Hybrid Vehicles

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By incorporating into the driveline of the vehicle the capability to generate electricity onboard the vehicle from a chemical fuel, a hybrid-electric vehicle has the characteristics of both an electric vehicle and a conventional internal combustion engine (ICE) vehicle and can be operated either on wall-plug electricity stored in a battery or from a liquid fuel (e.g., gasoline) obtained at a service station. This essay discusses technologies for hybrid-electric vehicles that can attain significantly higher fuel economy and lower emissions than conventional ICE vehicles of the same size, performance, and comfort.


There are a large number of ways an electric motor, engine, generator, transmission, battery, and other energy storage devices can be arranged to make up a hybrid-electric driveline. Most of them fall into one of two configurations—series and parallel. In the series configuration (Figure 1, Top), the battery and engine/generator act in series to provide the electrical energy to power the electric motor, which provides all the torque tot he wheel of the vehicle. In a series hybrid, all the mechanical output of the engine is used by the generator to produce electricity to either power the vehicle or recharge the battery. This is the driveline system used in diesel-electric locomotives. In the parallel configuration (Figure 1, Bottom), the engine and the electric motor act in parallel to provide torque to the wheel of the vehicle. In the parallel hybrid, the mechanical output of the engine can be used to both power the vehicle directly and to recharge the battery or other storage devices using the motor as a generator. In recent years, a third type of hybrid configuration, the dual mode, is being developed that combines the series and hybrid configurations. As shown in Figure 1b, the engine output can be split to drive the wheel (parallel mode) and to power a generator to produce electricity (series mode). This configuration is the most flexible and efficient, but it is also likely to be the most complex and costly.

A range-extended electric vehicle would most likely use the series configuration if the design is intended to minimize annual urban emissions. It would be designed for full-performance on the electric drive alone. The series hybrid vehicle can be operated on battery power alone up to its all-electric range with no sacrifice in performance (acceleration or top speed) and all the energy to power the vehicle would come from the wall plug. This type of hybrid vehicle is often referred to as a "California hybrid" because it most closely meets the zero emission vehicle (ZEV) requirement. The engine would be used only on those days when the vehicle is driven long distances.

Hybrid vehicles designed to maximize fuel economy in an all-purpose vehicle could use the series, parallel, or dual configurations depending on the characteristics of the engine to be used and acceptable complexity of the driveline and its control. Parallel hybrid configurations will require frequent on-off operation of the engine, mechanical components to combine the engine and motor torque, and complex control algorithms to smoothly blend the engine and motor outputs to power the vehicle efficiently. The parallel hybrid would likely be designed so that its acceleration performance would be less than optimum on either the electric motor or engine alone and require the use of both drive components together to go zero to sixty mph in 10–12 sec. Such a hybrid vehicle would not function as a ZEV in urban/freeway driving unless the driver was willing to accept reduced acceleration performance. The parallel configuration can be designed to get better mileage than the series configuration. A fuel cell hybrid vehicle would necessarily be a series hybrid because the fuel cell produces only electricity and no mechanical output. The dual mode hybrid is intended to maximize fuel economy and thus be designed like a parallel hybrid, but with a relatively small generator that could be powered by the engine. The engine in the dual mode hybrid would operate in the on/off mode, but be cycled on and off less frequently than in the parallel configuration.


Motor and Electronics

Recent advances in both motor and inverter electronics technologies have important implications for the design of high performance hybrid vehicles. In 2000 the size and weight of the motor and the electronics combined are significantly smaller (by a factor of two to three—see Table 1) than that of an engine and transmission of the same peak power in 1998. The size advantage of the electrical components will be even greater as the operating voltage of the electrical drive system is increased above the 300–400V that is common today. This means that in packaging the hybrid driveline, the electrical drive components take up only a fraction of the space available and finding room for the mechanical components, such as the engine, transmission or torque coupler, can present a difficult challenge.

