Abstract:
A variable arc gap plasma igniter element includes electrodes moveable relative to each other. The electrodes are preferably set to define a smaller air gap to initiate a plasma arc and later extended to obtain longer plasma arc.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ignition system and more particularly to a continuous plasma ignition system for a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     An ignition system for a gas turbine engine may include a plasma igniter. A plasma arc is generated across an air gap between two electrodes to light fuel in a combustion chamber. The size of the air gap between the two electrodes is a problematic in igniter design. Larger air gaps provide plasma arcs with higher energy but require higher breakdown voltages which can lead to failure in other locations such as at lead connections. Smaller gaps are subject to short circuit if carbon accumulates in the gap. 
     Therefore, there is a need for an improved plasma igniters. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an improved plasma igniter for gas turbine engines. 
     In accordance with one aspect of the present invention, there is a variable arc gap plasma igniter element which comprises a first electrode and a second electrode defining a gap therebetween, adapted to generate a plasma arc extending through the gap when an electric voltage is applied across the electrodes. There are means provided for moving the second electrode relative to the first electrode during arcing, from a first position to a second position. The second position increases the gap size relative to the first position. 
     In accordance with another aspect of the present invention, there is a variable arc gap plasma igniter element provided for gas turbine engines, which comprises a first electrode having an end exposed to a cavity, a second electrode moveable relative to the first electrode and spaced apart therefrom to define a variable-sized gap between the end of the first electrode and an end of the second electrode, and an apparatus adapted to move at least the second electrode to thereby vary the arc size during arcing. 
     In accordance with a further aspect of the present invention, there is a method provided for operating a variable arc gap plasma igniter element for gas turbine engines, which comprises setting first and second electrodes of a plasma igniter element in a close relationship to define a small arc gap therebetween, applying an electric voltage across the electrodes to initiate a plasma arc across the arc gap, increasing the arc gap size, and injecting a fuel flow adjacent the plasma arc to ignite the fuel. 
     The present invention advantageously provides extremely long plasma arcs with relatively low breakdown voltages, thereby reducing failure modes. This and other features and advantages of the present invention will be better understood with reference to that which is described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine, as an example showing an application of the present invention; 
         FIG. 2  is a schematic cross-sectional view of a plasma igniter element having a variable arc gap according to one embodiment of the present invention; 
         FIG. 3  is a schematic cross-sectional view of a plasma igniter element in a first phase for plasma arc ignition according to another embodiment of the present invention; 
         FIG. 4  is a schematic cross-sectional view, showing the plasma igniter element in a second phase to initiate the torch ignition process according to the embodiment of the present invention illustrated in  FIG. 3 ; and 
         FIG. 5  is a schematic cross-sectional view, showing the plasma igniter element in a third phase to withdraw the moving electrode from the combustion area according to the embodiment of the present invention illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A typical application of the present invention for turbofan engines illustrated schematically in  FIG. 1 , incorporates an embodiment of the present invention presented as an example of the application of the present invention, and includes a housing or nacelle  10 , a low pressure spool assembly seen generally at  12  which includes a fan  14 , low pressure compressor  16  and low pressure turbine  18 , a high pressure spool assembly seen generally at  20  which includes a high pressure compressor  22  and a high pressure turbine  24 . There is provided a burner seen generally at  25  which includes an annular combustor  26  and a plurality of fuel injectors  28  for mixing liquid fuel with air and injecting the mixed fuel/air flow into the annular combustor  26  for combustion. 
     A continuous plasma ignition system generally indicated by numeral  32  is provided in one location of the annular combustor  26  downstream of one of the fuel injectors  28 , for initiating a torch ignition process to start the combustion process. The continuous plasma ignition system  32  according to the present invention is adapted to vary the air gap between the electrodes in order to change the energy level of the generated plasma arc therebetween. 
