The manufacturing tolerance of diesel fuel injection equipment is extremely small. Therefore, if impurities in the fuel are not removed by the fuel subsystem, it may lead to premature damage. Most of the dirt found in the fuel is due to the condition of the fixed fuel tank, refueling operations and improper pre-filling techniques of the fuel filter by maintenance technicians.
The function of the fuel filter is to retain particulate matter (fine particles) in diesel. Although the filtration accuracy of some current secondary filters has reached a level sufficient to prevent free-state water from passing through the filter element, water separators are typically used to remove water. All diesel fuel systems require clean fuel, and the function of the filter in the fuel system is to ensure that the fuel is as clean as possible before being delivered to the fuel injection pump assembly.
A typical fuel subsystem with a primary circuit and a secondary circuit mostly uses a dual-filter layout, one for each circuit. Two basic types of filters are used: the currently more common spin-on disposable filter element and the tank-type replaceable filter element. Spin-on filters are obviously easier to maintain and are the preferred design for most manufacturers. Figure 19-7 shows some filter options and flow paths.
Figure 19-7 Various types of diesel filters
Define clean fuel
Fuel filters are classified according to their ability to retain particulate matter. Their working principle is similar to that of fishing nets, that is, particles smaller than a certain size will pass through the mesh - while those that are too large will be trapped. The unit we use to measure particle size is the micrometer. One micrometer (1μ, Greek letter mu) is equal to one millionth of a meter. Therefore, a typical secondary fuel filter rated at 5μ is designed to retain most of the solid particles of this size that attempt to pass through it. You should be aware that the human eye cannot see particles smaller than 40 microns. There is an International Organization for Standardization (ISO) standard for identifying these particles. Figure 19-8 shows the relationship between the cross-sectional dimensions of human hair and some of the dimensions we refer to when talking about clean fuel.
Figure 19-8 Interpretation of ISO Fuel Code and Definition of microns.
The ISO fuel standard involves the quantity of suspended particles in fuel at three key sizes: particles of 4 microns, 6 microns and 14 microns respectively. Caterpillar's definition of clean fuel is ISO standard 18/16/13. This refers to the count of these size particles arranged in this order as shown in Figure 19-8. Caterpillar also stipulates that the maximum water content in its fuel should not exceed 1,000 parts per million or 0.01%. Therefore, what this OEM means is that as long as the fuel cleanliness reaches 18/16/13 or better, the engine filtration system can complete the work required to clean the fuel. If the fuel contaminants are more serious than specified, the filter may get clogged. All fuel filters are classified according to their ability to retain particles of specific sizes. A filter with a rated value of 2 microns and an efficiency of 99% means that it will retain particles of 2 microns or larger in 99% of cases.
Primary filter
The primary filter represents the first stage of filtration in a typical two-stage fuel system. Therefore, the primary filter is usually in operation at a pressure lower than atmospheric pressure and is installed in series between the fuel tank and the fuel transfer pump. Their design purpose is to retain particles larger than 10 to 30 microns (depending on the fuel system). They achieve this by using a variety of media ranging from cotton thread fibers, synthetic fiber threads to resin-impregnated paper. Figure 19-9 shows a typical spin-on primary filter with a water separator function.
Figure 19-9 Spin-on primary filter and water separator.
Secondary filter
The secondary filter represents the second stage of filtration in the two-stage filtration. In a typical fuel subsystem, the secondary filter is supplied with fuel by the fuel transfer pump, which enables the use of more restrictive filter media. Therefore, the secondary filter is usually installed in series between the oil transfer pump or oil supply pump (the pump responsible for drawing oil from the oil tank and supplying oil to the fuel injection components) and the fuel injection device. In some diesel fuel subsystems that use two-stage filtration, both the primary and secondary filters may be located on the same circuit (usually the fuel supply circuit). In this case, two filters are installed on the same base, and the primary filter is connected in series to supply oil to the secondary filter. This layout is more commonly seen in off-road applications of diesel engines.
The current secondary filters may be specified to retain particles as small as 1 micron, but filtration efficiencies of 2 to 5 microns are more common. Secondary filters use a variety of media, including chemically treated pleated paper and cotton fibers. Figure 19-10 shows the secondary filter base assembly used on a newer Volvo engine equipped with a fuel pressure sensor.
Figure 19-10 Volvo secondary filter base assembly.
Water and secondary filter
Water in a free or emulsified state (terms will be defined later) cannot be pumped through most secondary fuel filters with current rated nominal values of 5 microns or less. This will cause the filter to clog due to water and shut down the engine by cutting off the fuel supply. The filter clogged with water should be replaced. As an emergency measure, you can clean the filter with methyl hydrate or other pure alcohol, and then refuel it with fuel.
Single-loop subsystem
In a fuel subsystem that is fully under suction (such as some Cummins systems), when multiple filters are installed in the circuit, the terms "primary" and "secondary" are not used to describe them. Because each filter device used in the fuel subsystem is at a pressure lower than atmospheric pressure (i.e., under "suction"), the inlet limit specification is of vital importance. If exceeded, it will lead to a decrease in power due to insufficient fuel supply.
Filter maintenance
Most fuel filters are replaced regularly according to a preventive maintenance plan, which is managed by road mileage, engine hours or calendar months. They are rarely tested to determine their maintainability. When testing filters, it is usually to determine whether they are clogged enough to reduce engine power due to insufficient fuel supply.
Test and maintain the primary filter
The inlet limit of the primary filter or the filter under suction should be tested using a negative pressure gauge calibrated to the inch mercury (Hg) limit. The negative pressure specification is based on the use of a mercury manometer. A mercury manometer is a U-shaped transparent tube column formed around a calibration scale measured in inches. Then fill the tube column with mercury or water (usually stained for easier reading) to the zero point of the calibration scale. When the pressure gauge is connected to the fluid circuit, it generates a reading based on the pulling force (vacuum circuit) or pushing force acting on the fluid in the pipe string. The negative pressure gauge produces the same reading, but there is no potential danger associated with mercury. The actual entry limit values vary depending on the fuel system and should always be referred to the relevant specifications.
The negative pressure gauge calibrated with mercury should be connected to the circuit between the filter mounting base and the oil transfer pump. Oil transfer pumps are usually positive displacement pumps (discharging a constant volume of fluid in each cycle), so the amount of fuel they pump is proportional to the increase in rotational speed. This means that effective test results can be obtained without loading the engine. When testing the loop limitations of the fuel subsystem that is completely under suction, the specifications can be quite strict. For some Cummins systems that pump large amounts of fuel, the circuit limit specification reading should be 4 to 6 inches of mercury when a new filter is installed, but 7 inches of mercury is the maximum specification. If this value is exceeded, it will result in insufficient fuel supply. Typical primary loop entry limits may be specified to be lower or higher than this value, so it is essential to review the OEM specifications.