Electrical Energy Storage

The key component in the hybrid driveline that permits it to operate more efficiently than the engine/transmission in a conventional car is the electrical energy storage unit. It must store sufficient energy (kWh) to provide the all-electric range of the vehicle or to permit the engine or fuel cell to operate near the average power required by the vehicle (in a load-leveled mode). It also must have sufficient power capability to meet on demand the peak power of the motor/electronics for vehicle acceleration or

    Emissions (gm/kWh) (Nominal)  
Engine Type kg/kW l/kW Maximum Efficiency (%) HC CO NOX Manufacturer/Developer
Spark Ignition
Valve Injection2.04.0323.0208.0Auto Companies
Direct Injection2.54.1385.04.03.5Auto Companies
Rotary.81.0303.04.05.0Moller International
Prechamber/Turbo2.64.235 .11.03.0Auto Companies
Direct Injection/Turbo3.04.4421.01.015.0Auto Companies
Gas Turbine
With Recup.2.55.330.08.30.03Capstone
Incl. generator (catalytic combustion) 
with Recup.
 (catalytic combustion) 
H2 Working Fluid/Wobble Plate Drive2.7-32-2.530- Thermal Motors

regenerative braking. In most cases, the energy storage unit in a hybrid vehicle is sized by the peak power requirement. Because the size (weight and volume) of the energy storage unit (often a battery) in the hybrid vehicle is smaller than the battery in a battery-powered electric vehicle (EV), the power density (W/kg and W/liter) requirements of the energy storage unit in the hybrid vehicle are greater than for the battery in an electric vehicle. For example, power densities of 200–300W/kg are satisfactory for use in an EV, but power densities of 500–1,000W/kg are needed for hybrid vehicles. Considerable progress has been made in the development of high power batteries (often referred to as pulse batteries) of various types. In hybrid vehicles, the energy density of the energy storage unit is of secondary importance; compromises in energy density have been made to reach the high power density of pulse batteries. For example, nickel metal hydride batteries designed for EVs have an energy density of 70Wh/kg and a power density of 250 W/kg, while those designed for hybrid vehicles have an energy density of 40–45Wh/kg and a peak power density of 600–700W/kg. Another important consideration for energy storage units for use in hybrid vehicles is the need to minimize the losses associated with transferring energy into and out of the unit, because in the hybrid, a reasonable fraction of the electrical energy produced onboard the vehicle is stored before it is used to power the vehicle. In order to minimize the energy storage loss, the round-trip efficiency (energy out/energy in) should be at least 90 percent. This means that the useable peak power capability of a battery is much less that the power into a match impedance load at which efficiency of the transfer is less than 50 percent. Useable power is only about 20 percent of the peak power capacity. This is another reason that the design of an energy storage units for hybrid vehicles is much more difficult than for battery-powered electric vehicles and energy storage technology is often described as the enabling technology for hybrid vehicles.

A new energy storage technology that is well suited for hybrid vehicles is the electrochemical ultracapacitor, often referred to as the double-layer capacitor. Ultracapacitors for vehicle applications have been under development since about 1990. The construction of an ultracapacitor is much like a battery in that it consists of two electrodes, a separator, and is filled with an electrolyte. The electrodes have a very high surface area (1,000–1,500 m2/gm) with much of the surface area in micropores 20 angstroms or less in diameter. The energy is stored in the double-layer (charge separation) formed in the micropores of the electrode material. Most of the ultracapacitors presently available use activated carbon as the electrode material. The cell voltage is primarily dependent on the electrolyte used. If the electrolyte is aqueous (sulfuric acid or KOH) the maximum cell voltage is 1V; if an organic electrolyte (propylene carbonate) is used, the maximum cell voltage is 3V. As in the case of batteries, high voltage units (300–400V) can be assembled by placing many ultracapacitor cells in series.

Batteries have much higher energy density and capacitors have much higher power capacity. The technical challenge for developing ultracapacitors for vehicle applications is to increase the energy density (Wh/kg and Wh/liter) to a sufficiently high value that the weight and volume of a pack to store the required energy (500 Wh for a passenger car) is small enough to be packaged in the vehicle. Power density and cycle life are usually not a problem with ultracapacitors. The cost ($/Wh) of ultracapacitors is presently too high—being about $100/Wh. It must be reduced by at least an order of magnitude (a factor of 10) before this new technology will be used in passenger cars. Nevertheless, ultracapacitors are a promising new technology for electrical energy storage in hybrid vehicles.

