marinediesels.co.uk Second engineers sample question December 2001 question 7

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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.