Oil and Water Cooled Injectors in Medium Speed Marine Engines

We have previously examined marine diesel engine fuel injection systems where we looked at the Common Rail Diesel Injection System (CRDI). When burning heavy fuel oil, the injector will require cooling for efficient operation. In many engine designs this is accomplished by circulating water or oil through additional passages in the injector assembly.

The next few sections deal with the cooling of the injectors, the first section gives an overview on how the injector operates.

The fuel injector consists of a steel body that has fuel oil supply and return channels drilled from the top down through the body ending in a chamber in the nozzle. There are also channels for supply and return of cooling fluid to the nozzle. A fine tolerance hole is drilled down the center of the valve to house a steel rod. A coiled spring holds it tight against the nozzle needle valve, with its load being adjusted by an external set of locknuts on the rod.

The nozzle is fitted to the fuel valve body, secured by the nozzle nut and containing a needle valve that is held against its seat by the rod and spring. The valve will operate when the pressure from fuel pump supplying the oil chamber overcomes the spring compression, lifting the shoulder of the needle valve instantaneously.

This allows the high pressure fuel in the chamber to be injected through the atomizer holes in the nozzle into the combustion chamber as an atomized spray. Here it is mixed with the combustion air at high temperature and pressure; igniting in the diesel principle of compression igniting.

At the end of the delivery of fuel, the needle valve will instantly close under the spring compression, thus cutting of the supply of fuel to the engine.

The injector is fitted with a spring-loaded non-return valve which permits any excess fuel after injection to pass through the spill passage in the valve body returning it to the buffer tank in the fuel oil system.

The next high pressure wave of the fuel pump immediately depresses the spring, ensuring that this spill passage is sealed off before the high pressure lifts the needle valve and injects fuel into the cylinder again.

A typical liquid cooled fuel injector is shown below:

The ideal position of the fuel injector is in the center of the cylinder cover, allowing a symmetrical, conical spray pattern in the combustion chamber.

In large engines with a centrally placed exhaust valve, the fuel injectors are placed symmetrically around the cover and charged from the common distribution connection to inject equal quantity of fuel oil simultaneously.

Engine fuel systems are designed for the normal working conditions. If an engine is running for a long periods at low power, combustion may be inefficient, thus leading to fouling and possible wear. When long periods of engine operation at low power are anticipated, a set of low-power injectors may be fitted which have reduced orifice area giving high atomisation and peak pressure.

It may also be necessary to adjust fuel pump timings. Engines with variable ignition timing fitted can be adjusted to low power automatically.They may also be fitted with fuel injectors which can maintain high efficiency over a wide range of operating conditions, including low speed.

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.

Author's Note

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.