THE C.

I FUEL-INJECTION SYSTEM
In the C.I. engine a special type of plunger pump and an injection are used to introduce fuel oil into the
hot, dense and turbulent charge of air in the combustion chamber. One pump and one injector are used
for each combustion chamber but the separate pumps are grouped into a common body and all are
driven from the one camshaft arranged in the lower body part. The pressure required to atomize the
liquid fuel and to force it to penetrate the dense air charge, varies between engines and depends upon
the compression ratio and the combustion chamber type-direct-injection chambers needing the higher
pressures. The injection pressure is determined by the spring loading upon the needle valve in the
injector.
As the pumps and injectors have to operate under these very high pressures, the clearances between
their moving parts have to be very small indeed. It is of vital importance that all traces of dirt and water
be removed from the fuel in order to protect these units from rapid wear and destruction. The fuel
vehicle system and the storage arrangements for bulk fuel must be designed and used in such a way that
dirt and water are rigorously excluded.

The Compression Ignition fuel system

On the vehicle, the fuel is carried in one or more tanks mounted at the side or vehicle rear. The tank has
internal baffles to prevent the fuel surging from damaging the tank and it may be arranged on rubber
pads. It must be vented to the atmosphere and the drain plug may carry a wire-gauze filter which
surrounds the lower intake pipe end to the lift pump. In many systems a cloth-element filter is fitted
between the rank and the lift pump. The lift pump may be a single or double-diaphragm type similar to
that used in petrol supply system but the diaphragms are resistant to fuel oil. The lift pump may be
driven from the engine or camshaft injection pump. They deliver fuel to one or two paper-element type
filters which are usually arranged above injection pump lever. These filters can extract extremely small
dirt particles and the elements are replaced at regular intervals. The cloth elements in the preliminary
filters can be washed a few times in clean petrol before they need replacing. These filters are fitted with
drain pugs and air vents and the paper types have pressure relief valves which permit excess fuel to
return to the tank when the lift pump delivers at a rate greater than is being injected.
The Compression Ignition Plunger lift pump
The fuel passes from the paper filter into the main injection pump gallery. it’s again filtered by small
replaceable elements built into the pump. From the gallery the fuel passes into the pump barrels and is
measured or metered out into the quantities appropriate to the speed and engine load and is also placed
under a high pressure. Fuel under high pressure is trapped between the pump and the injector at all
times and when the pump pressure exceeds the trapped pressure, the injector valve is opened and the
fuel is sprayed into the combustion chamber. Some fuel always escapes around the injector needle and
isn’t injected. This ‘leak-off’ fuel is collected from the injectors and is returned to either the tank or the
intake preliminary filter side. The fuel quantity must be taken to account when fuel consumption tests
are carried out. The pipes connecting each pump to its injector are made of steel and must have cold-
upset or welded nipples. Their characteristic is wall thickness to enable them resist very high pressure.
They should be of same length so as distortion, due to the pressure impulses, affects them all equally.

Fig.3: The Compression Ignition fuel filter

THE INLINE INJECTION PUMP

The injection pump has to:
a) Build up a pressure sufficient to atomize the liquid fuel and to force it the required
distance through the dense air charge.
b) Meter out the fuel quantities very accurately and vary these quantities to suit the air
weight induced at every engine speed and load combination.
c) Deliver the correct fuel quantities to the injectors at the correct moment relative to the
pistons position in the correct firing order and at equal crankshaft rotation angles.
Construction:
The injection pump is mounted on and driven by the engine. It’s driven in such a speed, timed to the
engine, that the full air charges expansion coincide with the pistons being in the best position to exert
maximum force on their crankpins. High-speed C.I. engines may have automatic devices which
advance the injection beginning as engine speed is increased. The pump consists of an aluminum-alloy
body which houses the separate pumps, their operating and control mechanisms and the governor. The
pumps are supplied with fuel from a common gallery and each is operated by its own cam on the single
camshaft driven by the engine. The cams are radically disposed around the shaft in firing order. All the
pumps are controlled from the same rack gear connected to the governor. Each pump in the assembly is
called a pumping element and each element consists of a very accurately machined plunger and barrel
supplied as mated pairs and must always be kept together. The barrel has two ports at its upper end
which communicate with the barrel is sealed off by a spring-loaded delivery valve of special shape.

Fig.4: The Plunger and barrier

The plunger is grooved both horizontally and vertically and also has a helical groove or helix which
begins near the upper vertical groove end and ends directly opposite at the upper horizontal groove
edge. The lower horizontal groove face is the sealing or pumping face and all the space between this
face and the delivery valve must be filled by fuel at all times. If air should enter the pump it must be
expelled by bleeding or venting. The lower plunger end is grooved to fit a spring retainer and this end
also has two lugs which engage with slots machined in the lower end of a cylindrical sleeve. The sleeve
fits around the lower barrel part and carries a gear quadrant which is meshed with the rank. The
quadrant is clamped to the sleeve in such a way that the rack movement results in the sleeve and the
plunger turning in relation to the barrel. The plunger is forced down by the spring action and up by the
cam action. A roller tappet is interposed between the cam and the plunger and the clearance between
the two is adjustable. The rack movement is controlled by the governor and not directly by the
accelerator pedal.

