The 2 Main Types of Fuel Injectors for Marine Diesel Engines

There are two types of fuel injectors in marine diesel engines. One type is a cooled injector, around which oil or water is circulated in a closed circuit system. The other type is an uncooled injector, which depends on the circulation of the fuel oils to maintain desirable temperatures at the atomizing nozzles. The primary difference between a cooled injector and an uncooled injector can be summed up as follows:

  • Cooled injectors, as their name suggests, directly serve to cool the engine.
  • Uncooled injectors do not actually cool the engine. Instead, they depend on the circulation of the fuel oil. (We’ll go into slightly more detail later.)

Fuel Type Used in Marine Diesel Engines

Marine diesel engines differ from their non-marine counterparts in several key ways, one of which is the type of fuel used in them. The most common type of fuel used in marine diesel engines is called heavy fuel oil (HFO). This fuel is almost tar-like and is a byproduct made through the refinement of crude oil. Before use, HFO needs to be treated to remove water and any solids. Once it has been treated, it is injected into the combustion chamber as an atomized mist.

Cooled Fuel Injectors

Fuel injectors that are cooled by recirculation are the type most often seen in modern diesel engines. Part of the reason for the popularity of cooled fuel injectors is that they are extremely efficient, particularly when paired with common rail fuel supply and engine management systems.

The modern-day cooled injector doesn’t have any of the typical cooling water passages to maintain its low temperature. Therefore, it is only able to maintain a cool temperature through the circulation of the HFO and the cylinder head cooling water down as it passes near the injector pocket. The cylinder head is made so that it has space for several water drillings in the area of the injector’s pocket that help to cool off the body of the injector.

Circulating HFO and the cylinder head both work to keep the injector cool without the assistance of an external fuel valve cooling system. The energy that is saved by not requiring the use of an external system improves overall efficiency, especially that of the engine.

Cooled fuel injectors are made up of a steel body and a nozzle. The body houses the spring an actuating rod, while the nozzle contains the needle valve, as well as its seat and atomizer holes. There is an upper chamber that receives a constant supply of HFO when the fuel pump cam is at the bottom of its stroke. This is the stage in which the fuel in the chamber is recirculated. When the cam comes back to the top of its stroke, the fuel pump’s increased pressure activates a relief valve, which supplies high-pressure fuel from the upper chamber to the lower chamber. This fuel lifts the needle valve and inserts atomized fuel through the nozzle holes into the combustion chambers.

Uncooled Fuel Injectors

Hydraulically-operated uncooled diesel fuel injectors are typically found in bigger two-stroke diesel marine engines. These fuel injectors are referred to as uncool because it is the fuel itself, and not the fuel injector, that provides the cooling effect.

Uncooled fuel injectors are all designed very similarly, regardless of the type of engine they are in. They have a needle valve that is spring-loaded and operated hydraulically to release fuel at high pressure through an atomizer nozzle.

These fuel injectors have two chambers — the upper and lower chamber. The upper chamber is charged with fuel oil from the fuel pump, which is sealed by the needle valve. The lower chamber has several small atomizer holes of a specific size and is sealed by the needle valve’s miter seat. This chamber is responsible for distributing the fuel to the combustion chamber.

The valve opens when the compression of the spring is overpowered by the pressure from the fuel pump. When the needle valve lifts, oil is then able to flow to the lower chamber. The needle lifts rapidly and enables the higher pressure fuel to travel through the atomizer holes into the combustion chamber. As the pressure is reduced, the spring compression causes the valve to close.

As reliable as these fuel injectors typically are, from time to time, as with anything, problems can crop up. Here are some of the problems that are seen most often, as well as solutions about how to best handle them:

The valve lead should function rapidly and positively without any oil leaking. There are a few places that you can inspect to ensure that there is no leakage and that rapid motion is retained. First, inspect the spring to see if it’s been damaged or is distorted in any way. You also need to confirm that the atomizer holes are clear and aren’t showing excessive wear. Finally, ensure that there is no damage to the lapped surface and that it is aligned properly.

A leaking needle valve is a common problem in a fuel injection system. A defective or damaged needle valve can lead to a host of problems. One problem is an excessively high exhaust temperature, which can be a fire hazard. The unburnt fuel caused by a leaking needle can lead to a carbon formation. Another common problem due to a leaking needle is decreased combustion efficiency. A leaking needle can lead to many problems and, in worst case scenarios, can be outright dangerous. Fortunately, if properly tested and maintained, the odds of these troubles coming about are minimal.

In Conclusion

There are many ways to provide a marine engine with fuel and, as of now, fuel injection is the best and most efficient. Advancements that have taken place over time have only enhanced the benefits that fuel injection systems provide to marine transportation, although not all fuel injection systems are equal. With just a little basic knowledge, you will be better able to take advantage of those benefits while knowing how to avoid many of the most potential problems with your marine fuel injection system.

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