Most of you never thought of a cylinder head as a condenser however at top dead center (where most of the burning is done) piston and cylinder head are only millimeters away from each other. That is why the mixture is in close contact with the cylinder head. E.G. a diesel running at lambda=3 with a cylinder head temperature of 90 C. Diesel fuel is a mixture with a mean of C10H22. From Raoults law we know all these different components will give one common vapour pressure. Let take C10H22 as the mean. At lambda =1 it would represent 1.7% of the moles in the mixture wich is also the partial pressure . At lambda =3 it is 0.5%. The vapour pressure at 80C below its boiling point will be 3% of a bar. This means that at 6 bars total pressure it starts condensing. I think a nice way to improve on the mileage is to add steam to process in such a way that the condensed fuel would land on top of the water. This would A. lower the pressure by condensating the H20 formed by the reaction before TDC (less NOx) B facilitate burning off from the cylinder head (giving better mileage and less soot). Most cars have a air weight sensor. so over the OBD you can acces this. I have a full description for gasoline cars but it is to lenghty for this forum