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HCCI may be the engine of choice before long

Diesel technology on gasoline engines
Monday, October 6, 2008


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 after treatment 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.

HCCI engine technology
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.

The video shows flame propagation in HCCI at the top and conventional spark ignition at the bottom

Because of the high compression ratios in a diesel, the engine must be more robust to withstand the loads and the temperature of the combustion tends to be high enough to cause the nitrogen in the air to react with the oxygen resulting in NOx. As the name implies, homogeneous charge compression ignition (HCCI) relies on the high temperatures generated by compressing the intake stream to cause the fuel to auto ignite just like a diesel. The difference is that an HCCI engine runs on gasoline (or ethanol) instead of diesel fuel and has a significantly lower compression ratio.

That lower compression ratio contributes to a lower combustion temperature and helps keep nitrogen oxide generation to a minimum. In order for this work, very precise metering of the fuel is required and that is now possible thanks to the latest direct injection technology. The fuel is injected directly into the cylinder and mixed with the air. Since gasolines vary in different regions and different times of the year, the timing and concentration has to be adjusted in real time. Having this capability built in also makes it easier to accommodate alternate fuels like ethanol.

In order to have smooth, consistent performance with varying fuels the engine management system needs to be able to vary the valve timing and lift which allows the compression ratio to be adjusted. Determining how to adjust the fuel and valve control requires a pressure sensor in the combustion chamber as well as fuel sensor like the ones already used on flex-fuel engines.

Because HCCI works best at relatively constant, partial-load conditions, the HCCI engines being developed right now are actually combination engines that can run as either spark ignition or HCCI. At higher speeds or loads, the engine runs as a normal SI type and then transitions to HCCI when the conditions warrant. The control software required to reliably detect when to operate in either mode as well as transitioning between modes is extremely complex and requires a lot of development. Most of the hardware necessary required to produce HCCI/SI engines exists now and the main stumbling block is getting reliable, cost effective cylinder pressure sensors.

All of this technology results in an engine that approaches the efficiency of diesel engines at a significantly lower cost. An HCCI engine provides a fifteen percent boost in fuel economy and reduced emissions compared to a conventional SI engine using pretty much the same exhaust after-treatment systems.

For the first media sampling of HCCI, GM provided an automatic transmission-equipped Saturn Aura and five speed manual Opel Vectra. Both cars had the same 2.2L Ecotec four cylinder modified to operate in HCCI mode at speeds up to 55 mph and partial loads. A display mounted on top of the dashboard shows a map of engine speed and fuel mass and indicates when the engine is in SI or HCCI mode.

On the test loop that we were able to drive, the transitions between SI and HCCI were largely transparent and far smoother than any of the current production hybrids when starting and stopping the engine. Performance felt pretty much the same as a regular Vectra or Aura. The only detectable difference was a slight audible ticking when the engine was in HCCI. The technology definitely works, the main problem now will be making the control software robust enough to deal with all real world weather, road and driver conditions.

It's critical to make sure that the fuel injection and valve timing and lift are managed correctly. If the fuel ignites too early, it can cause excessive noise or damage to the engine internals. If it happens too late, the engine can misfire or stall so the software and the cylinder pressure sensor have to be reliable. Currently GM is not giving a timeline for when HCCI engines will go into production, but it will probably be sooner rather than later.

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