ABSTRACT Conservation of natural resources has become a necessity in today’s World, especially in the new technology. In the automotive industry, maximum energy is lost during deceleration or braking. This problem has been resolved with the introduction of regenerative braking. Kinetic Energy Recovery Systems (KERS) is a type of regenerative braking system for recovering a moving vehicle's kinetic energy under braking. The recovered energy is stored in a reservoir (for example a flywheel or a battery or super capacitor) for later use under acceleration. It functions in such a way that it reduces the speed of the vehicle by converting some of its kinetic energy and storing it into a useful form of energy instead of dissipating it as heat as seen in conventional braking systems. This paper mainly highlights the different ways of recovering energy using flywheel, batteries, or even hydraulically.
Keywords:-kinetic energy recovery system, motor-generator unit, power control unit, batteries, flywheel, continuous variable transmission, reversible pump, accumulator.
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CONTENTS
TITLE
PAGE
LIST OF FIGURES………………………………………………................ iv LIST OF TABLES………………………………………………………...... v
1. INTRODUCTION………………………………………………………..…. 1 2. KERS…………………………………………………………......……….. 2 2.1 HISTORY OF KERS…………………………………...…………….… 3 2.2 KERS IN FORMULA ONE……………………….……………………. 4 2.3 KERS IN ROAD CARS…………………………....…………….….…. 6 3. BASIC COMPONENTS OF KERS………………………...…………………. 7 3.1 BASIC WORKING PRINCIPLE OF KERS…………..………………….. 9 4. TYPES OF KERS………………………………………………....……….… 11 4.1 MECHANICAL KERS……………………………..………………...… 11 4.2 ELECTRICAL KERS…………………………………………...……… 15 4.3 HYDRAULIC KERS …………………………………..…………….… 18 5. COMPARISON OF MECHANICAL, ELECTRICAL AND HYDRAULIC KERS.. 22 6. ADVANTAGES OF KERS…………………………………………….......… 24 7. DISADVANTAGES OF KERS………………………………….........…....… 25 8. APPLICATIONS OF KERS……………………………………………....….. 26 9. CONCLUSION…………………………………………………………....... 28 10. REFERENCES………………………………………………...........……… 29
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LIST OF FIGURES
FIGURE
TITLE
PAGE
3.1
A MOTOR GENERATOR UNIT
7
3.2
POWER CONTROL UNIT
8
3.3
KERS SCHEMATIC
9
3.4
KERS IN FORMULA ONE CAR
10
4.1
ROLLER BASED CVT
12
4.2
WORKING PRINCIPLE OF MECHANICAL KERS
13
4.3
ELECTRICAL KERS
16
4.4
WORKING PRINCIPLE OF ELECTRICAL KERS
17
4.5
VARIABLE DISPLACEMENT PUMP
19
4.6
HYDRAULIC KERS
19
4.7
WORKING PRINCIPLE OF HYDRAULIC KERS
20
4
LIST OF TABLES
TABLE
5.1
TITLE
PAGE
COMPARISON OF DIFFERENT TYPES OF KERS
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1. INTRODUCTION
With the growing demand for transportation from time to time, the number of vehicles owned in this world has increased exponentially, which in turn raises the demand for fuel. However, the supply for crude oil available on earth to provide fuel is diminishing quickly. Therefore change is needed to decrease the global reliance on oil and also to tackle the environmental problems caused by the usage of fuel, mainly the green house effects. As a solution to this, energy recovery systems were developed by the FIA. The FIA has decided to try and take formula one in a more environment friendly direction. As a result of this, the FIA feels that formula one car should more closely resemble hybrid cars available in the consumer market. There were two types of energy recovery systems. One was the heat energy recovery system (HERS) and the other was the kinetic energy recovery system (KERS). Heat Energy Recovery System captures the thermal energy generated from the engines exhausts which would be normally lost through the exhausts pipes. It consists of a turbocharger, intercooler and a motor-generator unit (MGU). The turbocharger comprises of a turbine and a compressor supported by bearings on the same axis. The exhausts energy is converted into mechanical shaft power by the turbine and it is used to drive a compressor and an MGU. The compressor draws in air and compresses it and fed it into the combustion chamber thus allowing more combustion and higher output. The intercooler lowers the intake air to a temperature suitable for combustion. Kinetic Energy Recovery System refers to the mechanisms that recover the energy that would normally be lost when reducing speed. This energy is then stored in a flywheel or a battery and retransmitted to the wheel in order to help the acceleration. Electric vehicles and hybrid have a similar system called Regenerative Brake which restores the energy in the batteries. The device recovers the kinetic energy that is present in the heat wasted during car’s braking process. It stores that energy and converts it into power that can be called upon to boost acceleration. By the touch of a push button provided on the car’s steering, this stored energy is converted back into kinetic energy giving the vehicle extra boost of power.
