EMERGENCY POWER UNIT FOR AIRCRAFT

Abstract: An aircraft emergency power unit having a high pressure air storage tank (22), an aviation fuel storage tank (24), and a combustor (26) which combusts pressurised air and aviation fuel in a stoichiometrically fuel rich ratio to produce a motive combustion gas.

Description: This invention relates to a system for providing emergency power on an aircraft. More specifically, this invention provides a stored energy subsystem arranged to yield a controlled, high temperature, high pressure gas. The gas produced by the system may then be used to drive a turbine wheel, which may provide power to drive such devices as engine starters, electrical generators, and hydraulic pumps. Generally, an aircraft has one or more primary engines which provide thrust for the aircraft, as well as pressurised bleed air for the environmental control systems. The primary engine also provides power to drive electric generators and hydraulic pumps, both of which are necessary for powering instruments and flight control systems. In addition, many aircraft also have an auxiliary power engine to provide electric and hydraulic power, as well as bleed air to the aircraft when the primary engines are not operating, for example when the aircraft is on the ground. The auxiliary power engine may also provide power to start the primary engines either on the ground or in flight. Both the primary engine and the auxiliary power engine operate on aviation fuel from the aircraft's main fuel tanks, mixed with air drawn from the atmosphere as the combustion components. For maximum fuel efficiency, these engines operate in a stoichiometrically air rich, fuel lean mode. In many instances, starting the auxiliary power engine requires an external power source such as a ground based start cart, a pressurised air tank, or an emergency power system. Since the auxiliary power engine is primarily designed to operate on the ground where the air is relatively dense, the auxiliary power engine may be incapable of operating at higher altitudes, for example above 16.754 m (55,000 feet). It is therefore evident that in many applications the auxiliary power engine would not be able to restart a failed primary engine above 16.764 m (55,000 feet) and in this event there would be no electrical or hydraulic power available. Also, since both the primary engine and auxiliary power engine operate on fuel from the main fuel tanks, if this fuel supply is depleted there will be no source of power for the electrical and hydraulic power systems to allow the pilot to control and land the aircraft. It is therefore desirable to have on an aircraft an emergency power system capable of operating independently of external conditions, which can provide emergency electrical and hydraulic power to the flight control systems and may be used to restart the auxiliary or primary engines. These are the minimum requirements of such an emergency power system. Since they are only operated in the event of an emergency, these systems remain stored and inactive for long periods of time, but are required to start instantly and provide a continuous power output for a prespecified duty cycle. Ideally, such an emergency power system should be compact, lightweight, highly reliable, easily maintained, require no special handling of materials or fuels, while providing a combustion process which is controllable and which produces a clean, non-toxic combustion gas. Presently, emergency power units primarily rely on liquid hydrazine based fuels sprayed into a catalyst bed to generate a pressurised gas. These units are in use on several aircraft and combine high performance with low weight. However, liquid hydrazine is highly corrosive and toxic, thereby requiring special handling procedures and design considerations. The catalyst material is expensive, and when the catalyst is depleted it must be replaced. Furthermore, the combustion gas which is produced is toxic and therefore limits ground testing of the emergency power unit. US-A-2873577 teaches a turbine engine starter which initiates combustion at a fuel rich air to fuel ratio in the range of 7,1:1 to 8,3:1 and after 3 seconds, the system adjusts to an air rich air to fuel ratio for normal operation. Thus normal operation of the system takes place in the air rich range past the stoichiometric combustion point of the air and fuel. It is specified that the combustion is continued for a set time interval or until the turbine reaches a preset speed, at which time combustion is stopped and decoupled from its load. Additionally, the system allows for two sets of specified air and fuel flow rates, corresponding to a combustion initiating ratio and a normal operating ratio. The system does not allow for controlled variation of the fuel and air flow rate in order to regulate the amount of combustion gases directed upon the turbine and provide variable turbine output power. It is an object of the present invention to provide an emergency power system method and apparatus for an aircraft, having an energy source which is readily available, non-toxic and clean burning and which is packaged in a compact, lightweight, reliable and easily maintained unit. Another object is to provide such a method and apparatus including a control system to initiate, maintain and shut off the delivery of the working fluids to the combustor. Another object is to provide such a method and apparatus including a control system which further monitors various output parameters