     It should be noted that similar components of the different embodiments shown in  FIGS. 2-5  are indicated by similar numerals for convenience of description of the present invention. Only those components different in one embodiment from the other will be separately described with reference to additional numerals. 
     Referring to  FIG. 2 , a continuous plasma ignition system  32  includes an igniter body  34  defining a cavity  36  therein. The cavity  36  is in fluid communication with a chamber of the annular combustor  26  of  FIG. 1 , or may be a part of the chamber of the annular combustor  26  when the igniter body  34  is a structural part of the combustor  26 . One fuel injector  28  is adapted to inject fuel into the cavity  36 . The fuel injector  28  is preferably affixed to the igniter body  34  but can also be otherwise attached to other structures of the engine, provided that the fuel injector  28  can inject fuel into the cavity  36  to a predetermined location. 
     A spark plug  38  which is connected to an electrical high voltage source such as an exciter, is affixed to the igniter body  34  at one side thereof. The spark plug  38  includes an electrode  40  having an end  42  thereof exposed to the cavity  36 . 
     A cylinder  44  is affixed to the igniter body  34  at an opposed side thereof, accommodating a slidable piston  46  therein. A solenoid  48  is attached to the cylinder  44  either inside thereof (as shown in this embodiment) or outside thereof. Piston  46  is connected at one side thereof to the solenoid  48  by a piston rod  50 . A spring  52  is provided within the cylinder  44 . The spring  52  is attached at one end thereof to the piston  46  and the other end is affixed to the cylinder  44  such that the piston  46 , when moving leftwards or rightwards, will cause the spring  52  to be pulled or pressed. An electrode  54  is attached to the piston at the opposed side thereof and extends across the cavity  36  towards electrode  40 . The electrode  54  is grounded in an electric circuit (not shown) supplying electric voltage to the spark plug  38 . The cylinder  44  is substantially aligned with the plug  38  therefore the electrodes  40  and  54  are aligned with each other. 
     The electrode  54 , piston  46 , piston rod  50  and the solenoid  48  are dimensioned and positioned to achieve the following operation of electrode  54 . In a first position, the solenoid  48  is activated, and moves the combination of electrode  54 , piston  46  and piston rod  50  towards the electrode  40  until an end  56  of the electrode  54  is in a proximity of the end  42  of the electrode  40  (the two ends of the respective electrodes preferably almost touch each other). Therefore, the air gap (not indicated) formed between the two ends  42  and  56  of the respective electrodes  40 ,  54  has a reduced resistance which requires a relatively low breakdown voltage to be applied to the spark plug  38  in order to initiate a plasma arc to extend through the air gap. 
     When the plasma arc is initiated, the solenoid  48  is deactivated. Because the spring  52  was pulled to extend when the piston  46  was driven leftwards to the first position by the solenoid  48 , the resilient force of the extended spring  52  now pulls the combination of electrode  54 , piston  46  and piston rod  50  to move rightwards, back to a neutral position which is referred to as a second position. During the movement of the electrode  54  from the first position to the second position, the plasma arc across the minimum air gap between the ends  42 ,  56  when the electrode  54  is in the first position, will follow the movement of the electrode  54  and extend because the initial ionization path established across the air gap between the ends  42  and  56  is still the preferred electrical route. In this manner, extremely long plasma arcs that would normally require extremely high breakdown voltages can be established using lower breakdown voltage levels. As the air gap between the end  42  and  56  of the respective electrodes  40 ,  54  increases, the resistance also increases. Therefore, the longer plasma arc extending through the air gap carries a high level electric energy compared to the initial plasma arc across the minimum air gap between the ends  42 ,  56  when the electrode  54  is in the first position. 
     Adequately determining the neutral position of the spring  52  (the second position of the electrode  54 ) can achieve generation of a plasma arc between the ends  42 ,  56  of the respective electrodes  40 ,  54  with a desired electric energy level for initiating a torch ignition process. The resilient properties of the spring  52  should also be adequately determined in order to assure controllable movement of the electrode  54  such that the plasma arc will follow and is maintained during the movement. 
