Circuit and method for spark-igniting fuel

Disclosed is a DC-powered spark-generating circuit for igniting fuel. Such circuit has two primary windings on a common transformer core and a separate transistor controlling power to each primary winding. Each transistor has a gate and a drain and such gates and drains are "cross-coupled." That is, the gate of the first transistor is coupled to the drain of the second transistor and the drain of the first transistor is coupled to the gate of the second. Also disclosed is a method for spark-igniting fuel. Such method includes applying a voltage across an air gap at a first frequency, establishing an arc across the air gap and maintaining the arc at least until the fuel ignites. Such arc is maintained at a second frequency which different from (and preferably higher than) the first frequency.

FIELD OF THE INVENTION 
This invention relates generally to electricity and, more particularly, to 
electrical circuits used for combustion. 
BACKGROUND OF THE INVENTION 
From the time liquid and gaseous fuels came into use for heating, various 
techniques have necessarily been employed to ignite such fuel. Probably 
one of the earlier techniques involved using a taper or match for 
ignition. 
Later (and particularly with gaseous fuels such as natural gas), gas-fired 
combustion equipment used a "standing" pilot light. Such pilot light 
burned continuously and was instantly available to ignite fuel. 
Disadvantages of a standing pilot light include unnecessary consumption of 
fuel, albeit in small amounts, and the practicality of using such standing 
pilot light only in stationary, permanent installations such as homes and 
offices. 
A more recent innovation involves so-called spark generating circuits which 
lend themselves well to automatic, e.g., thermostatic, control. That is, 
when the thermostat calls for heat, early steps in the combustion cycle 
include opening a pilot valve to allow gas to flow to a pilot light and 
energizing a spark generating circuit to ignite the raw flowing gas. Such 
circuits are in wide use today and like heating systems using standing 
pilot lights, are used mostly in buildings. 
Disadvantages of such spark generating circuits include the fact that most 
are configured to be powered by AC power, Yet another is that they are 
rather complex circuits and, relatively speaking, expensive. And although 
of generally modest size, they are physically rather large for 
applications involving portable heating equipment. 
One type of such heating equipment is generally referred to as pressure 
cleaning equipment or "pressure washers." Such equipment is mounted 
"dolly-like" on wheels and uses pressurized water, cold or hot, to clean 
manufactured parts, automobile bodies, building surfaces and the like. The 
invention is particularly well suited for use with pressure washers. 
One rather recent application for such pressure washers involves trucks 
which indiscriminately carry such diverse cargos as garbage and food 
products. In the New York City area, it was recently discovered that 
trucks hauling garbage would, after emptying and without benefit of 
intervening washing, pick up food products, e.g., fresh produce, for 
delivery. 
It is understood that operators of trucks handling such things as garbage 
are being ordered to wash such trucks before loading food products. 
Washing is with hot, high-pressure water from a portable hot water spray 
machine of the type made by Aqua Blast Company of Decatur, Ind. 
Certain patents in the field of spark generating circuits and in other 
fields are somewhat noteworthy. For example, U.S. Pat. No. 4,918,569 
(Maeda et al.) depicts a spark ignition circuit powered from a DC source 
which is switched on and off as is the nature of power source control in 
circuits for internal combustion engine spark plugs. 
The transformer has but a single primary winding, current to which is 
switched by a MOSFET. In turn, the MOSFET is controlled by a 
two-transistor driver circuit. There is also a detector for disabling the 
circuit based upon load current. Switching the circuit on and off (a 
necessity of operation) requires additional components and circuitry 
which, in view of the invention, is not required in other applications. 
U.S. Pat. No. 4,329,628 (Bohan, Jr.) involves an AC-powered spark ignition 
circuit. Such circuit has an energy storage capacitor which discharges 
through the primary winding of a spark transformer to develop a high, 
gas-igniting voltage at the transformer secondary. The energy storage 
capacitor is both charged and discharged during only one-half of the 
applied AC sine wave. To put it another way, the transformer core is 
"active" or in use only about one-half of the total operating time. 
