SAE TECHNICAL PAPER SERIES
2001-01-2501
Comparative Study of Hybrid Powertrain Strategies Jim Walters, Harry Husted and Kaushik Rajashekara Delphi Automotive Systems
Reprinted From: New Energy Systems and Environmental Environmental Impact Impact (SP–1637)
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2001-01-2501
Comparative Study of Hybrid Powertrain Strategies Jim Walters, Harry Husted and Kaushik Rajashekara Delphi Automotive Systems Copyright © 2001 Society of Automotive Engineers, Inc.
ABSTRACT Hybrid electric vehicles have the potential to reduce air pollution and improve fuel economy without sacrificing the present conveniences of long range and available infrastructure that conventional vehicles offer. Hybrid vehicles are generally classified as series or parallel hybrids. A series hybrid hybrid vehicle is essentially essentially an electric electric vehicle with an on-board source of power for charging the batteries. batteries. In a parallel hybrid vehicle, vehicle, the engine and and the electric motor can be used to drive the vehicle simultaneously. There are various possible configurations of parallel hybrid vehicles depending on the role of the electric motor/generator and the engine. In this paper, a comparative study of the drivetrains of five different hybrid vehicles is presented. The underlying design architectures are examined, with analysis as to the tradeoffs and advantages represented in these architectures. Where appropriate, comparisons to five popular non-hybrid vehicles are made to give the reader a sense of perspective. The primary drivetrain components of engine, transmission, electric machine, inverter, and battery pack are evaluated and changes to the user interface paradigm paradigm are highlighted. Based on the comparative study, hybrid drivetrain technology strategies for future vehicles are proposed.
INTRODUCTION Presently there are two production hybrid electric vehicles (HEV) on sale in the United States: the Toyota Prius and the Honda Insight. Both of these vehicles have unique design approaches that were incorporated in order to attain lower emissions and increased fuel economy. In addition, DaimlerChrysler, Ford, and General Motors have developed Technology Demonstration Vehicles (TDV) to satisfy the requirements of the Partnership for a New Generation of Vehicles (PNGV) [1]. [1]. The primary primary PNGV requirement was to develop a family sedan with a fuel economy of 80 mpg. The three concept vehicles that were developed for the project, ESX3, P2000 and Precept, each differ in terms of the hybrid architecture selected as well as the underlying electric drive technology.
These five vehicles represent hundreds of man-years of engineering and design effort by five of the world’s major automakers. In addition, they are in most cases the product of a sequence of hybrid vehicle learning experiences and the attendant prior effort. By our analysis, we hope to gain a view into the future of successful hybrid electric electric drivetrain strategies. Keeping in mind that these vehicles represent the subcompact/compact class of passenger cars, the conclusions drawn will be most relevant to this class of vehicle, versus, for example, large sport utility vehicles.
VEHICLE COMPARISON A comparison of the five hybrid vehicles selected for this paper must first start with the acknowledgement that these vehicles were developed with different goals in mind. Therefore, we should expect the resulting products to lie in somewhat different different directions. Both the Prius and the Insight were designed to be produced in volume, marketed, and sold to the public in Japan and the United States. States. In contrast, contrast, the PNGV vehicles were designed to meet a set of pre-established targets that included an especially aggressive fuel economy target. The build volume for each was a small handful of vehicles at most. most. However, if we keep these two paths in mind, we can hold open the possibility of charting a future path by balancing what is commercially available in today’s market against what is possible with HEV technology from the PNGV vehicles. To begin our study, a comparison to five non-HEV passenger cars is presented. This information is included to give the reader a sense of perspective by showing passenger cars currently sold in high volume in North America. The five conventional (non-HEV) vehicles are the 2001 Chevrolet Impala, DaimlerChrysler Concorde, Ford Taurus, Honda Accord, and Toyota Camry. The first three cars are the progeny of the three original 1994 PNGV reference vehicles. The latter two cars represent popular, high-volume cars in the U.S., and also represent non-HEV offerings by the two manufacturers currently offering HEV vehicles for sale in the U.S Figure 1 shows a vehicle size and mass comparison of the five non-HEV vehicles to the HEV vehicles in this study. Vehicle length is used as an approximation of
Vehicle Length vs. Curb Weight
Engine 3-cyl. CIDI
5.50 5.30 ) 5.10 m ( h 4.90 t g 4.70 n e L 4.50 e 4.30 l c i h 4.10 e V 3.90 3.70 3.50
Electric Machine 15kW IPM
5 speed Elec. Shifted M/T
Integral Differential
Front Wheels
(Dual Clutch) Inverter Electronics
Battery Pack
a. DaimlerChrysler ESX3
800
1000
1200
1400
1600
Engine 4-cyl. CIDI
Electric Machine 8kW AC Ind.