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2. KERS
The acronym KERS stands for Kinetic Energy Recovery System. KERS is a regenerative braking that stores the kinetic energy of a moving vehicle under deceleration. KERS works on the basic principle of physics that states, “Energy cannot be created or destroyed, but it can be endlessly converted.” This reduces the amount of energy needed for the engine to deliver for the vehicle to pick up which leads to better performance as well as fuel efficiency. KERS is a collection of parts which recovers some of the kinetic energy of a vehicle that is present in the wasted heat during the car’s braking process. It stores this energy in a reservoir (such as a flywheel or a battery) and releases it back into the drive train of the vehicle, providing a power boost to the vehicle. During braking, kinetic energy is wasted in the form of heat or sometimes as sound energy that is dissipated into the surroundings. Vehicles with KERS are able to harness some of this kinetic energy and in doing so will assist in braking. For the driver, it is like having two power sources, one of the power sources is the engine and the other is the stored kinetic energy. Basically, it’s working principle involves storing the energy involved with deceleration and using it for acceleration. That is, when a car brakes, it dissipates a lot of kinetic energy as heat. The KERS stores this energy and converts it into power. Upto 80 bhp of power can be obtained which lasts for approximately 6.67 seconds There are principally two types of system - battery (electrical) and flywheel (mechanical). Electrical systems use a motor-generator incorporated in the car’s transmission which converts mechanical energy into electrical energy and vice versa. Once the energy has been harnessed, it is stored in a battery and released when required. Mechanical systems capture braking energy and use it to turn a small flywheel which can spin at up to 80,000 rpm. When extra power is required, the flywheel is connected to the car’s rear wheels. In contrast to an electrical KERS, the mechanical energy doesn’t change state and is therefore more efficient. There is one other option available - hydraulic KERS, where braking energy is used to accumulate hydraulic pressure which is then sent to the wheels when required. The principle behind hydraulic KERS units, by contrast, is to reuse a vehicle’s kinetic energy by conducting pressurized hydraulic fluid into an accumulator during deceleration, then 2
conducting it back into the drive system during acceleration. But there are some fundamental problems here as well. One is the relatively low efficiency of rotary pumps and motors. Another is the weight of incompressible fluids. And a third is the amount of space needed for the hydraulic accumulators, and their awkward form factor. None of this matters too much in, say, heavy commercial vehicles but it makes this option unsuitable for road and racing cars.
