The Gasoline-Diesel Hybrid
September 11, 2007
General Motors recently invited journalists to test two prototype vehicles powered by HCCI, also known as “homogeneous charge compression ignition engines.” The HCCI engine is like a cross between a gasoline engine and a diesel engine—and can improve the fuel efficiency of a comparable gasoline engine by as much as 30 percent. For example, a Toyota Camry with a 4-cylinder HCCI engine could achieve highway gas mileage approaching 40 miles per gallon.
HCCI may not have a sexy name, but its potential to deliver substantial fuel savings is impressive. More importantly, the savings could come much sooner than other so-called “game-changing” technologies that receive a lot more press.
More Than Hot Air
In a gasoline engine, fuel and air are mixed together, compressed, and then ignited by a spark. In a diesel engine, only the air is compressed. Compressing the air causes it to heat up. When fuel is injected into the cylinder, the fuel combusts on contact with the hot air.
HCCI’s process begins like a conventional gasoline engine: fuel and air are mixed and compressed. But instead of lighting the mixture with a spark, the HCCI engine compresses the mixture to the point that it auto-ignites. In a conventional gasoline engine, auto-ignition is bad news since it can cause severe damage, but in an HCCI powerplant, the process is controlled and yields numerous benefits. HCCI deliver the efficiency of a diesel engine, but without generating the smog-inducing nitrogen oxides (NOx) that are inherent in diesel combustion. HCCI also can accommodate a range of fuels, including gasoline, diesel fuel, propane, E85, and natural gas.
HCCI faces a number of technical hurdles. One of the biggest challenges is the management of the combustion process over a diverse range of driving conditions. The HCCI process is fragile, and not yet tolerant of shocks such as cold starts or quick acceleration. To get around this limitation, companies like GM have built “hybrid” HCCI engines that use conventional spark-ignition combustion under some conditions, and HCCI under others. The result is a 15 percent increase in fuel economy in vehicles such as GM’s Saturn Aura HCCI concept vehicle. GM has yet to pair an HCCI engine with a hybrid-electric drivetrain, but the combination of the two technologies could increase gas mileage even further. In fact, some observers have suggested that an HCCI engine is perfect for a series-hybrid like the Chevrolet Volt, because a series-hybrid allows the engine to operate with little variation in engine speed and load.
Like many promising new technologies, HCCI has yet to make the leap from test track to production car. At least four major automakers are working on HCCI engines, and many are beginning to brand the technology—a sign that it is making its way toward mainstream vehicles. Unlike plug-in hybrids, electric cars, or fuel cell vehicles, HCCI represents a less radical shift in technology. But a 30 percent improvement in fuel economy is revolutionary, no matter how you get it.