Abstract:
A method of operating a fuel system to determine the extent of internal wear of a fuel pump thereof comprises driving the fuel pump, moving a metering valve member to a closed position in which the metering valve bears against a resilient stop, increasing the operating speed of the fuel pump to increase the pressure applied to the metering valve member urging the valve member towards the resilient stop to a point beyond which further movement of the metering valve member occurs, sensing when the further movement of the metering valve member occurs, and sensing the operating speed of the pump at the time when the further movement occurs.

Description:
FIELD OF THE INVENTION 
   The invention relates to a fuel system and in particular to a fuel system for use in supplying fuel to an aircraft engine. The invention relates, in particular to a method and apparatus for determining the operating characteristics, and in particular the extent of internal wear, of a fuel pump forming part of such a fuel system. 
   BACKGROUND OF THE INVENTION 
   A fuel system for use in the delivery of fuel to an aircraft engine typically comprises a primary constant displacement fuel pump arranged to supply fuel through a metering valve and a pressure raising and shut-off valve to the burners of an engine. A spill valve is arranged to maintain a substantially constant pressure drop across the metering valve by spilling fuel back to the inlet of the fuel pump. In some arrangements, a second pump is arranged to operate in parallel with the aforementioned primary pump to assist the primary pump during high demand phases of the engine operating cycle. 
   In order to ensure that a fuel pump is serviced or replaced at the optimum time, it is desirable to be able to monitor the extent of internal wear of the fuel pump without requiring the pump to be removed and dismantled. 
   Techniques are described in EP 1522731, EP 1138933 and U.S. Pat. No. 5,111,653 whereby the wear status of a fuel pump can be sensed by monitoring of the spill valve operation. However, the arrangements all require special designs of spill valve to be used. As a consequence they are unsuitable for retrofitting into existing fuel systems. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a method and apparatus whereby the disadvantages set out above can be overcome. 
   According to the present invention there is provided a method of operating a fuel system to determine the extent of internal wear of a fuel pump thereof comprising driving the fuel pump, moving a metering valve member to a closed position in which it bears against a resilient stop, increasing the operating speed of the fuel pump to increase the pressure applied to the metering valve member urging it towards the resilient stop to a point beyond which further movement of the metering valve member occurs, sensing when the further movement of the metering valve member occurs, and sensing the operating speed of the pump at the time when said further movement occurs. 
   The resilient stop is conveniently biased by a spring, and preferably comprises a dump valve. 
   The further movement of the metering valve member may be sensed by a position sensor used in normal control of the operation of the metering valve. 
   The said further movement of the metering valve member occurs when the pressure applied thereto exceeds a substantially constant predetermined pressure. By monitoring the operating speed of the pump at which this pressure is attained, an indication as to the extent of internal leakage wear of the pump can be gained. 
   Conveniently, the sensed operating speed is stored to allow a wear profile of the pump to be built up over time. The stored data may be used to allow estimation of the remaining operating lifetime of the pump and scheduling of its replacement. 
   Where a wear profile is built up over time, preferably other factors which may effect the pump operating speed required to cause the further movement of the metering valve member, for example altitude and fuel temperature, are taken into account. 
   The invention also relates to a fuel system comprising a fuel pump operable to supply fuel to a metering valve, the metering valve comprising a valve member movable under the influence of the fuel pressure applied thereto, a resilient stop limiting closing movement of the metering valve member when the pressure applied thereto is less than a predetermined level and allowing further movement of the metering valve member to occur when the applied pressure exceeds the predetermined level, sensor means for sensing said further movement of the metering valve member, and means operable to sense the operating speed of the fuel pump when said further movement of the metering valve member occurs. 
   The movable stop conveniently comprises a dump valve. 
   The method of the invention may be incorporated into the normal engine start procedure, and is suitable for retrofitting into existing systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will further be described, by way of example, with reference to the accompanying drawing ( FIG. 1 ) which is a diagram illustrating a fuel system in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The fuel system illustrated diagrammatically in  FIG. 1  comprises a low pressure pump  10  arranged to supply fuel through a filter  12  to the inlet  14  of a primary gear pump  16 . The pump  16  supplies fuel through a supply line  18  in which is located a flow washed filter  18   a  to an inlet port  20  of a metering valve  22 . 