5 speed Elec. Shifted M/T
Integral Differential
Front Wheels
Curb Weight (kg) Non-HEV
HEV
Linear Fit (Non-HEV)
Inverter Electronics
Battery Pack
Figure 1. Vehicle Length vs. Curb Weight
b. Ford P2000 vehicle size on this graph. All the HEV's HEV's are lower lower in mass than the non-HEV cars. The larger of the HEV's are comparable to the smaller of the conventional passenger cars. cars. A line was fitted fitted through the non-HEV vehicles and then extrapolated down to the minimum curb weight. It is notable that the 840-kilogram Honda Insight rests nearly on this extrapolated line, indicating it has the same length-to-mass ratio as the 2001 non-HEV vehicles. This would indicate that that it is not a lighter vehicle per se, just just smaller. smaller. The two HEV vehicles above the line are lighter per unit length than conventional vehicles, indicating that mass has been reduced while maintaining vehicle size.
Electric Machine 25kW IPM Inverter Electronics
Engine 3-cyl. CIDI
The DaimlerChrysler ESX3, Ford P2000, and Honda Insight represent represent the mild hybrids. These vehicles have a very similar engine and electric machine architecture, but differ in the area of transmissions. One difference in the Honda Insight architecture is the use of a conventional 12V cranking motor. motor. On the surface, surface, this hardware appears curiously redundant with the 10kW crankshaft-mounted machine. However, this choice allows battery pack size to be reduced by avoiding coldcranking issues. The medium hybrids are the GM Precept and Toyota Prius. These vehicles vehicles each use considerably considerably different different drivetrain arrangements. The GM Precept is unique in offering four-wheel drive through the use of a coaxial electric machine driving the front wheels in conjunction with an engine/IMG engine/IMG combination in the rear. Also unique is the fact that the heat engine is rear-mounted in the
Electric Machine 10kW IPM
HVAC Compressor
Front Wheels
Battery Pack
Inverter Electronics
DRIVETRAIN COMPARISON ARCHITECTURE - To begin the comparison of the hybrid drivetrains, we will examine the drivetrain architectures. The architectures architectures for the drivetrains drivetrains are shown in figure 2. As can be seen, seen, the vehicles fall into two categories: mild hybrids with electric machines mounted in the engine-transmission interface zone and medium hybrids with two electric machines and a higher level of electric propulsion capability.
Integral Reducer & Differential
4 speed Elec. Shifted M/T
Integral Differential
Rear Wheels
(No Reverse)
c. GM Precept Engine 3-cyl. Gasoline 12V Starter
Electric Machine 10kW IPM
CVT or 5 speed M/T
Inverter Electronics
Integral Differential
Front Wheels
Battery Pack
d. Honda Insight Inverter Inverter
Generator 10kW IPM Engine 3-cyl. CIDI
Planetary Gearset
Reduction Gearing
Electric Machine 33kW IPM
Front Wheels
e. Toyota Prius Figure 2. Drivetrain Architectures
Battery Pack
because more energy is being recaptured in the city cycle.
Peak Power-to-Weight Ratio 10.00 9.00
HEV
8.00 ) g k 0 0 1 / W k ( o i t a R
Avg.=8.1
Mild
Medium
7.00 Avg.=6.74
6.00
Non-HEV
5.00 4.00 3.00 2.00 1.00 0.00 800
900
1000
1100
1200
1300
1400
1500
1600
Vehicle Weight (kg) Peak Engine + Electric Ratio Engine-Only Ratio Non-HEV Engine-Only
Figure 3. Peak Power-to-Weight Power-to-Weight Ratio vehicle. This unconventional unconventional mounting choice allows for reduced exhaust system length and for significant gains in vehicle aerodynamics by allowing a full belly pan and rear-quarter cooling openings openings [2]. [2]. Because the front electric machine is adequately sized, it can provide the propulsion for the vehicle in the reverse direction, allowing elimination of reverse gear in the transmission. Finally, a unique drive scheme for the HVAC compressor on the rear electric machine allows heating and cooling when the engine is off and direct use of regenerative braking energy. The Toyota Prius architecture includes an internal combustion engine and two electric machines arranged to allow both both parallel and series hybrid operation. operation. This architecture uses a planetary gearset to allow “power splitting” of the engine power between the electric generator and the power going going to the wheels. The planetary gearset and reduction gearing take the place of a “conventional” multi-geared transmission and implement a continuously variable transmission (CVT) function. The generator can draw power from the engine to charge the battery pack or power the main propulsion 1 electric machine in a series hybrid configuration . The propulsion electric machine can drive the wheels simultaneously with the engine, thus implementing a parallel hybrid function [3]. The medium hybrids have the advantage of being able to more aggressively recapture vehicle kinetic energy because the electric machine, inverter, and battery are sized for higher power levels. More kinetic energy recaptured equates equates to increased driving efficiency. The penalty incurred for this benefit is increased system complexity, cost, cost, and mass. Such efficiency gains are mainly visible in city driving schedules and are less appreciable in highway cycles. For example, example, the 2001 Prius is rated at 52 mpg city, 45 mpg highway in the US [4]. A significantly lower highway rating is very very unusual in non-HEV vehicles, but is understandable in the Prius 1
In this series configuration the power travels through two inverters, incurring an extra efficiency loss.