to maximise the efficiency of the system. Another object is to provide such a method and apparatus including a combustor which can maintain the combustion of the working fluids in the prescribed fuel rich ratio for extended periods of time without being damaged or being rendered inoperative due to heat or carbon deposit build-up. Another object is to provide such a method and apparatus including a combustor having a nozzle which mixes the aviation fuel and air in the prescribed fuel rich ratio in such a way that the fuel is essentially completely atomised and the centre of the resulting flame of combustion is maintained along the central axis in the centre of the combustion chamber. Another object is to provide such a method and apparatus with the capacity to resupply the aviation fuel and compressed air tanks subsequent to use. Another object is to provide such a method and apparatus which can be retrofitted into aircraft which may or may not already have an emergency power unit. Another object is to provide such a method and apparatus which can be attached to an auxiliary power unit. Another object is to provide such a method and apparatus which can restart the aircraft's main engines. According to the invention, there is provided an aircraft emergency power unit for generating a controlled flow of motive gas, comprising a high pressure air storage tank, an aviation fuel tank, a combustor, an air conduit for delivering pressurised air from the air storage tank to the combustor, a fuel conduit for delivering aviation fuel from the fuel tank to the combustor, and a control system associated with the air and fuel conduits arranged to adjust the flow rate of air and aviation fuel to the combustor to a fuel rich ratio of between 1,5:1 and 7:1, the combustor being operable to sustain combustion of the air and aviation fuel while in the fuel rich ratio to generate the flow of motive gas at a temperature of at least 760°C (1400°F). According to another aspect of the invention there is provided a method for generating a controlled flow of motive gas to develop controlled, limited power in the event of an aircraft emergency, comprising the steps of: delivering a flow of high pressure air from a high pressure storage tank on the aircraft to a combustor; delivering a flow of aviation fuel from a dedicated aviation fuel storage tank on the aircraft to the combustor; adjusting the rate of flow of both the pressurised air and the aviation fuel to maintain the ratio of air to aviation fuel delivered to the combustor in a fuel rich ratio in the range of between 1,5:1 to 7:1; and combusting the air and aviation fuel while in the fuel rich ratio to generate the flow of motive gas at a temperature of at least about 760°C (1400°F). Thus, the present invention provides an emergency power system operating on a fuel rich mixture of aviation fuel and air and optimises the advantages of such a system to yield an emergency power system with the improved characteristics of relying on an energy source which is readily available, non-toxic, and clean burning, packaged in a compact, lightweight, highly reliable, and easily maintained emergency power unit. The invention provides an emergency power unit, gas generator apparatus and method which stores compressed air and aviation fuel, and delivers a specific ratio of each fluid to a combustor which ignites and burns the two fluids in a fuel rich mixture producing a combustion gas which may be used to drive a turbine. Thus, the power unit is capable of generating a controlled flow of motive gas to develop temporary, limited power in the event of an aircraft emergency, using a high pressure air storage tank on the aircraft for dedicated use substantially only by the emergency power unit and an aviation fuel tank for storing aviation fuel of the type which also operates the aircraft main engine, the aviation fuel tank being for dedicated use substantially only by the emergency power unit. Briefly, the invention contemplates the use of aviation fuel and compressed air as the stored energy source for an aircraft emergency power unit. When emergency power is required, the two fluids are allowed to flow through conduits which may include shutoff valves, temperature sensors, and flow control valves, to a preferred specialised nozzle which ejects the two fluids into a combustor. The oxidiser (air) to fuel (aviation fuel) ratio is in a stoichiometrically fuel rich range of approximately 1,5 to 1 or 2:1, up to approximately 7 to 1, and thereby, upon combustion within the combustion chamber, produce a motive combustion gas having a temperature above 760°C (1400°F). The motive combustion gas is preferably directed into a turbine to provide output shaft power. Preferably, the air conduit includes an air pressure regulator, a high pressure line connecting the air storage tank to the air pressure regulator and a regulated air pressure air line connecting the air pressure regulator to the combustor, the air pressure regulator being capable of receiving high pressure air from the air storage tank and reducing the air pressure to a lower, regulated, constant air pressure delivered via the regulated air pressure line to the combustor. Preferably, the control system comprises an air shutoff valve for starting and stopping the flow of compressed air through the high pressure air line, and an air flow control valve in the regulated air pressure line for varying the flow of compressed air through the regulated air pressure line. Preferably, the