Homogeneous Charge Compression Ignition Engine Improves Fuel Economy

MIT researchers have demonstrated how ordinary spark-ignition automobile engines can, under certain driving conditions, move into a spark-free operating mode that is more fuel-efficient and just as clean.

In a "homogeneous charge compression ignition" (HCCI) engine, fuel and air are mixed together and injected into the cylinder. The piston compresses the mixture until spontaneous combustion occurs. The engine thus combines fuel-and-air premixing (as in an SI engine) with spontaneous ignition (as in a diesel engine). The result is the HCCI's distinctive feature: combustion occurs simultaneously at many locations throughout the combustion chamber.

In a gasoline spark-ignition engine (left), combustion begins when a mixture of fuel and air is ignited by the spark plug. In a diesel engine (center), combustion begins when fuel is injected into hot, highly compressed air.  In a homogeneous charge compression ignition engine (right), well-mixed fuel and air are compressed until combustion occurs at multiple points throughout the combustion chamber. Diagram courtesy / MIT Laboratory for Energy and the Environment

That behavior has advantages. In both SI and diesel engines, the fuel must burn hot to ensure that the flame spreads rapidly through the combustion chamber before a new "charge" enters. In an HCCI engine, there is no need for a quickly spreading flame because combustion occurs throughout the combustion chamber. As a result, combustion temperatures can be lower, so emissions of nitrogen pollutants are negligible. The fuel is spread in low concentrations throughout the cylinder, so the soot emissions from fuel-rich regions in diesels are not present.

According to Professor William H. Green, Jr., of the Department of Chemical Engineering, ignition timing in an HCCI engine depends on two factors: the temperature of the mixture and the detailed chemistry of the fuel. Both are hard to predict and control. So while the HCCI engine performs well under controlled conditions in the laboratory, it is difficult to predict at this time what will happen in the real world.

A large part of their research has utilized an engine modified to run in either HCCI or SI operating mode. Students Morgan Andreae and John Angelos have been studying the engine's behavior as the inlet temperature and type of fuel are changedNot surprisingly, the range of conditions suitable for HCCI operation is far smaller than the range for SI mode. Variations in temperature had a noticeable but not overwhelming effect on when the HCCI mode worked. Fuel composition had a greater impact, but it was not as much of a showstopper as the researchers expected.

Using the results of their engine tests as a guide, the researchers developed an inexpensive technique that should enable a single engine to run in SI mode but switch to HCCI mode whenever possible. A simple temperature sensor determines whether the upcoming cycle should be in SI or HCCI mode (assuming a constant fuel).

To estimate potential fuel savings from the mode-switching scheme, Andreae determined when an SI engine would switch into HCCI mode under simulated urban driving conditions. Over the course of the simulated trip, HCCI mode operates about 40 percent of the time.

The researchers estimate that the increase in fuel efficiency would be a few miles per gallon. "That may not seem like an impressive improvement," said Green. "But if all the cars in the US today improved that much, it might be worth a million barrels of oil per day--and that's a lot."

The mode-switching capability could appear in production models within a few years, improving fuel economy by several miles per gallon in millions of new cars each year. Over time, that change could cut oil demand in the United States alone by a million barrels a day.

Comments

[pbean]

In hybrids designed to have gasoline engines with near-constant operating conditions, the HCCI mode could operate most of the time and thus be more valuable.

[kballs]

I agree, it would be far more efficient in a series hybrid/range extender genset... in a regular ICE connected to the wheels and a throttle pedal, I imagine this would perform more like cylinder deactivation: it wouldn't happen very often.

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Take advantage of the economy of scale of large power producers, and use liquid/gas fuels for when you can't plug in!!!

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Development of analytical pattern for evaluation of technical operation of gasoline homogeneous charge compression ignition engines and cost benefit analysis of fuel conservation.

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