to the wrong speed The override, if any, might not work
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Failsafe operation might not work with an active system (driver loses throttle control) Speed Limit Database Variable speed limits are not included Timing information for time dependent variable limits is incorrect (e.g. holidays) A speed limit changes but the digital map is not updated Road path (alignment) changes but the digital map is not updated Wrong speed limit is assigned Coordinates of map are incorrect/inaccurate Road is not mapped
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UNIT IV VEHICLE AUTOMATED TRACKS Preparation And Maintenance Of Proper Road Network The improved system, Road Mentor 4, as well as being more suited to network use, has been rewritten in Visual Basic and uses Microsoft Access tables to store data, both in order to be compatible with current operating systems. It can be run on PCs equipped with either Windows 98 or 2000. The main modules of the new Road Mentor 4 programme were completed early in 2002. At this time it had become evident during condition surveys carried out in 2001 that there would be considerable effort needed to populate the system with reliable and compatible data. Also, it was realised that the final features of Road Mentor cou ld only be determined after some use of the system. Consequently a 2nd phase of the project was carried out in which, with TRL assistance, the Road Mentor System would be implemented just within a single Zone. Experience gained in this exercise would be used to plan the subsequent implementation across the rest of the country. The Road Mentor 4 system includes data on road ordnance, road inventory, pavement condition, traffic and works history. It also includes a module which will identify homogeneous road sections and create a file of details for use with HDM-4 which will be used as the principal tool for strategic, programme and project analysis of maintenance DESCRIPTION OF ROAD MENTOR The Road Mentor system is based on a series of tables contained in an Access database. Data in these tables is viewed, input and edited via a programme written in Visual Basic. Whilst data held within the system can be viewed through various screens it is expected that the main means of viewing system data will be through a series of standard reports created using Crystal Reports software. Input Screens The User is presented with a number of screens at several levels. The first screen seen on entry to the system is the Highway Ordnance screen at Level 1, see Figure 1. From here the User can obtain access to Level 2 screens from a series of buttons on the Highway Ordnance screen. A further level of screens is available from the Inventory level 2 screen. The overall structure of the screens is shown in Figure 2. Highway Ordnance The Highway Ordnance table in the Road Mentor system holds the key reference to all roads and links in the system. In normal usage only the System Manager can edit existing data and add new data to this screen. During the substantial editing that has been necessary during the initial piloting of the system this has been relaxed to allow Data Technicians also to edit. There are built-in checks to ensure that both Node and Link numbers are not repeated Condition Data Paved road condition data consists of roughness, measured using the Road Measurement Data Acquisition System (ROMDAS) mounted in a special vehicle fitted with a bump integrator, and the 20 key rating keyboard for other visually assessed defects. The schematic presentation of the ROMDAS system is summarized Unpaved road condition data for unpaved roads are collected by visual assessment during a drive over survey recording the following data for each sub-link: Surface type, spot improvement needed, overall condition, drainage condition, side slopes condition, shoulder condition, carriageway shape, carriageway surface condition, need for urgent works, condition of culverts, need for culvert repairs, need for new bridges, need for bridge repair 58
and need for river protection. Export of HDM-4 network file The HDM-4 investment analysis model is used to investigate network lev el strategies and programmes and project level analyses. The HDM-4 model requires information on the network, which will be imported from the Road Mentor asset management system. HDM-4 export file module in the Road Mentor 4 is used to create homogeneous sections automatically by using a series of rules and thereby creating an HDM-4 export file. The export file created is compatible with the HDM-4 input requirements. The user will be able to specify the roads to be included in the following manner: • Road type (paved , unpaved or both) • Road class (trunk, regional or both) • Zone (one or all) • Specific road (one road or all) • Committed works (to be identified as specific sections or not) . The user will also have control over the Condition Sectioning criteria, which are: • Traffic range • Roughness range (IRI m/km) • Cracking parameter • Pavement age • Structural Number - considered, Yes or No • Carriageway width - considered, Yes or No • Traffic flow - considered, Yes or No The final stage of selection allows the user to combine sections with the same treatments. The number of sections resulting from the current criteria can be displayed and the criteria adjusted to produce more or fewer sections as required. Finally the details of the accepted homogeneous section is exported to an HDM4 compatible file containing all the section details required for full HDM-4 anal ysis. Project, programme and strategic analyses would be carried out in HDM-4 and results used in decision making process on various roads investments options and strategies.
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Reports Individual records can be viewed through the different screens, already described. However to get a more general impression of the various types of data it is expected that standard report such as a link in the Highway Ordnance or a sub-link record in the Inventory. A report can be generated for any length of road from as little as a part of a single link to the whole of Tanzania. A wide range of selection criteria has been built into the selection process that is carried out on three tab screens (see Figure 1). The user can select the roads to be reported, on the basis of • Location (Regions, Zones whole of Tanzania), • Road classes (Trunk or Regional), • Road numbers, Link numbers, • Pavement type • Pavement condition (roughness or overall condition. In general, two basic types of reports can be produced – summary and detailed reports. Summary reports are designed to present data for regions, a group of roads or a long road. Detailed reports will provide more data on individual roads or links. So far 18 standard reports have been created in the system plus two special reports used to assist in the control of data input. The reports are listed in Figure 2. Examples of the following reports described hereunder are given in Appendix A: Ordnance report This reports give the detailed information about the road network in a particular region. The information which is available in the ordinance report include; Road Number, link numbers, node names and numbers, link lengths, GPS coordinates and summary of road lengths and total length of road network for a particular region. This report form a basis for road referencing system and it can be used for planning purposes and for network condition surveys. Inventory Summary report This reports summarises inventory details information about the road network , it provide pavement lengths, Pavement area, shoulder length, shoulder area, Number of culverts, Number of sublinks with side drains, number of mitre drains, number of bridges, Number of road signs, guards rail lengths, number of junctions, number of railway crossings and number of sub links. All detailed information about these items are av ailable in the inventory detailed report. Inventory information are useful for planning, programming and b udgeting purposes. They are the basis and one of the components of RMMS. Paved roads Roughness report This report provide information about the condition of the road network in terms of roughness. This report is a result of roughness surveys and it represents roughness in terms of IRI m/km. It displays the roughness for sublinks for a particular region and thereby enabling planners and policy makers to understand the state of the network under their jurisdiction for planning and decision making. The pie chart could also be used to demonstrate the performance of a road network in subsequent years. Unpaved Roads Condition This report provide information about the overall condition of the road sub-network. The road sub-network condition is represented by numbers from 1to 5, meaning very good to very poor respectively. Again, this report provide useful information for planning and programming of maintenance works and it could be used to monitor the performance of the road sub-network 60
in following years These reports can be previewed from the Road Mentor system and then printed as required. It is also possible to export any of the reports in Excel, Word or PDF format. This will allow for more elaborate analysis, incorporation into other documents or to be transmitted electronically to a user without direct access to Road Mentor. New reports can easily be added as the system develops. IMPLEMENTATION IN CENTRAL ZONE During the implementation process, It was noticed that there was a compatibility problem between Ordinance, inventory and condition data. Being the case, a series of activities have been carried out to rectify the problem. Proper ordinance and inventory data are pre-requisite and basis for a good Road Maintenance Management System. The activities include: • Node Location survey • Verification of Road links and Link lengths • Inventory surveys 3.1 Node Location survey It was impractical on the grounds of costs and time to install a system of node markers. TANROADS therefore made the decision to describe in detail all the nodes and sketch plan for complex situations. In addition, a number of Garmin hand-held Global Positioning System (GPS) units were used coordinate all the nodes. 3.2 Verification of Links and link lengths All road links under TANROADS jurisdiction were verified and unce rtainties about ownership between TANROADS and Local Authorities were clarified. The ROMDAS system was used as a digital trip meter to measure distances for all the links. 3.3 Inventory surveys After Node location survey and verification of link lengths, Inventory surveys were carried out. Brantz precise odometers were used for distance measurements during inventory surveys. It is intended that all future surveys of any kind would be carried out using these devices. As the ultimate goal is to achieve a near-perfect match of Ordnance and Inventory sub-links (1km) with subsequent condition survey sub-links (1km), all survey lengths need to be measured to an accuracy of better than ± 0.5km. The Brantz odometers, with proper calibration, can indicate distance within ± 0.4%. This implies that a link length of 50km is approximately the maximum that can be accepted. On the shorter links, say 10km, this performance is easily achieved. 3.4 Quality assurance A Data Collection Manual was produced during the first phase of the Road Mentor Project and has since been revised to correct omissions and reflect small changes in the way the data is input to Road Mentor 4. An additional set of procedures has been developed to ensure that data collection and input is carried out correctly and efficiently. This covers such issues as: • Specifying of Surveys • Provision of Ordnance reports and Node descriptions to survey teams • Need to check Link lengths with Ordnance values • Proper referencing of data forms • Acceptable tolerances for GPS Nodes longitude and latitude values and Link lengths • Review of Completed Survey Forms before data input • Data input to Road Mentor • Final Review of data input 61
