What is Variable Injection Timing(VIT) and how it works? Disadvantages of VIT system | What actions are required in case of VIT system failure?

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variable injection timing

How fuel quantity and timing is controlled in marine diesel engines? Disadvantages of VIT system | What actions are required in case of VIT system failure?

VIT or Variable Injection Timing is a form of fuel pump control, that enables an engine to operate with the designated maximum cylinder firing or combustion pressure from approximately 75% power output to maximum power. This improves thermal efficiency and lowers fuel consumption.

VIT helps engines achieve maximum combustion pressure at a lower load, which can lower fuel consumption and improve engine efficiency.

The engine governor actuator output shaft rotates the fuel pump control shaft which in turn moves a rack to turn a pinion on each fuel pump.

The pinion rotates the fuel pump plunger thus varying the position of a helix machined into the plunger relative to the spill ports in the barrel. This alters the cut-off point of the fuel injection as the cam-driven plunger reciprocates in the barrel. 

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variable injection timing


As the governor output shaft moves, a linkage alters the position of a pivoted bar which alters the output of a pressure control valve. The output from this valve is led to Variable Injection servo motors fitted to each fuel pump.

The servo motor output drives a second rack which in turn rotates a pinion fixed to a threaded sleeve which acts to raise and lower the fuel pump barrel thus advancing or retarding the start of fuel injection. The varying output of the pressure control valve is adjustable by moving the valve, thus allowing adjustment for fuels of varying quality.

vit graph chart

Disadvantages of the VIT system:

  1. More complex pump unit which requires greater maintenance. 
  2. if Pmax is incorrectly adjusted, it will cause excessive pressure rise in cylinders, leading to high mechanical stress and shocks 
  3. Due to the precise mechanical components involved, VIT systems can be sensitive to wear and tear, which may affect their performance over time. 
  4. Poor maintenance and operation of VIT can reverse the result and even lead to major damage to the main engine parts.
  5. Maintenance and repair of VIT systems can be expensive, especially if specialized components need replacement or adjustment. 
  6. VIT systems may have a limited range of adjustments, which can restrict the engine’s ability to optimize performance under varying load conditions. 

What actions are required in case of VIT system failure?

In case you witness an operational main engine with one of the units cut off (for any reason), set the VIT to 0 or switch it off completely to avoid an uneven load distribution inside the engine.

In the event of a VIT system failure, the combustion peak pressure is no longer controlled by the pneumatic actuator. To address this situation, a distance sleeve must be installed to fix the neutral position of the VIT/FQS (Fuel Quality System).

This action helps stabilize the engine’s operation and ensures proper combustion performance despite the VIT system malfunction.

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