The hard part is controlling the pressure, temperature and air/fuel mixtures precisely enough to manage that combustion without causing excess noise and engine damage. When we first tried the HCCI prototypes a couple of years ago, the engines had a fairly narrow band of HCCI operation with the engine running in basic spark ignition mode the rest of the time. Thanks to a newly developed mixed-mode HCCI feature and external EGR, the engines can now run in HCCI from idle all the way to 60 mph!
We had a chance to drive a Saturn Aura with an HCCI engine based on the 2.2-liter EcoTec four-cylinder around the streets near the Tech Center. The engine ran smoothly and transitions between HCCI and spark ignition really couldn't be felt. The only indication of a transition was a slight ringing sound over the first couple of power cycles after transition.
The basic hardware for a production HCCI engine is in place now, with the only new piece of hardware being a combustion chamber pressure sensor. GM is continuing to work on the control software to make this a robust system and even adapting the homogeneous charge and pressure sensors to diesel engines to reduce NOx emissions. The HCCI engine achieves about a 15% improvement in fuel efficiency compared to a similar spark ignition engine at a much lower cost than a hybrid. GM hopes to have HCCI engines in production in about five years.
[Source: Green Fuels Forecast]