control system further comprises, and air temperature control valve in the high pressure air line between the air shutoff valve and the pressure regulator, an air temperature sensor in the regulated pressure line for sensing the temperature of the regulated air being delivered to the combustor and a heat exchanger for heating the pressurised air in the high pressure air line, the air temperature control valve being arranged to receive and restrict the flow of heated pressurised air from the heat exchanger, and also being arranged to mix the flow of pressurised air coming directly from the high pressure air tank with the flow of heated pressurised air having passed through the heat exchanger. Preferably, the control system further comprises a fuel shutoff valve in the fuel conduit for starting and stopping the flow of fuel through the fuel conduit a fuel flow regulator valve in the fuel conduit for varying the flow of fuel through the fuel conduit, and a fuel temperature sensor in the fuel conduit for sensing the temperature of the fuel within the conduit. Preferably, the control system includes an electrical controller having an input from the air temperature sensor for controlling the actuation of the air shutoff valve, the air flow control valve and air temperature control valve to control the temperature and flow rate of pressurised air to the combustor in a preselected manner to maintain the fuel rich ratio, the electrical controller also being connected to the fuel temperature sensor, the fuel shutoff valve, and the fuel flow regulator valve the electrical controller thus controlling the actuation of the fuel shutoff valve and the fuel flow regulator valve in order to control the flow of fuel to the combustor. A preferred combustor for an emergency power unit for aircraft comprises: a thermal liner defining a combustion chamber having openings at opposite ends thereof; a pressure shell enclosing the thermal line; an insulator disposed between the thermal liner and the pressure shell the insulator allowing the thermal liner to attain a temperature approximating to the combustion temperature while inhibiting heat transfer to the pressure shell whereby the shell operates at a greatly reduced temperature; means for initiating the combustion process within the combustion chamber; and an injector head secured to the thermal liner in an arrangement sealing one of the openings of the combustion chamber, the injection head being configured and arranged to deliver oxidant and fuel to the combustion chamber. Preferably the thermal liner is made from Inconel steel. Preferably, the relative locations and configurations of the insulator and thermal liner are such that the thermal liner retains sufficient heat to remain substantially free of carbon deposits on the interior surfaces thereof during operation of the combustor. A preferred air-fuel mixer for an emergency power unit for mixing the air and aviation fuel comprises a pressurised air inlet, an aviation fuel inlet, a first air plenum disposed radially around the fuel inlet and in open communication with the air inlet so that the first air plenum distributes the pressurised air radially around the fuel inlet, a second air plenum disposed radially around the fuel inlet in series with the first air plenum, and a plurality of air flow passages between the first air plenum and the second air plenum. Preferably the ratio of air to aviation fuel delivered to the combustor is approximately 3,5:1 and the motive combustion gas temperature is approximately 1010°C (1850°F). The aviation fuel is preferably JP.4. Preferably, the unit includes a fuel resupply system for replenishing the aviation fuel tank from the aircraft primary fuel tank, comprising a fuel resupply line from the aircraft primary fuel tank to the aviation fuel tank; a fuel pump disposed along the fuel resupply line and operable to pump aviation fuel from the aircraft primary fuel tank to the aviation fuel tank; and a check valve in the fuel resupply line to prevent flow of aviation fuel from the aviation fuel tank to the aircraft primary fuel tank. Preferably, the unit also includes a compressed air resupply system for compressing air and filling the high pressure air storage tank, the compressed air resupply system comprising: a positive displacement air pump for compressing ambient air to a high pressure; an air line from the positive displacement air pump to the air conduit; and a check valve to prevent the flow of pressurised air from the air conduit through the air line. The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of the emergency power unit embodying the principles of the present invention; Figure 2 is a block diagram of the electronic control unit hardware; Figure 3 is a schematic diagram of an alternative emergency power unit embodying the principles of the present invention; Figure 4 is a schematic diagram of a second alternative emergency power unit embodying the principles of the present invention; Figure 5 is a partially schematic partially cross-sectional view of the emergency power unit combustor; and Figure 6 is a partially cross-sectional view of the nozzle of the combustor of the emergency power unit. Referring now to the drawings, Figure 1 shows an emergency power unit 20 having a high pressure air tank 22 and an aviation fuel tank 24 as stored energy means. The air tank 22 is connected via a high pressure air line 28, an air pressure regulator 36, and a regulated pressure line 29 to a combustor 26. The