HYBRID VEHICLES INCORPORATING TURBOCHARGERS
`
`Inventors: ALEX J. SEVERINSKY
`THEODORE LOUCKES
`
`Cross-Reference to Related Application
`This application claims priority from Provisional Application
`Ser. No. 60/122,296, filed March i, 1999.
`
`Field of the Invention
`This application relates to improvements in hybrid vehicles,
`that is, vehicles in which both an internal combustion engine and
`one or more electric motors are provided to supply torque to the
`driving wheels of the vehicle, and wherein turbocharging is
`employed under certain circumstances. A preferred method of sizing
`the power-producing components of the hybrid vehicle is also
`disclosed.
`
`Background of the Invention
`This application discloses a number of improvements over and
`enhancements to the hybrid vehicles disclosed in U.S. patent
`5,343,970 (the "’970 patent") to one of the present inventors,
`which is incorporated herein by this reference. Where differences
`are not mentioned, it is to be understood that the specifics of the
`vehicle design shown in the ’970 patent are applicable to the
`vehicles shown herein as wel!. Discussion of the ’970 patent
`herein is not to be construed to limit the scope of its claims.
`Generally speaking, the ’970 patent discloses hybrid vehicles
`wherein a controllable torque transfer unit is provided capable of
`transferring torque between an internal combustion engine, an
`electric motor, and the drive wheels of the vehicle. The direction
`of torque transfer is controlled by a microprocessor responsive to
`the mode of operation of the vehicle, to provide highly efficient
`operation over a wide variety of operating conditions, and while
`providing good performance.
`The flow of energy - either electrical energy stored in a
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`substantial battery bank, or chemical energy stored as combustible
`fuel - is similarly controlled by the microprocessor. For example,
`in low-speed city driving, the electric motor provides all torque
`needed responsive to energy flowing from the battery. In high-
`5 speed highway driving, where the internal-combustion engine can be
`operated efficiently, it typically provides all torque; additional
`torque may be provided by the electric motor as needed for
`acceleration, hill-climbing, or passing. The electric motor is
`also used to start the internal-combustion engine, and can be
`operated as a generator by appropriate connection of its windings
`by a solid-state, microprocessor-controlled inverter. For example,
`when the state of charge of the battery bank is relatively
`depleted, e.g., after a lengthy period of battery-only operation in
`city traffic, the internal combustion engine is started and drives
`the motor at between 50 and 100% of its maximum torque output, for
`efficient charging of the battery bank. Similarly, during braking
`or hill descent, the kinetic energy of the vehicle can be turned
`into stored electrical energy by regenerative braking.
`The hybrid drive train shown in the ’970 patent has many
`advantages with respect to the prior art which are retained by the
`present invention. For example, the electric drive motor is
`selected to be of relatively high power, specifically, equal to or
`greater than that of the internal combustion engine, and to have
`high torque output characteristics at low speeds; this allows the
`conventional multi-speed vehicle transmission to be eliminated. As
`compared to the prior art, the battery bank, motor/generator, and
`associated power circuitry are operated at relatively high voltage
`and relatively low current, reducing losses due to resistive
`heating and simplifying component selection and connection.
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`30~~I~ Application S/rial Number 09/264,817 filed March 9, 1999
`(the "’817 applica~on"), also incorporated herein by reference,
`discloses and c~ims certain further improvements in hybrid
`vehicles, descried below, with respect to the vehicles of the ’970
`
`patent. T7 present patent application represents further
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`improve~nts
`application.
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`over the hybrid vehicle described in the ’817
`
`objects of the Invention
`It is an object of the present invention to provide further
`improvements over the hybrid vehicles shown in the ’970 patent and
`the ’817 application.
`It is a more specific object of the present invention to
`provide a hybrid drive system for vehicles that does not require
`the controllable torque-transfer unit shown in the ’970 patent,
`while providing the functional advantages of the hybrid vehicles
`shown in the ’970 patent and the ’817 application, together with
`further improvements.
`Other aspects of and improvements provided by the present
`invention will appear below.
`
`Summary of the Invention
`According to the invention of the ’817 application, the
`controllable torque-transfer unit shown in the ’970 patent is
`eliminated by replacing the single electric motor shown therein by
`two separate motors, both operable as generators when appropriate,
`connected by a functionally-conventional clutch or mechanical
`interlock operated by the microprocessor responsive to the
`vehicle’s mode of operation and to input commands provided by the
`operator of the vehicle. As in the ’970 patent, an internal
`combustion engine is provided, sized to provide sufficient torque
`for the maximum cruising speed desired without requirng a multi-
`speed transmission, and is used for battery charging as needed.
