Patent Publication Number: US-6209672-B1

Title: Hybrid vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Provisional Application Ser. No. 60/100,095, filed Sep. 14, 1998. 
    
    
     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. More particularly, this invention relates to a hybrid electric vehicle that is fully competitive with presently conventional vehicles as regards performance, operating convenience, and cost, while achieving substantially improved fuel economy and reduced pollutant emissions. 
     DISCUSSION OF THE PRIOR ART 
     For many years great attention has been given to the problem of reduction of fuel consumption of automobiles and other highway vehicles. Concomitantly very substantial attention has been paid to reduction of pollutants emitted by automobiles and other vehicles. To a degree, efforts to solve these problems conflict with one another. For example, increased thermodynamic efficiency and thus reduced fuel consumption can be realized if an engine is operated at higher temperatures. Thus there has been substantial interest in engines built of ceramic materials withstanding higher combustion temperatures than those now in use. However, higher combustion temperatures in gasoline-fueled engines lead to increase in certain undesirable pollutants, typically NO x . 
     Another possibility for reducing emissions is to burn mixtures of gasoline and ethanol (“gasohol”), or straight ethanol. However, to date ethanol has not become economically competitive with gasoline, and consumers have not accepted ethanol to any great degree. Moreover, to make an alternate fuel such as ethanol available to the extent necessary to achieve appreciable improvements in nationwide air quality and fuel conservation would require immense costs for infrastructure improvements; not only the entire nation&#39;s motor fuel production and delivery system, but also the vehicle manufacture, distribution, and repair system, would have to be extensively revised or substantially duplicated. 
     One proposal for reducing pollution in cities is to limit the use of vehicles powered by internal combustion engines and instead employ electric vehicles powered by rechargeable batteries. To date, all such “straight electric” cars have had very limited range, typically no more than 150 miles, have insufficient power for acceleration and hill climbing except when the batteries are substantially fully charged, and require substantial time for battery recharging. Thus, while there are many circumstances in which the limited range and extended recharging time of the batteries would not be an inconvenience, such cars are not suitable for all the travel requirements of most individuals. Accordingly, an electric car would have to be an additional vehicle for most users, posing a substantial economic deterrent. Moreover, it will be appreciated that in the United States most electricity is generated in coal-fired power plants, so that using electric vehicles merely moves the source of the pollution, but does not eliminate it. Furthermore, comparing the respective net costs per mile of driving, electric vehicles are not competitive with ethanol-fueled vehicles, much less with conventional gasoline-fueled vehicles. See, generally, Simanaitis, “Electric Vehicles”,  Road &amp; Track , May 1992, pp. 126-136; Reynolds, “AC Propulsion CRX”,  Road &amp; Track , Oct. 1992, pp. 126-129. 
     Brooks et al U.S. Pat. No. 5,492,192 shows such an electric vehicle; the invention appears to be directed to incorporation of antilock braking and traction control technologies into an otherwise conventional electric vehicle. 
     Much attention has also been paid over the years to development of electric vehicles including internal combustion engines powering generators, thus eliminating the defect of limited range exhibited by simple electric vehicles. The simplest such vehicles operate on the same general principle as diesel-electric locomotives used by most railroads. In such systems, an internal combustion engine drives a generator providing electric power to traction motors connected directly to the wheels of the vehicle. This system has the advantage that no variable gear ratio transmission is required between the diesel engine and the wheels of the locomotive. 
     More particularly, an internal combustion engine produces zero torque at zero engine speed (RPM) and reaches its torque peak somewhere in the middle of its operating range. Accordingly, all vehicles driven directly by an internal combustion engine (other than certain single-speed vehicles using friction or centrifugal clutches, and not useful for normal driving) require a variable-ratio transmission between the engine and the wheels, so that the engine&#39;s torque can be matched to the road speeds and loads encountered. Further, some sort of clutch must be provided so that the engine can be mechanically decoupled from the wheels, allowing the vehicle to stop while the engine is still running, and to allow some slippage of the engine with respect to the drive train while starting from a stop. It would not be practical to provide a diesel locomotive with a multiple speed transmission, or a clutch. Accordingly, the additional complexity of the generator and electric traction motors is accepted. Electric traction motors produce full torque at zero RPM and thus can be connected directly to the wheels; when it is desired that the train should accelerate, the diesel engine is simply throttled to increase the generator output and the train begins to move. 
     The same drive system may be employed in a smaller vehicle such as an automobile or truck, but has several distinct disadvantages in this application. In particular, and as discussed in detail below in connection with FIGS. 1 and 2, it is well known that a gasoline or other internal combustion engine is most efficient when producing near its maximum output torque. Typically, the number of diesel locomotives on a train is selected in accordance with the total tonnage to be moved and the grades to be overcome, so that all the locomotives can be operated at nearly full torque production. Moreover, such locomotives tend to be run at steady speeds for long periods of time. Reasonably efficient fuel use is thus achieved. However, such a direct drive vehicle would not achieve good fuel efficiency in typical automotive use, involving many short trips, frequent stops in traffic, extended low-speed operation and the like. 
     So-called “series hybrid” electric vehicles have been proposed for automotive use, wherein batteries are used as energy storage devices, so that an internal combustion engine provided to power a generator can be operated in its most fuel-efficient output power range while still allowing the electric traction motor(s) powering the vehicle to be operated as required. Thus the engine may be loaded by supplying torque to a generator charging the batteries while supplying electrical power to the traction motor(s) as required, so as to operate efficiently. This system overcomes the limitations of electric vehicles noted above with respect to limited range and long recharge times. Thus, as compared to a conventional vehicle, wherein the internal combustion engine delivers torque directly to the wheels, in a series hybrid electric vehicle, torque is delivered from the engine to the wheels via a serially connected generator used as a battery charger, the battery, and the traction motor. Energy transfer between those components consumes at least approximately 25% of engine power. Further, such components add substantially to the cost and weight of the vehicle; in particular, an electric motor capable of providing sufficient torque to meet all expected demand, e.g., to allow reasonable performance under acceleration, during hill-climbing and the like, is rather heavy and expensive. Thus, series hybrid vehicles have not been immediately successful. 
     A more promising “parallel hybrid” approach is shown in U.S. Pat. Nos. 3,566,717 and 3,732,751 to Berman et al. In Berman et al an internal combustion engine and an electric motor are matched through a complex gear train so that both can provide torque directly to the wheels. 
     In Berman et al, the internal combustion engine is run in several different modes. Where the output of the internal combustion engine is more than necessary to drive the vehicle (“first mode operation”) the engine is run at constant speed and excess power is converted by a first motor/generator (“speeder”) to electrical energy for storage in a battery. In “second mode operation”, the internal combustion engine drives the wheels directly, and is throttled. When more power is needed than the engine can provide, a second motor/generator or “torquer” provides additional torque as needed. 
     Berman et al thus show two separate electric motor/generators separately powered by the internal combustion engine; the “speeder” charges the batteries, while the “torquer” propels the vehicle forward in traffic. This arrangement is a source of additional complexity, cost and difficulty, as two separate modes of engine control are required. Moreover, the operator must control the transition between the several modes of operation. Such a complex vehicle is unsuited for the automotive market. Automobiles intended for mass production can be no more complicated to operate than conventional vehicles, and must be essentially “foolproof”, that is, resistant to damage that might be caused by operator error. Further, the gear train shown by Berman et al appears to be quite complex and difficult to manufacture economically. Berman et al also indicate that one or even two variable-speed transmissions may be required; see, e.g., col.  3 , lines  19 - 22  and  36 - 38  of U.S. Pat. No. 3,566,717, and col.  2 , lines  53 - 55  of U.S. Pat. No. 3,732,751. 
     Lynch et al U.S. Pat. No. 4,165,795 also shows an early parallel hybrid drive. Lynch argues that maximum fuel efficiency can be realized when a relatively small internal combustion engine is provided, such that when the engine is operated at an efficient speed, it produces approximately the average power required over a typical mission. The example given is of an engine producing 25 hp maximum and 17 hp at its most efficient speed, about 2500 rpm. 
     This is to be combined with an electric motor-generator of about 30 peak hp. This vehicle requires a variable-ratio transmission to achieve reasonable performance. It appears that the engine is to be run continuously, at a steady speed, with torque provided by the motor when needed and excess torque used to charge the batteries otherwise. In a first embodiment, torque provided by the motor is transmitted to the drive wheels through the engine, while in a second embodiment their respective positions are reversed. 
     Nishida U.S. Pat. No. 5,117,931 shows a parallel hybrid vehicle where torque from an electric motor may be combined with torque from an internal combustion engine in a “torque transmission unit” comprising paired bevel gears and means for controlling the relative rates of rotation of the motor and engine, so that the motor can be used to start the engine, absorb excess torque from the engine (by charging a battery), or provide additional propulsive torque. A variable-speed transmission is coupled between the torque transmission unit and the propelling wheels. Both the torque transmission unit and the variable-speed transmission are complex, heavy, and expensive components, the use of which would preferably be avoided. 
     Helling U.S. Pat. No. 3,923,115 also shows a hybrid vehicle having a torque transmission unit for combining torque from an electric motor and an internal combustion engine. However, in Helling the relative rates of rotation of the motor and engine input shafts are fixed; a flywheel is provided to store excess mechanical energy as well as a battery to store excess electrical energy. Albright, Jr. et al U.S. Pat. No. 4,588,040 shows another hybrid drive scheme using a flywheel in addition to batteries to store excess energy; various complicated mechanical connections are provided between the various components. Capacitors have also been proposed for energy storage; see Bates et al U.S. Pat. No. 5,318,142. 
     Fjällström U.S. Pat. No. 5,120,282 shows a parallel hybrid drive train wherein torque from two electric motors is combined with torque produced by an internal combustion engine; the combination is performed by a complex arrangement of paired planetary gearsets, and unspecified control means are alleged to be able to allow variation of road speed without a variable-ratio transmission. 
     Hunt U.S. Pat. Nos. 4,405,029 and 4,470,476 also disclose parallel hybrids requiring complex gearing arrangements, including multiple speed transmissions. More specifically, the Hunt patents disclose several embodiments of parallel hybrid vehicles. Hunt indicates (see col.  4 , lines  6 - 20  of the &#39;476 patent) that an electric motor may drive the vehicle at low speeds up to 20 mph, and an internal combustion engine used for speeds above 20 mph, while “in certain speed ranges, such as from 15-30 mph, both power sources may be energized. . . Additionally, both power sources could be utilized under heavy load conditions.” Hunt also indicates that “the vehicle could be provided with an automatic changeover device which automatically shifts from the electrical power source to the internal combustion power source, depending on the speed of the vehicle” (col.  4 , lines  12 - 16 ). 
     However, the Hunt vehicle does not meet the objects of the present invention, as discussed in detail below. Hunt&#39;s vehicle in each embodiment requires a conventional manual or automatic transmission. See col.  2 , lines  6 - 7 . Moreover, the internal combustion engine is connected to the transfer case (wherein torque from the internal combustion engine and electric motor is combined) by a “fluid coupling or torque converter of conventional construction”. Col.  2 , lines  16 - 17 . Such transmissions and fluid couplings or torque converters are very inefficient, are heavy, bulky, and costly, and are to be eliminated according to one object of the present invention, again as discussed in detail below. 
     Furthermore, the primary means of battery charging disclosed by Hunt involves a further undesirable complexity, namely a turbine driving the electric motor in generator configuration. The turbine is fueled by waste heat from the internal combustion engine. See col.  3 , lines  10 - 60 . Hunt&#39;s internal combustion engine is also fitted with an alternator, for additional battery charging capability, adding yet further complexity. Thus it is clear that Hunt fails to teach a hybrid vehicle meeting the objects of the 2-D present invention—that is, a hybrid vehicle competitive with conventional vehicles with respect to performance, cost and complexity, while achieving substantially improved fuel efficiency. 
