Hydromechanical control for a variable delivery, positive displacement fuel pump

A control system for providing a controlled amount of fuel to an engine, comprising:

• a variable delivery pump having an inlet and an outlet, the pump being operable to provide fuel at various predetermined quantities at the pump outlet;
• a metering valve having a metering window with first and second sides, the first side connected with the pump outlet, the metering window having fuel from the pump outlet flow through the metering window thereby creating a pressure drop of the fuel across the metering window from the first side to the second side;
• a pump control valve having a spool, the fuel from the first side of the metering window being connected to the spool to move the spool in a first direction, the fuel from the second side of the metering window being connected to the spool to move the spool in a second direction, the pump control valve having a first input orifice through which fuel passes to a first output orifice, an amount of opening of the first input orifice being controlled by a position of the spool;
• a bypass line having a first end connected with the first side of the metering window and having a second end connected with the pump inlet, in between the first and second ends of the bypass line is connected the first input and output orifices of the pump control valve, during a steady-state condition the pump control valve is operable to allow a predetermined amount of fuel to flow from the first end of the bypass line through the first input orifice and through the pump control valve and out the first output orifice to the pump inlet such that a constant pressure drop value across the metering window is maintained, during a transient condition in which the pressure drop across the metering window is no longer at the constant value the pump control valve is operable to allow an amount of fuel to flow from the first end of the bypass line through the first input orifice and through the pump control valve and out the first output orifice to the pump inlet to thereby vary the amount of fuel provided at the pump outlet such that the pressure drop across the metering window tends towards a constant value;
• and
• wherein the pump control valve further comprises a second input orifice and a third input orifice and second and third output orifices, the second input orifice being connected with a hydraulic fluid of a first predetermined pressure, the third input orifice being connected with a hydraulic fluid of a second predetermined pressure, the control system further comprising a pump control actuator operable to control the displacement of the pump to thereby control the amount of fuel delivered by the pump at the pump outlet, the pump control actuator having a moveable element whose position is controlled by application to the moveable element of the hydraulic fluids from the second and third output orifices of the pump control valve, during a steady-state condition the pump control valve is operable to keep constant the position of the moveable element of the pump control actuator thereby keeping constant the amount of fuel provided by the pump at the pump outlet to thereby keep constant the pressure drop across the metering window, during a transient condition the pump control valve is operable to vary the position of the moveable element of the pump control actuator thereby varying the amount of fuel provided by the pump at the pump outlet.