In the race to field energy-efficient vehicles, don’t count out internalcombustion technology just yet. GM recently debuted its homogeneous- charge-compression-ignition (HCCI) system in two drivable concept cars, a production-based 2007 Saturn Aura and Opel Vectra.

“HCCI was a dream of engine designers when I was an engineering student years ago,” says Tom Stephens, group vice president, GM Powertrain and Quality. “Today, using mathbased predictive analysis and other tools, we are beginning to make this technology real.“

HCCI is the capstone of an integrated suite of engine technologies that includes central direct-fuel injection, variable-valve lift, mechanicalcamshaft phasing, and individualcylinder pressure sensing. HCCI engines are said to use 15% less gas than conventional port-fuelinjected engines, and meet current emissions standards.

Unlike spark-ignition gas engines or compression-ignition diesel engines that have a combustion process characterized by growth of a flame front from a single point in the combustion chamber, HCCI produces a flameless, simultaneous release of energy throughout the entire combustion chamber. Lack of a flame and hot zones lowers combustion temperature and NOx emissions.

Fuel-air mixtures are comparatively lean, which helps the engine approach the efficiency of a diesel, but without the need for costly lean-NOx after-treatment systems. Burning less fuel at lower temperatures also cuts the amount of heat energy lost during combustion, boosting efficiency. HCCI engines have a compression ratio of 12:1 (similar to that of a conventional direct-injected gas engine), so they can run on regular pump gas and E85. GM says HCCI engines will cost less to build than diesels because the latter need stronger components to withstand compression ratios greater than 20:1.

“Perhaps the biggest challenge of HCCI is controlling the combustion process,” says Uwe Grebe, executive director for GM Powertrain Advanced Engineering. “With spark ignition, you can adjust the timing and intensity of the spark. But with HCCI’s flameless combustion, you must change the mixture composition and temperature in a complex and timely manner to get comparable performance.”

Having ample heat in the combustion chambers is key to making HCCI work. The engines use a conventional spark ignition for cold starts and when HCCI is disengaged. Fuel comes from conventional injectors located in the center of each combustion chamber. A controller uses special algorithms and feedback from the cylinder pressure sensors to adjust cam timing and fuel injection in the milliseconds between combustion events. “Going to HCCI mode from conventional spark ignition signals the fast-mechanical cam phasers and a variable-valve-lift mechanism to close the exhaust valves early in the exhaust stroke, trapping some of the hot residual combustion gases in the combustion chamber,” explains GM Global HCCI Program Manager Matthias Alt. “This helps maintain a high cylinder temperature to facilitate auto-ignition when the fresh airfuel charge is added next cycle.” Operation at cold ambient temperatures necessitates trapping more hot gas in the combustion chamber (earlier exhaust-valve closing), for example.

Currently, the GM demonstration prototypes can run in HCCI mode to about 55 mph, going to spark ignition at higher vehicle speeds and under heavy engine load. A goal of the program is to extend HCCI’s operating envelope through refinements to the control system and engine hardware. GM says HCCI will work on any gasoline engine in its inventory and could combine with hybrid technology. No release date has been set for production HCCI-engine cars.