MICHAEL M DE ANGELI, PC
`
`ATTORNEY AT LAW
`
`SUITE 330
`
`1901 RESEARCH BOULEVARD
`
`ROCKVILLE, MARYLAND 20850
`
`(301) ~ I 7-9585
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`FAX: (301) ~17-9345
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`,,~-~ ’~ REGISTERED PATENT
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`ATTORNEY
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`OF PA & MD
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`~ilds is a request for filing n PROVISIONAL APPLICATION >FOR PATENT< under 37 CFR I.$3 (b)(2).
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`TrT~E Oit TIlR INV]ENTION (lie elf.riflers ntis)
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`IMPROVEMENTS IN HYBRID VEHICLES
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`CORRESPOHDENCE ADDRESS :. (Including country It not United Slatel)
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`Michael de Angeli
`1901 Research Boulevard, Suite 330
`Rockville MD 20850
`KN~’LOSgD AIPIE’LICATION I’AR’rs p-~ ~ ~ ~PPb’)
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`The htventle~ ws~ wtsdt by *m alency of the United Slst~= Governmen/or under ¯ contrnet 3ndth an mlency of t~e United Ehlte~ Government.
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`Respectfully
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`.-1_7
`TYPED or PRINTED NAME . ch a e 1 ~ e e 1 i REGISTRATION NO. I
`(’~[ appropriate). 2 7,8 6 9
`Additional inventors are being nmned o~ separately numbered sheets attached hereto
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`PROVISIONAL PATENT APPLICATION
`of
`ALEX J. SEVERINSKY
`for
`IMPROVEMENTS IN HYBRID VEHICLES
`
`Field of the Invention
`This application relates to improvements in hybrid vehicles,
`that is, vehicles in w:hich both an internal combustion engine and
`one or more electric motors are provided to supply torque to the
`driving wheels of the vehicle.
`
`Background of the Invention
`This application discloses a number of improvements over and
`enhancements to the hybrid vehicles disclosed in the inventor’s
`U.S. patent 5,343,970 (the "’970 patent"), 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
`well. 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
`substantial battery bank, or chemical energy stored as combustible
`fuel - is similarly controlled by the microprocessor. For example,
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`in low-speed city driving, the electric motor provides all torque
`needed responsive to energy flowing from the battery. In high-
`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
`aspects and 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.
`
`Objeots of the Imvemtion
`It is an object of the present invention to provide further
`improvements over the hybrid vehicle shown in the ’970 patent.
`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,
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`while providing the functional advantages of the hybrid vehicle
`shown in the ’970 patent.
`Other aspects of and improvements provided by the present
`invention will appear below.
`
`Summary of the Invention
`According to the present invention, 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, and is used for battery charging as needed. 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
`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 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 speeds 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
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`essentially the same speed when the clutch is engaged, the clutch
`need not allow for slip therebetween. Accordingly, a friction
`clutch, as normally provided for road vehicles, is not required,
`and a less-expensive simple mechanical interlock can alternatively
`be employed. Engagement of the clutch is controlled by the
`microprocessor, e.g., by 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 is
`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
`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 oz" deceleration.
`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
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`accordingly.
`In the preferred embodiment, the engine and the two motors all
`rotate at the same speed when the clutch is engaged, avoiding
`intermediate gear trains or like mechanical components and the
`attendant cost, complexity, weight, audible noise, and frictional
`losses occasioned by their use. It is nonetheless within the scope
`of the invention to operate one or more of these components at
`differing rotational speeds; for example, the starter motor might
`drive the engine through a small pinion geared to a relatively
`large toothed flywheel, as conventional. Similarly, it might be
`desirable to provide the traction motor as a relatively high-speed
`unit, driving the road wheels through a belt or gear reduction
`unit. However, in all cases, the rotational speeds of the two
`motors and the engine, and of the road wheels, are fixed with
`respect to one another; no multi-speed transmissions between the
`motors and engine and the road wheels are required by the hybrid
`power train of the invention.
`Other improvements provided according to the invention include
`providing the batteries in two series-connected battery banks, with
`the vehicle chassis connected to the batteries at a central point,
`between the banks. This "center-point-chassis" connection reduces
`the voltage between wlrious circuit components and the vehicle
`chassis by half, significantly reducing the electrical insulation
`required and simplifying such issues as heat-sinking of power
`semiconductors used in the inverter circuitry. Providing dual
`battery banks and dual electric motors, as above, also provides a
`degree of redundancy, permitting certain component failures without
`loss of vehicle function.
