Plasma ignition and combustion assist system for gas turbine engines

An ignition and combustion assist system and method comprising a plasma igniter and electronic driver unit for use with gas turbine engines operating under low air densities, reduced voltage conditions and overall pressure ratios of 3:1 to 7:1. The plasma igniter has an inner chamber housing a centrally positioned and electrically isolated electrode attached to an electrical lead, driver unit, and AC or DC power supply. The electrode features a corner positioned near an outlet end of the igniter, where a plasma arc ignites a fuel-air mixture creating a flame extending into a primary burn region of a combustor of the gas turbine. The driver unit is in two embodiments and configured with low-cost microsecond voltage wave time periods or energy-efficient nano-second pulses. The method uses the plasma igniter and the electronic driver units described herein separately with other components or together.

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE EFS WEB SYSTEM

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to ignition systems for turbine engines. More particularly, the invention is a plasma ignition system with an electronic driver unit and one or more igniter components for use with a variety of gas turbine engine applications and particularly with miniature turbojets and miniature high-speed turbo generators with relatively low pressure ratios from 3:1 to 7:1 overall.

Background Art

All combustion engines have an air-fuel mixture inside the combustor that is ignited, the hot air generated in the combustor used to then turn blades of a turbine, piston, etc. Current ignition systems such as spark igniters rely on multiple factors for combustion to take place: stoichiometry, gas pressure, timing of the spark generated, and the voltage applied to create a sufficiently large spark must all be carefully calculated and calibrated to maximize performance and reliability.

One problem with current igniter systems is the fact that they are generally unsuitable for miniature lightweight engines, defined as those engines with diameters less than 16 inches and weights of less than 25 lbm, such as those used with lightweight drones, miniature missiles, airborne jamming devices, etc. as the cost, size (exciter electronics larger than a 3 inch cube) and weight (more than 1.0 lbm) of current igniter systems render them unsuitable. Smaller applications such as drones ideally need lighter and lower power components and accordingly favor ignition systems that are simple, lightweight, with relight/restart capability and are flexible according to the specific application needs.

The need for improved combustion system operability (stability over a wider range of fuel/air mixtures, at higher flame strain rates and shorter residence times), as well as limitations of prior art igniter systems have created intense interest in research using plasma igniters and suitable applications of this technology, including use with internal combustion engine and dual fuel engines. For small gas turbines such as those used in drones, and which operate with under low pressure ratios and low combustor inlet temperatures the performance of current igniter systems is lacking. The cost and limited performance of traditional spark ignition systems and pyrotechnic flare igniters have resulted in the need for a better system for achieving light-off, including re-light, with low-cost components.

What is needed is a plasma ignition and combustion assist system comprising a plasma igniter and an electronic drive unit that can be used with small gas turbine engines and a method of using a plasma igniter system having an ability to choose between more energy efficient applications and those that are more cost effective. What is also needed is a plasma igniter that generates a continuous electrical arc. What is also needed is an ignition system that can operate at lower voltage levels than conventional spark systems and yet deliver a higher energy output to the combustor. What is also needed is a plasma igniter that extends the plasma arc into the combustor's primary burn area, improving fuel burning efficiency. What is also still needed is an ignition system that has multiple re-light capability.

DISCLOSURE OF THE INVENTION

A plasma ignition and combustion assist system and method of using a plasma igniter and an electronic driver unit with a gas turbine engine operating under low temperature, low pressure ratios, and other conditions inappropriate for conventional spark systems. The plasma igniter and the electronic driver unit are lightweight and appropriate for use in drones, and other applications that require multiple re-light capability, as well as low-cost and higher efficiency options. The plasma igniter and the electronic driver unit in the method can be used together or separately with other igniter and driver units.

