Page 131 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Working
The feed pump plunger (1) (Fig 1 & Fig 2) is driven by the cam (2) provided on the F.I.P. camshaft (3). When the
plunger moves “downwards” by means of roller tappet (4) and pressure spindle (5) a portion of the fuel present in
the suction chamber (6) is delivered through the pressure valve (7) to the pressure chamber (8) and the plunger
spring (9) compressed in an intermediate stroke. Towards the end of this stroke the spring loaded pressure valve
closes again.

As soon as the cam or eccentric has passed its maximum stroke, plunger, pressure spindle and roller tappet
move “upward” due to the pressure exercised by the plunger spring. A portion of the fuel present in the pressure
chamber is thereby delivered to the fuel injection pump through filter. However, fuel is sucked simultaneously
from the fuel tank to the suction chamber through the primary filter provided in the feed pump and suction valve
(10). When the pressure in the feed pipe exceeds a specified, pressure the plunger spring lifts the plunger only
partially. The quantity of fuel delivered per stroke in this is comparatively smaller. When the fuel pipe line is full
and the F.I.P. does not need further fuel the feed pump should be put out of action. Due to the excess fuel in the
fuel outlet line the pressure in the pressure chamber, holds the plunger in the top position putting the feed pump
out of action. During this period only spindle works. The moment the pressure falls down the spring forces the
plunger down and the pumping action is resumed. This action during which fuel is not supplied by feed pump is
known as idling of feed pump.
Hand priming device
The hand priming device is screwed into the feed pump above the suction valve. When the engine is at rest, with
the aid of the hand priming device fuel can be pumped from the fuel tank through the filter to the F.I.P. In order to
operate the primer, the knurled knob is screwed out until the plunger can be pulled upwards causing the suction
valve to open for fuel to flow into the suction chamber. When the plunger is pressed down the suction valve closes
while the pressure valve opens and fuel flows through the feed pipe and the filter to the F.I.P. After the use it is
essential to screw the knob again in its original position.
Preliminary strainer
The preliminary strainer is usually attached to the feed pump. The function of the preliminary strainer is to prevent
the coarser impurities at a very early stage. It consists of a housing with a nylon/wire gauge insert or a wire mesh
sieve
Types
There are two types of fuel feed pumps.

• Mechanical
• Electrical
Mechanical type feed pump
A mechanical pump is mounted on the engine and is operated by a camshaft. This pump consists of an air
chamber divided in the Centre by a flexible diaphragm. Operation of feed pump (Fig.1) The rocker arm (1) is

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CITS : Automotive - Mechanic Diesel - Lesson 31 - 38