High energy ignitor power circuit

A constant energy, constant spark rate ignitor system suitable for use in high energy ignition systems. The ignitor system includes a vacuum interrupter switch through which an energy storage capacitor discharges to fire a surface gap spark plug. Discharge of the energy storage capacitor through the vacuum interrupter can only occur upon actuation of the switch. The quantity of energy stored in the energy storage capacitor and the timing of capacitor discharge is therefore not a function of ambient operating conditions or the dielectric properties of the switch and can therefore be independently controlled. A voltage comparator and spark rate clock are used to trigger closure of the vacuum interrupter switch and fire the plug when the capacitor contains the desired quantity of energy and a clock signal is present. A current proving circuit is also provided to verify that current passes through the spark plug.

BACKGROUND OF THE INVENTION 
This invention relates to high energy ignition circuits for igniting a 
fuel. Such ignitor circuits are frequently employed in power plants, steam 
generation boilers, jet engine ignition and other combustion engineering 
applications. To ensure that the fuel ignition is achieved, these 
applications require that the ignition circuit provide a spark at a spark 
rate and energy level sufficient ignition of the fuel. An uneven spark 
rate may prevent reliable ignition. In severe cases, uneven spark rates 
cannot obtain ignition of the fuel. 
FIG. 1 shows a block diagram of a conventional high energy ignitor circuit. 
The typical high energy ignition circuit consists of one or more 
capacitors 10 that are charged from a direct current (DC) power source 12 
through a series resistor 14. Capacitor(s) lo is coupled to a spark gap 
tube 20 which is in turn coupled to a spark plug 21. Capacitor 10 is 
charged exponentially by DC power source 12 until the breakdown voltage of 
spark gap 20 is obtained. At the breakdown voltage threshold of the spark 
gap, energy is transported across the electrodes and fires the surface gap 
spark plug. 
Conventional ignitor circuits may also contain a monitoring circuit 22 to 
verify that plug 21 is being fired. The monitoring circuit allows for the 
prevention of a fuel valve opening with a disfunctioning ignitor circuit. 
Unignited fuel presents a serious safety hazard; proof of ignition energy 
is often required by safety codes. 
FIG. 2 shows a cut away view of a conventional spark gap tube 20 used in 
the typical ignitor system described above. Spark gap tube 20 consists of 
two electrodes, 23 and 24 located within a housing 26 formed of glass or 
ceramic material. The interior 28 of housing 26 is a controlled 
environment of an inert gas. Electrodes 23 and 24 are separated a given 
distance, forming a gap 30. At a predetermined potential energy, current 
passes from electrode 23 to electrode 24 as arc 32. 
FIG. 3 contains a graph of gap breakdown voltage versus pressure. As is 
evident from curve 34, the breakdown voltage for a given gap is not 
constant but varies with pressure, and thus temperature. Because interior 
28 of spark gap tube 20 is not vacuous, the breakdown voltage is finite 
and varies across a range of operating conditions. 
Use of a spark gap tube therefore imposes several limitations on the 
typical high energy ignitor system described above. First, the capacitor 
discharge voltage is not independently variable, but is fixed by the 
breakdown voltage of the spark gap tube. The breakdown voltage of the 
spark gap, however, is not constant and varies with pressure and 
temperature as well as cycles of use. The discharge energy of capacitor 
10, therefore, must also vary as a function of temperature, contact 
erosion, gas 28 contamination, and self-heating, independently of the 
energy requirements for fuel ignition. To ensure sparking, capacitor 10 
must be sized large enough to provide sufficient energy for all 
environmental conditions and is thus sized to account for variations in 
the operating characteristics of spark gap tube 20. This fact imposes a 
design constraint which makes it difficult to optimize capacitor 10. 
In addition to governing the size of capacitor 10, the breakdown voltage of 
spark gap tube 20 fixes the discharge energy of capacitor 10 and prevents 
independent variation of the discharge energy to respond to changing uses 
or conditions. For example, the voltage required to fire a spark plug 
increases in proportion to the number of firings to which the plug has 
been subjected. To account for these variations over the life of the plug, 
the gap breakdown voltage and capacitor discharge energy of most ignitor 
circuits are set at an energy level sufficient to fire a high cycle plug. 
