[ASV]

you want the fuel and air in the cylinder to me as compact and dense as possible. A gas is less dense than a liquid mist so you could not have as much air in the cylinder and still have an effective fuel air ratio if you use fuel in a gaseous form.

it's the inert nitrogen that does most of the work on the piston absorbing heat and expanding.
pushing and simultaneously preventing the motor from melting. Having less nitrogen and oxygen from the fuel taking up more space means the nitrogen needs to get hotter to do the same work. but there is also less heat available because the fuel and oxygen must match and there is also less oxygen.

the actual ideal fuel air supply would have everything cryogenic.
liquid oxygen
liquid nitrogen
liquid fuel
With room temperature exhaust

I think 50% hydrogen peroxide water with an alcohol injection would be a fun motor to play with and a few steps closer to ideal