Assume that you can inject water on the downstroke following the normal
exhaust stroke, have it evaporate rapidly enough to furnish steam presure to
add some net power to the crankshaft. In addition to all the other
questions raised, I will add this one. You will now have TWO power strokes.
The second one will probably give you less power than the first. If it's a
lot less, then the engine will end up with less net horsepower, because the
gasoline power strokes will be spaced further apart. Then, if you can get
it to produce some steam power at full throttle, what will happen as you
throttle down? Will you still have enough heat in the cylinders to generate
steam pressure?