Variable injection timing (VIT) is a form of fuel pump control enabling an engine to operate with the designed maximum cylinder firing or combustion pressure from approximately 85% power output to maximum power.

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.

• The plunger moves vertically inside a matched barrel.
• The plunger is machined with helical grooves to control the end of injection.
• The spill ports and suction ports are located at the top of the barrel, allowing oil to flow into and out of the fuel pump.

Start of Injection:

• As the plunger moves upward during its stroke, it covers the spill port. This marks the beginning of injection as fuel pressure starts to rise.

End of Injection:

• As the plunger continues upward, the helical groove on the plunger aligns with the spill port. This alignment causes fuel to spill out, the pressure to drop, and the fuel injection to cease.

• The start of injection is adjusted by altering the height of the spill port relative to the plunger.
• This is achieved by raising or lowering the pump barrel:
  • A rack and pinion mechanism combined with a double-threaded sleeve moves the barrel up or down.
  • Moving the barrel upwards advances the start of injection.
  • Moving the barrel downwards retards the start of injection.
  Part (c)

  Determining Correct Fuel Quantity and Timing for Individual Fuel Pumps:

  A draw card (indicator diagram) is obtained for each cylinder. This diagram shows the pressure conditions throughout the engine cycle. By analysing the diagram, the start and end of injection can be identified and compared with engine design parameters. Any deviation from the expected timing indicates the need for adjustment.

  Checking Fuel Pump Lead (For Jerk-Type Pumps):

  Fuel pump lead is the vertical distance the plunger has risen above the spill port when the cylinder piston is at Top Dead Center (TDC).

  Procedure to Check Fuel Pump Lead:

  1. Shut off the fuel inlet to the pump and drain the fuel oil.
  2. Disconnect the control air line from the puncture valve and remove the valve.
  3. Unscrew the two plugs (forward and aft) on the pump’s top cover.
  4. Turn the engine until the concerned cylinder piston is at TDC.
  5. Place the measuring tool on the fuel pump cover, ensuring the two legs rest on the barrel.
  6. Push the measuring pin down until it rests on the top of the fuel pump plunger.
  7. Note the measurement (fuel pump lead) on the measuring tool.
  8. Compare this value with the manufacturer’s specifications in the manual, and a reference table may indicate the timing corresponding to the value.