Engines and Auxiliary Power Units

The characteristics of engines for hybrid vehicles are shown in Table 1. Most of the hybrid vehicles designed and built have used four stroke gasoline or diesel engines. Nearly all gasoline engines are now fuel-injected and both gasoline and diesel engines are computer controlled. Continuing improvements are being made in these engines in terms of size, weight, efficiency, and emissions. These improvements and the common use of computer control make it fairly easy to adapt the conventional engines to hybrid vehicle applications. The major difficulty in this regard is to find an engine of appropriate power rating for hybrid vehicle application. Most automotive engines have power of 60kW (75hp) and greater, which is too large for use in most hybrid vehicle designs. Experience has shown that good sources of engines for hybrid vehicles are the minicars designed for the small car markets on Japan and Europe.

Several advanced engines have been developed especially for hybrid vehicles. These include a high-expansion ration gasoline engine (Atkinson cycle) developed by Toyota for their Prius hybrid vehicle which they started to market in Japan in 1997. Stirling Thermal Motors (STM) under contract to General Motors (GM) designed and fabricated a Stirling engine (30kW) for use on GM's series hybrid vehicle built as part of the Department of Energy (DOE) hybrid vehicle program. Capstone Technology developed a 25kW recuperative gas turbine engine for use in a flywheel hybrid vehicle built by Rosen Motors. The characteristics of these engines are indicated in Table 1. The most successful of these engine development projects was the Prius engine of Toyota. The other two engines were too large and were not efficient enough to warrant further development for hybrid vehicle applications. The new Toyota engine in the Prius is a four cylinder (1.5 liter), four stroke gasoline engine that utilizes variable inlet valve timing to vary the effective compression ratio from 9.3 to 4.8 in an engine having a mechanical compression ratio of 13.5. Varying the effective volume of air during the intake stroke permits operation of the engine at part load with reduced pumping and throttling losses. This results in an increase in engine efficiency. The expansion ratio at all times is set by the high mechanical compression ratio of the engine. This new engine was optimized for operation in the hybrid mode and had a brake specific fuel consumption of about 235 gm/kWh for output powers between 10 and 40kW. This corresponds to an efficiency of 37 percent, which is very high for a four stroke gasoline engine of the same peak power rating.

The engine output in a hybrid vehicle can be utilized to generate electricity on board the vehicle or to provide torque to the driveshaft of the vehicle. In the first case, the engine output torque drives a generator and the combination of the engine and the generator is termed an auxiliary power unit (APU). The generator can be either an ac induction or a brushless dc permanent magnet machine. The size of the generator for a given power rating depends to a large extent on the voltage of the system and the rpm at which the generator rotates. For a 400V system and a maximum of 8,000–10,000, the size and weight of the APU are 0.7kg/kW and 0.8 liter/kW, respectively, including the electronic controls. The efficiency of the generator system will vary between 90 to 95 percent depending on the power output. The losses associated with the production and storage of the electrical energy onboard the vehicle in a series hybrid are significant (10 to 20%) and cannot be neglected in predicting the fuel economy of the vehicle.

Mechanical Components

The transmission, clutch, and other mechanical components needed in a hybrid vehicle to combine the output of the engine with the electric motor and generator and the main driveshaft of the vehicle are critical to the efficient and smooth operation of parallel and dual mode hybrid vehicles. The design of these components is relatively straightforward and not much different than that for similar components for conventional engine-powered vehicles. In a parallel hybrid (Figure 1b), the engine is connected to the drive shaft through a clutch that opens and closes as the engine power is needed. The speed ratio between the engine and the wheels is determined by the gear ratios in the transmission. Mechanical design of the engine clutch so that it has a long life and smooth operation is one of the critical tasks in the development of the parallel hybrid vehicle. Many hybrid vehicles are built with manual transmissions, because automatic transmissions with a torque converter have unacceptably high losses. A recent development is the use of a continuously variable transmission (CVT) in a parallel hybrid driveline. The operation of the CVT is like an automatic transmission from the driver's point-of-view with the advantages of lower losses and a wider range of continuous gear ratios, which result in efficient driveline operation in both the electric and hybrid modes. The disadvantages of the CVT are that control of the system is more difficult than with a manual transmission and the steel belt used in the CVT is much less tolerant of abuse (sudden changes in speed and torque). Nevertheless, it appears that in the future years CVTs will have application in parallel hybrid drivelines.

