Abstract:
A method and apparatus for conditioning a supply of bleed air from an aircraft engine to a system using such bleed air, including passing the bleed air to a turbine over which the air is expanded to produce a power output, and regulating the power output thereby to control the condition of the air downstream of the turbine.

Description:
BACKGROUND TO THE INVENTION  
         [0001]    This invention relates to a method of conditioning a supply of bleed air from an aircraft engine to an apparatus or system using such bleed air, to provide such air in a required condition.  
           [0002]    In referring herein to an aircraft engine it is to be understood that engines of the turbine type generally used for military or commercial civil aircraft may also be used in propulsion systems for other means of transport, and the invention is applicable to such other uses of the engines, as well as to their use in aircraft. Further, in relation to aircraft, we include all types thereof whether heavier or lighter than air and manned or unmanned.  
           [0003]    The term bleed air as generally understood and as used herein refers to pressurised air which is extracted from an aircraft engine after a or the compressor stage thereof.  
           [0004]    It is well known for most types of military and civilian aircraft to utilise bleed air from the aircraft&#39;s engine(s) for air conditioning and other purposes. Having been compressed in the engine(s) the bleed air is normally much too hot to be used directly and therefore has to be thermally conditioned by passing it through at least one heat exchanger. Conventionally the cooling medium in such a heat exchanger is ram air, i.e. ambient air caused to flow through the heat exchanger as a result of the forward speed of the aircraft.  
           [0005]    Such use of ram air to cool the bleed air has certain disadvantages. One is that the structure of the aircraft has to be designed to provide a ram air inlet or inlets, which sometimes have to be provided with adjustable doors or shutters to vary the size of the opening they afford: this entails the provision of actuators and controls. Also the internal structure of the aircraft has to provide space for the ram air to pass to and from the heat exchanger. Improvements in heat exchanger design and efficiency has enabled the size of heat exchangers to be reduced, but the use of ram air nevertheless creates aerodynamic drag on the aircraft, adversely affecting speed and fuel consumption. A further disadvantage pertaining to military aircraft is that compression of air at or in front of a ram air inlet opening creates a “hot spot” which can be detected by infrared-seeking anti-aircraft missiles.  
           [0006]    An alternative to the use of ram air, particularly in civil aircraft, is the use of low-pressure air impelled by a fan stage of the engine as the cooling medium, to cool the bleed air in a heat exchanger. Heated fan air downstream of the heat exchanger is discharged overboard.  
           [0007]    Whether ram air or fan air is used in a heat exchanger to reduce the temperature of compressor bleed air, a lot of power is wasted by the discharge overboard of heated air. Under certain conditions, possibly 300 kW might be wasted in the discharge of heated air. This also is a disadvantage in the case of military aircraft, since a hot outflow of air can be detected by a heat seeking missile. There are devices whereby the effect of discharge of a large quantity of hot air can be suppressed, but it would be desirable if the discharge could be avoided entirely.  
           [0008]    Some further disadvantages associated with the use of bleed air from an aircraft&#39;s engine(s) arise from the fact that the pressure of the bleed air varies as the engine compressor speed varies and/or as the aircraft&#39;s altitude changes. Such variations in pressure could cause operational problems if air conditioning apparatus is subjected to them. Hence there is a requirement for pressure reducing/regulating valves for supplying a generally constant (within operating limits) pressure downstream. Such pressure regulating valves are complex and contain sensitive components. The problem is greater if there are multiple engines and the outputs of bleed air therefrom have to be matched.  
         DESCRIPTION OF THE PRIOR ART  
         [0009]    It has been proposed in each of U.S. Pat. No. 3,764,814 and U.S. Pat. No. 5,899,085 to pass bleed air to an external turbine which in turn, drives a power generator, to generate electrical power for use in the aircraft in an effort to increase the efficiency of the system, but these proposals do not suggest how to accommodate variations in the pressure of the bleed air arising due to changing operational conditions.  
         SUMMARY OF THE INVENTION  
         [0010]    According to one aspect of the present invention, we provide a method of conditioning a supply of bleed air from an aircraft engine to a system using such bleed air, including passing the bleed air to a turbine over which the air is expanded to produce a power output, and regulating the power output thereby to control the condition of the air downstream of the turbine.  
           [0011]    Thus by regulating the power output, irregularities in the bleed air pressure are prevented or the effect of them in the system in which the bleed air is to be used, is reduced.  
