727 The incremental pressure stages
Inside the pump the fuel is contained in three compartments, each at a different pressure. These are: lift pump, supply pump (or high pressure) and injection pressure. Different vehicles may have different types of lift pump, but the supply pump, Fig. 7.56, is common to all installations of the VP44 and is of conventional design. Its vanes are forced radially outwards by centrifugal force, supplemented by coil type compression springs. The function of these springs is to ensure that, even when the engine is stopped, the radial vanes remain in contact with the eccentric ring around them.
Fuel, at lift pump pressure, passes from the low pressure chamber into the kidney-shaped slot in one end wall of the supply pump housing. The rotor picks it up and delivers it, at a pressure that is engine-speed dependent, through a diametrically opposite slot of similar shape in the other wall, and on through ducts into the high pressure chamber. To limit the maximum pressure, a spring-loaded relief valve, Fig. 7.57, screwed into the top of the casing, adjacent to the supply pump, returns the fuel in excess of requirements to the pump inlet.
From the high pressure chamber, the fuel passes through ducts in the distributor head into the centre of the hub of the radial plunger pump. When the plungers move inwards, they deliver the fuel, at injection pressure, through ducts in the distributor shaft and, in turn, each of the radial ducts in the distributor sleeve to a snubber valve in the outlet connection to the appropriate cylinder. The pressure at the nozzle is over 1500 bar.
Fig. 7.56 A radial vane type supply, or fuel lift, pump is common to all installations of the Bosch VP44 injection pump
Vanes
Vanes
Fig. 7.56 A radial vane type supply, or fuel lift, pump is common to all installations of the Bosch VP44 injection pump
Fig. 7.57 The location of this pressure regulating valve, on the top of the pump casing, can be seen in Fig. 7.55
Average user rating: 5 stars out of 1 votes
Post a comment