WORKING AND CONSTUCTION of auxiliary engine fuel pump

 JERK TYPE FUEL PUMP(B&W) make.

LETS TALK ABOUT CONTRUCTION OF FUEL PUMP                                                                     

The figure shows a fuel pump parts which is used in auxiliary engine of ship.The part no 87 shows a pump element complete, which consists of plunger 109 which resciprocates in barrel 099.

The key on the plunger fits over the sleeve 266. The sleeve on the outside has pinion  machined on it ,which engage with the rack. In this way the the plunger may be rotated by movements of rack. 

The lower part of plunger sits on the sloted peice 183 and thrust piece 205. The pump spring sits 291 sits on the thrust peice 205. The thrust peice 205 sits over the top of rollar guide 051 . The rollar pin 38 engages the rollar 26 to the the rollar guide 051. 

As the fuel cam rotates ,the rollar moves up . As the rollar is connected to rollar guide the rollar guide moves up with the thrust peice and compresses the spring ,and moving the plunger in barrel up . As the cam comes to base circle ,the spring force brings back the rollar to the base circle of the cam with the plunger to move down. The trailing flank of cam brings back the plunger to the bottom of its stroke to allow the chambor to refill. The plunger and barrel assembly house in the pump casing 229. 

The top part of the fuel pump is the constant pressure valve 146 in pressure valve housing 134. It is a non return spring loaded discharge valve arranged to reduce pressure on its discharge side as it closes ,ensuring positive Closing of the fuel injector needle and reducing Cavitation within the pump. 

Function of contant pressure valve- 

1.When the plunger of the fuel pump moving upwards the pressure of fuel keep on increasing , and at a time when it covers the spill port  the injection commences. But the time when short circuit take place that is  when helix comes in contact with spill port  so sudden pressure drop above the fuel pump plunger, so due to this the high pressure oil in the pipeline after the fuel pump try to come back to fuel pump.  But it is not possible because of discharge valve is of non return type. This high pressure oil then gets trapped into the high pressure line after the fuel pump and this high pressure fuel can lift the needle valve and  secondary injection can take place so this is the reason we have stagnation control valve.  This is a control valve which will not open during normal injection but if there is a high pressure on the discharge side then it will open and will maintain a constant pressure Above and below the high pressure pump . 

2.After the end of the delivery stroke as the plunger moves down and still it cover the spill port ,so no oil is present above the plunger, because fuel is not coming from spill port to the upper side of the plunger so whatever oil is present it start boiling because vaccum is created on top of the plunger so Cavitation can occur which can damage the plunger so in this case stagnation control valve helps and give oil from the high pressure line to the upper side of the plunger so that the space above the plunger should not go to  vaccum.

NOW LETS SEE HOW FUEL INJECTION TAKE PLACE.

A helix is machined on the plunger. The barrel has two ports one for inlet of the fuel oil and other is spill port . Let mark two points in helix- mark A the top part and mark B the lower part of helix. See the diagram above. 

As the plunger is moved up by the cam .

Start of fuel delivery -when the mark A of helix is alligned with the inlet and spill port. The pressurized oil above will go to the delivery valve and injection take place.

End of fuel delivery- when the mark B of helix alligned with the spill and inlet port. This allows the fuel pressure above the plunger to fall because the oil is given a path through the vertical slot and spill port.

The quantity of fuel delivered is regulated by the vertical lenght of helix where it is in line with the inlet and spill port. This setting may be  altered by rotating the plunger via sleeve 266 through rack and pinion arrangement. The rack is connected to the governor which moves the rack depending upon the load on the engine.

The fuel cam is designed to raise the plunger at the rate required to build up fuel pressure and maintain this for the corresponding period to operate the fuel injector . Since the pump only discharges on its upstroke ,only one flank of the cam operates the timing. The trailing flank of the cam returns the plunger to the bottom of its stroke to allow the chamber to refill.

Oil supply to the pump suction is by means of a continuously operating supply or surcharge pump which causes flooding of the fuel pump chambor as soon as the suction port is uncovered by the plunger.