GB2227795A

GB2227795A - Gas turbine engine fuel system

Info

Publication number: GB2227795A
Application number: GB8902641A
Authority: GB
Inventors: Barry Donald Miles; David Ian Jones
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1989-02-07
Filing date: 1989-02-07
Publication date: 1990-08-08
Also published as: GB8902641D0

Abstract

In an aircraft jet engine, on main engine shutdown unburned fuel in the main engine combustion system (124) is back purged into the main fuel metering unit (116) and is sprayed from the metering unit into the engine bypass duct (160) from whence it is subsequently expelled. This does away with the need for a separate onboard drainage tank for unburned fuel remaining in the engine on engine shutdown. Fuel is supplied from a tank 110 via a pump 112, and a gear pump 114, a line 103 feeding fuel via a pump 118, switch valve 120 and metering unit 122 to an afterburner system 126. <IMAGE>

Description

IMPROVEMENTS IN FUEL DRAINAGE SYSTEMS FOR GAS TURBINE ENGINES This invention relates to improvements in a fuel drainage system for a gas turbine engine, in particular the drainage of fuel from an aircraft gas turbine engine on engine shutdown or failed start.

A known fuel supply system for an aircraft gas turbine engine is shown in a simplified schematic layout in Figure 1 of the accompanying drawings.

Referring to Figure 1, fuel from a fuel tank 10 enters a fuel supply pump 12 from where a fuel line 13 branches via a gear pump 14 and fuel line 15 to a main fuel metering unit 16, and through a line 3 upstream of the gear pump 14 to an afterburner fuel pump 18. The fuel is pumped from the afterburner fuel pump 18 via line 11 to a T-junction pressure switch valve 20 from where it passes under normal operating conditions through line 19 to an afterburner fuel metering unit 22. Combustible fuel mixtures pass respectively from the main fuel metering unit 16 via fuel lines 17 to a main engine combustion system 24, and from the afterburner fuel metering unit 22 via fuel lines 23 to an afterburner combustion system 26.

A fuel return line 21 returns unburned fuel from the afterburner fuel metering unit 22 to line 3 upstream of the afterburner fuel pump 18 when the afterburner combustion system 26 is shut down. This unburned fuel is then pumped from the afterburner fuel pump 18 via line 11 to the T-junction pressure switch valve 20. At the normal, relatively high, fuel pressure in line 11, valve 20 switches the fuel through line 19 to the metering unit 22. When the pressure in line 11 drops to a predetermined level, valve 20 switches the fuel through a fuel return line 25 to a drain tank 28. The tank 28 is provided with a float valve 30. Similarly, on main engine shutdown, unburned fuel in the main fuel metering unit 16 is drained through the line 27 in the drain tank 28.An ejector pump 32 drains fuel from the drain tank 28 through the line 29 to the fuel supply pump 12 when the float valve 30 is actuated. A non-return valve 34 is provided between the ejector pump 32 and the drain tank 28 to prevent drainage of fuel through the line 29 back into the tank. Also, or in the alternative, provision may be made to vent drained fuel outside the aircraft by means of a vent line 31 leading from the drain tank 28 to an overboard vent 36. A second non-return valve 38 is provided in vent line 31.

Disadvantages of the system described above are that there is a potential fire hazard in hot fuel being collected in an engine mounted drains tank, and a drains tank and unburned fuel return system present a weight penalty.

Accordingly, objects of the present invention are to empty fuel from the engine manifold on shutdown or failed start, to avoid dumping that fuel outside the aircraft, and to obviate the need for an onboard drainage tank and fuel return system for that fuel, thereby effecting substantial weight and cost savings.

According to a first aspect of the present invention there is provided, in a gas turbine engine including a main engine combustion system, a main fuel metering unit for delivering fuel to the main engine combustion system, and an engine bypass duct, a method of draining unburned fuel from the main engine combustion system on engine shutdown or failed start comprising back purging unburned fuel from the main engine combustion system into the main metering unit and then draining unburned fuel from the main manifold unit into the engine bypass duct.

According to a second aspect of the present invention there is provided a gas turbine engine including a main engine combustion system, a main fuel metering unit for delivering fuel to the main engine combustion system, and an engine bypass duct, wherein there is further provided means for unburned fuel in the main fuel metering unit back purged from the main engine combustion system on engine shutdown or failed start into the engine bypass duct.

The invention will now be described by way of example only with reference to Figure 2 of the accompanying drawings which shows in schematic layout a fuel supply system according to the invention for an aircraft gas turbine engine.

Referring to Figure 2, in an aircraft gas turbine engine having a bypass duct 160, fuel from a fuel tank 110 enters a fuel supply pump 112 from where a fuel line 113 branches via a gear pump 114 and fuel line 115 to a main fuel metering unit 116, and through a line 103 upstream of the gear pump 114 to an afterburner fuel pump 118.

The fuel is pumped from the afterburner fuel pump 118 via line 111 to a T-junction pressure switch valve 120 from where it passes under normal operating conditions through line 119 to an afterburner fuel metering unit 122. Combustible fuel mixtures pass respectively from the main fuel manifold unit 116 via fuel lines 117 to a main engine combustion system 124, and from the afterburner fuel metering unit 122 via fuel lines 123 to an afterburner combustion system 126.

A fuel return line 121 returns unburned fuel from the afterburner fuel metering unit 122 to line 103 upstream of the afterburner fuel pump 118 when the afterburner combustion system 126 is shutdown. At the normal, relatively high, fuel pressure in line 111, valve 120 switches the fuel through line 119 to the metering unit 122. When the pressure in line 111 drops to a predetermined level, valve 120 switches the fuel through line 125 to an overboard vent 136. The pressure switching level is chosen so that the fuel released overboard is minimised. The vent 136 is a pipe with a small orifice so that fuel dribbles overboard.

On main engine shutdown (or a failure to start) unburned fuel in the main engine combustion system 124 back purges through the main fuel metering unit 116 and is drained therefrom by spraying via drain line 150 into the engine bypass duct 160, where it is subsequently entrained in air flow through the bypass and is expelled on shutdown.

Claims

1. In a gas turbine engine including a main engine combustion system, a main fuel metering unit for delivering fuel to the main engine combustion systems, and an engine bypass duct, a method of draining unburned fuel from the main engine combustion system on engine shutdown or failed start comprising back purging unburned fuel from the main engine combustion system into the main metering unit and then draining unburned fuel from the main metering unit into the engine bypass duct.

2. A gas turbine engine including a main engine combustion system, a main fuel metering unit for delivering fuel to the main engine combustion system, and an engine bypass duct, wherein there is further provided means for feeding unburned fuel in the main fuel metering unit back purged from the main engine combustion system on engine shutdown or failed start into the engine bypass duct.

3. A gas turbine engine as claimed in claim 2 wherein the means is a drain line providing a drainage connection between the main fuel manifold unit and the engine bypass duct.

4. A gas turbine engine as claimed in claim 2 wherein the means for feeding unburned fuel into the bypass duct is a pressure switch valve which directs the fuel into the engine bypass duct when the fuel pressure reaches a predetermined minimum level.

5. A method of draining unburned fuel from the main engine combustion system of a gas turbine engine, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

6. A gas turbine engine drainage system substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.