US 3556706 A
Description (OCR text may contain errors)
Jan. 19, 1971 v R. H. CAMPBELL 3,556,706
I OIL BURNER SPARK IGNITION SYSTEM Filed July 16, 1 969 66 5 I v 2 I 5s 44 z g INVEN'TORI RICHARD H. CAMPBELL BY: mason KolelqmovinQ-n Patented Jan. 19, 1971 US. Cl. 431265 3 Claims ABSTRACT OF THE DISCLOSURE An oil burner ignition system produces a tongue-like configuration of individual sparks extending from a pair of discharge electrodes and into the spray pattern of oil particles produced by the burner nozzle. The system includes a spark generating relaxation oscillator circuit capable of producing individual sparks at a frequency such that subsequent sparks are able to follow the elongating ionized air path established by previous sparks. The oscillator circuit includes a capacitor charged from an AC power source and repetitively discharged during alternate half cycles at rapid intervalsthrough the primary winding of a transformer by means of a controlled rectifier gated in accordance with the capacitor charge level.
The present invention relates to ignition systems for fuel oil burners.
A conventional fuel oil burner includes a nozzle for creating a spray pattern of oil particles in an air stream produced by a blower. In order to ignite the oil particles emerging from the burner nozzle, the traditional method is to mount a pair of spark electrodes upstream in the air stream from the spray pattern and to provide a substantially continuous stream of sparks with a large and expensive transformer coupled to an AC supply. Sparks produced in this manner are capable of forming a tonguelike spark extension in the air stream projecting from the electrodes into the spray pattern.
Due to the size, weight, expense and power consumption of conventional spark transformers, it has been proposed to provide solid state ignition of fuel oil burners. Some devices of this type introduced to date have not produced sparks capable of forming into a tongue-like extension, and as a result have required location of the spark electrode tips within the oil spray pattern. This approach is subject to several disadvantages including the fact that when the electrodes are located in the oil path, carbon forms on the electrodes eventually bridging the spark gap and resulting in unreliable ignition. In addition, impinging oil deflected from the spray pattern may reach the combustion area in an unatomized form, resulting in an undesirable smoky fire. Furthermore, interference by the electrodes with the spray pattern can disturb the uniformity of the pattern to such an extent that unstable, lopsided and noisy combustion takes place.
In order to overcome such disadvantages, it has been proposed to produce with asolid state ignition device a low frequency series of individual sparks each having a size and duration capable of being diverted by the air stream into the spray pattern. Devices of this character developed heretofore, however, have been subject to the disadvantages of high power consumption, expense and questionable reliability in operation.
Among the important objects of the present invention are to provide an improved oil burner ignition system characterized by small size, low cost, and reliability; and to provide an improved solid state fuel oil ignition system capable of producing a tongue-like extension of sparks in an air stream.
In brief, an oil burner ignition system constructed in accordance with the present invention serves to ignite an oil burner of the type producing a spray pattern of oil particles in an air stream. The ignition system includes a pair of spark discharge electrodes having spaced tip portions forming a spark gap and being located upstream in the air stream from the spray pattern and out of the path of the oil particles. In accordance with an important feature of the invention, a relaxation oscillator generates at the spark gap a series of individual sparks at a frequency such that a series of sparks traverse an ionized air path or region and are deflected from the spark gap into the spray pattern for ignition of the oil particles. A first circuit loop of the oscillator circuit includes a pair of power supply terminals adapted to be connected to an AC power source for charging a capacitor in a predetermined polarity during alternate half cycles of the power supply voltage waveform. A second circuit loop includes the primary winding of a transformer and the capacitor, together with the output terminals of a controlled rectifier. A trigger circuit for the rectifier places the rectifier in a conductive condition in response to a predetermined capacitor charge level, and the capacitor discharges in a current pulse through the primary winding thereby to produce a spark at the spark electrodes coupled to the secondary winding of the transformer. The controlled rectifier is returned to a nonconductive condition in response to production of the current pulse whereupon the capacitor recharges and subsequent sparks are produced. Since the system produces a train of individual short duration high frequency sparks rather than low frequency long duration sparks, the circuit design is simplified and the power consumption is significantly reduced.
