For many home mechanics the diesel injection pump is a bit of
a mystery. The Bentley and Haynes repair manuals doesn't describe its
internals, because it's not serviceable except by a few diesel
specialists. Learning some basics of how it works and what its internals
are could be of interest to the diesel owner, and the knowledge certainly
can't hurt when troubleshooting fuel injection problems, even if one isn't
about to take the pump apart.
The purpose of the fuel injection pump is to deliver an exact metered amount of fuel, under high pressure, at the right time to the injector. The injector, unlike in a gasoline engine, injects the fuel directly into the cylinder or a prechamber connected to the cylinder.
The VE in the name of the Bosch pump used in the VW diesels and many other small diesel engines stands for "Verteiler", which is German for distributor or divider. The other common kind of injection pump is the inline pump. The difference between them is that the "Verteiler" VE pump has one fuel metering plunger, and a mechanism (the "Verteiler"/distributor) to send the fuel to the right cylider. The inline pump has one plunger for each cylinder.
The Bosch VE has comparatively few moving parts, but what does
move does so in a complex way. The figure to the left is from a Yanmar
pump, which works and looks the same as the Bosch . On the leftmost end in
the picture is the fuel feed pump. This is a vane pump, just like the
vacuum pump on the VW diesel engine. Its purpose is to suck fuel from the
tank and deliver it to the metering pump. All the things shown on the
right in the figure have to do with the metering, timing and distribution
of fuel delivery. The figure below shows this part in detail. 
The plunger (right middle in the figure) in the VE pump both
rotates along its axis and performs a reciprocating translation in and
out. It is the translation that performs the high pressure pumping, while
the rotation is responsible for metering and sending the fuel to the
correct cylinder.
The cam disk is rigidly attached to the plunger. The drive shaft
rotates the cam disk. The cam disk rides on four rollers (only one shown
in this picture), and has four lobes. Thus for each revolution the plunger
will pump four times. Note that with this arrangement the plunger stroke
is constant. The metering (regulation of how much fuel is delivered) is
done not by changing the mechanical stroke, but by
To understand the function in some detail lets consider one stroke.
During the backward motion of the plunger, the rotation uncovers a
When the desired amount of fuel has been injected the spill port opens (located under the control sleeve in the figure), and the pressure quickly drops. This causes the delivery valve to close. During the rest of the stroke fuel is "spilled" through the spill port instead of being injected into the cylinder.
The position of the control sleeve controls at what angle the spill
port opens, and thus determines the amount of fuel injected, in other
words it controls the
Other functions
Some other functions of the fuel injection pump are:- Timing The timing is adjusted in response to engine RPM. At higher RPM s, the fuel pressure from the vane transfer pump is higher. Pressure changes effects a spring loaded plunger, and the resulting movement will move the cam rollers to either advance or retard the timing. There is also a cold start device which advances the idle timing manually.
- Governor A mechanical governor limits the maximum speed of the engine to 4800 rpm in the bus/vanagon application and 5350 rpm in newer passenger cars. It can be seen just above the cam disk in the middle figure.
- Stop A magnet valve or solenoid (shown in the figures) opens and shuts off the fuel channel between the feed pump and the metering pump.
- Aneroid An air inlet pressure sensor is used to determine maximum amount of fuel delivered on injection pumps for turbo engines. On newer ('89 and later) naturally aspirated engines a similar arrangement is used for altitude compensation.
