Information Sheet M3.3
DIESEL FUEL-INJECTION SYSTEM
Learning Objectives:
After reading this information sheet, you must be able to:
1. enumerate the purpose of the diesel fuel-injection system;
2. trace the flow of fuel in the diesel fuel-injection system;
3. identify the parts and purpose of the transfer pump and filter assembly;
4. explain the operation of the injection pump and its sub-assemblies;
Purpose of Diesel Fuel-Injection System
More and more demands are being made on the diesel engine’s injection
system due to the severe regulations governing exhaust and noise emissions,
and the demand for lower fuel-consumption. Basically speaking, depending on
the particular diesel combustion process (direct or indirect injection), in order to
ensure efficient air/fuel mixture formation, the injection system must supply the
correct quantity of fuel, time the fuel delivery, control the delivery rate, atomize
the fuel, and distribute the fuel evenly throughout the combustion chamber. The
optimum interplay of all these functions of the fuel-injection system is critical
for the faultless operation of the diesel engine.
In order to perform the above stated functions, the diesel fuel-injection
system will need a fuel tank, fuel transfer pump, fuel filter, an injection pump,
injectors or injection nozzles, and connecting hoses and pipes.
Figure 1 MAJOR COMPONENTS OF A DIESEL FUEL-INJECTION SYSTEM
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DIESEL FUEL INJECTION SYSTEM COMPONENTS
Let us now look at the path that the fuel takes as it flows through the
different components of the diesel fuel-injection system (figure 1). We will start
at the tank. The tank serves as the temporary storage area for the fuel which is
then supplied to the injection pump by the transfer pump. The transfer pump/
priming pump, usually assembled together with the filter, supplies fuel to the
injection pump at low pressure (figure 2). But before the fuel enters the injection
pump, it is first screened out of possible fine debris by the filter. In most
applications, a diesel fuel filter also contains a sedimenter that separates the
water from the diesel fuel. It uses the specific gravity difference between the
diesel fuel and water to separate the water before the fuel enters the injection
pump. Fuel can contain water in bound form (emulsion) or unbound form (e.g.,
condensation due to temperature changes). If this water gets into the injection
pump, corrosion damage can be the result.
Figure 2 PRIMING PUMP AND FUEL FILTER ASSEMBLY
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After the fuel passes thru the filter, it is then pressurized by the injection
pump. The injection pump also times, measures, and delivers the fuel under
pressure to each injection nozzles. There are many types of injection pumps
depending on diesel engine application but we will focus our discussion on axialpiston distributor pump.
Figure 3 AXIAL-PISTON DISTRIBUTOR PUMP
The axial-piston distributor pump has the following sub-assemblies as
shown on figure 3: (1) Vane type fuel-supply pump with pressure regulating
valve, (2) High-pressure pump with distributor, (3) Mechanical (flyweight)
governor, (4) Electromagnetic fuel shutoff valve, and (5) Timing device. Let’s
discuss each sub-assemblies in detail.
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Figure 4 VANE TYPE SUPPLY PUMP
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As shown on figure 4, the vane type supply pump is located around the
injection pump’s drive shaft. Its main purpose is to draw in fuel from the filter
and generate pressure inside the pump. Its impeller is concentric with the shaft
and connected to it with a Woodruff key and runs inside an eccentric ring
mounted in the pump housing. When the drive shaft rotates, centrifugal force
pushes the impeller’s four vanes outward against the inside of the eccentric ring.
The fuel between the vanes’ undersides and the impeller serves to support the
outward movement of the vanes. The fuel enters through the inlet passage and
a kidney-shaped recess in the pump’s housing, and fills the space formed by the
impeller, the vane, and the inside of the eccentric ring. The rotary motion causes
the fuel between adjacent vanes to be forced into the upper (outlet) kidneyshaped recess and through a passage into the interior of the pump. At the same
time, some of the fuel flows through a second passage to the pressure-control
valve.
Figure 5 VANE TYPE SUPPLY PUMP WITH IMPELLER ON THE DRIVESHAFT
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As stated earlier, the
pressure-control valve (Fig.
