Abstract:
An igniter for a gas turbine engine. An igniter generates a plasma, or spark, somewhat similar to an automotive spark plug. In the invention, an auxiliary electrode is provided, which is embedded in and covered by a solid insulator. During initial phases of operation, no part of the plasma generated contacts the auxiliary electrode. However, eventually, the covering insulation is eroded by the plasma, and the auxiliary electrode becomes exposed and available as a return path for the plasma. The igniter is constructed so that, when this erosion occurs, the lifetime of the igniter is near its end. Detection of this erosion, as by detecting the new current in the auxiliary ground electrode, indicates the approaching end-of-lifetime.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Application is related to subject matter in the following patent applications, which are of common inventorship and filed concurrently herewith: 
        SENSOR FOR DETECTION OF SPARK IN IGNITER IN GAS TURBINE ENGINE, Ser. No. __/___,___;     METHOD OF INFORMING PILOT OF AIRCRAFT OF SPARK DETECTED IN GAS TURBINE ENGINE, Ser. No. __/___,___;     PASSIVE, HIGH-TEMPERATURE AMPLIFIER FOR AMPLIFYING SPARK SIGNALS DETECTED IN IGNITER IN GAS TURBINE ENGINE, Ser. No. __/___,___;     INTEGRAL SPARK DETECTOR IN FITTING WHICH SUPPORTS IGNITER IN GAS TURBINE ENGINE, Ser. No. __/___,___; and     DETECTING SPARK IN IGNITER OF GAS TURBINE ENGINE BY DETECTING SIGNALS IN GROUNDED RF SHIELDING, Ser. No. __/___,___.       
 
     
    
     FIELD OF THE INVENTION  
       [0007]     The invention relates to gas turbine engines, and igniters therein.  
       BACKGROUND OF THE INVENTION  
       [0008]     This Background will explain why the lack of absolute certainty in lifetimes of igniters used in gas turbine aircraft engines can impose significant costs on the owners of the aircraft utilizing the engines.  
         [0009]      FIG. 1  is a highly schematic illustration of a gas turbine engine  3 , containing a combustor  6 . Fuel  9  is sprayed into the combustor. An igniter  12 , which functions in a roughly analogous manner to a spark plug in an automobile, produces a spark, or plasma discharge (not shown), which initially ignites the jet fuel.  
         [0010]     After initial ignition, the igniter  12  can be repeatedly sparked thereafter, primarily as a safety measure. That is, in a modern engine, under normal circumstances, it is extremely unlikely for a flame-out to occur in the combustor  6 . However, unexpected situations, such as an abrupt cross-wind, can affect the environment within the combustor, and resulting loss of flame.  
         [0011]     In addition, certain flight conditions make the unlikely event of a flame-out slightly more probable. Thus, for example, the igniter  12  may be activated when the aircraft enters a rain squall, or other situation which may disturb steady-state conditions in the combustor  6 .  
         [0012]     The igniters  12 , like all mechanical components, have useful lives which eventually expire, at which time the igniters must be replaced. However, this expiration-and-replacement can create a situation in aircraft which is expensive.  
         [0013]     A primary reason is that the approach of an igniter to the end of its lifetime is not marked by readily detectable events. That is, at some point, the igniter completely ceases to generate a plasma, or spark. However, prior to that point, the igniter may sporadically generate sparks.  
         [0014]     As explained above, the sparking is not, in general, required to maintain the combustor flame. Consequently, the sporadic sparking would only be noticed if an actual flame-out occurred, and if the sporadic sparking were ineffective to induce a re-light. Since such a combination of events is seen as unlikely, the sporadic sparking is not readily noticed. The impending expiration of the useful life of the igniter is similarly not noticed.  
         [0015]     Another reason is that, while all igniters may be constructed as identically as possible, nevertheless, those igniters do not all possess the same lifetimes. Nor do all igniters experience identical events during their lifetimes. Thus, it is not known exactly when a given igniter will expire.  