• Verification of input data with field conditions 3.5 Future Survey Cycle At the commencement of the 2nd phase of the project, discussions were held at Head Quarters, Zone and Regional levels to establish a tentative annual programme of condition surveys, data input, and preparation of the maintenance programme and release of the revised Road Mentor database to all TANROADS offices. The probable programme is: Ordnance and Inventory Revisions As far as possible any adjustments or re-surveys would be carried out and input to Road Mentor before December of each year so as to be available for the condition survey teams. Adjustments might come from any changes to the network ordnance, inventory and from the records taken from works programmes, but a typical re-survey interval would be set at 5 years interval • Paved Roads Condition Surveys These surveys would be carried out by HQ staff using the ROMDAS system in the period of December to February each year and would require minimal input from Regional staff. For resource reasons possibly not all would be included. If this was the case the choice of which roads were to be surveyed would be discussed with the Regional Managers. All data input would be by HQ staff and would be completed by the end of February each year. • Unpaved Roads Condition Surveys: These would be carried out by the Regional staff under the direction of the Zone Office, in December to February and would use precise odometers and GPS units to ensure a good match of survey data with the Ordnance and Inventory. All data input would be done at the Zone Office and also to be completed by the end of February and would probably involve data technicians seconded from one of the Regional Offices. • Preparation of Maintenance Programmes This would normally be carried out at HQ in the period March to April by using HDM-4 model. • Release of Revised Database At the end of April of each year a new national version of the Road Mentor Database will be released for all users. This will contain all the recently collected condition data and any Ordnance and Inventory revisions. HQ users will be able to obtain new versions via the TANROADS HQ network but in the short term CDs will have to be issued to Zonal and Regional users. •
National Highway Network With Automated Roads And Vehicles A major long-term element of Intelligent Transportation Systems research and development is Automated highway Systems (AHS). The AHS program is a broad international effort ―to provide the basis for, and transition to, the next major performance upgrade of the vehicle/highway system through the use of automated vehicle control technology‖ When the cruise control was first developed, there was much concern over the safety and user acceptance of the new system; however, it has become widely accepted and used. In the near future, obstacle and headway warning and Automatic Vehicle Identification (AVI) will be added to modern cruise control and existing communications infrastructure. The success of AHS
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depends on linking the power of cellular communications and the emerging range of high performance computers to the ongoing vehicle based developments. Ideally, the highway system can be divided into a number of ―cells‖ which contain local radio receivers or beacons that will be linked together through a fiber-optic network. Vehicles will also be equipped with a transceiver unit carrying several user services. The first applications of this technology are the Automatic Vehicle Identification (AVI) and Electronic Toll Collection (ETC). Obstacle and headway warning is the next step in AHS development in vehicles. Vehicle on-board radar (VORAD) systems in many commercial vehicles are already in use for the last two years. An important issue in warning systems is the capabilities of the sensor modules. Differentiating between a large vehicle and a small animal may not be possible using a simple system. A consequent application of the headway warning system is the automatic headway control. Adaptive cruise control systems are currently designed by many automobile manufacturers. Applications in advanced traffic management, traveler information and public transportation systems (ATMS, ATIS, APTS) will require more sophisticated vehicle location capabilities. In addition, the number of uses for vehicle-to-roadside communications will eventually increase. MAYDAY services, fleet tracking and automatic vehicle location (AVL) applications will use radio-location beacons as well as more sophisticated transceivers. As a result of AVL and AVI, processing real-time information on vehicle locations will be possible. Although the number of vehicles equipped with AVI/AVL technologies will initially be small, traffic management centers can effectively use a small percentage of vehicles as ―probes.‖
AHS Program Phases and the National Automated Highway Systems Consortium The AHS program in United States is planned around three broad phases: Analysis (1993-96), System Definition (1994-2001), and Operational Tests and Evaluation (starting in 2001). The National Automated Highway System consortium (NAHSC) is responsible for conducting the second phase.
Vehicle Control Vehicle control is probably the most important part of the advanced AHS applications. Implementation of AHS necessitates automatically controlled vehicles as mentioned p reviously. Achieving the optimal solution to congestion and safety problems requires extensive research in system modeling, lateral (steering) controls and longitudinal (speed and headwa y) controls. In a fully automated highway system, these control systems will rely on vehicle-to-vehicle communication, as information on velocity and acceleration of other vehicles will be utilized in individual vehicle controllers. The same information and much more (e.g., desired speed and lane) may also be received via vehicle-to-roadside communications. Here, we will briefly discuss the previous research on lateral, longitudinal and combined lateral and longitudinal control of vehicles.
Lateral Control Hessburg and Tomizuka designed a fuzzy rule-based controller for lateral guidance of a vehicle. This system is based on human-type reasoning. Advantages of such a controller include flexibility in the choices of input/outputs, and on-line/off-line training capability. Their focus was achieving good tracking for a variety of roadway curves over a range of longitudinal vehicle speeds. Simulations to demonstrate its performance under parameter variations and external disturbances gave satisfactory results. It concentrates the intelligence in the vehicle, using the visual sensing approach described In this model, no infrastructure modification is needed, but 63
considerable cost and complexity is added to each individual vehicle. With the current rate of technology improvement, this system may become feasible for production purposes. During the last five years, the research on lateral vehicle control and lane changing maneuvers was extensive. For a (non comprehensive) list of publications on the subject Besides the theoretical modeling and simulations for lateral control of vehicles, there are a few important experimental accomplishments: the use of magnetic markers, and the use of visual information for lateral position handling. The first method was designed by the PATH Program and employs magnetic markers imbedded into the road to detect the lateral displacement from the center of the lane. Current tests with a vehicle equipped with magnetic sensors on its front bumper are reported to be successful .The second application for lateral control uses visual data and on-board computing resources to obtain the steering command, and is designed by another NASHC participant. In order to locate the road ahead, the ―rapidly adapting lateral position handler‖ (RALPH) uses a template-based matching technique to find parallel image features such as lane markings or tire and oil markings. During the experiment called ―No Hands Across America,‖ the test bed vehicle equipped with the RALPH system drove 98% of the 2850 mile journey autonomously. An average speed of 63mph in conditions that included bright sunlight, dusk, rain and nighttime, and a maximum stretch of 69- miles autonomous driving are reported. A third application for lateral control consists of a vision-based system with a neural network learning from a driver.
Longitudinal Control Longitudinal control is an important aspect of the future AHS. One of the major concepts in this area is platooning, which is a formation of traveling vehicles that maintain close spacing at highway speeds. The concept requires inter-vehicle communication links to provide velocity and possibly acceleration information from the lead vehicle to each of the following vehicles, as well as the velocity and acceleration of the preceding vehicle in the platoon. Sheikholeslam and Desoer [Sheikholeslam89] showed that inter-vehicle communications increases the stability of the platoon formation in the case of identical vehicle platoons.
Combined Lateral and Longitudinal Control Although much of the research to date has focused primarily on either lateral or longitudinal control, an overall automated driving system combining both lateral and longitudinal control is vital for future automated highway systems. System models which incorporate longitudinal and lateral dynamics are very rare. Kachroo and Tomizuka [Kachroo95c] studied combined longitudinal and lateral control to investigate the resulting behavior of the coupled system. It is shown that longitudinal controllers that directly control the wheel slip are inherently more stable, especially during lateral maneuvers on very slippery road conditions. Their fault tolerant algorithms were found to be stable for a variety of faults such as braking, powertrain, and steering systems. combined control using partial state-measurements of longitudinal and lateral deviations, longitudinal velocity and yaw rate. The research on combined control of vehicles is moving toward more realistic systems. New control approaches for more platoon operations in more complex situations such as entry and exit maneuvers are being studied
Hierarchical Control Structure Varaiya introduced a structure for designing ITS functions and their relation to driver decisions The focus of AHS applications is mainly on the in-trip phase of the ITS activities. An automated vehicle has to (a) choose its route to reduce travel time, (b) plan its path to ensure a smooth traffic flow, (c) maneuver in coordination with other vehicles, and (d) regulate the proper spacing and steering to increase traffic flow in a safe manner According to Varaiya, the tasks for an automated vehicle-highway system to accomplish can be achieved by a four-layer hierarchical 64
control architecture. The layers of the architecture, from the top, are network, link, planning, and regulation. The network layer assigns a route to each vehicle as it enters the system. The link layer assigns each vehicle a path which balances traffic for all lanes, and assigns a target speed for each section of highway. This layer may also assign platoon size. The planning layer creates a plan which approximates the desired path. The regulation layer controls the vehicle trajectory so that it conforms to this plan. Below the regulation layer, a physical layer that provides sensor data and responds to actuator signals, is assumed.
Other AHS Issues Besides the automatic vehicle control, there are several important issues that need to be carefully considered for a successful implementation of an automated highwa y system. During the first few years of the AHS research efforts, the problems related to the issues given here were not investigated as much as vehicle control problems. However, as the AHS related research progressed, it expanded to the areas of sensing and communications, fault tolerance, and human factors. In this section, we will emphasize related research efforts on these areas.
