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GM Unveils an Advanced Combustion Engine that Reduces Fuel Use
General Motors Corporation (GM) has integrated a prototype advanced combustion engine into two drivable concept cars, a 2007 Saturn Aura and its European sibling, the Opel Vectra. The prototype GM engine uses homogeneous charge compression ignition (HCCI), a combustion technology in which the gasoline is pre-mixed with air, as in spark-ignition engines, but the engine combusts the mixture using compression, like a diesel engine. HCCI engines operate at lower temperatures than conventional gasoline engines, allowing them to achieve high efficiency and low emissions. The two concept cars feature 2.2-liter, four-cylinder HCCI engines that burn up to 15% less fuel while meeting current emissions standards.
HCCI engines have been under development for years, but they have an Achilles' heel: the timing of the combustion process is difficult to control. The GM prototype accommodates that timing problem at low speeds and under low loads through several advanced engine control technologies, including cylinder pressure sensing and variable valve timing. The engine employs traditional spark ignition when it is started cold, then switches to the HCCI mode once it warms up. Under load and at speeds greater than 55 miles per hour, the engine switches back to spark ignition. GM admits that the transition is "notable" in its prototypes, but the company believes such problems can be ironed out in future production engines. See the GM press release.
Coincidentally, researchers at the Massachusetts Institute of Technology (MIT) are working with Ford Motor Company on a very similar HCCI engine. Like the GM prototype, MIT's engine switches between spark ignition and HCCI, but the MIT engine has only been operated under test conditions, not in an actual vehicle. However, the researchers have simulated urban driving conditions and have found that the engine would run in HCCI mode about 40% of the time, yielding a fuel economy improvement of "a few miles per gallon." See the MIT press release.