I don't have any useful insight on the question, but a couple of comments:
1) In a *real* engine, there is also an overlap where the intake and exhaust valves are open - which helps purge the exhaust and draw more fresh mixture in.
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
The equation that pops into my mind is pv=nrt.. google then tells me: p=pressure, v=volume, n=number of moles (measure of molecules), R=gas constant, t = temperature.