The most common fuel pump used on auxiliary diesel engines is the Bosch type. This is a cam operated jerk pump with a helical groove on the plunger to control the fuel cut-off and therefore the quantity of fuel delivered to the cylinder for combustion.
These pumps can be arranged singly along the camshaft, with one at each cylinder position or they may be housed in a single block. Each pump unit contains a pump plunger and guide together with a spring loaded delivery valve and its seat. Plungers and guides are not interchangeable — they should be treated as combined units or elements.
Operation : The operation of helix type pumps is shown diagrammatically With the plunger E at the lower limit of its travel fuel enters the barrel from the surrounding suction chamber, through the two ports. As the plunger rises, some fuel is displaced through the ports until they are just closed by the top edge of the plunger. Fuel trapped above the plunger is now forced out through the delivery valve above the top of the pump barrel.
The pressure exerted by the rising plunger causes fuel to lift the valve and to enter the pipe which connects the pump to the injector. As the pipe is already full, the extra fuel which is being forced in, causes a rise in the pressure throughout the line and lifts the needle valve of the injector. This causes fuel to be forced into the combustion chamber in the form of a fine spray.
As the plunger continues to move upwards, the lower edge of the control helix uncovers the spill port, allowing fuel to be bypassed from the barrel suction chamber through the vertical drilled hole in the plunger or via a machined slot or channel This allows the delivery valve to shut under the action of its spring, and with the collapse of pressure in the pipeline, the injector also shuts. At the junction between the delivery valve and its guide, there is a short plane cylindrical piston which fills the seat aperture as the valve closes. This emphasizes pressure drop in the delivery pipe so that the injector closes sharply.
The actual plunger stroke is constant, but an effective stroke depends on which part of the helix is moving up and down in line with the spill port. The effective stroke can be set between maximum fuel and no fuel. The latter setting, means that fuel spills for the full length of the plunger stroke. The plunger is moved around to the required position by the rack and quadrant
The quadrant collar is on a sleeve which has two vertical slots at the bottom. Two lugs projecting from the lower part of the plunger move up and down in these slots as the plunger reciprocates. Rotary movement of the sleeve (which has no vertical motion) moves the plunger. The rack which meshes with the toothed quadrant is externally connected to suitable linkage from the governor and the manual control lever.
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