Sensors and Communication The realization of full AHS needs hardware both in infrastructure and the vehicle. Roadside monitors will measure traffic flow and speed, and vehicle paths will b e calculated based on this information. Such measurements are currently made with loop detectors, ultrasonic sensors, AVI tags or vision systems. Information may be communicated by infrared beacons, broadcast and cellular radio, or using emerging ultra wideband technologies. The vehicles need a longitudinal sensor to measure distance and relative speed of the preceding vehicle. Such sensors may be based on radar, ultrasound, or vision. Microwave radar sensors perform very well in fog and heavy rain, but they are very expensive. Laser radar systems are low-cost, but cannot handle low visibility conditions. Roadside-vehicle communications is also a critical aspect of AHS. Vehicles need to be identified, speed must be communicated to vehicles, and actions need to be coordinated for a fully automated system. In addition, there is a need for vehicle-to-vehicle communications because of the designed longitudinal control methods and coordination issues. Precise control can be obtained using full duplex communication. Networking represents a higher level of communications, in which traffic and hazard information detected by one group of vehicles can be communicated to other vehicles. There are a variety of methods for roadsidevehicle communications; most of them are discussed.
Safety and Fault Tolerance Although the solutions to most of the technical problems in vehicle control, traffic management, information systems, and communications have been found, the envisioned AHS will never be deployed unless the safety of the overall system can be verified. One issue which is often overlooked by researchers is the possibility of undesirable interaction of the systems. An example in mentions two devices which try to maintain vehicles at constant lateral spacing using side range sensing. If the devices (and the vehicles) have different dynamics, it is possible that one or both vehicles may become unstable, possibly resulting in a collision.
Human Factors In an advanced system such as AHS, the driver will be confronted with significantly more information, and possibly more controls, than are currently used in vehicles. In a system that uses complete automation during some segments of a trip, the safe transition from automated driving to manual driving is a difficult issue. Also, there exist several advanced vehicle control systems (AVCS) related problems such as the acceptance of a system which takes the control away from drivers, privacy issues related to AVI and AVL systems, and ―platooning claustrophobia.‖ For 65
users to accept the AHS, a successful AVC system must address important issues such as dealing with false alarms and system failures to gain acceptance and public confidence, displays and warnings with the right amount and content of information, and driver skills and attentiveness for smooth automated-manual switching.
An Experimentation and Evaluation Framework for AHS According to Center for Transportation Research‘s approach, the progression in AHS development from conceptualization to implementation has four steps. The intelligent controller described in this work is part of the four-block structure visualized at the center. As seen in Figure 2.1, the first block is computer software simulation (see Section 2. 6 for a detailed description) that is preceded by mathematical modeling. The second block consists of conducting small scale experiments in the Flexible Low-cost Automated Scaled Highway (FLASH) Laboratory. Then, hardware tests comprising the third block are performed on a test site with actual vehicles. The ―Smart Road‖ being built near Blacksburg, Virginia will be a suitable test bed for conducting such experiments using actual vehicles and controlled traffic conditions. The fourth block is the deployment of AHS on conventional highways.
These four blocks can be considered as the building blocks of a comprehensive testing and evaluation methodology for AHS. The input can be a hypothesis, a model, or technologies. The evaluation and testing procedure defined by this methodology is not seen as a single feed through four-step process, but as having some feedback and feed forward loops depending on the results obtained at each block. These loops represent the changes made to the hypothesis, model or the technological concept. Hardware tests are important since they provide the means to validate computer results or to modify them in the case of discrepancies, due to unmodeled or inadequately modeled dynamics. Without hardware testing, it would be foolhardy to jump into actual implementation. For instance, the FLASH Laboratory could be used to improve the computer simulation via scale model tests before starting the tests with full scale vehicles.
FLASH Laboratory The Flexible Low-cost Automated Scaled Highway (FLASH) Laboratory is visualized a precursor to a full-scale highway .It will provide a platform for experimental work as well as an arena for demonstration of AHS systems. In order to test highway situations with complex realistic scenarios of merging, splitting, exit, and entrance, small scale vehicles (1:10 to 1:15 scale) are designed in a modular fashion. Modularity will guarantee inexpensive and fast incorporation of different system configurations. The FLASH laboratory will be composed of tens of small vehicles (of different sizes) with a flexible highway system and communication network. The FLASH laboratory concept makes the construction of specifically designed scaled highway configurations possible in a short period of time, and at much less cost than it would be 66
required for the construction of a similar full scale test bed.
Smart Road When completed, the ‗Smart Road‘ will be a 6-mile roadway between Blacksburg and I-81 interstate highway in southwest Virginia, linking the Roanoke and New River Valleys. The first 2 miles is designated as a controlled test facility .It will be the first of its kind to be built from the ground up with an ITS infrastructure incorporated into the roadway. This full scale test bed will provide controlled testing of a variety of ITS technologies and concepts with varied terrain and environmental conditions. Research involving crash avoidance, driver behavior, vehicle dynamics, sensors, and automated vehicle control will take place under a broad range of carefully controlled testing conditions.
A Simulation Tool for AHS Systems: DYNAVI MTS During the last few years, many computer programs for microscopic and macroscopic level simulations of vehicles in an automated system have surfaced. Paramics (Parallel Microscopic Simulation) is currently used to find the reasons for large-scale congestions in the Controlled Motorway pilot project on the M25 highway in England, by dividing the the network into a number of regions. DYNAVIS is another dynamic visualization package specifically designed for evaluating automatic control of vehicles . This package is part of a bigger effort by the California PATH program, Smart Path. Smart Path is a simulation package for an automated highway system, designed to provide a framework for simulation and evaluation of AHS alternatives. It is again a micro-simulator, i.e., the functional elements and the behavior of each vehicle are individually modeled. Smart Path has two separate simulation and animation modules. An example of a macroscopic simulation is KRONOS developed at the University of Minnesota. It is a PC based simulation using a simple continuum modeling approach to evaluate the flow behaviors such as merging and diverging. Argonne National Laboratory‘s prototype simulation includes the modeling of automated vehicles with in-vehicle navigation units and a Traffic Management Center (TMC) using distributed computer systems or massively parallel computer systems. The Center for Transportation Research of Virginia Tech is developing a simulation package that models micro and macroscopic behavior of vehicles over a network. The software is envisioned to integrate the different research efforts at CTR into one comprehensive ITS software package. In the development of this tool, particular emphasis is given to modeling the AHS system based on the control architecture developed under the PATH program. The package is called DYNAVIMTS, Dynamic Visual Micro/Macroscopic Traffic Simulation Tool.
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UNIT V SUSPENSION, BRAKES, AERODYNAMICS AND SAFETY Air Suspension Air suspension is a type of vehicle suspension powered by an engine driven or electric air pump or compressor. This pump pressurizes the air, using compressed air as a spring. Air suspension replaces conventional steel springs. If the engine is left off for an extended period, the car will settle to the ground. The purpose of air suspension is to provide a smooth ride quality and in some cases self-leveling. Vehicles that use air suspension today include models from Maybach, Rolls-Royce, Lexus, Cadillac (GM), Mercedes-Benz, Land Rover/Range Rover, SsangYong, Audi, Subaru, Volkswagen, and Lincoln and Ford, among others. Citroën now feature Hydractive suspension, a computer controlled version of their Hydropneumatic system, which features sport and comfort modes, lowers the height of the car at high speeds and continues to maintain ride height when the engine is not running. The air suspension designs from Land Rover, SsangYong, Subaru and some Audi, VW, and Lexus models, feature height adjustable suspension controlled by the driver, suitable for clearing rough terrain. The Lincoln Continental and Mark VIII also featured an air suspension system in which the driver could choose how sporty or comfortable they wanted the suspension to feel. These suspension settings were also linked to the memory seat s ystem, meaning that the car would automatically adjust the suspension to the individual driver. The control system in the Mark VIII also lowered the suspension by about 25 m at speeds exceeding about 100 km/h for improved aerodynamic performance. Due to the many advantages air suspensions provide, and with the advancement of new materials and technologies, these systems are being designed on many future platforms. This is especially important as car manufacturers strive to improve gas mileage by reducing weight and utilizing active suspension technology to maximize performance. In addition to passenger cars, air suspension is broadly used on semi trailers, trains (primarily passenger trains) and buses, which are all transportation sectors that helped pioneer the use and design of air suspension. An unusual application was on EMD's experimental Aerotrain. Custom applications Over the last decade or so air suspension has become extremely popular in the custom automobile culture: street rods, trucks, cars, and even motorcycles may have air springs. They are used in these applications to provide an adjustable suspension which allows vehicles to sit extremely low, yet be able rise to a level high enough to maneuver over obstacles and inconsistencies in the roadways (and parking lots). These systems generally employ small, electric or engine-driven air compressors which sometimes fill an on-board air receiver tank which stores compressed air for use in the future without delay. High-pressured industrial gas bottles (such as nitrogen or carbon dioxide tanks used to store shielding gases for welding) are sometimes used in more radical air suspension setups. Either of these reservoir systems may be fully adjustable, being able to adjust each wheel's air pressure individually. This allows the user to tilt the vehicle side to side, front to back, in some instances "hit a 3-wheel" (contort the vehicle so one wheel lifts up from the ground) or even "hop" the entire vehicle into the air. When a pressure reservoir is present, the flow of air or gas is commonly controlled with pneumatic solenoid valves. This allows the user to make adjustments by simply pressing a momentarycontact electric button or switch. The installation and configuration of these systems varies for different makes and models but the underlying principle remains the same. The metal spring (coil or leaf) is removed, and an air bag, 68
