[VadeRetro]

What about tinkering with a car that had a second fuel injection system, which would inject very small quantities of water for each combustion stroke, perhaps at the projected end of the combustion, so that the resultant steam vapor would provide additional expansion of vapor?

Energy-wise, adding water to a combustion cylinder looks like "a wash," pun intended. If it is, the added mechanical complexity and corrosion issues are uncompensated. It figures to slow the release of energy down, but there are probably better ways to do that.

[NicknamedBob]

Inconceivable, hunh?

I do not think that word means what you think it means.