How are you modeling the combustion? Mass of fuel determines heat release?
If I were doing it, my approach would be along these lines:
track mass of water into cylinder
track phase of water into cylinder
use c.p of the water in to calculate heat contained
find phase of water as compression progresses (curve fit to tables in textbook, perhaps)
track energy "swallowed" by the water during compression
find phase of water at start of combustion
track energy "swallowed" by the water during combustion
find phase of water in exhaust stream
track phase of water as it moves through the turbo
etc...