DRIVETRAIN SIZING – One metric that can be used to evaluate the size of the powertrain for a vehicle is the power-to-weight ratio. For the purposes of comparison, the peak power of the engine and the electric machines that provide propulsion were added together to calculate a peak power for the the hybrid vehicles. Figure 3 shows the 2 peak power-to-weight ratios for the five hybrid vehicles 3 and the five non-HEV vehicles . For the HEV vehicles in this paper, the peak engine plus electric ratios were very consistent across all vehicles in a broad weight spectrum, with an average of 6.74 kW per 100 kg. of curb weight. This would seem to indicate indicate a similar set of performance goals that the vehicles were attempting to meet in the area of acceleration and hill climbing ability. As can be seen in figure 3, three of the five conventional cars have a very similar power-to-weight ratio as the hybrid vehicles. Two higher-output higher-output engines skew the non-HEV average to 8.1 kW per 100 kg. Overall, this is an indication that the hybrid powertrains are adequately sized to provide performance similar to, but slightly lower than, popular non-HEV cars in the North American market. By examining the engine-only power-to-weight ratios in figure 3, we can see a clear distinction between the three mild hybrids (the three leftmost sets of points) and the two medium hybrids (the two rightmost sets of points). The medium hybrids have significantly lower engine-only ratios, and use a larger electric motor to achieve the desirable power-to-weight power-to-weight ratio. ratio. This strategy allows the use of a relatively smaller internal combustion engine on these vehicles which can aid in higher fuel economy. The electric machine is used to supplement the engine’s power for more rapid accelerations and for grade climbing.
HYBRID SUBSYSTEMS As can be seen in the architecture diagrams in figure 2, all the vehicles have a common set of drivetrain subsystems. In this paper we will examine and compare the following major drivetrain subsystems: heat engine, transmission, electric machine(s), inverter, and battery pack. HEAT ENGINE – The combination of significantly reduced mass and electric hybridization allows the use of smaller engines in the HEV’s than is normal for this vehicle size. Table 1 shows a comparison comparison of the engines employed in the the five vehicles. vehicles. As can be seen in the the table, the engines all have 3 or 4 cylinders and range in displacement from 1.0 to 1.5 liters. The two production production vehicles use gasoline spark-ignited (SI) engines, while
2
A scaling of kilowatts per 100 kilograms of curb weight was used, as it provided conveniently scaled results. 3 Base models with base engines were used for all makes.
60
140
50
120
Turbocharged CIDI ) L / W40 k ( r e w30 o P c i f i c 20 e p S
) L / 100 m N ( e u 80 q r o T 60 c i f i c e p 40 S
C Engine capability C is higher than used in this application
10
20
0
0 0
0.5
1
1.5 2 2.5 3 Engine Displacement (Liters) Non-HEV
3.5
4
0
HEV
the three PNGV vehicles use compression-ignition direct-injection (CIDI) engines.
•
4
Audible noise reduction through the use of pilot injection Precise injection timing and quantity Reductant provision for the catalyst through the use of post injection
All three CIDI engines also employ turbocharging to significantly improve low-end torque and volumetric efficiency. While CIDI engines have a low application rate in North America, they are increasingly popular in the European car market. The two production hybrids, the Insight and Prius, use gasoline SI engines. The choice of a gasoline engine is in line with the US car market as a whole, which has a very low penetration penetration rate for for diesel engines in cars. (In 1999, only 13,600 diesel-engine cars were sold in the US, out of a total of 8.7 8.7 million cars [8].) [8].) Both engines also employ 4 valves per cylinder and variable valve timing schemes, with the Prius using Toyota’s VVT-i and the Insight using Honda’s VTEC-E. VVT-i offers continuously variable valve timing adjustment while VTEC-E offers two lobe profiles allowing switching between two distinct sets sets of timing and lift. lift. Both of these 4
Commonly known as “diesel” engines.
