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Getting to HCCI: Superior Performance for Gasoline Engines

General Motors is pursuing a number of approaches to providing more environ-mentally benign powertrain systems.

General Motors is pursuing a number of approaches to providing more environ-mentally benign powertrain systems. One of them could stop all of the discussion of whether there will be an increased number of passenger cars in the U.S. powered by diesel engines because while the engine has energy efficiency analogous to that of a diesel engine, because it is a gasoline-powered engine, it doesn’t require all of the expensive NOx aftertreatment gear that diesels have hung on them. It is called “HCCI,” which stands for “Homogeneous Charge Compression Ignition.” Essentially, this is ignition initiated by cylinder pressure rather than spark.

Inside the HCCI engine—and GM has produced drivable versions based on the 2.2-liter, 180-hp Ecotec architecture and placed them in both a 2007 Saturn Aura and an Opel Vectra, so we’re talking about conventional engine designs and conventional vehicles—gasoline is directly injected into the cylinder. Then through pressure, the gasoline is simultaneously burned. Because of this more-complete burning of the fuel, there are fewer emissions and improved fuel efficiency, sort of the best of all-possible worlds.

However, the HCCI engine doesn’t do away with spark plugs entirely because when there are cold starts, there needs to be something to provide the heat within the cylinders that’s greater than that achieved through pressure alone. According to Uwe Grebe, executive director for GM Powertrain Advanced Engineering, the trick of HCCI is that there needs to be careful adjustment of the fuel-air mixture in relation to temperature. He says that while there are but comparatively minor modifications to the engine design—this doesn’t require a wholesale reconfiguration of a conventional engine by any means (e.g., there needs to be improvements in the water cooling of the engine, which necessitates some modifications), which also means that existing manufacturing equipment and facilities are usable, thereby minimizing investment—the control technology and associated algorithms are the big challenges at present. Not only is it necessary to perform the aforementioned adjustments in real-time, but because the engine does use both HCCI and spark-ignition combustion depending on operating conditions, the controller must make the transition between the two imperceptible to the driver.

So while GM is evidently well on its way to HCCI under the hoods of its vehicles, Grebe estimates that it may not be until about 2015 that those engines are there.—GSV