Fuel system components

Reference has already been made to the various components of fuel systems, and the more common components will be covered in following sections of this chapter. Injection pumps, injectors, and particular systems with their components are treated later in separate chapters.

Fuel tank

Figure 19.4 shows the fuel tank of a truck. This is secured to the side rails by brackets. A fuel supply pipe and a leak-off return pipe are fitted.

Plain steel is used for the construction of most fuel tanks; aluminum is used with some larger vehicles to reduce weight. A galvanized steel tank must not be used for diesel fuel storage. The fuel has a chemical reaction with the galvanized (zinc) coating which forms powdery flakes. This would quickly clog the fuel filters and cause damage to the fuel pump and the injectors.

It is desirable that the fuel tank be refilled at the end of each day’s operation. A full tank will prevent condensation from forming on the surfaces of the tank and contaminating the fuel.


Fuel lift pumps

These are referred to as lift pumps, feed pumps, supply pumps, or transfer pumps. The function of the pump is to transfer fuel from the fuel tank through the filters to the injection pump. The pumps may be mounted on the engine block, and mechanically operated by a cam on the camshaft, or mounted on the injection-pump housing and operated by a cam on the injection-pump camshaft. Lift pumps are fitted with a priming lever, which enables the system to be primed or bled with out the engine running. Four types of lift pumps are: diaphragm, vane, gear, and plunger.

Diaphragm pumps

These pumps, when mounted on the engine block, are similar to those used on petrol engines, except for the addition of the priming lever. Diaphragm pumps mounted on the injection-pump body are a little more compact, but function in the same manner. They may, or may not, have a filter as part of the pump. A simplified drawing of a diaphragm fuel pump is shown in Figure 19.5. It consists of a flexible diaphragm (2) attached to a pull rod (3), which in turn is attached to the lever (5). The lever (or arm0 is caused to pivot about a pin by the action of a cam, so that the diaphragm flexes up and down, thus changing the diaphragm flexes up and down, thus changing the volume of the chamber above the diaphragm.

The pump operates as follows: Fuel in the tank has atmospheric pressure acting on its surface. AS the diaphragm is moved downwards under the action of the cam and lever, a low pressure is created above the diaphragm, and fuel is forced to flow from the tank through the inlet valve (1) into the pump chamber. When the diaphragm reaches the bottom of its stroke and the cam is moving away from the lever, the diaphragm is moved upwards under the action of the spring (8), which was compressed during the down stroke. The fuel in the pump chamber is now forced to flow through the outlet valve to the fuel filter. The pressure of fuel will close off the inlet valve and this prevents the fuel from returning to the tank. Rotation of the cam causes a flow of fuel from the pump.

The pressure of fuel that can be produced by the pump is determined by the strength of the pump spring, because it is the compressed spring that forces the diaphragm upwards to pump the fuel.

A sectional view of a complete fuel pump is shown in Figure 19.6. The pump consists of an upper and lower body held together by a number of screws. The upper half of the pump carries two caged valves – the inlet on the right and the outlet on the left side. A domed cover is fitted to the top of the pump. The lower half of the pump carries the diaphragm, its operating linkage and the diaphragm spring. A hand-priming lever is fitted to enable the pump to be operated by hand when bleeding the fuel system.

In operating, fuel enters the inlet port (not shown) and passes through the space at the top of the pump enclosed by the domed cover. Sediment traps and gauze filter are provided on the top of the pump to trap any foreign particles in the fuel, although these are not included in all pumps. Fuel then passes down through the intake valve into the pump chamber and, by diaphragm action, is discharged through the outlet valve.

The fuel pump has a greater capacity than the system needs and so pressure builds up in the system. Under these conditions, pressure prevents the diaphragm spring from forcing the diaphragm fully upwards and so the diaphragm operates around the lower end of its stroke. A free operating arrangement between the diaphragm link and the pump-operating lever allows the operating lever to follow the cam without moving the diaphragm.