There are several arrangements of the dual mode hybrid driveline. In the simplest arrangement (Figure 2), the generator can be used as a motor to start the engine and/or supplemental torque of the traction motor to drive the vehicle. In this dual mode configuration, the batteries can be recharged either by the generator or by the traction motor acting as a generator. This simple dual mode system does not require a transmission. A second dual mode system utilizes a planetary gear set to couple the engine, generator, and main driveshaft. In this arrangement, the speed ratio between the engine and the main driveshaft depends on the fraction of the engine power that is applied to the generator. This second arrangement is used by Toyota in the Prius hybrid car. This system is less flexible and less efficient than the first system in which the engine and generator are directly connected on the same drive shaft, but it does not require a clutch, which must be opened/closed smoothly and reliably under computer control. Operation of the Toyota dual mode hybrid driveline has proven to be smooth and reasonably efficient.

Fuel Cells

Fuel cells can be utilized in electric hybrid vehicles as the means of converting chemical fuel to electricity. Rapid progress has been made in the development of fuel cells, especially proton exchange membrane (PEM) fuel cells, for transportation applications. This progress has resulted in a large reduction in the size and weight of the fuel cell stack and as a result, there is now little doubt that the fuel cell of the required power (20–50kW) can be packaged under the hood of a passenger car. The primary question regarding fuel cells in light duty vehicles is how they will be fueled. The simplest approach is to use high pressure hydrogen as has been done in the most successful bus demonstration to date. This approach is satisfactory for small test and demonstration programs, but the development of the infrastructure for using hydrogen as a fuel in transportation will take many years. Considerable work is underway to develop fuel processors (reformers) to generate hydrogen onboard the vehicle from various chemical fuels (e.g., methanol or hydrocarbon distillates). Most of the hydrogen used for industrial and transportation applications is presently generated by reforming natural gas using well-developed technology. A promising approach to fuel processing to hydrogen (H+ and electrons) onboard the vehicle is direct oxidation of methanol within the fuel cell stack. When technology for the efficient, direct conversion of a liquid fuel to hydrogen within the PEM fuel cell is developed, the commercialization of fuel cells in light duty vehicles will occur rapidly.


Control Strategies for Series Hybrid Vehicles

The intent of the control strategy is to maintain the state-of-charge of the energy storage unit within a prescribed range regardless of the driving cycle and the resultant power demand on the driveline. This should be done so that the onboard electrical generator (engine/generator or fuel cell) is operated at high efficiency and low emissions. This is done more easily when the energy storage capacity is reasonably large as with a battery than when it is small as using ultracapacitors. The strategy used for vehicles having a significant all-electric range is to discharge the battery to a prescribed state-of-charge (20 to 30%) and then to turn on the engine to maintain the battery within 10 to 20 percent of that condition. Electrical energy is generated at a rate slightly greater than the average power demand of the vehicle to account for losses associated with storing the energy. In the case of an engine/generator, a minimum power level is set so that the engine is never operated below it. Proper selection of this minimum power can have an important effect on fuel economy. When the battery charge reaches the maximum permitted, the engine is turned off and it remains off until the battery state-of-charge falls to the engine turn-on state-of-charge. When the series hybrid is operated so that the battery is permitted to discharge to a relatively low state-of-charge, it is termed a charge depleting hybrid. If the battery is maintained at a high state-of-charge (60 to 70%), it is termed a charge sustaining hybrid and the battery is seldom, if ever, recharged from the wall-plug. A significant fraction of the energy used by charge depleting hybrid vehicles is from the wall-plug and their average annual emissions and energy consumption are dependent on the use-pattern (miles of travel per day) of the vehicle and how the electricity used to recharge the batteries is generated.