This same energy level is applied to all plugs no matter how many firing 
cycles the plug has actually endured and thus in disregard of the true 
energy requirements of the plug. This practice increases system energy 
costs and may accelerate plug wear. 
A second limitation of the typical ignitor system relates to control of the 
spark rate. The capacitor of the typical system will fire whenever the 
breakdown voltage of the gap is reached. Therefore, the spark rate will be 
governed by the time necessary to charge the capacitor to the breakdown 
voltage level. The firing rate of the circuit will thus vary according to 
changes in the spark gap breakdown voltage. Furthermore, line voltage 
variations can cause fluctuations in the time necessary for power supply 
12 to charge capacitor 10 to the breakdown voltage gap of 30 of spark gas 
tube 20. These fluctuations in the output of DC power source 12 will 
affect the spark rate unless expensive voltage regulators are used to 
regulate DC power source 12. 
A third limitation of the typical circuit stems from the fact that when the 
spark plug fires, the length of time that current flows through the spark 
plug circuit is very short, typically 50 microseconds. This short time 
period makes it impractical to directly operate an indicating device such 
as a relay that would require current for a number of milliseconds. 
Furthermore, the very low impedance of the spark plug, typically 20 
milliohms, would cause the insertion of an indicator device in series with 
the spark plug to rob most of the energy and reduce the circuit efficiency 
to an unacceptable level. Conventional designs commonly actuate an 
indicating device via the voltage drop across timing resistor 14 when the 
full wave rectified, unfiltered, output of high voltage supply 12 charges 
energy storage capacitor 10. This circuit suffers the disadvantage that a 
shorted capacitor 10 will provide the same effect as would the periodic 
discharge of energy storage capacitor 10 through spark plug 21 via spark 
gap tube 20, thus providing a false and dangerous indicating of current 
through the spark plug. Additionally, the relay used as the indicator 
device is a special type of relay with a delayed drop out characteristic. 
This special type of relay is less reliable and more expensive than a 
simple relay. 
SUMMARY OF THE INVENTION 
The present invention provides a high energy constant spark rate, constant 
energy ignitor system for use in a multitude of applications. According to 
one embodiment of the invention, the ignitor system comprises a vacuum 
interrupter, an electronic control circuit for controlling the operation 
of the vacuum interrupter and a high voltage DC power supply for charging 
the energy storage capacitor. The electronic control circuit monitors the 
voltage present across the terminals of the capacitor. When the voltage 
reaches a value representing the desired amount of energy, the high 
voltage power supply is turned off. The vacuum interrupter switch is then 
activated by the control circuit and the spark plug is fired. 
The vacuum interrupter contains a low resistance contact located in an 
evacuated housing. This switch is more efficient than the spark gap tube 
of the typical ignitor which relies on plasma discharge. The vacuum 
environment of the switch also means that the breakdown voltage of the gap 
between the switch terminals remains infinite, substantially constant, and 
does not vary with pressure, temperature or other environmental 
parameters. The discharge energy of the capacitor is therefore not 
governed by the gap breakdown voltage of the switch terminals. Discharge 
of the energy storage capacitor occurs only when the vacuum interrupter 
switch is closed and can thus be independently varied to respond to 
circuit conditions or changing applications. A voltage comparator tied to 
a reference voltage comprises one portion of the control circuit which 
controls charging of the capacitor by the high voltage DC power supply to 
the predetermined energy level. The control circuit thereby maintains a 
constant energy level for each discharge of the capacitor. 
According to another embodiment of the invention, the electronic control 
circuit further comprises a spark rate clock. In this embodiment, the 
control circuit does not engage the vacuum interrupter switch until the 
capacitor has been charged to the desired energy level and a clock signal 
is present. In this fashion, the spark rate of the ignitor remains 
constant without incurring the expense of regulating the power supply. The 
ignitor system is therefore not subject to AC line voltage variations. 