also referred to as an air spring, is inserted or fabricated to fit in the place of the factory spring. When air pressure is supplied to the air bag, the suspension can be adjusted either up or down (lifted or lowered). For vehicles with leaf spring suspension such as pickup trucks, the leaf spring is sometimes eliminated and replaced with a multiple-bar linkage. Th ese bars are typically in a trailing arm configuration and the air spring may be situated vertically between a link bar or the axle housing and a point on the vehicle's frame. In other cases, the air bag is situated on the opposite side of the axle from the main link bars on an additional cantilever member. If the main linkage bars are oriented parallel to the longitudinal (driving) axis of the car, the axle housing may be constrained laterally with either a Panhard rod or Watt's linkage. In some cases, two of the link bars may be combined into a triangular shape which effectively constrains the vehicles axle laterally. Often, owners may desire to lower their vehicle to such an extent that they must cut away portions of the frame for more clearance. A reinforcement member commonly referred to as a Cnotch is then bolted or welded to the vehicle frame in order to maintain structural integrity. Specifically on pickup trucks, this process is termed "notching" because a portion (notch) of the cargo bed may also be removed, along with the wheel wells, to provide maximum axle clearance. For some, it is desirable to have the vehicle so low that the frame rests on the ground when the air bags are fully deflated. Common air suspension problems Air bag or air strut failure is usually caused by wet rot, due to old age, or moisture within the air system that damages it from the inside. Air ride suspension parts may fail because rubber dries out. Punctures to the air bag may be caused from debris on the road. With custom applications, improper installation may cause the air bags to rub against the vehicle's frame or other surrounding parts, damaging it. The over-extension of an airspring which is not sufficiently constrained by other suspension components such as a shock absorber may also lead to the premature failure of an airspring through the tearing of the flexible layers.Failing of the Air bag may also result in completely immobilizing the vehicle. As the vehicle will rub against the ground or be too high to move. Air line failure is a failure of the tubing which connects the air bags or struts to the rest of the air system, and is typically DOT-approved nylon air brake line. This usually occurs when the air lines, which must be routed to the air bags through the chassis of the vehicle, rub against a sharp edge of a chassis member or a moving suspension component, causing a hole to be formed. This mode of failure will typically take some time to occur after the initial installation of the system as the integrity of a section of air line is compromised to the point of failure due to the rubbing and resultant abrasion of the material. An air line failure may also occur if a piece of road debris hits an air line and punctures or tears it. Compressor failure is primarily due to leaking air springs or air struts. The compressor will burn out trying to maintain the correct air pressure in a leaking air system. Compressor burnout may also be caused by moisture from within the air system coming into contact with its electronic parts. In Dryer failure the dryer, which functions to remove moisture from the air system, eventually becomes saturated and unable to perform that function. This causes moisture to build up in the system and can result in damaged air springs and/or a burned out compressor Regenerative braking A regenerative brake is an energy recovery mechanism which slows a vehicle by converting its kinetic energy into another form, which can be either used immediately or stored until needed. 69
This contrasts with conventional braking systems, where the excess kinetic energy is converted to heat by friction in the brake linings and therefore wasted. The most common form of regenerative brake involves using an electric motor as an electric generator. In electric railways the generated electricity is fed back into the supply system, whereas in battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Energy may also be stored via pneumatics, hydraulics or the kinetic energy of a rotating flywheel. The motor as a generator Vehicles driven by electric motors use the motor as a generator when using regenerative braking: it is operated as a generator during braking and its output is supplied to an electrical load; the transfer of energy to the load provides the braking effect. Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode. Early examples of this system were the front-wheel drive conversions of h orse-drawn cabs by Louis Antoine Krieger (1868 – 1951). The Krieger electric landaulet had a drive motor in each front wheel with a second set of parallel windings (bifilar coil) for regenerative braking. In England, the Raworth system of "regenerative control" was introduced by tramway operators in the early 1900s, since it offered them economic and operational benefits as explained by A. Raworth of Leeds in some detail. These included tramway systems at Devonport (1903), Rawtenstall, Birmingham, Crystal Palace-Croydon (1906) and many othe rs. Slowing down the speed of the cars or keeping it in hand on descending gradients, the motors worked as generators and braked the vehicles. The tram cars also had wheel brakes and track slipper brakes which could stop the tram should the electric braking systems fail. In several cases the tram car motors were shunt wound instead of series wound, and the systems on the Crystal Palace line utilized series- parallel controllers. Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911. Twenty years later, the regenerative braking system was reintroduced. Regenerative braking has been in extensive use on railways for many decades. The Baku-TbilisiBatumi railway (Transcaucasian railway or Georgian railway) started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerous Surami Pass. In Scandinavia the Kiruna to Narvik railway carries iron ore from the mines in Kiruna in the north of Sweden down to the port of Narvik in Norway to this day. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by their regenerative braking. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border. Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity. An Energy Regeneration Brake was developed in 1967 for the AMC Amitron. This was a completely battery powered urban concept car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile. Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack. Examples include the Toyota Prius, Honda Insight, the Vectrix electric maxi-scooter, and the Chevrolet Volt.
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Limitations Traditional friction-based braking is used in conjunction with mechanical regenerative braking for the following reasons: The regenerative braking effect drops off at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills. The friction brake is a necessary back-up in the event of failure of the regenerative brake. Most road vehicles with regenerative braking only have power on some wheels (as in a twowheel drive car) and regenerative braking power only applies to such wheels because they are the only wheels linked to the drive motor, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels. The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. No regenerative braking effect can occur if another electrical component on the same supply system is not currently drawing power and if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy. Under emergency braking it is desirable that the braking force exerted be the maximum allowed by the friction between the wheels and the surface without slipping, over the entire speed range from the vehicle's maximum speed down to zero. The maximum force available for acceleration is typically much less than this except in the case of extreme high-performance vehicles. Therefore, the power required to be dissipated by the braking system under emergency braking conditions may be many times the maximum power which is delivered under acceleration. Traction motors sized to handle the drive power may not be able to cope with the extra load and the battery may not be able to accept charge at a sufficiently high rate. Friction braking is required to dissipate the surplus energy in order to allow an acceptable emergency braking performance. For these reasons there is typically the need to control the regenerative braking and match the friction and regenerative braking to produce the desired total braking output. The GM EV-1 was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued two patents for this brake-by-wire technology.
Anti Skid Braking System INTRODUCTION Antilock braking system (ABS) prevent brakes from locking during braking. In normal braking situation the driver control the brakes, however during severs braking or on slippery roadways when driver the wheels to approach lockup, the antilock takes over here. The ABS modulates the brake line pressure independent of the pedal force to bring the wheel speed back to the slip level range that necessary to the optimal braking performance. The ABS does not allow full wheel lock under braking. In simple terms, during emergency of braking, the wheel does not get locked even if you push a full auto brake pedal and hence the skidding does not takes place. It allowed driver to control the car easier, even on roads with low adhesion, such a rain, snow and muddy road. The brain of antilock braking system consist Electronic Control Unit (ECU), wheel speed sensor and hydraulic modulator. ABS is a closed circuit, hence it used the feedback control system that modulates the brake pressure in response to the wheel deceleration and wheel angular velocity to prevent the controlled wheel from locking. SUBSYSTEM OF ANTILOCK BRAKING SYSTEM (ABS) 71
Wheel-Speed Sensors Each of the ABS wheel speed sensors detects the speed of the corresponding wheel. The sensor consists of a permanent magnet, coil and tone wheel. The magnetic flux produced by the permanent magnet changes as each tooth of the tone wheel (which rotates together with the wheel) passes in front of the magnet‘s pole piece. The hanging magnetic flux induces voltages at a frequency corresponding to the wheel speed. Electronic Control Unit (ECU)The work of ECU is to receive, amplifies and filter the sensor signals for calculating the speed rotation and acceleration of the vehicle. ECU also uses the speeds of two diagonally opposite wheels to calculate an estimate for the speed of the vehicle. The slip of each wheel is obtain by comparing the reference speed with the individual wheel. During wheel slip or wheel acceleration condition signal server to alert the ECU. The microcomputer alert by sending the trigger the pressure control valve of the solenoids of the pressure modulator to modulate the brake pressure in the individual wheel brake cylinders. The ECU reacts to a recognized defect or error by switching off the malfunctioning part of the system or shutting down the entire ABS.
Hydraulic Pressure Modulator/ Hydraulic Control Unit The Hydraulic pressure modulator is an electro-hydraulic device for reducing, restoring and holding the pressure of the wheel by manipulating the solenoid valve in the hydraulic brake system. ABS hydraulic modulator unit contain the valve, solenoid and piston. Under hard braking condition, this assembly control the holding and release of the different hydraulic brake circuit. During the normal condition the standard braking system used. Whenever hard braking situation occur, the system sense the change in the rotation of the speed sensor and decide whether to hold or release pressure to a brake circuit. A tire has its best traction just before it begin to skid, once it begins to skid a portion of traction and steering will be lost. ANTI-LOCK BRAKE TYPE Different schemes of anti-lock braking system uses depending upon the types of brakes use. Depending upon the channel (valve) and number of speed sensors the antilock brake are classified. Four Channel, Four Sensor ABS It is a more preferable type, the speed sensor on all the four wheels and contain separate valve for all four wheels. By using this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force. 72
Three Channel, Three Sensor ABS This type of system is can be found commonly in the pickup trucks with four wheel ABS, on each of the front wheels there is a valve and a speed sensor, and one valve and one sensor for both rear wheels. The speed sensor for rear wheels is located in the rear axle. To achieve the maximum braking force, this system provides individual control to the front wheels. The rear wheels, however, are controlled together; they are both have to start to lock up before the ABS will active on the rear. With the help of this system, it's possible that one if the rear wheels will lock during a stop, reducing brake effectiveness. One Channel, One Sensor ABS This Arrangement can be seen in a pickup trucks and heavy trucks with rear wheel ABS. It consist one valve, which operate both rear wheel, and one speed sensor located in the rear axle. This is quite similar as the rear end of a three channel system. The rear wheel are monitored together and they both have to lockup before ABS starts its action. In this system there is also probability that one of the rear wheels will lock, results reducing in brake effectiveness. This system is easy to identify, usually there will be one brake line going through a T-fitting to both rear wheels.