           [0012]    Preferably an electrical generator is driven by the turbine. Then the electrical power output of the generator can be regulated to control the expansion of the air in the turbine.  
           [0013]    Aircraft have complex electrical systems in which electrical power thus generated can be utilised. However, it would be within the broadest aspect of the invention if the turbine were to drive an hydraulic pump for example, whose power output could be utilised in an aircraft&#39;s hydraulic system.  
           [0014]    The method of the invention may include sensing at least one parameter of the air downstream of the turbine and changing a load on the power output depending upon the sensed parameter thereby to regulate the condition of the air.  
           [0015]    The system in which the bleed air may be used may be an air cycle air conditioning system and the bleed air is expanded and cooled as the bleed air passes over the turbine and may thus be used to cool a heat load.  
           [0016]    According to a second aspect of the invention, we provide apparatus for conditioning a supply of bleed air from an aircraft engine to a system utilising such bleed air, the apparatus including a turbine providing a power output as a result of expansion of the air, and a controller for regulating the power output and thereby controlling the condition of the air downstream of the turbine.  
           [0017]    Preferably the turbine drives an electrical generator, an electrical load on the generator being regulated by a controller in response to at least one parameter of the air downstream of the turbine.  
           [0018]    Preferably the sensor for providing an input to the controller is responsive to one or more parameters including, but not limited to, turbine speed, turbine inlet temperature, turbine inlet pressure, turbine outlet pressure, turbine outlet temperature, the controller responding to the or one or more of the inputs by regulating the electrical load to increase or decrease turbine speed so that the pressure of air downstream thereof is maintained in a generally steady state.  
           [0019]    The controller may also be responsive to signals relating to operational conditions, such as flight parameters, of which change would result in a change in the operating state of the engine or engines. Thus the controller may integrated with an engine control system and/or an aircraft control system.  
           [0020]    There may be a bypass through which at least a proportion of the bleed air can pass to the system where it is to be used other than by way of the turbine or other expansion device. The proportion of bleed air passing through the bypass may be determined by a valve whose operation preferably also is controlled by the controller.  
         BRIEF DESCRIPTION OF THE DRAWING  
         [0021]    Embodiments of the invention will now be described by way of example with reference to the accompanying drawing, which illustrates diagrammatically an apparatus in accordance with the invention for carrying out the method thereof.  
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    In the drawing, an aircraft engine is indicated at  10  and it is provided, in known manner, with an outlet  11  for bleed air, i.e. air taken from that compressed by the or a compressor stage of the engine prior to delivery thereof to the engine&#39;s combustion section. Such air is at a relatively high pressure and in consequence of its compression its temperature has been raised. The bleed air passes from the engine outlet  11  by way of a non-return valve  11   a  to a temperature control valve  12  which provides for the air to pass through an expansion turbine  13  having an outlet  14  and/or to bypass the turbine  13  by way of a bypass line  15 . The turbine outlet  14  and bypass line  15  meet to provide air under required temperature and pressure conditions to, for example, an air conditioning system indicated generally at  16  which includes a heat exchanger  17  which may for example be a cooling device for cooling a coolant which is circulated around components of a radar apparatus, or other heat load, indicated diagrammatically at  18 . From there the air may be otherwise used, partially recycled and/or ejected to the ambient atmosphere in any known manner.  
         [0023]    In a modified embodiment, the bypass line  15  may included a heat exchanger, as indicated in dotted lines at  15  in the figure, so that the bypass air may be cooled, for example by ram air acting as a coolant in the heat exchanger  15   a.  In this modified embodiment, the sensor indicated at  32  in the drawing, the function of which will be described below may need to be alternatively positioned, for example in the position indicated in dotted lines.  
         [0024]    The turbine  13  is preferably a radial flow turbine having a peripheral inlet and central axial outlet, with a wheel which preferably is of an aluminium alloy on account of its low density, the compressed and hot bleed air in use flowing over the turbine  13  causing the turbine to rotate, whilst the air is permitted to expand and thus cool. The turbine  13  may be of other materials e.g. a suitable steel or a ceramic material, as may be necessitated by the high temperature of the bleed air. The turbine  13  may have an inlet nozzle of fixed or variable geometry.  
         [0025]    The turbine has an output shaft  20  which is connected to an electrical generator  21 , and the electrical output from the generator  21  is delivered to a control and power regulating controller  22  which may be integrated with an engine  10  or aircraft control system if desired. Electrical power distribution lines extend from the controller  22  to wherever electrical power generated by the generator  21  may be consumed in the aircraft; by way of example power lines to the air conditioning system  16  are indicated at  23  and to the radar apparatus  18  at  24 . An electrical storage battery is indicated at  25 .  