         [0016]     Thus, the point in time when an igniter must be replaced is not known with certainty. One approach to solving this problem is to perform preventative maintenance, by replacing the igniters when they are still functioning. While the cost of a new igniter and the manpower required to install it is not great, the early replacement does impose another cost, which can be significant.  
         [0017]     The aircraft in which the igniter is being replaced represents a revenue source measured in thousands of dollars per hour. If the aircraft is rendered non-functional for, say, two hours during replacement of an igniter, the revenue lost during that time is substantial.  
         [0018]     Therefore, the uncertain lifetimes of igniters in gas turbine aircraft engines can impose significant losses in revenue.  
       SUMMARY OF THE INVENTION  
       [0019]     Normal operation of an igniter in a gas turbine engine causes erosion of an insulator inside the igniter. In one form of the invention, an auxiliary ground electrode is embedded within that insulator, and the erosion eventually exposes the auxiliary electrode. The igniter is designed so that the exposure occurs at the time when the igniter should be replaced.  
         [0020]     The exposed auxiliary ground electrode can be detected by the fact that, when a spark occurs, a small current travels through the auxiliary ground electrode. When that current is detected, its presence indicates the exposure. Alternately, the exposed auxiliary ground electrode can be visually detected by a human observer, perhaps by using a borescope. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a simplified schematic of a gas turbine engine.  
         [0022]      FIG. 2  illustrates an igniter  12 , shown in  FIG. 1 .  
         [0023]      FIGS. 3 and 4  are enlarged views of end E in  FIG. 2 .  
         [0024]      FIGS. 5 and 6  illustrate changes in geometry of end E which the Inventors have observed.  
         [0025]      FIG. 7  illustrates one form of the invention.  
         [0026]      FIGS. 8 and 9  are views resembling insert  84  in  FIG. 7 .  
         [0027]      FIG. 10  is a perspective view of part of  FIG. 7 .  
         [0028]      FIG. 11  is a perspective, cut-away view of one form of the invention.  
         [0029]      FIG. 12  is a cross-sectional view of the apparatus of  FIG. 11 .  
         [0030]      FIG. 13  is a perspective view of the apparatus of  FIG. 11 .  
         [0031]      FIG. 14  illustrates one form of the invention.  
         [0032]      FIG. 15  illustrates a sequence of events occurring in one form of the invention.  
         [0033]      FIG. 16  illustrates two distances D 9  and D 10 , over which two electric fields are generated.  
         [0034]      FIG. 17  illustrates one mode of constructing auxiliary electrode  72  in  FIG. 15 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]      FIG. 2  illustrates an igniter  12  used in the prior art. An electrical connector (not shown) is threaded onto threads  21 , and contains an electrical contact (not shown) which mates with the end  24  of electrode  27 . Insulator  30  isolates electrode  27  from the shell  33  of the igniter  12 .  
         [0036]     End E of the igniter  12  is shown in FIGS.  3  and  4 . A very simplified explanation of the physics involved in plasma generation will be given.  
         [0037]     In operation, a high voltage is applied to the electrode  27 , thereby creating a voltage difference, or potential difference, V between points P 1  and P 2  in  FIG. 3 . The electric field in that region equals the potential difference V divided by the distance D between the points P 1  and P 2 . For example, if the voltage is 20,000 volts and the distance D is 10 millimeters, or 0.01 meter, then the electric field equals 20,000/0.01, or 2 million volts per meter.  
         [0038]     The electric field is designed to exceed the dielectric breakdown strength of the material, or medium, lying between points P 1  and P 2 . That material is a mixture of air plus fuel. However, the field does not exceed the breakdown strength of insulator  30 , and that strength exceeds that of the air-fuel mixture.  
         [0039]     When breakdown occurs, the electric field strips electrons away from the atoms in the medium, producing positively charged ions and free electrons. The electric field drives the free electrons in a direction parallel with the electric field. However, during that movement, those temporarily free electrons will collide with other ions. Also, thermal motion of the ions and electrons will also bring them together in collisions.  
         [0040]     In the collisions, the electrons will be captured by the ions, and will drop to a lower energy state, releasing heat and light, in the form of an electric arc which is called a plasma, which is indicated as lightning bolt  40  in  FIG. 4 . This process continues as long as the electric field is present.  
