Cylinder Pressure

The diagram shows an "ideal"
pressure-cylinder volume curve
for a spark ignited 4-cycle engine
together with one approximating
actual engine (p,v) conditions.
In the ideal case, the inlet (1,2) &
exhaust (2,1) paths are at 1 atmosphere, the heat addition (3,4) is at constant volume as is the exhaust heat loss (5,6). The compression (2,3) and work (4,5) strokes are both adiabatic.

From, Reynolds & Perkins, McGraw Hill (1977)

In the "real" cycle, inlet and exhaust flow restrictions reduce and increase these pressures, respectively, the combustion process adds heat over a range of cylinder volumes, and the exhaust gases leave the engine above room temperature.