Fuel Feed Pumps (Automobile)


Fuel Feed Pumps

The fuel feed pump used for the diesel engine is similar to that of a fuel lift pump for the petrol engine. It delivers the fuel from the tank to the injection pump continuously and at a reasonable pressure. It is necessary because there is possibility of formation of vapour bubbles and subsequently cavitation in the pump due to suction of the rapidly moving plungers of the injection pump. This would lead to uncontrolled variations in the rate of delivery of fuel to the cylinders, causing rough running and possibly even mechanical damage to the engine. Also cavitation could cause mechanical damage in the injection pump. Generally delivery pressures of between about 29 and 98 kPa adequate for preventing vapour formation on the suction side of in-line type injection pumps. This pressure also ensures adequate supply of fuel for filling the plunger elements at high speeds in a rotary or distribution pumps.

Diaphragm Type Pumps

Diaphragm type fuel transfer pumps (Fig. 10.6) are similar to the lift pumps used for petrol engines. The AC Unitac pump (Fig. 10.6A) has a cam-and-lever actuation mechanism and its operation is same as described under section 9.4.1. Also the basic principle of operation of the AC pump shown in Fig. 10.6 B is similar to the above, except that it is designed for direct cam actuation, without a lever type follower. In this case the rod connected to the diaphragm is pulled up instead of pulled down by the cam mechanism. The return spring used for the diaphragm is weaker than that used for the push rod. Consequently, as the cam nose rotates past its top dead centre, the push-rod is pulled down by its return spring until the head formed at its upper end bears against the seating in the boss of the diaphragm carrier, so that the diaphragm is pulled down with it. The pressure differential in the chamber above the diaphragm causes the inlet
valve to open so that it is filled with fuel. On the return stroke, the inlet valve closes and the delivery valve opens, and the fuel is delivered by the action of the diaphragm return spring, however only at the rate to satisfy the demands of the engine.
AC lift pumps. A. Unitac model lever arm actuated. B. Directly actuated.
Fig. 10.6. AC lift pumps. A. Unitac model lever arm actuated. B. Directly actuated.
Plunger type pump (Bosch). A. Fuel transfer position. B. Fuel delivery condition.
Fig. 10.7. Plunger type pump (Bosch). A. Fuel transfer position. B. Fuel delivery condition.


Plunger Type Pumps

Two plunger type pumps are available; one single and the other double acting. The former has the disadvantage that in extreme conditions its delivery pressure can fall to zero between pumping strokes. One some models these pumps can be flange-mounted on the injection pumps,
and actuated by the cams. The single-acting version shown in Fig. 10.7 uses a cam-actuated piston in a cylinder flanked by non-return valve. The piston is lifted by its return spring, so that fuel is drawn into the pressure chamber through the port on the right and a filter and non-return valve. When the cam, acting on a roller-follower and push rod, forces the piston down again, the non-return valve on the inlet side closes and that over the transfer port on the other side opens, allowing the fuel to pass up into the chamber above the piston.
As the cam follower and the push rod rise, the piston is pushed upwards by its return spring generating pressure, which closes the non-return valve over the transfer port, displacing the fuel above it through the outlet port to the injection pump at the rate required by the engine.
The double-acting version has no transfer valve, but has two inlet and two delivery valves. With each stroke, fuel is drawn directly through the filter into the chamber on one side of the piston while, that on the other side it is delivered to the injection pump.

AC Universal Electronic Solenoid Fuel Pump

This pump was first introduced in 1981 as a diesel transfer pump. Subsequently it has been modified to make it suitable also for pumping petrol and other fuels. The measure modifications are induction of a more powerful coil, a special magnetic steel piston to improve its stroking and a teflon piston ring. The AC UES pump has few moving parts, which do not wear out. It can operate at high temperatures, is quiet, and reliable. The fuel inside the pump is confined to a central tubular core, so the coil assembly and electronic components around it are kept dry.
During the operation of the pump (Fig. 10.8) the coil is energised, which pulls the piston down against the resistance offered by the returned spring. At the same time, the outlet valve in the delivery port closes, and the inlet valve in the piston crown opens. As the piston moves downwards, fuel enters the chamber above it. The electronic system switches off the current to the coil, when the piston reaches the bottom of the stroke. The return stroke of the piston is powered by its spring, forcing the fuel above it up through the outlet valve. This sequence is repeated many times per second.
AC Universal electronic solenoid pump.
Fig. 10.8. AC Universal electronic solenoid pump.

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