`According to the invention of the ’817 application, a relatively
`high-powered "traction" motor is connected directly to the output
`shaft of the vehicle; the traction motor provides torque to propel
`the vehicle in low-speed situations, and provides additional torque
`when required, e.g., for acceleration, passing, or hill-climbing
`during high-speed driving. A relatively low-powered starting motor
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`is also provided, and can be used to provide torque propelling the
`vehicle when needed. This second motor is connected directly to
`the internal combustion engine for starting the engine. Unlike a
`conventional starter motor, which rotates an internal combustion
`engine at low speed (60 - i00 rpm) for starting, necessitating
`provision of a rich fuel/air mixture for starting, the starter
`motor according to the invention spins the engine at relatively
`high speed (typically 300 rpm) for starting; this allows starting
`the engine with a near-stoichiometric mixture, significantly
`reducing undesirable emissions and improving fuel economy at start-
`up.
`
`As noted, the two motors are separated by a functionally-
`conventional clutch, that is, a clutch which either joins the two
`motors together for rotation at the same speed, or separates them
`completely. As the motor shafts can be controlled to rotate at
`essentially the same speed when the clutch is engaged, the clutch
`need not allow for significant slipping before engagement.
`Accordingly, a friction clutch, as normally provided for road
`vehicles, may not be required, and a less-expensive simple
`mechanical interlock may alternatively be employed. Engagement of
`the clutch is controlled by the microprocessor, e.g., controlling
`a hydraulic actuator, responsive to the state of operation of the
`vehicle and the current operator input.
`For example, during low-speed operation, the clutch will be
`disengaged, so that the traction motor is disconnected from the
`engine; the vehicle is then operated as a simple electric car,
`i.e., power is drawn from the battery bank and supplied to the
`traction motor. If the batteries become depleted, the starter
`motor is used to start the internal combustion engine, which then
`runs at relatively high torque output (e.g., between about 50 -
`100% of its maximum torque), for efficient use of fuel, and the
`starting motor is operated as a high-output generator to recharge
`the battery bank. If the operator calls for more power than
`available from the traction motor alone, e.g., in accelerating onto
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`a highway, the starter motor starts the internal combustion engine,
`and the clutch is engaged, so that the engine and starter motor can
`provide additional torque. The engine is sized so that it provides
`sufficient power to maintain a suitable highway cruising speed
`while being operated in a torque range providing good fuel
`efficiency; if additional power is then needed, e.g., for hill-
`climbing or passing, the traction and/or starter motors can be
`engaged as needed. Both motors can be operated as generators,
`e.g., to transform the vehicle’s kinetic energy ito electrical
`power during descent or deceleration.
`It is also within the scope of the invention to provide power
`from the engine and starting motor to one pair of road wheels,
`through the clutch, and from the traction motor to another set of
`wheels; this provides all-wheel drive, when needed, without a
`transfer gearbox or drive shaft. See provisional patent
`application Ser. No. 60/122,478, filed March i, 1999, incorporated
`by reference herein. In a further embodiment, torque from the
`engine, starter motor, and a first traction motor can be provided
`to a first set of road wheels, and torque from a second traction
`motor to a second set of road wheels; this would provide maximal
`flexibility in control of the transfer of torque to the road,
`useful in low-traction conditions.
`In each of these aspects of the operation of the vehicle, and
`as in the ’970 patent, the operator of the vehicle need not
`consider the hybrid nature of the vehicle during its operation, but
`simply provides control inputs by operation of the accelerator and
`brake pedals. The microprocessor determines the proper state of
`operation of the vehicle based on these and other inputs and
`controls the various components of the hybrid drive train
`accordingly.
`According to the present invention, the engine is further
`provided with a turbocharger, also controlled by the
`microprocessor, and operated only under extended high-load
`conditions. In low-speed driving, the turbocharger is bypassed and
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`is inactive, so that the vehicle is operated as in the ’817
`application; similarly, when the torque provided by the engine is
`inadequate for short-term high loads, such as during overtaking on
`the highway, the traction motor is employed to propel the vehicle.
`The starting motor may also be employed to provide torque as
`needed. However, according to the present invention, when
`conditions demand production of high torque for extended periods,
`for example, when towing a trailer, climbing a long hill, or
`driving at sustained high speed, or when the battery bank is
`relatively discharged, the microprocessor activates the
`turbocharger, so that additional torque is produced by the internal
`combustion engine when needed.