7) is connected through a
passage to the upper
(outlet)
kidney-shaped
recess, and is mounted in
the immediate vicinity of
the fuel-supply pump. Its
main
purpose
is
to
maintain
the
internal
pressure of the fuel pump.
It is a spring-loaded spooltype valve with which the
pump’s internal pressure
can be varied as a function
of the quantity of fuel being
delivered. If fuel pressure
increases beyond a given
Figure 7 PRESSURE CONTROL VALVE
value, the valve spool
opens the return passage
so that the fuel can flow back to the supply pump’s suction side. If the fuel
pressure is too low, the return passage is closed by the spring. Using this valve,
it is possible to set a defined pressure for a given speed. The pump’s interior
pressure then increases in proportion to the speed, the higher the pump speed
the higher the pump interior pressure. Some of the fuel flows through the
pressure control valve and
returns to the suction side of
the supply pump.
Some fuel also flows
through
the
overflow
restriction (Fig. 6) and back
to the fuel tank. The overflow
restriction is screwed into the
injection pump’s governor
cover and connected to the
pump’s interior. It permits a
specified amount of fuel to
return to the fuel tank
through a narrow passage in
order to provide cooling and
self-venting for the injection
pump.
Figure 6 OVERFLOW RESTRICTION
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The fuel pressure needed for fuel injection is generated in the injection
pump’s high-pressure pump. (Fig. 3 & 8). The pressurized fuel then travels to
the injection nozzles through the delivery valves and the fuel-injection tubing.
Figure 8 HIGH-PRESSURE PUMP WITH DISTRIBUTOR
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Figure 9 HIGH PRESSURE PUMP COMPONENTS
The components of the HIGH PRESSURE PUMP are shown on figure 9. Let
us look at each component and their purpose.
The engine through its timing belt or gears, operates the injection pump’s
drive shaft. The rotary movement of the drive shaft is transferred to the
distributor plunger (5) via a coupling unit, whereby the dogs on cam plate (3)
and drive shaft engage with the recesses in the yoke (1), which is located between
the end of the drive shaft and the cam plate (3). The cam plate (3) is forced against
the roller ring (2) by springs (10), and when it rotates the cam lobes riding on the
ring’s rollers convert the purely rotational movement of the drive shaft into a
rotating-reciprocating movement of the cam plate (3). The distributor plunger (5)
is held in the cam plate (3) by its cylindrical fitting piece and is locked into
position relative to the cam plate by a pin. The distributor plunger (5) is forced
upwards to its TDC position by the cams on the cam plate (3), and the two
symmetrically arranged plunger return springs (10) force it back down again to
its BDC position. The plunger-return springs (10) abut at one end against the
distributor head (8) and at the other their force is directed to the plunger (5)
through a link element (6). These springs (10) also prevent the cam plate (3)
jumping off the rollers during harsh acceleration. The lengths of the return
springs are carefully matched to each other so that the plunger (5) is not
displaced from its centered position.
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As stated earlier, the pressure required for the actual fuel injection is
generated by the high-pressure pump. It is divided into four (4) phases which we
will call delivery phases and will be discussed in this section.
For a 4-cylinder engine the distributor plunger rotates through 90° for a
stroke from BDC to TDC and back again. In the case of a 6-cylinder engine, the
plunger must have completed these movements within 60° of plunger rotation.
As the distributor plunger moves from TDC to BDC, fuel flows through the
open inlet passage and into the high-pressure chamber above the plunger. At
BDC, the plunger’s rotating movement then closes the inlet passage and opens
the distributor slot for a given outlet port (figure 10).
Figure 10 DELIVERY PHASE A. Inlet Passage Closes
The plunger now reverses its direction of movement and moves upwards,
the working stroke begins. The pressure that builds up in the high-pressure
chamber above the plunger and in the outlet-port passage suffices to open the
delivery valve in question and the fuel is forced through the high-pressure line
to the injector nozzle (figure 11)
Figure 11 DELIVERY PHASE B. Fuel Delivery
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The working stroke is completed as soon as the plunger’s transverse cutoff
bore reaches the control edge of the control collar and pressure collapses. From
this point on, no more fuel is delivered to the injector and the delivery valve closes
the high-pressure line. During the plunger’s continued movement to TDC, fuel
returns through the cutoff bore to the pump interior. During this phase, the inlet
passage is opened again for the plunger’s next working cycle.
