In this type of compressor a cylinder bore encloses a moving piston. As the crankshaft
of the compressor rotate, the piston moves within the cylinder, similar to the piston in
a car engine. As the piston is pulled down, the volume increases, creating a lower
atmospheric pressure in the piston chamber. This difference in pressure causes air to
enter via the inlet valve. As the piston is forced upwards the volume of air reduces.
The air pressure therefore increases. Eventually the pressure forces the outlet valve to
open.