The process GM is testing, called homogeneous charge compression ignition, approaches the fuel efficiency of a diesel engine without the need for advanced pollution controls, the company said in a statement.

It is being tested in two drivable concept vehicles, a Saturn Aura and an Opel Vectra, and was to be shown to reporters Friday at GM's proving ground in Milford.

The process ignites an air-fuel mixture in the cylinders by compressing it, producing a low-temperature, flameless energy release in the combustion chamber, the company said. Since all the fuel in the combustion chamber is burned at the same time, the engine uses less fuel to produce power that is similar to conventional engines.

The company gave no estimate of when the technology might come to market, but said it still needs to work on controlling the combustion process.

"Additional development costs, including research and testing programs, are required to make the technology ready for the great variety of driving conditions that customers experience," Uwe Grebe, executive director for GM's powertrain advanced engineering, said in a statement.

GM shares fell 19 cents to $30.56 in morning trading.

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