air line 28 also includes an air shutoff valve 32 and an air temperature control valve 34, while the regulated pressure air line 29 includes an air temperature sensor 38 and an air flow control valve 40. Similarly, the fuel line 30 between the fuel tank 24 and the combustor 26 includes a fuel shutoff valve 42, a fuel temperature sensor 44 and a fuel control valve 46. An electrical controller 50 for the emergency power unit 20 is electrically connected to the air shutoff valve 32, the air temperature sensor 38, the airflow control valve 40, the fuel shutoff valve 42, the fuel temperature sensor 44, and the fuel control valve 46, as well as being electrically connected to an igniter 68, a pair of combustor temperature sensors 58, and a turbine speed sensor 56. The turbine speed sensor 56 is in relative proximity to, and senses the speed of an output shaft 54 which is attached to a turbine 52 which is driven by the combustion gases produced within the combustor 26. The output shaft 54 from the turbine 52 is connected to a gearbox 70 to drive a generator 72 and an hydraulic pump 74. Downstream of turbine 52, the combustion gases are directed through an exhaust duct 60 which may have an exhaust-to-air heat exchanger 62 mounted in heat exchange relationship thereto. The heat exchanger 62 is connected to a cool air conduit 64 which is connected at its opposite end to the high pressure air line 28 at a point between the air shutoff valve 32 and the air temperature control valve 34. The hot side of heat exchanger 62 is connected by a conduit 66 to the air temperature control valve 34. Prior to operation, the high pressure airtank 22 is filled to several thousand p.s.i. (a few hundred atmospheres) with compressed air and the fuel tank 24 is filled with aviation fuel. The fuel tank 24 may be a positive displacement pressurised piston or bladder type of fuel tank. When emergency power is required, the controller 50 opens the air shutoff valve 32 and the fuel shutoff valve 42 to start the flow of air and fuel through the respective air line 28 and fuel line 30. Subsequently, the temperature control valve 34 mixes air coming directly from the high pressure air tank 22 with air which has been circulated through the heat exchanger 62, delivering this controlled temperature high pressure air to the pressure regulator 36 which reduces the air pressure to an appropriate regulated pressure level. The controller 50 also uses the air temperature signal from the air temperature sensor 38 and fuel temperature signal from the fuel temperature sensor to direct the air flow control valve 40 and the fuel control valve 46 deliver a precise fuel to air ratio to the combustor 26. The fuel and air mix within the combustor 26 and the igniter 68 commences the combustion process. Once initiated, combustion is self-sustaining as long as fuel and air are delivered to the combustor. The controller 50 powers and controls the igniters 68, and receives signals back from the combustor temperature sensors 58, and the turbine speed sensors 56 to combine with the air and fuel temperature measurements in order to optimise the performance and efficiency of the emergency power system, while regulating the turbine speed to a nominal, normally very high speed by adjusting fuel and air flow rates to the combustor 26. It may be appreciated that as the compressed air supply within the high pressure air tank 22 is depleted, the temperature of the remaining air drops rapidly. Thus, the controller 50 must continuously monitor the regulated air temperature and the fuel temperature, which is also subject to variation in order to modulate the fuel to air ratio delivered to the combustor 26 properly. The desired air to fuel ratio is in the fuel rich range of between 1,5:1 and 7:1 to produce a combustion gas having a temperature above 760°C (1400°F). The optimum air to fuel ratio for the present system is approximately 3,5 :1, producing a combustion gas having a temperature of 1010°C (1850°F). Operating the emergency power unit 20 in a fuel rich mode minimises the amount of compressed air required and thus minimises the size of the high pressure air tank 22, and additionally maximises the amount of energy which is available to provide useful power due to the high temperatures of the combustion gas produced. Since the emergency power unit 20 must remain dormant for extended periods of time between operations, the reliability of the controller 50 is critical. Thus, the controller 50 has two independent digital Electronic Control Units (ECU) arranged in a master ECU 310 and a slave ECU 311 configuration. The master ECU 310 is shown in Figure 2, the slave ECU 311 having a substantially identical configuration and function; the ECU's 310 and 311 monitor the start/stop commands and the turbine operation. The controller 50 is provided with a Built In Test (BIT) to continually monitor the integrity of all of the electronic components. If the master ECU 310 fails to operate properly, or if major failure occurs to any of the electrical components of the system including the fuel shutoff valve 42, air shutoff valve 32, airflow control valve 40, fuel flow regulator 46, fuel temperature sensor 44, air temperature sensor 38, combustor temperature sensor 58 or the turbine speed sensor 56, the slave ECU 311 will activate, taking over control of the emergency power unit 20. The master ECU 310 remains active as long as power is applied to the unit via the aircraft battery system (not shown) or a backup power