   The metering valve  22  is of substantially conventional form and comprises a valve member in the form of a spool  24  movable within a bore  26  under the influence of the fuel pressures within chambers  28 ,  30  formed at the ends of the spool  24 . A servo valve arrangement  32  is used to control the relative pressures applied to the chambers  28 ,  30 . For example, if it is desired to lift the spool  24 , in the orientation illustrated, the chamber  30  is supplied with fuel at a higher pressure than that of chamber  28  by reducing the communication between the chamber  30  and the low pressure in line  32   a  and increasing the communication between the chamber  28  and the low pressure in line  32   a . Conversely, downward movement of the spool  24  is achieved by supplying chamber  28  with fuel at a high pressure than that of chamber  30  by increasing communication between chamber  30  and the low pressure in line  32   a  and reducing communication between chamber  28  and the low pressure in line  32   a . A position sensor  34 , for example in the form of an LVDT is provided to monitor the position occupied by the spool  24 , the output of the sensor  34  being used in the control of the operation of the servo valve  32  so as to allow the operation of the metering valve  22  to be controlled in a closed-loop fashion. 
   As illustrated, the metering valve  22  includes an outlet port  36  connected through a pressure raising and shut-off valve  38  to the burners of an aircraft engine. The spool  24  partially obscures the outlet  36 , and the rate at which fuel is able to flow through the metering valve  22  is dependent upon the position occupied by the spool  24 . It will be appreciated that by lifting the spool  24 , the fuel supply rate is increased, downward movement of the spool  24  reducing the fuel supply rate, the spool  24  being movable to a position in which the supply of fuel through the metering valve is terminated. 
   A combining spill valve arrangement  40  is provided and is operable to maintain a substantially uniform pressure drop across the metering valve  22 . In the event that the pressure drop across the metering valve  22  exceeds a predetermined level, a pressure drop control valve  42  causes the combining spill valve  40  to open thereby allowing fuel to escape along a return path  44  back to the inlet  14  of the fuel pump  16 . 
   As illustrated, the fuel system further comprises a second fuel pump  46  arranged to deliver fuel to the filter  18  and metering valve  22  through a non-return valve  48  under certain operating conditions of the engine. When delivery of fuel by the second fuel pump  46  is not required, then fuel delivered thereby is returned through the spill valve  40  and a return passage  50  to the inlet of the pump  16  thus unloading the second pump  46 . 
   A pressure relief valve  52  is provided between the outlet of the pump  16  and the inlet  14  thereof, the pressure relief valve  52  opening in the event of excessive pressures occurring at the outlet of the pump  16  in order to reduce the risk of damage thereto. 
   The fuel system illustrated in  FIG. 1  further comprises a dump valve  54  operable, on engine shut-down, to drain fuel from the burner manifold. The dump valve  54  comprises a valve member  56  movable by the spool  24  of the metering valve  22  when the spool  24  is moved to its fully closed position, such movement of the dump valve member  56  opening the dump valve to allow such escape of fuel. As illustrated, the dump valve member  56  is biased by a spring  58  towards a closed position and so other than when forced to its open position by the spool  24 , the dump valve  54  will remain closed. 
   In normal use of the fuel system, fuel is supplied by the pump  16  through the metering valve  22  and the pressure raising and shut-off valve  38  to the burners of the engine. The pump  16  is driven from an output shaft of the engine and rotates at a speed proportional to engine speed. It will be appreciated, therefore, that the rate of fuel supply by the pump  16  is governed by the speed of operation of the engine. The fuel supply to the engine is controlled by the metering valve  22  which, in turn, is controlled by the operation of the servo valve  32 . As described hereinbefore, the combining spill valve  40  together with the operation of the pressure drop control valve  42  maintains a substantially constant pressure drop across the metering valve  22  thus ensuring that the rate at which fuel is supplied by the metering valve  22  is governed by the axial position of the spool  24 . During this mode of operation, the fuel system operates substantially conventionally and so further detail of its operation is not included herein. 
