Abstract:
This injector with two valves ( 7, 10 ) which open at different fuel pressures so as to establish a primary flow regime and then a secondary flow regime is characterized in that the primary regime is enriched with fuel via an orifice ( 21 ) which adds a branch to the primary flow path of the fuel, but which is closed when the secondary valve is displaced so as to prevent enrichment of the fuel in the secondary regime. This invention applies in particular to certain aircraft engines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a two-stage fuel injector equipping the combustion chambers of gas turbines. 
     2. Discussion of the Background 
     It improves devices such as that described in French patent 2 832 492, the structure and operation of which can be summarized as follows. Two valves are arranged in series in an outer bushing of the injector and are biased separately into closed positions by respective springs. The fuel fed into the injector by a pump passes through a filter and weighs on the first valve, which is a primary valve biased by a weak spring. A moderate pressure is therefore enough to open said valve, and the fuel passes through it and then flows along a primary path which leads it out of the injector. The primary path comprises an annular portion between the outer bushing and an inner body, in which the other valve or secondary valve is housed in a sliding manner. The fuel also weighs on the secondary valve, which is biased by a much stronger spring than the previous spring. It therefore does not move until a pressure is reached which is greater than that necessary to displace the primary valve and to open the primary fuel path, but once it too has been opened it opens up a secondary fuel path which branches from the primary fuel path and passes through the secondary valve. The flow of fuel delivered by the injector is then increased by the additional flow which flows through this secondary fuel path. This secondary flow is used in certain regimes of the engine in which the injector is employed. 
     The idea here is to enrich the mixture delivered by the injector, i.e. to increase the fuel flow, but only in the primary flow regime. This enrichment might be required in order to improve the ignition capacity during high-altitude flight and also on the ground. It will concern only some of the injectors of the engine. The temptation will therefore be to modify existing injectors instead of using a completely new design or a different model. It will in particular be advantageous if the injector sought is identical to the existing injector with regard to the parts governing the secondary flow. 
     SUMMARY OF THE INVENTION 
     According to the invention, this is achieved by means of a two-stage fuel injector comprising two valves arranged in series and biased towards closed positions, namely a primary valve arranged on a primary fuel path and a secondary valve arranged on a secondary fuel path, the primary fuel path and the secondary fuel path branching downstream of the primary valve, and an inner body in which the secondary valve slides and which delimits the primary path, the secondary path comprising a central cavity of the secondary valve and at least one hole passing radially through the secondary valve between its periphery and the central cavity, characterized in that it comprises at least one orifice passing through the inner body between the primary fuel path and the secondary valve and opening into the hole when the secondary valve is in the closed position. 
     In order to achieve perfect enrichment of only the primary flow, it is advantageous if the orifice passing through the inner body opens into an end portion of the hole, in a direction towards the upstream end of the secondary fuel path. 
     In many customary injectors, however, the piercing of the orifice will produce an excessive primary flow. It will then be useful to replace, over part of its length, the annular portion of the primary flow path with another orifice which can be calibrated to the required diameter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will become apparent from the following detailed description, particularly when considered in conjunction with the drawings in which: 
         FIG. 1  shows a cross-section view of the injector according to an exemplary embodiment of the present invention with the primary flow alone; 
         FIG. 2  shows a cross-section view of the injector according to the embodiment of  FIG. 1 , with the primary flow and secondary flow; and 
         FIG. 3  shows a flow curve obtained as a function of fuel pressure for an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described in connection with  FIGS. 1 and 2 , which show an injector according to the invention in states with the primary flow alone and with the primary flow and secondary flow, respectively. 
     The injector in  FIGS. 1 and 2  comprises a cylindrical outer bushing  1  which is hollowed out in places between its two end faces. It contains at the upstream end a strainer-type fuel filter  2  and opens via a fuel inlet orifice  3 . The fuel filter  2  bears against a diaphragm  4  with a central orifice  5  which is designed to regulate the flow of fuel in the face of size variations produced during manufacture, according to the teaching of French patent 2 832 492 already cited at the start of this text. A bearing seat  6  for a primary valve  7  extends further downstream in the outer bushing  1 . A weak spring  28  biases the primary valve  7  against the seat  6  towards the upstream end of the injector. When the force of the spring  8  is overcome by the pressure of the fuel, the primary valve  7  moves downstream and allows the fuel to pass around it. 
     The outer bushing  1  also comprises an inner body  8  with a piercing  9  in which there slides a secondary valve  10  which is biased by a spring  11  towards the upstream end as far as a stop position, which can be produced by a collar  12  of the secondary valve  10  which is located at the downstream end and which bears against a corresponding seat portion of the inner body  8 . The secondary valve  10  comprises a central cavity  13 , although this does not pass all the way through it, and holes  14  of radial orientation which allow the central cavity  13  to communicate with the peripheral surface of the secondary valve  10  and which open in front of the wall of the piercing  9  of the inner body  8 . Slots  15  are cut on the secondary valve  10  so as to extend the holes  14  towards the downstream end in order to give the desired fuel flow in the secondary regime as a function of its pressure and the degree of closing of the secondary valve  10 . 
     The downstream end of the injector comprises a system of concentric conduits, the end of which penetrates into the outer bushing  1 . A downstream body  16  is housed therein and connects to the inner body  8  already mentioned. The downstream body  16  is pierced by a secondary fuel discharge conduit  17 , in the centre of which there is a primary fuel discharge conduit  18 . 
     After having passed the primary valve  7 , the fuel takes a primary flow path which passes around the primary valve  7  and then around the inner body  8 , into an annular slot  19  located between the inner body and the outer bushing  1 , and then around the downstream body  16  into an extension of this slot and finishing in the primary discharge conduit  18 . The fuel also flows around the secondary valve  10  and in its central cavity  13 , and it weighs against the secondary valve but does not displace it until it has reached a higher pressure. The state shown in  FIG. 2  is then reached: the holes  14  or the slots  15  meet the end of the piercing  9  of the inner body  10  and a secondary flow circuit is opened up which is established through the central cavity  13 , the holes  14  and the slots  15 , and a chamber  20  formed by the downstream body  16  below the inner body  8 ; the fuel finally reaches the secondary flow conduit  17 . 
     The characteristic elements of the invention will now be considered. An orifice  21  is pierced through the inner body  8  and extends from one of the holes  14  to the annular slot  19  in the state shown in  FIG. 1 . The primary fuel flow path therefore comprises a branching which passes through the central cavity  13  and the orifice  21  and gives rise to the desired enrichment in this regime. It must be emphasized that the orifice  21  opens immediately downstream of a solid portion of the secondary valve  10  in this position in which it rests against its seat. Thus, as shown clearly in  FIG. 2 , the orifice  21  is closed off by the secondary valve  10  as soon as it is displaced when the secondary flow is established, so that the enrichment then ceases. 
     Contrary to a previous design, the annular slot  19  is interrupted by a collar  23  of the inner body  8  which extends as far as the outer bushing  1 ; the primary fuel flow path is re-established by an orifice  22  passing through this collar  23  and joining the two portions of the annular slot  19 ; it is possible to calibrate said orifice to a very precise diameter, just like the orifice  21 , so as to perfectly control the primary fuel flow. 
       FIG. 3  shows the flow curve obtained as a function of the pressure of the fuel, with a first portion  24  representative of the flow rate in the primary regime obtained with the known injection, a second portion  25  representative of the primary flow obtained with the injector of the invention, and a portion  26  which is obtained in the secondary flow regime and which, according to the object of the invention, is identical for the new injector and for the old injector.