Fuel pumping apparatus

A fuel pumping apparatus comprising an injection pump which in use is driven in timed relationship with an associated engine, means including a fuel feed pump for supplying fuel under pressure to the injection pump, a relief valve for controlling the output pressure of the feed pump, said relief valve being located in a line operatively connected with said feed pump, and means including a fuel pressure operable piston for adjusting the timing of delivery of fuel by the injection pump to the associated engine, said piston being responsive to the output pressure of the feed pump, said relief valve comprising a first resiliently loaded valve element which is subjected to the output pressure of the feed pump;

• an orifice and a spill port located downstream of said first valve element, said first valve element when the pressure rises to a predetermined value opens to allow fuel flow through said orifice, a second resiliently loaded valve element subjected to the intermediate pressure of fuel between the first valve element and the orifice, said second valve element when subjected to said intermediate pressure acting to reduce the resilient loading on the first valve element, said second valve element acting to control the size of said spill port which is in parallel with said orifice and which starts to open when the second valve element has been moved a predetermined extent by the intermediate pressure, said second valve element having a greater area exposed to said intermediate pressure than the area of the first valve element which is exposed to the output pressure of the feed pump.