Control Strategies for Parallel Hybrid Vehicles

The control strategies for parallel hybrid vehicles are more complicated than those for series hybrids primarily because they are dependent on both vehicle speed and state-of-charge of the energy storage unit and should include a criteria for splitting the driveline torque between the engine and the electric motor. The intent of the strategy is to permit the electric motor to provide the torque if it can at vehicle speeds below a prescribed value and permit the engine to provide the torque at higher speeds. If the vehicle is operating in the all-electric mode, the motor provides the torque and the engine is not turned on regardless of the torque demand or vehicle speed. Since the all-electric range of a hybrid vehicle is usually less than 80 km, operation of the vehicle should change automatically to the hybrid mode when the all-electric range is exceeded. The control strategy in the hybrid mode can be either charge sustaining or charge depleting. In the case of charge sustaining, the battery state-of-charge is maintained at a near constant value by a control strategy using electrical energy produced by the engine and the motor acting as a generator and consequently little electrical energy is used from the wall-plug. For the charge depleting case, the control strategy permits the battery state-of-charge to decrease as the vehicle is driven and the battery is then recharged from the wall-plug at night. Parallel hybrids usually have a multi-speed transmission so the control strategy must also include a gear shifting algorithm that depends on whether the motor or engine or both are producing torque. A continuously variable transmission (CVT) would be particularly attractive for use in a parallel hybrid driveline.

In order to achieve high fuel economy with a parallel hybrid, it is necessary to avoid engine operation below some minimum engine torque (or effective throttle setting) where the engine brake specific fuel consumption (gm/kWh) is relatively high and to manage engine turn on and off carefully to minimize emissions and wasted fuel. In urban driving, the control strategies for parallel hybrids often result in the engine being turned on and off frequently because the vehicle speed and power demands vary rapidly in stop-and-go driving. The effects of this on-off engine operation on fuel usage and emissions for the parallel hybrids are neglected in most simulations at the present time, so further analysis and vehicle testing is needed to determine whether the high fuel economy and low emissions projected for parallel hybrids can be attained. The control strategies for parallel hybrids are necessarily more complex than those for series hybrids and the uncertainty in the simulation results for parallel hybrids are greater.

Control Strategies for Dual Mode Hybrid Vehicles

The control strategies for dual mode hybrid vehicles are a combination of those used for series and parallel hybrids. There are so many possible hardware arrangements and associated control strategies, it is not possible to summarize them in a simple manner as was the case for series and parallel hybrids. The objective of the dual mode operation is to use the possibility of battery charging simultaneously with the use of the engine and electric motor to power the vehicle as a means of maintaining engine operation at high efficiency at all times. At highway speeds, the engine can be used directly to power the vehicle with the engine operating at high efficiency. This mode of operation is essentially that of a parallel hybrid. At low vehicle speeds, when the battery does not need charging (state-of-charge greater than a specified value), the driveline would operate in an electric-only mode if the electric motor can provide the power required by the vehicle. If the power demand is greater than that available from the electric motor, the engine is turned on to assist the electric motor. At low speeds when the battery requires charging, the engine output is split between powering the generator and the vehicle. The possibility of splitting the engine output in this way at low vehicle speeds is the distinguishing feature of the dual mode hybrid configuration. This permits the engine to be operated near its maximum efficiency at all times and the battery to be recharged, when needed, regardless of the vehicle speed and power demand. The dual mode arrangement also reduces the need for on-off engine operation as required in the series and parallel control strategies. Dual mode hybrids are operated with the battery state-of-charge maintained in a narrow range (charge sustaining) and thus require no recharging of the battery from the wall-plug. The Toyota Prius hybrid vehicle uses this dual mode operating strategy.


