Scientists at the Massachusetts Institute of Technology 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 equally clean.
The MIT team says 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.
Switching a spark-ignition engine to a 'homogeneous charge compression ignition' mode pushes up its fuel efficiency.
In an 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.
While 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.
Most importantly, a HCCI engine is not locked into having just enough air to burn the available fuel, as is the SI engine. So when the fuel coming into an SI engine is reduced to cut power, the incoming air must also be constrained - a major source of wasted energy.
However, it is difficult to control exactly when ignition occurs in an HCCI engine. The ignition timing in an HCCI engine depends on two factors: the temperature of the mixture and the detailed chemistry of the fuel, both of which are difficult to predict and control. So while it performs well under controlled conditions in the laboratory, it is difficult to predict at this time what will happen in the real world.
Also if the engine does not start when the piston is positioned for the power stroke, the engine will not run right.
"It's like when you push a kid on a swing. You have to push when the swing is all the way back and about to go. If you push at the wrong time, the kid will twist around and not go anywhere. The same thing happens to your engine," Livescience quoted Prof. William H Green, Jr, of the Department of Chemical Engineering, as saying.
As of now, the researchers have 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).
The MIT team presented their latest results on July 23 at the Japan Society of Automotive Engineers / Society of Automotive Engineers 2007 International Fuel and Lubricants meeting.
