Variable Injection Timing
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Variable beginning and end-Variable Injection Timing (VIT) control This allows for ideal matching of load to injection timing for various qualities of fuel. The Barrel insert can be moved up and down by action of the Nut. This has the effect of altering the position of the spill port relative to the plunger stroke. Therefore the beginning of injection is altered. The end of injection is varied by its normal way of rotating the plunger. The Nut, which moves linearly, is controlled by the VIT rack, this is altered- continuously by the engine management. Pump adjustment-Individual pumps may be adjusted in order to account for wear in the pump itself or the entire range of pumps can be adjusted to suit particular loads or fuel ignition quality. Individual pumps are adjusted by means of the screwed links from the auxiliary rack to the nut, just as the main rack adjustment is carried out. Adjustment of all pumps is simply by movement of the auxiliary fuel rack. Problems associated with jerk pumps-the main problem with pumps of this type is that sharp edges on the plunger and around the spill port become rounded. As injection commences when the spill port is covered by the plunger, this means that later injection takes place. With the variable injection pump this can be accounted for by lowering the barrel insert and hence the spill port, so that it is covered at the required point. In older type pumps, adjustment required washes and shims to be placed between the plunger foot and cam follower or shims removed from below the pump body in order to lower it and the spill port relative to the plunger. Wear also causes leakage between the plunger and barrel but the only solution is replacement. Original timings must be restored. The period of fuel injection
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Variable Injection Timing
Transcript of Variable Injection Timing
Variable beginning and end-Variable Injection Timing (VIT) control
This allows for ideal matching of load to injection timing for various qualities of fuel. The Barrel insert can be moved up and down by action of the Nut. This has the effect of altering the position of the spill port relative to the plunger stroke.
Therefore the beginning of injection is altered. The end of injection is varied by its normal way of rotating the plunger.
The Nut, which moves linearly, is controlled by the VIT rack, this is altered- continuously by the engine management.
Pump adjustment-Individual pumps may be adjusted in order to account for wear in the pump itself or the entire range of pumps can be adjusted to suit particular loads or fuel ignition quality. Individual pumps are adjusted by means of the screwed links from the auxiliary rack to the nut, just as the main rack adjustment is carried out. Adjustment of all pumps is simply by movement of the auxiliary fuel rack.
Problems associated with jerk pumps-the main problem with pumps of this type is that sharp edges on the plunger and around the spill port become rounded. As injection commences when the spill port is covered by the plunger, this means that later injection takes place. With the variable injection pump this can be accounted for by lowering the barrel insert and hence the spill port, so that it is covered at the required point. In older type pumps, adjustment required washes and shims to be placed between the plunger foot and cam follower or shims removed from below the pump body in order to lower it and the spill port relative to the plunger. Wear also causes leakage between the plunger and barrel but the only solution is replacement. Original timings must be restored.
The period of fuel injection
Typical fuel pressure curve at outlet from pump
A-Pump spill closes (approx. 8o)B-Fuel injector opens (approx. -4o)C-Spill opens (approx. 12o)D-Fuel injector closes (approx. 16o)E-Reflected pressure waveF-Period of partial equilibrium i.e. the rate of delivery from the rising plunger in the barrel equals the flow out of the injector, therefore no pressure rise. Instability of the wave form can indicate too low viscosity fuel supplied.G- Injection period (approx. 20o)
It can be seen that the maximum pressure generated by the pump is far higher than the opening pressure by the injector ( 650 against 350 Kg/cm2). Engine monitoring equipment can be used to generate this graph allowing diagnosis of the fuel supply equipment. For example, the rate of rise of pressure before the fuel injector first opens indicates wear in the fuel pump. Period of equilibrium
This is the period between the beginning and end of stroke and can be divided into three periods.1. Delivery with no injection- being subject to high pressure the fuel reduces in volume, about 1 %. This causes a loss of effective plunger stroke and hence delays the start of injection. The main factor in this is the length of fuel pipe. The effect must be considered when advancing the fuel cam in relation to engine speed.2. Main injection period-This is directly related to the effective stroke of the fuel pump plunger and consequent engine load. The engine speed can alter the resilient pressure fluctuations in the fuel pipe and so alter the fuel delivery curve and cause irregular discharge from the injector.3. Secondary injection period-This is referred to as 'dribbling' and is due entirely to the resilient pressure fluctuations in the fuel piping and related to engine speed. The fuel oil passing to the injector has kinetic energy. At end of injection a low pressure wave passes through the fuel closing the needle valve in the injector. The kinetic energy in the fuel is converted to pressure energy and a pressure wave is formed. This can be seen below as the 'reflected pressure wave'. Avoided by fitting short, large diameter rigid fuel lines and having a sharp cut off at the fuel pump or an anti dribble device.
