Pump control using overpressure source

1. A rotary positive displacement pump for supplying pressurized working fluid to a driving device, the pump comprising:

a control valve comprising a piston moveable in a bore and a biasing spring for biasing the piston to act against a force created by pressurized working fluid from a first outlet port of the pump to reduce output pressure of the pump; and

a second outlet port wherein a pressure of the working fluid exiting the second outlet port is greater than a pressure of the working fluid at the first outlet port of the pump and varies with the speed of the pump, the greater pressure working fluid exiting the second outlet port is supplied to the control valve, creating a force which augments and combines with a force of the biasing spring acting on the control valve to counter the force of the pressurized working fluid from the first outlet port acting on the piston, and wherein the entire working fluid output of the second outlet port is supplied to the control valve.

2. The rotary positive displacement pump according to claim 1 , further comprising:

a pump body having a rotor chamber, a pump inlet communicating with an inlet port and a pump outlet communicating with the first outlet port;

a rotor set being rotatable in the rotor chamber to supply working fluid from the inlet port to the first and second outlet ports; and

wherein the piston is moveable in the bore, the piston having a head and a shaft, the head of the piston being exposed to pressurized working fluid from the first outlet port to urge the piston along the bore toward a first position to expose a discharge port in fluid communication with a low pressure working fluid sink which permits pressurized working fluid from the first outlet port of the pump to move to the low pressure working fluid sink and reduces pressure of the working fluid at the first outlet of the pump;

wherein the biasing spring biases the piston toward a second position wherein the discharge port is blocked by the piston which prevents pressurized working fluid from the first outlet port from moving to the low pressure working fluid sink;

and wherein the bore includes a chamber in which the shaft is located, the chamber in which the shaft is located being in fluid communication with the second outlet port of the pump to receive the pressurized working fluid from the second outlet port a thereby creating the augmenting force on the piston that additionally biases the piston toward the second position.

3. The rotary positive displacement pump of claim 2 wherein the pump is an internal-external gear pump and the rotor set comprises an inner rotor and an outer rotor which rotate within the rotor chamber.

4. The rotary positive displacement pump of claim 1 wherein the pump is a vane pump and the rotor set comprises a rotor head and a set of vanes extending radially from the rotor head.

5. The rotary positive displacement pump of claim 4 wherein the vane pump is a variable displacement vane pump with a control slide moveable to vary the displacement of the pump and wherein a control chamber is formed against the control slide and receives pressurized working fluid from the first outlet port of the pump, the pressurized working fluid from the first outlet port creating a force to move the control slide to a position of reduced displacement, the biasing spring biasing the control slide toward a position of maximum displacement and another chamber formed against the control slide receives pressurized working fluid from the second outlet port and creates a force on the control slide that augments the force of the biasing spring.

6. The rotary, positive displacement pump of claim 1 wherein the control valve further comprises a control sleeve movable to vary the displacement of the pump and further defining the bore around the piston.