Originally Posted By: weasley
The big change here is not GDI, it is the high compression ratio and the homogeneous charge ignition. Doing this reliably is the key.
It's not really homogeneous charge ignition. It's stratified charge. Mazda finally faced facts and gave up on HCCI because it was impractical. They have added spark to control the start of combustion, and stratified charge in the region of the spark plug so that the flame will propagate through the leaner charge in the rest of the chamber. According to the video, there is a homogeneous charge of 37:1 throughout the chamber, and a direct injector is used to make a charge stratification of 27:1 in the region of the plug right before it sparks. I think Mazda has done a great job in thinking outside the box, trying to bridge the gap between Otto and Diesel combustion. They would have had to figure out whole new methods of engine control to make it seamless to customers in all driving modes.