What is a fuel pump?

A fuel pump is used in the fuel system to deliver fuel from the fuel tank to the carburetor. 

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

For the vacuum test, connect a suitable vacuum gauge between the fuel tank and the pump; run the engine at idling speed. The gauge will indicate the vacuum created inside the pump to suck the fuel from the fuel tank. The vacuum must be retained for at least 10 seconds after closing the engine. The fuel pump found correct in the above three tests should be used in the fuel system.

Mechanical fuel pump

The diagram shows a diaphragm-type A.C. mechanical fuel pump. It is bolted to the engine block or crankcase .0 that the rocker’s arm slides on a pump operating cam on the engine camshaft or in front of the timing gear timing chain sprocket. It consists of a high-grade cotton diaphragm impregnated with synthetic rubber. The movement of the diaphragm sucks fuel from the fuel tank and pushes 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 valve to close and the outlet valve to open. The fuel through the outlet valve goes to the carburetor.

If the float chamber of the carburetor is filled up and there is no need to pump more fuel till some of it is consumed and the engine continues to run, the pump will build 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 the rocker’s arms spring which separates the rocker arm from the eccentric. Thus, although the cam is running continuously, the pump is not operated till the fuel pressure decreases it. In this manner, the fuel supply to the carburetor is regulated under its need.

Electrical fuel pump

This diagram shows the S.U. electric fuel pump. It also consists of a diaphragm, but it is operated electrically. By 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, causing the diaphragm to move to create pressure in the pump chamber. This causes the 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 the electric supply to the solenoid, and the same process is repeated, the pump continues to operate until the ignition switch is turned off.