The added complexity of the various hybrid vehicle designs relative to battery-powered electric vehicles and conventional engine-powered vehicles is evident. In order to justify this added complexity, hybrid vehicles must be more marketable than pure electric vehicles and have higher fuel economy and lower emissions than conventional engine-powered vehicles. All the vehicle types (electric, hybrid, and conventional) can be designed to have the same acceleration performance and top speed by the proper selection of the driveline components. Acceleration times of 0–96 km/hr (60 mph) in less than 8 sec and top speeds in excess of 120 km/hr for passenger cars have been demonstrated for both pure electric and hybrid vehicles. The primary advantage of the hybrid vehicle compared to the electric vehicle is that its range and refueling time can be the same or better than the conventional vehicle because it is refueled in the same manner—at the fuel pump. The key comparisons of interest are the fuel economy, emissions, and costs of hybrid and conventional vehicles.

Computer simulations have been performed for both midsize and compact, lightweight vehicle designs for driving on the Federal Urban Driving Schedule (FUDS) and the Federal Highway Driving Schedule (FHWDS). These driving cycles (speed vs. time) are intended to simulate vehicle operation in city and highway driving. For each vehicle type, computer simulations were run using gasoline fuel injected engines, diesel engines, and Stirling engines. Electricity was generated on board the hybrid vehicles by coupling the engines to a generator or by utilizing a fuel cell fueled using compressed hydrogen. In all cases, electrical energy was recovered into the energy storage unit during braking using the traction motor as a generator. The control strategies used in the simulations were essentially those previously discussed in the section on control strategies. In all cases, the engines and fuel cell were operated in an on-off mode to maintain the energy storage unit in the state-of-charge range specified by the control strategy. The minimum power setting of the engines and fuel cell when they were "on" were set so their efficiency was not outside the high efficiency portion of their operating maps.

Fuel Economy

Fuel economy simulation results for various engines in series hybrids are compared in Table 2 for the FUDS and FHWDS driving cycles. For both the midsize and compact cars, fuel economy depends significantly on the technology used in the driveline. The use of diesel engines results in the highest fuel economy (miles per gallon of diesel fuel); however, from the energy consumption (kJ/mi) and CO2 emission (gm CO2/mi) points-of-view, the advantage of diesel engine relative to gasoline-fueled engines should be discounted to reflect the higher energy and the carbon content per gallon of diesel fuel compared to gasoline. These discount factors are 15 to 20 percent. The simulation results also indicate that for the same type of engine, the fuel economy can be 10 to 20 percent higher using ultracapacitors in place of batteries as the energy storage device. The highest fuel economics are projected for vehicles using fuel cells. The fuel economies (gasoline equivalent) of the fuel cell vehicles using compressed hydrogen are

   Miles per Gallon
Vehicle Engine Energy Storage FUDS Highway
MidsizeHonda GasolineNi. Mt. Hy. Bat.36.145.4
 Direct Injection GasolineNi. Mt. Hy. Bat.47.356.0
 Sw. Ch. DieselNi. Mt. Hy. Bat.49.756.8
 Direct Injection DieselNi. Mt. Hy. Bat.60.571.1
 StirlingNi. Mt. Hy. Bat.50.057.2
 Honda GasolineCapacitor44.347.3
 Sw. Ch. DieselCapacitor62.365.7
 Fuel Cell (H2)Ni. Mt. Hy. Bat.89.2105.0
Lightweight CompactHonda GasolineNi. Mt. Hy. Bat.69.671.4
 Direct Injection GasolineNi. Mt. Hy. Bat.82.984.9
 Sw. Ch. DieselNi. Mt. Hy. Bat.98.295.6
 Direct Injection DieselNi. Mt. Hy. Bat.107.3110.4
 StirlingNi. Mt. Hy. Bat.89.592.7
 Honda GasolineCapacitor81.475.5
 Sw. Ch. DieselCapacitor109.8104.0
 Fuel Cell (H2)Ni. Mt. Hy. Bat.16.317.9

about twice those of hybrid vehicles with direct injected gasoline engines and about 80 percent higher than vehicles with diesel engines. All the fuel cell vehicle designs utilized a fuel cell load-leveled with a nickel metal hydride battery permitting it to operate at high efficiency at all times.

