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.
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.
Variable Injection Timing (VIT) control ،