2.1 HISTORY OF KERS The concept of transferring the vehicle’s kinetic energy using Flywheel energy storage was postulated by physicist Richard Feynman in the 1950s and is exemplified in complex high end systems such as the Zytek, Flybrid, Torotrak and Xtrac used in F1 and simple, easily manufactured and integrated differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS). Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a continuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle’s hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between the flywheel, drive wheel and road wheel. The FIA has defined the amount of energy recovery for the 2009 season as 400kJ per lap which translates to 6.6 seconds of 82hp speed boost. The transfer of power to a battery is the electronic KERS system. There is a mechanical KERS system also which uses a flywheel to store the wasted kinetic energy instead of a battery. Kinetic Energy Recovery Systems (KERS) were used for the motor sport Formula One's 2009 season, and under development for road vehicles. However, KERS has been abandoned for the 2010 Formula One season. The Formula One Teams that used Kinetic Energy Recovery Systems in the 2009 season are Ferrari, Renault, BMW and McLaren. One of the main reasons that not all cars use KERS is because it adds an extra 25 kilograms of weight, while not adding to the total car weight, it does incur a penalty particularly seen in the qualifying rounds, as it raises the car's centre of gravity, and reduces the amount of ballast that is available to balance the car so that it is more predictable when turning. FIA rules also limit the exploitation of the system. Eventually,
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during the season, Renault and BMW stopped using the system. Williams is developing a flywheel-KERS system. It was first introduced to the general public through the 2009 series of Formula one motor sport. KERS builders, Flybrid Systems demonstrated a working Formula One-spec device at the Autosport International show. (24kg, 400kj energy capacity, power boost-60kw).FIA introduced KERS in 2009 GP series to Increase Overtaking and also as defensive tool to block faster car. But many F1 teams opposed it, as it was an Expensive system, so it was banned in 2010 season. But with improvements and increase in manufacturers for KERS it was reintroduced in 2011. While the hybrid systems introduced for 2014 are altogether more far-reaching, the idea of the engine as a standalone source of propulsion in Formula One competition was consigned to history several years ago through the introduction of KERS (Kinetic Energy Recovery System) hybrid power in 2009 and from 2011 through 2013, with the Mercedes-powered McLaren of Lewis Hamilton taking the first ever hybrid F1 victory at the 2009 Hungarian Grand Prix. So, starting with the 2014 season, Formula 1 regulations shift drastically, and the most significant change is the switch from 2.4-litre normally aspirated V8 engines to 1.6litre direct-injected turbocharged V6 units. 1.6 turbocharged, and you kind of expect they'll be pretty similar to the 1.6-liter turbocharged engines in your family car nowadays. Not even close.
2.2 KERS IN FORMULA ONE Kinetic Energy Recovery System (KERS) is a very unique and a debated addition to F1 racing. The KERS is exemplified in complex high end systems such as the Zytek, Flybrid, Torotrak and Xtrac used in F1. The FIA (Federation Internationale Automobile) have authorized hybrid drivetrains in Formula 1 racing for the 2009 racing season. The intent is to use the engineering resources of the Formula 1 community to develop hybrid technology for use not only in motorsport but also eventually in road vehicles. The hybrid systems specifications have been kept to a minimum, especially the type of hybrid system. This was 4
done purposely to lead to the study and development of various alternatives for electrical hybrids which has been met with success. The Flybrid Kinetic Energy Recovery System (KERS) was a small and light device designed to meet the FIA regulations for the 2009 Formula One season. The key system features were:
A flywheel made of steel and carbon fibre that rotated at over 60,000 RPM inside an evacuated chamber
The flywheel casing featured containment to avoid the escape of any debris in the unlikely event of a flywheel failure
The flywheel was connected to the transmission of the car on the output side of the gearbox via several fixed ratios, a clutch and the CVT
60 kW power transmission in either storage or recovery
400 kJ of usable storage (after accounting for internal losses)
A total system weight of 25 kg
A total packaging volume of 13 litres.
With a focus on safety, the FIA have specified a limit on both the power rating of the hybrid system at 60kW and the quantity of energy transfer per lap at 400kJ. This translates into an extra 85bhp for just under seven seconds, which makes overtaking another vehicle on the track easier and the race much more interesting. Thus although a 0.3s boost to lap times, the system was ultimately limited in its potential to improve lap times. Thus no team could create a competitive advantage from a more powerful system. Then the weight of the system created issues, At a time when the wider front slick tyres demanded an extreme weight distribution of up to 49% weight on the front axle, the 25+Kg of a KERS system mounted behind the Centre of gravity, the handicapped teams being able to push weight forwards. Most teams dropping or not racing their system cited weight as the main reason for its loss. There is more than one
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type of KERS used in motorsports. The most common is the electronic system built by the Italian company Magnetti Marelli, which is used by Red Bull, Toro Rosso, Ferrari, Renault and Toyota. Although races have been won with this technology, KERS was removed from the 2010 Formula 1 season due to its high cost.
2.3 KERS IN ROAD CARS
Flybrid Systems are working with a number of OEM car makers including Jaguar and Volvo to develop flywheel hybrid systems for road cars. Available in a range of power and storage levels these systems are suitable for application to vehicles in all classes from small city cars to full size saloon cars and SUVs. Road car systems typically feature:
A design life of 250,000 kilometres with no performance degradation.
A better cost / benefit ratio than electric hybrid systems.
Optimised KERS transmission based on either CVT or CFT technology.
