Abstract:
A fuel injector for a turbine engine is provided. The fuel injector includes a body including an admissions chamber for admitting fuel under pressure, a stop valve mounted in the body downstream from the admission chamber and designed to open at a first determined fuel pressure and to remain open beyond that first pressure in order to feed a primary fuel circuit, and a metering valve mounted in the body downstream from the stop valve and designed to open above a second determined fuel pressure, greater than the first pressure, and to remain open above the second pressure in order to feed a secondary fuel circuit. The stop valve and the metering valve form a common movable assembly.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fuel injector for a turbine engine such as an airplane turboprop or turbojet. 
     Description of the Related Art 
     A turbine engine conventionally comprises an annular combustion chamber including regularly distributed fuel injectors at its upstream end, together with means for feeding air around the injectors. 
     There are two main types of injector, namely so-called “aeromechanical” injectors having two fuel circuits providing fuel flow rates that correspond to different operating stages of the engine (an ignition stage, a stage of operating at low power, and a stage of operating at full power), and so-called “aerodynamic” injectors that have only a single fuel circuit for all stages of operation of the engine. 
     Patent application FR 2 832 492 in the name of the Applicant describes an aeromechanical type injector having a primary fuel circuit serving for example during an ignition stage and a low power stage, and a secondary circuit that becomes involved during subsequent stages of operation at medium to full power, in addition to the primary circuit. 
     That type of injector comprises a body including admission means for admitting fuel under pressure, a stop valve mounted in the body downstream from the admission means and designed to open at a first determined fuel pressure P1 and to remain open beyond that first pressure in order to feed a primary fuel circuit, and a metering valve mounted in the body downstream from the stop valve and designed to open above a second determined fuel pressure P2, greater than the first pressure, and to remain open above the second pressure in order to feed a secondary fuel circuit. 
     The flow rate of fuel in the secondary circuit is adjusted by means of metering slots formed in the metering valve and presenting flow sections that vary as a function of the position of the valve, i.e. as a function of the fuel feed pressure. The greater the pressure of the fuel, the greater the flow sections of the slots. 
     In operation, several situations can arise. 
     In a first situation, the fuel pressure upstream from the stop valve is less than P1. The stop valve is then held in the closed position, e.g. by a return spring, and fuel flows neither in the primary circuit nor in the secondary circuit. 
     In a second situation, corresponding to a stage of ignition or of operation at low speed, the pressure of the fuel upstream from the stop valve is greater than P1 but the pressure of the fuel upstream from the metering valve is less than P2. The stop valve is then open and fuel can flow in the primary circuit. The metering valve nevertheless remains closed and fuel does not flow in the secondary circuit. 
     In a third situation, corresponding to a stage of operation at medium or full speed, the pressure of the fuel upstream from the stop valve is greater than P1 and the pressure of the fuel upstream from the metering valve is greater than P2. In this situation, the stop valve is open and fuel can flow in the primary circuit. The metering valve is also open and fuel can flow in the secondary circuit. 
     Such an injector requires two distinct valves to be used, which valves are movable independently of each other. Each valve is subjected to the action of a distinct return spring housed in a portion of the body. That injector presents considerable size and is heavy. It is also appropriate to improve the lifetime of such an injector. 
     Furthermore, studying the characteristic curve of the injector, i.e. the curve plotting fuel flow rate as a function of the pressure difference across the injector, reveals hysteresis in operation. This raises a problem of adjusting the injector and of ensuring that its behavior is predictable in operation. 
     Given the large number of parts in such an injector, this is also associated with the injector being subject to a certain amount of drift, i.e. to the characteristic curve of the injector changing over time. This drift is due in particular to its various parts being subject to wear. 
     BRIEF SUMMARY OF THE INVENTION 
     A particular object of the invention is to provide a solution to these problems that is simple, effective, and inexpensive. 
     To this end, the invention provides a fuel injector for a turbine engine such as an airplane turboprop or turbojet, the injector comprising a body including admission means for admitting fuel under pressure, a stop valve mounted in the body downstream from the admission means and designed to open at a first determined fuel pressure and to remain open beyond that first pressure in order to feed a primary fuel circuit, and a metering valve mounted in the body downstream from the stop valve and designed to open above a second determined fuel pressure, greater than the first pressure, and to remain open above the second pressure in order to feed a secondary fuel circuit, the injector being characterized in that the stop valve and the metering valve form a common movable assembly. 
     In this way, the stop and metering valves are movable together and not independently of each other, thereby limiting complexity and avoiding the above-mentioned problems of hysteresis. The assembly can be urged towards the closed positions of said valves by a single return member, thereby significantly limiting the weight and the size of the injector. Given the smaller number of parts that are separate and movable independently of one another, the lifetime of such an injector is also improved. 