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FUNCTION OF SENSORS ANDACTUATORS
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IMPORTANTCE OF ABS Stopping Distance The Stopping distance is a one of the important factor when it comes for braking. Stopping distance is the function of vehicle mass, its initial velocity and the braking force. Stopping distance can be minimize by increasing in braking force (keeping all other factors constant). In all types of road surface there is always exists a peak in friction coefficient. An antilock system can attain maximum fictional force and results minimum stopping distance. This objective of antilock systems however, is tempered by the need for vehicle stability and steerability. Stability The fundamental purpose of braking system is to decelerating and stopping of vehicle, maximum friction force may not be described in some cases like asphalt and ice (p-split) surface, such that significantly more braking force is obtainable on one side of the vehicle than on the other side. So when applying full brake on both the sides will result yaw or skidding moment that will tend to pull the vehicle to the high friction side and results vehicle instability. Here comes the concept of antilock system that maintain the slip both rear wheels at the same level and minimize two friction coefficient peaks, then lateral force is reasonably high thought not maximized. This contributes to stability and is an objective of antilock systems. Steerability Good peak frictional force control is necessary in order to achieve satisfactory lateral forces and, therefore, satisfactory steerability. Steerability while braking is important not only for minor course corrections but also for the possibility of steering around an obstacle. Tire characteristics play an important role in the braking and steering response of a vehicle. For ABS-equipped vehicles the tire performance is of critical significance. All braking and steering forces must be generated within the small tire contact patch between the vehicle and the road. Tire traction forces as well as side forces can only be produced when a difference exists between the speed of the tire circumference and the speed of the vehicle relative to the road surface. This difference is denoted as slip. It is common to relate the tire braking force to the tire braking slip After the peak value has been reached, increased tire slip causes reduction of tire road friction coefficient. ABS has to limit the slip to values below the peak value to prevent wheel from locking. Tires with a high peak friction point achieve maximum friction at 10 to 20% slip. The optimum slip value decreases as tire-road friction decreases. Retarders
Secondary Braking system Retarder is an external brake frequently used in trucks. The purpose of the retarder is to assist in stopping the vehicle, controlling vehicle downhill speed on a steep grade, extending the life of the vehicle service brakes, and enhancing vehicle control. There are several retarder technologies currently available. Two major kinds are Hydraulic retarders and Electromagnetic retarders. The hydraulic retarder: Hydraulic retarders use the viscous drag forces between dynamic and static vanes in a fluid-filled chamber to achieve retardation. A simple retarder would use vanes attached to a transmission driveshaft between the clutch and road wheels. They can also be driven separately via gears off a driveshaft. The vanes would be enclosed in a static chamber with small learances to the chamber's walls (which will also be vaned). When retardation is required, fluid (oil or water) is pumped into the chamber, and the viscous drag induced will slow down the vehicle. The working fluid will heat up, and will usually be circulated through a cooling system. The degree of retardation can be varied by adjusting the fill level of the chamber. 75
Electromagnetic retarders: The electric retarder uses electromagnetic induction to provide retardation force. An electric retardation unit can be placed on an axle, transmission, or driveline and consists of a rotor attached to the axle, transmission, or driveline and a stator securely attached to the vehicle chassis. There are no contact surfaces between the rotor and stator, and no working fluid. When retardation is required, the electrical windings in the stator are powered up from the vehicle battery, producing magnetic fields alternating in polarity. This induces eddy currents in the rotor, which slows down the rotor, and hence the axle, transmission, or driveshaft, to which it is attached. The rotor is engineered to provide its own air-cooling, so no load is placed on the vehicles cooling system, and the operation of the system is extremely quiet.
Safety Gauge Air Backs Air Bag: Since it is neither practicable nor desirable to build vehicles as strong as tanks, their basic structures must be designed to collapse in a controlled manner in an accident. A prime consideration is to prevent the steering wheel from being thrust back and crushing or penetrating the driver‘s chest or neck or, perhaps, even breaking his jaw. Among the measures originally adopted were the inclusion of telescopic or concertina type collapsible elements in the steering column. In some early instances, the lower end of the steering column tube was coarsely 76
perforated, so that it would collapse when subjected to heavy axial loading. Another of these measures was the incorporation of two universal joints, one at the lower end of a shortened steering column shaft and the other on the steering box, the section between them being set at an angle relative to the axis of the steering column. In the event of a front end impact, the section between the two universal joints would displace laterally instead of pushing the upper part of the column back towards the driver. Subsequently, two further changes were made. One was to increase the area of the hub of the wheel, to reduce the intensity of loading locally on the chest. The other was to reduce the stiffness of the rim of the wheel, so that, if the driver was thrown forward on to it, it yielded rather than severely damaging his rib cage. Later, gas-inflated bags were installed in the steering wheel hub, Fig.1 These are supplementary safety devices, as they are effective only in conjunction with correctly adjusted seat belts. They can be inflated by air but, to obtain rapid deployment, inflation using chemicals producing nitrogen or other gases are more commonly used. Correctly tensioning the belt is important, otherwise it will fail to guide the driver in a manner such that his face comes down on to the air bag instead of slithering over it and striking hard objects beyond. In the USA, failure of drivers to fasten seat belts has been the cause of serious injuries, which has led, unjustifiably , to doubts being expressed regarding the effectiveness of airbags. For the protection of front seat passengers, air bags are installed behind a panel in the dash fascia, and side air bags may be embodied in the seat squabs. An advantage of the latter site is that it moves with the seat when its position is adjusted, so the bag can be smaller than if it were stowed, for example, in the door. Moreover, in the door, it could be more vulnerable to impact damage. Mercedes has developed what they term window bags, 2 m long, for the protection of the heads of all the passengers, which otherwise could be injured either by hitting the side window or by intrusion. These are stowed in the sides of the roof, and deploy in 25 ms. Bags suspended from the cant rail and extending the full length for protecting the passengers in both the front to the rear seats are sometimes called curtain ba gs. In general, because the occupants‘ heads start further away from the bags than do their shoulders, side bags at or near shoulder height, for protection against side impacts, should open earlier than those for either window bags or those for frontal impacts. To meet this requirement, Toyota have developed a system in which pellets of a chemical that generates mostly argon gas are used for inflation. The sensors are mounted low in the centre pillars and the air bags are stowed in the front seat squabs.
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Since the primary impact may be over within 10 ms,all the bags have to deploy within 20 – 30 ms. To obtain rapid deployment, most manufacturers employ pellets of sodium azide which, when heated, produce large quantities of nitrogen to inflate the bags. Sodium azide is a salt of hydroazic acid (N3H3). Initially, air bag deployment was mostly triggered by deceleration force acting on some very simple form of mechanism, such as a ball in a tube, mounted adjacent to, or within, the steering wheel hub. Subsequently, electrically fired gas generators have been triggered by computers in response to its receipt of appropriate deceleration signals. The deceleration sensors are usually mounted on a front transverse member of the vehicle structure. An advantage of this system is that the whole sub-assembly, including the gas generator, can be housed compactly within the steering wheel hub assembly, and the deceleration sensor can be placed in any position where it will be most effective, Figs. 1 and 2
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Perforations in all bags allow the gas to leak out at a rate that increases with internal pressure, thus modifying their spring rates so that the occupants‘ heads do not rebound violently. This at least reduces, and hopefully even completely obviates, the possibility of spinal whiplash damage. Moreover, the deflation and collapse of the bag, within a few ms after inflation, leaves the steering wheel relatively clear of obstruction so that the driver will have a better chance of regaining control after the impact. In the event of a multiple collision, the airbags are, of course, effective in only the first impact. Crash Resistance: Material technologies are also expected to contribute to improving crash worthiness. In order to achieve a safe car body in the event of a collision, deformation of the cabin structure should be minimized to protect the occupants, and the collision energy should be absorbed in a short deformation length within the crushable zones, as shown in Figure 1. However, the reaction force generally exceeds an appropriate level when a material with higher strength is applied to an energy-absorbing location.
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Consequently, new structures and materials are required for building the ideal car body that can absorb the collision energy in a short span and with a constant reaction force. To meet the requirements for improved safety, thicker steel sheets or additional reinforcements are usually applied, which leads to a heavier body-in-white. Therefore, it is necessary to improve crash safety while at the same time lightening vehicles for better environmental performance. From the viewpoint of materials, both dynamic strength and static strength are important in designing parts for greater crash safety.
