24 August 2007
General Motors (GM) has demonstrated two driveable concept vehicles featuring homogeneous charge compression ignition (HCCI) combustion. The vehicles—a 2007 Saturn Aura and Opel Vectra—use mixed-mode gasoline HCCI engines, which operate in the HCCI mode at lower loads and revert to spark-ignition at higher engine loads.
Combined with such advanced technologies as direct injection, electric cam phasing, variable valve lift and cylinder pressure sensing, the HCCI combustion provided up to 15% fuel savings. In an integrated engine concept, gasoline HCCI with other advanced technologies approaches the engine efficiency benefit of a diesel, but without the need for expensive lean NOx aftertreatment systems, said GM.
The HCCI-powered concept vehicles—a production-based Saturn Aura and the Opel Vectra, both with a modified 2.2 L Ecotec 4-cylinder engine—drive like conventionally powered vehicles, but offer up to 15% improved fuel efficiency relative to a comparable port fuel-injected engine. The driveable concept vehicles represent some of the first tangible demonstrations of HCCI technology outside of the laboratory.
Currently, the GM demonstration prototypes can operate on HCCI up to approximately 55 mph, transitioning to spark ignition at higher vehicle speeds and during heavy engine load. An extended range for HCCI operation is intended as further refinements to the control system and engine hardware are made. A sophisticated controller, using cylinder pressure sensors, manages the HCCI combustion process, as well as the transition between HCCI combustion and conventional spark-ignition combustion. The transition between the combustion processes is notable in the demonstration prototypes, but production versions are intended to deliver an imperceptible transition while driving.
In 2005, GM announced a cooperative HCCI research program with Bosch and the Stanford University.