In comparisons between the fuel economies of conventional passenger cars and those using series hybrid divelines, the hybrid vehicles have the same weight and road load as the conventional cars. Still, the utilization of the hybrid driveline resulted in about a 50 percent improvement in fuel economy for the FUDS cycle and about a 10 percent improvement on the FHWDS (highway cycle). The fuel economy of the conventional cars was taken from the EPA Fuel Economy Guide corrected by 10 percent for the FUDS and 22 percent for the highway cycle. These corrections were made, because the actual dynamometer fuel economy test data had been reduced by those factors so that the published fuel economies would be in better agreement with values experienced in the real world.

The fuel economy of series and parallel hybrid vehicles are compared in Table 3 for both the compact, lightweight, and midsize cars. The series hybrids are assumed to operate only in the charge sustaining mode (no battery recharging from the wall plug), but the parallel hybrids can operate in either the charge sustaining or charge depleting mode. In the case of the parallel hybrid in the charge depleting mode, the fuel economy is given for gasoline alone and at the powerplant (pp) including energy needed to recharge the batteries from the wall plug. For hybrid vehicles using gasoline engines (port injected), the fuel economy of the parallel hybrid vehicles in the charge sustaining mode (batteries charged from the engine—not from the wall plug) is 9 to 12 percent higher than that of the series hybrids. For the powerplant efficiency (33%) assumed in the calculations, the parallel hybrids operating in the charge depleting mode (battery charged only from the wall plug) had only 1 to 4 percent higher equivalent fuel economy than the same vehicle operating in the charge sustaining mode. If the batteries were recharged using electricity from a higher efficiency powerplant, the fuel economy advantage of the parallel hybrid in the charge depleting mode would be lighter.

 Fuel Economy (mpg) Gasoline Engine Fuel Economy (mpg) Swirl Chamber Diesel
Vehicle FUDS Highway FUDS Highway
Small, light weight
Series Hybrid
Charge Sustaining62.471.175.785.2
Parallel Hybrid
Charge Sustaining68.279.575.988.8
Charge Depleting
gasoline alone90.186.195.694.0
Including power plant71.180.575.888.1
Midsize (1995 materials)
Series Hybrid
Charge Sustaining39.248.247.958.5
Parallel Hybrid
Charge Sustaining42.854.148.060.6
Charge Depleting
gasoline alone55.356.659.062.3
Including power plant45.454.749.160.6

Full Fuel Cycle Emissions

The full fuel cycle emissions of the hybrid vehicles are the total of all the emissions associated with the operation of the vehicle and the production, distribution, and dispensing of the fuel and electricity to the vehicle. The total emissions can be calculated for all the vehicle designs utilizing as inputs the vehicle simulation results for the electricity consumption, fuel economy, and exhaust emissions in the all-electric and hybrid modes and the upstream refueling, evaporative, and fuel production emissions based on energy usage—bothfuel and electricity. Both regulated emissions (nonmethane organic gases [NMOG], CO, NOx) and CO2 emissions can be calculated.

Regulated Emissions for Hybrid, Electric, and Conventional Cars

Hybrid vehicles operated in the charge depleting mode (battery charged from the wall plug) have total emissions comparable to those of electric vehicles if their all-electric range is 50 mi or greater. Hybrid vehicles operating in the charge sustaining mode have much greater NMOG emissions than electric vehicles when the refueling and evaporative emissions are included. The calculated total emissions of the electric vehicles are close to the equivalent zero emission vehicle (EZEV) emissions when the battery charging is done in the Los Angeles (LA) basin. These comparisons are based on the total NMOG, CO, and NOx emissions for the FUDS and highway driving cycles for electric vehicles and conventional ICE vehicles as well as hybrid vehicles. A baseline use pattern of 7,500 miles per year random, city travel, and a round trip to work of 15 miles was assumed.

