Fuel Economy of Hybrid Cars

Eleanor Hanson

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 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 drivelines, 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.

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