Researchers work toward spark-free, fuel-efficient engines

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