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 functionsSome other functions of the fuel injection pump are: