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`Toyota's Newly Developed
`Electric-Gasoline Engine Hybrid Powertrain System
`
`S. Sasaki
`T. Takaoka
`H. Matsui
`T. Kotani
`Toyota Motor Corporation
`1,Toyota-cho, Toyota, Aichi 471-71 Japan
`E-mail : sasaki@mickey.ve.toyota.co.jp
`
`Abstract
`Toyota has developed a hybrid powertrain system based on a new concept. This system includes a
`drivetrain which features a planetary gear mechanism for dividing the drive force and two
`motor/generators. Computer control is used to optimize engine fuel consumption, minimize exhaust
`emissions, stop and start the engine and apply regenerative braking. The system also uses newly
`developed, high-output Ni-MH batteries.
`Test vehicles fitted with this new hybrid system realized approximately twice the fuel economy of
`the conventional model and exhibited a high level of potential in terms of minimizing exhaust
`emissions.
`
`1. Background
`Man-made CO2 is thought to be one of the contributors to global warming. To minimize the
`automobile’s impact, industry continues to develop powertrains that consume less fuel and thus create
`less CO2. This has the added benefit of lessening the consumption of non-renewable fossil fuels. This
`trend is evidenced by examples such as the PNGV program in the United States and the 3-liter per 100
`km car project in Europe. At Toyota, research for creating a powertrain system that consumes less fuel
`suggested the possibility of adopting a hybrid system consisting of an electric motor and a gasoline
`engine.
`
`Previous hybrid systems that have been considered were generally classified as either series or parallel
`type. Our powertrain is based on the parallel type. However, to optimize the engine's operation point
`(for minimum fuel consumption), it includes a system that enables a series-like operation through the use
`of a generator and a motor that are independent of each other.
`
`2. System configuration
`Figure 1 shows the configuration of the Toyota Hybrid System. Following are sections describing the
`basic elements of the system.
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`GENERATOR
`
`INVERTER
`
`BATTERY
`
`ENGINE
`
`POWER SPLIT
` MECHANISM
`
`MOTOR
`
`AXLE SHAFT
`
`MECHANICAL
`POWER
`
`ELECTRICAL
`POWER
`
`Fig1 Toyota Hybrid System Configuration.
`
`2-1. Hybrid transmission
`The transmission consists of a single planetary gear set to divide the engine's output plus a generator
`and a motor. The planetary gear set divides the engine's drive force into two forces: one that is
`transmitted via the ring gear to drive the axle shaft and the other that drives the generator through the
`sun gear. The electrical force, produced in the generator, is re-converted into mechanical force through
`the motor. Since the motor is also connected directly to the ring gear, this force is transmitted to the
`axle shaft. In addition, batteries provide supplemental drive force to the motor as needed. The batteries
`also recover and reuse the braking energy through regenerative control.
`
`2-2. Engine
`As for the engine, we developed a new 1.5-liter gasoline engine that achieves high combustion
`efficiency through the application of the high expansion ratio Atkinson cycle. Because the electric
`motor can assist in meeting peak drive force requirements, we optimized the engine for a high level of
`efficiency rather than peak power output. Specifically, we designed an engine in which friction loss is
`dramatically reduced by lowering its maximum speed. Further, we used the VVT-i (Variable Valve
`Timing-intelligent) system to finely control the intake timing, thus ensuring maximum efficiency.
`
`2-3. Batteries
`The nickel metal hydride battery developed for the electric vehicle (EV) application features the
`superior characteristics of higher power density, lighter weight, and longer life. New batteries were
`developed building on these attributes but optimized for the hybrid application. The new battery has
`more than three times the power of the EV battery per unit volume.
`
`3. System operation
`Figure 2 shows a schematic diagram of the system. The engine is connected to the planet carrier, the
`generator is connected to the sun gear and the motor is connected to the ring gear. Ring gear output is
`connected to the axle via a differential and counter-gear.
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`GENERATOR
`
`MOTOR
`
`ENGINE
`
`PLANETARY
`GEAR
`
`AXLE SHAFT
`
` Fig.2 A schematic representation of the Toyota Hybrid System
`
`System operation is described below using the following symbols:
`Powertrain Component
`Engine
`Generator
`Ring
`Planet Element
`Carrier
`Sun
`Speed
`ne
`ng
`nr
`Torque
`Te
`Tg
`Tr
` : planeta ry gear ratio = number of s un gear t eeth / number of ring gear teet h
`Gr : reducti on gear r atio = nr / na
`
`Motor
`
`Axle
`
`nm
`Tm
`
`Ta
`
`3.1 Colinear relationship
`Relative shaft speeds for the planetary gear set elements are given by equation (1).
` * ng + nr = (1 + r ) * ne
`Generator and ring gear torque at steady state is obtained as follows:
`Tg = ( r
` / (1 + r )) * Te
`Tr = (1/(1 + r )) * Te
`
`(1)
`
`(2)
`(3)
`
`3.2 Relation between Engine output and axle shaft output (CVT function)
`Since the generator output is fed back into the motor, total power available at the axle can be
`expressed as shown in equation (4). (For simplification, the conversion efficiency from the generator to
`the motor is assumed to be 1).
