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DETROIT — General Motors Corp. executives are touting a new engine technology that could cut fuel consumption up to 15 percent.

The savings are the product of an engine-transmission system known as homogeneous charge compression ignition, or HCCI, which 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 put the engines in production vehicles.

GM is demonstrating the HCCI combustion process using two concept vehicles, a 2007 Saturn Aura and a 2007 Opel Vectra. Both vehicles are powered by modified 2.2-liter Ecotec four-cylinder engines. GM says the engines can generate 180 horsepower.

GM declined to say what fuel-efficiency figures the HCCI Aura might achieve in the Environmental Protection Agency tests that produce the window-sticker fuel-economy figures automakers advertise.

However, a 15 percent improvement over an Opel Vectra currently on sale in Europe works out at about 26 mpg in the city, 43 on the highway and 35 in mixed use on the European Union's standard fuel-economy test. That test is different from the EPA's procedure, but it suggests the HCCI engine would surpass the fuel economy of GM's 2008 Saturn Aura gasoline-electric hybrid.

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

David Cole, chairman of the Center for Automotive Research, agreed that it is a big deal.

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

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 in Germany 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.

Mercedes' engine uses a turbocharger, an electric hybrid system and a variable compression ratio in a 1.8-liter, four-cylinder engine. Mercedes calls the concept DiesOtto, a combination of the German names for diesel and gasoline engines.

Mercedes says its engine could generate 238 horsepower and achieve 30 mpg in a Mercedes S-class luxury sedan on the European test cycle that combines city and highway driving.

As automakers compete to be first to market an HCCI engine, however, they continue to work on many other technologies as well, and GM is expected to introduce the 14 new or significantly revised powertrains it will make available in the 2008 model year.

"For the most part, GM has a leg up on its competition in terms of fuel efficiency. They just get little credit for it," Merkle said. "They've really done some impressive things with fuel efficiency and performance."