     When the electrode  54  is in the second position and a longer plasma arc carrying a high electric energy level is established, the fuel injector  28  injects fuel into the cavity  36 . The injected fuel is lit by the plasma arc extending between the ends  42 ,  56  of the respective electrodes  40 ,  54 , thereby initiating a torch ignition process. Once the torch ignition process is initiated, all fuel injectors  28  of  FIG. 1  inject fuel continuously into the annular combustor  26  to start and maintain a combustion process. When the ambient air temperature and pressure increase to a certain level during the combustion process, the piston  46  is moved by hot air pressure further rightwards against the resilient force of the spring  52 , thereby moving the electrode  54  away from the combustion area in the cavity  36 . When the engine stops operation and the hot air pressure within the cavity  36  no longer exists, the spring  52  under its resilient force, moves from the compressed condition to regain the neutral position thereof, thereby moving the combination of the electrode  54 , piston  46  and the piston rod  50 , back to the second position thereof. 
     Referring to  FIGS. 3-5 , the continuous plasma ignition system  32 ′ according to another embodiment of the present invention incorporates a cooling system (not indicated) thereinto. Similar to the embodiment of  FIG. 2 , the system  32 ′ includes the igniter body  34  which defines the cavity  36  in a middle portion thereof. The cavity  36  further includes a ceramic liner  58  attached to the inner surface thereof. The body  34  further defines a cylindrical chamber  60  at one side thereof and a cylindrical chamber  62  at an opposed side thereof. A spark plug  38  affixed within the cylindrical chamber  60  includes the electrode  40 , and is electrically connected to a high voltage source. The cylindrical chamber  62  receives the piston member  46  slidably movable therein. The solenoid  48  is affixed to the igniter body  34  at the outside of the cylindrical chamber  62 . 
     The piston rod  50  extends through the solenoid  48  and is attached at an end opposed of the piston  46 , to an end member  64 . The piston rod  50  can be actuated to move to and remain in the first position as shown in  FIG. 3 , when the solenoid  48  is activated. In this position, the end member  64  compresses the spring  52  to store a resilient energy therewith. The spring  52  is affixed at one end thereof to the igniter body  34  and connected at the other end thereof to the end member  64 . The piston rod  50  is free to move through the solenoid  48  from the first position of  FIG. 3  to the second position of  FIG. 4  when the solenoid  48  is deactivated and the spring  52  returns to its neutral position, thereby moving the end member  64  outwards. 
     The piston rod  50  can be further moved through the solenoid  48  from the second position of  FIG. 4  to a third position shown in  FIG. 5  when the solenoid  48  is deactivated and the piston  46  is pressed by an air pressure differential which will be further discussed below. In this third position, the spring  52  is forced to be extended thereby storing energy in a resilient deformation thereof. When the air pressure differential does not exist, the stored energy of the extended spring  52  will pull the end member  64  and thus the entire combination of electrode  54 , piston  46  and piston rod  50 , back to the second position of  FIG. 4 . The electrode  54  connected to the piston  46  thus has three operative positions, which will be further described with reference to an ignition sequence below. 
     A cooling air circuit (not shown) is provided to the continuous plasma ignition system  32  and is preferably connected to a pressure air source such as the compressor air of the engine. A fluid passage  66  is provided in the igniter body  34 , for fluid communication between the cylindrical chamber  60  and the cooling air circuit. Fluid passages  68  and  70  are provided in the igniter body  34  for fluid communication between the cylindrical chamber  62  and the cooling air circuit. The fluid passages  68 ,  70  are positioned both at the righthand side of the piston  46  in the first position as shown in  FIG. 3 , and positioned at the opposite sides of the piston  46  when in the second and third positions as shown in  FIGS. 4 and 5 . The electrode  40  with the spark plug  38 , is affixed within the cylindrical chamber  60  of the igniter body  34  and is positioned such that the end  42  of the electrode  40  is exposed to the cavity  36  but does not protrude thereinto, in order to reduce the potential damage of the end  42  of the electrode  40  caused by the high temperature of combustion in the cavity  36 . 
