Question # 62

Briefly describe the principle of the fuel-injection pump and give reasons for the important points of design.

Briefly describe the principle of the fuel-injection pump and give reasons for the important points of design.

By: Dimitros
on Sept. 30, 2016, 10:52 a.m.

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Answer(s):

There is one pump for each cylinder. Fuel is pumped by a ram working in a sleeve, the ram being operated on the pumping stroke by a cam, and on the return stroke by a spring. It works at constant stroke, and the amount of fuel delivered is varied by varying the point in the stroke at which the pressure side of the ram is put into communication with the suction side. When this point is reached the pressure drops suddenly, fuel ceases to be delivered to the engine cylinder, and for the remainder of the stroke of the ram oil is merely pumped back to the suction side of the pump. The means of putting the pressure side of the ram into communication with the suction side is a helix-shaped groove on the side of .the ram, registering with a hole in the sleeve; according to the rotational position of the ram the helix-shaped groove will register with the hole early or late in the stroke of the ram.
The requirements that lead to fuel-injection pumps being of this design are as follows.
1 The need to build up to full pressure very rapidly at the beginning of injection so as to ensure full atomization immediately. Cams lend them¬selves to design for very rapid initial movement of the ram.
2 The need to cut-off ram pressure as suddenly as possible to prevent dribble. The opening of a port does this more effectively than the ending of a (variable) stroke.
3 The need to deliver extremely small quantities of fuel when the engine is running light, withlhe same requirements of rapid build-up, rapid cut-off, and full atomization. Cam drive and port cut-off is the most effective way of achieving this.
When the cut-off point occurs and fuel discharge stops, the rapid drop of pressure causes a shock on the inlet line to the pump. This may be damped out by a shock absorber fitted on the side of the fuel pump and connected with the
oil supply line to the pump. Guard plates are fitted round the bottom of the ram to prevent fuel oil leakages going into the engine lubrication system.
Some fuel pumps are fitted with delivery valves. Where delivery valves are fitted the wings on the valve are not cut through to the valve mitre; this part of the valve acts as a piston. When the delivery valve opens, a large lift is required to give it opening area; when delivery stops, the volume of the space between the pump and the injection valve is increased rapidly as the pump delivery valve
and prevents dribble.
The parts of a fuel pump are very robust to withstand the discharge pressures built up in the fuel system. These pressures may be up to 450 bars (approx. 6500 lb/in2 or 450 kg/cm2).
The rotation of the fuel ram to meter the volume of fuel delivered by the fuel pump may be controlled by a governor, in generators and alternators, or by the fuel lever, in propulsion engines.
By: Dimitros
on Sept. 30, 2016, 10:53 a.m.

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