In a first aspect of the plasma ignition system, the plasma igniter is comprised of a substantially cylindrical igniter body having a lead end and an outlet end, with an inner wall defining a chamber between the ends. An electrode having a proximal end and at least one of a conical and cylindrical distal end is housed centrally inside the chamber so as to be electrically isolated from the inner wall, forming an approximately annular air gap within the chamber around the electrode. The igniter body is electrically grounded to the combustor, or directly to the igniter driver electronics via an insulated wire. A diameter of the electrode is between about 0.125 and 2.0 inches. The electrode distal end is positioned towards the outlet end of the igniter body and is further formed with at least one corner having a corner radius ranging from zero to 0.15 inches. The corner in some embodiments is configured as a projection. An arc gap from the corner to the inner wall of the igniter body ranges from a shortest or smallest distance from the corner to the inner wall to a shortest or smallest distance measured to the inner wall at the outlet end. In some embodiments, the arc gap is between about 0.125 inches to about 0.75 inches, and in other embodiments, the arc gap measures between about 0.04 and 0.5 inches. An electrical lead connects to the electrode to the driver unit and power supply. An air feed through-hole in the igniter body allows air flow into the air gap and exit the outlet end, forcing a plasma arc generated at the arc gap into the primary burn region of a combustor of the gas turbine engine. In some embodiments, the air feed through-hole is sized and shaped to support an air injection velocity ranging from about 50 to 300 ft/sec.

In another aspect of the plasma igniter, at least one of a fuel feed port, which may be a simple orifice, and a fuel feed port and a fuel atomizing injector integral with the igniter body is included. In some embodiments, the fuel feed port is sized and shaped to control at least one of a fuel velocity of a quantity of fuel entering the arc gap ranging from about 5 to 300 ft/sec and an inlet pressure ranging from 2.5 psia to 100 psia. In other embodiments, the quantity of fuel entering the annular arc gap enters as fuel droplets with a mean diameter greater than 80 microns.

In yet another aspect of the plasma igniter, the air feed through-hole is positioned between the insulator and the outlet end of the igniter body, whereby air flow entering the air gap through the air feed through-hole forces an arc generated within the igniter body into the primary burn region of the combustor.

In yet another aspect of the plasma igniter has an igniter body selected from the group of igniter bodies including an extended length igniter body and a truncated igniter body.

In another aspect of the plasma igniter system, the driver unit comprises an input power controller, a voltage oscillator communicating with the input power controller, a transformer communicating with the voltage oscillator and the input power controller, an on-off switch communicating with the input power controller, and a power source providing at least one of alternating and direct current input to the driver unit. The driver unit provides an output of voltage and current to the electrode and is grounded to the engine or to the combustor. The input power is regulated, filtered and modulated by the input power controller. The voltage oscillator creates an electrical output waveform at a desired frequency and level. The transformer transforms the electrical output waveform generated by the voltage oscillator and generates a voltage level and voltage rate of change sufficient to create an electric arc.

In yet another aspect of the driver unit, the voltage and current supplied to the electrode are transient and a voltage wave time period is measured in at least one of nano-second pulses and micro-second pulses in a repetitive cycle.

In yet another aspect of the driver unit, the oscillating voltage output levels at the electrode range between about 250 Vrms to 7000 Vrms.

In still yet another aspect of the driver unit, the direct current power source with a voltage level between 10 Vdc and 120 Vdc to the driver unit provides current to a circuit generating a variable or constant frequency voltage wave at about 10 kHz to 10000 kHz.

In yet another aspect of the driver unit, the input power controller is at least one of a passive circuit with a single state for input and output and a voltage and current regulation system.

In yet another aspect of the driver unit, a voltage level increase of 100 to 1000 times the input voltage via a voltage transformer is produced by either an inductive electrical coil or a set of energy storage capacitors to achieve the oscillating voltage increase.

In a method of using the plasma igniter system having a plasma igniter and an electronic driver unit, the method comprises the steps of determining at least one of a desired size and weight of a plasma igniter based on engine size, space availability or kinetic application, determining a desired igniter electrode operating life, determining a desired power efficiency of the plasma igniter system, maintaining a power source compatibility of the plasma igniter system with that of the engine, determining engine pressure ratios, and determining whether the plasma igniter and driver unit will be operational only at initial ignition and start of the engine or at multiple times after initial ignition and start of the engine.

In another aspect of the method, the step of determining engine pressure ratios further comprises the steps of identifying engines having low pressure ratios between 3:1 to 7:1, small volumetric flow rates below 15 msec, and operating at temperatures below 400 Fahrenheit, and selecting electronic driver units with voltage outputs appropriate for at least one of the respective pressure ratios and volumetric flow rates, after the step of determining engine pressure ratios.

In yet another aspect of the method, the step of selecting electronic driver units further comprises the steps of sizing the arc gap in accordance to increased voltage requirements.