Fig.5: The Plunger and barrier-Two port element.

Fig.5: The Injection Pump-Element Assembly.
Element Operation:
The delivery valve is closed down to its seat, trapping fuel between it and the injector needle valve seat.
This fuel is under a pressure just below that required to lift open the needle valve. The barrel is full of
fuel and as the cam and tappet lift the plunger, fuel is displaced back to the gallery via the ports. Further
lifting results in the ports being closed by the plunger sides-and as the fuel is totally enclosed, the
pressure is increased. When this pressure exceeds that trapped in the pipe line and the injector, the
pressure difference opens the delivery valve and the higher pressure acts at once on the needle injector
valve-forcing it to lift from its seat and allow fuel to enter the combustion chamber. The spraying action
continues until the lifting of the plunger results in the helical groove uncovering its port (spill port). At
this instant the fuel is no longer enclosed and spills back into the gallery. The pressure in the element
collapses at once and reduces the pressure in the pipe and in the injector. The injector needle at once
returns to its seat and spraying or injection ceases as the delivery valve is also returned to its seat by its
spring, the two closing actions taking place so quickly that a high pressure is retained in the injector and
pipe line. This enables the next delivery to operate without having first to build up pressure at the
injector. The injector delivers the same fuel quantity as the plunger forces through the delivery valve-
but not the same fuel portion.

Fig.5: The Plunger and barrier-Single port element.

The fuel quantity delivered by the element is varied as the plunger moves up and down by the rack and
sleeve action. Rack movement turns the quadrant and sleeve; the sleeve turning the plunger via the slots
and lugs. As the plunger is turned the helical groove uncovers the spill port at different plunger-lifting
positions-so releasing the pressure sooner or later. Turning the sleeve and plunger clockwise increases
the fuel quantity delivered and vice versa. When the vertical groove is aligned with the spill port, there
can be no delivery at all i.e. cut-off is obtained. The gear quadrants are adjustable in their position on
their sleeves and they must all be set so that all elements make the same delivery at the same speed.
This adjustment is called ‘calibrating’ i.e. checking delivery size. The points at which the deliveries
are made must occur at equal crankshaft angle rotations. These points are adjusted by varying the tappet
clearances. The maximum delivery per plunger stroke varies with engine capacity. In others, the barrel
has only one port and the upper plunger end has one helical groove machined in its side. This groove
communicates, via a small hole, with a cylindrical hole in the plunger head centre. The metering control
operates like the two-port element. The rack and quadrant may be replaced by a rod-and-fork
mechanism but the turning principle the element to vary the fuel quantity delivered still applies. These
pumps are all constant-stroke types i.e. the plunger always lifts the same amount but the delivery is
variable. They are called ‘jerk’ pumps.

THE DELIVERY VALVE

Function:
The delivery valve acts as:
a) A non-return valve between the injector and the injector and the pumping element.
b) An injector anti-dribble device.

Construction:
The valve and its seat are arranged in the injection pump body immediately above the element barrel,
the lower seat face acting as the barrel end. The valve is spring loaded that it always tries to close down
to its seat, the sealing faces being beveled. Below the beveled face is a plain cylindrically portion, the
volume of which is so proportioned as to produce the correct drop in pressure as the valve closes. This
plain portion is of the same diameter as the grooved guide portion below it and both fit into an accurate
bore in the valve seat. Pressure generated in the pumping element acts upon the lower plain portion face
via the grooves in the guide.
Fig.6: The Plunger and Barrier- Valve Detail Fig.6: Delivery assembly
Operation
Start of delivery:
There must be a high standing pressure in the injector and pipe line to ensure an immediate injection
start. As the plunger is lifted, it closes the ports in the barrel and the resulting pressure acts on the
underside of the plain portion of the delivery valve. When this pressure exceeds that held in the pipe
line, the delivery is forced from its seat. The instant the lower plain face portion clears its bore; the
higher pressure is transmitted through the fuel to the needle injector valve. This valve is lifted against
its spring pressure and fuel is sprayed into the combustion chamber, the spraying continuing till the
plunger helix uncovers its port.
 Fig.7: The Plunger and barrier- Delivery valve Operation
End of delivery:
As the port is uncovered, the fuel is no longer enclosed and the pressure at once collapses. The delivery
valve is at once returned to its seat by spring action but in returning the plain portion, as it enters its
bore, acts as a plunger pump-suddenly increasing the effective pipe and injector volume. This in turn
results in a sudden pressure drop in the injector which enables its needle valve to close in one swift
movement i.e. without bounce and fuel dribble. The bevel face then contacts the seat and maintains a
pressure in the pipe and injection which is just less than that required to lift the needle injector valve.

