The MAN B&W MC Engine VIT Fuel Pump

Operational Information

The MAN B&W MC Engine VIT Fuel Pump.

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The pump is basically a jerk type with a plunger moving in a matched barrel, using two helical grooves machined in the plunger to control the end of injection by uncovering spill ports and causing the discharge pressure to drop rapidly, thus causing the needle valve in the injector to close.

Oil is supplied to the barrel via the spill ports and a suction valve. The suction valve, situated at the top of the barrel opens when the pressure in the barrel falls below the supply pump pressure; i.e. during downward stroke of plunger, while spill ports are covered by plunger.

Replaceable erosion plugs are fitted in the pump housing opposite the spill ports. The high pressure oil, spilling back, as the edge of the helix uncovers the spill ports at the end of injection, hit the plugs, which prevent damage to the pump casing

A puncture valve is fitted in the top cover of the pump. It is opened when compressed air from the control air system acts on top of a piston fitted in the top cover. Fuel oil from the discharge side is then returned to the suction side of the pump and no injection takes place. The puncture valve is operated in the event of actuation of the shut down system (all units), during the air start sequence or when excessive leakage is detected from the double skinned fuel pipes.

Fuel oil leakage past the plunger to the cam case is prevented by the use of an "umbrella" seal

A spring loaded damper is fitted to the side of the pump connected through to the suction side of the pump. This smoothes out the pressure fluctuations as the high pressure fuel spills back at end of injection.

VARIABLE INJECTION TIMING (VIT)

The pump is capable of Variable Injection Timing (VIT). This overcomes the disadvantage of the basic jerk pump, where although the end of injection is infinitely variable, the start of injection is fixed by the position of the spill ports, injection commencing shortly after the ports are covered by the top edge of the plunger.

Quantity Control

Using Fuel Rack

As well as having the normal fuel quantity control (i.e a rack which rotates the plunger in the barrel), the fuel pump is fitted with an adjustable barrel which has a large pitch thread machined on the bottom. The threaded barrel is located in a threaded sleeve which is rotated by a second rack. As the sleeve cannot move axially, and the barrel is prevented from rotating, then as the sleeve rotates, the barrel moves up and down, thus altering the position of the spill ports relative to the plunger, and varying the start of injection.

Timing Control

Using VIT Rack

Fuel Quantity and VIT Linkages on Engine

REASON FOR USING VARIABLE INJECTION TIMING

Graph Showing Effect of VIT on

Max Cylinder Pressure

The reason for using VIT is to achieve greater fuel economy. This is achieved by advancing the injection timing so that maximum combustion pressure (p_max) is achieved at about 85% MCR (maximum continuous rating).

The system is set up so that there is no change in injection timing at low loads (40%MCR). This is to avoid frequent changes of pump lead during manoeuvring.

As the engine load is increased above 40%, the start of injection advances. When the engine has reached approximately 85% MCR at which the engine is designed to have reached p_max, the servos retard the injection timing so that the maximum combustion pressure is kept constant between 85% and 100%MCR.

At 90% MCR a fuel saving of 4-5g/h.p.hour is claimed to be achieved.

Variable Injection timing also allows for small adjustments to the fuel pump timing to be made to allow for fuels of varying ignition qualities. Wear on the fuel pumps can also be compensated for as can changes in the camshaft timing due to chain elongation (up to 2 degrees)

HOW VARIABLE INJECTION TIMING IS ACHIEVED

Low pressure air is fed to the pressure control valve, the output of which is fed to the VIT servos on the fuel pump. A link from the governor output (or fuel pump control handwheel) moves a pivoted bar, the position of which determines the output of the pressure control valve.

The position of the control valve is adjustable which can be used to allow for fuels of varying ignition qualities and changes in the camshaft timing due to chain elongation.

The pivots are also adjustable for initial setting up of the VIT and adjustment of breakpoint position.