pact. In the active state, it will perform various health checks of itself, the sensors and the control system components while waiting for a start command. At frequent intervals, the master ECU 310 will activate the slave ECU 311 via the test enable signal in order to determine the health of the standby control system. Should the master ECU 310 locate any failures in either of the master or slave ECU's, it will raise an alarm signal, either via a discrete output that can be relayed to the cockpit, or via an optional data bus 314. The slave ECU 311 is normally in a dormant, unpowered standby state, waiting either for a start command or for a test enable signal from the master ECU 310. After activation, the slave ECU 311 executes a self-test routine and reads the test enable and start/stop commands to determine the proper course of action. A block diagram of the master ECU 310 hardware is shown in Figure 2. The heart of the master ECU 310 is a single chip microcontroller 315. This microcontroller 315 encompasses nearly all of the circuitry needed for a digital controller, including bit analog to digital converter, channel multiplexers, timers, high-speed pulse inputs, high-speed pulse outputs, pulse-width-modulated outputs, Random Access Memory (RAM), Read Only Memory (ROM), watchdog timer, hardware multiply/divide, serial input/output port, external memory interface, and discrete inputs/outputs. Using these microprocessor functions, only an input buffer 316, an output buffer 317, a sensor exitation circuit 318, and a switching power supply 319, are required to complete the ECU functions. An external memory 322 may also be required to supplement the memory of the microcontroller 315. The external memory 322 also provides an interface with the optional data bus controller 314. An optional battery 323, makes the low-power RAM of the microcontroller 315 nonvolatile, preserving test information for off-line servicing. An EPU start/stop buffer 324, and an altitude buffer 325 provide inputs to the microprocessor. To operate the master ECU 310, an external source of electrical power (not shown) provides +28 volts to the switching power supply 319 and the sensor exitation circuit 318. The switching power supply 319 uses a high frequency, pulse-width-modulation technique to provide isolated power for the gate drive circuits of the power switches within the microcontroller 315. The sensor exitation circuit 318 provides a reference voltage to the microcontroller 315, as well as power to activate the fuel temperature sensor 44, the air temperature sensor 38, and the combustor temperature sensor 58. Exitation power supplies are monitored by a self-test function of the micro-controller 315 to verify continuity of the sensor circuits. The test includes the turbine speed sensor 56, which is passive and thus does not normally require power. However, the sensor exitation circuit 318 may provide a voltage pulse to the turbine speed sensor 56 to verify circuit continuity. The output signals from the sensors 38, 44, 56 and 58 go through the input buffers 316 then to the microcontroller 315. The microcontroller 315 analyses signals and sends control signals via the output buffer 317 to the igniter 68, fuel shutoff valve 42, air shutoff valve 32, fuel flow regulator 46, and airflow control valve 40. The output buffer 317 also has a wraparound feedback loop to the microcontroller 315, and an external watchdog lockout 326. The combustor 26 of Figures 1, 3 and 4 is shown in a more detailed cross-sectional view in Figure 5. A combustion chamber 251 is enclosed by a generally cylindrically shaped thermal liner 253 which tapers at one end thereof to an attachment 267 of a nozzle box 261. The thermal liner 253 is wrapped in ceramic insulation 255 and enclosed by a pressure shell 257. The opposite end of the combustion chamber 251 is enclosed by an injector head 231 to which a plenum cover 229 and an injector assembly 227 are attached. Attached to the plenum cover 229 is an air inlet assembly 225 which delivers air to an outer air plenum 233 enclosed by the plenum cover 229. A plurality of air passages 237 in the injector head 231 deliver air from the outer air plenum 233 to an inner air plenum 235. An air swirler 239 mounted to the injector head 231 includes a plurality of vanes 241 causing the air to swirl as it passes from the inner air plenum 235 into the combustion chamber 251. The injector assembly 227 of Figure 5 as shown in Figure 6 includes a nozzle body 273, a tangential poppet 275, a filter screen 277, a compression spring 279 and a cylindrical insert 281. The combustor 26 is specifically designed to maximise the mixing of fuel and air as it enters the combustion chamber 251, to ignite the mixture, and to maintain the combustion process for extended periods of time without excessive carbon deposit build-up or melting of the combustor body. The aviation fuel flows to the injector assembly 227 through the centre of the insert 281 within the nozzle body 273 and is strained by the filter screen 277 before flowing around the tangential poppet 275 held in place by the wire compression spring 279. The aviation fuel flows out through an opening 274 at the end of the nozzle body 273. Simultaneously, air flows through the air inlet 225 into the outer air plenum 233 where it is distributed circumferentially about the injector assembly 227 before flowing through the air passages 237 into the inner air plenum 235 whereupon the vanes 241 of the air swirler 239 impart a