Fuel Pump: Types of Fuel Pump and Its Working Principle

Fuel Pump

In this article, you’ll learn fuel pumps, types of fuel pump, and their working principle.

The purpose of the fuel pump is to meter the correct quantity of fuel and deliver it at the correct time to the engine cylinder according to the varying load and speed requirements.

Construction and Working of a Fuel Pump

The plunger is driven by a cam and tappet mechanism at the bottom. The plunger reciprocates in the barrel. There are as many plungers as the number of cylinders in the engine. The plunger has a rectangular vertical groove.

The delivery valve is lifted off its seat under the pressure of the fuel against the spring. The fuel from the delivery valve goes to the injector. When the plunger is at the bottom of its stroke, the supply port and spill are uncovered, the fuel from a low-pressure pump after filtration is forced into the barrel.

Now the plunger is pushed up by the cam movement and both the parts are closed. On further movements of the plunger, the fuel above it is compressed which lifts the delivery valve and the fuel through it goes to the injector.

The plunger rises up still further, and at a particular moment, the helical groove connects the spill port, through the rectangular groove to the fuel in the upper part of the plunger. Consequently, there is a sudden pressure drop due to which the delivery valve falls back on its seat under the spring force. The pressure in the delivery pipe also drops. Thus the discharge from the nozzle of the injector is cut off suddenly. The cycle is repeated again and again.

During each stroke of the plunger, the duration of the delivery is more or less according to as the spill port is made to communicate earlier or later, the high-pressure fuel in the upper part of the barrel. This depends upon the position of the helical groove which can be changed by rotating the plunger by the rack.

The rack is connected to the accelerator. It meshes with a geared quadrant. The motion of the rack rotates the gear quadrant which ultimately rotates the plunger. The driver simply operates the accelerator which controls the fuel supply to the engine cylinder.

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Types of Fuel Pump

The fuel pump is used in the fuel system to deliver fuel from the fuel tank to the carburetor. Many types of fuel pumps are used in modern automobile vehicles.

Two general types of fuel pump are as follows:

1. Mechanical Fuel Pump
2. Electrical Fuel Pump

The performance of fuel is tested by pressure, volume and vacuum. A pump must produce certain pressure on the outlet side as specified by the manufacturer. To test a pump for pressure, connect a pressure gauge between the pump and the carburetor and run the engine at a specified speed. The gauge will indicate the pressure created by the fuel pump.

For the volume test, disconnect the fuel from the carburetor and run the engine at idling speed. Measure the volume of fuel coming out from the pump by collecting it in a separate container.

For a vacuum test, connect a suitable vacuum gauge between the gauge tank and then run the engine at idling speed. The gauge indicates the vacuum generated inside the pump to suck the fuel from the fuel tank.

The vacuum must be retained for at least ten seconds after closing the engine. The fuel pump found connected in the above three tests should be used in the fuel system.

Mechanical Fuel Pump

The mechanical fuel pump is operated by an eccentric on the engine camshaft. It is mounted on the side of the cylinder block inline engines. In some V-8 engines, it is fitted between the two cylinder banks.

Construction and Working of Mechanical Fuel Pump

The figure shows a diaphragm type A.C mechanical fuel pump it is bolted to the engine block or crankcase so that the rocker arm slides on a pump operating can on the engine camshaft or in front of the timing gear or timing chain sprocket.

It consists of a high-grade cotton diaphragm impregnated with synthetic rubber. The movements of the diaphragm suck fuel from the fuel tank and pusher it to the carburetor.

As the cam rotates, it operates the rocker arm which in turn pushes the diaphragm up and down. The downward movement of the diaphragm sucks the fuel through the strainer from the fuel tank. The upward movement of the diaphragm pushes the fuel up which causes the inlet to close an outlet valve to open. The fuel through the outlet valve goes to the carburetor.

If the float chamber of the carburetor is completely filled up and there is no need of pumping more fuel till some of it is consumed, and the engine continues to run, the pump will be built up excessive pressure in it which may damage the pump itself.

To avoid this, the connection between the rocker arm and the pull rod is made flexible with the help of springs. When the fuel pressure in the pump increases it compresses the diaphragm spring and rocker arm spring which separate the rocker arm from the eccentric.

Thus although the cam is running continuously the pump is not operated till the fuel pressure decreases in it. In this manner, the fuel supply to the carburetor is regulated in accordance with its need.

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Electric Fuel Pump

The electrical fuel pump is mounted in the fuel tank. It contains an impeller which is driven by an electric motor. This pushes the fuel through the fuel line to the carburetor. Another type of electric fuel pump is mounted in the engine compartment.

The figure shows an S.U.elcetric fuel pump. It also consists of a diaphragm, but it is operated electrically. But turning on the ignition switch, the solenoid winding generates magnetic flux, which pulls the armature and the diaphragm moves up.

The upward movement of the diaphragm creates suction, and the fuel is drawn into the chamber through the inlet valve. But as soon as the armature moves up it disconnects the electric supply, the magnetic flux dies and the armature falls down, causing the case’s outlet valve to open and the inlet valve to close.

The fuel goes out to the carburetor. The downward movement of the armature again sets electric supply to the solenoid, and the same process is repeated, the pump continues to operate until the ignition switch is turned off.

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