VIT or Variable Injection Timing is a form of fuel pump control, enabling an engine to operate with the designated maximum cylinder firing or combustion pressure from approximately 75% power output to maximum power. This improves thermal efficiency and lowers fuel consumption.
The fuel consumption for an engine at any load will be related to the expansion ratio of the combustion gases from their maximum pressure to the pressure at the commencement of exhaust blowdown.
The maximum cylinder pressure is a factor used in the design of the crankshaft and other important engine parts. In a normal engine the maximum cylinder pressure is reached only at full power operation, whereas with VIT the maximum cylinder pressure is reached at about 75% of the full load. The expansion ratio is therefore increased when the engine is operating under light loads right up to full load.
When the engine is equipped with a VIT+FQS system (Variable Injection Timing + Fuel Quality Setting) which permits an alteration of injection begin during operation, the system produces a maximum firing pressure over a wide power range close to the value for the Contracted
Maximum Continuous Rating (CMCR) and thereby reduces the fuel consumption. It is also possible to tune the VIT+FQS system following NOx emission. In this case the VIT angle is reduced and thereby the injection is retarded over a wide load range in order to reduce NOx emission.
Generally is used the indication VIT+FQS because VIT and FQS always act together, unless the data is specified only for VIT or only for FQS. The VIT+FQS control is integrated in the engine remote control and under normal operating conditions the VIT function is always on, but it can be switched off whereupon the actuator moves to the position corresponding to the FQS setting.
In its normal operating mode the VIT+FQS system produces earlier injection in the partial power range and therefore raises the maximum firing pressure. The standard VIT program calculates the necessary VIT angle for increased part load using scavenge air pressure and engine speed as input signals and therefore, a reduction of the fuel consumption is effected over the entire load range. The standard VIT program is individually adjusted during the shop trial of the engine. A typical curve can be seen on below diagram.
With the aid of a pneumatic cylinder and a mechanical linkage the suction valve and spill valve regulating are simultaneously altered. For example, the moving out of the pneumatic cylinder gives higher VIT angles, i.e. an earlier injection and therefore a higher maximum firing pressure.
With the manual adjustment of FQS, the influence of fuel quality is compensated. Fuels having a low ignition quality result in lower firing pressures for the same start of injection, i.e. for compensation, the FQS angle has to be increased. Fuels with better ignition quality result in higher firing pressure for the same start of injection, i.e. for compensation, the FQS angle has to be reduced.
For this purpose the FQS angle is defined as ’user parameter’ in the engine control and can be adjusted within the range of –3 deg. to +3 deg. The manual FQS adjustment supposes a firing pressure measurement before and after and it must further be established whether the firing pressure alteration is actually due to a fuel quality change. Alteration of firing pressure due to fouling or other causes may not be compensated with the FQS adjustment.
Into below videos you can learn on how the fuel pump’s injection timing can be adjusted on different type of engines
In the electronically controlled engines, the control of fuel injection is based upon volumetric injection control. Each cylinder electronic unit calculates the necessary injection timing for its own cylinder by processing the crank angle signal and the fuel command delivered by the master control module (MCM).
In these cases there is no need for adjustment as the VIT angle calculation depends on speed (RPM), charge air pressure and the fuel rail pressure. The (new) fuel rail pressure compensates for the differences in injection timing resulting from different fuel injection pressures in the fuel rail. Higher fuel pressure causes advanced injection and higher maximum pressure (Pmax). Thus the injection start angle is retarded by a small amount with increasing fuel rail pressure.
In conclusion, in order to run the engine efficiently and to avoid engine breakdowns and damage, proper maintenance and periodical checks must be carried out as per your company and manufacturer’s maintenance plan and instructions. Moreover, every engineer must be familiar with proper monitoring and how to take engine performance cards and how to properly adjust the fuel pump’s timing.
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