The fuel injection pump has a housing with at least one inlet and one discharge channel. The piston slides in the fuel chamber. The chamber capacity is increased/reduced on suction and discharge strokes respectively. There is an auxiliary piston sliding in the chamber which performs this change in capacity. A sensor determines the engine speed and a control system changes the working cycle for the pistons according to the engine speed. The overlap between the two piston cycles is increased to reduce the chamber capacity when the engine speed is low. The delay between the two cycles is longer to reduce the chamber capacity when the engine speed is high.
