DETROIT — General Motors Corp. executives will show off a new engine technology today that could cut fuel consumption by up to 15 percent.

The savings are the product of an engine-transmission system known as homogeneous charge compression ignition, or HCCI, that marries the high fuel economy of a diesel engine with the relatively low emissions of gasoline engines.

With the potential to deliver better fuel efficiency than even some of its gas-electric hybrids, GM calls HCCI "the most awaited advanced combustion technology of the past 30 years." Mercedes-Benz soon will show its own version of the technology, though neither automaker has said when it will make them in production vehicles.

"By combining HCCI with other advanced gasoline engine and control technologies, we can deliver a good fuel savings value for consumers," Tom Stephens, group vice president for GM Powertrain and Quality, said in a statement. "It is another initiative in GM's advanced propulsion technology strategy to lessen our dependence on oil. ... I am confident that HCCI will one day have a place within our portfolio of future fuel-saving technologies."

GM is expected to demonstrate the HCCI combustion process today using two vehicles, a 2007 Saturn Aura and a 2007 Opel Vectra. Both vehicles are powered by modified 2.2-liter Ecotec four-cylinder engines. GM said the engines generate 180 horsepower.

"This truly is something different," said auto analyst Erich Merkle of IRN Inc.

David Cole, chairman of the Center for Automotive Research, agreed.

"It's the kind of thing that will have a significant competitive dimension to it," Cole said.

GM's HCCI engine is among the first in a flood of high-efficiency power plants automakers are developing to combine the high fuel economy of a diesel with the low emissions gasoline engines produce.

Mercedes-Benz plans to show an experimental engine that adds three more technologies to the concept for even higher fuel economy at the Frankfurt auto show next month.

Mercedes has not allowed journalists to test drive any vehicle with its proposed engine yet and will not say when it hopes to have the engine in production.