The invention together with the above and other objects may be better understood from consideration of an illustrative embodiment shown in the accompanying drawing.
The single figure of the drawing includes a somewhat diagrammatic sectional view of a fuel oil burner together with a diagrammatic and schematic illustration of an ignition system for the burner.
Referring now more specifically to the drawing, there is illustrated part of an oil burner 10 equipped with an improved ignition system designated as a whole by the reference numeral 12 and constructed in accordance with the principles of the present invention. In accordance with important features of the present invention, the ignition system 12 is compact in size, light in weight, does not require a substantial amount of power for operation, and in operation serves to produce a series of individual high frequency sparks capable of establishing a deflectable ionized air path to the end that the ignition system 12 does not intrude upon or interfere with the oil particles emitted by the burner 10.
The burner 10 may be of conventional construction and includes a draft tube 14 Within which is centrally disposed a nozzle 16 supplied with pressurized fuel oil from a suitable pump or the like by means-"0f an oil conduit 18. Air is supplied to the draft tube 14 by a suitable fan or blower (not shown) and moves in a stream through a wall structure 20 defining an array of swirl vanes 22. The air stream emerges from the burner 10 through an aperture 24 in a choke plate 26, and the swirl vanes 22 and choke plate 26 combine to impart a turbulent swirling motion to the air stream.
Oil expelled from the nozzle 16 is atomized and forms a spray pattern of oil particles designated by the numeral 28. In the illustrated arrangement the spray pattern is substantially conical in shape. Interaction between the swirling turbulent air stream and the emerging oil particles in the region of the choke plate 26 causes some divergence of oil particles from the true conical array of particles,
and the actual interface between the spray pattern and the air stream is not well defined and may vary by as much as one-eighth inch or so,
In order to ignite the oil particles in the spray pattern 28, the ignition system 12 includes a pair of spark discharge electrodes 30 and 32 supported by a pair of insulators 34 and 36 carried by the wall structure 20. A pair of angularly offset electrode tip portions 30A and 32A define a spark gap 38 which may have a length in the neighborhood of one-eighth to three-sixteenth inch. Advantageously, the spark gap 38 defined between the tip portions 30A and 32A is spaced sutficiently from the spray pattern 28 so that the electrodes are not contacted by oil particles expelled from the nozzle 16. For example, the electrode tips 30A and 32A may be located onequarter inch upstream in the moving air stream from the normal boundary of the spray pattern 28 so that disturbances in the spray pattern caused by the turbulent air stream do not result in oil particles striking the elec trodes. As a result, the problem of carbon formation on the electrodes is greatly diminished, and since the electrodes do not interfere with the spray pattern, a clean burning, even and quiet combustion condition is maintained.
One important aspect of the present invention resides in the ability of the ignition system 12 to produce a high frequency series of individual sparks at the spark gap 38 in order that a tongue-like extension of sparks is deflected by the moving air stream from the gap 38 and into the spray pattern 28 for the ignition of the oil particles. The system 12 includes a relaxation oscillator circuit generally designated as 40 coupled by means of a transformer 42 to the electrodes 30 and 32 for producing the highly desirable high frequency spark action at the electrodes.
More specifically, the oscillator circuit 40 includes a pair of power supply terminals 44 and 46 adapted to be connected to a standard sixty cycle AC power supply having a rated line voltage in the neighborhood of 120 volts. During alternate half cycles of the voltage waveform when the terminal 44 is positive with respect to the terminal 46, the oscillator circuit 40 is effective to produce a series of individual sparks at a high frequency. When the terminal 44 becomes positive relative to the terminal 46, a capacitor 48 is charged by a current flowing in a first circuit loop through a current limiting resistor 50 and an inductor or choke 51. The capacitor 48 is permitted to become charged due to the initially nonconductive condition of a controlled rectifier 52.