According to yet another embodiment of the present invention, a current 
proving circuit is provided to sense current in the spark plug wire. This 
system ensures that the plug has fired and does not rely on inferences 
made from monitoring the energy storage capacitor charging voltage to 
detect an unsafe condition. In addition, the current proving circuit of 
the present invention enables use of a simple relay in lieu of the less 
reliable and more expensive special relay required in conventional 
circuits.

DESCRIPTION OF THE SPECIFIC EMBODIMENT 
The present invention includes a recognition of the limitations imposed on 
conventional ignitor circuits by the physics of the spark gap tube. The 
ignitor system of the present invention employs a vacuum interrupter 
instead of a spark gap tube to discharge the energy storage capacitor and 
enables design of a constant energy, constant spark rate ignitor system. 
Vacuum Interrupter General Principles 
FIG. 4 is a cutaway view of a vacuum interrupter switch 40. Vacuum 
interrupter 40 consists of two low resistance contacts 42 and 44, located 
within a glass or ceramic housing 46. The interior 48 of housing 46 is 
evacuated. Contacts 42 and 44 are depicted in an open circuit 
configuration and are separated by a gap 50. Contact 42 is moved in the 
direction 52 by means of the pressure differential between evacuated 
interior 48 and the atmosphere pressure exterior to housing 46. Mechanical 
operation of contact 42 can be controlled independently of conditions 
within interior 48 of vacuum interrupter 40. A model WL-35082 manufactured 
by Westinghouse of Pittsburgh, Pa., is an example of a vacuum interrupter 
suitable for use in the present invention. 
Because interior 48 consists of a vacuum, the ambient pressure inside 
housing 46 remains constant and substantially equal to zero. From the 
graph in FIG. 3, the breakdown voltage of gap 50 in a vacuum is 
essentially infinite. Current will only pass from contact 42 to contact 44 
when the switch is closed mechanically. Current will not pass from contact 
42 to contact 44 by arcing. Therefore, an energy storage capacitor can 
discharge through vacuum interrupter 40 only when the vacuum interrupter 
switch is closed. Unlike an energy storage capacitor coupled to a spark 
gap tube, discharge of an energy storage capacitor coupled to vacuum 
interrupter 40 is governed by the opening and closing of an independently 
operated mechanical switch and does not occur automatically, as upon 
attaining the breakdown voltage of a spark gap. The dielectric properties 
of gap 50 therefore do not govern the discharge energy level and discharge 
timing of the capacitor. Thus, timing and energy levels can be 
independently controlled by selectively configuring the remaining ignitor 
circuit elements. 
High Energy Ignitor Circuit System Overview 
FIG. 5 contains a block diagram of a high energy ignitor utilizing a vacuum 
interrupter and control circuitry according to an embodiment of the 
present invention. The ignitor system comprises a voltage comparator 80 
which is coupled to an energy storage capacitor 82 and to a reference 
voltage. When the voltage across energy storage capacitor 82 is below a 
value determined by the reference voltage of voltage comparator 80, 
voltage comparator 80 closes switch 84 via signal invertor 86. With switch 
84 closed, the high voltage DC power supply 85 charges energy storage 
capacitor 82 until the voltage across capacitor 82 exceeds a value 
determined by the voltage comparator 80 reference voltage. When the 
voltage across energy storage capacitor 82 exceeds this predetermined 
value, an input is provided to an AND gate 88, and switch 84 opens, 
thereby discontinuing charging of energy storage capacitor 82. 
A spark rate clock 90 determines the ignitor spark rate by providing a 
periodic output that enables AND gate 88. When both the output of voltage 
comparator 80 and the output of spark rate clock 90 are present at the 
input of AND gate 88, vacuum interrupter switch 40 closes. Closure of 
vacuum interrupter 40 transfers energy stored in energy storage capacitor 
82 to the spark plug 92, generating a spark to ignite the fuel. 