Bespoke Flybrid developed vacuum, oil and hydraulic pumps to reduce parasitic losses.
Powerful clutches to allow launch of the vehicle from rest under flywheel power alone with the engine turned off.
Fully automatic control systems that react to normal vehicle control pedal movements and require no additional inputs from the driver.
Packaging space similar to just the motor of an electric hybrid system.
These are full hybrid systems capable of kinetic energy recovery but also able to store energy when the vehicle is not braking in order to optimise the engine operating efficiency. The systems can also drive the car with the engine turned off and boost performance in addition to 6
the engine to aid engine downsizing. Using optimised strategies CO2 and fuel consumption savings of over 18% has been demonstrated on the NEDC test cycle and more than 22% has been demonstrated in real world conditions (quoted savings include the benefit of stop / start technology).
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3. BASIC COMPONENTS OF KERS The three main components of the KERS are as follows: 1. Motor/Generator Unit (MGU) 2. Power Control Unit (PCU) 3. Power Storage Unit (PSU) used to store and deliver quick energy.
Motor/Generator Unit (MGU) It is a single unit which has both motor-generator rotor coils wound around a single rotor, and both the coils share the same magnets. It is mounted to the front of the engine; this is driven off a gear at the front of the crankshaft. It works in two modes-braking mode and acceleration mode. That is it acts as a generator in braking mode and as a generator in acceleration mode. When electric current is supplied to the terminals of an MGU, it acts as motor and when the rotor of the MGU is rotated by means of a prime mover, it works as a generator. Running at high rpm and generating a significant dc current the unit runs very hot, so it is necessary to oil cool or water cool the MGU.
Fig 3.1: motor/generator unit (MGU)
Power Controlled Unit (PCU) 8
It is the central processing unit of the KERS system. It serves two purposes, firstly to invert & control the switching of current from the batteries to the MGU and secondly to monitor the status of the individual cells with the battery. Managing the battery is critical as the efficiency of a pack of Li-ion cells will drop if one cell starts to fail. A failing cell can overheat rapidly and cause safety issues. As with all KERS components the PCU needs cooling. It is directly linked to the vehicles electronic control unit.
Fig 3.2: Power controlled unit (PCU)
Power Storage Unit (Flywheel / batteries)
It stores power released by the MGU for immediate usage and gives power as and when required. Flywheels are used in Mechanical KERS and batteries are used in Electrical KERS. Being charged and discharged repeatedly during a lap, the batteries would run very hot and needed cooling. Lithium ion batteries and lead acid batteries are some of the examples. Ultra capacitors or super capacitors are also used.
3.1 WORKING PRINCIPLE OF BASIC KERS 9
When a car is being driven it has kinetic energy and the same energy is converted into heat energy on braking. It is the rotational force of the car that comes to stop in case of braking and at that time some portion of the energy is also wasted. With the introduction of KERS system the same unused energy is stored in the car and when the driver presses the accelerator the stored energy again gets converted to kinetic energy. This system takes the energy wasted during car’s braking process, store it and then reuse it to temporarily boost engine power. The following diagram show the typical placement of the main components at the base of the fuel tank, and illustrate the system’s basic functionality. A standard KERS operates by a ‘charge cycle’ and a ‘boost cycle’. In the charging phase, kinetic energy from the rear brakes is captured by an electric alternator/motor controlled by a central processing unit (CPU), which then charges the batteries.
Fig 3.3: KERS schematic In the boost phase, the electric alternator/motor gives the stored energy back to the engine in a continuous stream when the driver presses a boost button on the steering wheel. This energy 10
equates to around 80 horsepower and may be used for up to 6.6 seconds per lap. The location of the main KERS components at the base of the fuel tank reduces fuel capacity (typically 90100kg in 2008) by around 15kg, enough to influence race strategy, particularly at circuits where it was previously possible to run just one stop. The system also requires additional radiators to cool the batteries. Mechanical KERS, as opposed to the electrical KERS illustrated here, work on the same principle, but use a flywheel to store and re-use the waste energy.