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As defined in Equation 1, the average reactive force of a rectangular tube with a hat shaped cross section is related to the k-value, i.e., the dynamic/static ratio of yield strength: Average reactive force in crash deformation ∝ (kσy)3/2 × t5/3 where k = dynamic yield strength/static yield strength σy = static yield strength t = sheet thickness In general, the k-value decreases with increasing strength, as shown in Figure 2. To reduce vehicle weight effectively while improving safety, new materials with a higher k-value are needed. For example, substituting higher strength steel for pa rts made of 440-MPa steel sheet can reduce the weight, but a much larger weight saving would be possible by applying steels having higher k-values, as shown in Figure 3. Aerodynamics For Modern Vehicles Most of the information about car aerodynamics seems to be centered around generating downforce. While this may be needed for race cars, the average 3000+ pound car driving at speeds below 90 MPH does not need to be concerned with downforce. If you are trying to improve the efficiency of your vehicle, reducing the coefficient of drag (Cd) should be the main concern. Rationale In this day and age of expensive fuel and inefficient vehicles, it makes sense both economically and ecologically to conserve as much fuel as possible. To accomplish this, you could go out and buy another car with better mileage, but there are other options. This article focuses on how to optimize your current vehicle. The example vehicle is a 1998 Nissan Maxima. This is a rather boxy 4 door sedan with quite a lot of ground clearance and a 190hp 6 cyl engine, that is rated at 26MPG highway by fueleconomy.gov, but gets around 21MPG in mixed driving.
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For highway driving conditions, it is estimated that driveline uses about 15% of the total energy to required to push your vehicle down the highway, tire rolling resistance represents about 25%, and air drag is about 60%! While the traditional sources advocate saving fuel by driving less or driving slower, there are greater gains that can be made by modifying the aerodynamics, engine, and rolling resistance of the vehicle. These modifications are not without cost, but are within reach of even those of us with meager incomes. All of the aerodynamic modifications mentioned here can be performed for under $1000, providing you are willing to do the work yourself It may take a couple of years for the dollars expended in making the modifications to be paid for by the savings of gas, but a payback in that timeframe is easy to rationalize to yourself, and others. As seen in the table above, purchasing a 4cyl econobox or a 4cyl hybrid to replace your comfy (and paid for!) 6cyl sedan would save a bunch of money every year, but not enough to pay for the replacement. If you can afford it, it does make the best sense from an environmental point of view, but purchasing an expensive new car just to save $900 per year in gas is not an option many of us can afford. To most of us it makes more sense economically to keep driving our current gas guzzler. Modifying the sedan to get 25% better mileage, for under $1000 would start paying back after only two years. None of the modifications below in itself will provide a huge change in efficiency, but 3% here and 5% there all add up to big numbers eventually. The 25% mileage improvement figure above is an estimate based on results I have seen of a 70 MPG Honda Civic (Bryant Tucker), and a 32 MPG truck, (Phil Know). This would be an improvement in highway mileage only. The $1000 project cost estimate would be spent on: Eibach height adjustable springs - ~$300. Aluminum sheet and hardware to build a belly pan and other aero mods - ~$300 The remainder would be for other stuff like measuring the mileage. Manufacturers design most cars for looks, with aerodynamics as an afterthought. As such, much can be gained by tweaking the aerodynamics of these vehicles. The unit of measurement for aerodynamics is called the "coefficient of drag" or Cd. The Cd value tells us how efficiently the vehicle slips through the wind. Another common measurement multiplies the Cd times the total frontal area of the vehicle. This is called CdA. Check this site for the Cd value for different cars. Lower Cd means better Mileage! Here are things that can be done to improve your vehicle's aerodynamics: Lower the car - Lowering the car reduces the effective frontal area, increasing efficiency. Note that this only works up to a certain point. There will be an ideal ride height for each car. According to this article, 2.7" ground clearance is a good minimum height to shoot for. According to Mercedes, "Lowering the ride height at speed results in a 3-percent improvement in drag." Remove that wing - Many "sports" cars have a non-functional wing on the back. Removing it will improve the fuel economy. The exceptions are the small rear fairings that are designed to detach the airflow from a rounded trunk. Clean up the underside of the car. - Installation of a "body pan", while a labor intensive operation, will provide a significant improvement in mileage. More... If a body pan is not practical, an air dam will redirect air that would normally pile up under the car causing drag. Not as good as a body pan, but better than nothing. Should be combined with side fairings. Fair the wheel wells. - Yeah, this looks funny, but completely covering the rear wheel well will help improve efficiency. While the front wheel can not easily be completely
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faired due to clearances needed for turning, a partial fairing can be made. In addition, fairings can be added in front and behind the tires to help transition the air around these large appendages. Clean up the front of the car. Basically the smoother the better. If the car has a large air intake under the bumper, it may not need that opening above the bumper (they are often just styling cues). An aerodynamic plastic, composite, or foam and duct tape panel can be built to cover the opening. Remove the side view mirrors and instead use a remote camera system. Replace large whip antennas with smaller powered antennas. Vehicles with steep windshields can benefit from a hood fairing to help smooth the transition of air between the hood and windshield. A small "tail cone" can be affixed the the rear bumper to help transition the air from under the car. Side fairings can be used to clean up the lower half of the body between the tires. More... Additional mods for trucks : If you need the utility of a truck, there are things that can be done to improve their efficiency in addition to the items noted above. Most notably, cover the bed! A flat hard cover will help some, but a custom aero cover is much more efficient. Experimentation has shown that simple removal of the truck bed door does not provide better mileage. Additional mods for Vans and SU Vs:: A new spoiler design has been shown to reduce drag and lift significantly on bluff-backed vehicles such as minivans and SUVs. Simulations showed that aerodynamic drag on a mini-van moving at 67 mph were reduced by 5% when the new spoiler was attached. This rear spoiler acts like a diffuser when it is attached to the back of a vehicle, making the pressure on the back of the vehicle higher than without it. That's a good thing! Full technical paper Body Pans: A body pan fairs the underside of the vehicle. This becomes increasingly important as the vehicle gets closer to the ground. The pan ideally covers the entire underside of the car, but this may be impractical in many cases, so the idea is to make it as smooth as possible. Covering the exhaust system can lead to heat buildup between the belly pan and the floorboards. In general it's a good idea to create a heat shield/tunnel extending from the engine compartment to the rear of the vehicle. This will serve to seal in as much of the heat as possible. High pressure from the engine compartment will force air down the tunnel and out the rear of the car. Also, louvers may be cut into the body pan in areas where more heat needs to be released, such as along the route of the exhaust pipe. NACA ducts do not work well for this application as they are designed as devices to scavenge incoming air without disturbing the airflow, not as an air exhaust device. Engine airflow needs to be retained, but generally there are large enough opening between the engine compartment and the front wheels to give good engine airflow, even with the underside of the engine covered. Be sure to make the areas where maintenance will occur easily accessible, especially oil pan drain and oil filter access. The belly pad should be parallel to the ground until just past the rear axle, then it should gradually curve upward to meet with the underside of the rear fascia of the car. Even the most aerodynamic cars manufactured today, for example the Toyota Prius pictured here which is touted as having a full body pan, can be cleaned up extensively. Car side fairings - "ground effects": Most car bodies slope inward at the sides until they are inside of the tires toward the bottom of the vehicle, leaving a large gap between the tires. Mud
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flaps are spiffy but only serve to make the gaps bigger. This all adds up to a lot of aerodynamic inefficiency. Side fairings "fill the gap", transition the air around the tires and keep side winds from flowing under the car. If you are driving 60 MPH with a 20MPH side wind, 33% of the wind forces are on the side of the car, so making the side of the car aerodynamic is almost as important as improving the aero qualities of the car front. Stylists have created "ground effects" that claim to be aerodynamic, but really aren't. Instead, a flat panel slightly wider than the tires can be installed to help fair the sides of the car. Check out the side of NASCAR vehicles for reference. This panel should extend down to meet with the body pan. The corner where the two panels meet should be rounded if possible. The hardest part of this task will be the door cutouts and clearances. Side fairings also transition the air around those large appendages called tires. Turbulators, etc: In areas where the body transitions at a rate of more than 12 degrees, turbulator strips, vortex generators, diffusers, very short fairings or other devices can be used to "trip the airflow". The idea is that areas like the transition between the roof and rear window on the average car creates a large vortex. Any large vortices effectively grab the car and try to hold it back as it tries to slip through the air. If the air that makes up the vortex can be "tripped" before it leaves the back of the car, it will make smaller vortices, which will have a smaller effect on the overall aerodynamics of the vehicle. Measurement of the effects of these devices at highway speeds has been difficult to obtain. Tires: Tire rolling resistance (RR) also plays a large part in the mileage of a vehicle. Running your tire pressure at higher pressures will help somewhat (do not exceed rated pressures printed on the side of the tire), but specially designed low RR tires will help more. The typical 20% reduction in RR from a low RR tire can result in fuel savings of 2% to 4%. Here are some low rolling resistance tires tested by Green Seal and a report by the US government. Green Seal notes that a typical Ford focus can increase it's mileage by 2 MPG (from 30 to 32MPG) just by replacing the stock tires with low RR tires. A caveat however, is that low RR tires do not handle as well as normal "sport" tires. Wheel covers: Unfortunately, the coolest looking chrome spoked wheels are really bad aerodynamically. The best wheel cover is a slightly convex, completely smooth cover that fits flush with the tire. "Racing disks" like the one pictured here from JC Whitney or something similar can be snapped onto most wheels for a quick aero fix. Temperature Air temperature has a large effect on gas mileage. Part of this is due to rolling resistance. Because tires lose one PSI for every 10 degrees, and tires lose elasticity in colder weather, rolling resistance increases as temperature decreases. This means the tires don't roll as well when it's cold out. Air density also increases as temperature drops. Ralph Kenyon worked out the math to calculate how much this effects gas mileage here. His works suggests that gas mileage drops 2% for every 10 degrees F below 90 degrees due to air density alone. This means that at 40 degrees F there will be a 10% decrease in mileage. Engine efficiency : Modern engines are fairly efficient. Plenty of claims for products to improve your vehicles engine efficiency have been made, but few do anything worthwhile. The ones that do work are generally pricey. If you want to spend the bucks, you can:
Install headers or a "Y pipe" to scavenge the exhaust gasses. Do not remove the catalytic converter. Install efficient mufflers. Note that engines do require backpressure to function properly.
