Patent Publication Number: US-6217312-B1

Title: Ignition system for a gas appliance

Description:
FIELD OF THE INVENTION 
     The present invention is related to gas ignition systems. In particular, the present invention is related to gas ignition systems for gas appliances and heating equipment, including gas ranges. 
     BACKGROUND 
     Conventional gas appliances and heating equipment, such as gas ranges, often use silicon carbide (SiC) hot surface ignitors or spark ignitors. The conventional SiC ignitor is designed to survive in the gas range environment. The SiC ignitor is normally placed in series with the gas valve. The gas valve is designed to open when the current supplied to it exceeds a certain value. The SiC ignitor has a carefully controlled resistance versus temperature characteristic such that: (1) when current is initially supplied to the ignitor and the ignitor is cold, it has a relatively high resistance that keeps the current low enough so the gas valve stays closed; and (2) when the ignitor heats up, the resistance drops so the current becomes sufficiently large to open the gas valve. When the current reaches this threshold point, the ignitor is hot enough to ignite the gas. This resistance versus temperature relationship serves as a “fail-safe” in that the ignitor must reach a certain temperature before the gas valve opens, thus avoiding the situation of gas flowing to an ignitor which is not hot enough to ignite the gas. 
     Conventional SiC gas range ignitors are produced by several commercial vendors, including Surface Igniter Co. of Chagrin Falls, Ohio and Saint-Gobain/Norton Co. of Milford, N.H. Some of the problems with these conventional ignitors are that they are porous, fragile, and expensive. In addition, the resistance versus temperature characteristics of these conventional SiC ignitors may alter or drift over time, thereby adversely affecting their reliability. 
     Ignitor materials which are more mechanically robust than SiC have also been developed. One such ignitor, the Mini-Ignitor®, available from the Saint-Gobain/Norton Company of Milford, N.H., comprises a pressure sintered composite of aluminum nitride (“AlN”), molybdenum disilicide (“MoSi 2 ”), and silicon carbide (“SiC”), and is designed for 8 volt through 48 volt applications. However, the resistance versus temperature characteristics of the pressure sintered composite material is different from the resistance characteristics of conventional ignitor materials such as SiC. Generally, the pressure sintered composite material has a resistance which increases with temperature (e.g., a metallic resistance characteristic). Accordingly, pressure sintered composite ignitors are generally not compatible with existing conventional ignition systems which rely on a resistance fail safe region. 
     Thus, there is a need for a reliable ignition system which does not rely on a resistance fail safe region and which is not susceptible to performance degradation due to temperature drifts. 
     SUMMARY 
     The present invention provides an ignition system for gas appliances comprising an ignition controller coupled to a power source to receive a current from the power source. The ignition controller is coupled to an ignitor. The ignition controller is also coupled to a current actuated valve that releases a flow of gas when the current is greater than a first predetermined current value and less than a second predetermined current value. 
     The present invention further provides a gas oven comprising ignition control means. An ignitor is coupled to the ignition control means. The ignition control means is also coupled to a current actuated valve that releases a flow of gas when the current is greater than a first predetermined current value and less than a second predetermined current value. A burner is also coupled to the gas valve to receive the flow of gas. 
     The present invention provides a method for controlling the ignition of a burner with an ignitor. A current (I) is provided to the ignitor. A valve that releases a flow of gas is opened when the current (I) is greater than a first current value (I 1 ) and less than a second current value (I 2 ), where I 1  is less than I 2 . Thus, the ignitor ignites gas flowing from the burner when I 1 &lt;I&lt;I 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows one embodiment of an ignition system incorporated in a gas oven according to an exemplary embodiment of the present invention. 
     FIG. 2 shows another embodiment of the ignition system. 
     FIG. 3 shows another embodiment of the ignition system. 
     FIG. 4 shows the resistance versus temperature characteristics of silicon carbide and pressure sintered SiC—MoSi 2 —Al 2 O 3 . 
     FIG. 5 shows one embodiment of an ignitor. 
     FIG. 6 shows another embodiment of an ignitor. 
     FIG. 7 shows another embodiment of an ignitor. 
     FIG. 8 shows an alternative embodiment of the ignition system. 
     FIG. 9 shows an alternative embodiment of the ignition system. 
     FIG. 10 shows an alternative embodiment of the ignition system. 
     FIG. 11 shows an alternative embodiment of the ignition system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to an ignition system for gas appliances and heating equipment. An ignition system according to one embodiment of the present invention is shown in FIG.  1 . The ignition system  10  includes a controller  16 , an ignitor  20 , a main burner  26 , and a current actuated valve  22 . Ignition system  10  is coupled to a power source  12  to provide current for the ignition system. For example, power source  12  can be a standard 120 volt alternating current (AC) power source. Alternatively, the power source  12  can be an 80 volt power source or a 240 volt power source. Line  13  couples power source  12  to ignition controller  16  within ignition system  10 . 
