Double-acting fuel supply pump: 1, nonreturn valve (suction side); 2, nonreturn valve
In the double-acting supply pump shown in Figure above, two additional non retum valves make the suction chamber and the pressure chamber of the single acting supply pump into two combined suction and pressure chambers. The pump does not execute an intermediate stroke. On each stroke of the double-acting supply pump the fuel is drawn into one chamber and simultaneously delivered from the other chamber of the injection pump. Each stroke is, therefore, a delivery and suction stroke. In contrast to the single-acting supply pump, the fuel delivery can never be reduced to zero. For this reason, the delivery line or the fuel filter must be provided with an overflow valve through which the excess fuel can flow back to the fuel tank.