The ignitor circuit of the present invention may also contain a current 
proving circuit 93 to sense if current periodically flows through plug 92. 
Once current is proven fuel flow for ignition is initiated. The initial 
flow of fuel without an ignition source presents a dangerous situation. 
The output of the current sensing circuit is forwarded to a control center 
which monitors operations and prohibits initiation of fuel flow in the 
event no current flows to plug 92. Typically, once fuel is ignited the 
ignitor circuit is de-energized. Flame stability can be monitored and 
reported to the control center by separate combustion detectors. 
Description of High Energy Ignitor System Operation and Components 
FIG. 6 contains a top level schematic of an embodiment of the present 
invention. In the embodiment of FIG. 6, a single phase of AC line voltage 
is transformed to approximately 28 volts root mean square (rms) by a low 
voltage DC power supply 96 at the secondary of transformer 98. Diodes 
100-103 are configured to form a full wave bridge circuit. The DC output 
of the bridge circuit is filtered by capacitor 104, to provide an 
unregulated 28 VDC for operation of the device which effects mechanical 
closure of vacuum interrupter switch 40. The center top 105 of transformer 
98 output is filtered by capacitors 106 and 108 and regulated to 12 volts 
by voltage regulator 110 and capacitor 112. The 12 volt DC output of power 
supply 96 powers the remaining elements of the circuit. 
Operational amplifier 120 is configured to form voltage comparator 80. The 
level of energy stored in energy storage capacitor 82 is determined by the 
voltage comparator reference voltage. When the potential across energy 
storage capacitor 82 is below a value determined by the reference voltage 
of voltage comparator 80, charging switch 84 is closed and high voltage DC 
power supply 85 charges capacitor 82. Capacitor 82 has been charged to the 
desired level when the potential across energy storage capacitor 82 
exceeds a value determined by the reference voltage of voltage comparator 
80. Charging switch 84 is then opened to halt charging of capacitor 82. 
A fixed reference voltage may be used to establish a stable and fixed 
discharge energy for storage capacitor 82. A fixed reference voltage is 
easily obtained using any number of known circuit designs. In the 
schematic of FIG. 6, voltage comparator 120 comprises an output node 121, 
an inverting input 122 and a noninverting input 123. Output node 121 is 
coupled to switching circuit 84. Noninverting input 123 is coupled to a 
voltage divider circuit consisting of resistors 126-130 and feedback 
resistor 132. The noninverting input is further coupled to energy storage 
capacitor 82 at node 134. The potential of energy storage capacitor 82 
thus appears at noninverting input 123 as V.sub.div which is a known 
function of resistors 126-130 and 132. Inverting input 122 is coupled to 
resistors 136 and 137 which are coupled to V.sub.cc and ground 
respectively to present a fixed reference voltage (V.sub.ref) at node 122. 
FIG. 7 diagrams a variation of the voltage divider circuit depicted in FIG. 
6. In this circuit resistors 127-130 are represented as a single resistor 
140 and a resistor 142 has been added in series. As will be apparent to 
those of ordinary skill in the art, a large number of circuit designs can 
be used to establish a fixed reference voltage and a corresponding range 
of fixed discharge energy levels for capacitor 82. 
In many high energy ignitor applications, however, a variable, 
user-selected, discharge voltage is desireable to accommodate changed 
operating conditions or a worn spark plug 92. The energy stored in storage 
capacitor 82 before discharge, can be increased by increasing the voltage 
comparator 80 reference voltage 122. Conversely, the energy stored in 
storage capacitor 82 can be decreased by decreasing the voltage comparator 
80 reference voltage. 
FIG. 8 contains one circuit embodiment suitable for selecting from among 
several possible reference voltages. In FIG. 8, resistors 144-152 are 
connected in series and coupled to ground and to V.sub.cc. The output of 
the circuit at node 122 equals V.sub.ref. The circuit is also coupled to 
ground via node 153 through Zener diode 153a. Switches 154, 156 and 158 
remove resistors 146, 148, and 150 from the series network by closing the 
switch to create a short circuit around the resistor(s). The table 
associated with the schematic of FIG. 8 shows an example of how energy 
storage levels for capacitor 82 can be varied by opening and closing 
switches 154-158 for a given set of resistor values. 