INJECTION PUMP TESTING

Two main tests and adjustments have to be carried out in the servicing of the injection pumps. They are
called calibration and phasing.

Calibration
Function:
This test or operation is carried out to ensure that all the separate pumping elements are delivering the
same, correct fuel quantity at each point over the engine speed. As each individual delivery or shot is so
very small, it is common practice to measure, in cubic centimeter, the total delivery. This delivery must
be made at specified rack-opened positions and at specified pump speeds. The test figures and the limits
allowed for differences from them are supplied by the pump manufacturers and are different for
different models and makes of pump.

Fig.8: Pump-Testing Machine

The machines used in the testing and injection pumps adjusting vary in design but usually incorporate
the following features:
a) Variable speeds of rotary drive.
b) A revolution indicator or tachometer.
c) A set of test injectors.
d) A set of test tubes calibrated in cubic centimeters.
 e) A protractor scale, which can be moved, on the drive dog.
f) A fuel tank and filter with either a gravity or pump feed to the injection pump.

Method
a) Setting up:
The inspection injection pump plate is removed to allow access to the plunger tappets and the sleeve-
adjusting quadrants. Each element in turn is then checked to ensure that a clearance exists between the
plunger top and the delivery valve bottom. This is done by turning the pump drive dog till the plunger
is at the stroke top. It should then be possible to lift the tappet and the plunger very slightly by
screwdriver means. This clearance may be made or adjusted by altering the tappet clearance by either
the adjusting or locknuts or by shims. The drive pump dog is then engaged with the drive dog of the
machine and the pump clamped to the machine bedplate. Delivery equal length pipes are connected
between the machine injectors and the pumping elements. The fuel supply pipes are connected between
the machine and the pump. The rack ‘stop’ position is then used to make certain that fuel isn’t delivered
when the ‘cut-off’ control is operated.
b) Calibrating:
The rack is set at exactly 12mm open from the stop position and is locked. The pump is then driven and
the deliveries are collected in the calibrated tubes below the injectors, one tube for each injector. The
individual deliveries are then compared with each other and against that specified by the pump
manufacturer. Any adjustments found necessary are made by repositioning the quadrant upon its sleeve
in each case. These adjustments are a trial-and-error process and are continued until all of the elements
deliver quantities within the limits specified by the pump manufacturer. When elements prove
incapable of being adjusted to come within these limits, the plunger and barrel must be replaced by a
new set. An erratic or varying delivery is usually an indication of a faulty delivery valve which has to
be replaced. The test is concluded with a series of checks at 200rev/min: if the results in deliveries
below those specified the cause are excessive wear and the barrels and plungers must be replaced.
Adjustments are made only at the start of the series of checks. When all elements are delivered, fuel
within the specified limits, the sleeves and quadrants should be marked by a freshly scribed line. This
makes easier any later adjustments or correction if the quadrant should move on the sleeve during
service.

Phasing
Function:
This operation is carried out to ensure that the pump makes its deliveries at equally spaced camshaft
rotation intervals. The intervals will be 90° for a four-element pump and 60° for a six-element pump.
Method:
This operation is carried out after calibration. The delivery pipes are removed from the pump, together
with the delivery valve. A swan-necked pipe is connected into the delivery valve holder and the pump
rack is set and locked from the stop position. The drive from the machine is disconnected to enable the
pump to be turned by hand. The pump drive dog is slowly rotated by hand until fuel flows freely from
the swan-necked pipe, being expelled by the plunger’s upward movement. A point can then be reached
where by very slow and careful turning; the last oil drop can be made to move in and out of the pipe
end. This is the instant of spill cut-off i.e. the instant when the plunger closes off the ports in the barrel
as it is lifted by the tappet.
More accurate methods are now available in which the phasing is checked electronically, the first fuel
particles to leave the injector operating an electronically controlled valve which results in the revolving
pointer illumination. The pointer indicates the phase angle. When the spill point is obtained, the
protractor reading is noted the protractor scale may be moved around the drive dog and set to zero. The
delivery valve and spring are replaced and the operation is repeated for the other elements in firing
order. The protractor readings at the spill point of each are noted and the intervals are compared. The
pump is considered to be correctly phased and requires no adjustments if intervals are equal. If
adjustments are necessary they are made by adjusting the tappet clearance until the phase angles are
correct. This method of finding the spill cut-off is used when timing the pump to the engine, it being
considered for this purpose that spill cut-off is the same point as the commencement of injection.