Fig 3.4: KERS unit in Formula One car
4. TYPES OF KERS
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Advanced transmissions that incorporate hi-tech flywheels are now being used as regenerative systems in such things as formula-1 cars, where they're typically referred to as kinetic energy recovery systems (KERS). The types of KERS that have been developed are:
Mechanical KERS Electrical KERS Hydraulic KERS
The mechanical system utilizes a rotating mass (or flywheel) as the energy storage device, whereas electrical system utilizes rechargeable batteries as the storage medium and electric motor/generator systems as the energy transfer and control media. The hydraulic KERS uses variable displacement hydraulic pump/motor together with a hydro pneumatic accumulator.
4.1 Mechanical KERS
The concept of transferring the vehicle’s kinetic energy using flywheel energy storage was postulated by physicist Richard Feynman in the year 1950. The mechanical KERS system has a flywheel as the energy storage device but it does away with MGUs by replacing them with a transmission to control and transfer the energy to and from the driveline. The kinetic energy of the vehicle ends up as kinetic energy of a rotating flywheel through the use of shafts and gears. To cope with the continuous change in speed ratio between the flywheel and road-wheels, a Continuously Variable Transmission (CVT) is used, which is managed by an electrohydraulic control system. A clutch allows disengagement of the device when not in use. The CVT is used because the ratios of vehicle and the flywheel speed are different during a braking or an acceleration event. The only mechanism for controlling energy into or out of the flywheel is by controlling the ratio of CVT. The 'variator', as the CVT portion of the KERS unit is called, operates at over incredible 90 percent mechanical efficiency. The components within each variator include an input disc and an opposing output disc. Each disc 12
is formed so that the gap created between the discs is ‘doughnut’ shaped; that is, the toroidal surfaces on each disc form the toroidal cavity. Two or three rollers are located inside each toroidal cavity and are positioned so that the outer edge of each roller is in contact with the toroidal surfaces of the input disc and output disc. One disc connects to the engine. This is equivalent to a driving pulley. Another disc connects to the drive shaft. This is equivalent to the driven pulley. As the input disc rotates, power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc. The angle of the roller determines the ratio of the Variator and therefore a change in the angle of the roller results in a change in the ratio. So, with the roller at a small radius (near the centre) on the input disc and at a large radius (near the edge) on the output disc the variator produces a low ratio. The transfer of the vehicle’s kinetic energy to the flywheel reduces the speed of the vehicle and increases the speed of the flywheel. Similarly the transfer of flywheel’s kinetic energy to the vehicle increases the speed of the vehicle and reduces the speed of the flywheel. The flywheel is made of high strength steel and carbon fibre material and rotates at 60,000 rpm.
Fig 4.1: A roller based CVT
Working principle of mechanical KERS
Storage cycle:
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During braking of the cars, the CVT connects the KERS flywheel system with the drive shaft. The flywheel starts rotating and absorbs kinetic energy from the wheels.
Boost cycle:
During this stage when the driver pushes the boost button, the flywheel acts as propulsion motor and discharges the energy to the wheels. The Flywheel rotor is decelerated during boost discharge mode and the energy is converted back. Thus flywheel acts as a generator and sends energy back to the power train of the vehicle.
Fig 4.2: Working principle of mechanical KERS
Pros of Mechanical KERS
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KERS Compact weight and size – The entire system (the CVT, the flywheel and the housing) is roughly half the weight and packaging of a battery hybrid system.
Twice as efficient – Electrical KERS lose kinetic potential during the conversion of energy from mechanical to electrical to chemical, and then back again. It’s a fundamental of the Second Law of Thermodynamics: transforming energy from one form to another introduces losses. Electrical KERS are approximately 34 percent efficient. Flywheel drives are all mechanical and suffer no conversion losses. Most of the energy loss that does occur comes from normal friction between moving parts. These systems are about 70 percent efficient.
Lower cost – Smaller size and weight and reduced complexity make these arrangements about one quarter the cost of a battery-electric system.
Cons of mechanical KERS
Less range – The power stored by these systems has much less potential vehicle range than normal battery-electric hybrid units.
Complex construction – To maintain high efficiency, flywheel storage units require high strength materials, nearly friction-free magnetic or vacuum bearings, and sometimes, multiple individual flywheels.
Frictional effects – Friction produced in the bearings and seals cause the flywheel to slow down and loose energy.