U.S. Pat. No. 4,414,491 (Elliott) is directed to a power supply for a 
discharge lamp. Such power supply uses transformer core saturation to 
control the frequency at which the two-transistor invertor circuit 
operates. The transformer core has what is described as a shunt flux flow 
path with an air gap. For certain applications not involving a discharge 
lamp, the transformer is unnecessarily complex. 
An improved igniter circuit overcoming some of the aforementioned problems 
and shortcomings would be an important advance in the art. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a new circuit and method for 
spark-igniting fuel which overcomes some of the problems and shortcomings 
of the prior art. 
Another object of the invention to provide a new circuit and method for 
spark-igniting fuel which operates from a vehicle electrical system. 
Another object of the invention to provide a new circuit and method for 
spark-igniting fuel powered from a DC source which is continuously-applied 
when the circuit is in operation. 
Another object of the invention to provide a new circuit and method for 
spark-igniting fuel which uses a relatively simple transformer structure. 
Another object of the invention to provide a new circuit and method for 
spark-igniting fuel which makes more efficient use of the transformer 
structure. How these and other objects are accomplished will become more 
apparent from the following descriptions and from the drawing. 
SUMMARY OF THE INVENTION 
The invention involves a DC-powered spark-generating circuit for igniting 
fuel such as atomized fuel oil. Such circuit includes (a) a transformer 
with a first primary winding and a secondary winding on a common core and 
(b) a first transistor controlling power to the primary winding. 
In the improved oscillatory circuit, the transformer includes a second 
primary winding and the circuit has a second transistor controlling power 
to the second primary winding. Each transistor has a gate and a drain and 
the gate of the first transistor is coupled to the drain of the second 
transistor. 
More specifically, such gates and drains are "cross-coupled." That is, the 
gate of the first transistor is coupled to the drain of the second 
transistor and the drain of the first transistor is coupled to the gate of 
the second. The circuit is thereby in a push-pull configuration and 
oscillates. Preferably, such gate-and-drain coupling is by a separate 
resistor-capacitor coupling network for each gate/drain combination. 
In other aspects of the inventive circuit, the gate of the first transistor 
is also coupled to the second primary winding. Similarly, the gate of the 
second transistor is coupled to the first primary winding. During 
operation, the primary windings alternate as to conducting and 
non-conducting state. That is, when the first primary winding is 
conducting, the second primary winding is in a non-conducting state. And 
when the second primary winding is conducting, the first primary winding 
is in a non-conducting state. 
As each transistor conducts, the transformer core flux increases and over a 
short period of time, such core becomes substantially saturated with 
magnetic flux. As saturation approaches, the inductance of the transformer 
primary falls rapidly and the current of the transistor controlling the 
particular winding causing saturation also rises. In consequence, the 
voltage at the drain of that transistor, e.g., the first transistor, is at 
a first, relatively high drain voltage. 
Such relatively high drain voltage is coupled to the gate of the second 
transistor, turning it on. The second transistor thereupon conducts and 
its drain is thereupon at a relatively low second drain voltage. That is, 
the first drain voltage is higher than the second drain voltage. 
The circuit also includes a feature preventing electro-magnetic energy in 
the core from being dissipated as heat in the resistance of the 
transformer winding and in resistors in the circuit. Clearly, such energy 
dissipation is to be prevented if energy is to be transferred to the 
transformer secondary and thence to a spark gap to ignite fuel. 
In the inventive circuit, each primary winding is coupled to a DC power 
supply, e.g., +12 VDC, and a diode is interposed between each primary 
winding and the power supply. Stored transformer energy (which would 
otherwise change from flux to electrical current circulating in the 
transformer primary windings) is forced to the secondary winding by such 
diodes. 
Other aspects of the invention include a new method for spark-igniting 
fuel. Such method includes applying a voltage across an air gap at a first 
frequency, establishing an arc across the air gap and maintaining the arc 
at least until the fuel ignites. Such arc is maintained at a second 
frequency which different from (and preferably higher than) the first 
frequency. 