     Kawakatsu U.S. Pat. Nos. 4,305,254 and 4,407,132 show a parallel hybrid involving a single internal combustion engine coupled to the drive wheels through a conventional variable-ratio transmission, an electric motor, and an alternator, to allow efficient use of the internal combustion engine. As in the Hunt disclosure, the engine is intended to be operated in a relatively efficient range of engine speeds; when it produces more torque than is needed to propel the vehicle, the excess is used to charge the batteries; where the engine provides insufficient torque, the motor is energized as well. 
     A further Kawakatsu patent, No. 4,335,429, shows a hybrid vehicle, in this case comprising an internal combustion engine and two motor/generator units. A first larger motor/generator, powered by a battery, is used to provide additional torque when that provided by the engine is insufficient; the larger motor-generator also converts excess torque provided by the engine into electrical energy, to be stored by the battery, and is used in a regenerative braking mode. The second smaller motor/generator is similarly used to provide additional torque and additional regenerative braking as needed. 
     More particularly, the latter Kawakatsu patent asserts that a single electric motor sized to provide sufficient torque to propel the vehicle would not be capable of providing sufficient regenerative braking force; see col.  1 , line  50  - col.  2  line  8 . Accordingly, Kawakatsu provides two separate motor/generators, as noted; a separate engine starting motor is also provided. See col.  6 , lines  22 - 23 . In the embodiment shown, the larger motor/generator is connected to the wheel drive shaft, while the engine and the smaller motor/generator are connected to the wheels through a complex mechanism comprising three separately-controllable clutches. See col.  5 , lines  50 - 62 . 
     Numerous patents disclose hybrid vehicle drives tending to fall into one or more of the categories discussed above. A number of patents disclose systems wherein an operator is required to select between electric and internal combustion operation; for example, an electric motor is provided for operation inside buildings where exhaust fumes would be dangerous, and an internal combustion engine provided for operation outdoors. It is also known to propose a hybrid vehicle comprising an electric motor for use at low speeds, and an internal combustion engine for use at higher speed. The art also suggests using both when maximum torque is required. In several cases the electric motor drives one set of wheels and the internal combustion engine drives a different set. See generally Shea (4,180,138); Fields et al (4,351,405); Kenyon (4,438,342); Krohling (4,593,779); and Ellers (4,923,025). 
     Many of these patents show hybrid vehicle drives wherein a variable speed transmission is required, as do numerous additional references. A transmission as noted above is typically required where the internal combustion engine and/or the electric motor are not capable of supplying sufficient torque at low speeds. See Rosen (3,791,473); Rosen (4,269,280); Fiala (4,400,997); and Wu et al (4,697,660). Kinoshita (3,970,163) shows a vehicle of this general type wherein a gas turbine engine is coupled to the road wheels through a three-speed transmission; an electric motor is provided to supply additional torque at low speeds. 
     For further examples of series hybrid vehicles generally as discussed above, see Bray (4,095,664); Cummings (4,148,192); Monaco et al (4,306,156); Park (4,313,080); McCarthy (4,354,144); Heidemeyer (4,533,011); Kawamura (4,951,769); and Suzuki et al (5,053,632). Various of these address specific problems arising in the manufacture or use of hybrid vehicles, or specific alleged design improvements. For example, Park addresses certain specifics of battery charging and discharge characteristics, while McCarthy shows a complex drive system involving an internal combustion engine driving two electric motors; the torque generated by the latter is combined in a complex differential providing continuously variable gear ratios. Heidemeyer shows connecting an internal combustion engine to an electric motor by a first friction clutch, and connecting the motor to a transmission by a second friction clutch. 
     Other patents of general relevance to this subject matter include Toy (3,525,874), showing a series hybrid using a gas turbine as internal combustion engine; Yardney (3,650,345), showing use of a compressed-air or similar mechanical starter for the internal combustion engine of a series hybrid, such that batteries of limited current capacity could be used; and Nakamura (3,837,419), addressing improvements in thyristor battery-charging and motor drive circuitry. Somewhat further afield but of general interest are the disclosures of Deane (3,874,472); Horwinski (4,042,056); Yang (4,562,894); Keedy (4,611,466); and Lexen (4,815,334); Mori (3,623,568); Grady, Jr. (3,454,122); Papst (3,211,249); Nims et al (2,666,492); and Matsukata (3,502,165). Additional references showing parallel hybrid vehicle drive systems include Froelich (1,824,014) and Reinbeck (3,888,325). 
     U.S. Pat. No. 4,578,955 to Medina shows a hybrid system wherein a gas turbine is used as the internal combustion engine to drive a generator as needed to charge batteries. Of particular interest to certain aspects of the present invention is that Medina discloses that the battery pack should have a voltage in the range of 144, 168 or 216 volts and the generator should deliver current in the range of 400 to 500 amperes. Those of skill in the art will recognize that these high currents involve substantial resistance heating losses, and additionally require that all electrical connections be made by positive mechanical means such as bolts and nuts, or by welding. More specifically, for reasons of safety and in accordance with industry practice, currents in excess of about 50 amperes cannot be carried by the conventional plug-in connectors preferred for reasons of convenience and economy, but must be carried by much heavier, more expensive and less convenient fixed connectors (as used on conventional starter and battery cable connections). Accordingly, it would be desirable to operate the electric motor of a hybrid vehicle at lower currents. 
     U.S. Pat. No. 5,765,656 to Weaver also shows a series hybrid wherein a gas turbine is used as the internal combustion engine; hydrogen is the preferred fuel. 
     U.S. Pat. No. 4,439,989 to Yamakawa shows a system wherein two different internal combustion engines are provided, so that only one need be run when the load is low. This arrangement would be complex and expensive to manufacture. 
     Detailed discussion of various aspects of hybrid vehicle drives may be found in Kalberlah, “Electric Hybrid Drive Systems for Passenger Cars and Taxis”, SAE Paper No. 910247 (1991). Kalberlah first compares “straight” electric, series hybrid, and parallel hybrid drive trains, and concludes that parallel hybrids are preferable, at least when intended for general use (that is, straight electric vehicles may be useful under certain narrow conditions of low-speed, limited range urban driving). Kalberlah then compares various forms of parallel hybrids, with respect to his FIG. 4, and concludes that the most practical arrangement is one in which an internal combustion engine drives a first pair of wheels, and an electric motor the second; more particularly, Kalberlah indicates that mechanical combination of the torque from an internal combustion engine and an electric motor is impractical. 
     Gardner U.S. Pat. Nos. 5,301,764 and 5,346,031 follow Kalberlah&#39;s teachings, in that Gardner shows separately driving at least two pairs of wheels; one pair is driven by a first electric motor, and the second by a second electric motor or alternatively by a small internal combustion engine. Three different clutches are provided to allow various sources of drive torque to be connected to the wheels, and to a generator, depending on the vehicle&#39;s operation mode. The internal combustion engine is run continuously, and provided the driving torque when the vehicle is in a cruise mode; at other times it is used to charge the batteries powering the electric motors. 
     Bullock, “The Technological Constraints of Mass, Volume, Dynamic Power Range and Energy Capacity on the Viability of Hybrid and Electric Vehicles”, SAE Paper No. 891659 (1989) provides a detailed theoretical analysis of electric vehicles in terms of the loads thereon, and a careful analysis of the various battery types then available. Bullock concludes that a vehicle having two electric motors of differing characteristics, driving the wheels through a variable-speed transmission, would be optimal for automotive use; see the discussion of FIG.  8 . Bullock also suggests the use of an internal combustion engine to drive battery charging, but does not address combining the engine&#39;s torque with that from the motors; see pp. 24-25. 
     Further related papers are collected in  Electric and Hybrid Vehicle Technology , volume SP-915, published by SAE in February 1992. See also, Wouk, “Hybrids: Then and Now”; Bates, “On the road with a Ford HEV”, and King et al, “Transit Bus takes the Hybrid Route”, all in  IEEE Spectrum , Vol. 32, 7, (July 1995). 
     Urban et al U.S. Pat. No. 5,667,029 shows two embodiments of parallel hybrids; a first embodiment is shown in FIGS. 1-9 and  11 , and a second in FIGS. 12-17. Both embodiments have numerous common features, including similar operating modes. Referring to the first embodiment, an internal combustion engine provides torque to the road wheels or to a generator; two electric motors can provide torque to the road wheels, or charge batteries during regenerative braking. Torque from the engine and motors is combined at the input shaft to a variable-ratio transmission. Overrunning clutches are provided, e.g., to allow the engine&#39;s torque to be applied to the road wheels without also rotating the motors. 
     As indicated at col.  6 , lines  25 - 54 , certain transitions between various operating modes are made automatically, responsive to the position of the accelerator pedal; for example, if the operator does not depress the pedal beyond a given point, only the internal combustion engine is employed to propel the vehicle; if the operator depresses the pedal more fully, the electric motors are also energized. Other changes in the operational mode must be made by the operator directly; for example, the vehicle may be operated as a “straight electric” vehicle, e.g. for short duration trips, by the operator&#39;s making an appropriate control action. See col.  7 , lines  49 - 56 . 
     The Urban et al design appears to suffer from a number of significant defects. First, the internal combustion engine is stated to provide all torque needed to accelerate the vehicle to cruising speed under normal circumstances (see col.  5 , lines  3 - 10 ), and also to propel the vehicle during cruising (see col.  6 , lines  48 - 54 ). The electric motors are to be used only during rapid acceleration and hill-climbing; col.  5 , lines  10 - 13 . A 20 horsepower engine, operated through a continuously variable-ratio transmission and a torque converter, is stated to be adequate for this purpose. Such components are clearly rather complex and expensive; further, torque converters are notoriously inefficient. Moreover, using the internal combustion engine as the sole source of power for low-speed running would require it to be run at low speeds, e.g., at traffic lights, which is very inefficient and highly polluting. (Various additional references suggest that excess torque can be used to charge batteries; if this were incorporated in the Urban system, the engine might be run at a reasonably efficient output level while the vehicle was stationary, but this would lead to high levels of noise and vibration. In any event Urban does not appear to consider this possibility.) 
     On the other hand, Urban does suggest that the vehicle can be operated as a “straight electric” under low-speed conditions, but this requires the operator to provide an explicit control input; this complexity is unacceptable in a vehicle intended to be sold in quantity, as would be required in order to reach Urban&#39;s stated goals of reduction of atmospheric pollution and reduced energy consumption. As noted, hybrid vehicle operation must be essentially “foolproof”, or “transparent” to the user, to have any chance of commercial success. 
     Urban&#39;s second embodiment is mechanically simpler, employing but a single “dynamotor”, through which torque is transmitted from the engine to the variable-ratio transmission, but suffers from the same operational deficiencies. 
     A second Urban et al U.S. Pat. No. 5,704,440, is directed to the method of operation of the vehicle of the &#39;029 patent and suffers the same inadequacies. 
     Various articles describe several generations of Toyota Motor Company hybrid vehicles, stated soon to be available commercially. See, for example, Yamaguchi, “Toyota readies gasoline/electric hybrid system”,  Automotive Engineering , July 1997, pp. 55-58; Wilson, “Not Electric, Not Gasoline, But Both”, Autoweek, Jun. 2, 1997, pp. 17-18; Bulgin, “The Future Works, Quietly”, Autoweek Feb. 23, 1998, pp. 12 and 13; and “Toyota Electric and Hybrid Vehicles”, a Toyota brochure. A more detailed discussion of the Toyota vehicle&#39;s powertrain is found in Nagasaka et al, “Development of the Hybrid/Battery ECU for the Toyota Hybrid System”, SAE paper 981122 (1998), pp. 19-27. According to the Wilson article, Toyota describes this vehicle as a “series-parallel hybrid”; regardless of the label applied, its powertrain appears to be similar to that of the Berman patents described above, that is, torque from either or both of an internal combustion engine and an electric motor are controllably combined in a “power-split mechanism” and transmitted to the drive wheels through a planetary gearset providing the functionality of a variable-ratio transmission. See the Nagasaka article at pp. 19-20. 