4.2 Electrical KERS
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In electrical KERS, braking rotational force is captured by an electric motor / generator unit (MGU) mounted to the engines crankshaft. This
MGU takes the electrical energy that it
converts from kinetic energy and stores it batteries. The boost button then
summons the
electrical energy in the batteries to power the MGU. The system consists of three main components: 1. Motor Generator Unit (MGU): It is situated between the fuel tank and the engine, linked directly to the crankshaft to deliver additional power. It consists of the rotor coils of the motor and generator wound around the same rotor. They share the same magnets. 2. Storage Unit: It consists of high voltage lithium ion batteries which are capable of storing and delivering energy rapidly. 3. Power Control Unit (PCU): It manages the behaviour of the MGU when charging and releasing energy. It performs two functions. It inverts and controls the switching between the battery and MGU and it also monitors the status of individual cells in the battery. This is essential because the efficiency of the Lithium ion battery cell may decrease leading to battery failure and subsequently raid overheating, leading to safety issues. It is linked to the car’s standard electronic control unit. The electric motor works as an alternator to convert the kinetic energy into electrical as well as to release the stored electrical energy to the wheels. Additionally, the motor also has a built in water cooling system. The motor is attached to the front of the engine and is driven by a reduction gear off the crankshaft. Batteries become hot while charging, so many of the KERS cars have more cooling ducts since charging will occur multiple times throughout a race. Super-capacitors can also be used to store electrical energy instead of batteries, since they run cooler and are more efficient.
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Fig 4.3: Electrical KERS
Working principle of Electrical KERS
Storage cycle:
When the brakes are applied, the gearbox output shaft rotates the electric Motor /Generator Unit (MGU) and hence it acts as an electric generator. The generator produces electrical energy to the MGU. The PCU unit transfers the electrical energy to store it in the battery unit.
Boost cycle:
When the additional acceleration in required, the PCU unit releases the stored electrical energy to the MGU. The MGU now acts as an electric motor. The motor converts the electrical energy to rotational energy. The kinetic energy is the transferred to the drive wheels through the gear box.
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Fig 4.4: working principle of electrical KERS
Pros of Electrical KERS:
The electric systems allow the teams to be more flexible in terms of placing the various components around the car which helps for better weight distribution which is of vital importance in formula one.
The specific energy of lithium-ion batteries in comparison is unrivalled as they can store considerably more energy per kg which helps reducing the size.
Cons of electrical KERS:
Lithium-ion batteries take 1-2 hours to charge completely due to low specific power (i.e. rate to charge or discharge) hence in high performance F1 cars more batteries are required which increases the overall weight of the car.
Chemical batteries heat up during charging process and this takes place a number of times in KERS units which if not kept under control could cause the batteries to lose energy over the cycle or worse even explode. 18
The specific power is low as the energy needs to be converted at least two times both while charging or discharging causing energy losses in the process.
4.3 Hydraulic KERS A hydraulic KERS would use a pump in place of the MGU and an accumulator in place of the batteries. Simple valves would route the fluid into the accumulator or to the pump to either generate or reapply the stored power. It consists of four main components:
A working fluid A low pressure reservoir Hydraulic pump/motor (variable displacement machine) A high pressure accumulator.
In the hydraulic kinetic energy recovery system, the brake energy is used as prime moving source for hydraulic pump; the low pressure fluid will be pumped from the low pressure reservoir to the high pressure accumulator which functions as a high pressure hydraulic energy storage device. Based on the state of charge of hydraulic accumulator, the required acceleration demanded, the stored energy will be used to propel the vehicle later on via hydraulic motor. When the driver steps on the brake, it uses the movement of the wheels to compress hydraulic fluid, thus reducing the truck’s speed. When the truck accelerates again, the energy returns to the wheels. This is a hydraulic recovery system. The principle behind hydraulic KERS units is to reuse a vehicle’s kinetic energy by conducting pressurized hydraulic fluid into an accumulator during deceleration, then conducting it back into the drive system during acceleration. In some systems, hydraulic transformer is also installed for converting output flow at any pressure with a very low power loss. A variable displacement pump is a device that converts mechanical energy to hydraulic energy. The displacement or amount of fluid pumped per revolution of the pumps input shaft can be varied. Many variable displacement pumps are reversible. That is they can act as a 19
hydraulic motor and convert fluid energy into mechanical energy. It consists of several pistons in cylinders arranged parallel to each other rotating around a central shaft. A swash plate is present at one of the end and is connected to pistons. As the driving shaft rotates the angle of the swash plate causes the piston to reciprocate inside the cylinder. Thus sucking the fluid from the low pressure reservoir and delivering pressurised fluid to high pressure accumulator.