`In the preferred embodiment, both the traction and starting
`motors are AC induction motors of four or more poles and the
`accompanying power circuitry provides current of three or more,
`preferably five, phases, allowing the vehicle to function even
`after failure of one or more components.
`During substantially steady-state operation, e.g., during
`highway cruising, the control system operates the engine at varying
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`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.
`
`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 shows a block diagram of the principal components of
`the drive system of the invention, illustrating various control
`signals provided;
`Fig. 3 shows a partial schematic diagram of the battery bank,
`inverter, and motor circuitry;
`Fig. 4 is a diagram illustrating a typical control strategy
`employed during highway driving; and
`Fig. 5 is a generally similar diagram illustrating a typical
`control strategy employed during low-speed city driving.
`
`Description of the Preferred Embodiments
`As indicated above, this application discloses certain
`modifications, improvements, and enhancements of the hybrid
`vehicles shown in the inventor’s U.S. patent 5,343,970; where not
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`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 prowided by the
`latter, as detailed herein.
`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
`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
`vehicle 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
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`frictional losses. According to the present invention, the
`controllable torque-transfer unit is eliminated. Instead, two
`electric motors are provided, each separately controlled by a
`microprocessor controller responsive to operator commands and
`sensed operating conditions.
`As shown in Fig. 1 of the present 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 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
`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. 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
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`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.
`As shown in Fig. 2, shaft encoders 18 and 19 may be mounted on
`the shafts 15 and 16 of starting motor 21 and traction motor 25,
`respectively, to provide signals to microprocessor controller 48
`indicative of the relative rotational speeds of the shafts, and
`their respective rotational positions. Such shaft encoders are
`well-known and commercially available. Alternatively, signals
`indicative of the rotational speeds of the shafts may be derived
`from the inverter control signals, in accordance with well-known
`principles of control of "sensorless" motor drives (see, for
`example, Bose, "Power Electronics and Variable Frequency Drives",
`IEEE, 1996). However, provision of encoders 18 and 19 will allow
`better low-speed torque characteristics of motor 21 and 25, and
`thus reduction in cost.
`In either embodiment, the microprocessor controller 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 be an ordinary automotive friction clutch (as
`illustrated schematically in Fig. i), provided to allow slipping
`before the shafts are fully engaged. Alternatively, clutch 51 may
`be a simple self-aligning mechanical interlock (as illustrated
`schematically in Fig. 2), wherein positive mechanical connection is
`made between the shafts 15 and 16 upon engagement; such a
`mechanical interlock is much simpler and less expensive than a
`friction clutch.
`Additional signals input to microprocessor controller 48, as
`shown in Fig. 2, include operator input commands, typically
`acceleration, direction, and deceleration commands. Position-
`sensing encoders 71 and 72 (which could be configured as rheostats,
`Hall-effect sensors, or otherwise) provide signals to
`microprocessor controller 48 over lines 67 and 68 upon motion of
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`accelerator and brake pedals 69 and 70 (Fig. i) 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 to make
`decisions concerning the proper operation of the vehicle according
`to the invention.
`For example, suppose the vehicle has been operated in city
`traffic for some time, that is, under battery power only.
`Typically the operator will only depress the accelerator slightly
`to drive in traffic. If the operator then depresses accelerator
`pedal 69 significantly farther than he or she had, for example, the
`prior few times acceleration was required, this may be taken as an
`indication that more torque will be required, and so starting motor
`21 will be used to start engine 40. If the operator does not
`depress pedal 69 rapidly, a heater 63 will be used to preheat a
`catalytic converter 64 provided in the engine exhaust system 62, so
`that any fuel that is not combusted during starting of the engine
`40 will be catalytically combusted; however, if the operator
`depresses the pedal 69 rapidly, indicating an immediate need for
`full acceleration, the preheating step may be omitted. 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, monitored by shaft encoders 18 and 19 as discussed
`above, 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
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`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, although the duplicate control lines required are
`not illustrated in Fig. 2. Inverter/charger operation is discussed
`further below in connection with Fig. 3.
`Other elements of the system as illustrated in Figs. 1 and 2
`are generally as discussed in the ’970 patent, including supply of
`fuel 36 from tank 38, air filter 60, and throttle 61.
`As in the ’970 patent, engine 40 is sized so that its full
`torque output is adequate to drive the vehicle at a desired top
`speed, so that the engine is operated at high efficiency during
`highway cruising. During highway cruising, 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. Fig. 4, discussed below,
`illustrates operation in this mode in further detail.
