Abstract: A passive compressor is disclosed which includes a free piston or a flexible membrane whose movement is controlled by pressure variations in a main chamber. The pressure in the main chamber changes according to the pressure in the motor crankcase, as the main chamber is coupled to the crankcase by a crankcase transfer passage duct.As the free piston or flexible membrane moves towards the main chamber, it causes a pressure decrease in the air chamber which subsequently admits external air through an inlet valve. When the free piston or the flexible membrane moves in the direction of the air chamber, the increased pressure on the piston or membrane forces the external air into the motor cylinder, through an air transfer passage duct, and towards a port on the cylinder that is opened due to the position of the motor piston. The inlet valve prevents the loss of pressure in the air chamber. Alternatively, the main chamber may be directly attached to the motor crankcase.