     The movable assembly may be formed as a single piece or as a plurality of parts that are assembled together so as to move together in a manner in which they are not independent of one another. 
     Thus, according to a characteristic of the invention, said movable assembly may be urged towards a closed position of the stop valve and of the metering valve by means of a single resilient return member, e.g. such as a helical compression spring. 
     Furthermore, the stop valve may be designed, in a closed position of said stop valve, to bear against a seat of the body, which seat is fitted with an O-ring. 
     This makes it possible to provide very good sealing for the stop valve, i.e. to ensure a leakage rate that is very small or even zero. 
     Advantageously, the stop valve is formed by a first end of the movable assembly, the metering valve being formed by a second end of the movable assembly. 
     Under such circumstances, the body may include an inner tubular portion in which the movable assembly is slidably mounted, the outer surface of the movable assembly including at least one slot having both a first end opening out into an inner chamber of the body situated downstream from the stop valve and in fluid flow connection with the primary fuel circuit, and also a second end of varying section, the second end of the slot being spaced apart from the second end of the movable assembly by a sealing section of the movable assembly, which sealing section co-operates with a complementary sealing section of the tubular portion so as to form the metering valve. 
     The lengths of the sealing sections of the tubular portion and of the movable assembly are designed so as to enable the stop valve to be opened by the movable assembly moving through a first stroke while keeping the metering valve closed, and then to enable the metering valve to be opened over a consecutive second stroke. The metering valve becomes open when the above-mentioned slot opens out beyond the sealing section of the tubular portion, e.g. into a chamber situated downstream therefrom and in fluid flow connection with the secondary fuel circuit. 
     According to another characteristic of the invention, the inner chamber of the body is defined between the tubular portion and an annular wall of the body located radially outside said tubular portion, the resilient member being housed at least in part in said inner chamber and bearing at a first end against a portion of the movable assembly, and at a second end against a bottom of said inner chamber. 
     Under such circumstances, a leakage channel may be formed in the body and opens out at a first end in the sealing section of the tubular portion, and at a second end in the primary fuel circuit. 
     Given the structure of the metering valve, obtained by the complementary shapes of the sealing sections of the tubular portion and of the movable assembly, laminar leaks of fuel exist that occur between the complementary surfaces of the above-mentioned sections when the metering valve is closed. These leaks feed the secondary fuel circuit. 
     The above-mentioned leakage channel serves to reinject all or some of this laminar leakage flow into the primary fuel circuit. In practice, it has been calculated that at least 50% of the laminar leaks can thus be reinjected into the primary circuit. This is made possible in particular by the pressure difference that exists between the primary circuit and the secondary circuit. In order to increase this pressure difference, i.e. in order to reduce the pressure in the primary circuit, a diaphragm may be provided in the primary circuit, upstream from the opening of the above-mentioned leakage channel. 
     In addition, the movable assembly may include an abutment suitable for bearing against the inner tubular portion of the body when the stop valve and the metering valve are open. 
     The invention also provides a turbine engine, such as an airplane turboprop or turbojet that includes at least one injector of the above-specified type. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention can be better understood and other details, characteristics, and advantages of the invention appear on reading the following description made by way of non-limiting example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a longitudinal section view of a prior art fuel injector; and 
         FIGS. 2 to 4  are longitudinal section views of a portion of an injector in an embodiment of the invention, shown in three different positions of the movable assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A fuel injector  1  as disclosed in patent application FR 2 832 492 in the name of the Applicant is shown in  FIG. 1 . 
     This injector  1  is of the aeromechanical type and it comprises a primary fuel circuit e.g. for use during a starting stage and at low power, and a secondary circuit that becomes involved during subsequent stages of operation, at medium to high power, in addition to the primary circuit. 
     The injector  1  comprises a hollow body  2  with a fuel admission orifice  3  for receiving fuel under pressure coming from a fuel pump (not shown) and leading into a pre-admission chamber  4  after passing through a filter strainer  5 . 
     The body  2  also has an admission chamber  6  situated downstream (in the flow direction of fuel through the injector) from the pre-admission chamber  4  and separated therefrom by a stop valve  7 . A diaphragm  8  is placed between the pre-admission chamber  4  and the stop valve  7 . 
     The stop valve  7  has a head  9  and a stem  10  movably mounted in a tubular portion  11  of an annular support  12  that is stationary relative to the body  2 . The annular support  12  rests on a tubular bushing  13  extending downwards and itself resting on another tubular support  14  having a metering valve  15  mounted therein. The support  14  finally rests on a part  16  that defines a reception chamber  17  situated under the metering valve  15  and serving to support two coaxial tubes  18  and  19 . 