         [0041]     The Inventors have observed one result of the operation just described. As indicated in  FIG. 5 , the insulator  30  becomes eroded from the phantom shape  50  to the curved shape  53 . In addition, the electrode  27  becomes eroded from the phantom shape  56  to the solid shape  59 . Corners  33 A also become eroded.  
         [0042]     The Inventors believe that one or more of the following agencies are responsible for the erosion. One agency is the corrosive nature of the plasma: free electrons are very reactive, and seek to bind to any available atoms or ions which are nearby. Also, the generation of free electrons from oxygen, which is present in the air, creates ionized oxygen, which is also highly reactive.  
         [0043]     A third agency is that the plasma creates a high-temperature environment. A high temperature, by definition, represents agitated atoms and molecules with high velocities. High-velocity atoms and molecules react more readily with stationary objects when they collide with the objects.  
         [0044]     Possibly a fourth agency is the fact that the plasma generates high-frequency photons, in the ultra-violet, UV, and perhaps into the X-ray regions of the spectrum. It is well known that UV and X-radiation can damage numerous types of material.  
         [0045]     Irrespective of the precise causes of the erosion, the erosion illustrated in  FIG. 5  eventually causes the igniter  12  to eventually stop functioning. A primary reason is illustrated in  FIG. 6 . Previously, prior to the erosion, voltage was applied between points P 1  and P 2  in  FIG. 6 . However, after the erosion, point P 2  has effectively moved to point P 3 . Distance D has now become longer distance D 2 . The electric field, which causes the ionization and thus the plasma, is now weaker.  
         [0046]     Continuing the example given above, if distance D 2  is 20 millimeters, then the electric field becomes 20,000/0.020, or one million volts per meter, half its original value. Eventually, distance D 2  becomes so great that the electric field does not reliably exceed the dielectric breakdown strength of the air-fuel mixture, and ionization ceases to occur.  
         [0047]      FIG. 7  illustrates one form of the invention. An auxiliary electrode  72  is embedded in the insulator  75 . The tip  78  is covered by the insulator-material in region  81 , as indicated by the insert  84 . Auxiliary electrode  72  may be connected to the shell  33 , as at region  90 .  
         [0048]     Initially, current enters electrode  27  as indicated by arrow  84 , jumps to the shell  33  through the plasma  85 , and exits the shell  33  into the engine, through multiple paths, such as through its mounting threads, as indicated by arrow  86 .  
         [0049]     As erosion occurs, the insulator  75  departs from its initial shape indicated by phantom lines  92  in  FIG. 8 . Tip  78  of the auxiliary electrode  72  now becomes exposed. Now, when a high voltage is applied to the igniter, two paths exist for a plasma to follow. One is the usual path P 5  in  FIG. 9 . The other path is indicated as P 6  of  FIG. 9 , and runs from the central electrode  27  to the now-exposed auxiliary electrode  72 .  
         [0050]     Restated, two current-return-paths are available to the central electrode  72 . Path P 5  runs to the shell  33 , in the usual manner. Path P 6  runs to the now-exposed auxiliary electrode  72 . Eventually, further erosion will lengthen path P 5 , and cause plasma formation along that path to terminate. That is, path P 5  in  FIG. 9  initially can be represented by distance D in  FIG. 6 . After sufficient erosion, path P 5  in  FIG. 9  will be represented by distance D 2  in  FIG. 6 , and, as explained above, no plasma will be generated along path P 5  when distance D 2  becomes sufficiently large.  
         [0051]     However, auxiliary plasma path P 6  is still available in  FIG. 9  at this time. A plasma can still be generated, and the lifetime of the igniter has been increased.  
         [0052]     The preceding discussion presented the auxiliary electrode  72  in  FIG. 7  in the form of a rod.  FIG. 10  illustrates such a rod in perspective view, surrounded by insulator  75 .  