     The electrode  54  with its associated components is designed to meet the following requirements: in the first position as shown in  FIG. 3 , the end  56  of the electrode  54  is in close proximity with the end  42  of electrode  40 ; in the second position as shown in  FIG. 4 , the end  56  of the electrode  54  is located in a predetermined position for generating a plasma arc between the two electrodes  40 ,  54 , having an electric energy level predetermined for initiating a torch ignition process; and in the third position as shown in  FIG. 5 , the electrode  54  is withdrawn from the cavity  36 , and the end  56  of electrode  54  does not protrude into the cavity  36 . 
     The ignition sequence of a gas turbine engine using the continuous plasma ignition system  32 ′ includes three phases. At the initiation of the ignition sequence which is the first phase, the solenoid  48  is activated thereby moving the combination of the electrode  54 , piston  46 , piston rod  50  and end member  64  against the spring  52  inwardly towards the fixed high voltage electrode  40  until the ends  42 ,  56  of the respective electrodes  40 ,  54  almost touch each other. This results in a dependable low power ionization path development across the electrodes  40 ,  54  when a relatively low breakdown voltage is applied over the electrodes, regardless of the insulation around the cavity  36  or the electrodes  40 ,  54 . An initial plasma arc is thus generated, as shown in  FIG. 3 . 
     When the initial plasma arc is generated the ignition sequence enters the second phase as shown in  FIG. 4 . In this second phase, the solenoid  48  is deactivated and the retracting forces of the compressed spring  48 , pulls the ground connected electrode  54  away from the high voltage electrode  40  with the initial plasma arc following the movement of the electrode  54 . The resistance of the air gap between the ends  42 ,  56  of the respective electrodes  40 ,  54  increases as does the electric energy input into the extending plasma arc. The length of the air gap may be much longer than could be crossed by a plasma arc from a static condition, because the ionization path developed in the initial small air gap helps establish the initial arc and promotes its further growth. Once the spring  52  reaches its neutral position, the electrode  54  reaches a predetermined position at which the working air gap is set. Fuel is then injected from the fuel injector  28  into the plasma arc in order to initiate the torch ignition process. Once the torch ignition is initiated, a stable combustion process is started and maintained in a combustion area  72  within the cavity  36 , provided that all fuel injectors  28  of the engine continuously inject fuel into the annular combustor  26  of  FIG. 1 . 
     The engine ignition sequence is part of an engine starting process. Prior to and during the first and second phases of the engine ignition sequence as shown in  FIGS. 3 and 4 , the engine high pressure compressor  22  of  FIG. 1  is rotated by a starter (not shown). Therefore pressure air is generated and introduced through the fluid passage  66 ,  68  into the respective cylindrical chambers  60 ,  62  to cool the respective electrodes  40  and  54 . In the first phase as shown in  FIG. 3 , the pressure air enters the cylindrical chamber  62  from the fluid passage  68  and exits from the fluid passage  70 , having little pressure effect on the piston  46 . In the second phase of the ignition sequence as shown in  FIG. 4 , piston  64  moves to the middle of the cylindrical chamber  62 , thereby blocking the cooling air path from the passage  68  to  70  through the cylindrical chamber  62 . Thus, the pressure air entering the left side of the cylindrical chamber  62  through the fluid passage  68  builds up a pressure differential over the opposed sides of the piston  46 . Nevertheless, at this stage, the high pressure compressor  22  of  FIG. 1  is driven by an engine starter at a limited speed and cannot generate high pressure air. Therefore, the air pressure differential over the opposed sides of piston  46  is not enough to significantly move the piston  46  outwardly against the resilient force of the spring  52 . 