rotational motion to the air flow as it enters the combustion chamber 251. It is at this point that the air flow and the fuel flow mix and the fuel is atomised as the mixture enters the combustion chamber 251. A spark from the igniter 68 commences the combustion process. Due to the uniform mixing of the air and fuel resulting from the duel plenum design, the combustion flame is maintained in the centre of the combustion chamber 251. However, the thermal liner 253 rapidly heats up to approximately the combustion temperature. The ceramic insulation 255 prevents the conduction of heat to the pressure shell 257, so that during normal operation the temperature of the pressure shell 257 does not exceed 260°C (500°F) while the thermal liner 253 may attain temperatures up to 982°C (1800°F). The combustion gases then flow out of the combustor 26 into the nozzle box 261. The nozzle 261 distributes the combustion gases through one or more nozzles 263 to impinge upon the turbine 52, and provide a motive force to the turbine 52 causing it to rotate about the output shaft 54 and thus drive the generator 72 and/or the hydraulic pump 74. The unique design of the walls of the combustor 26 promotes and stabilises the combustion flame while minimising the weight of the combustor system. The thermal liner 253 which is made of Inconel steel and has a very thin cross-section, rapidly heats up to the temperature of the combustor flame. Once the thermal liner 253 has heated sufficiently the hot thermal liner 253 tends to stabilise the combustion process which is self-sustaining. The ceramic insulation 255, which is made of a woven ceramic cloth, prevents the conduction of heat to the pressure shell 257, which is also made of Inconel steel, and thereby minimises the thickness of the cross-section of the pressure shell 257 which is required to contain the combustion pressures. A further benefit of operating the thermal liner 253 at combustion temperatures is that it effectively prevents the build-up of carbon deposits on the walls of the combustion chamber 251. An alternative embodiment of the invention is shown in Figure 3. This system is basically similar to the system of Figure 1, however, the system of Figure 3 additionally includes apparatus for compressing air to replenish the high pressure air tank 22 or for supplying compressed air to the combustor 26, as well as apparatus for either refilling a pressurised bladder fuel tank 124 or delivering fuel to the combustor 26 directly from a primary fuel tank 48. This apparatus includes a positive displacement air pump 176, an air resupply line 127 including pressure relief valve 178, and an air check valve 180 in parallel with the high pressure air tank 22, having an air service valve 188, a second pressure regulator valve 110, and a secondary air shutoff valve 184. These two sources of pressurised air are attached to an accumulator 182 which is then connected via a high pressure air line 128 to the air shutoff valve 32, the air pressure regulator valve 36 and an air and fuel master shutoff valve 112, and from there to the combustor 26. The fuel supply side of emergency power unit 120 includes the pressurised bladder type of fuel tank 124 having a nitrogen service valve 199 and pressure gauge 196 and the fuel valve 42 in parallel with a fuel pump 186 which can pump fuel directly from the primary fuel tank 48 of the aircraft through a fuel resupply line 187 and a fuel check valve 192. The two sources of fuel are then merged upstream of fuel flow regulator valve 46 which regulates fuel to the master shutoff valve 112, immediately prior to the combustor 26. This system also includes an electrical controller 150 which is electrically connected to the secondary shutoff valve 184, the air pump 176, the air shutoff valve 32, the air pressure regulator valve 36, the fuel shutoff valve 42, the fuel pump 186, and the master shutoff valve 112, as well as to the igniters 68, the turbine speed sensors 56 and an aircraft cockpit console 18. The remainder of the system of Figure 3 is essentially the same as that shown in Figure 1. The electrical controller 150 performs all of the same functions, and additionally turns on the air pump 176 and fuel pump 186 when the high pressure air tank 22 and the pressurised bladder fuel tank 124 are nearly empty in order to provide a continuous source of emergency power after the stored fuel and compressed air has been consumed. With this system, the air pump 176 and the fuel pump 186 may also be used to refill the high pressure air tank 22 and the fuel tank 124 when the emergency power unit 120 has been used and then shutoff while still in flight or on the ground. Another embodiment of the invention is shown in Figure 4. This system includes the high pressure air tank 22, a pressurised piston fuel tank 224, the combustor 26, and the turbine 52 in a similar manner to the systems of Figures 1 an 3. The high pressure air line 28 leads from the air tank 22 to an air regulator valve 31. The air regulator valve 31 includes the air shutoff valve 32, the pressure regulating valve 37, the check valve 80 and the relief valve 78 components. The regulated air pressure line 29 from the air regulator valve 31 is connected to a fuel tank air line 59 leading to the pressurised piston fuel tank 224, and the cool air conduit 66 which leads to the heat exchanger 62, and the air temperature control valve 34. The air temperature control valve 34 also has a connection to the hot air conduit 64 returning air from