FUEL PUMP FAULTS

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Barrel/plunger wear

Although internal wear will always occur, the wear rate is strongly influenced by the cleanliness of the fuel. The purifier and filtration units should remove abrasive particles in the fuel. A worn pump will:-

·                     Reduce the max fuel pressure achieved by the pump which influences the atomisation achieved,

·                     Inject a reduced amount of fuel, resulting in lower power developed by that cylinder,

·                     Retard the fuel injection.

Pump wear can be compensated for by increasing the fuel rack setting and retiming the fuel pump using indicator cards as the datum.

Cavitation damage

This is caused by an excessive pressure drop at the spill ports, and produces erosion damage just above the helix control edge. This is usually a design problem (fuel flow reflected back against plunger), and will increase plunger wear, reducing sealing efficiency. The only option for the engineer with this fault is to ensure that the fuel pump surcharge pressure is correct, and change the plunger/barrel when excessive damage is present. (Double helix/baffle screw/erosion plug/ shock absorber are all designs to help reduce).

Excess leakage of fuel into camshaft system

Occurs when the barrel is excessively worn, or the sealing arrangement of the barrel is leaking with the drainage holes blocked. Obviously less of a problem with umbrella type and space beneath plunger is exposed (NSD and later MAN B&W), but can introduce low flash point oil with abrasive particles into camshaft/sump system. This will increase cam wear, and increase risk of crankcase explosion. Regular tests of the lube oil system should monitor for fuel ingress, and the seals and possible barrel change will repair this fault.

Wear at discharge valve

The discharge valve is provided to maintain the required pressure in the discharge pipe between fuel pump and injector. If this pressure is too high then the reflection of the pressure pulse in the fuel line against the closed discharge valve will reopen the injector causing secondary injections. These injections will increase the trumpet formation at the tip, and smoke levels from the engine. However if the pressure is too low then there will be an excessive injector delay, which may even prevent fuel injection at low loads. The control of this pressure is achieved by the discharge valve, which causes a controlled reduction in the discharge pipe contents by means of the piston fitted beneath the valve sealing faces. If the valve piston is worn this will reduce the quantity of fuel it removes from the pipeline when seating (& increase seating velocity), and hence secondary injections are possible. If valve leaks or spring is broken, then a greater quantity of fuel will `escape' and hence less fuel will be injected (compared to other cylinders) at next injection, producing cylinder imbalance. Both faults can be easily be repaired by exchanging the valve and spring.

Seizure

Fuel pump seizure may occur if the clearances between the plunger and barrel are reduced, water in fuel or no fuel (the lubricant!) is present. Obviously the fuel should never be closed to an operating fuel pump to stop fuel delivery, but the most frequent seizure cause is excessive temperature changes when changing between fuel grades. MAN B&W recommend that the rate of fuel temperature change should not exceed 2^o C per minute, and this can only be ensured if the temperature control is measuring temperature rather than viscosity. If there is only a viscosity control on your vessel, then a manual change of the fuel temperature should be carried out.

A fuel pump element that has either started or is fully seized should be replaced.

 

 

 

OTHER FUEL INJECTION SYSTEMS

Pilot injection system

A number of manufacturers have investigated a pilot injection system in which two pulses of fuel are injected by the one injector. The first small charge has a long delay period, but when it burns it heats up the cylinder so that the second charge burns with much reduced delay, which produces a smoother combustion sequence. This can be achieved either by an electronic control of the injector, or by a double injection profile on the single Bosch type fuel pump plunger.

Twin injection system

This system is used by Wärtsilä and utilises two fuel injectors per cylinder, a small valve for the pilot injection, and the larger main injector. The pilot injector injects the same amount of fuel each cycle (just below that require to idle the engine under no-load) and is used to reduce diesel knock and ignition delay on the main charge. This mechanism uses the pilot charge to heat up the main charge, similar to pilot injection, and allows the engine to operate for `unlimited' periods on minimum load, and burning of the highly aromatic fuels in medium speed engines. Note that medium speed engines are much more sensitive to changes in fuel quality (i.e. CCAI index)

Twin fuel pump barrels

This system is used by Wärtsilä on its largest medium speed engine 640mm bore, and utilises two fuel pumps per cylinder. The main reason is to ensure high pressures for the fuel injection, which they say can not be achieved using a single pump plunger. Thus two pump plungers operate in parallel, and deliver to the same fuel injector. However one plunger controls the start of the fuel injection (timing), and the other controls the end of fuel injection (quantity).