Question

With Reference to Main Engine Fuel Pumps:

(a) Explain how the setting of a variable injection timing fuel pump is checked and adjusted. (10)

(b) State why it may be necessary to adjust the settings of a variable injection timed fuel pump. (6)

Answer 1

Part (a)

A VIT fuel pump varies the injection timing (start of injection) with the fuel index/ load to optimise combustion (maintain the correct injection timing and peak pressure over the load range).
Setting/ checking: The VIT mechanism (a control that shifts the pump plunger/ cam or the pump body to advance/ retard the timing) is set so that at a given index/ load the injection timing is correct. The timing is checked at several index positions using a timing gauge/ dial indicator on the plunger, and the VIT control is adjusted (e.g. by adjusting the VIT linkage/ cam) so the timing follows the maker's VIT curve (timing vs index).
Adjustment: The VIT control is adjusted by altering the linkage/ cam position so that at each index the injection start is at the correct crank angle. The adjustment is verified by re-checking the timing at the index points and by observing the peak pressure/ combustion.

Part (b)

To maintain optimum combustion (correct injection timing and peak pressure) over the load range, improving efficiency and reducing fuel consumption.
To compensate for changes in fuel quality/ properties (e.g. different fuel viscosity/ cetane number).
To correct for wear of the pump/ injector components, which changes the timing/ delivery.
To optimise the engine for different operating conditions (e.g. part load, manoeuvring).
To reduce emissions (NOx) by optimising the injection timing.
To correct for a change in the engine's condition (e.g. after overhaul, or a change in the turbocharger/ charging).

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