         [0026]    In operation of the apparatus, the bleed air which is expanded as it passes over the turbine  13  has its pressure and temperature reduced. By way of example, the pressure may drop to the order of 30-100 psig, dependent on what use the air may be put to downstream of the turbine  13 . The extent to which the air is expanded and thus cooled over the turbine  13  is controlled by regulating the electrical load on the generator  21  and this regulation is performed by the controller  22 .  
         [0027]    The controller  22  may be operated in response to any number of operating parameters of the apparatus and/or of the engine  10  and/or the aircraft. The drawing shows sensors as follows whose electrical output signals are provided to the control and regulating means  22 :  
         [0028]    a sensor  30  measuring engine speed (rpm),  
         [0029]    a sensor  31  measuring air pressure and/or temperature at the inlet to the turbine  13 ,  
         [0030]    a sensor  32  measuring temperature and/or pressure at the outlet of the turbine  13 ,  
         [0031]    a sensor  33  measuring turbine/generator shaft speed,  
         [0032]    a sensor  34  measuring the temperature at an appropriate position in relation to the heat load  17 ,  
         [0033]    a sensor  35  measuring electrical power consumption of the radar apparatus  18 ,  
         [0034]    The above sensors are indicated by way of examples and other parameters associated with operation of the system and not necessarily all those mentioned above. Further parameters which may be sensed and used in control of the apparatus may include, by way of example, aircraft surface temperature (sensor(s)  36 ); ambient air temperature (sensor  37 ) and ambient air pressure (sensor  38 ).  
         [0035]    The controller  22  may in addition be responsive to one or more signals derived from a flight control system of the aircraft and/or a control system for the aircraft&#39;s engine(s). In the drawing an aircraft control system is indicated diagrammatically at  40 , responsive to manual flight control input signals as indicated at  42  and automated flight control input signals as indicated at  43 . An engine control system is indicated diagrammatically at  41 . Signals from both these control systems are provided to the control means  22  to be utilised as required in regulating the electrical load on the generator  21  and thus controlling the expansion of the bleed air in the turbine  13 . The signals from sensors  36 - 38 , and the flight input signals  42 ,  43 , enable the requirements for cooling air to be predicted to some extent and thus facilitate control of the conditioning of the bleed air to meet such requirements.  
         [0036]    The controller also controls the valve  12 .  
         [0037]    Thus, in principle, the invention provides that the electrical load on the generator  21  is regulated by the controller  22  to increase or lower the speed of the turbine and maintain its outlet pressure in a generally steady state regardless of the operating conditions of the engine  10 . This principle of operation and the apparatus above described may be at the heart of an integrated air conditioning management system for an aircraft with one or a number of engines, which would enable predicted heat load requirements to be responded to in a better manner than is presently possible. The power consumed by electrical and electronic equipment may be taken into consideration to help calculate predicted heat loads, and aircraft surface temperatures and ambient temperatures may also be taken into consideration.  
         [0038]    In the case where an aircraft has two or more engines, a respective turbine arrangement for conditioning of the bleed air may be provided for each engine. The respective power outputs of the generators associated with respective turbines may be regulated separately so as to balance and match them as required.  
         [0039]    Possibly the expansion of the bleed air could take place in successive turbines to provide two (or more) stage expansion, and two different temperature/pressure supplies of conditioned bleed air could thus be obtained yet a further possibility is that expansion could take place in turbines in parallel with one another.  
         [0040]    Although a simple heat load has been referred to at  17 ,  18 , it would be possible that one or more air or vapour cycle cooling systems could be arranged to use conditioned air supplied in accordance with the invention. The invention may have elements in common with and/or integrated into such systems, and a common control system could be used.  
         [0041]    If insufficient bleed air pressure is available from the engine to satisfy heat load demands, a compressor (not shown) may be operated by energy derived from operation of the engine, and/or by battery power and/or other electrical power sources, e.g. fuel cells. Thus ambient air may supplement the bleed air to cool the heat load.  
         [0042]    Finally it is pointed out that, although the invention is described above in relation to an aircraft, aircraft engines can be used for other means of transport or indeed for other purposes. Reference herein to an aircraft engine should not be construed as limiting the invention in its broadest aspects to its use in aircraft.