HCCI is difficult enough--high compression ratio requires strengthened materials. Plus there&#x27;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&#x2F;lean burn can lead to high NOx which is pretty bad if you need a urea injector system.<p>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&#x27;t all switch to pure electric...