In still yet another aspect of the method, the method is used with a turbojet with thrust ranging from about 15 to 600 lbf.

In yet another aspect of the method, the method is used with a turbo-generator having a 5 to 100 kW electrical power output.

In yet another aspect of the method, the method is further comprised of the steps of operating the plasma igniter to sustain combustion or increase combustion efficiency when conditions where mixing and reaction times are short or where the fuel-air mixture in the combustor burn zone is outside conventional lean and rich flammability limits, after the step of determining whether the plasma ignition system will be operational at initial ignition and start of the engine only, or multiple times after initial ignition and start.

DRAWINGS LIST OF REFERENCE NUMERALS

The following is a list of reference labels used in the drawings to label components of different embodiments of the invention, and the names of the indicated components.100plasma igniter100atruncated body igniter100bextended body igniter10igniter body10aoutlet end10blead end10couter wall12retention cap10emount16insulator18lead wire retention material20electrical lead20apower end of electrical lead20belectrode end of electrical lead22crimp or braze joint or solder24electrode24adistal end of electrode24bproximal end of electrode24cterminal end or vertex of electrode24dcorner26arc gap28inner wall of chamber of igniter body30air gap32air feed hole34fuel feed hole36fuel feed line40combustor42primary burn region of combustor44combustor recirculation zone50electronic drive unit or electronic driver unit or driver unit or drive unit or driver or unit52on/off trigger54input power (AC or DC)56input power controller58zero-voltage switching block60flyback transformer62high voltage output64voltage oscillator and transformer block70flame

DETAILED DESCRIPTION

A plasma ignition and combustion assist system for use with a gas turbine engine is comprised of a plasma igniter100in two embodiments, shown inFIGS.1-10and an electronic driver unit or drive unit or driver unit50, in two embodiments, shown inFIGS.11-13.

The plasma igniter100is comprised of an igniter body10defined by an outer wall10cand a pair of opposed open ends. The igniter body10is grounded to either the engine or a combustor40of the gas turbine engine. The igniter body10has an inner wall28defining an approximately cylindrical inner chamber having a lead end10benclosed by a retention cap12positioned over the lead end10band an opposed outlet end10a. The retention cap12is formed with a hole sized and shaped to receive an electrical lead20.

An electrode24having an approximately cylindrical shape, with a proximal end24band a distal end24a, is connected at the proximal end24bto an electrode end20bof the electrical lead20by a solder or braze or crimp joint22, with the joined electrode-electrical lead positioned inside the chamber through the hole in the retention cap12. Note that the crimp joint22includes any other suitable connection and use of the term “crimp joint” is not meant to be limiting. The electrode24and electrical lead20are positioned centrally within the chamber and electrically isolated from the inner wall28of the igniter body10by a lead wire retention material or retention material18, typically a quantity of potting or solder sandwiched between the inner wall28and the crimp joint22, with a position of the electrical lead20secured by the retention material18around a perimeter of the electrical lead20and attached at one end to an interior side of the retention cap12. An approximately annular air gap30is thus created between the inner wall28and the electrode24positioned centrally within the chamber. One or more through-holes or air feed holes32are formed into the igniter body wall leading from outside the igniter body10and into the air gap30. A steady quantity of air generated by an air compressor is fed into the air gap30through the air feed hole32. In some embodiments, an alternative or additional fuel feed port34is also formed into the igniter body10and leading into the air gap30. The air feed hole32and the fuel feed port34are typically formed near the proximal end24bof the electrode24however they may in fact be positioned anywhere between the outlet end10aand the insulator16inside the chamber of the igniter body10. The fuel feed port34may be configured as a simple orifice or may include with the port34a fuel atomizing injector integral with the igniter body10. If the fuel feed port34is present, a fuel feed line36supplying fuel into the air gap30is affixed to the fuel feed port34. A power end20aof the electrical lead is attached to a power supply input54for supplying power to the igniter100.

