Fuel Pump in Marine Diesel Engine Working, Types And Functions

                The function of the fuel pump is to control the quantity and timing of the fuel injected into the combustion space and to provide the high fuel pressure required to hydraulically operate the fuel injector. Late or early injection will lead to a lack of power and damage to the engine.               Fuel injection is done with the help of cam and camshaft. Cams mounted on the camshaft, which is driven by the crankshaft are used to operate the fuel pumps, one of which is provided for each cylinder. The speed of the camshaft is the same as the engine speed in case of two stroke engine and half the speed in case of 4 stroke engine.

Basic Working principle of fuel pump

              As the cam rotates it operates a spring loaded ram (the plunger) which moves up and down in a cylinder (the barrel). As the plunger moves up the barrel, the pressure of the fuel in the barrel above the plunger rises very quickly. The high pressure fuel then opens the fuel valve (injector) and is sprayed into the cylinder in tiny droplets known as atomization. It is important to note that the injection only takes place when the plunger is moving up the cam slope. 

         But actual engine’s fuel pumps are something different, because the above explained principle will always deliver the same amount of fuel. In actual engines fuel injection varies according to the load ie, variable timing at the beginning and end of the injection.

         There are two different methods are used in marine diesel engine

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>  Port-Controlled Helix Jerk Pump  

         It is commonly used in MAN B&W engines as well as 4-stroke engines

         The pump consists of a cam operated single acting plunger with fixed stroke. The plunger has a helix machined into it which also forms a vertical groove and an annular groove at the base of the helix. The plunger reciprocates in a barrel, located in the pump body which has spill ports, connected to the suction side of the pump, drilled so that they are above the top of the plunger when the cam is on the base circle. The plunger is keyed to a sleeve which has a gearwheel (pinion) machined into it. The pinion meshes with a rack which can rotate the plunger relative to the barrel. The rack is connected to the engine governor.

        Pump barrel fill up with oil during downward stroke through the suction port which is uncovered as in fig A. During the upward stroke, the plunger covers the suction and spill ports as in fig.B. The beginning of injection is constant and is achieved by the fuel pressure rising above the spring loaded delivery valve preset pressure. The delivery ends when the helical edge uncovers the spill port as in fig. C. Beginning of injection is initially set and constant. It starts when the top edge of the plunger covers the suction ports and the pressure is greater than the delivery valve setting. End of injection is variable and is controlled by the helical edge uncovering the spill port. (This can be varied by moving a rack and pinion mechanism which rotates the plunger and helix). The spill port spills fuel back to the suction side.

      This is the commonly used pump.

 

> Suction and Spill Controlled Fuel Pump

           A plain plunger reciprocates in a barrel. As the  plunger moves up and down, two pivoted levers operate push rods which open the suction and spill valves. When the cam follower is on the base circle of the cam, the suction valve is open and the spill valve is closed. As the plunger moves up the barrel, the suction push rod moves downwards and the suction valve closes. Injection then commences and fuel is delivered via a non return valve to the injectors. As the plunger continues upwards the spill push rod will open the spill valve, the pressure above the plunger will fall and injection will cease.