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Install Under-drive pulley. Note that this will reduce engine cooling and and battery recharging. Most vehicles are designed for worst case scenarios though, so this is usually ok unless you have a 3 kilowatt stereo. Install a cold air intake. Most air intake systems are designed to b e quiet, not efficient. Install a high flow air filter. If the radiator fan is driven off of the engine by belts, replace it with thermostatically controlled electric fans. Install a transmission with taller gears. Once you have made your vehicle more aero, it won't need the power that the extra RPMs provided. Taller gears mean that the engine RPMs will be lower, which equates to less gas used. Note that due to differences in how engines operate, changing the intake or exhaust system may not help the mileage. Generally they don't hurt it, but you may get lower mileage due to the tendency to drive more aggressively when you can hear the engine making cool noises. Measuring is key. Measuring your mileage : So, you have decided to terrorize your car, and are not too concerned about what your neighbors will think. Now, how do you figure out if what you did helps or hurts your mileage? You have a couple choices. Record the amount of gas and your mileage and do the math. Here's how: 1) Fill up your car. Record the mileage. 2) Next time you fill up, record the mileage and the amount of gas. 3) Latest mileage minus original mileage = number of miles driven 4) Number of miles driven divided by amount of gas = miles per gallon This is the cheapest thing to do, but takes a long time and is not very granular. Buy a mileage measurement device. I like the Scangauge II. $159 and it just plugs into the OBD port of your car. It works on almost all cars newer than 1995. New is the PLX Kiwi MPG device for only $90, though they seem to always be on backorder.
Safety Systems One of the most sophisticated technological advances on our planet has become a crucial part of everyday human life. The progression and development of the automobile has led to an increase in the dangers related to its operation. Today‘s modern vehicles are equipped with many devices that help prevent serious injury in the event of a crash, or help avoid an accident all together. Most average drivers may recognize the names of some common safety devices in their vehicle, but many lack the knowledge of how these devices effectively work in providing them with a safe driving experience each day. They key feature of this project concept incorporates the display of actual automobile safety devices. These devices will serve the purpose of allowing a user to touch and visualize each device in its true-form. A monitor screen driven by a hidden computer will display relevant information in the form of videos, diagrams, audio tracks, etc. Each device will be represented by a user-triggered button that will activate the presentation to focus on that particular chosen system. The Seatbelt System
The Seatbelt System was chosen for its importance in automobile safety, despite its simplicity. The Seatbelt has become one of the oldest automobile safety devices that are still used today. It plays a crucial role in the safety of its occupant during the operation of a vehicle. The Seatbelt has become far more than just a belt used to restrain an occupant during an automobile collision. The seatbelt system has developed through the years to become a part of the Safety-Restraint 85
System (SRS), which works in effort with the Airbag system to minimize the risk of fatal injury during an automobile accident. The Airbag System
The Airbag System has become a major technological advancement in the automotive history. The effectiveness of the Airbag in an automobile collision has made this technology a standard in all modern vehicles produced today. It works in tandem with the Seatbelt System to form the Safety-Restraint System (SRS), which minimizes the risk of fatal injury during an automobile accident. The Anti-Lock Braking System
The Anti-Lock Braking System is a safety system designed to prevent the wheels of a motor vehicle from locking up while breaking. It functions by rapidly applying a brake on and off in the event of wheel lockup. It utilizes the Engine Control Unit (ECU) along with sensors in each wheel to detect and decide when to apply anti-lock braking to a wheel. Headlights, Turn Signals, & Crumple Zones
Headlights function to illuminate the road in front of an automobile when visibility is limited. They are a crucial component to the safe operation of automobiles when visibility is limited. Advancements in headlight technology have led to adaptive headlights, which adjust the angle of the beams to upcoming curves on a road. Turn signals function to alert other automobiles of an upcoming change in direction of your automobile. They are essentially headlights that flash when activated until a turn completion is detected. Crumple Zones function to redistribute energy from a collision away from occupants. They were a significant development in preventing serious injury to occupants during a collision by collapsing, crumpling, deforming, and crushing on impact. Electronic Traction & Stability Control
The Electronic Traction Control System functions to keep automobile wheels from losing traction when accelerating. It is the Anti-Lock Braking System applied to acceleration rather than braking. It is controlled by the ECU and shares sensors and control mechanisms with the ABS. The Stability Control System functions to ensure an automobile stays on its intended path through turns. It utilizes the Anti-Lock Braking System to prevent skidding during turns as well as potential rollovers. Both of these systems are modern automobile safety achievements that have significantly reduced automobile collisions The following list of safety devices were chosen as areas in which the community may be least knowledgeable about and therefore need a better understanding of their purpose and functionality to develop a level of respect for these life-saving devices. Safety-Restraint System (SRS) Seatbelts Air Bags
Anti-Lock Braking System (ABS) Anti-Lock Disc Brakes
Passive Safety Systems o Bumper Crumple Zones Headlights Turn Signals
Engine Control Unit (ECU) 86
Electronic Traction Control Electronic Stability Control
Hydrogen storage method Hydrogen storage is a materials science challenge because, for all six storage methods currently being investigated, materials with either a strong interaction with hydrogen or without any reaction are needed. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are reviewed.
The goal is to pack hydrogen as close as possible, i.e. to reach the highest volumetric density by using as little additional material as possible. Hydrogen storage implies the reduction of an enormous volume of hydrogen gas. At ambient temperature and atmospheric pressure, 1 kg of the gas has a volume of 11 m3. To increase hydrogen density, work must either be applied to compress the gas, the temperature decreased below the critical temperature, or the repulsion reduced by the interaction of hydrogen with another material
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Storing hydrogen as a gas Three isotopes of hydrogen are known, hydrogen or protium (H), deuterium (D), and the unstable tritium (T). All the isotopes of hydrogen form covalent molecules like H2, D2, and T2, respectively, because of the single electron in the atom. Hydrogen has an ambivalent behavior towards other elements, occurring as an anion (H-) or cation (H+) in ionic compounds, forming covalent bonds, e.g. with carbon, or even behaving like a metal to form alloys or intermetallic compounds at ambient temperature The safety of pressurized cylinders is a concern, especially in highly populated regions. It is envisaged that future pressure vessels will consist of three layers: an inner polymer liner overwrapped with a carbon-fiber composite (which is the stress-bearing component) and an outer layer of an aramid-material capable of withstanding mechanical and corrosion damage. Industry has set itself a target of a 110 kg, 70 MPa cylinder with a gravimetric storage density of 6 mass% and a volumetric storage density of 30 kg·m-3. Hydrogen can be compressed using standard, piston-type mechanical compressors. The theoretical work for the isothermal compression of hydrogen is given by the equation:
where R is the gas constant, T the absolute temperature, p and p0 the end pressure and the starting pressure, respectively. The error of the work calculated with eq 3 in the pressure range of 0.1-100 MPa is less than 6%. The isothermal compression of hydrogen from 0.1-80 MPa therefore consumes 2.21 kWh·kg-1. In a real process, the work consumption is significantly higher because compression is not isothermal. Compression ratios of greater than 20:1 are possible3 with final pressures >100 MPa. The relatively low hydrogen density together with the very high gas pressures in the system are important drawbacks of this technically simple and, on the laboratory scale, well established high pressure storage method Liquid hydrogen storage Liquid hydrogen is stored in cryogenic tanks at 21.2 K at ambient pressure. Because of the low critical temperature of hydrogen (33 K), the liquid form can only be stored in open systems, as there is no liquid phase existent above the critical temperature. The pressure in a closed storage system at room temperature (RT) could increase to ~104 bar. The simplest liquefaction cycle is the Joule-Thompson cycle (Linde cycle). The gas is first compressed and then cooled in a heat exchanger, before it passes through a throttle valve where it undergoes an isenthalpic Joule- Thomson expansion, producing some liquid. The cooled gas is separated from the liquid and returned to the compressor via the heat exchanger4. The Joule-Thompson cycle works for gases, such as nitrogen, with an inversion temperature above RT. Hydrogen, however, warms upon expansion at RT. For hydrogen to cool upon expansion, its temperature must be below its inversion temperature of 202 K. Hydrogen is usually precooled using liquid nitrogen (78 K), therefore, before the first expansion step occurs. The free enthalpy change5 between gaseous hydrogen at 300 K and liquid hydrogen at 20 K is 11640 kJ·kg-1. The necessary theoretical energy (work) to liquefy hydrogen from RT is Wth = 3.23 kWh·kg-1, the technical work6 is about 15.2 kWh·kg-1, almost half of the lower heating value of hydrogen combustion. The boil-off rate of hydrogen from a liquid storage vessel because of heat leaks is a function of its size, shape, and thermal insulation. Since boil-off losses as a result of heat leaks are proportional to the surface-to-volume ratio, the evaporation rate diminishes as the storage tank 88
