Hydraulic mechanism

Claim 1

A liquid fuel pumping apparatus for supplying fuel to an internal combustion engine comprising a piston movable in a bore, said piston having a body and a flange surrounding said body, said flange extending outward from said body and defining a surface, a fluid accumulator chamber at one end of the bore and into which the piston can move, an end wall to said accumulator chamber, biasing means engaging said flange and biasing the piston toward contact with said end wall with predetermined biasing force, a control port in said end wall, a seating surface defined on said piston for engagement with said end wall surrounding said control port, said control port being arranged so that fluid in said control port acts on said plunger to cause an accumulator chamber opening force on said piston which is in a direction to force said piston away from contact with said end wall, a reciprocable pumping plunger slidable in a cylinder, an outlet from said cylinder, the outlet in use communicating with a fuel injection nozzle of an internal combustion engine, passage means connecting said cylinder with the accumulator chamber to fluidically connect said chamber and said cylinder so that fluid pressure in said cylinder is communicated to said chamber to establish a first chamber pressure, a low pressure pump for supplying liquid fuel to said cylinder, with said first chamber pressure acting on said flange surface in co-operation with said biasing means so that said predetermined biasing force and said first chamber pressure cooperate to form a combined accumulator chamber closing force on said piston which exceeds said accumulator chamber opening force to maintain said piston seating surface in contact with said end wall, said first chamber pressure being such that fluid from said cylinder flows through said outlet, valve means for controlling the flow of fuel into the cylinder during outward movement of the plunger, and a control valve means operable to connect said control port to the chamber, the arrangement being such that during inward movement of the pumping plunger and with the piston seating surface in contact with the end wall, the pressure in said chamber will be said first chamber pressure and fuel will be displaced through said outlet until said control valve means is operated, whereupon the pressure of fuel in the chamber will act upon the area of the end of the piston enclosed by the seating surface to increase said accumulator chamber opening force to a second value, with the area enclosed by said seating surface being sized so that said accumulator chamber opening force second value exceeds the combined accumulator chamber closing force to move the piston away from said end wall to lower the pressure in said accumulator chamber to a second pressure which is below that of the pressure in the outlet so that fuel from the cylinder will flow into the accumulator chamber rather than to the outlet, said piston moving outwardly of the chamber, the outward movement of the piston causing the accumulator chamber to absorb the further fuel displaced by the plunger after operation of the control valve means to accumulate such further fuel in said accumulator chamber to terminate the flow of fuel through the outlet.