In order to control discharge of the capacitor 48, there is provided a trigger circuit 54 coupled between the capacitor and the gate electrode of the controlled rectifier 52 for rendering the controlled rectifier '2 conductive when a predetermined capacitor charge level is attained. The trigger circuit includes a resistor 58 and a variable resistor 60, the setting of which determines the firing point of the controlled rectifier 52. A resistor 56 provides stabilizing gate bias on the controlled rectifier for preventing spurious triggering due to internally generated leakage currents. A capacitor 62 provides a slight delay phase shift, and a diode 64 is included to prevent an excessive and possibly damaging voltage drop between the cathode and gate electrodes of the controlled rectifier 52.
When the controlled rectifier 52 is rendered conductive, the capacitor 48 rapidly discharges in a current pulse through a second circuit loop including the capacitor 48, a current limiting resistor 66, the anode-cathode circuit of the controlled rectifier 52 and a primary winding 68 of the transformer 42. A secondary winding 70 of the transformer 42 is connected to the electrodes 30 and 32 and includes substantially more turns than the primary 68 to provide a high voltage for generation of an arc at the spark gap 38. The transfer of energy into the spark gap may be substantially improved by choosing the electrical values of the circuit so that resonance takes place at a frequency approximating that of the generated pulse.
Upon discharge of the capacitor to produce a spark, the controlled rectifier 52 is returned to its nonconductive condition in preparation for another cycle of operation. More specifically, when the controlled rectifier 52 becomes conductive, the inductance 51 acts as a choke to prevent rapidly increasing current flow from the power supply terminals 44 and 46. The collapsing field associated with the primary winding 68 of the transformer 42 imposes a reverse voltage through the capacitor 48 across the anode and cathode of the controlled rectifier 52, thereby to return it to its nonconductive condition. Preferably the rectifier is subjected to a substantial reverse bias to provide positive turnoff and to increase the frequency of operation. As a result, the capacitor 48 again is permitted to become charged until the trigger circuit 54 again renders the controlled rectifier 52 conductive to produce another spark.
The sparking operation continues repetitively throughout the half cycle of the power supply waveform until the power supply voltage is incapable of charging the capacitor to the extent necessary for operation of the trigger circuit 54. The oscillator then remains inactive during the follow-ing half cycle of reverse polarity and operates once more in the second following half cycle. Since the oscillator is energized directly by an AC supply, rather than by DC, the reverse polarity excursions of the power supply render the controlled rectifier 52 nonconductive in the event that the negative voltage pulse normally turning off the rectifier is interrupted or the like. Thus, the reliability of the circuit is improved because the oscillator is effectively returned to its initial condition prior to each positive excursion of the power supply voltage waveform.
It is critical in carrying out the present invention that the frequency of operation of the relaxation oscillator circuit 40 be sufficient so that a tongue-like extension of spark is created in the air stream at the spark gap 38. Having reference to the mechanism of the arc inthe air stream at the gap 38, when an arc is struck by an initial high voltage jump across the shortest path between the two spaced electrode tips 30A and 32A, the air path traversed by the arc is ionized and consequently exhibits an abrupt drop in impedance. The arc continues over the ionized path, and due to the movement of air through the region of the spark gap the ionized path is displaced downstream and becomes elongated. At some point the combination of decreasing voltage at the spark gap and increasing length of the low impedance ionized air path causes the arc to become extinguished. For a period of time after the arc is extinguished, the ionized region persists adjacent the electrodes. If the subsequent spark is produced soon enough, the arc is reestablished in the region of ionized air remaining from the previous spark. Since the region of ionized air is continually moved downstream, the arc can be extended into the spray pattern 28 to ignite the oil particles ,even though the electrode tips 30A and 32A are located as much as one-quarter inch or more from the spray pattern region.
The frequency of operation is important to carrying out this result. For example, many known solid state ignition devices produce sparks at the rate of one spark during each half cycle of the power supply AC waveform. With such an extended duration of time between sparks, it is impossible for subsequent sparks to produce arcing across the ionized air path established by previous sparks, and the tongue-like spark extension of several individual sparks cannot be achieved.