So long as the input signal V.sub.div is slightly less than the voltage 
V.sub.ref, energy storage capacitor 82 is not at the desired potential. 
Under these conditions, the output signal appearing at node 121 is 
deasserted and switching circuit 84 remains closed. With switching circuit 
84 closed, high voltage DC power supply 85 charges energy storage 
capacitor 82. Once V.sub.div becomes slightly larger than V.sub.ref, 
capacitor 82 is at the desired potential. The output signal at node 121 is 
asserted and switching circuit 84 opens thereby discontinuing charging of 
capacitor 82. 
Switching circuit 84 comprises an optically coupled triac driver 190 which 
operates triac 192; two enhancement mode MOS transistors 193 and 194; and 
NPN transistor 195. Optically coupled triac driver 190 and triac 192 
operate as an AC switch to open and close the capacitor charging circuit. 
Use of an optically coupled triac driver isolates the high voltage level 
circuitry from the lower voltage circuitry in the remainder of the 
circuit. When voltage comparator 80 asserts the signal to open the 
charging switch, NPN transistor 195 becomes conductive. Resistors 196a and 
196b operate as signal inverter 86, such that, once NPN transistor 195 
turns on, optically coupled triac driver 190 turns off and shuts down 
triac 192, thereby halting the charging of capacitor 82 from power supply 
85. 
Operation of the AC switch, however, requires a zero point crossing of the 
AC waveform before effecting the switch opening or closing. This 
requirement can cause a delay of over 8 milliseconds before switch 84 
opens. During the delay, capacitor 82 continues to charge beyond the 
desired energy level. For these reasons, a faster DC switch consisting of 
transistors 193 and 194 connected in series is also provided. Transistors 
193 and 194 are coupled to the secondary of the high voltage power supply 
85 transformer 197. When NPN transistor 195 turns on, transistors 193 and 
195 turn off preventing charging of capacitor 82 until such time as the AC 
switch fully activates. 
Transformer 197 and diodes 198a-d comprise high voltage power supply 85 
used to charge energy storage capacitor 82 through resistor 199 when 
switch 84 closes. Transformer 197 receives the AC line voltage and 
converts to a higher voltage for conversion to DC through diodes 198a-d. 
Because the spark rate is determined by spark rate clock 90 and not by the 
time required for high voltage DC power supply 85 to charge energy storage 
capacitor 82 to the breakdown voltage of the gap as in conventional 
ignitor circuits, the spark rate is unaffected by AC line voltage 
variations. High voltage DC power supply 85, therefore, need not be 
regulated in order to obtain a constant spark rate as is the case with 
prior art high energy power units. For these reasons, the expensive 
ferroresonant transformers of the conventional ignitor circuit are not 
required and transformer 197 can consist of a less expensive simple 
transformer. Optionally, however, if independent timing of the spark rate 
via spark rate clock 90 is not desired, spark rate can be determined by 
the time necessary to charge capacitor 82 to the desired voltage. In such 
a system, power supply 85 must be regulated to account for AC line voltage 
variations. 
In the embodiment of FIG. 6, however, operational amplifier 200 is 
configured to form spark rate clock 90 for independent timing of the 
ignitor system. Any number of known circuit designs may be used. In this 
embodiment, amplifier 200 is a free running oscillator. The inverting 
input 201 of operational amplifier 200 is coupled to a capacitor 202 
connected in parallel with resistor 204. Resistor 204 controls the 
discharging of capacitor 202 and thus the oscillation of the circuit. To 
adjust the spark rate clock frequency, the value of resistor 204 can be 
altered by configuring resistor 204 as a variable resistor or by 
substituting a resistor of a different value. Decreasing the resistance of 
resistor 204 increases the speed with which capacitor 202 discharges and 
thus increases the spark rate. 
