It’s a function for adjusting the combustion pressure.
VIT = Variable Injection Timing (Variable staring of fuel injection + Variable ending of fuel injection)
Not Variable Injection Period (It only vary the injection timing i.e. if ignition starts early then closes early also)
It’s a special arrangement or system of large 2-stroke, slow speed Marine Diesel Engine for injection of fuel.
It is attached or connected with Fuel Pump.
VIT control is totally load dependent concept.
Credit for VIT goes to SULZER (Leading marine engine manufacturer). First time used in 1978 for RLA Series.
There are two broad classification of fuel injection system for large, slow speed diesel engine: Jerk system & Common rail system. Fuel injection can be controlled at starting as well as ending. But, earlier fuel pumps of marine engine are simple Jerk Type, in which only ending of fuel injection was controlled. Due to this arrangement, Engine loses too much combustion pressure and temperature at low loads.
Disadvantage: It leads to decreased peak pressure and lower efficiency at low loads. And it results in higher fuel consumption during higher price of fuel market.
There are two major rivals of engine manufacturing i.e. Sulzer (Wartsila) & MAN Diesel. Both introduced their special design for adjusting the combustion pressure by controlling injection timing. Actually both manufacturers have different fuel pump for their Engines i.e. Sulzer uses Valve controlled type and MAN Diesel uses Helix type.
Sulzer introduced VIT for valve controlled type pump and MAN Diesel introduced VIT for helix type pump. Earlier design of MAN Diesel’s VIT for helix type is called Conventional VIT.
Later, Sulzer introduced VIT with FQS (Fuel Quality System) and MAN Diesel introduced Super VIT.
Before VIT, fuel pumps have fixed starting of fuel injection and variable ending of fuel injection i.e. there is constant beginning and variable ending of injection of fuel. To overcome this disadvantage or to make variable starting of injection of fuel VIT was implemented. It controls the injection timing of fuel in the fuel delivery pump and advances the fuel pump injection timing. It simply increases the maximum compression pressure in engine.
Methods for variable starting of Fuel Injection:
For MAN Diesel Engines: Alter the barrel position with respect to the plunger or Use fuel pump with two plunger or Alter the fuel cam or position of cam follower.
For Sulzer Engines: Use special design of fuel pumps (arrangement with valve controlled type pump).
VIT (Sulzer VIT)
(Adjustable fuel injection timing + FQS)
In Sulzer VIT system i.e. (VIT + FQS) controls the beginning or starting of injection according to engine load and cares for fuel consumption for different quality of fuels.
It is used in RTA Series and later designs of Sulzer (Wartsila).
Injection timings are varied by lowering or raising the position of the spill valve and suction valve with the help of eccentric.
Raising of valves result in early injection timing and lowering of valves result in later injection timing.
Suction valve is responsible for beginning of injection and Spill valve is responsible for ending of injection.
Advancing of fuel injection timing: (Lowering of suction valve + Raising of spill valve)
Retarding of fuel injection timing: (Raising of suction valve + Lowering of spill valve)
FQS (Fuel Quality Setting): It’s an arrangement for burning low quality fuel. And it is integrated with VIT in valve controlled type pump for increasing cylinder compression pressure with low quality of fuel. There is a FQS Lever (a manual adjusting lever) for setting of various fuel qualities. Output shaft of governor is connected to the VIT control and matched with the FQS.
(Adjustable Injection + Fixed Break Point): It has only adjustable fuel injection timing.
It is used in older designs of MAN Diesel engines like L35MC, L42MC engines. It is obsolete now for larger engines.
It has fixed break point.
Plunger of the fuel pump is profiled i.e. it has extra oblique cut. Due to this extra oblique cut, starting of injection and ending of combustion is not free for adjusting during running period of the engine.
It works at control air pressure greater than 0.5 bar.
It does not have FQS arrangement.
(Adjustable fuel injection timing + Adjustable break point)
It is used in LMC, KMC, SMC and later designs of MAN Diesel engines.
Due to better fuel economy, good combustion tuning at different loads and advantage of emission control, MAN Diesel named it Super VIT. It’s nothing but advanced version of conventional VIT system.
Super VIT mainly comprises of double threaded fuel pump, VIT regulator, position servo, control air (for pneumatic operation), regulating shaft, FQS lever. It doesn’t need profile or extra oblique cut on plunger of the pump.
Simply, vertical position of the pump barrel is lowered or raised for altering the injection timing using rack and pinion mechanism with double thread of the pump.
In double threaded pump, upper thread regulates the suction port and thus starting of the fuel injection. Lower thread regulates the spill port and thus ending of the fuel injection.
Advantage of VIT or Super VIT
Thermal efficiency has increased.
Combustion quality has improved.
SFOC has reduced.
Emissions has controlled.
Thermal stresses has decreased.
Hot corrosion has reduced.