Power plants – Combustion products used as motive fluid – Combined with regulation of power output feature
Reexamination Certificate
1998-12-18
2001-01-23
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Combined with regulation of power output feature
C060S734000
Reexamination Certificate
active
06176076
ABSTRACT:
This invention relates to a fuel system for a gas turbine engine, such as a jet engine, primarily for use in aircraft, the system incorporating a pressure raising valve in the metered fuel supply path serving to maintain a minimum level of system pressure, thereby providing sufficient pressure to enable ancillary items of engine control equipment to function correctly.
In a fuel system for an aircraft jet engine, it is common practice to supply pressurised fuel from an engine-driven fixed displacement pump. The system is required to provide a pressure gradient in the fuel flow from the pump to the engine combustor so that flow can be metered to the engine and the engine variable geometry actuators controlled. As a result of the pressure rise generated by the pump and the correct functioning of the pump, metering system and external actuators, there will be a flow of pressurised fuel, known as parasitic flow, back to the pump inlet. The magnitude of this flow is a function of the pressure rise that exists from the inlet to the outlet of the pump. When the difference between the free flow delivered by the pump, which is a function of its size, and the parasitic flow is less than that required to sustain combustion in the engine, the engine cannot be started. It is therefore imperative to minimise the pump pressure rise at engine start-up, especially at low speed re-light conditions, consistent with the correct operation of the aforesaid system components, to provide sufficient fuel flow to the engine to allow the engine to be started at very low speeds. In addition to this, it can be advantageous to shape the characteristic of pump pressure rise at other operating conditions within the flight envelope of an aircraft fitted with the system.
An object of the invention is to provide a fuel system for a gas turbine engine, in which the aforesaid requirements are fulfilled in a simple and convenient manner.
According to the present invention, a fuel metering system comprises a fuel pump operable to supply fuel at high pressure to a metering valve from which metered fuel is fed, in use, via a pressure raising valve to a gas turbine engine, a spill valve connected between the high pressure supply upstream of the metering valve and low pressure, and a restrictor device connected between the spill valve and low pressure, the pressure arising at or upstream of the restrictor device, which varies according to spill flow through the device from the spill valve, being applied to the pressure raising valve, such that the pressure required to open the pressure raising valve varies in response to the amount of fuel spilled by the spill valve.
Preferably, the pressure arising at or upstream of the restrictor device is applied to the pressure raising valve in opposition to the metered fuel pressure.
The use of a restrictor device in the aforesaid manner can produce acceptable results provided that the engine windmill re-light speed is not reduced beyond a pre-determined minimum value. If the re-light speed is reduced further, it is desirable to limit the system pressure rise to relieve the demand on the pump.
This may conveniently be achieved by providing a pressure device operatively associated with the pressure raising valve and movable under pressure to assist opening of the pressure raising valve. Such assisting movement of the pressure device may conveniently be limited so that the effect of the device is nullified after movement through a predetermined distance.
Preferably, the pressure device is a piston of which the movement is preferably limited by engagement with a stop.
REFERENCES:
patent: 4493187 (1985-01-01), Hansen
patent: 4817376 (1989-04-01), Brocard et al.
patent: 5209058 (1993-05-01), Sparks et al.
patent: 5490379 (1996-02-01), Wernberg et al.
Andrus Sceales, Starke & Sawall
Casaregola Louis J.
Lucas Industries PLC
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