The spark rate established by the spark rate clock can be varied over a 
wide range. At one extreme the spark rate can have a period of a fortnight 
or more. At the other extreme the spark rate is limited by the reaction 
time of the solenoid actuation device used to close the vacuum 
interrupter. In typical applications the spark rate will have a frequency 
on the order of eight hertz. 
Operational amplifier 240 is configured as AND gate 88. When either voltage 
comparator 80 or spark rate clock 90 signals are not asserted, the signal 
asserted by resistor 243 is overridden. When both voltage comparator 80 
and spark rate clock 90 output signals are asserted and present at node 
242, the output 244 of AND gate 240 is deasserted and signals activation 
of vacuum interrupter 40. Activation of vacuum interrupter 40 causes 
capacitor 82 to discharge and fire plug 92. Optionally, AND gate 88 can be 
omitted and activation of vacuum interrupter 40 can be governed solely by 
the output of the spark rate clock. Such an arrangement would produce a 
constant spark rate ignitor but not constant energy ignition. 
Activation of vacuum interrupter 40 is effected by a solenoid 246 which is 
controlled by MOS transistor 248. Vacuum interrupter 40 is held in the 
open circuit position when Solenoid 246 is energized. When the output of 
AND gate 88 is deasserted, in response to signals from spark rate clock 90 
and voltage comparator 80, MOS transistor 248 stops conducting and 
deenergizes solenoid 246. Deenergizing solenoid 246 closes vacuum 
interrupter 40. Energy storage capacitor 82 then discharges through vacuum 
interrupter 40 and fires plug 92. Alternatively, a triac and optically 
coupled triac driver may be used to deenergize solenoid 246 and close 
vacuum interrupter 40. 
FIGS. 9A and 9B contain a top and end view, respectively, of a vacuum 
interrupter solenoid actuation device assembly suitable for use in the 
present invention. Solenoid 246 comprises a solenoid and plunger assembly 
251 with conical taper configured for DC operation. Two leads, 252 and 253 
connect solenoid 246 to the remainder of the ignitor circuit. Plunger 251 
connects to one end of a lever arm 254 with tension pin 255. The other end 
of lever arm 254 connects to the vacuum interrupter insulated actuating 
rod 256 via swivel rod end 258. Actuating rod 256 is fabricated from 
insulating material. A flexible lead 262 couples terminal 42 of vacuum 
interrupter switch 40 (shown in FIG. 4) to energy storage capacitor 82. 
Terminal 44 (also shown in FIG. 4) of vacuum interrupter switch 40 couples 
to spark plug 92 from the output stud 264. Two insulating washers 266a and 
266b insulate output stud 264. The entire actuation assembly is mounted on 
a steel plate 280, which may include a tab 282 for mounting. 
Alternatively, plate 280 may be secured to a mounting surface with tapped 
holes or the like. Solenoid 246 attaches to plate 280 with mounting screws 
284. 
When solenoid 246 is energized, vacuum interrupter 40 is held in the open 
position. Plunger 251 is drawn in the direction of arrow 286, which causes 
lever arm 254 to pivot on pivot pin 287, causing swivel rod end 
258/insulated actuating rod 256 to move in a direction opposite arrow 288 
via the lever arm linkage. The terminals of vacuum interrupter 40 are 
thereby separated by a gap 50, as shown in FIG. 4, and the switch is open. 
Vacuum interrupter 40 is held in this open configuration so long as 
solenoid 246 remains energized. 
When solenoid 246 deenergizes in response to the signal from AND gate 88, 
the force holding vacuum interrupter 40 in the open position is released. 
Plunger 251 moves in a direction opposite to arrow 286. This motion of 
plunger 251 causes lever arm 254 to pivot about pivot post 287. 
Atmospheric pressure acting upon interrupter 40 terminal 42 in the 
direction of arrow 288 pulls on actuating rod 256, allowing terminal 42 of 
vacuum interrupter 40 to come into contact with terminal 44. Energy 
contained in capacitor 82 is then immediately conveyed from input lead 262 
across the terminals and output to spark plug 92 via output stud 264. 
