The low pressure piston 30 has a delivery chamber 65 into which high pressure fuel is introduced through bore 66 provided in the spacer 67. At forward end of the delivery chamber 65 is a delivery orifice 68 provided in an insert 69. The orifice 68 is shown closed by needle type delivery valve 70 which seats against the insert 69 under the action of delivery spring 71. When the pressure of fuel in the delivery chamber 65 is sufficiently great, the needle valve 70 moves against the action of delivery spring 71 and opens the orifice 68 and fuel is injected through the orifice 68 into the associated engine cylinder. The commencement of injection through orifice 68 causes an immediate drop in fuel pressure in delivery chamber 65 and the needle valve 70 will tend to close the orifice 68 again. This, in turn, will allow pressure in delivery chamber 65 to rise and again open needle valve 70. This process continues so that the needle valve 70 opens and closes the orifice 68 at high speed. This action is known as "buzzing" of the delivery needle valve 70 and causes the fuel to be injected through orifice 68 in waves and this is believed to improve fuel combustion efficiency.

The needle valve 70 has a shank 75 which moves within a guide 76. The end 77 of the shank 75 remote from the delivery orifice 68 closes a control chamber 78. Control chamber 78 communicates through (aligned) bores 79, 80 provided in the spacer 67 and low pressure piston 30 respectively and through the space 81 around the outside of the low pressure piston 30 with the low pressure chamber 37. Thus the control chamber 78 is normally in communication with low pressure fuel allowing the needle valve 70 and shank 75 to move away from the insert 69 to open the orifice 68 under the pressure of fuel in the delivery chamber 65.