     Furutani U.S. Pat. No. 5,495,906 describes a vehicle having an internal combustion engine driving a first set of wheels through a variable-ratio transmission and an electric motor driving a second set of wheels. The engine is apparently intended to be run continuously; at low speeds, it drives a generator to charge batteries providing energy to the motor, and at higher speeds the engine or both engine and motor propels the vehicle. In some circumstances the transmission may not be required; compare, for example, col.  3 , lines  4 - 8  with col.  5 , lines  59 - 64 . 
     U.S. Pat. No. 5,842,534 to Frank shows a “charge depletion” control method for hybrid vehicles; in this scheme, the internal combustion engine is essentially used only when the state of the batteries is such that the vehicle cannot otherwise reach a recharging point. See col.  3 , lines  50 - 55 . In normal operation, the batteries are recharged from an external power source. Frank also discusses two-mode brake pedal operation, wherein mechanical brakes are engaged in addition to regenerative braking when the pedal is depressed beyond a preset point. 
     U.S. Pat. No. 5,823,280 to Lateur et al shows a parallel hybrid wherein the shafts of an internal combustion engine and first and second electric motors are all coaxial; the engine is connected to the first motor by a clutch, and the first motor to the second by a planetary gearset, allowing the speeds of the motors to be varied so as to operate them in their most efficient range. See col.  4 , line  57  - col.  5 , line  60 . 
     U.S. Pat. No. 5,826,671 to Nakae et al shows a parallel hybrid wherein torque from an internal combustion engine is combined with that from a motor in a planetary gearset; a clutch is provided therebetween. The specific invention relates to sensing of engine warmup conditions, so as to limit emission of unburnt fuel and thus lower emissions. 
     U.S. Pat. No. 5,846,155 to Taniguchi et al shows a parallel hybrid wherein torque from an internal combustion engine and a motor is again combined in a planetary gearset; the specific improvement appears to be the use of a continuously-variable transmission. 
     It will be appreciated by those of skill in the art that there are significant limitations inherent in the use of planetary gearsets as a means for connecting different sources, e.g., an internal combustion engine and an electric motor, to the drive wheels of a vehicle, namely, that unless the planetary gearset is effectively locked (anathematic to its use as a continuously-variable transmission, e.g., in the Toyota vehicle) it is capable of additive combination of shaft speeds, but not of output torque. Hence, the principal advantage of the parallel hybrid drivetrain, additive combination of the output torque of both the electric motor and the internal combustion engine, is only available when the planetary gearset is locked. This fact is acknowledged by Lateur, for example, at col.  6 , line  27 . 
     Additional disclosures of possible interest include U.S. Pat. No. 5,845,731 to Buglione et al; this patent issued Dec. 8, 1998, and therefore is not necessarily available as a reference against the claims of the present invention. The basic powertrain shown by Buglione et al includes an internal combustion engine  12 , coupled through a first clutch  18  to a first electric motor  20 , coupled to a second electric motor  26  through a second clutch  24 ; the wheels are (apparently; see col.  3 , line  8 ) driven by the second motor  26 . The overall hybrid operational scheme provided by Buglione et al is illustrated in FIG.  4 . At low speeds one or both motors may be used to propel the vehicle, with the engine off, idling, or running to drive one motor as a generator. During low-speed cruising the second motor propels the vehicle, while during high-speed cruising, the engine propels the vehicle. When acceleration is required at high speed, the engine and both motors may be used to propel the vehicle. Buglione et al also indicates that a variable-ratio transmission may be unnecessary, col.  3 , line  9 , and that the first motor can be used to start the engine, col.  4 , lines  8 - 15 . 
     U.S. Pat. No. 5,586,613 to Ehsani, showing an “electrically peaking hybrid” vehicle is also of interest. Ehsani&#39;s vehicle is shown in several embodiments; in each, an engine is apparently to be run continuously, with excess torque used to charge the batteries, and one or more motors used to provide additional propulsive torque when the engine&#39;s output torque is inadequate. A transmission is provided in some embodiments of the Ehsani vehicle. An embodiment involving two motors is shown in FIG. 7, and can be modified as discussed in the text at col.  9 , lines  4 - 5 . 
     FIG. 7 itself shows driving a first set of wheels by a first “electric machine”, i.e., a motor capable of operation as a generator. This drive arrangement is independent of a second drive arrangement, whereby a second set of wheels is driven by an engine connected through a first clutch to a second electric machine, connected to the second set of wheels by a second clutch. 
     Ehsani suggests at col.  9 , lines  4 - 5  that the drive shaft otherwise coupled to the first electric machine could also be driven by the engine. Although it is not made explicit that the first electric machine is to be retained, this seems likely; otherwise, the modified FIG. 7 embodiment would be the same as Ehsani&#39;s FIG. 1, modified to have all four wheels driven by a common driveshaft. 
     This application discloses a number of improvements over and enhancements to the hybrid vehicles disclosed in the inventor&#39;s U.S. Pat. No. 5,343,970 (the “&#39;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 &#39;970 patent are applicable to the vehicles shown herein as well. Discussion of the &#39;970 patent herein is not to be construed to limit the scope of its claims. 
     Generally speaking, the &#39;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, according to the operating scheme of the hybrid vehicle disclosed in the &#39;970 patent, 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 &#39;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. 
     It can thus be seen that while the prior art, including the &#39;970 patent, clearly discloses the desirability of operating an internal combustion engine in its most efficient operating range, and that a battery may be provided to store energy to be supplied to an electric motor in order to even out the load on the internal combustion engine, there remains substantial room for improvement. In particular, it is desired to obtain the operational flexibility of a parallel hybrid system, while optimizing the system&#39;s operational parameters and providing a substantially simplified parallel hybrid system as compared to those shown in the prior art, again as including the &#39;970 patent. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide an improved hybrid electric vehicle realizing substantially increased fuel economy and reduced pollutant emissions as compared to present day internal combustion and hybrid vehicles while suffering no significant penalty in performance, operating convenience, cost, complexity, or weight. 
     It is an object of the invention to provide an improved hybrid electric vehicle realizing substantially increased fuel economy and reduced pollutant emissions as compared to present day internal combustion and hybrid vehicles, which can be operated efficiently by an operator accustomed to conventional vehicles without special training, and while not requiring modification of the existing infrastructure developed over the years to support conventional vehicles. 
     More specifically, it is an object of the invention to provide such an improved vehicle that operates on fuel now widely available and uses batteries already well understood and widely available, so that the operator need not learn new driving techniques, deal with new fuel supply arrangements, nor be obliged to be attentive to maintenance of batteries employing complex new technologies. 
     It is a more particular object of the present invention to provide an improved series-parallel hybrid electric vehicle wherein an internal combustion engine and two separately-controlled electric motors can separately or simultaneously apply torque to the driving wheels of the vehicle, controlled to realize maximum fuel efficiency at no penalty in convenience, performance, or cost. 
     It is a further object of the invention to provide a series-parallel hybrid electric vehicle comprising two electric motors together providing output power equal to at least 100 percent of the rated output power of the internal combustion engine, and more preferably up to about 150-200 percent thereof, so that the engine operates under substantially optimum conditions in order to realize substantial fuel economy and reduced emission of undesirable pollutants in operation. 
     More particularly, it is an object of the invention to provide a series-parallel hybrid electric vehicle wherein the internal combustion engine is sized to efficiently provide the average power required for operation at moderate and highway speeds, with two separately-controlled electric motors together sized to deliver the additional power needed for acceleration and hill climbing. 
     Still another object of the invention is to provide a series-parallel hybrid electric vehicle wherein the electric motor and battery charging circuits operate at no more than about 30-50 amperes continuous current (although significantly greater currents may flow for short periods, under peak load conditions), whereby resistance heating losses are greatly reduced, and whereby inexpensive and simple electrical manufacturing and connection techniques can be employed. 
     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 &#39;970 patent, while providing the functional advantages of the hybrid vehicle shown in the &#39;970 patent. 
     It is another specific object of the invention to provide a control strategy that controls the hybrid drivetrain to respond differently to given instantaneous control inputs depending on recent control inputs, for example, to distinguish between gentle and rapid depressing of the accelerator pedal by the operator. 
     It is a more specific object of the invention to employ the control flexibility provided by the improved hybrid drive train of the invention to allow starting of the engine at comparatively high RPM, while controlling the fuel:air mixture supplied during starting, throttling the engine, and providing a preheated catalytic converter, minimizing emission of unburned fuel and further improving fuel economy. 
     It is a more specific object of the invention to employ the control flexibility provided by the improved hybrid drive train of the invention to allow employment of a motor producing constant torque up to a base speed, and constant power thereafter, as the engine starting motor, so that torque produced thereby can also be used to propel the vehicle. 
     Other aspects and objects of the invention will become clear as the discussion below proceeds. 
     SUMMARY OF THE INVENTION 
     As discussed above, the &#39;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. See FIGS. 3-11 thereof. 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 m microprocessor. 
     According to the present invention, the controllable torque-transfer unit shown in the &#39;970 patent is eliminated by replacing the single electric motor shown the rein by two separate motors, both operable as generators and as traction motors when appropriate. See FIGS. 3 and 4 hereof. The engine is connected to the drive wheels by a clutch operated by the microprocessor responsive to the vehicle&#39;s mode of operation and to input commands provided by the operator of the vehicle. As in the &#39;970 patent, an internal combustion engine is provided, sized to provide sufficient torque to be adequate for the range of cruising speeds 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. 
     According to the present invention, 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 (e.g., 60-200 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 speeds (300-1500 rpm) for starting; this allows starting the engine with a much less fuel-rich fuel:air mixture than is conventional, significantly reducing undesirable emissions and improving fuel economy at start-up. A catalytic converter provided to catalytically combust unburnt fuel in the engine exhaust is preheated to an effective working temperature before starting the engine, further reducing emissions. 
     In the embodiment discussed in detail, the two motors are controllably connected by a clutch for transfer of torque. The motor shafts are controlled to rotate at essentially the same speed when the clutch is engaged, and to bear a predetermined angular relationship, so that the clutch need not allow for extensive slip therebetween; by comparison, the clutches normally provided for road vehicles must slip sufficiently to allow the engine to propel the vehicle from rest. A clutch capable of such extensive slipping is normally not required according to the invention, and a less-expensive simple mechanical interlock may alternatively be employed. Engagement of the clutch is controlled by the microprocessor, e.g., controlling an electrical or 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 “straight” electric car, i.e., power is drawn from the battery bank and supplied to the traction motor. Should the batteries become relatively depleted (e.g., become discharged to 70% of full charge), 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. 
     Similarly, when 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; when it reaches an engine speed at which it produces useful torque, the clutch is engaged, so that the engine and starter motor can provide additional torque. As noted above, the engine is rotated at relatively high speed for starting, so that the engine rapidly reaches a useful speed. 
     As in the 1970 patent, the engine is sized so that it provides sufficient power to maintain the vehicle in a range of suitable highway cruising speeds, 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&#39;s kinetic energy into electrical power during descent or deceleration. Also as in the &#39;970 patent, the peak power of the two motors together at least equals the rated power of the engine, as is necessary to provide good performance without employment of a variable-speed transmission or the equivalent. 
     In each of these aspects of the operation of the vehicle, and as in the &#39;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. 
     It is also within the scope of the invention to operate one or both of the motors at differing rotational speeds than the engine, so that each can be optimized for the demands thereon. More specifically, motors can in general be made smaller if they can be operated at relatively high RPM. Motors operating at up to 9000-15,000 RPM appear appropriate for the present application. However, operating the internal combustion engine at this speed would likely lead to undesirable levels of noise and vibration, and might constrain its performance characteristics in an undesirable manner. Accordingly, for example, the starter motor might drive the engine through a pinion geared to a larger 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 chain, belt, or gear reduction unit. It is also within the scope of the invention, as noted above, to operate the engine and the two motors 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. 
