VARIABLE INJECTION TIMING

VARIABLE INJECTION TIMING IS A METHOD IN WHICH THE TIMING OF FUEL INJECTION IS ALTERED FOR BETTER THERMAL AND POWER EFFICIENCY AND LOWER THE SPECIFIC FUEL CONSUMPTION.

NOW THE QUESTION ARISES IS WHY DO WE NEED VIT. 

THE ANSWER IS SIMPLE, TO GIVE A LEAD TO THE FUEL W.R.T BURNING OF FUEL SO THAT FUEL MAY MIX PROPERLY AND REACHES ITS IGNITION TEMPERATURE WITH BETTER ATOMIZATION AND MIXING. IF A FUEL HAVE A HIGHER IGNITION LAG THEN VIT INJECTS THE FUEL BEFORE SO AS TO GIVE IT MORE TIME TO MIX AND HENCE GIVE GOOD COMBUSTION AND HENCE MORE POWER.

MANY A TIMES A QUESTION IS ASKED :

WHAT IS THE DIFFERENCE BETWEEN A VIT AND A SUPER VIT?

*A CONVENTIONAL VIT VS SUPER VIT :-

1. ONLY A FUEL RACK IS PRESENT NO VIT RACK.
2. BREAKPOINT IS FIXED W.R.T PUMP INDEX.
3.  NO FUEL QUALITY SETTING PRESENT IN CONVENTIONAL VIT.

CONVENTIONAL VIT:-

as the pump plinger moves to pumping on load the top helix of plunger controls the effective stroke of the pump and this is lenthened at both ends of injection “a to b” and “f to e”.

from “b to c” the pump runs from 60-80% load. if the load is further increased by turning the plunger beyond c the peak pressure rises. the portion of the top edge from “c to d” is again profiled so that VIT index is lowered i.e pump lead is reduced and maximum pressure in cylinder doesnot exceed. the plunger is pre-profiled with helical cut at both ends for control of fuel injection.

the profiled plunger principle of both end control of fuel injection (VIT control) is not fixed w.r.t pump index.

SUPER VIT:-

SUPER VIT HAS A MOVABLE BARREL CONNECTED TO VIT RACK VIA A RACK AND PINION ARRANGEMENT. THE VIT RACK CAN BE ADJUSTED INDEPENDENTLY FOR DIFFERENT FUEL PUMP.

VIT RACK IS ACTUATED BY THE HELP OF A SERVO CYLINDER AS SHOWN IN PIC ABOVE. THE SERVO CYLINDER HAS CONTROL AIR CONNECTION WHICH RECIEVE 0.5 TO 5.5 BAR PRESSURE FROM POSITION SENSOR WHICH IN TURN IS ACTUATED BY A CONTROL LEVER . THIS VIT RACK GETS ACTUATED AFTER 40% LOAD AND AS ITS ACTUATED IT PRESSES THE POSITION SENSOR TO SEND MORE AIR PRESSURE TO SERVO CYLINDER LEADING TO MOVEMENT OF VIT RACK INSIDE AND IN TURN MOVING BARREL DOWN. HENCE FUEL GETS INJECTED BEFORE. THIS HAPPENS TILL 85%MCR AFTER WHICH CONTROL LEVER MOVES AWAY AND CONTROL AIR PRESSURE DECREASES AND VIT RACK MOVES AWAY LEADING TO MOVEMENT OF BARREL OF FUEL PUMP UP ,HENCE LEADING TO DECREASE IN POWER.

VIT OPERATES BETWEEN 40-85% MCR. AT LOWER LOADS VIT IS NOT MADE FUNCTIONAL COZ AT THAT SPEED THE LUBRICATION OF BEARINGS IS NOT ENOUGH AND IF MORE LOAD IS PUT IN BEARINGS THAT CAN LEAD TO FAILURE OF BEARING.