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In order for the fuel injection system to fulfill its purpose, fuel must be transferred to it from the fuel tank. This is the role of the low-pressure fuel system components. The low pressure side of the fuel system consists of a number of components including the fuel tank, one or more fuel supply pumps and one or more fuel filters. In addition, many fuel systems contain coolers and/or heaters to better control fuel temperature. Figure 1 illustrates two examples of schematics for low pressure fuel systems, one for a heavy-duty diesel truck and one for a light-duty diesel passenger car .
The fuel tank is a reservoir that holds the fuel supply and helps maintain its temperature at a level below its flash point. The fuel tank also serves as an important means of dissipating heat from the fuel that is returned from the engine . The fuel tank should be corrosion-resistant and leakproof to pressures of at least 30 kPa. It must also use some means to prevent excessive pressure accumulation such as a vent or a safety valve.
The fuel supply pump, often referred to as the lift pump, is responsible for drawing fuel from the tank and delivering it to the high pressure pump. Modern day fuel pumps can be electrically or mechanically driven by the engine. Using an electrically driven fuel pump allows the pump to be placed anywhere in the fuel system including inside the fuel tank. Pumps driven by the engine are attached to the engine. Some fuel pumps may be incorporated into units that serve other functions. For example, so called tandem pumps are units that incorporate a fuel pump and a vacuum pump for the brake booster. Some fuel systems, such as those based on a distributor type pump, incorporate a mechanically driven supply pump and the high pressure pump in one unit.
Fuel pumps are commonly sized to deliver more fuel than is consumed by the engine at any particular operating system. This extra fuel flow can serve a number of important functions including providing extra fuel to help to cool injectors, pumps and other engine components and maintaining a more constant temperature of the fuel in the entire fuel system. Also, the excess fuel that is heated by its contact with hot engine components can be returned to the tank or fuel filter to improve the vehicle’s low temperature operability.
Trouble-free operation of a diesel injection system is possible only with filtered fuel. Fuel filters help reduce damage and premature wear from contaminants by retaining very fine particles and water to prevent them from entering the fuel injection system. As shown in Figure 1, fuel systems can contain one or more stages of filtration. In many cases, a course screen is also located at the fuel intake located in the fuel tank.
Two stage filter system typically uses a primary filter on the inlet side of the fuel transfer pump and a secondary filter on the outlet side. The primary filter is required to remove larger particles. The secondary filter is required to withstand higher pressures and remove smaller particles that can damage the engine components. One-stage systems remove larger and smaller particles in a single filter.
Filters can be a box-type or replacement element design, as shown in Figure 2. The box-type filter is that which can be completely replaced as needed and does not require cleaning. Filters with a replaceable element have to be thoroughly cleaned when replacing elements and care must be taken to avoid any dirt residue that could migrate to the intricate parts of the fuel injection system. Filters can be constructed of metal or plastic.
Common materials for modern fuel filter elements are synthetic fibres and/or cellulose. Micro glass fibres can also be used but because of the risk of migration of small glass fibre pieces broken off from the main element into critical fuel system components, their use in some applications is avoided . In the past, pleated paper, packed cotton thread, wood chips, a mixture of packed cotton thread and wood fibres and wound cotton have also been used .
The degree of filtration required depends on the specific application. In general, when two filters are used in series, the primary filter retains particles down to about 10 - 30 µm, while the secondary filter is capable of retaining particles greater than 2-10 µm. As fuel systems evolve, clearances and stresses on high pressure components increase and the need for clean fuel becomes event more critical. Both the capability of fuel filters to keep up with demands for cleaner fuel  as well as methods quantifying acceptable fuel contamination levels have needed to evolve .
In addition to keeping solid particles out of the fuel supply and injection equipment, water in fuel must also be prevented from entering critical fuel injection system components. Free water can damage fuel lubricated components in the fuel injection system. Water can also freeze in cold temperature conditions and ice may block small fuel injection system passages thus cutting off the fuel supply to the rest of the fuel injection system.
Water can be removed from the fuel using two common approaches. The incoming fuel can be subject to centrifugal forces that separates the denser water from the fuel. Much better removal efficiencies can be achieved with a filter media that separates water. Figure 3 shows a filter using a combination of media-type and centrifugal approaches.
Different water separation media operate under different principles. Hydrophobic barrier media, such as silicone treated cellulose, rejects water and causes it to bead up on the upstream surface. As the beads become larger, they run down the face of the element into a cup under the force of gravity. Hydrophilic depth coalescing media, such as glass micro-fibre, has a high affinity for water. The water in the fuel associates with the glass fibres and over time as more water enters from the upstream side, massive droplets are formed. The water moves through the filter with the fuel and on the downstream side, falls out of the fuel flow into a collection cup.
Increased use of surface active fuel additives and fuel components such as biodiesel have rendered conventional separating media less effective and filter manufacturers have needed to develop new approaches such as composite media and ultra-high surface area coalescing media . Methods of quantifying fuel/water separation performance have also been affected .
Fuel filters can also contain additional features such as fuel heaters, thermal diverter valves, de-aerators, water-in-fuel sensors, filter change indicators.
A fuel preheater helps minimize the accumulation of wax crystals that can form in the fuel as it cools to low temperatures. Common heating methods use electric heaters, engine coolant or recirculated fuel. Two approaches that use warm return fuel to heat the incoming fuel are shown in Figure 1.
Fuel overflow and leak-off fuel returning to the tank also carries air and fuel vapor. The presence of gaseous substances in the fuel can cause difficulties in starting as well as normal engine operation in high temperature environments. Therefore, bleeder valves and de-aerators are used to rid the fuel supply of vapors and air to ensure trouble-free engine operation.