FUEL INJECTOR FAULTS

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Over heating/under cooling

If cooling of the injector is reduced, either by fuel valve cooling system or poor heat transfer to the cylinder head, then the working temperature of the injector will rise. This can cause:-

·         Softening of the needle and seat which increases the possibility of nozzle leakage and/or,

·         Fuel to expand/boil out of the fuel sac, leading to carbon trumpet formation, and increased levels of HC and smoke in the exhaust gases.

Over cooling

More common on older vessels with separate fuel valve water cooling systems. When the injector is over cooled, the tip of the injector falls below the condensation temperature and acid corrosion due to the sulphur in the fuel oil occurs. This can severely corrode the injector tip, causing the spray pattern to be affected.

Nozzle (needle/seat) leakage

This fault will produce carbon trumpets as the dribble of fuel burns close to the tip and the carbon deposits remain. The formation of the trumpets will have a progressive affect by influencing the spray pattern of the fuel, and this can be detected in the increased exhaust gas temps and smoke levels.

Nozzle leakage can sometimes be identified by a seat defect, i.e. the seat is no longer narrow in appearance, and is caused by:-

·         Insufficient cooling,

·         Dirt within the fuel damaging/abrading the seating area,

·         Excessive needle valve hammering, due to excessive time in service, excessive needle lift or spring force.

Weak spring

This will cause the injector to open and close at a lower pressure. Thus the size of the fuel droplets will increase during these injection periods.

Increased droplet size at the start of combustion will decrease the maximum cylinder pressure (late combustion), whilst increased droplet size at the end of combustion will increase the exhaust temperature and smoke (afterburning).

Causes of a weak spring are usually metal fatigue, due to an excessive number of operations.

Slack needle

Slight leakage between the needle valve and its body is required to provide lubrication of the moving parts. However excess leakage due to a slack needle will allow a greater quantity, and larger size of fuel particle to pass between the valve and body.

The quantity of leakage should not influence injector performance unless excessive, but dirt particles between the needle and body can increase friction and make the needle action sluggish.

The cause of a slack needle is usually poor filtration of the fuel causing wear between needle and body.

Poor atomisation

This will increase the size of the fuel droplets, which will increase the time required for combustion. Thus engine noise, exhaust smoke, exhaust temperatures, etc will increase. Poor atomisation can be caused by low injection pressure (fuel pump wear), high fuel viscosity and nozzle hole obstruction such as carbon trumpets.

Poor penetration

This will reduce the mixing which occurs between the fuel and air, and will increase the over-rich areas in the centre area of the cylinder. Thus only following combustion in the centre area will the expanding gases move the fuel charge into the air rich outer ring of the cylinder where the greatest mass of air is present.

This will increase the time required for combustion as the fuel/air mixture is not correct in many areas, and hence afterburning, exhaust temps, and smoke will increase.

Causes of poor penetration is reduced injection pressure, and nozzle hole blockage such as trumpets or sac deposits.

Over penetration

This will occur when the air density within the cylinder is reduced, or with over-size holes. The liquid stream travels too far into the cylinder, so that a high level of liquid impingement on the liner wall takes place. This will remove the liner lubrication, and once burning will greatly increase the liner wall temperature, and its thermal stress.

If this over penetration is caused by prolonged low power operations, then “slow speed” nozzles should be fitted.

Slow steaming nozzles can be used when regular and prolonged engine operation is required between 20-50% power.

The nozzle hole diameter is reduced to

1.         Reduce the penetration that will occur into the less dense cylinder air

2.         Keep the atomisation level and injection pressure sufficient, as mass flow rate is reduced.

If the engine is operated for long period on low levels of power/speed with `normal' size injector nozzles, then the atomisation will reduce, thus engine noise, mechanical loading, exhaust smoke, exhaust temps, and fuel consumption will increase.