1
1.5 2 2.5 3 Engine Displacement (Liters)
3 .5
4
HEV
Figure 5: Specific Torque vs. Displacement
The three PNGV vehicles use compression ignition (CIDI) engines because a study of fuel economy projections showed a CIDI parallel hybrid as the only drivetrain with the potential to offer 80 mpg, an important PNGV goal [5]. These engines offer inherently inherently higher higher cycle efficiency efficiency compared compared to gasoline SI engines. The P2000 engine boasts a peak efficiency of 43% [6] and the Precept engine achieves greater than 40% efficiency at high loads [7]. [7]. The use of electronically electronically controlled controlled common rail fuel injection gives these engines a number of benefits:
•
0 .5
Non-HEV
Figure 4: Specific Power vs. Displacement
•
Normally Aspirated Gasoline SI
schemes are in production on non-HEV cars, but are beneficial to the HEV's because they offer the ability to design for improved fuel economy in normal rpm ranges and higher power at higher rpm. Figure 4 shows the specific power for the five HEV engines and the five non-HEV engines from the 2001 cars mentioned mentioned earlier. It is quickly apparent apparent that the HEV engines are much lower in displacement, with the closest non-HEV engines being the base 4-cylinder engines on the Accord and Camry. In terms of maximum power per liter they are roughly on par with their non-HEV counterparts. This might be expected for the gasoline engines since they use very similar engine technologies (4 valves per cylinder, variable valve timing, etc.). This is also laudable since the the primary goals for the HEV's were fuel economy and and emissions. Power is still very important - the vehicles need a certain power-toweight ratio for acceptable acceleration and gradeability. Specific torque of the engines is shown in figure 5. The higher torque of the turbocharged diesels is clearly apparent. They are over 30% higher than the highest highest SI engine in specific torque. torque. This is explained by the natural benefits of turbocharging as well as the higher cylinder pressures associated with CI engines. The gasoline SI engines offered in the hybrids are comparable in specific torque to their non-HEV counterparts. TRANSMISSION – The transmission choices for the hybrid vehicles fall i nto two groups: continuously-variable transmissions (CVT) and automatically-shifted manual (ASM) transmissions. transmissions. Both production production HEV's come come with 5 a CVT , which provides a smooth application of torque as the vehicle accelerates and allows the engine to be held at desirable operating speeds relatively independent 5
The Insight entered the U.S. market with a manual transmission and will be offered with a CVT in 2001. In Japan, both transmissions are available.
Engine Type # of Cylinders Displacement (Liters) Peak Power (kW) Peak Torque (Nm) Compression Ratio Engine Details
ESX3 CIDI 3 1.5 55 kW
P 2 0 00 CIDI 4 1 .2 55 k W 153 Nm
Turbocharger with intercooler, common rail fuel injection, all-aluminum;
4V/cyl., Turbocharger with variable geometry, common rail fuel injection 110kg.
Insight Gas 3 1.0 50 k W 90 N m 10.8:1 SOHC, 4V/cyl., VTEC-E variable valve lift & timing
Engine Mass
113 kg.
Transmission
6 speed Automaticallyshifted manual with 2-clutch design Parallel IMG PM (15 kW) N/A
5 speed Automaticallyshifted manual
Parallel IMG IM (8 kW) N/A
Parallel IMG PM (10 kW ) N/A
Resolver
90 tooth quadrature
3 hall sensor and target wheel
2 80 V Regen Braking Starter/Alt. Motor Boost Firing pulse Smoothing N/A
IGBT 14 4 V Regen. Braking Starter/Alt.(warm) Motor Boost Firing pulse Smoothing N/A
Configuration Machine #1 Type Machine #2 Type Sensor Switch Technology Bus Voltage Vdc Machine #1 Functionality
Machine #2 Functionality
IGBT 15 0 V Regen. Braking Starter/Alt. Motor Boost Firing pulse Smoothing N/A
80 kg. (including elec. machine) CVT or 5 speed M/T
Precept CIDI 3 1.3 40 k W 170 Nm 17.6:1 DOHC, 4V/cyl., Turbocharger with variable geometry & intercooling, common rail fuel injection
Prius Gas 4 1.5 53 kW 11 2 N m 13:1 DOHC, 4V/cyl., VVT-i Variable valve timing
4 speed Automatically-shifted manual, (no reverse)
E-CVT
Series/Parallel PM (25 kW ) PM (10 kW )
Series/Parallel PM (33kW) PM
2 Resolvers IGBT 35 0 V Vehicle Launch Regen. Braking Reverse Acceleration Boost Starter/Alt. Regen. Braking Motor Boost AC Compressor Drive Synchronize gear shifting
IGBT 288 V Vehicle Launch Regen. Braking
Alternator Motor Boost Starter
Table 1: Summary of Hybrid Drivetrain Technology by Vehicle of vehicle speed to achieve economy or performance. The three PNGV vehicles all use ASM transmissions. This type of transmission offers the automatic control of shift selection that is desirable for fuel economy and performance, while providing the near-perfect transmission efficiency of a manual transmission. The DaimlerChrysler ASM is unique in offering a dual clutch arrangement with alternating gears on two shafts (even/odd). This scheme allows for continuous application of torque, with no interruptions during shifting [9]. Conventional single-clutch ASM transmission experience a torque interruption during shifting, which can dissatisfy the driver [10]. The GM Precept has a novel transmission feature that allows the air conditioning compressor to be driven directly off the input shaft of the transaxle [11]. This allows the compressor to be powered by the engine or the electric machine, which means the electric machine can continue to run the HVAC compressor even after the engine shuts off. This scheme also allows regenerative regenerative
