Auxiliary pump means of an advance arrangement of a high pressure piston pump

#This invention relates to an advance arrangement for use in adjusting the timing of fuel delivery of a high pressure fuel pump.

#In a known high pressure fuel pump, a rotor is provided with one or more bores, pumping plungers being reciprocable within the bores. The outer end of each pumping plunger has a shoe and roller arrangement associated therewith, the rollers being engageable with the cam surface of a cam ring. As the rollers ride over cam lobes forming part of the cam surface during rotary motion of the rotor with respect to the cam ring, the plungers are pushed inwardly into the respective bores, pressurizing and displacing fuel from the bores to permit fuel to be supplied under pressure to the cylinders of an associated engine.

#It will be appreciated that the timing of fuel delivery by the pump is dependent upon the position of the cam lobes, and hence upon the angular position of the cam ring. The timing of fuel delivery can therefore be advanced or retarded by moving the cam ring. One way of moving the cam ring is to provide the cam ring with an outwardly extending peg which is received within a recess or opening provided in an advance piston. The piston is slidable within a bore, and is spring biased towards one end of the bore. Fuel under pressure is supplied to the bore, the fuel pressure acting on a surface of the piston to apply a force to the piston acting against the spring. In such an arrangement, variations in the fuel pressure applied to the bore result in the advance piston occupying different axial positions, the engagement between the piston and the peg transmitting axial movement of the piston to the cam ring, adjusting the angular position of the cam ring.

#The invention will be described, by way of example, with reference to the accompanying drawing (FIG. 1) which is a diagrammatic view of an advance arrangement in accordance with an embodiment of the invention.

#The advance arrangement illustrated in the accompanying drawing is intended for use with a high pressure fuel pump of the type comprising a distributor member rotatable within a sleeve, the distributor member including a plurality of radially extending bores within which pumping plungers are reciprocable. The outer end of each plunger engages the shoe of a shoe and roller arrangement, the roller of which is cooperable with the cam surface of an angularly adjustable cam ring 10. The cam ring 10 includes a plurality of inwardly extending cam lobes 10a, and as the rollers ride over the cam lobes 10a, it will be appreciated that the plungers are pushed inward into the bores, pressurizing fuel within the bores. Appropriate inlet and outlet ports are provided whereby fuel can be supplied at relatively low pressure to the bores from a feed pump, and whereby during inward movement of the pumping plungers, fuel can escape from the bores at high fuel pressure to be supplied to the cylinders of an associated engine, in turn. The timing of fuel delivery by the high pressure fuel pump can be adjusted by adjusting the angular position of the cam ring 10. Such angular movement of the cam ring 10 is achieved by means of an outwardly extending peg 12 provided on the cam ring 10 which is received within a recess 14 provided in an advance piston 16. The advance piston 16 is slidable within a bore 18 provided in a housing, the piston 16 being biased towards an end of the bore 18 by a spring 20. The piston 16 is a sufficiently good fit within the bore 18 that substantially no fluid is able to flow from one end of the piston 16 to the other end thereof. In order to permit movement of the piston 16 within the bore 18, it will be appreciated that the chambers 27, 28 defined between each end of the piston 16 and the bore 18 need to be vented, and appropriate vent passages 22, 24 are provided.

#In order to adjust the axial position of the piston 16 within the bore 18, a fuel pump 26 is provided, the fuel pump being arranged to supply fuel to the chamber 28 defined between the end of the piston 16 remote from the spring 20, and the bore 18.

#The fuel pump 26 comprises a piston member 30 slidable within a bore. The piston member 30 is coupled to an armature 32 which is reciprocable under the influence of the electromagnetic field generated by a coil 34. A return spring 36 engages the piston member 30, biasing the piston member 30 towards a position in which the armature 32 is spaced from the coil 34. The piston member 30 and bore together define a pumping chamber 38 which communicates through an inlet, spring biased non-return valve 40 with a supply of fuel at low pressure, and through an outlet, spring biased non-return valve 44 with the chamber 28.