Figure 12 DELIVERY PHASE C. End of Delivery
During the plunger’s return stroke, its transverse cutoff bore is closed by
the plunger’s rotating stroke movement, and the high-pressure chamber above
the plunger is again filled with fuel through the open inlet passage.
Figure 13 DELIVERY PHASE D. Entry of Fuel
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The high pressure pump is equipped with delivery valves in order to ensure
precise closing of the injection nozzle at the end of the injection process (figure
3 and figure 14). The delivery valve closes off the high pressure line from the
pump. It has the job of relieving the pressure in the line by removing a defined
volume of fuel upon completion of the delivery phase.
The delivery valve is a plunger-type valve. It is opened by the injection
pressure and closed by its return spring. Between the plunger’s individual
delivery strokes for a given cylinder, the delivery valve in question remains
closed. This separates the high-pressure line and the distributor head’s outletport passage.
Figure 14 DELIVERY VALVE
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During delivery (figure 15), the pressure generated in the high pressure
chamber above the plunger causes the delivery valve to open. Fuel then flows via
longitudinal slots, into a ring-shaped groove and through the delivery-valve
holder, the high-pressure line and the nozzle holder to the injection nozzle.
Figure 15 Delivery valve open
As soon as delivery ceases (transverse cutoff bore opened), the pressure in
the high pressure chamber above the plunger and in the high pressure lines
drops to that of the pump interior, and the delivery valve spring together with
the static pressure in the line force, the delivery valve plunger moves back onto
its seat again
Figure 16 Delivery valve close
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So far you have learned the flow of fuel inside the injection pump. You
learned that diesel fuel is drawn in from the filter and is pressurized inside the
pump by the vane-type supply pump. From the vane-type supply pump, fuel
pressure is further increased, which is required for the actual fuel injection, by
the high-pressure pump. From the high-pressure pump, the highly pressurized
fuel is then delivered to the injector nozzles (figure 17). The injection nozzle
converts the high pressurized fuel into a mist and is injected into the combustion
chamber. The diesel engine directly injects the fuel into the combustion
chamber, which is different from the gasoline engine which makes the air-fuel
mixture in advance. Thus, in a diesel engine, the time for mixing with the air is
much shorter. Therefore, the fuel is injected at high pressure and high speed to
create a mist that mixes easily with the air, thus improving the ignition
performance.
Figure 17 INJECTOR NOZZLE
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The injector nozzle is composed of the following components shown in
figure 18.
Figure 18 INJECTOR NOZZLE COMPONENTS
The nozzle needle (7) is centered in the nozzle body (8) and fastened using
the nozzle-retaining nut (9). When the nozzle holder (1) and retaining nut (9) are
screwed together, the distance piece (6) is forced against the sealing surfaces of
nozzle body (8) and retaining nut (9). The distance piece (6) serves as the nozzle
needle-lift stop and with its locating pins centers the nozzle needle (7) in the
nozzle body (8).
The spring (4) is centered in position by the pressure pin (5), whereby the
pressure pin (5) is guided by the nozzle-needle (7). The spring (4) applies pressure
to the nozzle needle (7) through the pressure pin (5). The spring’s initial tension
defines the nozzle’s opening pressure which can be adjusted using a shim (3).
On its way to the nozzle seat, the fuel passes through the nozzle-holder (1)
inlet passage, the distance piece (6), and the nozzle body (8). When injection
takes place, the nozzle needle (7) is lifted by the injection pressure and fuel is
injected through the injection orifice into the combustion chamber. Injection
terminates as soon as the injection pressure drops far enough for the nozzle
spring to force the nozzle needle (7) back onto its seat. Any excess fuel will return
to the tank via the overflow pipe (2).
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