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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| Image Credit: http://www.wikimarine.net |
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
#Cover Image credit: http://www.marinesite.info
Author: Amit Article Requested by: suzi karan
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