Fig 4.5: variable displacement pump An accumulator is an energy storage device. The output of the accumulator will be continuous in nature. It stores high pressure fluid delivered by the pump in during braking. The accumulator is pre-charged with nitrogen gas. A low pressure reservoir is used to store the low pressure liquid after it has done the work on motor.
Fig 4.6: A hydraulic KERS
Working principle of Hydraulic KERS 20
Storage cycle:
When the driver steps on the brake, the vehicle’s kinetic energy is used to power a reversible pump. This reversible pump sends the working fluid from a low pressure reservoir to a high pressure accumulator of the vehicle. The fluid compresses the nitrogen gas in the accumulator and pressurizes the system. This slows the vehicle speed. The fluid remains under pressure in the accumulator, until the driver pushes the accelerator pedal again.
Boost cycle:
During acceleration, the fluid in high pressure accumulator is metered out to drive the pump as a motor. Thus the system propels the vehicle by transmitting torque to the driveshaft. The vehicle accelerates and the pump moves the fluid back to the low pressure reservoir, ready to charge the accumulator again.
Fig 4.7: working principle of hydraulic KERS
Pros of hydraulic KERS
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Efficient – In many applications, especially those where high power densities are required, hydro-pneumatic systems offer a more efficient alternative to system driven
by electric motors. Faster – They can be used to capture and transfer high levels of energy extremely quickly compared with similarly sized electric systems, which generally require long
periods over which batteries have to be charged. Longer life– They are also likely to have a longer operating life than battery-powered systems.
Cons of hydraulic KERS
Weight – The main disadvantage of a hydraulic KERS would be its weight, which is attributed to by weight of hydraulic fluid, accumulator material (steel), and the fact that in application it would be necessary to have separate high and low pressure accumulators.
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Limited power – hydraulic systems are limited where consistent levels of power are required for extended periods at near constant speeds, such as long-distance cruising.5.COMPARISON
ELECTRICAL
OF MECHANICAL,
AND HYDRAULIC KERS
5.1 COMPARISON OF DIFFERENT TYPES OF KERS
Factors
Mechanical
Electrical
Hydraulic
Very compact in size. Heavier due to It is also heavier due to
Weight and size
The entire system is the presence of the installation of pump, nearly half the weight MGU,
inverters accumulators
and
and batteries but reservoirs.
of electrical system.
it provides more They
Efficiency
are
flexibility. more They
efficient than other potential
lose More efficient than during electrical systems and
systems since there the conversion of has an efficiency of are
no
energy mechanical
to about 60 to70%.
conversion losses and electrical and to are about 70 to 75% chemical. Hence efficient.
they
are
less
efficient and are about 30 to 35%
Power storage
The power stored by
efficient. The specific
Limited power.
flywheels has much
energy of
Consistent level of power
less potential it lasts
batteries is high
is required for extended
for only about 20 to
compared to other periods such as long
30 seconds.
systems and it lasts for about 30
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distance cruising.
minutes.
Operating speed
Cycle life
Faster in receiving
They take 1 to 2
Quicker compared to
and delivering
hours to charge
electrical systems. They
charges.
completely due to
can be used to capture
low specific
and transfer high levels
Longer service life
power. Less service life
of energy quickly. Longer operating life
than all other
compared to
than electrical systems.
systems.
flywheel and hydraulic
Cost
Fuel consumption
Smaller size and
systems. Higher cost due
They are also costlier due
reduced complexity
to inverter,
to installation of the
makes the system’s
batteries, and
pump, reservoir and
cost one fourth of the
MGU.
accumulator.
battery system. Reduced fuel
About 15% of
18% of fuel consumption
consumption of about fuel consumption
is reduced which is better
27%.
than electrical system.
is reduced. They have least due to shorter life and less conversion capacity.
