Question

(a) Describe how a main engine fuel pump would be set and checked for: (6)

(i) Timing;

(ii) Quantity

(b) Explain how a setting of a variable injection timing fuel pump is checked and adjusted. (5)

(c) State why it be necessary to adjust the settings of a variable injection timed fuel pump. (5)

Answer 1

Part (a)

(i) Timing:

The fuel pump timing (the point at which injection begins) is set by the position of the pump plunger/ cam relative to the crankshaft. The pump is set so that injection starts at the correct crank angle (e.g. a few degrees before TDC) as per the maker's specification.
Procedure: Bar the engine to bring the cylinder to the correct position (e.g. the injection-start point). With the pump drive/ cam in the correct position, adjust the pump (e.g. by adjusting the pump plunger/ tappet, or the pump body position) so that the plunger begins to lift (or the spill/ delivery valve opens) at the set crank angle. This is checked using a timing gauge/ dial indicator on the pump plunger or by observing the fuel delivery (e.g. a timing mark on the pump/ camshaft).
The timing is verified by checking the crank angle at which injection starts, using the flywheel/ crankshaft markings and a timing pointer.

(ii) Quantity:

The fuel quantity (the amount of fuel injected per stroke) is set by the pump's effective stroke, which is controlled by the pump's fuel rack/ index position (the position of the control rack that rotates the plunger to vary the effective stroke).
Procedure: With the pump at a given index/ rack position, the quantity is checked by measuring the fuel delivered (e.g. by a measuring/ calibration test, or by the pump's index-to-delivery relationship). The pump is calibrated so that at a given index the delivery matches the maker's curve. The quantity is adjusted by setting the rack/ index linkage so that all pumps deliver equal quantities at the same index (balancing the pumps).
The quantity is verified by comparing the fuel index/ rack position with the actual delivery (e.g. by a fuel-flow test or by the engine's fuel consumption/ power relationship).

Part (b)

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 (c)

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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