The VIT fuel pump is designed to adjust both the timing and quantity of fuel injected into the engine. It operates using a plunger that moves up and down in a barrel, driven by a cam. The movement of the plunger controls two valves: the suction valve and the spill valve. These valves are actuated through pivoted levers and push rods.

Injection Start (Suction Valve Control):

• When the cam follower is on the cam’s base circle, the suction valve is open. As the cam rotates, the plunger moves upward, causing the suction valve push rod to move downward and close the suction valve. This marks the beginning of fuel injection.

Injection End (Spill Valve Control):

• As the plunger continues its upward movement, the spill valve push rod opens the spill valve. This releases the fuel pressure above the plunger, ceasing fuel injection.

Adjustment for Variable Injection Timing (VIT):

• The timing and quantity of fuel injection are controlled by altering the positions of the pivot points for the suction and spill valve levers:
  • Adjusting the suction valve pivot changes the timing of injection start.
  • Adjusting the spill valve pivot changes the amount of fuel injected.
  Part (b)

  Reasons for Changing Injection Timing

  Advancing the injection timing ensures the maximum cylinder pressure (Pmax) is reached at around 85% Maximum Continuous Rating (MCR), improving combustion efficiency and reducing fuel consumption.

  • Up to 40% MCR, the injection timing remains constant to avoid frequent adjustments during low-speed operations or maneuvering.
  • As the load increases beyond 40%, the timing advances until 85% MCR. Between 85% and 100% MCR, Pmax is maintained constant.

  Furthermore, VIT accommodates variations in fuel ignition quality and compensates for wear in the fuel pump and minor camshaft timing changes due to factors like chain elongation.

  Part (c)

  Adjustment of Injection Timing While the Engine is Running (SULZER engine):

  Injection timing adjustment during engine operation is achieved through a pneumatic control system integrated with the electronic governor. Low-pressure air, regulated by a pneumatic control valve, is supplied to the VIT servos on the fuel pump. The governor's output signal, via a linkage and pivoted lever, dictates the pneumatic control valve's output pressure. This pressure signal, ranging from 0.5 to 5 bar, is generated from an electronic signal within the governor by a converter. The pneumatic pressure then directly adjusts the position of the suction valve lever to alter the start of injection, while the spill valve lever's position (and thus fuel quantity) may be adjusted independently