     However, in all cases, the rotational speeds of the two motors and the engine are fixed with respect to one another, and to the speed of the road wheels; 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 various 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 phases and the accompanying power circuitry provides current of more than three, preferably five, phases, allowing the vehicle to function even after failure of one or more components. 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 &#39;970 patent; that is, the motors should produce constant torque up to a base speed 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 1. 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. 
     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&#39;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&#39;s torque requirements exceed the engine&#39;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&#39;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&#39;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&#39; state of charge. 
     The vehicle is operated in different modes, depending on its instantaneous torque requirements, and the state of charge of the battery, and other operating parameters. The mode of operation is selected by the microprocessor in response to a control strategy discussed in detail below; the values of the sensed parameters in response to which the operating mode is selected may vary depending on recent history, or upon analysis by the microprocessor of trips repeated daily, and may also exhibit hysteresis, so that the operating mode is not repetitively switched simply because one of the sensed parameters fluctuates around a defined setpoint. 
     The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood if reference is made to the accompanying drawings, in which: 
     FIG. 1 is a plot of output power versus rotational speed (RPM) for a typical internal combustion engine, illustrating the relative fuel consumption of the engine as used in a conventional automobile in gallons/horsepower-hour; 
     FIG. 2 is a similar plot describing operation of a relatively small internal combustion engine used in the present invention under circumstances similar to those depicted in FIG. 1; 
     FIG. 3 shows a schematic diagram of the principal components of a first embodiment of the hybrid vehicle drive system according to the invention; 
     FIG. 4 shows a block diagram of the principal components of the drive system of the invention in a second embodiment, differing in certain mechanical arrangements from that of FIG. 3, and illustrating various control signals provided in both embodiments; 
     FIG. 5 shows a partial schematic diagram of the battery bank, inverter, and motor circuitry; 
     FIG. 6 is a diagram illustrating differing modes of vehicle powertrain operation, plotted on a three dimensional chart, illustrating that the mode of vehicle operation is a function of the state of charge of the battery bank, the instantaneous road load, and time; 
     FIG. 7, comprising FIGS. 7 ( a )-( c ), and extending over two sheets, is a timing diagram showing road load, engine torque output, the state of charge of the battery bank, and engine operation as functions of time, thus illustrating a typical control strategy employed during low-speed city driving, highway cruising, and extended high-load driving; 
     FIG. 8, comprising FIGS. 8 ( a )-( d ), are diagrams indicating the flow of torque and of energy among the components of the hybrid powertrain of the invention, in various modes of operation; 
     FIG. 9 is a simplified flow chart of the algorithm employed by the microprocessor to implement the control strategies provided by the vehicle according to the invention; 
     FIG.  9 ( a ) is a flow chart of an engine starting subroutine employed in the flowchart of FIG. 9; 
     FIG.  9 ( b ) is an alternate version of one of the steps of the flowchart of FIG. 9, implementing a modification to the vehicle control strategy; 
     FIG.  9 ( c ) is an alternate version of another of the steps of the flowchart of FIG. 9, similarly implementing a modification to the vehicle control strategy; and 
     FIG. 10 illustrates the preferred torque versus speed characteristics of the electric starting and traction motors, and of the internal combustion engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring specifically to FIG. 1, which is reproduced here from the &#39;970 patent for convenience, curve  10  represents the output power versus engine speed (RPM) of a typical spark ignition gasoline-fueled internal combustion engine as used with an automatic transmission in a typical sedan of 3,300 pounds. As can be seen, the maximum engine power available is about 165 horsepower at about 5,000 RPM. Also shown in FIG. 1 by curve  12  are the average power requirements of such a vehicle. Points C, S and H on curve  12  show average fuel consumption in city, suburban and highway driving, respectively. Point C on curve  12  shows that the average power required in typical city driving is less than 5 hp. Point S shows that the average power consumed in suburban driving is 10 hp, and point H shows that the power needed for steady-speed highway driving is only about 30 hp. Thus, the vehicle is vastly overpowered at all times except during acceleration or hill-climbing. 
     FIG. 1 also includes curves indicating the relative fuel consumption of the engine. As can be seen, reasonable fuel efficiency, that is, at least about 105 percent relative fuel consumption (100% being ideal), is reached only when the engine is operated at between about 2,000 and 4,000 RPM, and when producing between about 75 and 150 horsepower. FIG. 1 thus indicates that the typical internal combustion engine is operated with reasonable efficiency only when producing between about 50 and about 90% of its maximum output power. The typical automobile only requires such substantial power under conditions of extreme acceleration or hill climbing. Thus, only during relatively brief intervals is the engine&#39;s output torque used efficiently; more specifically, at lower power outputs, losses due to friction and pumping consume larger fractions of the engine&#39;s total torque, so that a lower fraction is available to propel the vehicle. As can be seen, during typical highway driving, shown by point H on curve  12 , the relative fuel consumption is on the order of 190 percent of that required during the most efficient operation of the engine. The situation is even worse in suburban driving, where the relative fuel consumption is nearly 300 percent of the most efficient value, and in city driving, where the relative fuel consumption is almost 350 percent of that required at most efficient operation. 
     FIG. 1 thus demonstrates that an internal combustion engine having sufficient horsepower for adequate acceleration and hill climbing capability must be so oversized with respect to the loads encountered during most normal driving that the engine is grossly inefficient in its consumption of fuel. As noted, FIG. 1 further shows that only about 30 horsepower is needed to cruise on the highway even in a relatively large car. 
     FIG. 2 (again reproduced from the &#39;970 patent for convenience) is similar to FIG. 1, and illustrates the operational characteristics of the same 3,300 pound car if driven by a relatively small engine having a maximum horsepower rating of about 45 horsepower at 4,000 RPM. The power requirement of the vehicle during highway cruising, shown by point H on curve  14 , is in the center of the most efficient region of operation of the engine. However, even with this small engine thus optimized for highway cruising, there is a substantial gap between the engine operating power line  16  and the average power requirement line  14 . That is, even this small engine produces substantially more power at low RPM than needed for city driving (point C) or for suburban driving (point S). Accordingly, even with a small engine sized appropriately for highway cruising, substantial inefficiencies persist at lower speeds. Moreover, of course, such a vehicle would have unsatisfactory acceleration and hill climbing ability. Therefore, the answer is not simply to replace large internal combustion engines with smaller internal combustion engines. 
     The prior art recognizes that there are substantial advantages to be gained by combining the virtues of a gasoline or other internal combustion engine with those of an electric motor running from a battery charged by the internal combustion engine. However the prior art has failed to provide a solution which is directly price and performance competitive with vehicles now on the market. 
     As indicated above, “straight” electric vehicles, that is, vehicles having electric traction motors and batteries requiring recharge at the end of each day&#39;s use, do not have sufficient range and require too much time to recharge to fully replace conventional automobiles. Further, the operational costs of such vehicles are not competitive with internal combustion vehicles operated on fuels derived from renewable resources such as ethanol, and are even less competitive with gasoline-fueled automobiles. 
     A first type of series hybrid vehicles, involving a gasoline engine driving a generator charging a battery powering an electric traction motor, are limited in acceleration and hill climbing ability unless the electric motor is made very large, costly, and bulky. The alternative series hybrid approach, involving a transmission between a relatively smaller electric motor and the wheels to provide the torque needed to accelerate quickly, loses the virtue of simplicity obtained by elimination of a multi-speed transmission. These vehicles fail to realize the advantages provided by the parallel hybrid system in which both an internal combustion engine and an electric motor provide torque to the wheels as appropriate. 
     However (apart from the &#39;970 patent) the prior art relating to parallel hybrid vehicles fails to disclose a system sufficiently simple for economical manufacture. The art further has failed to teach the optimum method of operation of a parallel hybrid vehicle. Moreover, the art relating to parallel hybrids (again, apart from the &#39;970 patent) does not teach the appropriate operational parameters to be employed, relating to the relative power outputs of the internal combustion engine and the electric motor; the type of electric motor to be employed; the frequency, voltage, and current characteristics of the motor/battery system; the proper control strategy to be employed under various conditions of use; and combinations of these. 
     According to one aspect of the invention of the &#39;970 patent, the internal combustion engine of a hybrid vehicle is sized to supply adequate power for highway cruising, preferably with some additional power in reserve, so that the internal combustion engine operates only in its most efficient operating range. The electric motor, which is substantially equally efficient at all operating speeds, is used to supply additional power as needed for acceleration and hill climbing, and is used to supply all power at low speeds, where the internal combustion engine is particularly inefficient, e.g., in traffic. 
     As indicated above, this application discloses certain modifications, improvements, and enhancements of the hybrid vehicles shown in the inventor&#39;s U.S. Pat. No. 5,343,970; where not otherwise stated, the design of the vehicle of the present invention is similar to that shown in the &#39;970 patent. Components commonly numbered in this application and the &#39;970 patent are functionally similar in the corresponding systems, with detail differences as noted. The advantages of the system shown in the &#39;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. 
     As shown in the &#39;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 &#39;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 approximate 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 &#39;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 frictional losses. According to one aspect of 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. 
     In this connection, while the term “microprocessor” is used throughout the present application, it is to be understood that this term as used herein includes various types of computerized control devices not always referred to as “microprocessors” per se, such as computers themselves incorporating microprocessors, digital signal processors, fuzzy logic controllers, analog computers, and combinations of these. In short, any controller capable of examining input parameters and signals and controlling the mode of operation of the vehicle according to a stored program, as discussed below in detail, is considered to be a “microprocessor” as used herein. Furthermore, the electronic fuel injection and electronic engine management devices shown in FIGS. 3 and 4 as separate elements might also be integrated within the “microprocessor” as used herein. 
     FIG. 3 of the present application shows a first embodiment of the present invention; FIG. 4, discussed below, shows a second embodiment illustrating certain alternative mechanical arrangements; overall the two embodiments are very similar, and functionally they are substantially identical. 
     In the FIG. 3 embodiment, 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, connected between the motors and battery bank  22 . At present, essentially conventional lead-acid batteries are preferred for battery bank  22 , since these are widely available and understood. More advanced batteries may be used if and when they become widely available and economically competitive. 
     Motors  21  and  25  are controllably connected for torque transfer by a clutch  51 , mechanically interlocking the shafts  15  and  16  of motors  21  and  25  respectively. As discussed further below in connection with FIG. 4, 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 . Accordingly, clutch  51  need not be an ordinary automotive friction clutch (as illustrated schematically in FIG.  1 ), as conventionally provided to allow extensive relative slipping before the shafts are fully engaged. More particularly, as slipping of clutch  51  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 (as shown in FIG.  4 ), 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. 
     Clutch  51  is operated by microprocessor  48 , e.g., through a known electric or hydraulic actuator  53 , together with the other components of the system, in accordance with the operational state of the vehicle and the operator&#39;s input commands. 
     The respective positions of motor  21  and engine  40  with respect to clutch  51 , motor  25 , and wheels  34  could be reversed as compared to their positions in FIGS. 3 and 4 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. 
     As shown in FIG. 4, 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  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. 
     Thus being provided with signals indicative of the rotational speeds of shafts  15  and  16 , microprocessor  48  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.  3 ), as conventionally provided to allow extensive slipping before the shafts are fully engaged. According to this aspect of the invention, and particularly if microprocessor is made capable of ensuring that shafts  15  and  16  bear a desired relative angular relationship, clutch  51  instead may be a simple, relatively inexpensive self-aligning mechanical interlock (as illustrated schematically in FIG.  4 ), wherein positive mechanical connection is made between the shafts  15  and  16  upon engagement. 
