The New York Times has a nice summary on a form of combustion all major automakers are actively pursuing, called HCCI, short for homogenous charge compression ignition. HCCI was discovered quite some time ago through investigation of so-called "run-on", the continued running of an engine after the ignition was switched off, common with engines of late 1970's vintage. Gasoline engines can be designed for very low emissions, while diesel engines deliver great fuel economy. To make gasoline engines more fuel efficient, and diesel engines cleaner, money needs to be spent on either advanced engine technologies or expensive after-treatment devices. HCCI, which promises low emissions and high efficiency, uses a premixed fuel and air mixture like a gasoline engine, but the mixture is ignited through compression, as in a diesel engine. According to Dr. Uwe Grebe from General Motors, every automaker is working on the HCCI combustion process, since it deals with mileage and emissions challenges inside the engine, which would make it a more cost effective solution.
[Source: The New York Times, free subscription required]

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