         [0053]     In an alternate embodiment, a cylinder is used.  FIG. 11  is a cut-away view of one embodiment. Central electrode  27  is surrounded by an insulator  100 , which itself is surrounded by a conductive tube or cylinder  103 , which is then surrounded by another layer of insulator  105 .  FIG. 12  illustrates the system in cross-sectional view, with similar numbering.  
         [0054]      FIG. 13  illustrates the insulator  100  in its initial configuration, after manufacture or just after installation. A tip  110  of central electrode  27  is exposed, and surrounded by the conical surface  113  of the insulator  100 . Cylindrical auxiliary electrode  103  is embedded within the insulator  100 , and no tip or edge is exposed, as indicated by distance D 8  in  FIG. 12 .  
         [0055]     The preceding discussion stated that the auxiliary electrode  72  may be connected at region  90  in  FIG. 7 . In another embodiment, the auxiliary electrode  72  of  FIG. 14  is also connected to ground, but through a detector  150 . Detector  150  looks for a current in auxiliary electrode  72 . Current detectors are well known.  
         [0056]     If no current is detected, it is inferred that the auxiliary electrode  72  is still embedded within insulator  75 , as in  FIG. 7 , and is electrically isolated from central electrode  27 .  
         [0057]     In contrast, if a current is detected, it is inferred that the auxiliary electrode has become exposed through erosion, as in  FIG. 9 . The detected current is attributed to a plasma following path P 6 . When the current is detected, detector  150  issues a signal, sets a flag, or otherwise indicates the inference that erosion has exposed auxiliary electrode. A human technician at that time, or a prescribed time afterward, replaces the igniter.  
         [0058]     An alternate mode of detection is to remove the igniter and visually examine the end corresponding to end E in  FIG. 2 . If a smooth surface of the insulator  100  is seen, as in  FIG. 13 , then it is concluded that the igniter is still functional. However, if the auxiliary electrode  72  is seen, as in  FIG. 8 , then it is concluded that replacement may be required.  
         [0059]     In another embodiment, the auxiliary electrode is designed to become exposed, and then to erode rapidly.  FIG. 15 , viewed left-to-right, illustrates first a newly installed igniter  160 . After a period of usage, igniter  165  exposes its auxiliary electrode  72 . Now a plasma P 6  extends to the auxiliary electrode  72 .  
         [0060]     However, as stated above, the auxiliary electrode  72  is designed to erode rapidly. For example, as insert  170  indicates, the auxiliary electrode  72  is fabricated with a pointed end. Plasma  6  causes the pointed end to become rapidly eroded, as indicated by the small particles in frame  170 . This operation causes a specific sequence of two events.  
         [0061]     One is that, when the auxiliary electrode becomes first exposed, a current passes through the it. The current is detected, as by detector  150  in  FIG. 14 . Next, after the auxiliary electrode fractures or erodes, no current passes through it.  
         [0062]     One reason for this sequence is illustrated in  FIG. 16 . Initially, the voltage V spans distance D 9 , creating an electric field equal to V/D 9 . After fracture or erosion, the same voltage V spans distance D 10 . The electric field equals V/D 10 , a smaller value. The latter electric field is insufficient to create a plasma, while the former is.  
         [0063]     In one embodiment, the occurrence of the two events just described occurs prior to the termination of the lifetime of the igniter. Thus, that termination is signalled by the occurrence of a current through the auxiliary electrode  72 , followed by a termination of that current. The onset of the current indicates the approach of the termination of the lifetime, but with time remaining to operate the engine. The subsequent termination of the current indicates that less time remains, and that replacement of the igniter becomes more important.  
         [0064]      FIG. 17  illustrates one embodiment of the auxiliary electrode  72 . A neck, or groove,  190  is provided, which facilitates the breakage schematically illustrated in the insert  170  in  FIG. 15 . The groove  190  is a region of mechanical weakness intentionally built into the auxiliary electrode  72 . Prior to the erosion indicated in  FIG. 8 , that weakness is not important, because mechanical support to the electrode is supplied by the insulator  75 .  
         [0065]     The discussion above stated that a high voltage is applied to electrode  27 . It is possible that a low voltage applied to the electrode  27  can accomplish the same function of generating a plasma.  
         [0066]     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.