`More specifically, in the ’817 application, during
`substantially steady-state operation, e.g., during highway
`cruising, the control system operates the engine at varying torque
`output levels, responsive to the operator’s commands. The range of
`permissible engine torque output levels is constrained to the range
`in which the engine provides good fuel efficiency. Where the
`vehicle’s torque requirements exceed the engine’s maximum efficient
`torque output, e.g., during passing or hill-climbing, one or both
`of the electric motors are energized to provide additional torque;
`where the vehicle’s torque requirements are less than the minimum
`torque efficiently provided by the engine, e.g., during coasting,
`on downhills or during braking, the excess engine torque is used to
`charge the batteries. Regenerative charging may be performed
`simultaneously, as torque from the engine and the vehicle’s kinetic
`energy both drive either or both motors in generator mode. The
`rate of change of torque output by the engine may be controlled in
`accordance with the batteries’ state of charge.
`According to the present invention, the above control
`strategy is retained and employed under substantially all "normal"
`driving conditions; addition of a turbocharger controlled by the
`microprocessor according to the invention allows additional control
`flexibility. More specifically, when conditions demand power in
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`excess of the engine’s normally-aspirated maximum output for a
`relatively long period of time, e.g., for climbing long hills, for
`towing, or when driving at high speed, the turbocharger, which is
`normally bypassed and thus inactive, is energized by supply of the
`engine’s exhaust gas stream. The engine then produces additional
`torque as required.
`As compared to turbochargers as conventionally employed, which
`are constantly active, the turbocharger according to the present
`invention is used only when needed, that is, as noted, only when
`torque in excess of that available from the engine when in
`"normally-aspirated" mode is required. This allows both the engine
`and turbocharger to be designed to meet relatively well-defined
`objectives, providing further efficiency in use of fuel.
`Furthermore, because according to the invention theturbocharger is
`employed in a hybrid vehicle having one or more electric motors
`available to provide additional torque substantially immediately
`upon demand, the vehicle overall does not exhibit the slow response
`time of conventional turbocharged vehicles.
`In a further refinement, the amount of time during which the
`motors will be used to supply torque in excess of that available
`from the engine in normally-aspirated mode before the turbocharger
`is activated is controlled responsive to the state of charge of the
`vehicle battery bank. More specifically, in general, when power in
`excess of the engine’s normally-aspirated capacity is required for
`a shorter period of time, e.g., when passing, at least the traction
`motor, or both of the electric motors, are energized by power from
`the battery. However, it is also within the scope of the invention
`to employ the turbocharger under circumstances calling for maximum
`acceleration, or when the state of charge of the battery is such
`that the electric motors will not be adequate to supply the torque
`required.
`In addition to the operational advantages noted, provision of
`an engine having a "turbocharger-on-demand" in a hybrid vehicle
`allows the engine to be smaller than otherwise, that is, to provide
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`adequate highway performance in a vehicle of a given weight. As
`the starting motor/generator must be sized such that when it is
`driven by the engine to charge the batteries (e.g., in extended
`city driving) the engine is loaded adequately to be operated
`efficiently, employment of a smaller engine allows use of a smaller
`starting motor/generator. For similar reasons, provision of a
`smaller engine allows it to be used to efficiently propel the
`vehicle in highway driving commencing at lower average speeds,
`resulting in turn in better fuel economy.
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`Brief Description of the Drawings
`The invention will be better understood if reference is made
`to the accompanying drawings, in which:
`Fig. 1 shows a schematic diagram of the principal components
`of the hybrid vehicle drive system according to the invention;
`Fig. 2 is a diagram in which engine torque is plotted against
`engine speed for a typical engine in normally-aspirated and
`turbocharged modes of operation, and also shows typical road loads
`encountered;
`< ji
`Fig. 3 is a diagram illustrating differing modes of vehicle
`20 ~
`~ powertrain operation, plotted on a three-dimensional chart,
`illustrating that the mode of vehicle operation is a function of
`%~i} the state of charge of the battery bank, the instantaneous road
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`~i load, and time; and
`Fig. 4 is a timing diagram showing road load, engine torque
`output, the state of charge of the battery bank, and engine
`operation in normally-aspirated and turbocharged modes of operation
`as functions of time, thus illustrating a typical control strategy
`employed during low-speed city driving, highway cruising, and
`extended high-load driving.
