Fuel Injection Pump

In a diesel engine the fuel-and-air mixture is formed within the cylinder itself, after compression by the piston. The fuel is injected into the highly compressed hot air and burns.

The fuel injection pump has to provide the requisite high pressure (80-300 atm. = 1175 4400 lb/in.2), which must be higher than the pressure developed in the cylinder at the instant of injection, and ensure that a certain quantity of fuel is injected into the combustion chamber at the correct time and with the desired velocity. The injection nozzle distributes and atomises the fuel.

It is mounted in the nozzle holder, and the nozzle needle is pressed against the valve seat by a spring. The needle has a larger cross-section at its top part than at the bottom. The pump pressure acts upon the transition surface (taper) between the two cross-sections, so that, when the spring pressure is overcome, the needle is raised some distance and the fuel is injected into the combustion chamber through the aperture thus formed.

The nozzle holder is provided with a screw by means of which the spring pressure and therefore the injection pressure (i.e., the pressure needed to overcome the counteracting force exerted by the spring) can be varied. The nozzle holder is connected to the injection pump by a delivery pipe. The length of this pipe, and indeed its diameter and wall thickness, affect the pressure distribution in the pipe and thus also affect the injection and the combustion process.

Like every reciprocating pump the injection pump comprises a piston, a suction valve and a delivery valve. In addition, it is provided with a device for regulating the quantity of fuel delivered. In general, this regulation is effected by keeping the suction valve open for a certain length of time while the piston is performing its stroke.

As a result, a proportion of the delivered quantity is returned to the suction pipe. In Bosch pumps the delivery rate is varied by rotating the piston. The compression chamber over the piston is connected to the space below the chamber of the piston by a longitudinal groove or hole. So long as the surface covers the opening on the right, the pump delivers fuel; when the opening is uncovered, however, delivery ceases because then the rest of the fuel, which is displaced above the piston, can flow away through the longitudinal groove into the space below, whence it is discharged through the overflow pipe.

On rotation of the piston, the ‘red’ area will cover the opening for a longer or shorter time, so that the delivery rate is varied. This adjustment by rotating the piston is performed by hand (through the agency of a toothed rack) or by a centrifugal governor.

In order to achieve a rapid drop in pressure in the delivery pipe from the pump to the nozzle, and thus to ensure immediate closure of the latter, the pressure valves of Bosch injection pumps are fitted with a special device. On completion of delivery, the relief piston first plunges into the valve hole and closes the delivery pipe.

Then the taper of the valve body is lowered on to a seat. As a result, the volume available to the fuel in the delivery pipe undergoes an increase equal to the volume of the relief piston, and the delivery pipe is quickly relieved of pressure. Dribbling and the resultant carbonisation (coking) of the nozzle are thereby obviated.