The new method is preferably practiced using an ignition circuit having a 
transformer with a secondary winding defining a tuned circuit having a 
resonant frequency. That is, the inductance of such secondary winding 
(represented by the symbol "L") and the turn-to-turn capacitance of such 
winding (represented by the symbol "C") form a resonant LC circuit. 
The arc maintaining step includes maintaining the arc at a second frequency 
substantially equal to the resonant frequency. The two transistors 
controlling the primary windings of the transformer are switched at a 
frequency and the voltage applying step includes switching such 
transistors at a switching frequency substantially equal to the first 
frequency. 
In a preferred method, the second frequency is higher than the first 
frequency, preferably at least twice the first frequency. Most preferably, 
the second or resonant frequency is several times the first or switching 
frequency. 
Other details of the invention are set forth in the following detailed 
description and in the drawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Before describing the inventive igniter circuit 10, it will be helpful to 
have a more complete understanding of an application (certainly not the 
only application) for which the circuit 10 may be used. Referring to FIG. 
1, a pressure washer 11 is dolly mounted and has a water pump 13 and a 
prime mover 15, e.g., an electric motor, for driving the pump 13. When the 
pump 13 is driven and the controls properly manipulated, water heated in 
the tank 17 is expelled through a wand-mounted nozzle (not shown). A 
burner (not shown) and the circuit 10 used to ignite such burner are 
beneath the tank 17. 
Referring next to FIG. 2, a general description of the operation of the 
igniter circuit 10 will be provided. Such circuit 10 operates from a 12 
volt DC source 19 (such as a motor vehicle battery) and uses a transformer 
21 to significantly "step up" the voltage from the transformer primary 
windings 23, 25 to the secondary winding 27. Such high voltage is applied 
to a pair of electrodes 29 which are spaced apart slightly to define an 
air gap 31. The high voltage spark "jumps the gap 31" and in so doing, 
ignites nearby fuel. In the pressure washer application described above, 
the fuel is atomized kerosene, fuel oil or the like. The operation of the 
circuit 10 to ignite fuel is not unlike the operation of the ignition 
circuit and spark plug to ignite atomized gasoline, all as used in an 
automobile engine. 
Persons of ordinary skill in the art recognize the general understanding 
that transformers do not operate on direct current (DC) power. As 
described in detail below, the new circuit 10 directs DC power alternately 
along two paths so that, to the transformer 21, it "looks like" 
alternating current (AC) power. 
In the following discussion, reference is made to "open circuit" and "short 
circuit" operating conditions and the new igniter circuit 10 operates 
under both such conditions. The open circuit condition is that prevailing 
between the instant 12 VDC power is applied to the circuit 10 and the 
instant the air in the air gap 31 ionizes and the spark jumps such air gap 
31 and establishes the fuel-igniting arc. This interval may be a second or 
two. 
The short circuit condition is that prevailing between the instant the 
spark jumps the air gap 31 to establish an arc and the instant at which 
the 12 VDC power is removed from the circuit 10 if the circuit 10 is 
connected for intermittent powering. There are several ways to operate the 
circuit 10. 
In a pressure washer application, one way is to use a separate control 
which initiates the circuit 10 whenever the operator actuates a spray 
nozzle to spray hot water. Another way is to use a manual "start" button 
and the manually-operated contact 33 symbolically represents such button 
or any of the several available ways to start the circuit 10. 
Referring also to FIG. 3, the transformer 21 used with the circuit 10 has 
first and second primary windings 23, 25, respectively, wound one atop the 
other on the iron core 35 in a flux-opposing relationship. (Windings 23, 
25 are shown "side-by-side" for easier understanding.) That is, when DC 
current flows into the winding 23 along the lead 37 and in the direction 
of the arrow 39, such current produces electromagnetic flux illustrated to 
propagate around the core 35 in the direction of the arrow 41. 
