To add to your comment, the SkyActiv engines are Atkinson-cycle engines, which improves efficiency by holding the intake valve during the beginning of the compr

To add to your comment, the SkyActiv engines are Atkinson-cycle engines, which improves efficiency by holding the intake valve during the beginning of the compression stroke, so that the exhaust compression ratio is greater than the intake compression ratio, extracting more work out of the exhaust gases. The disadvantage of this is that the engine has lower power density (it won't get the same HP/liter ratings that a similar Otto-cycle engine will). This can be made up for by adding an electric motor to make it a hybrid car, or by adding a supercharger (which makes it a Miller cycle engine). 2 strokes are terrible engines generally, because they have very high emissions, usually because they release unburned fuel in the exhaust.