FIG.1shows a truncated body embodiment100aof the igniter100, with the distal end24aextending beyond the outlet end10aand into a primary burn region42of the combustor40. The distal end24ais generally conical in shape, with a terminal end or vertex24cpositioned beyond the outlet end10aof the igniter body10. A base of the distal end24ais further formed with a corner24dhaving a radius between zero and 0.15 inches, with a smaller radius being preferable. The inclusion of the corner24dshortens a distance between the electrode24and the inner wall28, and thus creates a constriction of the air gap30at the outlet end10a. The corner24dmay in fact be of a uniform size and shape about the circumference of the distal end24aas inFIG.1or may be formed as a regular series of protrusions about the circumference of the base of the distal end24aas shown inFIGS.6-8.

An arc gap26, shown in the Figures as a squiggly line, is typically a shortest distance measured from the electrode24to the inner wall28, as most clearly shown inFIG.1with the truncated body igniter100a, and may in fact be a same measurement as the air gap30. Formation of a plasma arc can occur anywhere along a length of the electrode24to the inner wall28, but the inclusion of the corner24dshortens the air gap30distance and encourages formation of the plasma arc at the corner24dto the inner wall28at that shortest distance. Hence, in the drawings, the arc gap26shown as the squiggly line in the Figures is also generally indicative of the plasma arc. This is advantageous for ignition of the discharge as well as for ensuring the discharge is disposed proximal to zones of at least one of a flame holding or a combustor recirculation zone44. In all the Figures, the arc gap26is shown as emanating from the corner24dto the inner wall28at the outlet end10aof the igniter body10and inFIG.2, this distance is not the shortest distance between the corner24dand the inner wall28.FIG.2shows the location of the arc gap26when there is air flow present in the air gap30, with the air flow extending the arc gap26distance to the inner wall28at the outlet end10a, and thus effectively moving the plasma arc formation towards the outlet end10a. If no air is actively being fed through the air gap30, the arc gap26inFIG.2would in fact be a shortest distance from the corner24dto the inner wall28as shown inFIG.1. Hence, the arc gap26distance may change depending on whether there is air flow within the air gap30and exiting the outlet end10a. The inventors note that instead of having the corner24dbe configured as a protrusion to encourage arcing at that position, in other embodiments, the inner wall28could instead or also be designed with a flange or other protrusion constricting the outlet end10aof the igniter body10with a same end result of encouraging plasma arc formation at the constriction point in the igniter body10. The inclusion of the corner24don the electrode24at the outlet end10aand the air flow towards the outlet end10aoptimize the plasma arc formation at the outlet end10asuch that a resulting flame70produced will be positioned in the primary burn region42of the combustor40. When the electrode24receives current from the driver unit50, the current jumps the space between the electrode24to the inner wall28at the arc gap26, and when there is air flow in the air gap30, the flame70extends from the arc gap26and outwards beyond the outlet end10ainto the primary burn region42.

The truncated body igniter100amay also have a fuel port24and fuel line26or be unfueled. For embodiments with the fuel port24and fuel line26, a fuel-air mix from the fuel port24and the air feed hole32enters and swirls through the air gap30around the electrode24. The plasma arc formed at the arc gap26ignites the fuel, creating the flame70, and the moving air pushes the arc and the flame70caused by the burning fuel beyond the outlet end10aof the igniter100aand into the combustor40. The igniter body10and the centrally positioned electrode24are heated by the passage of electrical current through both components and this heating enhances the processes of evaporation and break-up of fuel injected into the air gap30.

The outer wall10cof the truncated body igniter100ais further formed with a widened body mount10e, having a larger diameter compared to the outer wall10cand sized and shaped to allow the igniter body10to be more easily secured to the combustor40.

InFIG.2, an extended igniter body100bembodiment is shown, where the distal end24aand vertex24cof the electrode24do not extend beyond the outlet end10aof the igniter body10. InFIG.2, the corner24dis at the base of the conical distal end24a, but the corner24dis formed without any protrusions and the cylindrical electrode24at the corner24dinstead tapers to the vertex24c. The air gap30has a uniform distance from the inner wall28to the body of the electrode24. The arc gap26is shown from the corner24dto the inner wall28at the outlet end10aof the igniter body10and is a longer distance as compared to the arc gap26in the truncated body igniter100aembodiment inFIG.1. As previously discussed, when the igniter100bis in use, air pumped into in the air gap30via the air feed hole32flows out the outlet end10aof the igniter body10and extends the arc gap26distance and thus the plasma arc to what is shown inFIG.2, with the arc gap26and plasma arc indicated by the squiggly line.

