Difference between VIT & Super VIT

Modern engines MAN B & W low speed 2-stroke have large Super VIT mechanism to lower the fuel injection and increases the maximum pressure loads, which optimize fuel consumption. But this lead to a another question " Was there a normal vit before super vit ?", is there any major significance of super in VIT ?

Back in the days of conventional VIT, A time-variable injection is obtained through unique profiles in the piston of fuel pumps . Therefore, it was a solid link in-between the injection time and fuel index. Therefore, it was impossible adjusting fuel indexes of each pumps without any much change in injection timing. Therefore, the super VIT was developed to make it possible to regulate the fuel flow & injection time and can be set independently of each other.

VIT = Variable beginning of injection + Variable ending of injection = Adjustable timing of injection.
Therefore, the maximum combustion pressure reaches a load range of 75-100%.

Super VIT
Super VIT = Adjustable Timings + Adjustable Break Point

breaking point

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It is the load point at which the maximum cylinder pressure is reached and the injection time is advanced. Above the breakpoint, the injection time is gradually decelerated until a load of 100% MCR reaches the initial setting. Usually 85% MCR load.

What is the variable injection time and how does it work?
In modern engines, both the start and the end of the injection are varied due to economic considerations. With the start of constant injection, 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 consumption, it is necessary to maintain the maximum pressure , even at low load. This is achieved by advancing the fuel injection time at low load, called VIT.
The cylinder bottom has a thread cut in. The cylinder is moved by a rack and pinion. The cylinder can move freely upwards and downwards in the pump housing. This has changed the effect of the discharge opening position relative to the piston stroke. Therefore, changes are made at the start of the injection. The end of the injection is changed in the normal manner, the rotation of the piston.

The reason for using VIT is to achieve more economical fuel consumption. No change in the injection time at low load (up to 40% MCR). As the load increases, the fuel timing is advanced to about 85% of the MCR, with the engine Pmax being reached. The synchronization is then delayed to maintain a combustion at constant pressure between 85% and 100% of the MCR.

The VIT also allows small adjustments in time for the fuel pump for different fuel ignition qualities. The wear of the fuel pump and changes the timing of the camshaft due to the extension of the chain can be compensated to some extent by VIT.

The vit can be achieved by:

1. Mechanical - Pneumatic: Low-pressure air is fed to a pressure control valve, the output of which is supplied with the VIT Pump Servos. At the output of the link controller, a pivot rod moves, the position of which determines the output of the control valve.
The position of the control valve which can be used to allow various degrees of ignition of the fuel is adjustable and the time of the camshaft changes due to the extension of the chain. A link from governor output moves a pivoted bar and the position of the contol valve is adjusted.
2. Electro pneumatic: The air signal at the fuel pump Actuators VIT Operating VIT racks are realized in the electronic control as an electrical signal of 4 to 20 mA. The signal is sent to an IP converter to generate the pneumatic control signal between 0.5 bar (minimum setting VIT) and 5 bar (maximum value VIT).
The difference between the two above is the use of the breakpoint. Mechanical break point is fixed, with the electrical switching point is variable depending on the pressure recovery. While operation adjustments are easier since the correction values ​​are entered directly into the controller. The change in the fuel quality or wear of the pump may require adjustment of the VIT.

Super Vit:

In VIT, adjustable times are obtained by a special profile in the fuel pump pistons. Therefore, there is a fixed relationship between the timing of the fuel injection and the fuel index. Therefore, you must adjust the injection time without changing the fuel index. But this is possible with the new Super Vit. For this reason, the super-VIT, where it is possible to introduce the fuel flow and the injection time, is set independently of each other.

Super-VIT is available in both mechanical and electronic versions. In the electronic version an I / P air control of the converter supply pressure relative to the servo single cylinders instead of the pilot valve actuated by the fuel frame in the mechanical version. The I / P converter receives the pilot signal control.

The advantage of the electronic version is that the break point is calculated from the real conditions, the engine load is calculated by the engine speed and the speed of the fuel, while the compression pressure is calculated by pressurized purge air. Based on these calculations, the controller calculates output to I / P.

Has the Sulzer engine uses Super VIT? If so, what is the difference between VIT and VIT Super in Sulzer?
At Sulzer, VIT was able to set the start and end of the fuel injection independently, that is the same characteristics as the MAN B & W Super Vit. Therefore it is from a "Super-VIT" in Sulzer engine conversation. The only change they made was to replace the mechanical control Sulzer VIT to electronic control for the following engines.

#Cover Image credit: http://www.marinesite.info 

Author: Amit                                                                                      Article Requested by: suzi karan

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