General Motors Striving to Bring HCCI Engines to Mass-Market Vehicles

Rex Roy

If you think that companies like General Motors only think about fuel economy when new CAFE standards are making news, you'd be wrong. GM and other manufacturers are always looking for new technologies to more efficiently power the next generation of transportation.

Need proof? You could look to the 1978 Eldorado GM built in the 1980s that ran on coal dust. The engine was a turbine. Engineers labeled that technology "disruptive" and they couldn't get the NOx under control, but it goes to show the imagination and vision of automotive engineers.

A less disruptive and more promising technology is one that been around for a few years, Homogenous Charge Compression Ignition (HCCI). Several manufacturers have produced working prototypes, including Mercedes-Benz. The technology promises fuel-saving and emissions-reductions.

According to GM, an HCCI engine, when combined with other advanced technologies, provides up to 15 percent greater fuel economy than a comparable, non-HCCI engine by radically altering the combustion process.

In general terms, the heat and pressure within an HCCI engine's cylinders are used to ignite the air/fuel mixture without the aid of a traditional spark-generated flame - roughly analogous to the combustion process of a diesel engine. Heat is a necessary enabler for the HCCI process, so traditional spark ignition is used when the engine is cold to generate heat within the cylinders and instigate "auto-ignition" of the mixture.

More than a laboratory experiment, HCCI has been successfully demonstrated in prototype models in North America and Europe. Last spring, GM took the technology on the road, putting journalists and others in an HCCI-equipped Saturn Aura for real-world drives in Los Angeles, Washington, D.C., and New York. The vehicle featured the breakthrough of auto-ignition (HCCI) from idle to 60 mph, which significantly advances the benefits of HCCI and the viability for production.

"HCCI delivers enhanced fuel savings without sacrificing the performance consumers have come to expect," said Prof. Dr. Uwe Grebe, executive director for GM Powertrain Advanced Engineering. "It is a great example of how GM is developing advanced engine technology for consumers that squeezes more miles per gallon of gas and reduces emissions."

HCCI's efficiency comes from reduced pumping losses, burning fuel faster at lower temperatures and reducing the heat energy lost during the combustion process. Consequently, less carbon dioxide is released because the engine's operation in HCCI mode is more efficient.

During HCCI mode, the engine approaches the efficiency of a diesel, but unlike a diesel, it requires only a conventional exhaust system. Diesel engines require more elaborate and more expensive exhaust "aftertreatment" to reduce emissions.

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