the heat exchanger 62. The regulated air line 29 coming from the air temperature valve 34 goes through an air control valve 39 before terminating at the combustor 26. The air control valve 39 includes a secondary air shutoff valve 84 and an airflow control valve 40. The fuel line 30 from the fuel tank 224 to the combustor 26 has along it a fuel trim valve 94, a filter 98, a pressure relief valve 90, the fuel shutoff valve 42 and the fuel control valve 46. The pressurised piston fuel tank 224 of this embodiment is a positive displacement piston type of tank having an enclosed cylinder 81, with one end having a connection to the fuel line 30 and the other end having a connection to the fuel tank air line 59. Within the cylinder 81, a free floating piston 83 separates the tank into a fuel chamber 85 and an air chamber 87. When the air shutoff valve 32 is opened the air chamber 87 is pressurised to the regulated air pressure within the air lines 29 and 59. Pressurising the air chamber 87 forces the piston 83 to pressurise the fuel within the fuel chamber 85 to the same pressure as to regulated air pressure. Since the piston 83 is free to move within the cylinder 81, the pressure on the fuel is maintained as the fuel is depleted. This system may also include a fuel pump 86 for supplying fuel from the primary fuel tank 48 to the pressurised piston fuel tank 224 via the fuel resupply line 187, to resupply the depleted pressurised piston fuel tank 224 after use of the emergency power unit 20. A check valve 92 prevents the flow of fuel from pressurised fuel tank 224 through fuel resupply line 187 to the primary fuel tank 48. This system of Figure 4 is similar in operation to the systems of Figures 1 and 3 with the electrical controller 50 initiating the flow of aviation fuel and compressed air to the combustor 26, regulating the ratio of air to fuel delivered, and using sensor signals to optimise the efficiency of the system. As in the other systems shown in Figure 1 and Figure 3, the motive combustion gases produced in the combustor 26 are delivered to the turbine 52. In addition, a bleed air system 16 from the main engines of the aircraft also provide a source of pressurised air which may be used to drive the turbine 52. Within this system, the emergency power unit 20 must be capable of supplementing the bleed air system 16 in driving the turbine 52, as well as provide the total power required by the turbine 52 in the event of main engine failure. Thus, the controller 50 may have to pulse the combustor 26 by supplying discrete amounts or air and fuel, and selectively powering the igniter 68. The controller 50 relies on feedback from the turbine speed sensors 56 to determine the emergency power load requirements and then sets the duration, interval, and magnitude of each combustion pulse, thereby providing an averaged power output to the turbine.

Claims: An aircraft emergency power unit for generating a controlled flow of motive gas, which comprises a high pressure air storage tank (22), an aviation fuel tank (24), a combustor (26), an air conduit (28,29) for delivering pressurised air from the air storage tank (22) to the combustor (26), a fuel conduit (30) for delivering aviation fuel from the fuel tank (24) to the combustor (26), and a control system (50) associated with the air and fuel conduits (28,29,30) arranged to adjust the flow rate of air and aviation fuel to the combustor (26) to a fuel rich ratio, characterised in that the fuel rich ratio is between 1,5:1 and 7:1, the combustor (26) being operable to sustain combustion of the air and aviation fuel while in the fuel rich ratio to generate the flow of motive gas at a temperature of at least 760°C (1400°F). An emergency power unit as claimed in Claim 1 characterised in that the air conduit (28,29) includes an air pressure regulator (36), a high pressure line (28) connecting the air storage tank (22) to the air pressure regulator (36) and a regulated air pressure air line (29) connecting the air pressure regulator (36) to the combustor (26), the air pressure regulator (36) being capable of receiving high pressure air from the air storage tank (22) and reducing the air pressure to a lower, regulated, constant air pressure delivered via the regulated air pressure line (29) to the combustor (26). An emergency power unit as claimed in Claim 2 characterised in that the control system (50) comprises an air shutoff valve (32) for starting and stopping the flow of compressed air thorough the high pressure air line (28), and an air flow control valve (40) in the regulated air pressure line (29) for varying the flow of compressed air through the regulated air pressure line (29). An emergency power unit as claimed in claim 3 characterised in that the control system further comprises, an air temperature control valve (34) in the high pressure air line (28) between the air shutoff valve (32) and the pressure regulator (36), an air temperature sensor (38) in the regulated pressure line (29) for sensing the temperature of the regulated air being delivered to the combustor (26) and a heat exchanger (62) for heating the pressurised air in the high pressure air line (28), the air temperature control valve (34) being arranged to receive and restrict the flow of heated pressurised air from the heat exchanger (62), and also being arranged to mix the flow of pressurised air coming directly from the high pressure air tank (22) with the flow of heated pressurised air having passed through the heat exchanger (62). An emergency power unit as claimed in any preceding claim characterised in that the control system further comprises a fuel shutoff valve (42) in the fuel conduit (30) for starting and stopping the flow of fuel through the fuel conduit (30) a fuel flow regulator valve (46) in the fuel conduit (30) for varying the flow of fuel through the fuel conduit (30), and a fuel temperature sensor (44) in the fuel conduit (30) for sensing the temperature of the fuel within the conduit (30). An emergency power unit as claimed in claim 4 and Claim 5, characterised in that the control system (50) includes an electrical controller (50) having an input from the air temperature sensor (38) for controlling the actuation of the air shutoff valve (32), the air flow control valve (40) and the air temperature control valve (34) to control the temperature and flow rate of pressurised air to the combustor (26) in a preselected manner to maintain the fuel rich ratio, the electrical controller (50) also being connected to the fuel temperature sensor (44), the fuel shutoff valve (42), and the fuel flow regulator valve (46) the electrical controller (50) thus controlling the actuation of the fuel shutoff valve (42) and the fuel flow regulator (46) in order to control the flow of fuel to the combustor (26). An emergency power unit as claimed in any preceding claim including a fuel resupply system for replenishing the aviation fuel tank from the aircraft primary fuel tank, comprising a fuel resupply line (187) from the aircraft primary fuel tank (48) to the aviation fuel tank (224); a fuel pump (86) disposed along the fuel resupply line (187) and operable to pump aviation fuel from the aircraft primary fuel tank (48) to the aviation fuel tank (224); and a check valve (92) in the fuel resupply line (187) to prevent flow of aviation fuel from the aviation fuel tank (224) to the aircraft primary fuel tank (48). An emergency power unit as claimed in any preceding claim characterised by compressed air resupply system for compressing air and filling the high pressure air storage tank, the compressed air resupply system comprising: a positive displacement air pump (176) for compressing ambient air to a high pressure; an air line (127) from the positive displacement air pump (176) to the air conduit (128); and a check valve (180) to prevent the flow of pressurised air from the air conduit (128) through the air line (127). An emergency power unit as claimed in any preceding claim, characterised in that the combustor (26) comprises: a thermal liner (253) defining a combustion chamber (251) having openings at opposite ends thereof; a pressure shell (257) enclosing the thermal line (253); an insulator (255) disposed between the thermal liner (253) and the pressure shell (257) the insulator (255) allowing the thermal liner (253) to attain a temperature approximate to the combustion temperature while inhibiting heat transfer to the pressure shell (257) whereby the shell (257) operates at a greatly reduced temperature; means for initiating (68) the combustion process within the combustion chamber (251); and an injector head (231) secured to the thermal liner (253) in an arrangement sealing one of the openings of the combustion chamber (251), the injection head (231) being configured and arranged to deliver oxidant and fuel to the combustion chamber (251). An emergency power unit as claimed in Claim 9 characterised in that the thermal liner (253) is made of Inconel steel, the pressure shell (257) is made from Inconel steel, and the insulator (255) comprises a woven ceramic insulation cloth. An emergency power unit as claimed in Claim 9 or claim 10 characterised by an air-fuel mixer for mixing the air and aviation fuel, including a pressurised air inlet (225), an aviation fuel inlet, a first air plenum (233) disposed radially around the fuel inlet and in open communication with the air inlet so that the first air plenum (233) distributes the pressurised air radially around the fuel inlet, a second air plenum (235) disposed radially around the fuel inlet in series with the first air plenum, and a plurality of air flow passages (237) between the first air plenum (233) and the second air plenum (235). A method for generating a controlled flow of motive gas to develop controlled, limited power in the event of an aircraft emergency, comprising the steps of: delivering a flow of high pressure air from a high pressure storage tank (22) on the aircraft to a combustor (26); delivering a flow of aviation fuel from a dedicated aviation fuel storage tank (24) on the aircraft to the combustor (26); adjusting the rate of flow of both the pressurised air and the aviation fuel to maintain the ratio of air to aviation fuel delivered to the combustor in a fuel rich ratio; and combusting the air and aviation fuel while in the fuel rich ratio to generate the flow of motive gas, characterised in that rates of flow are adjusted so that the fuel rich ratio is maintained in the range of between 1,5:1 and 7:1 and the temperature of the flow of motion gas is at least about 760°C (1400°F). A method as claimed in Claim 12 characterised in that the ratio of air to aviation fuel delivered to the combustor is approximately 3,5:1 and the motive combustion gas temperature is approximately 1010°C (1850°F)

Inventors: KOERNER, MICHAEL S., NESS, RONALD J., WEIGAND, FRANCIS K.
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1987
Main classification: B64D41
IPC codes: F02C7, F02C9, F23D11, F23R3, B64D41, F02K99