braking energy to go directly to the compressor, avoiding the electrical round-trip losses through the battery. ELECTRIC MACHINE – The mild hybrid vehicles use an integrated motor generator (IMG) system while the higher power applications use a dual-machine approach. An IMG system has an electric machine attached to the end of the transmission such that the rotor is linked to the crankshaft. Due to driveline volume constraints, the challenging thermal environment and the engine’s dynamics, this configuration requires careful attention to design considerations. In the dual-machine strategy, a motor either driving a separate axle or integrated through a planetary gear system to the engine is used. These systems have an additional machine that acts primarily as a motor/generator and is interfaced to the engine through a gear assembly. Table 1 summarizes the hybrid systems for the vehicles evaluated.
Of the vehicles that use an IMG system, the permanent magnet (PM) machine has been more commonly selected. This choice can be understood from an efficiency and energy density perspective. perspective. Efficiency is important to the overall vehicle but can also impact the motor design since less heat will be dissipated in a more efficient machine. In addition, the PM machine’s high energy density helps to meet the demanding volume constraints of the the IMG IMG systems. systems. Nevertheless, care must be taken to make sure the machine does not become too small for a given power level since aircooling may become inadequate. The ESX3 uses a conventional 12 pole interior PM machine. This motor design utilizes both magnet and reluctance torque to to increase the torque/current torque/current ratio. In order to further reduce phase currents over the operating range, field-weakened field-weakened operation is used [12]. This mode of operation reduces the inverter currents for low and moderate speeds but requires more attention to the machine controller’s performance under high-speed conditions since loss of field-weakening can cause an over-voltage condition on the power bus. The Insight machine is a 12-pole surface mounted magnet design featuring featuring a bobbin-wound stator. This unique construction helps to simplify construction and reduce manufacturing costs costs [13]. The peak torque of the the Insight machine is inadequate for cold temperature engine cranking at at –29C [14]. During these conditions conditions a conventional starter is used. This design choice was likely made to help mitigate the challenge of designing a machine with a high-torque/low-speed region and a wide constant power range. Ford’s P2000 utilizes an induction induction machine (IM). (IM). A PM machine was not selected due to high-temperature demagnetization concerns [15]. IMG systems are located close to the heat engine as well as the transmission, which can cause high operating temperatures under heavy loads. The P2000 machine has a rather large torque capability of 300 Nm for cranking. Due to this high cranking torque, there is likely a rather large field weakened range of operation for this vehicle at higher higher speeds. Ford had previously previously indicated the possibility of using a winding change from series to parallel to change the torque speed characteristic of the machine [16]. This idea does not appear to to have been pursued further. For the dual-machine systems, permanent magnet machines were the universal selection for both the main propulsion machine and the motor/generator machine. The Precept utilizes a 25 kW interior permanent magnet (IPM) machine for the front axle and a 10 kW surface mount PM machine with the the rear engine [17]. The Prius also utilizes two PM machines. The two PM machines are linked to the engine by a planetary planetary gear system. system. The original Prius motor was an 8 Pole 30 kW interior design with a torque capability capability of 300 Nm. Nm. The new Prius has a torque capability of 350 Nm [18]. Since the main propulsion motors are not directly tied into the driveline
there is more axial space available which allows a lower pole number machine machine and higher torque capability. The Prius system has an advantage in that both machines and their their inverters are located close together. This allows for simplicity in the cooling system as well as the wiring system. For both the Precept and the Prius, liquid cooling was used for the propulsion machine and the motor/generator. motor/generator. Liquid cooling cooling helps to increase increase the the continuous power rating and the length of time that peak power can can be provided. However, the use of liquid cooling does add cost, mass, and complexity. A PM machine may appear to have less-challenging cooling needs due to its higher efficiency. efficiency. But, due to the higher energy density, cooling could become complex because the heat generation is more concentrated. concentrated. In summary, summary, the need for a liquid-cooling system