In the previous article we have discussed Port-Controlled Helix Jerk Pumps And Suction and Spill Controlled Fuel Pumps. In those pumps, injection timings are controlled by VIT & Super VIT.
What is VIT and Why is it required?
If there is no VIT in the engine, the maximum firing pressure of the engine decreases as the engine power is reduced, which reduces the thermal efficiency. To improve the thermal efficiency and the specific fuel oil consumption, it is necessary to keep the maximum pressure even at low loads. It is achieved by advancing fuel oil injection timing at low load is called VIT
Working Of VIT in Sulzer RTA engine
VIT= Variable beginning of injection + Variable ending of injection = Adjustable timing of injection
In this method the injection timing is varied so as to have maximum combustion pressures, Pmax, at low loads. Thereby helps in achieving maximum combustion pressure at 75-100 % load range.
A plain cylinder plunger reciprocates inside the barrel. As the plunger moves up and down, two pivoted levers operate suction valves and spill valve push rods which open the suction and spill valves as shown. When the cam follower is on the base circle of the cam, the suction valve is open at the same time the spill valve is closed. As the plunger moves up in the barrel, the suction valve rod moves downwards and the suction valve closes. Fuel injection then starts and the fuel is delivered through a non return valve to the fuel injectors. As the plunger continues upwards, the spill valve push rod will open the spill valve, the pressure above plunger will fall and injection will cease.
The Pump injection timings can be varied by raising or lowering the position of the suction and spill valves. Raising or lowering of the suction and spill valve positions are done by changing the position of the eccentric. Raising the valve implies earlier timings, while lowering the valve implies later timings. The suction valve controls the beginning of ignition i.e. the timing of injection, while the spill valve controls the end of injection i.e. the quantity of fuel
Super VIT
It is a VIT method only, but used on B&W large engines like S-MC, K, L series engines.
Super VIT = Adjustable Timings + Adjustable Break Point
> Break Point
It is the load point where the maximum cylinder pressure has been reached and injection timing has been advanced the most.
Above break point injection timing is gradually retarded back until it reaches its original setting at 100% MCR load. Generally it is 85% MCR load.
Super VIT incorporates both adjustable timing as well as adjustable break point. The super VIT mechanism consists of a jerk type pump with double thread, VIT regulating lever, VIT position servo, control air signal, a position servo unit with input from the governor, FQS (fuel quality setting) liver and regulating shaft.
In this method the jerk type fuel pump does not have a profile i.e. no extra oblique cut on the plunger. The vertical position of the pump barrel is raised or lowered to change the starting of injection by a rack and pinion mechanism and a double thread. Upper threads control the suction ports i.e. commencement of injection by changing the vertical position of the pump barrel with respect to the plunger. Lower threads control the spill port i.e. the fuel quantity and end of injection by rotating the Helix scroll of the plunger with respect to spill Port.
The VIT rack setting is controlled according to the engine load via the regulating shaft and the governor. The VIT rack position setting is done by means of a control signal supply, which pushes the VIT rack position servo. Control air position sensor valve gets its input from the governor, the FQS lever and the regulation shaft.
What Is Fuel Quality Setting (FQS)
FQS is a manual adjustment of fuel injection timing. This setting is used when we get fuel of different ignition lag.
One of the most important property is ignition lag. A fuel with high ignition lag will take much time to burn and lead to decrease in maximum pressure in the cylinder while the fuel with less ignition lag will burn quickly and will lead to maximum pressure. Both these situations disturb the proper combustion of fuel.
To Overcome this problem , FQS setting is provided in the VIT system by which the fuel oil admission angle can be advanced or retarded.
Does the Sulzer engine have Super VIT? If yes, then what is the difference between VIT and super VIT in Sulzer?
When Sulzer introduced their VIT it had the possibility of adjusting the beginning and end of fuel injection independent of one another i.e. it had the same features as the MAN B&W super VIT. Hence there is no question of a "super VIT" in a Sulzer engine. The only change the Sulzer made was to replace the mechanical control of the VIT by an electronic control in their later engines.
Advantages:-
1. Reduced
fuel consumption especially in the important 65-85% power range
2. Adjustments to fuel timing can be
easily carried out, and without stopping engine, which can:-
i) allow balancing of individual engine
cylinder pmax levels,
ii) allow fuel quality effects to be
countered.
Disadvantages:-
A. More complex pump unit, requiring
greater amount of maintenance. Most VIT pump units do not operate,
B. If incorrectly adjusted, pmax
will become excessive, which results in high mechanical stresses and shock
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