Table No 5.1: Comparison of Different Types of Kers
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6. ADVANTAGES OF KERS
It is an alternative source of power. Reduced wastage of power during deceleration. Reduced fuel consumption and carbon monoxide levels. Longer system life up to 250,000 km’s. An ancillary flywheel is particularly suited to start-stop driving situations when real world fuel economy is often at its worst. In heavily congested traffic where a car is frequently stopped and restarted, the system can help alleviate the heavy fuel consumption and emissions of green house gases normally associated with these
conditions. High efficiency and easy storage and recovery. High power capability. It is truly a green solution. Low cost in volume manufacture. Mechanical KERS are of lower cost than electric systems due to small in size and weight.
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7. DISADVANTAGES OF KERS The energy recovery system is functional only when the car is moving. The recovery system must be controlled by the same electronic controlled unit. If in case the KERS is connected between the wheels and the differential, the torque
applied to each wheels must be the same. Malfunction of the system might result in fire hazard. The system is bulky. It also requires complex mechanisms and electrical components. In mechanical KERS that uses flywheel, large gyroscopic forces are induced in them. Requires high investment in developing the system as it is relatively new and under development.
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8. APPLICATIONS OF KERS
Auto part makers
Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include a lithium ion battery with scalable capacity or a flywheel, a four to eight kilogram electric motor (with a maximum power level of 60 kW (81 hp)), as well as the KERS controller for power and battery management. Bosch also offers a range of electric hybrid systems for commercial and light-duty applications.
Bicycles
KERS is also possible on a bicycle. This is achieved by mounting a flywheel on a bike frame and connecting it to the back wheel. By shifting the gear, 20% of the kinetic energy can be stored in the flywheel, ready to give an acceleration boost by re-shifting the gear.
Motorcycles
KTM racing boss Harald Bartol revealed that the factory raced with a secret kinetic energy recovery system fitted to Tommy koyama’s motorcycle during the 2008 season-ending 125cc Valencia grand prix. This was illegal, so they were later banned from using it in the 27
future. The Lit C-1 electric motorcycle will also use a KERS as a regenerative braking system.
Modern day cars It is used in few hybrid cars,buses and lorries.
Locomotive and metro trains When most trains are powered either directly or indirectly by electricity, it might seem like a foregone conclusion that hybrid technology is the right choice for braking energy recovery. In fact this is not true and a mechanical KERS equipped train could save around twice as much of the normally wasted energy savings as an electric hybrid one. Braking energy storage and recovery on trains also has a number of significant secondary advantages such as greatly reduced brake wear and reduced heat generation.
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9. CONCLUSION KERS is an effective type of regenerative braking which can fulfil the main purpose of hybrid vehicles i.e., storing and re-using energy lost while braking. The various types of KERS show different ways of storing and converting energy from one form to another. It’s a technology for the present and future because it’s environment-friendly, reduces emissions, increases efficiency and is highly customizable and modifiable. Adoption of a KERS may permit regenerative braking and engine downsizing as a means of improving efficiency and hence reducing fuel consumption and CO2 emissions. The KERS have major areas of development in power density, life, simplicity, effectiveness and first and foremost cost of the device. Applications are being considered for small, mass-production passenger cars as well as luxury cars and trucks. With the increase in prices of the crude oil and its depletion, it is necessary to implement suitable measures to make the automobiles highly efficient. This is where KERS comes into the 3account. This system has its own limitations with the technology available currently. With the developments in various fields radical advancement in this system can be achieved and the limitations will be overcome one day. This is a technology which is still in its early development stages, so future energy storage devices and mechanisms might be quite different compared to the currently available ones.
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10. REFERENCES www.howstuffworks.com/KERS.html www.flybridsystems.com/F1System.html www.gizmag.com/formula-one-kers.html http://blog.mechguru.com/machine-design/kinetic-energy-recovery-system-kers
works thewptformula.com/kers.html https://scarbsf1.wordpress.com/2010/10/20/kers-anatomy www.formula1–dictionary.net/kers.html www.racecar-engineering.com/articles/f1/flywheel-hybrid-systems-kers www.formula1.com/inside-f1/understanding_the sport/8763.html. www.racecar–engineering.com/articles/technology/311644/Williams-f1-kers-
explained.html http://www.iaeng.org/publication/WCE2013/WCE2013_pp1969-1973.pdf http://www.slideshare.net/harshgupta161/kinetic-energy-recovery-system-kers
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