is a clear disadvantage relative to the air-cooled IMG systems. With PM machines, there are some safety/protection concerns that that need to be addressed. If a large fieldweakening range is used, the electronics need to be protected in case case of a controller controller fault. This is because the induced electromotive force (EMF) in the machine exceeds the Power Bus voltage throughout the fieldweakened region. The protection can be provided by ensuring that the short circuit current causes enough voltage drop in the machine to protect the electronics or by over-designing the inverter using higher-voltage capacitors and switches. There is also a concern of the amount of torque that will be produced during a machine or inverter fault. fault. This torque is generally quite small small due to the fact that it is out of phase with the motor currents. At low speeds, the in-phase component will increase but the current level is much lower. Hence, the loss of fieldweakening control issue as generally perceived is not a major concern in the PM machine. INVERTER - The IMG systems with a PM machine use a voltage near 150 V while the higher power systems use a higher voltage in the 288-360 288-360 V range. Ford’s inductionbased IMG vehicle uses a 280 V system, which may have been required due to the lower energy density of the IM and/or and/or a desire to limit the phase currents. The higher voltage reduces the phase current level required, but forces an IGBT switch technology selection, rather than MOSFETs. The higher voltage also requires greater attention to vehicle safety. All the inverters, with the exception of the Insight, appear to be liquid cooled with a single integrated power switch module. Considerable focus has been given to developing integrated electronic switch packages to reduce cost, mass mass and volume. The Insight is unique unique in its forced air cooling approach, which saves the mass and volume of a coolant pump, hoses, heat exchanger, and coolant. There is also a significant distinction between the IMG and dual-machine systems. systems. The Prius and Precept Precept each require two inverter power stages, but with different
ratings. Either two different different inverter designs are are used, as is the case in the Prius, or one of the inverters is significantly over-rated, as is the case in the Precept [19]. The two inverters may lead to an increase in the cost of the total system. Machine Controller- Within the inverter, the machine controller is the intelligence that monitors sensors and performs sophisticated closed-loop control of the electric machines. The controller’s controller’s sophistication varies based on the application. For dual-machine vehicles, the dynamic requirement of the main propulsion machine is generally small. Thus, the need for high-performance control algorithms, such as vector control strategies, may not be required. By contrast, two machine controllers as well as two sensor strategies are used due to the separate propulsion and motor/generator functions of the machines. In regard to the reduced controller requirements, the new 6 Prius design incorporates the ability in the fieldweakened range to move to 6-step operation with phase angle control. control. The new Prius also also appears appears to use trapezoidal operation for cranking and then switches to sinusoidal operation at higher speeds. This may explain explain why more expensive resolvers are used for position sensing, as opposed to hall-effect sensors. Like the Prius, the Precept’s front motor fits the slow dynamic characteristic due to its connection to the wheels. The rear motor generally has slow dynamics but it is also used for gear synchronization during shifting which can require some additional control capability. The IMG vehicles have challenges that require greater control capability. Operating modes to suppress driveline oscillations or to cancel engine firing pulse vibrations require greater dynamic performance. Driveline accelerations due to harsh shifts are also more severe than wheel wheel accelerations. Ford and Chrysler Chrysler both appear to have chosen vector control strategies for their vehicles. The sensors for the controllers were were likewise selected to maintain maintain high dynamic performance. performance. The ESX3 was initially designed to use a reluctance-based resolver [20]. The P2000 uses a quadrature 90-tooth wheel which, because of resolution limitations, can compromise cranking performance and cause torque ripple across the speed range unless other compensation techniques are used. The Honda Insight's motor control strategy is the most interesting of the three IMG vehicles as it uses three separate hall-effect position sensors as would typically be used for trapezoidal operation. operation. Trapezoidal operation operation during engine starting allows for higher torque capability. Depending on the presence of a field-weakened range this scheme can provide adequate performance with the possible exception of the harmonic cancellation mode. The three sensors could also be used in an incremental 6
The original Prius was introduced for sale in Japan in 1997. A significantly improved improved version was introduced to the US market in 2000. See [3] for more more information.
