A hybrid within a hybrid?
That's the promise of the Homogeneous Charge Compression Ignition engine -- a sort of super-efficient "hybrid" internal combustion engine, one that is part conventional gasoline and part diesel, that could serve alongside an electric motor in a hybrid automobile powertrain.
With HCCI, the intake charge of gasoline and air is ignited not by a spark plug as in an Otto cycle engine, but with compression like a diesel engine. HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously, producing power similar to today's conventional gas engines, but using less fuel to do it.
Though HCCI engines are sometimes referred to as "cold cumbustion" engines, heat is necessary for the HCCI process, so a traditional spark ignition is used when the engine is started, to generate heat within the cylinders and quickly heat up the exhaust catalyst.
During the compression ignition mode, the air/fuel mixture is lean, meaning there is a larger percentage of air in the mixture. This helps the HCCI engine approach the efficiency of a diesel, but requires only a conventional automotive exhaust after-treatment, such as a catalytic converter, rather than the more elaborate systems fitted to today's clean diesels.
As HCCI engines don't respond as robustly as conventional gasoline engines during a sudden demand for full acceleration, pairing HCCI engines with electric motors would seem to be an excellent way to boost performance, or help get the HCCI system going when the engine is cold.
Real World Use
GM demonstrated its latest HCCI technology during a media test drive in Washington, D.C. and New York City in 2009. Journalists drove an HCCI-enabled concept car on public roads to get a real-world feel for the efficient and low-emissions engine. GM says that HCCI provides up to a 15% fuel savings when combined with several additional advanced technologies, while meeting current emissions standards.
Dr. Rolf Leonhard, executive vice president of the Engineering Gasoline Systems Division of Robert Bosch GmbH, believes that HCCI could be a very promising technology for future generations of gasoline engines.
"It is simply a better way to combust, providing large gains in fuel economy," Leonhard said.
Daimler too has been showing off its HCCI work in an exotic-looking research touring sedan called the F700. The car's 238hp 1.8-liter DiesOtto (for Diesel and Otto) four-cylinder engine combined with a 20hp electric motor has achieved 44.3 mpg and propelled the car to a 7.5-second 0-to-62 mph dash.