In conventional oil burner apparatus the velocity of the moving air stream may range from as high as fifty feet per second to a much smaller value. In order to produce the desirable result of the present invention, it is necessary for the frequency of spark production to be no less than about several thousand sparks per second. As a practical matter, it is believed that under ordinary conditions about twenty-five hundred sparks per second is the minimum rate which will produce the desired spark tongue extension, while a considerably higher frequency is desirable in order to produce sparks having a desirable hot, fat appearance.
Another advantage of the high frequency of operation of the relaxation oscillator circuit 40 is that the cost of the transformer 42 is reduced. More specifically, at a high operating frequency the impedance of the transformer is such that smaller wire and fewer turns can be used than would be required at a lower frequency.
In an oil burner ignition apparatus constructed in accordance with the present invention, the following specific values, dimensions and details of construction produced highly satisfactory results. These specific details of the illustrated embodiment are given by way of example only, and other arrangements may be used as well. The spark gap 38 between the electrode tips 30A and 32A had a length of about one-eighth inch, and the gap was spaced about one-quarter inch from the normal position of the spray pattern 28. The circuit was operated from a standard sixty cycle 120 volt AC power supply, and the controlled rectifier 52 comprised a General Electric Company type C-106B1 semiconductor controlled rectifier. The transformer 42 included a ferrite core having a primary winding 68 with 18 turns and a secondary winding 70' with 891 turns. The remaining components of the relaxation oscillator circuit had the following values:
Resistor 50-70 ohms Resistor 5651O ohms Resistor 5833,000 ohms Variable resistor 60250,000 ohms, maximum Resistor 66 4.0 ohms Capacitor 48-.68 microfarad Capacitor 62.O68 microfarad Inductance 52335 millihenries It was found that the circuit constructed with these specific components performed in a successful manner and was capable of producing individual sparks at a spark pro duction frequency of about five thousand to about eight thousand sparks per second during the intervals when the oscillator circuit 40 operated. Consequently, a desirable tongue-like extension was produced at the spark gap extending well into the region of the oil particles in the spray pattern 28.
What is claimed and desired to be secured by Letters Patent of the United States is:
1. An ignition system for use with a fuel oil burner of the type producing a spray pattern of oil particles in an air stream, said system comprising:
a pair of spark discharge electrodes having tip portions spaced apart to form a spark gap;
said tip portions being disposed upstream in said air stream from said spray pattern and out of the path of the oil particles;
an oscillator circuit for producing a series of spark discharges at said spark discharge electrodes and including a pair of power supply terminals adapted to be coupled to an AC voltage source;
a transformer having a secondary winding coupled to said electrodes and having a primary winding;
a first circuit loop including said power supply terminals for charging said capacitor in accordance with the voltage source;
a second circuit loop including said primary winding and said capacitor;
a controlled rectifier having a pair of output electrodes and a control electrode;
said output electrodes being connected in said second circuit loop for controlling the discharge of said capacitor through said second circuit loop;
a trigger circuit coupled between said capacitor and said control electrode for rendering said controlled conduction device conductive in response to a predetermined capacitor charge level thereby to discharge said capacitor in a current pulse through said second circuit loop to produce an are at said spark discharge electrodes;
and means responsive to production of the current pulse for returning said controlled rectifier to the nonconductive condition;
said oscillator being constructed and arranged to pro duce sparks at a rate sufiicient to provide a tongue of sparks in said air stream extending into said spray pattern for ignition of said oil particles.
2. The ignition system of claim 1, the rate of spark production being more than about twenty-five hundred sparks per second during operation of said oscillator.
3. The ignition system of claim 1, the rate of spark production being in the approximate range of about five thousand to about eight thousand sparks per second during operation of said oscillator.
References Cited UNITED STATES PATENTS 2,789,632 4/1957 Smits 317-96X 2,817,395 12/1957 Lutz et al 431265X 3,142,332 7/1964 Weber et al 317-96X 3,207,953 9/1965 Smith et al. 317-96 3,318,358 5/1967 Potts 317-96X 3,377,125 4/ 1968 Zielinski 431-74 3,384,440 5/1968 Mayer 431-66 3,385,527 5/1968 Drewry 431265X 3,457,456 7/1969 Dietz 315-206 VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R.