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`Description of the Preferred Embodiments
`As indicated above, this application discloses certain
`modifications, improvements, and enhancements of the hybrid
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`vehicles shown in U.S. patent 5,343,970 (the "’970 patent") to one
`of the present inventors, which is incorporated herein by this
`reference; where not otherwise stated, the design of the vehicle of
`the present invention is similar to that shown in the ’970 patent.
`Components commonly numbered in this application and the ’970
`patent are functionally similar in the corresponding systems, with
`detail differences as noted. The advantages of the system shown
`in the ’970 patent with respect to the prior art are provided by
`that of the present invention, with further improvements provided
`by the latter, as detailed herein. The present application also
`represents further improvements over the hybrid vehicle described
`in Application Serial Number 09/264,817 filed March 9, 1999 (the
`"’817 application"), also incorporated herein by reference.
`As shown in the ’970 patent with reference to Figs. 1 and 2
`thereof, typical modern automobiles operate at very low efficiency,
`due principally to the fact that internal combustion engines are
`i~!~ very inefficient except when operating at near peak torque output;
`this condition is only rarely met. (The same is true, to greater
`or lesser degree, of other road vehicles powered by internal
`combustion engines.) According to an important aspect of the
`invention of the ’970 patent, substantially improved efficiency is
`afforded by operating the internal combustion engine only at
`relatively high torque output levels, typically at least 35% and
`preferably at least 50% of peak torque. When the vehicle operating
`conditions require torque of this appoximate magnitude, the engine
`is used to propel the vehicle; when less torque is required, an
`electric motor powered by electrical energy stored in a substantial
`battery bank drives the vehicle; when more power is required than
`provided by either the engine or the motor, both are operated
`simultaneously. The same advantages are provided by the system of
`the present invention, with further improvements and enhancements
`described in detail below.
`In the system of the ’970 patent, torque from either or both
`the engine and motor is transferred to the drive wheels of the
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`vehicll by a controllable torque-transfer unit. This unit also
`allows torque to be transferred between the motor and engine, for
`starting the engine, and between the wheels and motor, for
`regenerative battery charging during deceleration of the vehicle.
`This unit, while entirely practical, comprises gears for power
`transfer, which are inevitably a source of audible noise and
`frictional losses. According to the invention of the ’817
`application, and the present invention, the controllable torque-
`transfer unit is eliminated. Instead, two electric motors are
`provided, each separately controlled by a microprocessor controller
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`responsive to operator commands and sensed operating conditions.
`As shown in Fig. 1 of the present application, and as also
`shown in the ’817 application, a traction motor 25 is connected
`~ directly to the vehicle differential 32, and thence to the road
`wheels 34. A starting motor 21 is connected directly to the
`15
`@ internal combustion engine 40. The motors 21 and 25 are functional
`as motors or generators by appropriate operation of corresponding
`inverter/charger units 23 and 27, respectively. The two motors are
`controllably connected for torque transfer by a clutch 51,
`conventional in the sense that it is either engaged, wherein the
`shafts of motors 21 and 25 are connected and rotate together, or
`disengaged, wherein the shafts may rotate separately. (The
`respective positions of motor 21 and engine 40 with respect to
`clutch 51 could be reversed as compared to their positions in Figs.
`1 and 2 without affecting the function of the system, although as
`engine 40 would then require torque transmitting connection at both
`ends of its crankshaft, some additional complexity would result.)
`The clutch 51 is operated by microprocessor 48, e.g., through a
`known hydraulic actuator 53, together with the other components of
`the system, in accordance with the operational state of the vehicle
`and the operator’s input commands.
`As in the case of the hybrid vehicle system shown in the ’970
`patent, and as indicated above, the vehicle of the invention is
`operated in different modes depending on the torque required, the
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`state of charge of the batteries, and other variables. For
`example, during low-speed operation, such as in city traffic, the
`vehicle is operated as a simple electric car, where all power is
`provided to road wheels 34 by traction motor 25; engine 40 is run
`only as needed to charge battery bank 22. (In this connection,
`essentially conventional lead-acid batteries are currently
`preferred for battery bank 22, since the infrastructure to provide
`and dispose of these batteries is already in place; that is, lead-
`acid batteries are widely available, readily recycled, and so on.
`More advanced batteries may be used if and when they become widely
`available and economically competitive.) Under these circumstances,
`the charging current is provided by starting motor 21, operated as
`a generator by appropriate operation of inverter/charger 23.
