Conventional gasoline engines work by pistons compressing a mixture of fuel and oxygen that is ignited by a spark. Diesel engines work by injecting fuel into hot, compressed air which then ignites. HCCI is a mixture of the two methods. Air and fuel are mixed, injected into cylinder, then ignited by pressure from the piston.
One MIT researcher says HCCI could get 25% better fuel economy over a gasoline engine, while running cleaner than most diesels.
The article from MIT's Technology Review says several automakers are currently researching HCCI engines. Volvo has been testing an engine that can switch between spark ignition and HCCI. AutoblogGreen just recently wrote that Mercedes is working on what they call DiesOtto, which is basically HCC, as welI.
[Source: MIT Technology Review]