Internal combustion in cars and pressure volume diagram

The picture below explains how the pressure and the volumes behave inside the combustion chamber:

Intake: during the intake stroke the piston moves down sucking the mixture of air and gas into the chamber, this increases the volume inside the chamber while the pressure will stay constant.

Compression: during the compression stroke the piston moves up while the valves are closed, compressing the mixture which will lead to an increase in pressure and a decrease in volume.

Ignition: the spark will ignite the mixture, which will be formed into gases. the gases ignited will cause more pressure inside the chamber while the volume will stay constant.

Expansion: Also called the power stroke. in this stroke the piston will move down making more volume inside the chamber and leading the pressure to decrease.

Exhaust: during the exhaust stroke the piston will move up firing all the gases from the chamber leading to decrease the volume while the pressure will stay constant.