`Accordingly, clutch 51 is disengaged, so that the road speed of
`the vehicle is independent of the speed of engine 40; engine 40 can
`thus be operated at high torque, for fuel efficiency. The
`operating modes of the vehicle of the invention are further
`discussed below.
`As shown in further detail in the ’817 application, the
`microprocessor 48 is provided with signals indicative of the
`rotational speeds of shafts 15 and 16, and controls operation of
`engine 40, motor 21, and motor 25 as necessary to ensure that the
`shafts are rotating at substantially the same speed before engaging
`clutch 51; therefore, clutch 51 need not necessarily be an ordinary
`automotive friction clutch (as illustrated schematically in Fig.
`i), provided to allow extensive relative slipping before the shafts
`are fully engaged. More particularly, as slipping of the clutch is
`not required to propel the vehicle initially from rest, as is the
`case in conventional vehicles, clutch 51 need not allow for
`extensive slipping when being engaged; in some cases it may be
`satisfactory to provide clutch 51 as a simple self-aligning
`mechanical interlock, wherein positive mechanical connection is
`made between the shafts 15 and 16 upon engagement. Such a
`mechanical interlock is simpler and less expensive than a friction
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`clutch. However, it is within the scope of the invention to
`provide a friction clutch.
`Additional signals input to microprocessor 48 include signals
`indicative of the state of charge of the battery bank 22, and
`operator input commands, typically acceleration, direction, and
`deceleration commands. The operator input commands may be provided
`to microprocessor 48 by position-sensing encoders 71 and 72
`providing signals to microprocessor 48 over lines 67 and 68 upon
`motion of accelerator and brake pedals 69 and 70 (Fig. i)
`I0 respectively. The microprocessor monitors the rate at which the
`operator depresses pedals 69 and 70 as well as the degree to which
`pedals 69 and 70 are depressed. The microprocessor uses this
`information, and additional information to make decisions
`~ concerning the proper operation of the vehicle according to the
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`~~ For example, suppose the /v~hicle has been operated in city
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`(!~ "traffic for some time, tha~ is, under battery power only.
`,~{i Typically the operator will ~hly depress the accelerator slightly
`~!i:~i to drive in traffic. If t~e/operator then depresses accelerator
`20~ pedal 69 significantly far, her/ than he or she had, for example, the
`~{~ prior few times accelera~ion/Z _was required, this may be taken as an
`i!]i indication that more t~que will be required, and so starting motor
`!~i~i 21 will be used to,tart! engine- 40. If the operator does not
`i!i~i depress pedal 69 r~idly,~/~ a heater 63 will be used to preheat a
`25
`catalytic converte~64 provided in the engine exhaust system 62, so
`I_
`that any fuel th~ is not combusted during starting of the engine
`40 will be caialytically~ combusted;_ however, if the operator
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`depresses the~edal 69 rapidly, indicating an immediate need for
`/.
`full acceleration, the traction and starting motors may be driven
`i
`beyond their/norms! rated power briefly, so that adequate power is
`I
`provided (~s may be necessary for safe operation) while the
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`catalytic~onvetrer is heated. Copending provisional application
`Ser. No./ 60/122,477 filed March i, 1999, incorporated herein by
`I
`provides full details of the preheating of the
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`catalytic convert~r, while the catalytic converter itself is
`preferably manu~acturedl according to the invention disclosed in
`copending application Ser. No. filed . (Attorney’s
`I
`Docket NOV P~I~995). ¯
`Similarly, if the operator depresses the brake pedal 70
`relatively gently, all braking may be provided by regenerative
`charging of the batteries; if the operator instead presses ,rapidly
`on brake pedal 70, both mechanical and regenerative braking will be
`provided. Mechanical braking is also provided on long downhills
`when the batteries are fully charged.
`In addition to engine and starting motor speed and traction
`motor speed, battery voltage, battery charge level, and ambient
`temperature are also either monitored directly or derived from
`monitored variables. In response to these inputs, and the operator
`inputs, microprocessor controller 48 operates a control program and
`provides output control signals to engine 40, by commands provided
`to its electronic fuel injection unit (EFI) 56 and electronic
`engine management system (EEM) 55, and to starting motor 21, clutch
`51, traction motor 25, inverter/charger units 23 and 27, and other
`components. As indicated, the control signals provided to
`inverter/chargers 23 and 27 allow control of the current to be
`provided, of direction of rotation of the motor 25, allowing
`reversing of the vehicle, as well as control of operation of the
`motors 21 and 25 in motor or generator mode. Inverter/chargers 23
`and 27 are separately controlled to allow independent operation of
`motors 21 and 25. See the ’817 application for further details.