     The inner tube  18  forms a duct  20  for passing a primary fuel flow, with the annular space between the two tubes  18  and  19  forming a duct  21  for passing a secondary fuel flow. 
     An annular space  31  forming part of the primary circuit is defined between the outer wall of the bushing  13  and the body  2 . The inner wall of the bushing  13  also defines an inner chamber  32  situated upstream from the metering valve  15 . 
     The stop valve  7  is held in the closed position by a return spring  22 , with the stop valve  7  being opened when the pressure of the fuel upstream from the valve exceeds a first predetermined value P1. 
     The metering valve  15  is also held in the closed position by a return spring  23 , with the metering valve  15  being opened when the pressure of fuel upstream from the valve  15  exceeds a second predetermined value P2, greater than the above-mentioned first value P1. 
     The metering valve  15  has a bottom end forming a head for resting on a seat  24  of the corresponding support, and a top end where a cup  25  is fastened. The return spring bears firstly against the cup  25  and secondly against a radial surface  26  on the support  14 . 
     The metering valve  15  includes a central axial hole  27  and radial openings  28  opening out into the central hole  27  and into metering slots  29  of appropriate shapes that are formed in the outer surface of the metering valve  15 . 
     The metering valve  15  is movable between two extreme positions, respectively a completely closed position in which its head rests against the seat  24  of the support  14  under the action of the corresponding return spring  23 , and a completely open position in which the cup  25  comes into abutment against the top end  30  of the tubular support  14 . 
     In the completely closed position of the metering valve  15 , as shown in  FIG. 1 , the openings  28  and the slots  29  are situated facing the tubular support  14 , the bottom ends of the slots  29  not opening out into the reception chamber  17 . As a result, in this position, the fuel present in the chamber  32  cannot flow into the reception chamber  17  and into the secondary duct  21 . 
     When the pressure of the fuel situated in the chamber  32  increases, then this pressure causes the metering valve  15  to move towards its open position, i.e. downwards, against the force exerted by the return spring  23 . 
     When this pressure exceeds the second valve P2, the slots  29  open out into the reception chamber  17  and fuel can flow into the secondary duct  21 . 
     The shapes of the slots  29  are such that the flow sections of the slots  29  vary as a function of the position of the metering valve  15 . In particular, the higher the pressure of the fuel in the chamber  27 , the greater the flow sections of the slots  29 . 
     In operation, several situations can arise. 
     In a first situation, the pressure of the fuel in the pre-admission chamber  4  is less than P1. The stop valve  7  is then held in its closed position by the return spring  22  and fuel flows neither into the primary circuit  20  nor into the secondary circuit  21 . 
     In a second situation, corresponding to a stage of ignition or of operating at low speed, the pressure of the fuel in the pre-admission chamber  4  is greater than P1, but the pressure of the fuel in the chamber  32  is less than P2. The stop valve  7  is then open and fuel can flow into the annular space  31  and then into the primary duct  20  (primary circuit). The metering valve  15  nevertheless remains closed, and fuel does not flow in the secondary duct  21 . 
     In a third situation, corresponding to a stage of operating at medium or full speed, the pressure of the fuel in the pre-admission chamber  4  is greater than P1 and the pressure of the fuel in the chamber  32  is greater than P2. The stop valve  7  is open and fuel can flow into the annular space  31  and then into the primary duct  20  (primary circuit). In addition, the metering valve  15  is also open and fluid can flow through the chamber  32 , the openings  28 , the slots  29 , the reception chamber  17 , and then the secondary duct  21  (secondary circuit). 
     As mentioned above, such an injector is of considerable size and weight. It is also appropriate to improve the lifetime of such an injector. Furthermore, studying the characteristic curve of such an injector reveals the presence of hysteresis in operation. This is in addition to the injector being the subject of a certain amount of drift over time. 
       FIGS. 2 to 4  are diagrams showing a portion of an injector in an embodiment of the invention in which the injector  100  has a body  102  that comprises, as above, a pre-admission chamber  104  situated downstream from a filter or a strainer and upstream from a stop valve  107  formed by a first end (referred to below as a top end) of a movable member  133 . The body  102  also has an admission chamber  106  defined radially between an inner tubular portion  134  and an outer tubular portion  135  of the body  102 . A channel  136  forming part of a primary fuel circuit is arranged radially outside the annular portion  135 , and its top end opens out into the admission chamber  106 . A diaphragm  137  is situated level with the opening of the channel  136  of the primary circuit into the chamber  106 . 