In comparison to the truncated body igniter100a, typically the extended body igniter100bincludes a fuel port34and fuel line36along with the air feed hole32.

The plasma igniter100and its embodiments100a100bare supplied voltage and current by the electronic driver unit50, shown inFIGS.12and13as schematics of two typical embodiments of the driver units50and their component blocks. The driver unit50provides electrical power to the plasma igniter100100a100b. The driver unit50supports the generation of a sufficiently high electric field so that an electrical arc is generated between the electrode24and the inner wall28of the igniter torch body10. The arc gap26and an electrode geometry are design parameters that affect the voltage level required. The larger the arc gap26, the higher the voltage requirement. The timing for triggering the plasma arc is not a critical requirement in these systems since the frequency of the voltage wave and resulting arcs are effectively continuous, as shown inFIG.11. Wave time, measured as 1/frequency, is shorter than the effective combustor residence time, typically 0.5 to 30 msec. Electrical power and voltage are dependent on the required ignition energy for the combustor40. Typical levels range from 100 to 500 Watts (RMS power input to the system). The delivered energy at the arc gap26is typically 5 to 10 times lower than the power input. However, the inventors note these parameters may vary without limitation and one with ordinary skill in the art would recognize that changes in these parameters outside of these typical levels do not negate the novelty or usefulness of this invention.

FIG.12shows a first embodiment of the driver unit50, with a schematic of the plasma ignition driver unit50with component blocks. The power input52can be either alternating or direct current. Most aircraft systems use 28 Vdc power, and thus this is a preferred power input. A voltage output62is dependent on the driver unit50output frequency, which can range from 5 kHz to 100 MHz. Voltage output levels will range from 250 Vrms to 7000 Vrms. Peak voltage levels may be significantly higher. As shown inFIG.12, there are two basic component blocks in the driver unit50, an input power controller56to regulate, filter and modulate the input power52and a voltage oscillator64to create a voltage waveform at the correct frequency and level with a transformer to generate the high voltage required for arcing at the igniter100. The driver unit50also has a triggering input52, configured as an on/off signal in a simplest form, and connections with safety fuses and EMI shielding to reduce electromagnetic noise from the driver unit50.

FIG.13shows a second embodiment of the driver unit50. In this system, a simple zero-voltage switching unit58is used along with a voltage step-up transformer or flyback transformer60. This circuitry is common and used in applications where different DC voltage levels are needed. The transformer60steps up the voltage from the primary side voltage to a higher voltage necessary for generating plasma. This step up ratio can be customized to best suit engine operating parameters and is controlled by the primary to secondary windings ratio of the transformer, the flux coupling and inductive effects. The circuit can be tuned to a particular frequency, and a higher frequency is usually preferred for plasma igniter systems. The simplicity and availability of low-cost components in the driver unit50make this a desirable means for plasma igniter power input. The input power controller56could be a complex voltage and current regulation system, or a simple passive circuit with a single state for input and output.

The driver units50supply voltage and current to the plasma igniter100100a100bsuch that there is a transient rate of voltage rise sufficient to create the electrical arc from the corner24dof the electrode24to the inner wall28of the grounded igniter body10. The two types of driver units50used in this system include a low-cost AC driver unit50with a microsecond voltage wave time period, and a high-cost, energy-efficient nano-second pulse driver unit50. Either driver unit50shown inFIGS.12and13can be configured to be low-cost (microsecond voltage wave) or energy-efficient (nano-second pulse), however the high-cost, energy-efficient driver unit50requires relatively more expensive electronics with faster switching capability and may or may not require a physically larger unit. Thus, either igniter100a100bmay be driven by either electronic driver unit50. A single engine may have multiple igniters100and driver units50, with a minimum one igniter100and one driver unit50per engine. The type of driver unit50used is dependent on the desired cost and use of the system. The AC driver unit50is low-cost and more appropriate for engine ignition where activation is required for only a short period of time, typically less than one minute. The nano-second pulse driver unit50is more power-efficient. Either driver unit50can be used for ignition, re-start and combustion sustainment, efficiency enhancement, and in conditions where mixing and reaction times are short or where the fuel-air mixture in the combustor burn zone is outside conventional lean and rich flammability limits. These systems are capable of multiple engine starts, unlike pyro-technic or flare devices used in conventional military unmanned systems and are desired for periods longer than 5 minutes and where the system cost trades well against improved engine operational envelope and fuel efficiency.

