To inject a determinable amount of fuel at very high pressure, and at a given time, through the injectors into the combustion chamber.
The pump elements are responsible for the high-pressure developement. A pump element consists of a small piston which is moved by the camshaft of the in-line injection pump by a roller tappet after TDC, and a spring after BDC. It is fitted into its cylinder with a tolerance of 1/1000th of a millimetre. In TDC position (the first and 3rd figures from the left) it mostly opens two fuel-inlet passages through which fuel can enter the space above the piston. As soon as the piston seals the fuel inlet passages (the second and 4th figure), the fuel is highly compressed, the pressure-release valve at the upper end of the cylinder opens, and the fuel flows in the form of a pressure wave to the injector.
Their springs determine the nozzle opening pressure. The end of the delivery is reached when the lower control edge opens the fuel inlet passage (5th figure). The rest of the stroke, up to TDC, is an idle stroke. A considerable vacuum develops. Air of course, is not present. Thus, fuel is, for a short time, brought into a gaseous state (extruded). The technical term is 'cavitation'. Of course this vacuum helps a great deal in the filling of the piston space in BDC position again.
In in-line injection pumps, because of the direct-injection, several sharp injection jets are provided almost exclusively by hole-type nozzles. The high-pressure delivery ends when the lower control edge of the piston opens one of the fuel inlet passages. The control edge serves the fuel-volume regulation.
Due to the low tolerances, the fuel must pass through a fine pored filter before reaching the pump.