VARIABLE INJECTION TIMING

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The next stage of adjusting the fuel pump timing was to allow the fuel pump timing to be adjusted whilst the engine is running. This is the Variable Injection Pump that is used on all the slow speed engines, and some medium speed engines.

The traditional fuel pump helix had a flat top, which meant that fuel injection was commenced at the same time irrespective of the engine load. However improvements in engine efficiency would occur if the fuel were injected into the cylinder earlier, which would lead to higher cylinder temperatures and pressures. The limit imposed on the maximum pressure was dictated by engine design strength, but once the engine operated below maximum power this maximum pressure would also fall. Hence for an engine operating away from the MCR, there was the opportunity of injecting the fuel into the cylinder earlier to improve the specific fuel consumption of the engine.

The first fuel pumps to achieve this had the upper part of the fuel pump plunger modified to provide early fuel injection from 50% to 85% engine load. If the fuel is injected too early at low powers, then the engine could stall, whereas at high loads early fuel injection would cause excessive cylinder pressures.

This method can control the start point of injection from outside the fuel pump, and therefore is adjusted whilst the engine is running for different operating conditions. The fuel pump is predominately a fixed start of injection, but whilst operating in the `upper load range’ (50-85%), the start of injection is automatically advanced. This produces an increase in maximum pressure from 85-100%, and a constant 100% maximum pressure from 85% to 100% engine load.

The diagram shows the main features of the fuel pump that will allow adjustments to be made in service.

Main design points:

·         Fuel pump quantity automatically (from govr) adjusted by standard method of rotating the plunger (rack position). This allows the effective length of the fuel pump stroke to be increased.

·         Individual/manual adjustments of the quantity can be made at the turnbuckle arrangement at the input/rack to each fuel pump.

·         Fuel pump timing is changed by moving the fuel pump timing control lever in and out of the fuel pump. This timing is automatically adjusted by the air servo fitted at each fuel pump. The air pressure for this servo is controlled by the VIT timing servo fitted at the fuel rack. When the fuel rack is rotated to admit more fuel into the cylinder, the output pressure of the VIT timing servo will increase as its piston is depressed. This will advance the fuel timing. However the construction of this VIT timing servo will mean that the change in fuel pump timing is dictated by the cut off/break point of the pivot arrangement and can be adjusted.

·         Individual timing adjustment of the fuel pump to correct for worn pumps or to balance Pmax will be via the turnbuckle adjustment at the timing rack of each fuel pump.

Advantages:-

            1.         Reduced fuel consumption especially in the important 65-85% power range

            2.         Adjustments to fuel timing can be easily carried out, and without stopping engine, which can:-

                        i)          allow balancing of individual engine cylinder p_max levels,

                        ii)         allow fuel quality effects to be countered.

Disadvantages:-

            A.        More complex pump unit, requiring greater amount of maintenance. Most VIT pump units do not operate,

            B.        If incorrectly adjusted, p_max will become excessive, which results in high mechanical stresses and shock.

Another type of VIT unit is used by the MAN B&W medium speed engines. In this unit the separate fuel camshaft is effectively rotated radially by the application of oil pressure.