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.
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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 spilling
some of the fuel through spill ports, and thus changing the
effective stroke . This is done by uncovering a
spill port under the control sleeve at
a particular angle of rotation.
The other purpose of the rotation is to deliver the fuel to the
correct cylinder. This is done by having four four delivery
valves (only one shown in the figure), one for every 90 degrees
of rotation. During a full revolution the plunger makes
four strokes, one at 0, 90, 180 and 270 degrees. During each stroke
the delivery port in the middle of the plunger is connected to
a particular delivery valve.
To understand the function in some detail lets consider one stroke.
During the backward motion of the plunger, the rotation uncovers a
fill port (to the right in the figure, just below the magnet valve
(solenoid)), and the plunger barrel is loaded with fuel. At bottom
dead center the fill port is closed. On the forward pressure stroke
fuel is pressurized (to over 120 bar). At this time the Plunger
barrel is connected to a particular delivery valve through the
channel in the center of the plunger, and a port in the side.
When pressure builds up to the delivery valve opening pressure,
the valve will open and deliver high pressure fuel to the injector.
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 metering.
The control sleeve is moved in response to a combination of
accelerator position and engine speed. The latter is determined
by a mechanical governor.
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.
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