Vane pumps

This type of pump rotates and is usually made as part of a distributor-type injection pump. The rotor, which carries the vanes in slots, is mounted off-center in the pump body. As the pump rotates, the vanes are able to slide back and forth in the rotor so that they can follow the shape of the shallow cylinder in which they are being rotated. As the vanes rotate, they increase the size of the cavity near the inlet port. This causes a low pressure and draws fuel into the pump. Further rotation decreases the size of the cavity the pump through the outlet port. In this way, fuel is carried through the pump by the action of the vanes. A vane pump is shown in Figure 19.7

Vane pumps are usually part of a distributor type injection pump and closed in the injection pump housing.

Gear pumps

These are also rotary pumps. They consist of two gears in a housing – a driven gear and an idler gear. Fuel enters the inlet port and is carried through the pump in the space between the teeth of the gears and the body of the pump. The Detroit diesel uses a gear-type lift or transfer pump, and the Cummins PT fuel pump has a gear-type pump within its housing.

Plunger pump

A plunger pump is used with some fuel systems. This may be used as the lift pump, or as a hand-operated priming pump for priming and bleeding the system.

Figure 19.8 illustrates a plunger pump, which also includes a hand-priming pump. The lift pump is mounted on the side of the injection-pump housing and operated by a cam or eccentric on the injection pump camshaft.

The hand primer is fitted to the inlet side of the lift pump. To operate, the plunger is unscrewed and operated up and down by hand.

In operation, the eccentric on the injection pump camshaft operates against a roller tappet to move the plunger back and forth against the plunger spring. Fuel is drawn into the intake on the right and discharged from the outlet on the left. The arrows show the path of fuel through the pump.

Fuel filters

Because of the very small clearances in the fuel injection pump and injectors, fuel used in diesel engines must be clean. Precautions must be taken to ensure that fuel put into the fuel tank is free from contamination by foreign matter, including water. The fuel system is fitted with a series of filters, which sometimes commence with wire gauze at the tank filer and end with a small edge filter at the injector itself. The filters in the system should be seen as safeguard for the fuel injection components and not as a means of removing contamination from the fuel.


The need for fine filters can be seen when considering clearances between working parts of fuel injection equipment. The clearance between injection pump barrels and plungers is about 2 to 4 microns. A micron is one-thousandth of a millimeter (0.001 mm). Figure 19.9 shows the size do dust particles in relation to the diameter of a human hair and gives an appreciation of the need for line filters in the fuel system. A medium size particle that can be floating in the air is about one-tenth of the diameter of the air.

The efficiency of a filter can be related to its cut-off size as well as to the amount of particles that it removes. The term “cut-off” refers to the maximum particle size that can pass through the filter element. Particles above this size are “cut-off”. Different filtering materials have different cut-off sizes.

In addition to filtering solid particles, fuel filters also prevent water from passing through the system. Water is unable to break down into small enough particles and so is blocked at the filter.

Types of filters

Fuel filters can be divided into two general types: those that use depth filtration and those, which use edge filtration. Depth filtration employs some form of filter element, which allows particles to accumulate without blocking up the filter. Edge type filters tend to build up a layer of particles, which can restrict the fuel flow through the filter.

The names given above really relate to the method of filtering, and the type of filtering material employed more often refers to depth-type filters.

Pleated paper

The filter element, of special filter paper, is a very effective filter, with a large area. It is capable of filtering very fine particles. It has a cut-off size of around 5 microns. The element cannot be cleaned and is renewed during servicing of the fuel system. The paper filter is in common use. An example of this is shown in Figure 19.10.

Cotton and felt

Felt is cloth made by rolling and pressing wool or another suitable textile accompanied by the application of moisture or heat, which causes the fibres to mat together.

Both materials were used but have largely been replaced by more efficient paper filters. A cloth filter has a cut-off size of around 25 microns, while a felt filter has one of about 17 microns. The elements of these filters can usually be cleaned.

Sintered metal

These are porous metal filters of sintered alloys such as bronze. Powdered metal is fused together to form a porous material which allows clean fuel to pass, but filters to a cut-off size of 10 to 20 microns. A variation of this type of filter is compressed metal gauze.