     FIG. 4 also shows additional signals provided to microprocessor  48  in both the FIG.  3  and the FIG. 4 embodiments. These include operator input commands, typically acceleration, direction, and deceleration commands, as shown. The acceleration and deceleration commands may be provided by position-sensing encoders  71  and  72  (which could be configured as rheostats, Hall-effect sensors, or otherwise) connected to microprocessor  48  by lines  67  and  68 , to inform the microprocessor of the operator&#39;s commands responsive to motion of accelerator and brake pedals  69  and  70  (FIG. 3) 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 operator may also provide a “cruise model” signal, as indicated, when a desired cruising speed has been reached. The microprocessor uses this information, and other signals provided as discussed herein, in accordance with the operational strategy discussed in detail below in connection with FIGS. 6-9, to properly control operation of the vehicle according to the invention by appropriate control signals provided to its various components. 
     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 pedal  69  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 an amount of torque that can efficiently be provided by engine  40  will be required; microprocessor will then initiate the sequence whereby starting motor  21  will be used to start engine  40 . 
     Upon initiation of the engine starting sequence, a heater  63  (FIG. 3) will first 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 and subsequent running of the engine  40  will be catalytically combusted, reducing emission of undesirable pollutants. As noted above, engine starting is performed with the engine running at a higher speed than is conventional, so that a the fuel:air ratio need only be slightly (e.g., 20%) richer than stoichiometric. The engine is still throttled during starting, to control the total amount of air drawn in. By comparison, in convetional vehicles, very significant fraction of the total pollutants emitted during a given trip are emitted during the first 30-60 seconds of operation, due to the extremely rich mixtures normally supplied during starting, and to the ineffectiveness of the catalyst until it has been heated by the exhaust. 
     If the operator depresses the pedal  69  rapidly, indicating an immediate need for full acceleration, the preheating step may be omitted; however, a preferable alternative may be to allow the traction and starting motors to be driven at or slightly beyond their rated power, providing adequate torque, for a short time sufficient to allow the catalyst to be warmed and the engine started. Further details concerning the operation of the vehicle so as to reduce emissions to ultra-low levels are found in copending provisional application Ser. No. 60/122,477, filed Mar. 1, 1999, incorporated herein by reference. 
     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 , and/or presses brake pedal  70  beyond a predetermined point, both mechanical and regenerative braking will be provided. Mechanical braking is also provided on long downhills when the batteries are fully charged, and in case of emergency. 
     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 (see the high-level flowchart of an exemplary control program provided as FIG.  9 ), 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 in FIG. 4, the control signals provided to inverter/chargers  23  and  27  by microprocessor  48  allow control of the current (represented as I) to be provided, of the direction of rotation of the motor  25  (represented as +/−), allowing reversing of the vehicle, and the frequency of switching (represented as f), 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 . Inverter/charger operation is discussed further below in connection with FIG.  5 . 
     As noted above, the FIGS. 3 and 4 embodiments of the system of the invention differ in certain mechanical arrangements, intended to illustrate variations within the scope of the invention, and FIG. 4 also provides more detail concerning the specific control signals passing between various elements of the system. 
     Referring to the differing mechanical arrangements, it will be observed that in FIG. 3 the shafts of motors  21  and  25  are illustrated as coaxial with that of engine  40 ; this is the simplest arrangement, of course, but would require the engine  40  and starter motor  21  to rotate at the same speed at all times, and at the same speed as traction motor  25  when clutch  51  is engaged. As noted above, it may be preferable to design motors  21  and  25  to have maximum speeds of 9000-15,000 rpm, so that they could be made smaller, lighter, and less costly than slower-rotating motors. However, it is envisioned that a preferred maximum speed for engine  40  is 6000 rpm; internal combustion engines running at substantially higher speeds wear rapidly and tend to have limited torque at low speed; higher frequency engine noise and vibration can also be difficult to absorb. It is within the scope of the invention to provide the motors coaxial with the engine shaft, as illustrated in FIG. 3, but to provide a planetary gearset(s) between the shafts of either or both of traction motor  25  and starting motor  21  and the output shaft to permit differing engine and motor speeds. 
     FIG. 4 illustrates an alternative construction, also permitting differing engine and motor speeds. In this case, the output shaft of starting motor  21  is shown connected to that of engine  40  by spur gears and traction motor  25  is connected to the output shaft  55  by drive indicated at  54 . Numerous other arrangements will occur to those of skill in the art. However, in each case there is no variable-ratio transmission between the sources of torque—that is, the motors  21  and  25 , and the engine  40 —and the road wheels  34 . 
     Additional possible embodiments of the hybrid vehicle drivetrain according to the invention abet forth in copending provisional application Ser. No;. 60/122,478, filed Mar. 1, 1999 incorporated herein by reference. More specifically, it is within the scope of the invention to connect the traction motor to one set of wheels, and to connect the combination of the engine  40  and starting motor  21  to another set of wheels through clutch  51 , thus providing a four-wheel drive vehicle with differing power sources for the alternate pairs of wheels. In this embodiment, the torque from the traction motor  25  is effectively combined with that from engine  40  (and from starting motor  21 , when used as a source of propulsive torque) by the road surface, rather than by mechanical connection, as in the FIGS. 3 and 4 embodiment. Both embodiments are within the scope of the invention, and the control strategy is essentially the same as to both. 
     Other elements of the system as illustrated in FIGS. 3 and 4 are generally as discussed in the &#39;970 patent, including supply of fuel  36  from tank  38 , air filter  60 , and throttle  61 . 
     Control of engine  40  by microprocessor  48  is accomplished by way of control signals provided to electronic fuel injection (EFI) unit  56  and electronic engine management (EEM) unit  55 ; control of starting of engine  40 , and using either or both of starting motor  21  and traction motor  25  as motors, providing propulsive torque, or as generators, providing recharging current to battery bank  22 , by microprocessor  48  is accomplished by way of control signals provided to inverter/charger units  23  and  27 . 
     Under deceleration, for example, during descents, or as needed for braking, or when the engine&#39;s instantaneous torque output exceeds the vehicle&#39;s current torque requirements, either or both of motors  21  and  25  are operated as generators, proving regenerative recharging of battery bank  22 . FIG. 7, discussed below, illustrates this aspect of the operation of the vehicle of the invention in further detail. 
     Thus, as indicated above, when microprocessor  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  69 , 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 (as determined by monitoring the operator&#39;s commands), 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, that is, as in the &#39;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. It is also known to control auxiliary motors, such as conventional starter motors, to absorb or add torque to that provided by an associated internal combustion engine, to damp out vibration caused by fluctuation of the torque provided by the engine; doing so herein using either or both of motors  21  and  25  is within the scope of the invention, and is simplified by virtue of the direct connection of the engine  40  to the drive wheels through motors  21  and  25  according to the invention. 
     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 300 and 2000 rpm, as compared to the 60-200 rpm starts conventionally provided. As is generally known in the art (see Simanaitis, “What goes around comes around”,  Road &amp; Track , November 1998, p. 201) high-rpm starting allows substantial 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, particularly from cold. 
     More particularly, in conventional low-rpm starts, a rich mixture comprising up to on the order of 6 to 7 times the stoichiometric amount of fuel is provided, to ensure that some fraction of the fuel is in the vapor phase, as only fuel in the vapor phase can be ignited by a spark. Most of the excess fuel condenses as liquid on the cold cylinder walls, and thus does not burn efficiently, if at all, and is immediately emitted unburned. By comparison, at high starting speeds according to the invention, turbulence in the combustion chamber is sufficient to ensure the presence of vapor, so that a near-stoichiometric mixture, typically including only 1.2 times the stoichiometric amount of fuel, can be provided to engine  40  during the starting phase. 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. 
     Furthermore, 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 to an effective working temperature of at least about 350° C. before starting the engine, to prevent even this relatively small emission of unburned fuel. See copending provisional application Ser. No. 60/122,477, filed Mar. 1, 1999 incorporated herein by reference, for further details of ultra-low emission engine starting strategies that may be employed according to the invention. 
     Thus, the primary consideration in selecting the torque of starting motor  21  is that it be capable of rotating the engine  40  at about 300-2000 rpm for starting, and that it be capable of accepting at least about 30% of the engine&#39;s maximum torque output when operated as a generator, so that the engine can be efficiently employed when charging the battery bank during extended low-speed operation; the main consideration in specification of the torque of engine  40  is that it provides sufficient power for highway cruising while being operated at high efficiency, i.e., that its maximum power output be sufficient to cruise in 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 &#39;970 patent, the total torque provided by motors  21  and  25  should be at least equal to that produced by engine  40 , in order to provide adequate low-speed performance under motor alone, and without necessity of a variable-ratio transmission. 
     At the same time, motors  21  and  25  are also sized to be capable of recovering almost all of the vehicle&#39;s kinetic energy when operated as generators in the regenerative braking mode. A particularly high fraction of the vehicle&#39;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 the rated power produced continuously by the engine, but to the rated peak power of the motors, as is generally conventional in the art. Further, the motors are specified assuming the direct-drive embodiment of FIG. 3; if the motors run at higher speeds, their ratings would be reduced accordingly. 
     Engine  40 : 40 to 50 horsepower at 6000 rpm 
     Starting motor  21 : 10-15 horsepower at approximately 1500 rpm and higher speeds 
     Traction motor  25 : 50-75 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 75-100 horsepower. 
     In both cases, the total power available from the electric motors together should equal, and preferably exceeds, the maximum power available from the engine. 
     In the hybrid vehicle of the invention, which as noted does not require a complex, heavy, and costly variable-ratio transmission, these components would provide acceleration much superior to that of typical similarly-sized automobiles of United States manufacture, together with far better fuel economy and substantially reduced emission of pollutants. 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 controlling them in response to control signals from the microprocessor (as discussed further below), should be chosen and operated such that the motors have torque output characteristics varying as a function of rpm as illustrated by curve A in FIG.  10 . That is, the motors are operated by the inverter/chargers, in response to control signals from the microprocessor, so as produce constant torque up to a base speed C, typically 1500 rpm for a motor having a top speed of 6000 rpm, as employed in the direct-drive embodiment of FIG. 3, and should produce constant power at higher speeds; accordingly, the torque drops off at speeds above the base speed C, as shown. The ratio of the base to maximum speed, 4:1 in this example, can vary between about 3 to 1 and about 6 to 1. This torque output characteristic essentially allows the vehicle of the invention to provide quite acceptable performance, especially acceleration, without the weight, complexity and cost of a variable-ratio transmission. 
     By comparison, the series-wound DC motors conventionally used as automotive 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. 
     FIG. 10 also shows the torque curve of a typical internal combustion engine at B; as noted, the torque is zero at zero rpm, so that a clutch is required. FIG. 10 shows at D typical curves for torque as measured at the wheels of a vehicle propelled by a typical internal combustion engine driving the vehicle through a four-speed transmission, used to provide additional torque at low speeds; the vertical spaces between sections of curve D represent changes in gear ratio, that is, the vehicle will be shifted within the cross-hatched regions shown between the sections of curve D. The desired torque characteristics of the starting and traction motors discussed above allow the vehicle of the invention to provide low-speed performance comparable to or better than a conventional vehicle, while eliminating the necessity of a variable-ratio transmission. 
     The ratio between the base speed and maximum speed of the motors as thus described is thus comparable to the ratio between the lowest and highest gears of a conventional transmission; for passenger cars, the latter ratio is typically between 3 and 4:1, so that the engine&#39;s torque is relatively well matched to the road load over a reasonable range of road speeds. (“Overdrive” top gears are sometimes provided, to reduce engine speed during highway driving, broadening this range somewhat, but typically do not permit adequate acceleration.) Of course, if it is desired to employ the hybrid vehicle powertrain and control strategy of the invention in a vehicle requiring a wider range of speeds (e.g., heavy trucks may have multiple gearboxes, permitting the driver to choose between 18 or more gear ratios, allowing the engine&#39;s maximum torque to be available at a very wide range of road speeds), it is within the scope of the invention, where not excluded by the appended claims, to also provide a variable-ratio transmission. However, this should not be necessary with respect to passenger cars, light trucks, and similar vehicles. 
