Oil and Water Nozzle Cooling
The nozzles, with the cooling medium being oil or water, have to be cooled because of the high temperatures they are exposed to. The coolant is circulated through passages in the injector assembly, being directed around the injector nozzle. Oil cooling is favored because of its corrosion mitigation, however additives can be added to the cooling water to prevent corrosion.
The cooling medium is supplied to the injectors by a dedicated pump that circulates the medium around the nozzles and through a sea water cooler, to ensure that optimum operating temperature is maintained.
As we have seen, the operating temperature of the cooling medium is controlled by the seawater cooler seawater inlet and outlet valves. It is part of the watch keeping engineer’s duties to maintain the correct temperature of the cooling medium.
Problems with injectors
1. Overheating of the fuel injector nozzle will lead to the nozzle injector holes becoming clogged with carbon and operating inefficiently.
2. The helical spring can become worn and loose part of its tension, this will lead to late cut-off of fuel, the delivery being extended. This is one cause of scavenge fires.
3. The nozzle needle valve/seat can start passing due to carbon build-up or incorrect spring setting. This will cause the HFO to drip from the nozzle after injection, once again promoting scavenge fires.
Fuel valves used on modern two-stroke cross-head marine diesel engines are not water or oil cooled. Instead they are kept cool by several water cooled channels machined into the cylinder head adjacent to the fuel valve insert.
The valves are also cooled by recirculating the fuel oil around the fuel valve when not under pressure or when the engine is stopped.
This also keeps the fuel oil hot and viscous, ready for the next injection.