Uncooled Fuel Injectors - An Overview

Hydraulically operated uncooled diesel fuel injectors are fitted in large two stroke diesel marine engines. The main characteristic feature of these types of fuel injectors used in the main engine fuel injection system of ships is that cooling effect is not provided to the injectors, by the fuel oil. In this article, we will take a look at the construction and working of such diesel injectors. Take a look at the sketch below before you read further, in order to understand it in a better manner.

The design is similar for most engines and consists of a spring loaded non-return NEEDLE VALVE operated hydraulically by a fuel pressure wave from the fuel pump, to discharge fuel at high pressure through an atomizer nozzle.

A typical fuel injector consists of valve body or nozzle holder with the nozzle or atomizer being secured by a retaining nut. Needle valve is lapped into the bore of the atomizer and these must be kept as matched unit.

It has two chambers:

1. UPPER chamber being charged with fuel oil from fuel pump sealed by needle valve.
2. LOWER chamber or sac is sealed by mitre seat of the needle valve and has number of small atomiser holes of correct size and pattern to atomise and distribute the fuel spray into the combustion chamber.

The valve will open when the pressure from fuel pump acting in the shoulder of the needle valve overcomes the spring compression. As the needle valve lifts oil flows to the lower chamber in the atomiser.

Needle lifts rapidly allowing the fuel at higher pressure to pass through the atomiser holes into the combustion chamber. When the fuel pump cuts off pressure, the valve will close under spring compression.

This section lists a few of the commonly found defects in the fuel injectors and how to go about correcting the same.

Action of the needle valve must be rapid and positive with no oil leakage. The following checks should be sufficient to ensure that rapid action is retained without any leakage

• Lapped surface must be free of damage and correctly aligned
• Spring to be inspected for distortion
• Atomiser holes must be clear and unworn

Defects in injector or leaking needle valve may cause :

1. High exhaust temperature
2. Surging of turbo charger
3. Scavenge fire
4. Carbon formation due to unburnt fuel
5. Lifting of cylinder relief valve
6. Reduction of combustion efficiency

Defects in fuel injectors while in use mainly occur due to the following reasons:

• A possible choking due to dirt in the fuel
• Carbon build up at the atomiser

So normally after the assembly the fuel injector is tested with a test pump to see it operating pressure and spray pattern are checked and there must be no leakages.

In the upcoming article we will take a look at cooled fuel injector used in medium and high speed engines.