     As discussed above, while it is within the scope of the invention to operate the motors  21  and  25  and the internal combustion engine  40  at the same maximum speed, so that no gearing is required to couple these elements, it is presently preferred that the maximum speeds of the motors  21  and  25  be at least 150% of the maximum speed of the internal combustion engine  40 , and possibly significantly higher. The maximum speed of the internal combustion engine is preferably limited to on the order of 6000 rpm to limit wear, noise and vibration, which increase with higher operating speeds, and because engines capable of higher-rpm operation tend to have narrow ranges of rpm within which they produce substantial torque; the latter characteristic would be undesirable in a vehicle not having a variable-ratio transmission and intended to cruise powered solely by the internal combustion engine, according to the invention. 
     By comparison, operating the motors  21  and  25  at maximum speeds of 9000-15,000 rpm allows them to be made smaller, lighter, and less costly; whether this advantage overcomes the added complexity of chain, gear, or belt drives, or other mechanical means allowing combination of torque from the motors with that from the engine, is a matter of engineering choice that may vary from one model of vehicle to the next. Both are accordingly within the present invention. If each of the torque-producing components (that is, engine  40  and starting and traction motors  21  and  25 ) is to be operated at the same speed, a maximum speed of approximately 6000 rpm is preferred, as this represents a good compromise between cost, weight, and size of the key components. 
     In either case, no variable-ratio transmission between the torque-producing components and the road wheels is provided. The speed of rotation of the torque-producing components is reduced to a suitable speed for the road wheels by gear or belt drive reduction in (or incorporated with) differential  32 ; the reduction ratio can be chosen to provide substantially any desired top speed. 
     As discussed above, it is preferred that motors  21  and  25  have more than two poles, and be operated by current applied over more than three phases, so that failure of some components—such as the power semiconductors used in the inverter/charger units, as discussed below—can be tolerated without total failure of the vehicle. It is also desired that the battery bank be divided into two, with the vehicle chassis connected between them, halving the voltage between given components and the vehicle chassis, and thus simplifying their construction, insulation, and connection. FIG. 5 shows a partial schematic diagram of a circuit providing these attributes. 
     The functions of the inverter/chargers  23  and  27  (separate inverter/chargers being required to allow independent operation of motors  21  and  25 ) include control of motors  21  and  25  to operate as motors or as generators; operation of traction motor  25  in the opposite direction for reversing the vehicle; conversion of DC stored by the battery bank to AC for motor operation; and conversion of AC induced in the motors when operated as generators to DC for battery charging. Essentially similar functions were provided by the solid-state switching AC/DC converter  44  in the &#39;970 patent; where not specified to the contrary, the discussion thereof is applicable to the inverter design shown in FIG. 5 hereof. 
     As illustrated in FIG. 5, traction motor  25  is embodied as a five-phase AC induction motor; starting motor  21 , which is not fully illustrated, as indicated, can be but is not necessarily generally similar. Other motor types, such as permanent magnet brushless DC motors or synchronous motors, might also be employed. The motors are operated as multiphase devices, having three phases or more, permitting employment of smaller and overall less costly semiconductors, and allowing operation even if some of the semiconductors fail. Use of motors operated at relatively high frequency, e.g., more than 60 Hz, also permits motors of a given power output to be smaller. As shown in FIG. 5, it is currently preferred that at least traction motor  25  be wired in the “wye” arrangement shown, rather than the “delta” arrangement shown in Provisional Application 06/100,095; it is found that certain undesirable harmonics are reduced by the “wye” arrangement. Both are well known in the art, and both are within the scope of the invention. 
     As illustrated in FIG. 5, each of the windings  78  of motor  25  is connected to a pair of semiconductor switching elements  80  collectively making up inverter/charger  27 . Inverter/charger  27  is correspondingly configured as a set of ten power semiconductors  80  controlled by switching signals A through J provided by a pulse generator  88  responsive to frequency, polarity and current signals received from microprocessor  48  (FIGS.  3  and  4 ). Typical operating frequencies can be up to 200, 400 or 600 Hz; the transfer of power between the battery bank  22  and motors  21  and  25  is then controlled by pulse-width modulation, that is, by controlling the semiconductors  80  to conduct during portions of the power waveform, the duration of the conducting portions varying in accordance with the power required. Semiconductors  80  may be any type suitable for handling relatively high voltages and currents; satisfactory insulated-gate bipolar transistors (IGBTs) are currently available and are presently preferred. As conventional, each of the semiconductors  80  is paralleled by a freewheeling rectifier diode  82 . 
     Design of the inverter/chargers  23  and  27 , and of pulse a generator  88  to provide suitable control signals A through T so that the inverter/chargers perform the functions listed above, is within the skill of the art; again, see, for example, Bose, “Power Electronics and Variable Frequency Drives”, IEEE, 1996. 
     The current drawn from the battery bank  22  during long-term operation of the traction and starting motor(s) to propel the vehicle should be limited to 30-50 amperes, to reduce the size of the conductors and other components required, as discussed in the &#39;970 patent; these components are satisfactory to carry currents of up to 200 amperes, as may be encountered during full-power acceleration, as this condition will not persist for more than 10-15 seconds. 
     As indicated, the battery bank  22  comprises two substantially similar battery assemblies  84 ; in one embodiment, each battery assembly will comprise eight 48-volt batteries, such that 384 volts is provided by each. The battery assemblies  84  are connected in series, so that 768 volts are provided across the circuit “rails”  86 ,  88 . However, the vehicle chassis connection is taken from between the series-connected battery assemblies, so that only 384 volts is present between any given circuit component and the vehicle chassis; this “center-point-chassis” connection significantly reduces various insulation and heat-sinking requirements. More specifically, the conductors, connectors, relays, switches and like elements can be as approved by the National Electrical Manufacturers&#39; Association (NEMA) for 600 volt service; such elements are widely available, and are much more easily employed and much less expensive than those needed for continuously carrying current at, for example, 300 volts and 300 amperes. 
     Preferably, as indicated by FIG.  5 ( a ), illustrating a detail of a portion of one of the battery assemblies  84 , the 48-volt batteries  85  are connected by normally-open relays  87 , so that the batteries  85  are isolated from one another under fail-safe conditions; for example, if the vehicle is involved in an accident, power to the relays is cut off, so that the maximum open voltage anywhere in the vehicle is 48 volts, reducing the danger of fire. Similarly, the relays open when the vehicle&#39;s “ignition” is shut off by the operator. 
     Turning now to detailed discussion of the inventive control strategy according to which the hybrid vehicles of the invention are operated: as in the case of the hybrid vehicle system shown in the &#39;970 patent, and as discussed in further detail below, 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. In the following, the relationships between these modes are illustrated using several different techniques, to ensure the reader&#39;s full understanding of various aspects of the vehicle control strategy; some of these are seen more clearly in one form of illustration than another. 
     FIG. 6 illustrates the several modes of vehicle operation with respect to the relationship between the vehicle&#39;s instantaneous torque requirements or “road load”, the state of charge of the battery bank  22 , and time, while FIG. 7 shows variation in, and the relationship between, road load, engine torque output, and the state of charge of the battery bank over time, that is, during an exemplary trip. FIGS.  8 ( a )-( d ) show simplified schematic diagrams of the vehicle of the invention in its principal modes of operation, showing the flow of energy, in the form of electricity or combustible fuel, by dot-dash lines, and the flow of torque by dashed lines. Finally, FIG. 9 provides a high-level flowchart, showing the principal decision points in the algorithm according to which the microprocessor operates the various components of the hybrid vehicle drivetrain according to its the invention, and FIGS. 9 ( a )-( c ) show details and modifications thereof. 
     As noted, the preferred control strategy of the invention is illustrated in several different ways by FIGS. 6-9. The same specific numerical examples for various significant control variables, data items, and the like are used throughout for clarity. It will be understood that these examples would normally be expressed as ranges; although ranges are not used in the following, to simplify the discussion, it should be understood throughout that these numerical examples are exemplary only, and that the invention is not to be limited to the exact values of the control variables mentioned herein. 
     Further, it should be realized that certain of these control variables need not be restricted to specific numbers; in some cases, the decision points may be “fuzzy”, i.e., so-called “fuzzy logic” may be employed, so that while the operating scheme retains its overall characteristics, the specific values against which the control variables and data items are tested in implementation of the control strategy according to the invention may vary from time to time. Examples of this practice—amounting in many circumstances to modifying certain specific values depending on other data items not discussed in detail, or by monitoring the vehicle&#39;s actual usage patterns over time—are given below. 
     Given these several different explanations of the relationship between the various operating modes of the vehicle of the invention, and specifically these different illustrations of the combinations of conditions in response to which the microprocessor controls mode selection, one of ordinary skill in the art would have no difficulty in implementing the invention. 
     As noted, during low-speed operation, such as in city traffic, the vehicle is operated as a simple electric car, where all torque is provided to road wheels  34  by traction motor  25  operating on electrical energy supplied from battery bank  22 . This is referred to as “mode I” operation, and is illustrated in FIG.  8 ( a ). The same paths of energy and torque may also be employed under emergency circumstances, referred to as mode III operation, as discussed below. 
     While operating at low speeds, e.g., when the vehicle&#39;s torque requirements (“road load”, or “RL”) are less than 30% of the engine&#39;s maximum torque output (“MTO”), engine  40  is run only as needed to charge battery bank  22 . Starting motor  21  is first used to start engine  40 , and is then operated as a generator by appropriate operation of inverter/charger  23 , so that charging current flows to battery bank  22 . 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 relatively high output torque level, for fuel efficiency. This “mode II” operation is illustrated in FIG.  8 ( b ); as indicated, clutch  51  is disengaged, so that engine operation to charge battery bank  22  through starting motor  21 , and propulsion of the vehicle by traction motor  25 , are completely independent of one another. 
     As in the &#39;970 patent, engine  40  is sized so that its maximum torque is sufficient to drive the vehicle in a range of desired cruising speeds; this requirement ensures that the engine is operated at high efficiency during normal highway cruising. Therefore, when a sensed increase in the road load (e.g., by a continued operator request for more power) indicates that the preferred operating mode is changing from low-speed to highway cruising operation, the microprocessor controls starting motor  21  by way of inverter/charger  23  to start engine  40 . When engine  40  is essentially up to speed, clutch  51  is engaged, so that engine  40  drives road wheels  34  through the shafts of motors  21  and  25 . When the operator releases pressure on the accelerator pedal, indicating that a desired cruising speed has been reached, traction motor  25  is accordingly depowered. The highway cruising mode is referred to as “mode IV” operation, and the flow of energy and torque are as illustrated in FIG.  8 ( c ). 
     If extra torque is needed during highway cruising, e.g., for acceleration or hill-climbing, either or both of motors  21  and  25  can be powered. This “mode V” operation is illustrated in FIG.  8 ( d ); energy flows from tank  38  to engine  40 , and from battery bank  22  to traction motor  25 , and possibly also to starting motor  21 ; torque flows from either or both motors and engine to wheels  34 . 
     FIG. 6, as indicated above, is a diagram illustrating differing modes of operation of the hybrid vehicle powertrain of the invention; the modes of operation, indicated by numerals I-V, are plotted on a three dimensional chart, illustrating that the mode of vehicle operation as controlled by microprocessor  48  is a function of the state of charge of the battery bank, the instantaneous road load, and time. FIG. 7, discussed below, further illustrates the inventive mode of vehicle operation. 
     FIG. 6 shows on one axis the state of battery charge extending from 80% at the origin outwardly to a minimum value shown of 10%. Normally the batteries are maintained at at least 30% of full charge. Preferably, the batteries are not charged to more than 70-80% of their full charge; if a number of series-connected batteries were all charged to 100% of their nominal full charge, some would likely be overcharged due to manufacturing variation, local temperature variation and the like, which would significantly shorten their service life. 
