Fuel injection pump is another important part of fuel injection system in diesel engine. It is used in the individual pump system of a solid injection and consists of a plunger driven by a cam and tappet mechanism at the bottom. The plunger reciprocates in the barrel. The quantity of plunger varies with the number of cylinder in the engine. The plunger has a rectangular vertical groove which extends from the top to another helical groove.

When the plunger is at the bottom of its stroke and supply as well as spill ports are uncovered, oil from fuel feed pump, after filtration is forced into the barrel. The supply and the spill port are sealed at certain stage when the plunger is pushed away with the help of cam and tappet mechanism which is provided at bottom.

During the further movement of plunger the fuel above it gets compressed and the high pressure developed, lifts the delivery valve off its seat and fuel starts flowing through the passage to the atomiser. As the plunger rises up still further, at a particular moment the helical groove connects the spill port through the rectangular vertical groove to the fuel in the upper part of the plunger.

Consequently, there is a sudden pressure drop due to which the delivery valve falls back on its seat under the force of spring. This increases the volumetric capacity of the delivery system resulting in sudden pressure drop in the delivery pipe. Thus the discharge from nozzle of the injector is cut off suddenly and the fuel is prevented from dribbling into the cylinder even after the injection ceases. The cycle is repeated frequently.

When engine is at full load, the position of the helical groove on the plunger is kept below the passage. At partial load or normal delivery, the plunger is rotate to a position nearby passage, in which the delivery takes place for shorter period. When the engine is stopped, the plunger is rotate to the position where helical groove is opposite to the passage, in which rectangular slot is in line with the spill port. There is no pressure built up above the top of the plunger, the delivery valve is not at all lifted and hence no delivery of fuel to the atomiser takes place.