10.3.
Fuel Lines
Low-Pressure Line.
The piping from the tank to the high-pressure pump through feed pump and filters constitutes the low-pressure line. The pipe-line dealt in section 9.3 holds for low-pressure line. Bends of small radius should be avoided and the pipes should slope conÂtinuously upwards from the tank to avoid trapping of air and accumulation of water in loops,
where water could freeze and block the system. Some vehicle manufacturers rationalise on a single bore tube of about 6 mm for low-pressure pipes.
High-Pressure Line.
Delivery of fuel from the high-pressure pump to the injector requires a high-pressure line. The high-pressure line should be as short, straight, and stiff as practicable to avoid excessive vibration, and the bores should be smooth to minimise friction. In general, the straight lengths between each end fitting and the nearest bend should be short to keep the natural frequency of vibration of this section well above the normal frequency range of engine vibrations. Bends should be of as large a radius as possible. Clamps of rubber lined metal strip supplanted by plastic fittings are slid into T-grooves in the components to which the pipes are to be secured and then are snapped shut around a tube or bunch of tubes. The number and position of clips is of course critical for limiting amplitudes of vibration.
These pipes are subjected to violently fluctuating pressures, which, in modern engines, can be well over 98100 kPa and are superimposed with shock waves therefore major considerations are strength and cavitation. Cavitation damage is avoided by providing smooth pipe bores, protection against corrosion, as large a bend as possible, and no discontinuities of bore between the pipes, their end fittings, parts of the pump and injector to which they are connected. At the bends, the availability in excess of-1.0 and 0.2 mm should be avoided.
The most commonly used pipe materials is mild steel, which is zinc or tin plated for protection against corrosion. Seamless tubes with accurate bore are used. The harder the material, more it is resistant to cavitation erosion. Stainless steel pipes may be the alternative to steel pipes. The majority of engine manufacturers use 6 mm outside diameter tubing, and the inside diameter vary about 1 to 3 mm to suit the application.
Pipe End Fittings and Connections.
Proper design of the end fittings and connections is necessary to avoid leakage and fatigue failure. The lower injection pressure bell-mouth end of the tube and seal is used with an olive (Fig. 10.2A). This type of fitting is simple and has the advantage of self-aligning capability but damage can occur if the pipe and component to which it is to be connected are badly out of line. During service, mechanics with dirty fingers could leave particles of foreign matter trapped between the sealing surfaces of the olives. The most common arrangement is to bump up the end of the tube to form a spherical seating (Fig. 10. 2B).
Fig. 10.2. Tube end fittings.
To eliminate the problem of local crushing of the edge of a misaligned tube on one side of the connection, tapered seating have been used instead of spherical pipe flare and seating. If injection pressures are vary high a smaller area of seating are obtained by making the angle of the female taper slightly more acute than that of the male part (Fig. 10.2C). This provides higher seating pressure and hence more secure sealing. A sealing compound is applied to the joints to have more reliably fuel-tight. However, only a slight smear of the liquid sealant must be applied so that it does not leak away into the pipeline and pass on to the injectors. When the joints are tightened the liquid is squeezed from between the faces and sealant is left only in surface depressions where metal fails to contact metal.