In short, the low-cost and high-cost (energy efficient) designs are both DC powered. The circuitry to drive the arc formation is the difference between these driver units. In the low-cost design, the arc is produced by an AC voltage switching circuit with a simple step-up transformer. In the high-cost design, the voltage step up is done with high frequency switching components with different voltage amplifiers (solid state devices). The high-cost design is used for higher efficiency and better performing arc characteristics, such as faster, easier arcing with more active ion generation.

For driver units50with a direct current power source with a voltage level between 10 Vdc and 120 Vdc, the driver unit50provides current to a circuit generating a variable or constant frequency voltage wave at about 10 kHz to 10000 kHz. The input power controller56can be configured as a passive circuit with a single state for input and output or as a voltage and current regulation system. For the driver unit50inFIG.12, a voltage level increase of 100 to 1000 times the input voltage via the voltage transformer64is produced by an inductive electrical coil, or alternatively by a set of energy storage capacitors to achieve the oscillating voltage increase.

The plasma ignition and combustion assist system is applicable to a wide range of gas turbines. The full range includes both ground power systems as well as aircraft engines. The system described herein is expected to be lower cost than conventional spark ignition systems. In 2021, a low-cost system is approximately less than $500 USD and a high-cost system is more than $2500 USD. In comparison, a conventional spark ignition system in 2021 costs between $4000-7000 USD. The plasma igniter100100a100band driver unit50of the plasma assist system described herein produces a continuous or pulsed arc that does not require expensive nor complex triggering electronics, and the voltage required to sustain the plasma arc is several factors lower than for spark ignition systems, which reduces the need for complex isolation leads and connectors.

Plasma assist ignition is best applied to small or miniature gas turbines, which must be capable of operation with short combustor residence times. These engines are characterized by low pressure ratio with low combustor inlet pressure (pressure levels below about 125 psia) and temperatures below 400 F, and with overall residence time (volume/volumetric-flow-rate) below about 15 msec. Larger, higher pressure ratio engines having overall pressure ratios above 7:1 would benefit from the plasma igniter100and driver unit50described herein but typically have higher voltage, single-spark, systems. Plasma assist systems based on the plasma igniters100and the driver units50described herein can benefit large ground power systems mostly by running continuously during operation thereby improving lean stability and allowing stable operation at conditions consistent with lower nitrogen oxides (NOx) and carbon monoxide (CO) emissions.

Typical engines for which the plasma ignition and combustion assist system is useful include the following:

1. Miniature turbojets with thrust ranging from 15 lbf to 600 lbf. These are generally used in flight systems (such as miniature missiles, surveillance aircraft/drones, airborne jamming devices and commercial drones). These engines use a range of heavy distillate fuels including Jet-A, Diesel and JP-10. Current engine models suitable for use with the plasma assist system include the ATI070, and all derivatives of the B300STG turbojet and the ATI200, a 200 lbf thrust turbojet. For these engines, the plasma ignition and combustion assist system would be used for ignition and engine start/operational envelope expansion with low engine speeds and/or power levels at altitude. The plasma ignition and combustion assist system allows for multiple starts and/or re-starts in flight; and

2. Miniature high speed turbo-generators in the 5 to 100 kW electrical power output range. These generators are most commonly airborne power systems used for small, unmanned commercial and military aircraft where high power/weight is needed. Other suitable applications of the plasma ignition and combustion assist system include those where high power in a small, lightweight package is required, including small ground power units. Current engine models suitable for use with the plasma assist system include the ATI010e, a derivative of the B140TG and SP10e, which are both 10 kWe turbo-generators, the ATI35e, a derivative of the B300STG 35 kWe turbo-generator, and the SP75e, a 75 kWe turbo-generator. For these engines, the plasma ignition and combustion assist system is used for ignition and starting. The system in this application allows for multiple starts and/or re-starts during flight.

Engines suitable for use with the plasma ignition and combustion assist system have relatively low overall pressure ratios of 3:1 to 7:1, and where plasma arcing in air is relatively easy due to low air densities and reduced voltage required for electrical arc initiation.

It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the present invention.