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`~ Other elem~ of the system as illustrated in Fig. 1 are
`generally as~scussed in the ’970 patent and ’817 application,
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`including ~ply of fuel 36 from tank 38, air filter 60~ and
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`30 throttle~. . .
`As in the ’970 patent, engine 40 is sized so that its full
`torque output is adequate to drive the vehicle in a desired range
`of cruising speeds, so that the engine is operated at high
`efficiency during highway cruising. During highway cruising,
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`therefore, clutch 51 is engaged; engine 40 then drives road wheels
`34 through the shafts of motors 21 and 25. If extra power is
`needed, e.g., for acceleration or hill-climbing, either or both of
`motors 21 and 25 can be powered. Similarly, under deceleration,
`either or both of motors 21 and 25 are operated as generators,
`proving regenerative recharging of battery bank 22.
`According to the ’817 application, when the microprocessor
`controller 48 detects an operator requirement for additional power,
`such as during transition from slow-speed to highway operation, or
`by measuring the rate at which the operator depresses accelerator
`pedal 70, engine 40 is started using starter motor 21 and brought
`up to speed before clutch 51 is engaged, to ensure a smooth
`transition. As cruising speed is reached, power to traction motor
`25 (and to starter motor 21, if also used to accelerate the
`vehicle) is gradually reduced. Provision of the clutch 51 and
`separate starter motor 21, as compared to using the single traction
`motor to start engine 40 while simultaneously accelerating the
`vehicle, as in the ’970 patent, provides much simpler operation.
`Provision of the clutch 51 and separate starter motor 21 also
`allows another important improvement to be provided according to
`the ’817 application, namely starting engine 40 at high speed,
`e.g., at least about 300 rpm, as compared to the 60 - i00 rpm
`starts conventionally provided. High-rpm starting allows
`significant reduction of the usual necessity of providing a fuel-
`rich fuel:air mixture to start engine 40, reducing emission of
`unburned fuel and improving fuel economy at start-up. More
`specifically, in conventional low-rpm starts, an extremely rich
`mixture must be provided to ensure that some fraction of the fuel
`is in the vapor phase; only fuel in the vapor phase can be ignited
`by a spark. At high starting speeds, turbulence in the combustion
`chamber is sufficient to ensure the presence of vapor, particularly
`if the fuel is atomized by high-pressure fuel injection, as is
`preferred, so that a near-stoichiometric mixture can be provided to
`engine 40 during the starting phase. As noted, the avoidance of
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`rich mixtures at starting significantly reduces emission of
`unburned fuel - since most of the fuel provided to a conventional
`engine at starting is immediately exhausted unburnt - and provides
`some improvement in fuel efficiency. Further reduction in
`emissions are discussed in the ’817 application, and copending
`provisional application Ser. No. 60/122,477 filed March i, 1999.
`Accordingly, the primary considerations in selecting the
`torque of starting motor 21 is that it be capable of rotating the
`engine 40 at about 300 rpm for starting, and that it be capable of
`accepting at least about 30% of the engine’s maximum torque output
`when operated as a generator, so that the engine can be be
`efficiently employed when charging the battery bank during extended
`low-speed vehicle operation; the main consideration in
`specification of the torque of engine 40 is that it provide
`sufficient power for highway cruising while being operated at high
`efficiency, i.e., that its maximum power output be approximately
`equal to that required to maintain a range of desired cruising
`speeds; and the principal consideration defining the power required
`of the traction motor 25 is that it be sufficiently powerful to
`provide adequate acceleration in combination with the engine 40 and
`starting motor 21. Stated differently, the total power available
`provided by all of these torque-producing components should be at
`least equal to and preferably exceeds the peak power provided by
`the internal combustion engines of conventional vehicles of similar
`intended use, both as measured at the wheels. Moreover, as set
`forth in the ’970 patent, the total torque provided by motors 21
`and 25 should be at least equal to that produced by engine 40; this
`is important is achieveing adequate low-speed performance in a
`vehicle not including a variable-ratio transmission.
`At the same time, motors 21 and 25 are also sized to be
`capable of recovering 65 - 90% of the vehicle’s kinetic energy when
`operated as generators in the regenerative braking mode. A
`particularly high fraction of the vehicle’s kinetic energy can be
`recovered during low-speed operation; as compared to high-speed
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