On Homogeneous Continuous Compression Ignition, of HCCI for short.
Toyota (and Ford) call it Controlled Auto Ignition, or CAI.
GM is working feverishly on it and will have proto's to drive next year, go here for the story:
http://www.sae.org/automag/techbriefs/02-2007/1-115-2-22.pdf
It has been the subject and hot topic of many SAE papers and symposiums the last few years.
Think of it as the weight of a piston engine with almost the fuel economy of a diesel that works good on low octane fuel.
It works in a narrow band, fuel economy can increase as much as 40% and emissions drop 80%.
In my opinion it is the "Holy Grail" for engines and will make diesels boat anchors. The question has been asked on the Green Car Blogs, will GM try to incorporate HCCI into the "Prime Mover" engine on the "Volt". Since it will run at steady state, you could potentially set it up to stay in the HCCI mode all the time. If they do, Katie Bar the Door....
In my opinion, the fuel to watch, Propane....
Also a very interesting technology you may want to google: "Spark Ignited Heavy Fuel"
More later....