Last summer we had the chance to meet up with GM powertrain researcher Paul Najt and drive GM's prototype HCCI engines. Homegeneous charge compression ignition engines basically take some of the technical concepts behind diesel engines and combine it with a gasoline engine. The result is a new power system that achieves most of the benefit of diesel efficiency without the problems of soot and NOx emissions. As features like variable valve timing, direct fuel injection, cylinder pressure sensors and more powerful control systems have become available in engines, HCCI has become more practical. HCCI doesn't work well under all operating conditions, but engineers are learning how to blend HCCI and normal spark ignition operation seamlessly. The guys at Popular Mechanics had a chance to visit the GM research lab where the HCCI engines are being developed. There is a good interview with Najt and some explanation of how the engines work in a video on their site.
[Source: Popular Mechanics]