VIT AND SUPER VIT A DISCUSSION

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Modern large slow speed MAN B&W 2-stroke engines have Super VIT mechanism fitted to advance fuel injection at lower loads for increasing maximum firing pressure resulting better fuel economy. Details of Super VIT are available in the engine manuals.

Why use the term "Super"? was there an "ordinary" VIT earlier, which has been changed to Super VIT?
If yes, can I have a brief description of how the ordinary (non-Super) VIT worked?

Before the Super VIT was introduced variable injection timing was obtained by means of a special profile on the fuel pump plunger. Hence there was a fixed relationship between the injection timing and the fuel index. Thus it was not possible to adjust the fuel index of the individual pumps without also altering the injection timing. For this reason the Super VIT was introduced, where it is possible to adjust the fuel index and the injection timing independently.

Is it possible to achieve Super VIT action by means of mechanical linkage or can it be only done if we have electronic (microprocessor) controlled fuel injection system. Or is it right to say that most efficient Super VIT can only be achieved by electronic fuel injection?

The Super VIT is available in both a mechanical and an electronic version. In the electronic version an I/P converter supplies the pilot air pressure to the individual servo cylinders, instead of the pilot valve activated by the fuel rack used in the mechanical version. The I/P converter receives its pilot signal from the governor system.

The advantage of the electronic version is that the break-point is calculated from the actual conditions, why the ambient conditions are taken into account. The engine load is calculated from the engine speed and the fuel index, while the compression pressure is calculated from the scavenging air pressure. Based on these calculations the governor calculates the output to the I/P converter

Am I right in assuming break point is where max cylinder pressure has been reached before MCR which is about 85% MCR when using VIT, after which injection timing is retarded back to its original setting at 100% MCR

Also, does the MAN B&W engine have a fuel quality setting lever on the VIT control similar to that used on the Sulzer RTA, or must the fuel pumps be adjusted individually if a fuel of differing ignition quality is bunkered.

The breakpoint is the point where the maximum cylinder pressure has been reached and the injection timing is advanced the most. Above the breakpoint the injection timing is gradually retarded back until it reaches its original setting at 100% MCR load. The position of the breakpoint is determined by the layout of the engine. Formerly it was generally considered to be at approximately 85% MCR load, but it also has to be ensured that the maximum pressure rise from compression to maximum cylinder pressure is 35 bar or less (recommended by MAN B&W Diesel A/S). For this reason the breakpoint has tended to be somewhat higher on the latest engines (approximately 90% MCR load).

In order to compensate for fuel related differences in the maximum cylinder pressure it is possible to adjust the VIT according to the experience with the different fuels.
In case of the mechanical VIT an offset is introduced by moving the pilot valve bracket horizontally towards or away from the lever, by means of the adjusting screws.
In case of the electronic VIT it is possible to adjust an offset value on the governor panel.

What the term fuel index and how is it adjusted?

The fuel index is an indication of the active stroke of the fuel pump. This is controlled by push rods on the individual fuel pumps connected to the fuel rack, which in turn is controlled by the engine governor.

When the engine is given a speed command the governor increases the fuel index, subject to certain limitations, untill the requested engine speed is reached

if i am not wrong, the vit pilot line pressure increases to approx 3 kg at break point and then reduces till the mcr keeping the pmax const.
i would like to draw your attention to the inlet pressure of this vit pilot valve which is 7 kg (control line pr).
i had experienced sticking of this pilot valve and full 7 kg outlet pressure was fed to the servo positioners. luckily it was noticed immediately and corrective actions were taken.
suppose this had happened during ums period in an engine room, would any enine alarm sound?
what would the peak pressure be during that limited time with pilot line pressure at 7 kg and an engine operating at 90 % mcr?

If the servo signal increases way beyond the normal operating range, e.g. as you state the worst case is equal to the control system pressure of 7 bar, this will naturally result in an increased pressure rise from the compression pressure to the maximum pressure.
No alarm is provided for this malfunction, as it does not directly influence the operation of the engine. The increased pressure rise will actually improve the performance of the engine, but will over time overstress the piston rings. If such a condition is allowed to exist for a long time, cylinder condition problems are thus likely to occur. However, an abnormal servo signal ought to be discovered by the engine crew during their routine inspection rounds - even with the engine room under UMS conditions.

But if we observe the VIT index at break point, this would be around 6 notch and the peak pressure developed at this point would be the design 140 bar in the cylinder. If 7 bar is admitted to the servo the VIT index becomes around 14 notch. and as each notch gives an increase in peak pressure of 1 bar, the peak pressure may exceed by 8 bars. this difference may increase more if the engine is operated above the break point. Wouldn’t this effect the load on the bearings and other running gear if the engine is operated in this condition for more than 8 hrs.
My doubt is , why isn’t a reducer fitted before the vit pilot valve, when the max pressure needed is 3.5 bars, which is given during engine starting and astern movements.
the data above are speculated and may not be fully correct, please correct if wrong.

The actual VIT index at the break point depends on a variety of factors, e.g. fuel properties and ambient conditions. The latter is taken into account in the calculations of the electronic VIT version, as the scavenging pressure is used in the calculations. Consequently, it is not possible to state default values for the VIT index break point setting or rate of change.

In any case, the mechanical limits of the VIT system also have to be considered. The travel of the servo actuators is designed to be going from a minimum at a servo signal of 0.5 bar to a maximum at a servo signal of 5.0 bar, why the mechanical limit is reached earlier than a servo signal of 7 bar would otherwise indicate. In case of the mechanical VIT system, the break point is determined by the point where the lever rests on both pivot points. In this case it is not possible to depress the pressure adjusting valve further, thus preventing a too high servo signal as the control air inlet will always press the piston of the pressure adjusting valve against the lever.

Even if the maximum pressure is over the specified maximum, the engine is not overloaded due to this. The engine load is determined by the power required to turn the propeller at the requested revolutions and a malfunction of the VIT system does not change this and e.g. the auxiliary systems are not affected by this either. Consequently, the only real impact on the engine is the increased pressure differential over the piston rings, due to the too high pressure rise from the compression pressure to the maximum pressure, and a very limited increase to the bearing load.

As stated earlier, the over stressing of the piston rings may possibly require maintenance, if the malfunction is allowed to persist, but does not affect the possibility to safely manoeuvre the engine. Hence, it has not been deemed necessary to introduce an alarm or other measures to prevent or limit the duration of such a malfunction.