Edge filters

One filter of this type has laminated discs of metal or composition. The edges of the discs, although set slightly apart to enable fuel to flow between them, are close enough to act as a filter.

A type of edge filter is used with some injectors. This is a high-pressure filter that is mounted on the injector. It consists of a small metal bar which fits into a bore in its housing. The bar has very little clearance in the bore. The ends of the bar are circular, but the remainder is of edges on the bar, which, because of their close fit in the bore, form an edge filter. Fuel entering the hole in the filter is forced to flow across the edge of the filter (between the edges of the filter and its bore) before it can pass through the filter to the injector.

Filter design

There are various designs of filters, which are arranged to filter out particles of dirt and small quantities of water. Filters fitted to the system ahead of the lift pump are known as primary filters, while those fitted after the lift pump are known as secondary, or sometimes as main filters. These may also be referred to as suction filters and pressure filters respectively. Faulty sealing of filters on the suction side of the lift pump will allow air to enter the fuel system, while faulty sealing of pressure filters will cause fuel leaks.

An arrangement of the fuel-supply portion of a fuel system is illustrated in Figure 19.11. This includes a primary filer (sedimentary) to separate water and solids from the fuel: a hand primer to enable air to be from the system after the filters have been serviced; a lift pump which operates while the engine is running to supply the injection pump with fuel; a secondary filter to filter the fuel before it reaches the injection pump; and a distributor-type injection pump. The function of the filters is as follows


This is a primary separator or filter designed to remove water and solid particles from the fuel. Its name is derived from its action, which causes droplets of water and particles of solid matter to settle or form sediment in the bottom of the sediment chamber. Sediment is matter that settles to the bottom of a liquid and in the sedimenter this occurs because water and small solid particles in the fuel are heavier than the fuel itself.

A section through a sedimenter is shown in Figure 19.12. The unit is constructed of three parts: the filter head, the sediment element, and the sediment chamber or bowl. A drain plug at the bottom of the chamber can be removed to drain or flush away-accumulated water and sediment. The three parts of the unit are held tighter by a central bolt in the filter head. Sealing rings between the parts prevent leaks.

Arrows in the illustration show the fuel flow through the sedimenter. Fuel enters through the inlet connection and then flows over and around the conical section of the element, which acts as a diffuser to spread the fuel flow.

Fuel then passes down through the narrow gap between the outer edge of the conical section and it’s housing into the sediment chamber where it flows in a radial direction back towards the center of the unit and then out through the outlet connection.

During the period that the fluid is flowing in a radial direction, any particles of solid matter or water (being heavier than the fuel) fall to the bottom of the sediment chamber and so separate from the fuel. The sediment and water can be removed through the drain plughole, or the unit can be dismantled for cleaning by removing the central bolt on the filter head.

Water warning devices

Sedimenter can include a device, which provides a warning when the level of the water in the bowl of the sedimenter rises to the extent that the unit could become ineffective. The bowl has a probe, which senses the water level; when the sedimenter requires draining, the probe provides an electrical signal, which is used to light a warning lamp or sound a buzzer in the driver’s compartment.

In addition, some sedimenter units incorporate a float and valve in the bowl. The float rises as water collects in the bowl until the conical point on the top of the valve enters the valve seat and cuts off the fuel. This type of sedimenter is given the name of “water-stop”. A warning signal can be used in conjunction, so that a warning is given before the water reaches the level where the fuel supply will be cut off.

Secondary filter

The filter in Figure 19.11, which is fitted between the fuel lift pump and the injector pump, is called ”agglomerate” by its manufacturer. It has a paper filter element with a deep bowl or sediment chamber beneath it to catch water. This type of filter is shown in more detail in Figure 19.13. The fuel flow, as shown by arrows – in through the inlet connection and downwards through the filter elements – is designed for both filtration and water separation. The filter retains the solid particles but the fine water droplets, which are forced through the pores in the filter element, agglomerate or coagulate – that is, they combine to form larger droplets, which are deposited by sedimentation into the sediment chamber at the base of the unit. The filtered fuel, free of water, flows up the center tube of the element and from the filter through the outlet connection.

