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Diesel Fuel System Components


Before discussing the various types of fuel injection systems, let’s spend some time looking at the basic components that are necessary to hold, supply, and filter the fuel before it passes to the actual injection system. The basic function of the fuel system is to provide a reservoir of diesel fuel, to provide sufficient circulation of clean filtered fuel for lubrication, cooling and combustion purposes, and to allow warm fuel from the engine to recirculate back to the tank(s). The specific layout and arrangement of the diesel fuel system will vary slightly between makes and models.

The basic fuel system consists of the fuel tank(s) and a fuel transfer pump (supply) that can be a separate engine-driven pump or can be mounted on or inside the injection pump. In addition, the system uses two fuel filters—a primary and secondary filter—to remove impurities from the fuel. In some system you will have a fuel filter/water separator that contains an internal filter and water trap.

Tank and Cap
Fuel tanks used today can be constructed from black sheet steel, or for lighter weight, aluminum alloy is used. Baffles are welded into the tanks during construction. The baffle plates are designed with holes in them to prevent the fuel from sloshing during the movement of the vehicle. The fuel lines (inlet and return) should be separated by a baffle in the tank to prevent warm return fuel from being sucked right back up by the fuel inlet line. Both the inlet and return lines should be kept 2 inches above the bottom of the tank, so sediment or water is not drawn into the inlet.

A well-designed tank will contain a drain plug in the base to allow for fuel tank drainage. This allows the fuel to be drained from the tank before removal for any service. Many tanks are equipped with a small low-mounted catchment basin so that any water in the tank can be quickly drained through a drain cock, which is surrounded by a protective cage to prevent damage.

The fuel tank filler cap is constructed with both a pressure relief valve and a vent valve. The vent valve is designed to seal when fuel enters it due to overfilling, vehicle operating angle, or sudden jolt that would cause fuel slosh within the tank. Although some fuel will tend to seep from the vent cap, this leakage should not exceed 1 ounce per minute.

The diesel fuel tank is mounted directly on the chassis of construction equipment because of its weight (when filled) and to prevent movement of the tank when the equipment is operated over rough terrain. Its location depends on the type of equipment and the use of the equipment. On equipment used for ground clearing and earthwork, the tank is mounted where it has less chance of being damaged by foreign objects or striking the ground.

The electric gauges used in the diesel fuel system are the same types as used in the gasoline fuel system. Some manufacturers use a bayonet type gauge permanently attached to the filler cap of the fuel tank or installed under the fuel cap. These are graduated and the fuel level is checked by the same method as oil in an engine.

Fuel Filters
The purpose of any diesel fuel filter is mainly to remove foreign particles as well as water. The use of a suitable filtration system on diesel engines is a must to avoid damage to closely fitted injection pump and injector components. The components are manufactured to tolerances as little as 0.0025 mm; therefore, insufficient fuel filtration can cause serious problems. Six principal filter elements have been used for many years:

  1. Pleated paper
  2. Packed cotton thread
  3. Wood fibers
  4. Mixtures of packed cotton and wood fibers
  5. Wound cotton or synthetic yarn
  6. Fiber glass

Filter ability will vary between the type and manufacturer. On diesel engines a primary and secondary filter are used. The primary filter is capable of removing dirt particles down to 30 microns and the secondary filter between 10 to 12 microns. Secondary filters are available between 3 and 5 microns, which are used in severe service operations. The primary is usually located between the tank and the supply pump and the secondary filter between the supply pump and the injection pump. Diesel fuel filters are referred to as full-flow filters, because all the fuel must pass through them before reaching the injection pumps.

Some filters use an internal replaceable element inside a bowl or shell; these are commonly referred to as a shell and element design (fig. 5-8). However, most filters used today are of the spin-on type, which allows for faster change out since the complete filter is a throwaway. Fuel filter elements or cartridges should be replaced at the recommended interval designated by the manufacturer’s service manual.


Should the engine run rough after a fuel filter change, it is likely that air is trapped in the system. Bleed all air from the filter by loosening the bleed screw. In the absence of a bleed screw, individually loosen the fuel lines until all air has been vented.

Water Separators
The purpose of a fuel filter is mainly to remove foreign particles as well as water. However, too much water in a fuel filter will render it incapable of protecting the system. So to ensure this does not happen, most diesel engine fuel systems are now equipped with fuel filter/water separators for the main purpose of trapping and holding water that may be mixed in with the fuel. Generally, when a fuel filter/water separator is used on a diesel engine, it also serves as the primary filter. There are a number of manufacturers who produce fuel filter/water separators with their concept of operation being common and only design variations being the major difference. Basic operation is as follows:

  • The first stage of the fuel filter/water separator uses a pleated paper element to change water particles into large enough droplets that will fall by gravity to a water sump at the bottom of the filter.
  • The second stage is made of silicone-treated nylon that acts as a safety device to prevent small particles of water that avoid the first stage from passing into the engine.

Supply Pump
Fuel injection pumps must be supplied with fuel under pressure because they have insufficient suction ability. All diesel injection systems require a supply pump to transfer fuel from the supply tank through the filters and lines to the injection pump. Supply pumps can be either external or internal to the injection pump.

The two types of supply pumps used on diesel engines today are the gear type and the vane type.

The remaining task to be accomplished by the fuel system is to provide the proper quantity of fuel to the cylinders of the engine. This is done differently by each manufacturer and is referred to as fuel injection.

Figure 5-8.—Fuel filter assembly with replaceable element.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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