`na * Ta = nr*Trnm*Tm
`nr * Tr nr * ( 1 / (1+))*Te
`nm*Tm Ng * Tg ng * (/ (1+))*Te
`
`(4)
` (5)
`(6)
`
`Consequentl y, usi ng formula ( 1), and formulas ( 4) through (6) re sult i n:
`na * Ta ( 1+) *ne*(1 / (1+))*Te
` ne * Te
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`r
`r
`

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`Thus, it i s evident that the engi ne output has been converted into a xle shaf t output . Moreover, based on formula
`(1), by fi xing ng, the rel ationshi p between na and ne wi ll be:
`Gr * na (1+) * ne * ng
` The refore, between axl e shaft speed na and engine speed ne stands the speed conversion in which the
`parameter is g enerator speed ng.
` As described above, due to t he output saving f unction gi ven in formula (7) and the speed conversion functi on
`given in formula (1)’, it i s evident that this composi tion i s that of a CVT ( Continuous Vari able Tra nsmi ssion).
`
`(1) ’
`
` 3.3 Engine start/stop
` To attai n the target engine spe ed ne^ reg ardless of whether the vehi cle i s at a standst ill or moving, it wil l suff ice
`to control the gener ator spe ed by applyi ng formula (1).
` Hence , render ing ng^ as the t arget generator speed, e xecuting the gener ator spee d control so th at:
` (8)
`ng^ ((1+) * n e^- nm) /
` The n, it will become possibl e to r ender the engine s peed to be ne^, thus ena bling t he engine to be started by
`inj ecting a nd ignit ing the fuel.
`
`4. System Control
`Figure 3 shows the configuration of the hybrid control system.
`
`Accelerator
`
`Shift Position
`
`Brake Pressure
`
`Charge Request
`
`Vehicle_ECU
`
`Power
`Command
`
`/ Vehicle
` Control
`
`/ Engine
` Operating
` Point
`
`/ Motor
`Torque
`
`Torque
`Command
`
`Torque
`Command
`
`/Generator
`Speed & Torque
`
`Pressure
`Command
`
`Vehicle Speed
`
`/ Brake
` Pressure
`
`Engine_ECU
`Engine & Throttle
` Control
`
`
`Motor_ECU
`Motor Control
`
`Generator_ECU
`Generator Control
`
`Brake_ECU
`Brake Fluid
`Pressure Control
`
`Fig.3 Hybrid Control System
`
`Throttle
`Angle
`
`Motor
`Current
`
`Generator
`
`Current
`
`Control Valve
`
`Current
`
`Throttle Actuator
`
`Motor Inverter
`
`Generator
` Inverter
`
`Pressure
`control
`Valve
`
`Vehicle Control (Vehicle ECU)
`Based on accelerator pedal angle, shift position, brake pedal effort and battery condition this ECU
`calculates the engine output, motor torque, and generator speed commands for the respective control
`devices. Furthermore, during braking, this ECU gives a regenerative control command to the motor and
`simultaneously gives a pressure reduction command, equivalent to the regenerative control command, to
`the hydraulic servo valve to provide the total braking force required.
`
`Engine Throttle Control (Engine ECU)
`Based on the engine power command from the Vehicle ECU, this ECU gives the actuator a throttle
`opening angle command.
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`Motor Control (Motor ECU)
`The Motor ECU commands the motor to provide that portion of the total axle torque not met by
`the engine direct path torque. Hence, the sum of the engine direct path and the motor torque meets the
`accelerator pedal demand.
`
`Generator Control (Generator ECU)
`To control the engine speed, the Generator ECU controls the generator torque to achieve the target
`generator speed that was defined by the Vehicle ECU.
`
`Hydraulic Brake Control (Brake ECU)
`The Brake ECU commands the pressure control valve to provide brake energy in excess of
`regenerative potential to meet the total brake pressure requirement calculated by the Vehicle ECU.
`
`This hybrid control system is extremely flexible and offers the following features:
`• The engine can be started and stopped any time while the vehicle is being driven
`• The CVT function enables the engine operating point to be controlled
`• To meet braking demand, the system balance regenerative and hydraulic operation
`
`Accelerator
`Pedal Angle
`
`Axle Shaft
`Torque &
`Driving Power
`
`PeÅ{
`
`Ta Å{
`
`Maximum
`Engine
`Efficient
`Point
`
`Target engine speed
`Å»
`Ne
`
`Generator Torque
`Command
`Å»
`
`Tg
`
`Vehicle
`Speed
`
`Battery
`Charge
`Demand
`
`Brake
`
`Target
`Generator
`speed
`
`Speed
`Control
`
`Pchg
`
`RegenerAtive
`Control /
`Hydraulic
`
`Å{
`
`Å|
`
`Axle Shaft
`Torque
`
`Motor
`Torque
`
` Motor Torque
`
`Command TmÅ»
`
`Hydraulic Brake Control
`
`Generator
`ECU
`
`Generator
`Speed
`
`Motor
`ECU
`
`Hydraulic
`Servo
`Valve
`
`Fig4. Control Logic Block Diagram
`
`Figure 4 is a block diagram of the control logic. Followings are features of the control system from
`the perspective of fuel consumption:
`
` Engine operation and/or electric motor operation
`Because engines are less efficient in the low-load region, it is more effective to stop the engine and
`operate the vehicle on its batteries when the driving power demand is low. Therefore, below a defined
`power demand, the firing of the engine is stopped.
