Reciprocating air compressors are positive displacement machines, it means increase the pressure of the air by reducing its volume. In other words they are taking in successive volumes of air which is restrained within a closed space and elevating this air to a higher pressure. The reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing and displacing element.

Load reduction is achieved by unloading individual cylinders. Typically this is accomplished by throttling the suction pressure to the cylinder or circumventing air either within or outside the compressor. Capacity control is achieved by changeable speed in engine-driven units through fuel flow control.