Pulse Width Modulated Valves for Liquid Control

Liquid flow rate has been controlled in the past by varying flow area and differential pressure mechanically. The pulse-width-modulated valves used for metering liquid fuel to the engine in the automotive industry are operating at engine cycle frequency and a low supply pressure. The fuel delivered each cycle is proportional to the pulse width with constant flow area and differential pressure. Higher operating frequency is desired when metering fuel to a combustor of a turbine engine, to dampen the pressure ripples created by the valve pulsation. However, the fuel rate turn down ratio will become inevitably small and unacceptable. Electronically controlled, variable area, cavitating venturi fuel control valves which have been used in the turbine engine industry, are expensive and relatively large. The variable area is either produced by a dc motor or a movable servo-actuated pintle. Liquid flow rate can be controlled by varying the flow area through pulse width modulation. The flow rate will be directly proportional to the flow area which varies with pulse width, independent of pressure variation. By exploiting the knowledge gained during the development of the Servojet Pulse Width Modulated gaseous fuel injector and the Proportional Pressure Relief Valve, the concept of controlling liquid flow rate by varying flow area through pulse width modulation without feedback position transducer, becomes feasible.