mode with additional calculations to provide a better angle estimate to enhance performance to allow vector control or phase angle control. control. The sensor selection is also unique due to the fact that three separate sensors are used rather than three elements in one package. This creates cost and sensor location variability issues. 800
Lithium 25oC 700 600
NiMH 25 oC 500
PbA 25 oC
Lithiu Lithium m -29oC
400 300 200
NiMH -29 oC
100
o
PbA -29 C
0 30
40
50
60 70 State of Charge (SOC)
80
90
100
Figure 6: Battery Technology Comparison BATTERY PACK - The vehicles considered in this study chose either a nickel metal hydride (NiMH) or lithium ion technology. These two technologies technologies are preferred preferred to the traditional lead acid technology and to nickel cadmium technology for reasons of energy density, power density, and power output at low state of charge. charge. Figure 6 shows the specific power versus state of charge characteristics for three technologies at two different temperatures. Clearly lithium ion and NiMH have much higher specific power than lead acid (PbA) technology, meaning that they will result in a much lighter battery pack for a given set of specifications. The higher specific power power also means that NiMH and lithium ion are better able to accept the high peak power levels associated with regenerative braking. This is often a factor that drives a lead acid battery pack to increased size and weight. At the present time, NiMH is four to five times more expensive than lead acid. acid. Lithium ion, while being expensive at present, is viewed as having the potential to become cost competitive in high volumes over time. Lithium ion does require more control monitoring than NiMH due to undesirable overcharge and overtemperature characteristics. NiMH is more desirable from this standpoint due to its inherent internal charge balancing, although there are still thermal runaway characteristics that require cooling systems. NiMH also demonstrates severe output power degradation at low temperatures as shown in figure 6 with the –29C curve. Battery pack design is extremely important and requires careful system analysis in order to determine the proper strategy. In sizing battery packs for hybrid vehicles, the required peak power power becomes a dominant concern. The ability to handle regenerative braking as well as peak motoring demands sets the power rating that the battery pack must meet. The power requirement is further further defined by expected cold temperature performance.
This is in contrast to electric vehicles where the energy capability drives the battery pack design due to the desire to maximize maximize range. Voltage is also important to consider since for a fixed power requirement, a higher voltage system will lower the current requirements which can reduce losses, lower required battery volume and save cost in connectors and wires. The IMG systems have used distinctly different strategies to solve solve the battery pack question. The Insight system is a 144 V NiMH system designed to handle a peak machine load of 10 kW [21]. The P2000 system is a 288 V NiMH 22 kW system designed to handle a peak load of 8 kW [22]. The large disparity in battery pack ratings is interesting since both systems have similar functionality. The lower power power rating rating of the the Insight Insight system may be due to the presence of a separate 12V starter motor for cranking cranking requirements. requirements. Since NiMH battery performance is poor at low temperature, a NiMH pack would have to be made significantly larger to meet the cold-cranking requirements of the engine. By using a 12 V lead acid battery and cranking motor, this cost is avoided. In terms of the dual-machine vehicles, significantly more power and energy energy are required. The Prius uses a 288 V 21 kW NiMH system. The need for higher power is due to the different different requirements requirements of these systems. Vehicle launch requires substantially more torque than engine cranking. Both also need extra capacity for assisting the the engine during extended hill climbing. The Prius battery system was modified for the year 2000 US introduction, which has substantially reduced the volume, complexity and weight of the pack [23]. [23]. The Precept was designed for a 42 kW power rating and a 3 kWh energy rating. The battery pack was selected based on the need for repeated accelerations and extended gradeability, which drive a relatively high energy requirement. Two options were considered for the vehicle. For the near future, a NiMH system is proposed while lithium ion technology is considered for the future [24]. Both systems are liquidcooled and require significant significant vehicle space. The size, cost and weight of a 42 kW battery pack provides significant challenges in terms of cost, mass, and volume. With NiMH technologies, technologies, the challenge challenge of cold performance is clearly seen in terms of having an overdesigned battery pack or of having a separate starting system. Lithium ion packs packs offer the opportunity to reduce battery size by alleviating the cold power performance issue at the expense of more careful battery management.
USER INTERFACE TO THE HEV DRIVETRAIN Interface to the driver is an additional area that must be considered as the vehicle changes from a conventional drivetrain to a hybrid hybrid engine/electric drivetrain. The existing paradigm for monitoring is a gauge set typically showing vehicle speed, engine rpm, oil pressure, engine temperature, and fuel tank level. Many vehicles also offer a "trip computer" that shows the driver additional information such as fuel remaining, range, and other
statistics. The existing paradigm for for control is a foot pedal to command power and vehicle speed and a shifter to select transmission mode. The hybrid architecture adds several new components (electric machine, power inverter, battery) which in turn offer several possible needs for monitoring. The Honda Insight offers additional displays to the driver to show battery state of charge (SOC) and electric machine charge/assist magnitude, which essentially shows the operating state of the electric machine and inverter. For the control interface, all the hybrid vehicles maintain the familiar pedal and shifter arrangement. arrangement. Behind the scenes, a new controller performs the function of a hybrid system supervisor by monitoring the accelerator 7 pedal and then parceling out torque commands to the engine, transmission, and inverter based on the pedal input and the optimum balance of key factors. factors. This new control architecture is needed to optimize system performance while maintaining an interface to the user that is consistent with existing non-HEV vehicles. For the interface to the ASM transmission, the DaimlerChrysler ESX3 uses an automatic transmission PRNDL-like shifter interface, thus helping the user understand this new transmission as similar to today's automatic transmission function.