     The road load is shown in FIG. 6 on a second axis as varying from 0 at the origin to 200% of the engine&#39;s maximum torque output. (Negative road load, occurring during descents or under braking, is not shown in FIG. 6 due to the difficulty of illustration. This circumstance is discussed in connection with FIG. 7, below.) Time is shown on the third axis extending from an arbitrary point at the origin; that is, FIG. 6 shows the mode of the vehicle&#39;s operation over the next short period of time (on the order of 30-60 seconds) from a present instant at the origin. Stated differently, according to one aspect of the invention, the microprocessor  48  controls the vehicle&#39;s mode of operation at any given time in dependence on “recent history,” as well as on the instantaneous road load and battery charge state. 
     More specifically, FIG. 6 shows that during city driving (mode I), defined in this example as driving where the vehicle&#39;s instantaneous torque requirements, or “road load”, is up to 30% of the engine&#39;s maximum torque, the vehicle is operated as a “straight electric” car, the clutch being disengaged and energy from the battery bank  22  being used to power traction motor  25  to propel the vehicle, as long as the battery remains charged to between 50 and 70% of its full charge. If the charge falls to below a given value, which may vary over time as indicated by the curved line defining the extent of mode II, mode II is entered as indicated, the engine is started, and the starter motor  21  is operated as a generator to charge the battery to substantially full charge. As indicated in mode III, operation of the vehicle as an electric car may also be permitted when the battery falls to below 30% of full charge, for example, if there is a fault in the engine or charging system, but only on an emergency basis; such deep discharge is harmful to battery life. 
     During highway cruising, region IV, where the road load is between about 30% and 100% of the engine&#39;s maximum torque output, the engine alone is used to propel the vehicle. Accordingly, when the microprocessor detects that transition between regions I and IV is required (e.g., the microprocessor can effectively determine the road load by monitoring the response of the vehicle to the operator&#39;s command for more power), it causes the starting motor  21  to spin the engine  40  to relatively high speed; when a desired starting speed, typically 300-1500 rpm is reached, the electronic engine management unit  55  and electronic fuel injection unit  56  are controlled to fire the spark plugs and supply fuel, respectively, starting the engine. 
     Thus starting the engine at relatively high rpm allows a near-stoichiometric fuel:air mixture to be used, as compared to the much richer mixtures normally used for starting. Emissions of unburned hydrocarbons are thus substantially reduced, and fuel economy improved copending provisional application Ser. No. 06/122,477, discusses improvements in catalytic converter operation that further reduce emissions on engine starting. 
     When the speed of the engine output shaft substantially matches that of traction motor  25 , clutch  51  is engaged; the power produced by motor  25  is reduced as that produced by engine  40  is increased, so that the transition between modes I and IV is smooth and essentially undetected by the operator. When the operator reduces pressure on the accelerator pedal  69 , indicating that the desired cruising speed has been reached, power to motor  25  is reduced to zero. 
     If the operator then calls for additional power, e.g. for acceleration or passing, region V is entered; that is, when the microprocessor detects that the road load exceeds 100% of the engine&#39;s maximum torque output, it controls inverter/charger  27  so that energy flows from battery bank  22  to traction motor  25 , providing torque propelling the vehicle in addition to that provided by engine  40 . Starting motor  21  can similarly be controlled to provide propulsive torque. 
     As indicated above, during highway cruising, where the torque required to propel the vehicle varies as indicated by the operator&#39;s commands, the control system operates the engine at correspondingly varying torque output levels. The range of permissible engine torque output levels is constrained to the range in which the engine provides good fuel efficiency. Where the vehicle&#39;s instantaneous torque requirement exceeds the engine&#39;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&#39;s torque requirements are less than the torque then being produced 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 occur simultaneously, as torque from the engine and the vehicle&#39;s kinetic energy both drive one or both motors operated in generator mode. The rate of change of torque output by the engine may be controlled to reduce emissions, and in accordance with the batteries&#39; state of charge. FIG. 7 illustrates these relationships. 
     As mentioned above, FIG. 7, comprising FIGS.  7 ( a )-( c ), and extending over two sheets, is a timing diagram showing the relationship between road load, engine torque output, the state of charge of the battery bank, and operation of the engine as these vary over time, during low-speed city driving, highway cruising, and extended high-load driving, thus further illustrating the control strategy employed according to the invention. 
     FIG.  7 ( a ) shows the vehicle&#39;s instantaneous torque requirement, that is, the “road load”, by a solid line, and the engine&#39;s instantaneous output torque by a dashed line, as these vary over time. (The engine&#39;s instantaneous output torque is repeated in FIG.  7 ( c ), for clarity, and in order to clearly show certain additional aspects of the inventive control strategy.) The road load is expressed as a function of the engine&#39;s maximum torque output. Where the road load exceeds the engine&#39;s instantaneous output torque, the cross-hatched areas between these two lines represent torque provided by the traction motor; where the road load is less than the engine&#39;s instantaneous output torque, the cross-hatched areas represent charging of the batteries. 
     It will be appreciated that positive vehicle torque demands correspond to steady-state cruising, acceleration, hill-climbing, or the like, while negative vehicle torque requirements correspond to deceleration or descent. The engine&#39;s output torque is constrained to the range of efficient operation; as illustrated in FIG. 7 ( a ) and ( c ), this range is controlled to be between 30% and 100% of the engine&#39;s maximum torque output (“MTO”). As mentioned above, it will be appreciated that the 30% figure, as well as similar figures mentioned herein, may vary without departure from the scope of the invention. 
     In the example of vehicle operation shown in FIG. 7, initially the vehicle is operated only at road loads below 30% of MTO, that is, in traffic, as indicated at A. Accordingly, all the torque required is provided by the traction motor  25 , and the state of charge of the battery bank  22  (“BSC”), as illustrated by FIG.  7 ( b ), corresponds directly to the road load; when the road load is negative, BSC increases as the battery bank is charged by regenerative braking. 
     At point B, the road load exceeds 30% of MTO for the first time on this particular trip. When this is detected by microprocessor  48 , starting motor  21  spins the engine  40  at relatively high speed, and the catalytic converter  64  is preheated, causing a short drain on BSC, as shown at C. When the engine reaches the desired starting speed, e.g. 300-2000 RPM, and the catalyst reaches a minimum effective operating temperature, e.g. at least about 350° C. (as discussed more fully in application Ser. No. 06/122,477, the engine is started by supply of fuel and firing of its spark plugs, and the clutch is then engaged. As the engine is already rotating at relatively high speed, and has been warmed by compression of air in its cylinders during the starting process, it begins to produce useful torque almost immediately, as indicated at D. 
     Thereafter, when the vehicle&#39;s torque requirement exceeds the instantaneous engine output torque, as at points E-G and P, one or both of the traction and starting motors  25  and  21  are powered to provide additional torque to the road wheels, that is, the vehicle is operated in mode V. While the road load RL remains within the engine&#39;s efficient operating range, e.g., while 30% MTO &gt;RL&gt;100% of MTO, the vehicle is operated in mode IV. During mode IV operation, if the engine&#39;s instantaneous torque output exceeds the vehicle&#39;s torque requirement, but the battery is relatively fully charged, as at point H, the engine&#39;s torque output is reduced to match the road load; when MTO exceeds the road load, and BSC falls below a predetermined level (see FIG.  7 ( b )), as at I and J, the excess torque available from engine  40  is used to charge the batteries, as indicated at K and L (FIG.  7 ( c )). When the vehicle&#39;s torque requirement is less than the minimum permissible engine torque output, as at M, the engine is again used to charge the batteries, and regenerative braking is also performed, further charging the batteries. If the batteries become substantially fully charged, e.g., during a long descent, as at N, the engine may be shut off entirely, as seen at Q in FIG.  7 ( c ). 
     The rate of change of the engine&#39;s torque output is limited, e.g., to 2% or less per revolution, as indicated by noting that the dashed line in FIG.  7 ( a ), indicating the instantaneous engine output torque, lags the solid line indicating the vehicle&#39;s instantaneous torque requirement. Thus limiting the rate of change of engine output torque is preferred to limit undesirable emissions and improve fuel economy; that is, as the stoichiometric fuel/air ratio varies somewhat as the load changes, simply opening the throttle and causing additional fuel to be injected (as is typically practiced) upon the operator&#39;s depressing the accelerator pedal would result in non-stoichiometric, inefficient combustion. According to this aspect of the invention, the rate of change of engine torque is limited; this provides sufficient time for the essentially conventional electronic engine management and electronic fuel injection systems, which comprise a “lambda sensor” for monitoring the oxygen content of the exhaust gas stream as an indication of stoichiometric combustion, to respond as the load changes, preserving stoichiometric combustion and reducing emission of unburned fuel. 
     The maximum permissible rate of change of engine output torque also may be varied in accordance with the state of charge of the batteries; more specifically, if the batteries are relatively discharged, it may be preferable to allow the engine&#39;s output torque to ramp-up more quickly than otherwise, in order to limit the amount of electrical power drawn from the batteries in response to an acceleration command. More generally, it is preferred to operate the engine so as to limit the amount of power drawn from the batteries, as there are unavoidable losses attendant on conversion of energy stored in the batteries to motor output torque, and during the corresponding recharging period. 
     A further improvement in the control strategy provided according to the invention may be implemented if the engine  40  as fitted with a turbocharger, controlled by the microprocessor to be operated only when torque in excess of 100% of MTO is required for a period of time T, which period T may vary in accordance with BSC. This improvement is discussed in detail in copending provisional application Ser. No. 60/122,296, and in copending complete application Ser. No. 09/392,743, filed Sep. 9, 1999, filed Mar. 1, 1999 incorporated herein by reference. 
     As mentioned above, FIG. 9 is a high-level flowchart of the principal decision points in the control program used to control the mode of vehicle operation. Broadly speaking, the microprocessor tests sensed and calculated values for system variables, such as the vehicle&#39;s instantaneous torque requirement, i.e., the “road load” RL, the engine&#39;s instantaneous torque output ITO, both being expressed as a percentage of the engine&#39;s maximum torque output MTO, and the state of charge of the battery bank BSC, expressed as a percentage of its full charge, against setpoints, and uses the results of the comparisons to control the mode of vehicle operation. 
     As noted above, certain control decisions involved in the inventive control strategy illustrated in FIG. 9, and described therein as being determined in response to precise criteria (in order to clearly present the main features of the inventive operating strategy), may instead be usefully somewhat “fuzzy”; in the present application, this term is intended to indicate that the value of a setpoint (for example) may vary somewhat in response to recent history, or in response to monitored variables not discussed above. As mentioned above, it is also to be understood that the values given above for various numerical quantities may vary somewhat without departing from the invention. Specific alternatives are provided below for steps set forth in FIG. 9 that implement certain of these alternatives. 
     For example, in the example of the inventive control strategy discussed above, it is repeatedly stated that the transition from low-speed operation to highway cruising occurs when road load is equal to 30% of MTO. This setpoint, referred to in the appended claims as “SP”, and sometimes hereinafter as the transition point (i.e., between operation in modes I and IV) is obviously arbitrary and can vary substantially, e.g., between 30-50% of MTO, within the scope of the invention. 
     It is also within the scope of the invention for the microprocessor to monitor the vehicle&#39;s operation over a period of days or weeks and reset this important setpoint in response to a repetitive driving pattern. For example, suppose the operator drives the same route from a congested suburban development to a workplace about the same time every morning; typically the road load might remain under 20% of MTO for the first few minutes of each day, then vary between 0 and 50% of MTO for another few is minutes as the operator passes through a few traffic lights, and then suddenly increase to 150% of MTO as the operator accelerates onto a highway. It is within the skill of the art to program a microprocessor to record and analyze such daily patterns, and to adapt the control strategy accordingly. For example, in response to recognition of a regular pattern as above, the transition point might be adjusted to 60% of MTO; this would prevent repetitive engine starts as the road load exceeded 30% of MTO for a few hundred yards at a time, as might often occur in suburban traffic. Similarly, the engine starting routine might be initiated after the same total distance had been covered each day. 
