GM has developed a new type of cylinder combustion process which improves fuel efficiency by 15% and is compatible with all commercially available gasoline and E85 fuels. Known as Homogeneous Charge Compression Ignition or HCCI, ignition is produced by compressing the air/fuel mixture in the combustion chamber without the use of a spark plug. The result is a flameless low-temperature burn which produces less CO2 and consumes less fuel. As a result, the HCCI engine has the efficiency of a diesel engine but without the NOx after treatment system. Despite the ability of the HCCI process to function without spark, it still needs heat to get started and therefore uses a conventional spark ignition system for cold starts, heavy engine load conditions and during speeds above 55 mph.
HCCI combustion is managed by a sophisticated control unit which orchestrates central direct injection system, with variable valve lift on both the intake and exhaust sides, dual electric camshaft phasers and individual cylinder pressure transducers. “Perhaps the biggest challenge of HCCI is controlling the combustion process,” said Prof. Dr. 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 need to change the mixture composition and temperature in a complex and timely manner to achieve comparable performance.”
The two test vehicles, Saturn Aura and Opel Vectra, used modified 2.2L Ecotec four-cylinder engines producing 180 hp and 170 lb/ft of torque (230 Nm) and maintain normal drivability except for a noticeable change in driving behavior when the combustion process transitions from HCCI to conventional spark ignition. Like all new models, GM assures that further refinements will be made in all weather environments and the HCCI to spark transition will be unnoticeable.