FUTURE HYBRID DRIVETRAIN STRATEGIES Most existing passenger cars are already a showcase of packaging efficiency and cost effectiveness in a very competitive global sector. sector. The challenge in engineering a hybrid vehicle is that components and systems are only being added to this showcase – none are being removed (although some can be reduced in size, mass, and output). By definition, the the HEV design team must at a minimum add an electric machine, a power inverter, and 8 a battery pack . In doing so, so, they add the cost, mass, mass, and volume of these components. Future hybrid drivetrain strategies will be driven not only by the availability of reliable technologies, but also by the 9 goals and incentives of the target markets . For example, producing and selling hybrid vehicles in the state of California will be a requirement for the seven major automakers to continue selling conventional passenger cars there. there. In other markets, markets, the combination of higher fuel prices and government incentives can make the higher fuel economy of a hybrid vehicle costeffective to to the end consumer. consumer. The hybrid drivetrains must also be designed to meet the performance requirements of the market, which may mean several drivetrain solutions are viable, each for a particular 7
Note that in conventional vehicles the accelerator pedal input goes to the engine or powertrain controller. 8 Or some other suitable energy storage device such as an ultra-capacitor or an inertial storage device. 9 For example, Toyota Prius sales have been aided by a government incentive in the form of a 250,000 yen payback and tax acquisition reduction [3].
market. For example, the 1997 version of the Toyota Prius met market requirements requirements in Japan. Japan. However, the 2000 version designed for the US market was modified for improved acceleration, improved passing performance, extended hill climbing, and improved electric-only operating region through increased power output, improved inverter cooling and technology, and an improved battery pack [25]. ENGINE - The future engine strategy will provide a very efficient, low-mass, high-output engine. Presently, turbocharged CIDI engines with common rail fuel injection look attractive as long as they are not excluded by emissions regulations regulations in the target target market. Presently, challenges still exist in reducing oxides of nitrogen (NOx) and particulate particulate matter [26]. A gasoline direct injected engine, perhaps turbocharged, may also be competitive by allowing leaner operation and higher performance without the challenges of NOx and particulate matter removal. TRANSMISSION – The future transmission strategy will be one that gives very high transmission efficiency, has customer-pleasing characteristics, and allows a high degree of computer control to allow optimization of the engine and electric machine operation. An automatically-shifted manual fits this description, and the DaimlerChrysler dual-clutch scheme appears to solve the torque interruption that is normally associated with shifting. High-efficiency vehicles such as the Volkswagen Lupo already use an ASM transmission in production [27]. ELECTRIC MACHINE – The future machine will have to be a cost effective and have high efficiency. efficiency. Presently, the permanent magnet machine and the induction machine are both commonly considered for hybrid applications. As magnet costs decrease in the future, future, a shift towards PM machines is likely. The high efficiency and power density of the machine make it a better candidate. INVERTER – The strategy for future power inverters will have to move towards the ideal inverter with minimized losses and a small package size. Unless new device technologies emerge, high voltage applications will use IGBT technology due to the relatively high switching frequency capability and low losses. Low voltage systems will consider MOSFETs for the low resistance characteristics and lower cost. The future machine controller will control the machine to operate as efficiently as possible and ensure that the desired response characteristics of the drive can be met. met. For many many applications a vector control based strategy is appropriate due to the need for relatively fast response characteristics. As the Toyota Prius has demonstrated, demonstrated, there are other systems that do not require that level of response where a possible combination of several control strategies can be employed. BATTERY PACK – The battery pack for future vehicles would ideally have a high power-to-weight ratio, be
inexpensive, thermally robust and have a high coulombic efficiency to allow maximum reuse of recaptured kinetic energy. Lithium ion technology technology has most of these characteristics with the exception of the battery management. This however however is being worked worked on by industry and assuming no material regulation issues arise it promises to be an attractive technology.
CONCLUSION The future of hybrid electric vehicles is dependent on market economics and incentives. A future HEV for the the US market may employ a CIDI engine in the rear with a permanent-magnet electric machine and a small, but powerful lithium-ion battery battery pack. In addition to its hybrid hybrid drivetrain, this five-passenger vehicle may have a lightweight plastic-molded body with superior aerodynamics and a combination of creative approaches to achieving efficient heating and cooling of the passengers.
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CONTACT Jim Walters, Senior Project Engineer, Delphi Energy and Chassis. Email:
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