Leo Lichtman wrote:
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.

To be picky about: you would add water at the top of the exhaust stroke.
You would meter the amount of water so that it would all boil (flash).

... 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.

True, but the gas use is also less & given you're getting *some* power
from the steam stroke, your efficiency will be higher. Putting it the
other way around: if you increase the RPM so that your gas use is the
same as a 4-cycle (same number of combustion strokes per second), you
will get more power.

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?

You'll have enough heat to produce *some* steam. You just "throttle"
the water along with the gas.

Bob