   When it is desired to shut down the engine, the pressure raising and shut-off valve  38  is closed by a shut-off servo valve  38   a  to terminate fuel supply to the engine burners. The metering valve  22  is also closed by supplying the chamber  28  of the metering valve  22  with fuel under pressure, the chamber  30  being connected to low pressure by the servo valve  32 . It will be appreciated that under these circumstances the spool  24  is forced downwardly in the orientation illustrated, thus closing the outlet  36 . The operation of moving the spool  24  downwards brings the spool  24  into engagement with the dump valve member  56 , forcing the dump valve member  56  away from its seating against the action of the spring  58  and opening the dump valve  54  thereby allowing fuel to escape from the burner manifold. Engine shut down in this manner is, again, substantially conventional and will not be described in further detail. 
   To start the engine, the engine is turned by an external drive, resulting in the fuel pumps  16 ,  46  being driven. The fuel pressure to which the combining spill valve  40  is exposed is relatively low so the spill valve  40  occupies a closed position in which the second pump  46  is unloaded and in which none of the fuel supplied by the primary pump  16  and the secondary pump  46  is returned through the lines  44  and  50 . The servo valve  32  is held in a position in which the chamber  30  is held at a lower pressure than chamber  28 . The chamber  28  is exposed to a pressure related to the pressure of the fuel supplied by the pump  16  to the line  18 . It will be appreciated, therefore, that the spool  24  is urged downwardly towards its closed position by the pressure differential. Initially, the difference in pressure is relatively small and so the spool  24 , although urged into engagement with the dump valve member  56 , is unable to move the dump valve member  56  against the action of the spring  58 . 
   The speed at which the engine, and hence the pump  16 , is driven is increased, with the result that the fuel pressure in the line  18 , and hence in the chamber  28 , increases and a point will be reached at which the pressure applied to the spool  24  is sufficient to overcome the action of the spring  58 , thus the spool  24  is able to move further in the downward direction. This movement is accompanied by opening movement of the dump valve member  56  but as no fuel is being supplied to the burners at this point, such operation of the dump valve  54  is of no effect. 
   The further movement of the spool  24  is sensed by the sensor  34 , and the engine/pump operating speed at the time of the further movement being sensed is measured, for example by sensing the operating speed of the engine or the external drive used to rotate the engine at this time, or using sensors  16   a ,  46   a  associated with the pumps. 
   As the force applied by the spring  58  is substantially constant over the working life of the fuel system, it will be appreciated that the technique described hereinbefore can be used to provide an indication of the pump operating speed necessary to develop a substantially fixed pressure. Increases in the required operating speed are representative of internal wear and hence leakage of fuel from the outlet side of the pump, in this case the combination of the primary and secondary pumps and so the leakage could be in either or both of these pumps, to the inlet side thereof. 
   Although the measured speed may be used directly to provide an indication of the wear status of the pump, it is envisaged to store the measured speed data over time to allow a wear profile to be built up which can be used to monitor pump wear and to allow scheduling of pump replacement at the optimum time. The speed data may be stored with other data which could impact upon the pump operating speed required to develop the required pressure, for example altitude and fuel temperature. 
   After having sensed the pump wear status, the metering valve  22  can be opened to commence fuel supply to the pressure raising and shut-off valve  38  to allow engine start-up to continue in the usual manner. 
   Although described as part of the engine start-up procedure, the method could form part of a separate pump test procedure if desired. 
   It will be appreciated that the invention may be retrofitted or implemented in existing fuel systems without requiring the replacement or modification of significant components thereof. It therefore is of wide ranging application. 
   A number of modifications or alterations may be made to the arrangement described hereinbefore. For example, although the dump valve conveniently forms the resilient stop, particularly when being retrofitted, arrangements are possible in which a separate spring biased resilient stop is provided. Further, although described as part of a system in which fuel is supplied by a pumping unit having two pumps arranged in parallel to sense the wear status of the pumping unit, the invention is also applicable to single pump systems.