HCCI is difficult enough--high compression ratio requires strengthened materials. Plus there's the hydrocarbon issues that come up with low temps. Maybe the hea

HCCI is difficult enough--high compression ratio requires strengthened materials. Plus there's the hydrocarbon issues that come up with low temps. Maybe the heat retained by not having a cylinder head would be able to compensate? High temp burn/lean burn can lead to high NOx which is pretty bad if you need a urea injector system. Mazda, GM and others are known to be working on CI engines for gas targeting 2020, so this might be the next step in 10 years. Assuming we don't all switch to pure electric...