Total CO2 Emissions

The difference in the CO2 emissions between operating a hybrid vehicle in charge depleting and charge sustaining modes, regardless of its all-electric range, is not large using nickel metal hydride batteries. The CO2 emissions of the gasoline and diesel engine powered hybrids vary only about 25%—not as much as might be expected based on the differences in their fuel economies—because of the higher energy content and the higher carbon-to-hydrogen ratio of the diesel fuel. The fuel cell powered, hydrogen fueled vehicles are projected to have the lowest CO2 emissions by 25 to 30 percent when compared to the most efficient of the engine powered vehicles even when the hydrogen is produced by reforming natural gas. The CO2 emissions of the conventional ICE vehicles are directly proportional to their fuel economy, which is projected to be significantly less than the hybrid vehicles. ICE powered vehicles will have low CO2 emissions only when their fuel economy is greatly increased. CO2 emissions of vehicles are highly dependent on the technologies used to power them and can vary by a factor of at least two for the same size and weight of vehicle.


There have been relatively few tests of hybrid vehicles in recent years. One example of such tests is that of the Toyota Prius by the EPA. Special care was taken in those tests to account for changes in the net state-of-charge of the batteries on the vehicle. The simulation results were obtained using the same hybrid vehicle simulation program used to obtain the fuel economy projections given in Table 2. There is good agreement between the measured and calculated fuel economies for the Prius. The EPA emissions data indicate that the CO and NOx emissions of the Prius are well below the California ULEV standards and that the NMOG emissions are only slightly higher than the 0.04 gm/mi ULEV standard. The fuel economy of the Prius on the FUDS cycle (in city driving) is 56 percent higher than a 1998 Corolla (equipped with a 4-speed lockup automatic transmission) and 11 percent higher for highway driving. These two improvements in fuel economy for a hybrid/electric car compared to a conventional ICE car of the same size are consistent with those discussed earlier in the section on fuel economy.


The societal advantages of the hybrid-electric vehicles will come to fruition only when a significant fraction of vehicle purchasers decide to buy one of them. This will occur if the purchase of the hybrid vehicles makes economic sense to them and the vehicle meets their needs. Otherwise vehicle buyers will continue to purchase conventional ICE-powered vehicles. The key to any workable marketing strategy is the availability of hybrid driveline technologies that make the transition from engine-based to electric-based drivelines manageable and attractive to the consumer with only modest financial incentives. The state of development of the new driveline technologies at the time of introduction must be such that vehicles meet the needs of the first owners and they find vehicles to be reliable and cost-effective to operate. Otherwise the market for the new technologies will not increase and the introduction of the new technologies at that time will be counterproductive. Even after the technical and economic feasibility of a new technology is shown in prototype vehicles, a large financial commitment is needed to perform the preproduction engineering and testing of the vehicles before the vehicles can be introduced for sale.

Starting in the fall of 1997, Toyota offered for sale in Japan the Prius Hybrid at a price close to that of the comparable conventional car. The initial response of the public was enthusiastic and the production rate quickly rose to more than 1,000 vehicles per month. Toyota is planning to introduce a redesigned Prius in the United States in the fall of 2000. Honda began selling a subcompact hybrid/electric car, the Insight, in the United States in the fall of 1999 at a price of less than $20,000, which is about $5,000 higher than the Honda Civic. According the EPA tests, the corporate average fuel economy (CAFÉ) (combined city and highway cycles) of the Honda Insight is 76 mpg.

Recent advances in exhaust emission technologies have resulted in the certification by several auto manufacturers of conventional gasoline fueled cars that can meet the California ULEV and SULEV standards. It seems important that the driving force for the eventual introduction of the hybrid-electric and fuel cell cars will be improved fuel economy and lower CO2 emissions and not lower regulated emission standards. It appears likely that a significant increase in the price of energy (either because of scarcity or higher taxes), regulation (for example, the CAFÉ standard), or financial incentives to purchase and license hybrid vehicles will be necessary before advanced technology hybrid vehicles become popular.

Andrew Burke

See also: Capacitors and Ultracapacitors; Drive-trains; Electric Motor Systems; Electric Vehicles; Engines; Fuel Cell Vehicles; Fuel Cells.


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