Bosch VE-type diesel injection pump. Like all diesel engines, the purpose of the injectionpump is to deliver a precisely-metered charge of diesel fuel to each cylinder injector, in thefiring order of the engine. The Bosch VE injection pump uses a single pump plunger toproduce these high-pressure fuel charges. The pump mechanism includes a distributorsection to direct each successive charge to the appropriate cylinder injector, in the requiredorder.The actual fuel charge delivered to the injectors is proportional to the pump stroke. Theeffective stroke is continuously adjusted according to the throttle position and the enginespeed (rpm). This function is performed by the mechanical fly-weight governor mechanismwithin the pump. In a naturally-aspirated diesel (or assuming the boost pressure remainsconstant in a turbo-charged engine), the governor will adjust the pump stroke to try tomaintain a particular engine rpm at any given throttle setting. That is, unlike a carburettedpetrol engine where the throttle directly alters the quantity of air/fuel mixture drawn into thecylinders, in a diesel the throttle merely adjusts the 'rpm setpoint' of the engine.At a fixed throttle position, as the load changes (e.g. the road rises or falls slightly) the flyweightgovernor will increase or decrease the fuel charge to attempt to keep the engine rpmconstant, within limits of course. A bit like a very basic 'cruise control'. In practice, theengine rpm and hence the vehicle speed will vary quite a bit as the load changes, despite thebest efforts of the governor. [If you want to get technical, this is because a fly-weight governor is a‘proportional action’ type of controller and will settle at a different ‘steady state’ point for each differentload on the engine.]Boost CompensationNow, the description above stated "assuming theboost pressure remains constant in a turbochargedengine..." Of course, it rarely remainsconstant for very long. For this reason, theBosch VE injection pump used on 200Tdi andnon-EDC1 300Tdi LR engines has a boostcompensator (also called a manifold-pressurecompensator or 'aneroid') to enable it to furthercontrol fuel delivery in proportion to the boost1 Electronic Boost Compensation
Now, the description above stated "assuming theboost pressure remains constant in a turbochargedengine..." Of course, it rarely remainsconstant for very long. For this reason, theBosch VE injection pump used on 200Tdi andnon-EDC1 300Tdi LR engines has a boostcompensator (also called a manifold-pressurecompensator or 'aneroid') to enable it to furthercontrol fuel delivery in proportion to the boost varies greatly as the boost pressure increases from zero to full boost. At low boostpressures, the cylinder air mass is much smaller and is not sufficient to fully combust themaximum fuel charge. Therefore, the boost compensator’s job is to reduce the maximumfuel charge when the boost pressure is less than maximum. A tube from the turbo air outletto the diaphragm chamber on the top of the boost compensator transmits the pressuresignal.In order to consistently achieve low smoke emissions on every vehicle leaving the assemblyline, it seems the 'standard' settings of the boost compensator are very conservative. Thatis, they severely restrict the fuel delivery at less than full boost in order to ensure low smokeemissions. This seems to result in the legendary off-idle sluggishness of the Tdi engines.By carefully adjusting and optimising the boost compensator settings for a particular engine,significant improvements can be had in off-boost and low to mid-range rpm performance,without excessive smoke emissions. [By the way, if you are making black smoke you are justwasting fuel, not making more power. The object is to have the engine just on the brink of makingsmoke when under full throttle, at any combination of rpm and boost pressure.]
Boost compensation works by automaticallyadjusting the position of a pump StrokeLimiting Pin within the pump. This pin lieshorizontally through the pump body and it's endis visible at the bottom of the boostcompensator well. When in the rearwardposition (away from the front or drive end of thepump) the pump delivers it's 'high boost' fuelcharges to the injectors, as defined by theinternal design of the pump. When this pin ispushed forward, the fuel charge, for any givenrpm and throttle setting, is reduced tocompensate for the reduced air charge to thecylinders when the boost pressure is lower.The stroke limiting pin is positioned by the upand down movement of an eccentric ControlCone attached to a diaphragm which sensesboost pressure. When there is effectively noboost, such as when idling, thediaphragm/control cone is held in the fully upposition by its spring and sits against the stopscrew in the top cover. In this position, thelimiting pin is bearing against the thickest(bottom) part of the control cone.
Therefore it ispushed forward to it's most restrictive position.As boost pressure increases, the pressureabove the diaphragm begins to overcome thespring force and the control cone is pusheddownwards. The stroke limiting pin now bearsagainst a narrower part of the control cone and is allowed to move backward slightly,allowing increased fuel delivery. Eventually, when maximum boost is achieved, thediaphragm/control cone no longer moves any further downward, the limiting pin ceases tomove further rearward and fuel delivery is solely determined by the governor, with noadditional restriction from the boost compensator.
Boost Compensator AdjustmentsThere are three adjustments available in the VE pump boost compensator:1. The rotational position of the eccentric control cone at the bottom of the diaphragm/coneassembly, adjusted by rotating the assembly. (See Fig. 3)2. The diaphragm spring pre-load, adjusted by rotating the 'starwheel' on the lower springseat 3. The diaphragm/cone rest position, adjusted using the Torx stop screw and locknut in thediaphragm cover .The following sections shall examine each of these adjustments in detail
and explain theireffect on fuel charge delivery,