Many modern cars have a multiple valve system, electronic spark ignition, and electronic fuel injection.
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?
One might need a somewhat longer stroke, so the engine would run more slowly, but it should also run cooler, with less waste heat.
It would undoubtedly be a good idea to run the engine warmly enough to complete the evaporation of the water, rather than accept its corrosive presence, but I see no basic reason this would not be ultimately more efficient.
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.