     Once the torch ignition is initiated and the combustion process is started, the electrical voltage applied over the electrodes  40 ,  54  is withdrawn and no plasma arc further exists between the ends  42 ,  56  of the respective electrodes  40 ,  54 . In the third phase of the ignition sequence as shown in  FIG. 5 , the combustion in the annular combustor  26  of the engine of  FIG. 1  is stable and the engine reaches a certain power level, which results in the capability of the high pressure compressor  22  of  FIG. 1  to generate compressor air at a predetermined pressure level. At this stage, the air pressure differential built over the opposed sides of the piston  46  is enough to overcome the resilient forces of the spring  52 , thereby moving the piston  46  to the third position as shown in  FIG. 5 . In this position the piston  46  abuts a stop shoulder (not indicated) of the cylindrical chamber  62  and the end  56  of the electrode  54  does not protrude into the cavity  36 , thereby being protected from the high temperature of the combustion area  72  within the cavity  36 . In this way, both electrodes  40 ,  54  are withdrawn from direct exposure to fuel and combustion gases, thereby increasing the life of the electrodes. The electrode  54  remains withdrawn until the air pressure differential over the opposed sides of the piston  46  falls and the extended spring  52  returns the combination of the electrode  54 , piston  46 , piston rod  50 , back to the neutral position which is the second position shown in  FIG. 4 . 
     It is preferable to include in the electric circuit of the continuous plasma ignition system  32 ′, means (not shown) for detecting the absence of electrical current in the ground circuit resulting from plasma arc process failure due to electrode deterioration. When such a situation is detected a warning signal is generated and sent to the engine display panel. 
     The present invention advantageously provides the apparatus for a standard method for a continuous plasma ignition system for gas turbine engines, which requires lower electrical insulation for dependable plasma arc initiation, and which provides a, higher power plasma arc than the conventionally available plasma arc from conventional static electrodes. In accordance with the present invention, a variable arc gap plasma ignition system can be operated under severe operative conditions. For example, a plasma arc can be initiated with adequate breakdown voltages even when the air gap is flooded with liquid fuel or water because the flooded gap can be adjusted to a minimum to reduce the resistance between the electrodes. The present invention further advantageously provides longer electrode life for plasma ignition systems. 
     By moving the electrodes close together to initiate the process, the minimum air gap becomes the lowest resistance path and an initial arc will arise there, even under conditions which would cause plasma arc initiation failure of conventional plasma igniters, as discussed in the background of the invention. Once the initial plasma arc is started, the electrodes move away from each other and the plasma arc will follow because the initial ionization path is still the preferred electrical route. 
     It should be noted that the above-described embodiments are merely part of an ignition system of gas turbine engines, and especially addresses the problem associated with the failure to ignite a plasma arc between electrodes in those systems. Therefore, the present invention is applicable to any continuous plasma ignition system, and is not limited to the above-described embodiments. The present invention is also applicable to any type of gas turbine engine, not being limited to the turbofan engine taken as an example to illustrate the application of the present invention. The particular motive and biasing systems disclosed for moving the electrode(s) are but of a multitude of possibilities which will become apparent to the skilled reader, and thus is intended to be merely exemplary, and the motive and biasing means need not be separated, either. Likewise the cooling system disclosed is merely one of many possibilities now within the ordinary skill in the art in light of this description. Although described as 3 distinct phases, it will be understood that the phases may overlap or occur more or less at the same time. For example, the second and third phases ( FIGS. 4 and 5  may be integrated into a single step, such that the second phase is rather an element of phase three. Although the embodiments described include a fixed electrode and a moveable electrode, both may be moved if desired. 
     Still other modifications to the above-described embodiments of the present invention will be apparent to those skilled in the art without departing form the principles disclosed. For example, the solenoid and spring in the above-described embodiments can be replaced by linear actuators of any type, such as a linear electric motor, linear hydraulic motor, or a motor with gears and racks, etc. Therefore, the foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.