It can be seen that with this type of filter, a bowl is not used to cover the filter element. The element is clamped by the central bolt between the filter head at the top and the sediment chamber on the filter base at the bottom. Other designs, such as the one shown in Figure 19.10, use a bowl that completely encloses the filter element.


This is a name given to filters, which are fitted ahead of fuel lift pump. The sedimenter previously described is a large pre-filter. A pre-filter is fitted to the lift pump as in Figure 19.2. Another type of pre-filter is illustrated in Figure 19.14. This consists of a small glass or metal bowl, which can be removed for cleaning y loosening the stirrup fastener and moving it to one side. A gauze filter is fitted into the filter head.

A pre-filter not only filters the fuel before it enters the lift pump, but also assists the main filter by removing water and larger solids which could build up within the main filter element. Water can be removed much easier by the pre-filter on the suction side of the suction side of the lift pump as it will be present in large droplets. Any water passing through the lift pump will be emulsified (mixed) with the fuel and much more difficult to separate from the fuel in this form.

Dual filters

Two or more main filters are sometimes fitted. Two filters can be arranged in series, or in parallel. When installed in series, all the fuel passes first through one filter and then the other. In parallel, the fuel flow is divided so that other. In parallel, the fuel flow is divided so that each filter carries half the fuel. This arrangement is suitable for large engines which have a high rate of fuel flow in the system.

Two filters in series can be seen in Figure 19.15, one of which is being removed for servicing. With this arrangement, the first filter can be of the “agglomerate” type, with a sediment bowl on its base, to assist in removing water from the fuel.

Water in fuel

While minute quantities of water may be dissolved in the fuel and probably cause little harm, water in droplets can cause damage by corrosion, wear, and pitting of highly finished surfaces.

Water can be accidentally introduced into the vehicle’s fuel tank, or it can condense on the walls of the fuel tank or fuel storage tank by precipitation due to changes in temperature. It has been estimated that, in a 5000 liters storage tank, as much as 20 litters of water can condense from the air during one year. Normally, this water would be drained away periodically through drain plugs provided for this purpose. There are other precautions recommended when handling fuel, such as storing drums on an angle and drawing from the high side of the drum. The installation requirements for both underground and overhead tanks include provision for draining water and also any include that might accumulate in the bottom of the tank.

Servicing filters

Filters may be of the “throw-away” type, where the complete filter is renewed, or of the replaceable-element type where only the element is renewed.

The throw-away type is unscrewed from its mounting and disposed of. The filter mounting should be cleaned and new filter and seal installed. After installation, and following priming and bleeding of the system, the filter should be checked for leaks.

The procedure for a disposable element filter, which is illustrated in Figure 19.15, is as follows:

  1. Remove all external dirt from the assembly. If fitted with a drain cock, the filter should be drained.
  2. Unscrew the center bolt (1); at the same time support the base of the filter assembly and prevent it from turning.
  3. Remove the filter base or bowl together with the element.
  4. Discard the filter element. Clean out the filter base or bowl and rinse out with clean fuel.
  5. Clean the inside of the filter head with a lint-free cloth or clean brush and diesel fuel. Particular attention should be paid to the groove that houses the sealing ring.
  6. Inspect the sealing rings and replace if any damage or imperfections can be seen.
  7. Fit the element to the base or into the bowl, depending on the filter type.
  8. Fit the base, or bowl, to the filter head, making sure that the seals are correctly in position.
  9. Tighten the center bolt to the correct torque. This should not be over tightened, as it will cause damage. A torque of around 8 to 10 Nm is usually specified. This could be considered as being firm or just tight.
  10. Vent or bleed the system and check the filter for leaks.


The importance of clean fuel and cleanliness of the fuel-injection system cannot be overstressed. The fuel supplied from the oil company will be clean and ready for use. Care must be taken to ensure that poor handling and storage practices do not cause contamination before the fuel reaches the engine. Cleanliness is important for all liquid fuels, but it is particularly important for diesel fuels. Contamination will damage the injection components resulting in costly repairs. Only lint-free rags should be used for cleaning the parts of the fuel system. Cotton waste should not be used.



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