Green Car Advisor

By Robert E. Calem, Contributor

General Motors Corp. says it has achieved a new milestone in the development of an experimental fuel combustion technology called homogenous charge compression ignition, and can now operate an HCCI engine at much lower speeds than previously possible.

HCCI technology mimics a diesel engine, igniting a mixture of fuel and air by compressing it in the cylinder, but it works with gasoline like a traditional spark ignition engine.

And unlike either of those other engine technologies, HCCI burns the fuel at a low temperature and throughout the entire combustion chamber – yielding the power of a gasoline engine and the torque of a diesel with greater fuel economy and lower carbon dioxide emissions than either.

GM, which previously was unable to operate an experimental engine in HCCI mode at speeds below 15 miles per hour,  demonstrated operation in the fuel-saving mode at idle this week in a specially modified Saturn Aura test vehicle.

Demonstrations in Washington, D. C. and White Plains, N.Y., also marked the first time journalists were permitted to drive a vehicle with an HCCI engine on public streets.

The prototype engine operates in both HCCI and regular spark-ignition modes, but did not operate in HCCI mode at idle when initially demonstrated to the press last August at a GM test track.

The ability to operate at idle is a technological breakthrough, and GM – one of several major automakers developing HCCI engines -- believes it is the first to achieve it, said Paul M. Najt, senior engine systems group manager at GM’s powertrain systems research lab in Warren, Mich.

Despite the great technological leap, Najt said HCCI still needs a lot more work before it can be commercialized.

He promised that final milestone would be reached “within a decade.”

Among the future improvements GM is pursuing: Enabling the engine to operate in HCCI mode at speeds faster than 55 MPH, the present upper limit.

Meanwhile, GM is continuing to refine its standard diesel engines, as well. Sharing the spotlight with the HCCI-equipped Aura at this week’s press event was a compact new Duramax 4.5-liter V-8 turbo-diesel engine that contains 70 fewer parts than other diesel engines. It will debut in the 2010 Chevrolet Silverado and GMC Sierra light-duty pickup trucks.

Don’t look for diesel to be introduced in GM passenger cars in North America anytime soon, though, said Charles E. Freese, executive director of diesel engineering, reiterating a position the company has long taken.

Although others, including Volkswagen and Mercedes-Benz, are introducing clean diesel passenger cars here, Freese said GM believes diesel is not a “rational” choice for the passenger car segment, given the fuel's high price and the relatively modest gain in fuel economy it yields in light-duty vehicles.