`When the microprocessor controller 48 detects a continued
`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
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`’970 patent, provides much simpler operation. In a particularly
`preferred embodiment, both motors 21 and 25 and clutch 51 may be
`provided in a single sealed housing, possibly bathed in oil for
`cooling and protection from dust and the like. Connection of the
`engine 40 to the drive wheels through motors 21 and 25 also serves
`to damp out any vibration of engine 40.
`Provision of the clutch 51 and separate starter motor 21 also
`allows another important improvement to be provided according to
`the present invention, namely starting engine 40 at high speed,
`e.g., between about 500 and 2000 rpm, as compared to the 60 - 200
`rpm starts conventionally provided. High-rpm starting allows
`elimination of the usual necessity of providing a fuel-rich
`air/fuel mixture to start engine 40, reducing emission of unburned
`fuel and improving fuel economy at start-up. More specifically, in
`conventional low-rpm starts, a 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, so that a near-stoichiometric
`mixture can be provided to engine 40 during the starting phase. As
`noted, the avoidance of 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.
`More specifically, in normal low-rpm starting, a fuel/air
`mixture comprising on the order of 6 to 7 times the stoichiometric
`amount of fuel is provided; most of the excess fuel is immediately
`emitted unburned. According to the invention a mixture typically
`including only 1.2 times the stoichiometric amount of fuel is
`provided for starting, greatly reducing these emissions. As noted
`above, whenever possible - that is, whenever the engine is started
`except when immediate full torque is required by the operator - a
`catalytic converter 64 is preheated before starting the engine, to
`prevent even this relatively small emission of unburned fuel.
`Thus, the primary consideration in selecting the torque of
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`starting motor 21 is that it be capable of rotating the engine 40
`at about 500 - 2000 rpm for starting; the main consideration in
`specification of the torque of engine 40 is that it provide
`sufficient power for highway cruising at high efficiency, i.e.,
`that its maximum power output be approximately equal to that
`required to maintain maximum vehicle speed; 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.
`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
`operation, where air resistance and road friction consume a
`relatively large fraction of the total energy required, in low
`speed operation much energy is lost by conventional vehicles as
`heat released during braking.
`Given the above considerations, the following are typical
`power specifications for the engine 40, starting motor 21 and
`traction motor 25 of a 3000 pound vehicle having performance
`approximately equivalent to that of a "mid-size" sedan of United
`States manufacture; it should be understood that in these
`specifications, reference is made to rated peak power of the
`various components, that is, the components are rated at the
`combination of torque and rpm yielding the maximum torque produced
`per unit time.
`Engine 40:40 to 50 horsepower at 6000 rpm
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`Starting motor 21: I0 - 15 horsepower at approximately 1500
`rpm and higher speeds
`Traction motor 25:50 - 60 horsepower from 1500 to 6000 rpm.
`The same starting motor would be satisfactory for a larger,
`4000 pound sedan, but the engine would typically provide 70 - 90
`horsepower at 6000 rpm and the traction motor 75 - i00 horsepower.
`In both cases, the total power available from the electric
`motors should equal, and preferably exceeds, the maximum power
`available from the engine.
`These components would provide acceleration much superior to
`that of typical similarly-sized automobiles of United States
`manufacture. It will be apparent that these specifications may
`vary over relatively wide ranges depending on the intended use of
`the vehicle of the invention, and should not be construed to limit
`the scope of the invention.
`As indicated above, in the preferred embodiment, both the
`starting and traction motors are AC induction motors, although
`other types may also be employed. These motors, and the
`inverter/chargers driving them, should be chosen and operated such
`that the motors have torque output characteristics varying as a
`function of rpm as illustrated in Fig. 14 of the ’970 patent; that
`is, the motors should produce constant torque up to a base speed,
`typically 1500 - 2000 rpm, and should produce constant power at
`higher speeds. The ratio of the base to maximum speed can vary
`between about 3 to 1 and about 6 to i. By comparison, the series-
`wound DC motors conventionally used as engine starting motors
`provide very high torque, but only at very low speeds; their torque
`output drops precipitously at higher speeds. Such conventional
`starter motors would be unsatisfactory in the present system.
`As noted, each of the torque-producing components (that is,
`engine 40 and starting and traction motors 21 and 25) preferably
`operate at the same speed, so that no gear-reduction units or
`similar heavy and power-dissipating intermediate components are
`needed. A maximum speed of approximately 6000 rpm is preferred, as
`this represents a good compromise between cost, weight, and size of
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`the key components. However, as noted above, it is within the
`scope of the invention to operate the starter motor, traction
`motor, and engine at differing rotational speeds, and connect these
`torque-producing components to one another and to the road wheels
`by fixed-ratio gear or belt drives. In each case, no variable
`speed transmission between the torque-producing components and the
`road wheels is provided.