     The movable member  133  is mounted to slide along an axis X inside the inner tubular portion  134  of the body  102 . The top end of the movable member  133  has a surface  138  perpendicular to the axis of said member  133  and forming the stop valve. Specifically, this surface  138  is for bearing in leaktight manner against an O-ring  139  housed in the end of a countersink formed in the body  102 . 
     The movable member  133  also has a head  109  that is offset axially downwards relative to the first end  138 , said head  109  having regularly distributed holes  141  passing therethrough. 
     A resilient member  142 , e.g. such as a helical compression spring, is mounted in the chamber  106  and bears against the bottom face of the head  109  and against the bottom  143  of the chamber  106 . 
     The bottom portion of the movable member  133  is in the form of a stem of axis X. Slots, referred to as “metering” slots  129 , are formed in the outer surface of the bottom portion of the movable member  133 , i.e. under the head  109 . These slots  129  extend from the head  109  into a zone situated in the proximity of the second end or bottom end of the movable member  133 , but offset from said bottom end. Thus, the movable member  133  has a sealing section  144  with a cylindrical outer surface that is situated between the bottom ends of the slots  129  and the bottom end of the movable member  133 . The sealing section  144  of the movable member  133  co-operates with a sealing section  145  of complementary shape of the tubular portion  134  of the body  102  that is situated at the bottom end of said tubular portion  134  and that presents an inside surface that is cylindrical. In spite of there being a small amount of assembly clearance between said sealing sections  144 ,  145 , the second end of the movable member  133  forms a metering valve  115  that can be considered as being in a closed position when the sealing section  144  of the movable member  133  is situated in the sealing section  145  of the body  102 , and that is in an open position when the metering slots  129  open out beneath the sealing section  145  of the body  102  in a reception chamber  117  similar to that described above and connected to or forming part of a secondary fuel circuit. 
     The bottom ends of the slots  129  are of varying section. More particularly, the sections of the slots  129  decrease going towards the second end of the movable member  133 . 
     The inner wall of the tubular portion  134  has a middle zone  146  of greater diameter, and two end zones  147 ,  145  of smaller diameter (including in particular the sealing zone  145 ), in order to form bearing surfaces for guiding the movable member  133 . This serves to provide short guidance for the movable member  133  and also to provide better sealing in the sealing sections  144 ,  145  and to reduce friction between the movable member  133  and the tubular portion  134 . 
     The body  102  also has a leakage channel  148  extending radially and opening out firstly into the sealing section  145  of the inner wall of the tubular portion  134  and secondly in the channel  136 . 
     The pressures of fuel in the pre-admission chamber  104  that enable the stop and metering valves  107  and  115  to open are referenced P′1 and P′2 respectively. These pressures are functions in particular of the area of the surface  138 , of the spring constant (written K) of the return spring  142  of the movable member  133 , and of the stroke (written x) of the movable member  133 . 
     In operation, when the pressure of the fuel upstream from the stop valve  107  is greater than the pressure P′1, the movable member  133  is in the position shown in  FIG. 2  where sealing is provided at the stop valve  107  by the surface  138  bearing against the O-ring  139 . It should be observed that the leakage flow rate past the stop valve  107  is very small or zero. 
     When the pressure is greater than P′1 and less than P′2, the movable member  133  is moved through a stroke x 1  enabling the stop valve  107  to be opened while keeping the metering valve  115  closed, i.e. while maintaining at least a portion of the sealing section  144  of the movable member within the sealing section  145  of the body  102  ( FIG. 3 ). 
     In this situation, the fuel from the pre-admission chamber  104  penetrates into the admission chamber  106 , in particular through the holes  141 , and then into the channel  136  so as to feed the primary fuel circuit. It should be observed that a small laminar leakage flow exists between the cylindrical surfaces of the sealing sections  144 ,  145 , with a fraction of this leakage flow being reinjected into the primary circuit via the leakage channel  148 . This is made possible by the existence of a small pressure difference between the two ends of the leakage channel  148 . This pressure difference is generated in particular by the diaphragm  137 . 
     Finally, when the pressure of the fuel is greater than P′2, the movable member  133  is moved through a stroke x 2  so as to open both the stop valve  107  and the metering valve  115 . 
     In this situation, fuel penetrates into the admission chamber  106  and is directed both towards the channel  136  and the primary circuit and also towards the chamber  117  and the secondary circuit, via the slots  129 . The flow rate of fuel brought into the secondary circuit is a function of the flow section of the slots  129 , i.e. of the position of the movable member  133 . The further the movable member  133  is moved downwards, the greater this flow section (and thus the greater the flow rate of fuel in the secondary circuit). After moving through a certain distance, the head  109  of the movable member  133  comes into abutment against the top end of the tubular portion  134  of the body  102 , as shown in  FIG. 4 . Movement of the movable member  133  is thus limited by this abutment.