On the other hand, when DC current flows into the winding along the lead 43 
and in the direction of the arrow 45, such current produces 
electromagnetic flux illustrated to propagate around the core 35 in the 
direction of the arrow 47. As will be seen, the circuit 10 is uniquely 
arranged so that the DC currents represented by the arrows 39, 45 are 
caused to flow alternately at relatively high frequency. 
Flux propagating in opposite directions around the core 35 cuts across the 
secondary winding 27 in opposite directions. Therefore, an AC voltage is 
induced across the electrodes 29. And since the ratio of the number of 
turns of the secondary winding 27 to the number of turns of either primary 
winding 23, 25 is quite high, such AC voltage across the electrodes 29 is 
quite high in the open circuit condition, i.e., about 10,000 VAC, for 
rapid air ionization and sparking. In the preferred igniter circuit 10, 
the turns ratio is in the range of 300:1 to 400:1 with about 340:1 being 
highly preferred. 
It should be appreciated that the amount of flux in the transformer core 35 
is a function of the number of turns of the windings 23, 25 and the 
magnitude of the current flowing through the windings 23, 25. However, the 
core 35 is not a "flux pipeline" of infinite capacity. Quite the contrary. 
There is a limit to the amount of flux a given transformer core 35 can 
carry and when that limit is reached, the core 35 is said to be 
"saturated." Core saturation is used in a unique way in the circuit 10. 
A more detailed description of the operation of the circuit 10 follows. 
However, such description will be more easily understood by first having a 
general understanding of how a metal oxide semiconductor field effect 
transistor (MOSFET) operates. Referring again to FIG. 2, each MOSFET 49, 
51 is of the N-channel type and has a source 53a, 53b, respectively, a 
drain 55a, 55b, respectively, and a gate 57a, 57b, respectively. Each 
MOSFET 49, 51 can be considered as a switch which can be open or closed to 
block electric current or permit current to flow. 
If a MOSFET, e.g., MOSFET 49, is conducting (such MOSFET 49 is functioning 
as a closed switch), current flows from the 12 VDC supply 19, through a 
diode 59 and a winding 23, and through the drain 55a to the source 53a. On 
the other hand, when the MOSFET 49 functions as an open switch, no current 
flows. Whether or not either MOSFET "switch" is open or closed depends 
upon the voltage at the gate 57. 
If the voltage at the gate 57 is zero or negative, the MOSFET 49, 51 is 
nonconductive, i.e., the "switch" is open. If the gate voltage rises to an 
appropriate positive value, e.g., 5 VDC or so, the MOSFET 49, 51 conducts 
and functions as a closed switch. 
The circuit 10 includes two MOSFETs 49, 51 in what is referred to as a 
push-pull configuration. "Push-pull" is another way of referring to the 
alternating conduction of the two MOSFETs 49, 51 as described above. Each 
MOSFET 49 and 51 controls the flow of current through a separate primary 
winding 23 and 25, respectively, of the ignition transformer 21. The drain 
55 of each MOSFET 49, 51 is coupled to the gate 57 of the other MOSFET 51, 
49 through a resistor-capacitor coupling network 61 or 63. This 
arrangement provides positive feedback, with a loop gain of much greater 
than one. The circuit 10 oscillates as the MOSFETs 49, 51 are alternately 
closed and opened. 
From the foregoing, it will be appreciated that generally stated, the 
voltage at the drain 55a of a MOSFET, e.g., MOSFET 49, controls the gate 
57b of the other MOSFET 51 and vice versa. The specification below 
includes an explanation of how and why the drain voltage of a particular 
MOSFET 49, 51 goes up and down. 
The circuit 10 uses characteristics of the transformer 21 (along with 
selected values of resistors and capacitors) to control the rate at which 
the circuit 10 oscillates. 
Continuing reference to FIG. 2 and referring also to FIGS. 4 and 5, this 
part of the specification assumes that a "switch" is closed and that 
MOSFET 49 is conducting current as shown in the region 65 of FIG. 4. 
