Compression ignition comes to gas engines

Oct. 25, 2007
What many consider the Holy Grail of combustion technology is one step closer to prime time.

In the race to field energy-efficient vehicles, don’t count out internal combustion 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, mechanical camshaft phasing, and individual cylinder pressure sensing. HCCI engines are said to use 15% less gas than conventional port-fuel injected 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.

HCCI first drive
Machine Design Editors Lawrence Kren and Robert Repas recently drove an HCCI-engine Opel Vectra and Saturn Aura at GM’s Milford Proving Grounds in Milford, Mich. Driving the cars at modest speeds and accelerations automatically engages HCCI mode, accompanied by a diesel-like clatter from the engine. The clatter was less pronounced in the Opel Vectra, however. GM engineers credit a special diesel noise-abatement package in the European-spec Vectra with the lower cockpit sound levels. The package includes an insulated engine cover and additional firewall soundproofing. Transitions to HCCI mode from conventional spark ignition were abrupt and gave a noticeable shudder. GM says such transitions will be imperceptible in production vehicles, similar to the deactivation performance of the company’s production Active Fuel Management system. AFM in GM V8s runs the engines on four cylinders under low loads to save fuel.

HCCI engine differences aren’t visible from the outside. The heat from trapped exhaust gases in the chamber promotes instant combustion at piston TDC without using a spark.

Conventional spark ignition forms a flame that burns outward from the sparkplug. HCCI ignites the entire air-fuel mixture at once.

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