FUEL VALVE PUMP

         


              

FUEL VALVE PUMP :

The duty of the pump is to time & meter the delivery while the nozzle

breaks up the fuel & distributes into the combustion chamber .

Marine diesel engines mostly employ mostly  a non turbulent

Chamber . An injection system should be able to maintain a constant

pressure before the nozzle during the full time of injection .If this  is

achieved atomisation  is more uniform & a large quantity of fuel can be admitted within a given period . At low speed operation the injection should take  place within the range of maximum accelaration of the plunger .

          Such a system adopts itself well to speed & load fluctuations .

The injection pressure , the rate of the fuel admission remaining more or less constant at all engine speed the effective plunger stroke

Is varied .There are however other factors such as compressibility of

Oil elasticity of material of the fuel delivery pipes etc…., that  would

Somewhat  lower  the  actual performance but it is still reasonable

Close  to ideal.

          The  pump barrel is completely filled with fuel under positive pressure ( gravity or low pressure pump )during the downward stroke of the plunger . The delivery stroke commences as soon as cam

Contour comes in contact with the roller . No pressure  is however buildup in the pump barrel as long as the suction valve remains lifted from its seat . when the pump plunger advances in its pumping stoke

The suction valve is seated due to the push  rod receding sufficiently downwards . A pressure is immediately developed inside the pump barrel which lifts the delivery valve & the fuel is delivered to the pipeline upstream . This timing in relation to  the piston travel is the commencement of injection . The position is adjustable by making the suction valve to sea early or late  . The   termination of injection is affected by reaching the plunger reaching its maximum travel  . This  timing is not adjustable by regulation mechanism .

          The part of the plunger stroke beginning from the commencement  of delivery to the termination is called the “ effective stroke “ . it is that that part of the plunger stroke which is utilised  for the purpose of injection of fuel . Evidently  it is the effective stroke that is varied with the load on the engine , the plunger  stroke remaining constant throughout .

         

In this case the effective  stroke of the plunger is altered by closing the suction valve early or late during the pumping stroke of the plunger . The push rod operating the suction valve is connected to the plunger  movement by a lever mounted on an eccentric shaft .When the shaft is turned , the lever end operating the push rod is raised or lowered  thus making the suction valve  seat late or early .

When seated late in the plunger stroke the effective stroke is shortened  & lengthened when the suction valve is seated early.

          The pump has employed a constant beginning & a variable  end of injection . the beginning of the injection is initially set  on the eccentric shaft operating the push rod of suction valve through the lever . The  length  of the push rod & the position of eccentric shaft determine  the beginning of the injection . The positions of these  once set are not altered by regulating mechanism with the  variation of  load on engine .

          The filling of the barrel is commenced by the pressure of the supply fuel oil opening the suction opening the suction valve as the plunger performs its return stroke . Towards the later part of the stroke the push rod is raised sufficiently to lift   the suction valve

From its seat . The upward pumping stroke commences  with the roller ridding the cm  profile from the base circle . The injection of fuel begins only after the plunger has  advanced further in its stroke when the suction valve is seated . The delivery is continued till it is terminated by the spill valve lifted mechanically  near the end of the plunger stroke . The timing of opening of spill valve depends on the engine load &  is variable by the regulating mechanism . The delivery takes place through a spring loaded valve which keeps keep pipe line full at all times but with a reduced residual pressure