Mechanical bounce of the switch is not a problem with this apparatus since 
the discharge of energy across the terminals takes place before bounce 
rebound occurs. 
The embodiment of FIG. 6 also includes a safety feature to ensure that 
current flows from capacitor 82, through vacuum interrupter 40 and into 
plug 92. Current proving circuit 93 comprises an inductance coil 300 which 
senses spark plug current at node 302. The inductance coil must be located 
on a wire which carries only spark plug 92 current. Inductance coil 300 
outputs a voltage which is rectified by a full wave bridge circuit 
comprised of diodes 304-307 and converted to a DC voltage by capacitor 
309. This DC voltage is supplied to the noninverting input 311 of 
operational amplifier 312. Operational amplifier 312 is configured as a 
voltage comparator having its inverting input tied to a reference voltage 
at node 314. When the DC voltage at node 311 exceeds the reference voltage 
at node 314 the output of operational amplifier 312 is asserted and causes 
a transistor 315 to become conductive. Transistor 315 then turns on 
optically coupled triac and triac driver 316, which in turn activates 
current proving relay 318. Terminals 320 of relay 318 connect to various 
user system control circuits (not shown) which monitor combustion 
operations. A light emitting diode 322 lights whenever triac 316 is 
activated to indicate that spark plug current has been proven. 
Energy acquired during the brief spark plug discharge period is stored in 
capacitor 309, diode 322 and resistor 324 form a circuit to protect OPams 
312 noninverting input 311 from excessive (damaging) voltages. Resistor 
326 sets a time constant (with capacitor 309) for how often the plug must 
fire. This fact allows the current proving system to verify and not merely 
infer that current in fact passes through the plug. In addition, the 
current sensing scheme of the ignitor of the present invention disposes 
with the complicated and expensive components necessary in conventional 
circuits to sense and process the voltage fluctuations of the energy 
discharge capacitor. The current proving circuit of the present invention 
therefore permits a reduction in cost of the relay and requires only the 
inclusion of a simple inductance coil 300 to sense plug 92 current. 
Description of High Energy Ignitor Operation Using Illustrative Example 
The operation and advantages of the ignitor system of the present invention 
are best illustrated by way of an hypothetical combustion engineering 
application. For example, a newly installed ignitor plug is used in 
conjunction with the ignitor circuit of the present invention to fire an 
aircraft jet engine. The aircraft ferries passengers from Denver to Miami 
during the winter months. During the first flight, an engine flameout 
occurs and the engine must be restarted. The new plug of this example 
requires that energy discharge capacitor 82 supply only 8 joules of energy 
to fire the plug. As the plug is subjected to repeated firings, the energy 
demands of the plug increase until finally, the high cycle plug requires a 
12 joule setting to fire. Towards the end of the plug's expected life, but 
prior to a routine maintenance replacement, the plug incurs a fault which 
prevents the plug from firing. In addition, the AC line voltage supplied 
by the engine compressor can vary .+-.10%. 
In the hypothetical airplane example, the discharge energy of storage 
capacitor 82 of the ignitor would be set at 10 joules by maintenance 
personnel when the new plug is installed. To set the ignitor system of the 
representative embodiment to the desired energy level, maintenance 
technicians close switch 158 as described in FIG. 8. At a later date, the 
energy level can be changed by maintenance personnel to the 12 joules 
necessary to account for the high cycle plug by closing switch 156 in FIG. 
8. 
FIG. 10 contains a flow chart which summarizes the logical operation of the 
embodiment of the high energy ignitor system as described above during 
operation of the aircraft engine. In the first logical operation step 400, 
the potential of energy storage capacitor 82 is checked against the 
preestablished reference voltage necessary to discharge the desired 8 
joules of energy. If capacitor 82 is not at the desired energy level, the 
output of voltage comparator 80 initiates step 402 causing charging switch 
84 to close (or remain close). High voltage DC power supply 85 charges 
capacitor 82 to the desired level. 
