Gasoline engines emit fewer particulate and lower levels of nitrogen oxide emissions that diesel engines. Why? Because gasoline engines typically run at a lower operating temperature. However, those high operating temperatures make Diesel engines more fuel-efficient. On the downside Diesel engines emit more nitrogen oxide emissions and particulates.
Most auto manufacturers are working on HCCI engines. The only manufacturers talking so far are General Motors, Daimler and Volkswagen. An HCCI engine runs on gasoline, but operates like both engines. When running at full load or starting, it runs like a traditional gasoline engine with a spark ignition. However, in its most efficient mode, part throttle, part load it operates like a diesel with compression ignition. In this Diesel mode, the HCCI engine can be as fuel efficient as a Diesel -- delivering up to 15 percent more fuel economy than an indirect gasoline spark ignition engine.
The challenge for engineers is this: gasoline engines compress the air/fuel mixture to approximately 102 pounds of pressure per square inch before igniting it with a spark. A Diesel engine compresses the mixture to approximately 319 pounds of pressure per square inch. This heats the air enough to instantly ignite the fuel injected into the combustion chamber. "Proper combustion control is our biggest challenge," said Uwe Grebe, executive director of GM Powertrain in an interview with Automotive News Europe. "The change from spark-ignition mode to self-ignition mode still needs a lot of development," he added. Rough transitions from one mode to the other continue to plague the HCCI engine.
Electronic engine controls that allow variable valve timing; controlled injection timing and mixture temperature will be part of the solution. Daimler has added a variable compression ration with an adjustable crankshaft stroke to control all the variables.
Daimler calls their engine the DiesOtto and it's a 1.8-liter four-cylinder, rated at 238 horsepower. It was shown in the Mercedes F700 concept vehicle at the Frankfurt auto show last September.
GM is working on a 180 horsepower HCCI version of its 2.2-liter Ecotech engine and VW is working on a 1.6-liter FSI gasoline engine. No horsepower figures for either engine were distributed. Volume production of HCCI engines is not expected until 2020, but some special models might be available by 2015.
