16 August 2005

General Motors, Robert Bosch, and Stanford University announced a $2.5 million joint program to accelerate the development of Homogeneous Charge Compression Ignition (HCCI)—a combustion technology that has the potential to dramatically improve the emissions of NOx and PM from the diesel engine and to improve the fuel efficiency of the gasoline engine. GM would use the HCCI technology in place of both the conventional diesel and gasoline engines, including those in hybrid vehicles.

GM said it intends to develop and demonstrate the viability of HCCI within the next few years. Under the joint program, Bosch would be responsible for the development of sensors and actuators needed for the HCCI engine. The Bosch Research and Technology Center (Bosch RTC) in Palo Alto, CA, in conjunction with Stanford, have already been working on advanced system technology for HCCI combustion control.

In the HCCI engine, fuel is uniformly mixed with air, as in a spark-ignition engine, but at a lean air-to-fuel ratio. Rather than using a spark plug to ignite the air-fuel mixture, the mixture is compressed by the piston until rising temperatures inside the chamber ignites it spontaneously—a process similar to that used in the diesel engine, but at a much lower temperature. The lower combustion temperature combined with the high ratio of air to fuel, virtually eliminates NOx emissions. Compared to the gasoline engine, HCCI lowers throttling losses, which leads to a significant boost in fuel economy.

The major obstacle in commercializing HCCI engines is the difficulty in controlling HCCI combustion, especially at high engine load conditions. HCCI combustion—dicovered over a hundred years ago—can be made possible only with the use of advanced electronic engine control technology and systems to make it robust. “The development of critical sensors and actuators, along with control systems, will allow us to maximize the performance of the combustion system during transient operation, and comprehend the variation in fuels found around the globe,” said GM.

Another drawback of HCCI are increased CO and HC emissions, which occur at low exhaust temperatures that make it difficult to use catalysts to control them.

Faced with the challenges in achieving stable HCCI combustion, several engine manufacturers also indicated the possibility of developing mixed-mode engines, which would operate in the HCCI regime at lower load, and switch to the conventional diesel mode at high engine loads. This type of engines have been considered for meeting the US Tier 2 emission standards in light-duty vehicles. In heavy-duty engines, mixed-mode HCCI may be an option for meeting the US 2010 and the JP 2009 emission standards.

Source: General Motors