Hybrid electric vehicles (HEV) have two power sources. One converts fuel into useable energy, and the other -- an electric motor powered by an advanced energy storage device -- lowers the demand placed on the first power source.
When the two HEV power sources are arranged in parallel, one or both can be used, depending on the situation. The electric motor often can power the HEV alone in city driving or over flat terrain. When the hybrid is accelerating and climbing hills, the two power sources can work together for optimal performance.
"Direct-injection" (DI) engines, in which fuel is injected directly into each engine cylinder, show the greatest promise for near-term hybrids. Because the DI engine works in concert with an HEV's electric motor, the engine can be smaller and turned off automatically when not needed, thus increasing mileage and reducing emissions. DI engines can be fueled by either gasoline or diesel fuel.
As with electric vehicles, the total GHG emission rate of an HEV is somewhat dependent upon the mix of fuels used to generate electricity. By using a small efficient internal combustion engine, however, there should almost always be gains compared to a conventional gasoline fueled car. Based on a US national mix of power plants generating the electricity, GHG emissions reduction on the order of 15% seem likely. Using a western U.S. generating mix, reductions should approach one-third and only slightly less in the Northeast.
HEVs have several advantages that make commercialization prospects good. First, HEVs allow car manufacturers to continue selling engines. Second, HEVs provide the range of a traditional gasoline vehicle. Third, the development of HEVs will continue to advance the development of electric drive trains, an outcome for which a number of interests are pushing. Honda has demonstrated very high fuel efficiency with a hybrid car, which it introduced to the US market last year.
Toyota is selling an electric/gasoline hybrid car, at a premium of about 35%. They have stated that they could sell the vehicle at approximately the same price as a conventional vehicle and break even if production increases to about 200,000 units annually.
Toyota's "Prius" is the first mass-produced gasoline-electric hybrid vehicle in the world. The Prius is currently for sale in Japan, the US and Europe where it boasts nearly double the fuel efficiency of conventional gasoline engines. It consists of a 1.5-liter gasoline engine and battery. In addition to conserving fuel, the Prius reduces CO2 by 50% and CO, HC and NOx by 90% below Japanese standards based on the Japanese test procedure. Testing by EPA indicates significant benefits under U.S. type driving conditions as well.
Cost is currently the largest barrier HEVs face. In addition, low sulfur fuel is necessary to maximize the overall emissions reductions from the gasoline engine.
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