1998-02-23

New Concept of a Low Cost, Fast Acting Electronically Controlled Fuel Metering System With Back-Up Capability 980814

Typical fuel metering systems for small gas turbine engines consist of a metering valve and a bypass valve which is maintaining a constant differential pressure across the metering valve orifice. The metering valve is operated by the compressor pressure signal and the bypass valve acts automatically.It is proposed to bring both the metering and the bypass valves under the control of two digital linear actuators. The first one would move the metering plunger according to the compressor pressure and the second would move the bypass plunger in such a way, that the differential pressure across the metering orifice would be maintained constant only during steady state engine operation. During the engine transient processes, however, it would be increased or decreased as required, to speed up the change in the nozzle flow rate and consequently the engine dynamic response.Independent actuation of the metering and bypass valves gives the opportunity to meter the fuel flow to the nozzles only by one of the valves, in case if the second valve would fail. When the metering valve fails, the nozzle flow is controlled by the change of the differential pressure across a fixed metering orifice. When the bypass valve fails, the nozzle flow is controlled by the variation of the metering orifice flow area at constant fuel supply pressure. The change from the first mode of operation to the second one is provided by the rotation of both plungers which open or close the particular orifices.The new fuel metering system design is explained at all three modes of operation. It is computer simulated and its dynamic response is investigated on an engine model to determine the most effective control strategy.

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