During current conduction, there is very little voltage "drop" across the 
MOSFET 49 (measured from drain 55a to source 53a) and such voltage is as 
shown in the region 67 of FIG. 5. As illustrated in the region 65 of FIG. 
4, the average current flowing through the MOSFET 49 (as represented by 
the dashed line 69) increases rather gradually until the transformer 21 
saturates. 
While the MOSFET 49 is conducting current, the current flowing through the 
MOSFET 51 is essentially zero as shown in the region 71 of FIG. 6. And the 
drain-to-source voltage across MOSFET 51 is relatively high (but 
declining) as shown in the region 73 of FIG. 7. 
Upon saturation due to rising average current flowing through MOSFET 49, 
the transformer inductance falls sharply with an attendant sharp rise in 
MOSFET current as shown at 75. The drain voltage of the conducting MOSFET 
49 also rises rapidly as shown at 77. Since high drain voltage at MOSFET 
49 controls "gating" of the MOSFET 51, the latter turns on and the MOSFET 
49 turns off. 
While the MOSFET 51 is conducting, its current and voltage replicate those 
of the MOSFET 49. In other words, its current and voltage are like those 
shown in regions 65 and 67 of FIGS. 4 and 5, respectively. 
At the transition when one MOSFET, e.g., MOSFET 49, stops conducting and 
the other MOSFET, e.g., MOSFET 51 starts conducting, a large amount of 
electromagnetic energy (transferred to the transformer core 35 by the 
primary windings 23, 25) is stored in the core 35. But for the inclusion 
of a pair of diodes 59, 60 blocking negative current flow, that energy 
stored in the core 35 would cause the drain 55a or 55b of the 
then-conducting MOSFET 49, 51 to be driven negative with respect to ground 
79. 
The diode 81a, 81b internal to a MOSFET 49, 51, respectively, would conduct 
and the energy would be dissipated in the resistance of the winding 23, 
25, the diodes 81a, 81b, and the source resistors 83a, 83b. The blocking 
diodes 59, 60 prevents such "back feeding" and causes the electromagnetic 
energy to be transferred to the transformer secondary 85. 
The transformer secondary 85 is a tuned circuit and in a highly preferred 
embodiment, the tuned frequency of such secondary 85 is several times the 
switching frequency of the MOSFETs 49 and 51. As alternating conduction of 
MOSFETS 49 and 51 continues, an open circuit AC voltage is produced across 
the electrodes 29. (The open circuit voltage is the electrode-to-electrode 
voltage before the air in the gap 31 between the electrodes 29 is ionized 
and before current flows therebetween.) 
The open circuit electrode voltage in the regions 87 of FIG. 8 results from 
conduction of MOSFET 49 while the electrode voltage in the regions 89 
results from conduction of MOSFET 51. The open circuit electrode voltage 
is high, on the order of 30,000 VAC peak-to-peak, and with the component 
values mentioned below, the frequency is in the range of 2-4 KHz. During 
open circuit operation, the current is a very low "trickle" current. 
It is now assumed that the aforementioned open circuit electrode voltage 
has ionized the air in the gap 31 between the electrodes 29. When that 
occurs, there is a substantial reduction in the resistance between the 
electrodes 29. As a result, the current flowing between the electrodes 29 
increases dramatically to about 20-40 milliamperes peak-to-peak and at 
that time, the maximum current in the primary windings 23, 25 is about 12 
amperes. At the same time, the voltage across the electrodes 29 drops 
sharply to about 3000 VAC. The transformer secondary 85 is then said to be 
short-circuited and the impedance of the transformer secondary 85 is 
sharply reduced. 
The reason for this sharp reduction in inductive impedance is better 
understood by considering an inductive impedance equation. In such 
equation, inductance equals [a constant x turns x flux], all divided by 
current. Since the value of current appears in the denominator (and 
recalling the aforementioned sharp increase in the current flowing between 
the electrodes 29), it is now apparent why the impedance of the secondary 
85 is sharply reduced. 