`
`Maximizing engine efficiency
`Since the engine's maximum efficiency is usually obtained at its wide-open-throttle operating point,
`this data is stored and used to control the generator speed and the throttle opening angle.
`
`Regenerative control
`The brake system prioritizes regenerative braking by storing a table of available regenerative force
`and only demanding excess effort to the hydraulic brakes based on brake pedal effort.
`
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`5. Vehicle fuel consumption
`Figure 5 shows the fuel consumption rate during driving of the test vehicle on a chassis dynamometer
`over the Japanese 10-15 mode test. Figure 6 shows efficiency and cumulative operation by band over
`the same cycle.
`Depending on the driving situation, the fuel consumption rate of the hybrid vehicle appears to be
`favorable when the driving force is extracted from its batteries, and unfavorable in the opposite situation.
`Because the fuel consumption rate when the net electrical current is at equilibrium (zero) can be
`considered to be the actual fuel consumption rate, in figure 5, the hybrid vehicle achieved approximately
`twice the 10-15 mode fuel efficiency of the conventional vehicle with an identical amount of cylinder
`displacement (14 km/l). Figure 6 makes it evident that the engine use frequency is controlled at an
`operating point in which a high level of engine efficiency is realized.
`
` Fuel
`consumption
`
` [ km/L ]
`
`35
`
`30
`
`25
`
`20
`
`-0.2
`
`0.1
`0.0
`-0.1
`Charge balance [Ah]
`
`0.2
`
`Fig5. Fuel consumption and charge balance
`
`40
`
`thermal
`efficiency
`
`20
`
`[ % ]
`
`30
`
`accumurate
`time
`
`[ sec ]
`
`0
`30
`
`
`
`00
`
`0
`
`10
`20
`Engine output (kw)
`Fig.6. Engine thermal efficiency
` and operation frequency
`
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`6. Exhaust gases
`The advantages and disadvantages of the hybrid vehicle are summarized below from the perspective
`of cleaning the exhaust gases.
`
`Advantages
`(1) The light-load region in which the exhaust temperature is low and the level of concentration of HC
`emissions is high can be avoided by use of the motor.
`(2) By distributing the load to the motor, fluctuations in engine load, such as those which occur during
`sudden acceleration or deceleration, can be minimized. Consequently, the transient enrichment of the
`engine can be minimized, enabling the air-fuel ratio to be regulated in a stable manner.
`
`Disadvantages
`(1) Because the engine is used in the high efficiency region, the exhaust temperature is relatively lower
`than that of the conventional vehicle.
`(2) There is a risk of emitting unburned fuel and lowering the catalyst temperature due to the increase in
`the frequency of stopping and starting the engine.
`
`To address the first disadvantage noted above, we adopted a rearward exhaust configuration in which
`the exhaust is placed toward the vehicle cabin and located the catalytic converter closer to the exhaust
`outlet. This improves catalyst light-off and better maintains catalyst temperature. The second
`disadvantage is minimized by the exhaust gas flow being cut off when the engine is stopped, thus
`reduction of the temperature of the catalytic converter is minimized to maintain the proper activation
`temperature. Figure 7 shows the engine out emission level during 10-15 mode driving. Reductions are
`noted for CO and NOx, although the amount of HC emitted is the same level as that of the
`conventional vehicle.
`
`HC,
`CO,
`NOx,
`
`[ g/km ]
`
`1.2
`
`0.8
`
`0.4
`
`0
`
`10-15 mode
`
`8
`
`HV Conventional
`
`4
`
`0
`
`CO
`
`HC
`
`Fig7. Engine out emission
`
`4
`
`2
`
`0
`
`NOx
`
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`7. Conclusion
`This development has demonstrated that a highly efficient hybrid powertrain can be realized through
`the combination of a very efficient engine, a drive force dividing mechanism using planetary gears, a
`generator, a motor, and high-power batteries, and that such a system offers a high level of potential in
`terms of minimizing exhaust gases.
`
`References
`1.Gelb ,G.H.,et al , ”An Electromechanical Transmission for Hybrid Vehicle PowerTrains Design and Dynamometer
`testing “, SAE Paper 710235,1971
`2.Yamaguchi,K.,Miyaishi,Y.,Kawamoto,M.,”Dual System Newly Developed Hybrid System”, Proc.of 13rd EVS
`Ohsaka,1996
`3..Kawashima.Y,et al, “Development of Toyota “COASTER HYBRID EV” ”, Proc.of 13rd EVS Ohsaka,1996
`4. Takaoka , et al,”A High Expansion Ratio Gasoline Engine for the Hybrid Electric Vehicle”, Toyota Technical Review
`,1997
`
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`020359.813;me
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