The resulting force drives the piston downward in the cylinder. The volume is increased and the preasure is decreased. No more energy is added and because of this the internal energy of the gas is decreased as so as the temperature. At BDC the exhaust valve is opend and the piston moves up the cylinder. The pressure drops near the preasure outside the cylinder because of the opened exhaust valve.
Exhaust gas leaves the cylinder.
The volume is decreased.