The impedance of the primary windings 23, 25 is therefore also reduced and 
the MOSFETs 49 and 51 commence switching at a frequency higher than that 
in the open circuit mode. In the illustrated circuit 10, the frequency of 
switching when the transformer 21 is short-circuited is in the range of 
3-5 KHz. 
FIGS. 9-12 show the voltage and current of MOSFET 49 and 51 and FIG. 13 
shows the output current, all while the transformer secondary 85 is short 
circuited and output current flows between the electrodes 29. Regions 91 
and 93 of FIGS. 9 and 10 show the current and drain voltage, respectively, 
of MOSFET 49 while such MOSFET 49 is conducting and regions 95 and 97 of 
FIGS. 11 and 12 show the current and drain voltage, respectively, of 
MOSFET 51 while it is conducting. FIG. 13 shows the current between 
electrodes 29. The current in the regions 99 is when the MOSFET 49 is 
conducting while the current in the regions 101 is when the MOSFET 51 is 
conducting. 
Current flowing between the electrodes 29 (as shown in FIG. 13) manifests 
itself as a hot spark (represented by the symbol 103) across the air gap 
31. In the exemplary application of the circuit 10 involving the pressure 
washer 11, the spark 103 is closely proximate atomized fuel and such fuel 
ignites. Upon ignition, the operator opens the contact 33 since combustion 
is self-sustaining until the fuel supply is shut off. 
To recap, the new igniter circuit 10 has a number of features which include 
a dual primary winding 23, 25 with current-controlling transistors 49, 51 
operating in push-pull oscillation. The frequency of oscillation is a 
function of the characteristics of the ignition transformer 21 as well as 
of the values of the resistors and capacitors in the gate R-C coupling 
networks 61, 63. 
The diodes 59, 60 alternately block negative current flow in the primary 
windings 23, 25 and cause high energy transfer to the transformer 
secondary 85. It should be noted that conventional "flyback" ignition 
circuits use only about one-half of the flux-carrying capability of the 
core since flux does not reverse. In the depicted push-pull configuration, 
flux reverses and the flux-carrying capability of the core 35 is used 
substantially continuously. 
In the new circuit 10, output frequency in the short-circuit ("arc 
established") mode is higher than the output frequency in the open-circuit 
mode. And there is an automatic, substantial drop in secondary output 
voltage after the ignition arc is established. 
Other aspects of the invention include a new method for spark-igniting 
fuel. Such method includes applying a voltage across an air gap 31 at a 
first frequency, establishing an arc 103 across the air gap 31 and 
maintaining the arc 103 at least until the fuel ignites. Such arc 103 is 
maintained at a second frequency which different from (and preferably 
higher than) the first frequency. 
The new method is preferably practiced using an ignition circuit 10 having 
a transformer 21 with a secondary winding 27 defining a tuned circuit 
having a resonant frequency. That is, the inductance of such secondary 
winding 27 (represented by the symbol "L") and the turn-to-turn 
capacitance of such winding 27 (represented by the symbol "C") form a 
resonant LC circuit. 
The arc maintaining step includes maintaining the arc 103 at a second 
frequency substantially equal to the resonant frequency. The two 
transistors 49, 51 controlling the primary windings 23, 25, respectively, 
of the transformer 21 are switched at a frequency and the voltage applying 
step includes switching such transistors 49, 51 at a switching frequency 
substantially equal to the first frequency. 
In a preferred method, the second frequency is higher than the first 
frequency, preferably at least twice the first frequency. Most preferably 
the second or resonant frequency is several times the first or switching 
frequency. 
The following components have been found useful in the circuit 10: 
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COMPONENT VALUE 
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R1, R2, R6, R8 1 Kohm 
R5, R7 680 ohm 
R3, R4 0.1 ohm 
C1, C2 0.0015 .mu.F 
Primary windings 30 turns each 
Secondary winding 5100 turns 
D1, D4 Zener Diode 
D2, D3 Conventional Diode 
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While the principles of the invention have been shown and described in 
connection with a specific embodiment, it is to be understood clearly that 
such is by way of example and is not limiting.