Wednesday, February 22, 2012

LOW PRESSURE FUEL SYSTEM

LOW PRESSURE FUEL SYSTEM


83. An L.P. system (fig.10-13) must be provided to supply the fuel to the engine at a suitable pressure, rate of flow and temperature, to ensure satisfactory engine operation. This system may include an L.P. pump to prevent vapour locking and cavitation of the fuel, and a fuel heater to prevent ice crystals forming. A fuel filter is always used in the system and in some instances the flow passes through an oil cooler (Part8). Transmitters may also be used to signal fuel pressure, flow and temperature (Part 12).

FUEL PUMPS


84. There are two basic types of fuel pump, the plunger-type pump and the constant-delivery gear-type pump; both of these are positive displacement pumps. Where low pressures are required at the fuel spray nozzles, the gear-type pump is preferred because of its lightness.



Plunger-type fuel pump


85. The pump shown in fig. 10-14 is of the single-unit, variable-stroke, plunger-type; similar pumps may be used as double units depending upon the engine fuel flow requirements.

86. The fuel pump is driven by the engine gear train and its output depends upon its rotational speed and the stroke of the plungers. A single-unit fuel pump can deliver fuel at the rate of 100 to 2,000 gallons per hour at a maximum pressure of about 2,000 lb. per





square inch. To drive this pump, as much as 60 horsepower may be required.

87. The fuel pump consists of a rotor assembly fitted with several plungers, the ends of which project from their bores and bear on to a non-rotating camplate. Due to the inclination of the camplate, movement of the rotor imparts a reciprocating motion to the plungers, thus producing a pumping action. The stroke of the plungers is determined by the angle of inclination of the camplate. The degree of inclination is varied by the movement of a servo piston that is mechanically linked to the camplate and is biased by springs to give the full stroke position of the plungers. The piston is subjected to servo pressure on the spring side and on the other side to pump delivery pressure; thus variations in the pressure difference across the servo piston cause it to move with corresponding variations of the camplate angle and, therefore, pump stroke.

Gear-type fuel pump

88. The gear-type fuel pump (fig. 10-12) is driven from the engine and its output is directly proportional to its speed. The fuel flow to the spray nozzles is controlled by recirculating excess fuel delivery back to inlet. A spill valve, sensitive to the pressure drop across the controlling units in the system, opens and closes as necessary to increase or decrease the spill.


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