Once energy storage capacitor 82 is charged to the desired level, the 
output of voltage comparator 80 causes the charging switch to open and 
halts charging of the capacitor in step 406. 
The signal output by voltage comparator 80 to halt charging of capacitor 
also appears at AND gate 88. In step 408, AND gate 88 ANDs the voltage 
signal with the signal from spark rate clock 90. Spark rate clock 90 is 
set according to the spark rate necessary to start the aircraft engine. If 
both the spark rate clock signal and the voltage comparator signal are 
asserted, then in step 410, a signal is forwarded to solenoid 246, closing 
the vacuum switch and sending current into spark plug 92 causing it to 
fire. The spark appearing at plug 92 ignites the fuel and the engine 
starts. 
During the airplane flight, the ignitor circuit experiences severe changes 
in operating conditions due both to the extreme differences in the winter 
climates in Miami and Denver and also due to altitude temperature changes 
as the plane travels along its flight path. When the engine flames out 
during the flight, the ignitor system must respond consistently to ignite 
the fuel and restart the engine. In conventional ignitor systems, the 
breakdown voltage of spark gap tube 20 varies in response to these changes 
in operating environment. As the breakdown voltage varies, the energy 
stored in the energy storage capacitor before discharge drifts away from 
the desired energy level of 8 joules. Furthermore, since the spark rate of 
a conventional ignitor system is determined by the time needed to charge 
the energy storage capacitor to the breakdown voltage of gap 30 of spark 
gab tube 20, the spark rate fluctuates as the breakdown voltage varies. 
Unlike conventional ignitors, the ignitor system of the present invention 
is unaffected by these drastic changes in operating conditions. Because 
interior 48 of vacuum interrupter 40 is evacuated, the breakdown voltage 
of gap 50 remains infinite despite these changes in climatic conditions. 
Capacitor 82 therefore cannot discharge until voltage comparator 80 
determines, in step 400, that capacitor 82 is at the desired energy level. 
The discharge energy of the capacitor remains constant at the desired 8 
joules throughout the trip. 
The climatic changes also do not affect the spark rate, since spark rate is 
controlled by spark rate clock 90. Before the plug is fired in step 410, a 
clock signal must be present in step 408. Nonetheless, even in the absence 
of a spark rate clock, spark rate would be unaffected since the time 
needed to charge the capacitor to the fixed energy level of 8 joules would 
remain constant if a regulated voltage supply 85 were used to account for 
the .+-.10% variations experienced in the AC line voltage. 
The ignitor system of the present invention, however, does not require that 
power supply 85 be regulated to account for the line voltage variations 
experienced during flight. Any fluctuation in the charging time of 
capacitor 82 caused by these voltage variations is small compared with the 
period of the spark rate clock. Therefore, the ignitor system spark rate 
is insensitive to any line voltage variations experienced and remains 
constant at the desired rate throughout the trip. The ignitor system of 
the present invention can therefore deliver the same constant energy, 
constant spark rate performance during flight to restart the engine as on 
the ground. 
According to the example, after several trips to and from Miami, the now 
high cycle spark plug fails. When this failure occurs, current no longer 
flows through the plug. Current proving circuit 93 therefore no longer 
senses current in the spark plug wire. Current proving relay 318 then 
forwards a signal to the flight engineer and/or pilot who can then either 
request specific engine maintenance or engage a backup ignitor system to 
prevent a possible internal explosion of the engine in the event of 
relighting an in-flight flameout. 
As is apparent from the foregoing, the present invention provides a high 
energy ignitor circuit with constant spark rate and constant energy. 
Furthermore, a feature improvement provides an indication of circuit 
operation with a higher degree of integrity. Presently preferred 
embodiments of the present invention have been described. Variations and 
modifications will be readily apparent to those skilled in the art. For 
example, variable resistors can be used to control the spark rate clock 
and vary the reference voltage of the comparator circuit. In addition, a 
number of power supply designs known to those skilled may be used.