size increases. For double-walled, vacuum-insulated spherical dewars, boil-off losses are typically 0.4% per day for those with a storage volume of 50 m3, 0.2% for 100 m3 tanks, and 0.06% for 20 000 m3 tanks. The large amount of energy necessary for liquefaction and the continuous boil-off of hydrogen limit the possible use of liquid hydrogen storage systems to applications where the cost of hydrogen is not an issue and the gas is consumed in a short time, e.g. air and space applications Physisorption of hydrogen Resonant fluctuations in charge distributions, which are called dispersive or Van der Waals interactions, are the origin of the physisorption of gas molecules onto the surface of a solid. In this process, a gas molecule interacts with several atoms at the surface of a solid. The interaction is composed of two terms: an attractive term, which diminishes with the distance between the molecule and the surface to the power of -6, and a repulsive term, which diminishes with distance to the power of -12. The potential energy of the molecule, therefore, shows a minimum at a distance of approximately one molecular radius of the adsorbate. The energy minimum7 is of the order of 0.01-0.1 eV (1-10 kJ·mol-1). Because of the weak interaction, significant physisorption is only observed at low temperatures (<273 K). Once a monolayer of adsorbate molecules is formed, gaseous molecules interact with the surface of the liquid or solid adsorbate. The binding energy of the second layer of adsorbate molecules is, therefore, similar to the latent heat of sublimation or vaporization of the adsorbate. Consequently, a single monolayer is adsorbed at a temperature equal to or greater than the boiling point of the adsorbate at a given pressure8. To estimate the quantity of adsorbate in the monolayer, the density of the liquid adsorbate and the volume of the molecule is required. If the liquid is assumed to consist of a close-packed, face-centered cubic structure, the minimum surface area, Sml, for one mole of adsorbate in a monolayer on a substrate can be calculated from the density of the liquid, liq, and the molecular mass of the adsorbate, Mads:
where NA is the Avogadro constant. The monolayer surface area for hydrogen is Sml(H2) = 85 917 m2·mol-1. The amount of adsorbate, mads, on a substrate material with specific surface area, Sspec, is given by mads = Mads·Sspec/Sml. In the case of carbon as the substrate and hydrogen as the adsorbate, the maximum specific surface area of carbon is Sspec = 1315 m2·g-1 (single-sided graphene sheet) and the maximum amount of adsorbed hydrogen is mads = 3.0 mass%. From this approximation, we may conclude that the amount of adsorbed hydrogen is proportional to the specific surface area of the adsorbent with mads/Sspec = 2.27 × 10-3 mass%·m-2 g, and can only be observed at very low temperatures. Materials with a large specific surface area like activated or nanostructured carbon and carbon nanotubes (CNTs) are possible substrates for physisorption. The main difference 89
between CNTs and high surface area graphite is the curvature of the graphene sheets and the cavity inside the tube. In microporous solids with capillaries, which have a width of less than a few molecular diameters, the potential fields from opposite walls overlap so that the attractive force acting upon adsorbate molecules is increased compared with that on a flat carbon surface9. This phenomenon is the main motivation for the investigation of the hydrogen-CNT interaction. Most work on the theoretical absorption of hydrogen in carbon nanostructures uses the Feynman (semiclassical) effective potential approximation to calculate the adsorption potential10 or the grand canonical Monte Carlo simulation11,12. The adsorption potential is 9 kJ mol-1 Metal Hydrides Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides. The electropositive elements are the most reactive, i.e. Sc, Yt, lanthanides, actinides, and members of the Ti and Va groups. The binary hydrides of the transition metals are predominantly metallic in character and are usually referred to as metallic hydrides. They are good conductors, have a metallic or graphite-like appearance, and can often be wetted by Hg. Many of these compounds, (MHn), show large deviations from ideal stoichiometry (n = 1, 2, 3) and can exist as multiphase systems. The lattice structure is that of a typical metal with hydrogen atoms on the interstitial sites; and for this reason they are also called interstitial hydrides. This type of structure is limited to the compositions MH, MH2, and MH3, with the hydrogen atoms fitting into octahedral or tetrahedral holes in the metal lattice, or a combination of the two. The hydrogen carries a partial negative charge, depending on the metal, but an exception27 is PdH0.7. Pt and Ru are able to adsorb considerable quantities of hydrogen, which becomes activated. These two elements, together with Pd and Ni, are extremely good hydrogenation catalysts, although they do not form hydrides28. Especially interesting are the metallic hydrides of intermetallic compounds, in the simplest case the ternary system ABxHn, because the variation of the elements allows the properties of these hydrides to be tailored. Element A is usually a rare earth or an alkaline earth metal and tends to form a stable hydride. Element B is often a transition metal and forms only unstable hydrides. Some well defined ratios of B:A, where x = 0.5, 1, 2, 5, have been found to form hydrides with a hydrogen to metal ratio of up to two. The reaction of hydrogen gas with a metal is called the absorption process and can be described in terms of a simplified one-dimensional potential energy curve29 (Fig. 4). The hydrogen atoms contribute their electron to the band structure of the metal. At a small hydrogen to metal ratio Complex hydrides
In 1996, Bogdanovic and Schwickardi38 showed, for the 90
first time, adsorption and
desorption pressure-concentration isotherms for catalyzed NaAlH4 at temperatures of 180°C and 210°C. The isotherms, which have a nearly horizontal pressure plateau, do not show hysteresis. Furthermore, the catalyzed system reversibly absorbs and desorbs hydrogen of up to 4.2 mass%. The mechanism of the two-step reaction was also described. A more detailed study of NaAlH4 with an improved catalyst has been conducted more recently39. A desorption hydrogen pressure of 2 bar at 60°C was found and the enthalpy for the dissociation reaction was determined to be 37 kJ·mol-1 and 47 kJ·mol-1 for the first and second dissociation steps of Ti-doped NaAlH4, respectively, according to the reactions: The equilibrium hydrogen pressure at RT, therefore, is approximately 1 bar. Furthermore, the reaction is reversible, a complete conversion to product was achieved at 270°C under 175 bar hydrogen pressure in 2-3 hours40. Storage via chemical reactions Hydrogen can be generated by reacting metals and chemical compounds with water. The common experiment, seen in many chemistry classes, where a piece of Na floating on water produces hydrogen, demonstrates the process. The Na transforms to NaOH in this reaction. The reaction is not directly reversible, but NaOH can be removed and reduced in a solar furnace back to metallic Na. Two Na atoms react with two H2O molecules and produce one hydrogen molecule. The hydrogen molecule produces a H2O molecule in combustion, which can be recycled to generate more hydrogen gas. However, the second H2O molecule necessary for the oxidation of the two Na atoms has to be added. Therefore, Na has a gravimetric hydrogen density of 3 mass%. The same process carried out with Li leads to a gravimetric hydrogen density of 6.3 mass%. The major challenge of this storage method is reversibility and control of the thermal reduction process in order to produce the metal in a solar furnace. The process has been successfully demonstrated with Zn45. The materials science challenge of hydrogen storage is to understand the interaction of hydrogen with other elements better, especially metals. Complex compounds like Al(BH4)3 have to be investigated and new compounds of lightweight metals and hydrogen will be discovered. Hydrogen production, storage, and conversion has reached a technological level, although plenty of improvements and new discoveries are still possible. Six different hydrogen storage methods have been described here. Alongside well-established, high-pressure cylinders for laboratory applications and liquid hydrogen storage methods for air and space applications, metal hydrides and complex hydrides offer a safe and efficient way to store hydrogen. Further research and technical development will lead to higher volumetric and gravimetric hydrogen density. The best materials known today show a volumetric storage density of 150 kg·m-3, which can still be improved by approximately 50% according to theoretical estimations.
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AUTOMATED GUIDED VEHICLE SYSTEM (AGVS)
AGVS is a Computer-Controlled, Non-manned, Electric Powered Vehicle Capable of Handling Material. •
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Clear floor space No floor deck construction
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Simple installation
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High availability/reliability
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Flexible performance increments
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Short installation times
Simple expansion Vehicle functions •
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Man / vehicle functions
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Inputs made via operator panel with its keyboard and display
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Destination input to the vehicle
Plug-in manual control and diagnosis module Route (destination) finding •
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High vehicle intelligence
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Travel route topology stored in the vehicle
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Destination code processing
Load-sensing and empty location recognition Guide track following •
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Guided movements using:
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optical track
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inductive track
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"free-flight" (partly guide-trackless)
"free-navigation" (guide-trackless Data exchange •
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Infrared
Radio Special functions •
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Battery reserve monitoring
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Control of battery charging 92