     It is also within the scope of the invention to make the setpoint SP to which the road load is compared to control the transition from mode I to mode IV somewhat “fuzzy”, so that SP may vary from one comparison of road load to MTO to the next depending on other variables. For example, as discussed above, if during low-speed operation the operator depresses the accelerator pedal rapidly, this can be treated as an indication that full power will shortly be required, and the engine-starting operation begun before the road load reaches any particular setpoint SP. 
     The value of the transition point may also vary in dependence on the mode of operation in effect when the road load equals a given setpoint SP. For example, suppose the setpoint at which the mode of operation is controlled to change from the low-speed mode to the highway cruising mode is normally set to 30% of MTO, as in the examples discussed above. If traffic conditions were such that the road load fluctuated around this value, and engine operation were controlled solely in response to road load, the engine would be repeatedly started and shut off as the road load exceeded 30% of MTO for a few hundred yards at a time, and then fell back below 30% of MTO, as might often occur in suburban traffic. Repeated restarts might also occur if the road load averaged over 30% of MTO but occasionally dropped below this value, as might occur in moderate-speed, flat-road cruising. 
     By monitoring the road load over time, and comparing it to different setpoints accordingly, much of this undesirable repetitive sequence of engine starting and shut-off can be eliminated. It might be preferable to commence mode IV operation upon the occurrence of differing conditions; for example, mode IV might be entered from mode I only after the road load exceeded a first, lower setpoint SP for an extended period of time, so that the engine would be run for extended low-speed cruising, but to start the engine immediately if the road load exceeded a higher setpoint SP 2 , e.g. 50% of MTO, as during acceleration to highway speed. Similarly, the engine might preferably be shut down only if the road load was less than a minimum setpoint for mode IV operation for an extended period of time. Thus providing “hysteresis” in the mode-switching determination would limit repetitive engine starts in certain types of driving. These limits could be further adjusted as the driving pattern became clear, i.e., as discerned by the microprocessor. 
     In a further refinement, the setpoint at which the engine is shut off as the road load dropped below the usual minimum value for mode IV operation could vary dependent on BSC; if the batteries were substantially fully charged, the engine might be shut off as road load dropped below 30% of MTO, but if their charge was lower the engine might be controlled to continue to run, even at at a stop, i.e., zero road load, to charge the batteries. Of course, the clutch would still have to be disengaged at when the road load fell below 20-30% of MTO, in order that the engine could run at an efficient speed for production of torque. 
     FIG. 9 thus shows the main decision points of the control program run by the microprocessor, with the transition point between mode I, low-speed operation, and mode IV highway cruising, set at a road load equal to 30% of MTO. Examples are then given for some of the various options discussed above, by substituting various of the decision points with alternatives indicated below. Other optional points not specifically shown but discussed herein are within the scope of the invention. 
     The control program is entered at step  100 , where the microprocessor determines whether the road load RL is less than 30% of MTO. If the answer is yes (“Y”), the clutch is disengaged if necessary as indicated at steps  103  and  105 . The state of charge of the battery bank BSC is then tested at step  110 ; if BSC is between 50 and 70% of full charge, the vehicle can operate for some time as a straight electric vehicle, and mode I is accordingly entered, as indicated at  115 . A “mode I” loop is then established, including steps  100 ,  103 , and  110 ; as long as all conditions tested in these steps remain stable, the vehicle continues to be operated in mode I. 
     However, if at step  110  it was determined that BSC was less than 50% of its maximum value (“N”), the engine should be run, if possible, to charge the battery bank, up to, for example, 75% of its maximum charge, as tested at step  120 . If the engine is already running, as tested at step  125 , the battery is charged as indicated at  130 , and a stable “mode II” loop, as noted at  135 , is established including steps  100 ,  103 ,  110 ,  120 ,  125 , and  130 . (Normal operation of step  110  would be bypassed or disabled in this mode to prevent battery charging from being stopped when BSC reaches 70%). If the engine is not running, an engine starting subroutine (shown separately, by FIG.  9 ( a ), is entered, as indicated at step  140 . 
     In the engine starting subroutine, beginning with the ‘enter’ block  141 , the clutch is disengaged if necessary at steps  142 - 143 , and the catalyst temperature is tested at  145 , to determine whether it is at least about 350° C.; the catalyst is heated as necessary, as indicated at  150 . When the catalyst is heated suitably, the engine is then spun by the starter motor until a desired starting speed is reached, as indicated by the loop including blocks  155  and  160 . When the engine reaches its desired starting speed, it is started at step  165 , by supply of fuel and firing of its spark plugs, concluding the engine starting subroutine as indicated by ‘return’ block  170 . As noted, further details of the engine starting process are found in copending provisional application Ser. No. 60/122,477, filed Mar. 1, 1999. If the engine starting subroutine was entered from the mode II loop, as above, the battery bank may then be charged as indicated at  130 . 
     If in performance of step  120  it appeared that BSC was less than 50%, step  175  may be performed; thus, if 10%&lt;BSC&lt;30%, the vehicle may be operated in mode III as an electric car, to provide emergency operation. However, this should be strictly limited to avoid deep discharge of the battery bank, tending to shorten its useful life. As indicated at  177 , the vehicle is completely disabled if BSC falls below 10%. 
     If RL is determined to exceed 30% of MTO in step  100 , the program goes to step  180 , where the term 30%&gt;RL&gt;100% is evaluated; that is, the microprocessor determines whether the road load is appropriate for highway cruising in mode IV. If so, and if the engine is running, as tested at step  190 , a stable loop including steps  180  and  190  is established; the system remains in mode IV, as indicated at  185 , until the state of one of these tests changes. 
     If in step  190  it is determined that the engine is not running, the engine start subroutine, starting with step  140  as discussed above, is entered as indicated at  195 ; upon return, at  200 , the clutch is engaged at  210 , and the loop including steps  180  and  190  is entered. 
     As noted, in step  180  it is determined whether RL is between 30 and 100% of MTO; if not, it is determined in step  220  whether RL is greater than 100% of NTO. If so, mode V is entered, and the traction motor (and optionally the starting motor) is powered to provide additional torque propelling the vehicle, as indicated at  230 . A loop including steps  220  and  230  is thus established, so that mode V remains stable until the state of the test performed in step  220  changes. 
     When in performance of step  220 , it appears that RL is now less than 100% of MTO, it is then determined in step  215  whether RL is less than 30% of MTO. If so, the engine is shut off, as indicated at  240 , and the program returns to step  100 ; if not, the program is returned to step  180 . 
     It will be appreciated that according to the FIG. 9 flowchart, it is possible for the system to proceed directly from mode I to mode V, that is, from step  100  to step  220 , if the road load rapidly increases from less than 30% of MTO to more than 100% of MTO. Permitting the operator to thus operate the system is an important safety feature, for example when fast acceleration from a stop is required to merge into highway traffic. In these circumstances the engine would not be running during initial operation in mode V, necessitating a significant drain on the battery bank and overdriving the traction motor. Accordingly, steps equivalent to steps  190 ,  195 , and  210  (including the engine starting subroutine) are to be understood to follow step  220  and precede step  230 . That is, in the event mode IV was effectively omitted in passing directly from mode I to mode V, the engine is started and the clutch engaged as soon as possible; these duplicate steps are not shown, for clarity. 
     In the above discussion of FIG. 9, it was assumed that the transition point between low-speed and highway operation is set so that the transition occurs when the road load is equal to 30% of MTO under all circumstances. However, as discussed above, it may be desirable to operate the system so that the vehicle goes from the low-speed mode I to the highway-cruising mode IV at a higher road load, e.g., 50% of MTO, than the road load at which the low-speed mode is reentered, e.g., when road load in mode IV falls to below 20%. This “hysteresis” of the mode switching point—that is, allowing the vehicle to accelerate in mode I up to road loads of up to 50% of MTO, but not shutting the engine off, ending mode operation, until road load falls below 20% of MTO—avoids excessive mode-switching during periods of fluctuating road load. 
     For example, in typical suburban traffic, one commonly accelerates past 30% of MTO, to what might otherwise be a normal cruising speed, but must stop again shortly thereafter; it would be inefficient to thus repetitively stop and restart the engine as the load fluctuates around 30%. Hysteresis might similarly be useful in avoiding needless mode switching in moderate-speed, flat road cruising in mode IV, when the road load might well occasionally drop below 30%; again, it would be inefficient to repeatedly shut off and restart the engine. 
     Thus providing differing mode switching points depending on the direction of the change in road load can be accomplished readily by monitoring the road load RL as a function of time, and taking appropriate control action. For example, if the system is maintained in mode I until RL exceeds the “normal” 30% of MTO mode switching point for a period of, for example, 30 seconds, and without exceeding 50% of MTO, the excessive mode switching otherwise likely to be encountered in suburban traffic can be largely avoided. FIG.  9 ( b ) shows a step  100 ′ replacing step  100  in FIG.  9  and implementing this “low-speed hysteresis”. As indicated, the system remains in the low-speed mode I as long as RL is less than 30% of MTO, or unless RL exceeds 30% of MTO for more than 30 seconds, or exceeds 50% of MTO; if either of the latter conditions occurs, the program goes to step  180 , initiating mode IV operation. 
     Similarly, hysteresis in mode IV cruising, in order to implement excessive mode shifting that might otherwise occur if the road load fluctuates around a fixed mode switching point, can be implemented by simply providing that the system remains in mode IV as long as RL remains between 30 and 100% of MTO, unless RL is less than RL for more than 30 seconds, or exceeds 100% of MTO. This can be implemented as shown in FIG.  9 ( c ); a revised step  215 ′ replaces A step  215  of FIG. 9, and provides that, if the system is in mode IV, unless RL is less than 30% of MTO for more than 30 seconds, step  180  is re-entered, thus preserving the “mode IV loop”; when RL is less than 30% of MTO for more than 30 seconds, the engine is shut down, at step  240 , control is passed to step  100 , and mode I reentered. 
     Numerous further modifications to the detailed control strategy of the invention as illustrated in FIGS. 6-9 will occur to those of skill in the art, and are within the scope of the invention. For example, it may be desirable to vary the operation of the system insofar as responsive to BSC in accordance with monitored variables indicative of battery temperature, ambient temperature, and the like; e.g., on a hot day it may be advisable to avoid charging the battery bank to more than 60% of full charge, as this may cause overheating. Further, as noted above the transition points between modes I, IV, and V in particular may vary in accordance with the operator&#39;s commands, so as to provide maximum vehicle responsiveness for safety and ease of consumer acceptance, and over periods of days or weeks, as the microprocessor builds up a detailed historical record of the vehicle&#39;s usage pattern, from which an optimized control strategy may be derived. 
     It may also be possible to provide the microprocessor with useful control information from the operator, without requiring the operator to understand the workings of the system in detail. For example, operators are now well-accustomed to set a “cruise control” when a desired cruising speed is reached; thereafter, existing engine management systems control the instantaneous engine torque output with respect to variation in the road load to maintain vehicle speed substantially constant. It would be a simple matter for the microprocessor to accept a desired cruising speed thus input by the operator, as indicated in FIG.  4 . The operator would then be relieved of continuous throttle control, and the microprocessor would similarly control the instantaneous engine torque output with respect to variation in the road load to maintain vehicle speed substantially constant, both as conventional; however, according to the invention, the microprocessor would also reset the transition point so that the system would remain in cruising mode IV until the operator had so indicated. 
     It will be appreciated that the hybrid vehicle and operational strategy therefor of the invention provide numerous advantages over the prior art discussed herein, and that further improvements and modifications thereto are within the skill of the art. Accordingly, while a preferred embodiment of the invention has been disclosed, and various alternatives mentioned specifically, the invention is not to be limited thereby.