Engineers Motor EKs
December 2001 Question 7.
Describe, with the aid of a sketch, a VIT (variable injection
timing) fuel pump as fitted to a main propulsion engine. (10)
Describe how the timing of the fuel pump described in Q.7(a) is
adjusted during operation of the pump. (6)
The sketch shows the working principle of a
fuel pump as fitted to large Sulzer two stroke engines.
(one per cylinder)
A cam operated plunger is a closely matched fit
in a barrel in which it reciprocates. When the cam
is on the base circle, fuel is admitted into the space
above the plunger via a suction valve The suction valve is
lifted by a push rod which is operated from a
pivoted lever connected to the fuel pump plunger.
As the plunger moves upwards the suction valve pivoted
lever moves anti clockwise and the suction valve closes.
High pressure fuel is delivered by the pump to the
injectors via the delivery valve. At the end of injection
a spill valve also controlled by a pivoted lever opens and
the pressure above the plunger drops.
The position of the pivots for the suction and spill
valves are variable, thus varying the timing of the start
and end of injection. The variation of the timing of the
start of injection gives the pump VIT, whilst the
variation of the spill timing varies the quantity of fuel
delivered, and thus the power developed by the engine.
The position of the
suction valve adjustable pivot is controlled by the fuel
quality regulating shaft. The position of this shaft is
determined by the fuel quantity setting which is controlled by
the governor. As well as altering the spill valve timing, the
output from the governor is led to a profiled segment, the
output from which controls the position of the suction valve
control lever pivot. The usual arrangement is to advance the
start of injection from about 40% load until about 85% load when
the max cylinder pressure is achieved. The timing is then
retarded to maintain max cylinder pressure at the same value
until the timing is back to its normal setting at 100% MCR.
Also attached to the
profile segment is a fuel quality control lever which retards or
advances the fuel pump injection timing (i.e. the closing of the
suction valve). This is for hand adjustment so that the timing
can be easily altered for differing quality fuels.
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