Fuel Pump

FUEL PUMP

“A fuel pump is a device which deliver the fuel inside the fuel atomizer or fuel injector.”

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.

Parts of Fuel Pump

The Fuel pump consists of following parts.
1. Plunger
2.Barrel
3. Delivery Valve
4. Ports
5. Spring
6. Rack

Construction of Fuel Pump

Its consists of Plunger which is driven by the cam & tappet mechanism, Barrel in which plunger reciprocate, Delivery Valve which lift off its seat under the liquid fuel pressure & spring force. The fuel pump is connected to the fuel injector through the passage P. SP & Y are the spill & supply port respectively.

Types of Fuel Pump

Two general types of fuel pump are as follows:
1. Mechanical Fuel Pump
2. Electrical Fuel Pump

1. 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. 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.
Working of Mechanical Fuel Pump

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.

2. Electrical 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.

Working of Electric Fuel Pump

The figure shows an S.U. electric fuel pump. It also consists of a diaphragm which is operated electrically. When 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 cases the outlet valve to open and 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.

Working of Fuel Pump

When the plunger is at bottom stroke Port SP & Y are uncovered, oil from low pressure pump after being filtered is forced into barrel. When the plunger moves up due to cam & tappet mechanism, a state reach when both Port SP & Y are closed & further upward movement of the plunger the fuel get compressed. The high pressure thus developed lift the delivery valve off its seas & fuel flow to injector through Passage P. With further rise of the plunger at certain moment, the port SP is connected to the fuel in the upper part of the plunger through the rectangular vertical grooves by the helical groove, as the result of which A sudden drop in pressure occurs & delivery valve falls back & occupied its seat against the spring force. 
 
When the engine is stopped, the plunger is rotated, at this position the rectangular slot in the line with spill port & there is no possibility of pressure build up above the plunger as the upper part of the plunger always remain in the connection with spill port. Therefore there is no delivery of the fuel.

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