HCCI prototypes combine spark, diesel technologies
August 24, 2007 - 11:37 am EST
General Motors is developing an engine that burns gasoline without a spark plug at speeds as high as 55 mph.
The engine sips fuel like a diesel engine but produces low emissions like a gasoline engine.
GM today demonstrated two working concepts of its homogeneous charge compression ignition engine. The engines can improve fuel economy by as much as 15 percent compared with a conventional gasoline engine, GM said.
Volkswagen AG and DaimlerChrysler AG also are developing engines that will use diesellike compression ignition with gasoline. DaimlerChrysler is expected to disclose more details of its DiesOtto engine at the Frankfurt auto show in September.
The engines allow automakers to reduce emissions of carbon dioxide, which cause global warming, without messy diesel exhaust, which is expensive to clean.
A conventional gasoline engine uses a spark to ignite the air-fuel mixture in each cylinder. Catalytic converters easily and cheaply clean the exhaust to meet tough emissions standards.
A diesel, or compression ignition engine, relies on high pressure inside the cylinder to generate heat to cause the air-fuel mixture to burn. It burns as much as 25 percent less fuel than a conventional engine because it has a higher ratio of air to fuel - ideal for reducing CO2 emissions.
Soot and NOx
But the combustion process generates soot and high levels of oxides of nitrogen, or NOx - a leading cause of ozone pollution. Cleaning that exhaust to meet stringent emissions rules calls for expensive technology, such as advanced catalysts and urea injection. Those costs are on top of the extra costs of $2,000 or more for a diesel engine.
The GM engine, which it calls HCCI, relies on an advanced computer controller and a pressure sensor for each engine. The fuel is injected directly into the cylinder, where it mixes with air. The engine also uses variable valve timing.
The HCCI engine runs at the lower cylinder pressure of a gasoline engine. In HCCI mode, the air-fuel mixture in the combustion chamber burns at lower temperatures than a diesel engine. The lower temperature helps prevent formation of NOx.
The HCCI engine, which burns gasoline or E85 ethanol, runs in compression ignition at speeds as high as 55 mph, GM said. At higher speeds, or under heavy loads, the engine switches to spark ignition mode.
Strategy adapts
The combustion strategy can be adapted to most existing engine architectures, GM said.
GM said it is continuing to refine the technology but did not commit today to a production timetable for the HCCI engine.
"Perhaps the biggest challenge of HCCI is controlling the combustion process," Uwe Grebe, executive director of advanced engineering for GM Powertrain, said in a statement.
"With spark ignition, you can adjust the timing and intensity of the spark, but with HCCI's flameless combustion, you need to change the mixture composition and temperature in a complex and timely manner to achieve comparable performance."
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Source:
Dale Jewett
Automotive News
