Fuel Conditioning (Automobile)


Fuel Conditioning

For diesel engines, good filtration is critical because of contamination. Organic sludge, inorganic abrasive debris, water and wax crystals are the contaminates, which are to be guarded against. Debris due to corrosion or wear can cause severe problem in both pumps and injectors. The clearances between the pump plungers and barrels and between the moving parts of the injectors are normally 1 to 2 um due to which particles of microscopic size can cause wear, scoring, or seizure thereby damage the system. Also, wear can occur due to the shearing of the abrasive particles between the closing edges of the plungers and the delivery and spill ports. If the injectors fail to sit properly, whether due to debris on the seats or sticking of the moving parts, dribbling may occur and consequently, carbon build-up may take place leading even to total blockage.
Distributor type pumps are more sensitive than in-line types to abrasive particulate matter like debris and corrosive influences in the fuel. A sedimentor is normally installed between the tank and feed pump, and a fine filter capable of arresting particles down to 5 um diameter between the feed pump and engine, is adequate for removing the harmful substances from the fuel. Under very cold conditions, the choking of filters by ice or wax can be overcome by using electric heater elements in the water separators and filter assemblies.

Solid Particles and Sludge

A filter or filters are installed in the fuel feed system to protect the injection equipment from dust. Normally a fuel filter which can pass at least 4546 litres of fuel before chocking can last for about 80,000 km; however the actual figure depends upon the fuel consumption. During service period, a filter should retain at least 80% of particles of about 7 um diameter. The useful life of the filter is a function of the area and porosity of the filtering element. The useful life increases with increase in element area. Therefore, paper elements provide the best compromise between particle retention capability and long life of the filter. The type of paper used, its porosity, the method of production and treatment of the fibbers, such as resin impregnation to bond them together, are of course critical. Generally, the paper is creped to provide adequate fuel flow path between the layers of paper.
Filtration areas of 3 cm /cm volumes are normal for the elements, and 10 cm /cm are practicable. They are generally produced in one of the three forms; pleated, star shape elements or coiled V-construction. The engine and injection equipment manufacturers usually judge the intervals for the change of filter element. Under certain severe operating conditions, short replacement intervals may be necessary.

Removal of Water

Even with the care normally taken in both keeping water out of storage facilities and draining off the collected water in the storage tank, some inevitably enters into the fuel tank due to condensation from the air entrapped in it and precipitation from that already present in solution before delivery from bulk storage. The quantity of water in the fuel for high-speed diesel engines should not exceed 0.05%. Under poor conditions of storage, it can exceptionally, however, increase to as high as 2%. With or without acid the water can damage the pumping elements in two ways. First, because of the poor lubrication properties of water as compared with diesel oil, its presence can cause scuffing and rapid wear. Secondly, when water is trapped between the rubbing surfaces of the elements corrosion can occur. Also contamination with water can cause pitting of the cams used to actuate the pumping plungers, failure of springs and rusting of other steel components.
Combined filter and water separators are in use where the fine droplets of water agglomerate in the filter element forming larger drops, which fall into the base of the unit. In such units,
agglomeration always occurs on the clean side of the filter. A simpler primary water separator is a barrier type vertical flow sedimentor fitted upstream of the pump. In this the fuel, after being delivered into the sedimentation bowl, passes vertically up to a disc of very fine gauge or some other thin filtering medium. This traps water droplets, which drop down to the base of the bowl. In other type of simpler sedimentors, the fuel passes through a plenum chamber, or bowl to reduce the velocity of flow, so that the water droplets fall to the bottom.

The second category of separators are the radial flow types, in one of which the fuel enters at the centre, flows radially outwards over a disc or conical baffle (Fig. 10.3), and then drops down through the clearance around its periphery into the sedimentation bowl, where the water drops to the bottom. From here, the cleaned fuel flows radially inwards and then passes up to the outlet port. This arrangement is simple and compact, because both the inlet and outlet ports can be provided in the filter head casting.
Sedimentors effectiveness is a function of the horizontal cross-sectional area of the bowl, and all types can be designed for removal of water droplets down to 100 um. Droplets below a certain diameter fail to drop down through the fuel into the base of the bowl. This limiting diameter is obtained from the formula based on Stokes Law as follows.
A typical radial flow sedimentor.
Fig. 10.3. A typical radial flow sedimentor.
Solid particles sediment in a bowl at a rate calculated on the basis of Stoke’s law. Those remaining in the fuel are trapped in the filter placed downstream. The minimum size of particles trapped depends on the material and construction of the filter. Provision is incorporated for periodical draining of water. An electrical high water level sensor is installed in the base of the sedimentation bowl, which illuminates a warning lamp on the dash board to indicate the driver for draining off the water in the bowl. Alternatively a float type water level indicator is installed or, a transparent bowl is fitted for visual checking.

Removal of Wax

Small crystals of wax formed in very cold weather, block the pipeline or clog the filter, thereby adversely affect cold operation of the engine. Under extreme conditions, the engine may not even start. For operation of vehicle under extremely low temperatures climatic conditions, it is advisable to install a fuel heater element, either in or upstream of the filter. This makes the fuel warm before the engine is started. However, because of the risk of blockage by wax crystals, it is inadvisable to use a wire mesh strainer on the lower end of the fuel pick up in the tank. With fuel of reasonably good quality, simply mounting the filter close to the engine and fitting radiator blinds can avoid wax precipitation. The specification of cold filter plugging point for diesel fuel depends upon local climatic conditions.

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