The plunger reciprocates in the barrel. The quantity of plungers varies with the number of cylinders in the engine.
The plunger has a rectangular vertical groove that extends from the top to another helical groove.
The delivery valve can be lifted off its seat under the pressure of the liquid fuel. And also against the force of the spring. The passage is connected to the fuel atomizer.
When the plunger is at the bottom of its stroke and supply as well as spill ports are uncovered. The oil from the fuel feed pump, after filtration, is forced into the barrel.
The supply and the spill port are sealed at a certain stage when the plunger is pushed away. This is done with the help of the cam and tappet mechanism which is provided at the bottom.
During the further movement of the plunger, the fuel above it gets compressed and the high pressure developed.
It lifts the delivery valve off its seat and fuel starts flowing through the passage to the atomizer.
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 a sudden pressure drop in the delivery pipe.
Thus the discharge from a nozzle of the injector is cut off suddenly. Hence the fuel is prevented from dribbling into the cylinder even after the injection ceases. The cycle is repeated frequently.
During each stroke of a plunger of a Fuel Injection Pump, the duration of the delivery is more or less.
According to the spill, a port is made to communicate, earlier or later, with the high-pressure fuel in the upper part of the barrel.
This depends upon the position of the helical groove which can be changed by rotating the plunger with help of a rack.