Diesel or Gasoline? This Clever HCCI Engine is a Bit of Both

In the future, you're going to hear much more about HCCI, or homogeneous charge compression ignition. Besides universities and national laboratories, virtually every major automaker is working on this promising technology. Research on HCCI, underway for about three decades, is now producing tangible results.

General Motors has shared the results of its considerable research in this area with two drivable concept vehicles, a 2007 Saturn Aura and Opel Vectra, each with a 2.2-liter Ecotec four-cylinder engine incorporating HCCI. General Motors claims up to a 15 percent fuel savings with the 180-horsepower engine while meeting current emissions standards. This results from combining HCCI technology with other enabling, and already proven, technologies like direct injection, electric cam phasing, and variable valve lift. The GM technology is compatible with today's gasoline and E85 fuels.

HCCI combines the best of both the spark-ignition (gasoline) and compression-ignition (diesel) worlds. With HCCI, the fuel and air mixture is ignited by compression rather than by spark plugs. Unlike either a spark ignition or diesel engine, HCCI produces a low-temperature and flameless release of energy throughout the entire combustion chamber, so all the fuel in the chamber is burned simultaneously. Lean air/fuel mixtures also help the engine approach the efficiency of a diesel.

Compression ratios are substantially lower than in a diesel, which means lower-combustion temperatures that minimize the production of nitrogen oxide (NOx) emissions. The result is diesel-like fuel economy, but without the need for the more elaborate and expensive aftertreatment devices required to clean up diesel exhaust emissions.

Another important advantage of HCCI is that because a gasoline engine doesn't need to operate with the high compression ratio required by diesel, engine blocks, cylinder heads, and other components can be much lighter, resulting in less-expensive engines. Thus, the HHCI engine approaches diesel efficiency at a significantly lower cost and with weight savings that contribute to better fuel efficiency as well.

While General Motors has shown that HCCI definitely works, there's still much to be done before HCCI vehicles show up in new car showrooms. Control software must be able to handle weather and altitudes anywhere on the globe, as well as differing road and driver conditions. HCCI's flameless combustion requires much more complex composition and temperature control than just the traditional methods of adjusting spark timing and intensity. Fuel injection, valve timing, and lift are critical. If the fuel ignites too early it can cause excessive noise or engine damage, and if it's too late the engine could misfire or stall.

As an example of its complexity, GM uses spark ignition when starting a cold HCCI engine to produce needed heat to warm the exhaust catalyst and enable HCCI operation. Also, the engine operates in the HCCI mode up to 55 mph and under partial loads. At higher speeds or heavier loads, the engine runs with normal spark ignition. HCCI works best at relatively constant, partial-load conditions. Since gasolines vary with location and season, valve timing and lift must be adjusted in real time for precise compression ratio control. Another big challenge is reliable, cost-effective cylinder pressure sensors. All of this is expected to come together as developmental work continues throughout the auto industry on this bright new engine technology.