Abstract: An air supply system for a fuel injection system of an internal combustion engine wherein a compresosr is arranged to deliver air to the fuel injection system by an air circuit including an air chamber, and a control valve to selectively communicate the air conduit with the air chamber, the said air control valve being arranged to isolate the air chamber from the air conduit when the pressure in the air conduit falling below a predetermined pressure, and to selectively open the valve during engine start up to provide air to the circuit from the chamber.