     Ignitor  20  is coupled to ignition controller  16  via line  19 . Ignitor  20  can comprise a pressure-sintered composite material or other material which has a metallic resistance characteristic, as will be discussed in more detail below. Ignition controller  16  is also coupled to current actuated valve  22  via a line  21 . Main burner  26  is adapted to be supplied fuel, such as natural gas, propane, etc., from a fuel source (not shown) via a gas conduit  24 . Ignitor  20  is disposed adjacent to burner  26 , which can be housed inside an oven chamber  18 . Alternatively, burner  26  can be located atop a conventional range. In addition, a conventional gas regulator (not shown) can be disposed in conduit  24  between the fuel source and valve  22 . When valve  22  is open, fuel flows to burner  26 . Generally, the ignitor remains energized whenever the gas valve  22  is open. Valve  22  can be any type of suitable valve such as a conventional solenoid valve, which can be inexpensive and has good reliability. 
     Optionally, an ignition indicator  27  can also be housed in oven chamber  18  and adjacent burner  26 . Ignition indicator  27  can be a thermostat, a thermocouple, a resistance temperature device, a light sensor, or other flame sensitive device. Indicator  27  can be used to determine when flames are present. 
     Ignition controller  16  is able to control the opening and closing of valve  22  as well as the energization of ignitor  20 . Ignition controller  16  can be adapted to receive a selection or control signal from a user-operated control knob (not shown), which can cause the ignition of gas at burner  26  and set a desired temperature within oven chamber  18 . When the user-operated control knob is in an “off” position, current is not available to ignitor  20  from power source  12 . 
     FIG. 2 shows one embodiment of ignition controller  16  that produces acceptable results. Ignition controller  16  is designed such that gas valve  22  is opened only when the ignition temperature is reached. A suitable ignition temperature is realized when the current (I) reaching ignitor  20  is of a predetermined level. In this embodiment, ignition controller  16  comprises relays  32  and  36  that are placed in series and couple line  13  to line  21  (power source to gas valve). Relays  32  and  36  typically comprise current actuated or driven switches. 
     In the embodiment shown in FIG. 2, relay  32  is normally closed. Relay  32  opens only if the current I is greater than an upper current level I 2 . When relay  32  is open, line  13  is not connected to line  21  and gas valve  22 . 
     Relay  36  is a current actuated relay that is normally in the open position, as shown in FIG.  2 . When relay  36  is in the open position, line  13  is not connected to line  21  and gas valve  22 . Relay  36  closes when the current I is greater than a threshold current level I 1 . 
     Line  21  is only coupled to power source  12  via line  13  when I 1 &lt;I&lt;I 2 . When the current level is too low (I&lt;I 1 ; temperature too low) or the current level is too high (I&gt;I 2 ; temperature too high), gas valve  22  will be shut off, thus providing a safety feature to the gas appliance. The minimum current limit I 1  protects against an open circuit condition which may have been caused by ignitor burnout, for example. The maximum current limit I 2  protects against a short across the ignitor or elsewhere, for example. 
     Alternatively, relays  32  and  36  can be changed in position without affecting operation of ignition controller  16 . Further, the present invention is not limited to the use of solenoid relays. Other current sensitive circuit components such as switches and diodes can be utilized in ignition controller  16  as will be apparent to those of skill in the art given the present description. 
     According to one embodiment of the invention, the ignition system  10  will provide gas to burner  26  when the current is at a level corresponding to an ignitor temperature of between 800 degrees and 1500 degrees centigrade. Typically, a temperature range of between 1100 and 1400 degrees centigrade is utilized. The actual values for the lower and upper current levels (i.e., I 1  and I 2 ) can depend on a number of factors including, but not limited to, the voltage source utilized, the resistance characteristics of the ignitor, and the physical size of the ignitor. Accordingly, the upper and lower current levels can be selected based on these factors, as would be apparent to one of skill in the art given the present description. 
     Relays  32  and  36  can be conventional solenoid relays, which can be purchased from a variety of commercial vendors such as Newark Electronics Corp., of New Jersey. For example, relays  32  and  36  can be two-way spring loaded contact relays. The relays can be adapted to operate with a variety of power sources, as would be apparent to one of skill in the art. Further, ignition controller  16  can be adapted to control the ignition of additional burners and the opening of additional valves as would be apparent to one of skill in the art given the present description. 
     According to another embodiment of the present invention shown in FIG. 3, relay  32  can be removed from the circuit altogether. Relay  36  is a current actuated relay that is normally in the open position, and closes when the current I is greater than a threshold current level I 1 . A fuse  35 , such as a conventional fuse, can be placed in line  19  proximate to the ignitor  20 , such that if the current through line  19  exceeds a upper current limit I 2 , the fuse  35  is blown, and the current in line  19  goes to zero. When the current in line  19  goes to zero, the relay  36  opens, which deactivates the gas valve  22 . Fuse  35  can be a timed fuse, such as a “slow-blow” fuse, available from a variety of commercial electronics vendors. Alternatively, fuse  35  can be designed according to the current characteristics of the ignition system being utilized. 
     According to one embodiment of the present invention that produces acceptable results, the ignitor  20  comprises a material which has a metallic resistance characteristic in which resistance increases with temperature. As mentioned above, conventional ignitors, such as silicon carbide ignitors, are implemented in conventional ignition systems based on their resistance characteristics. As the temperature of the SiC ignitor increases, its resistance decreases. An example of this relationship is depicted in FIG. 4, wherein the Y axis represents resistance, and the X axis represents temperature. Resistance curve  42  represents an exemplary SiC ignitor used in conventional gas appliances. The resistance curve  42  for the SiC ignitor drops to a resistance of about 30 to 40 ohms (Ω) at temperatures approaching 1200 degrees centigrade. As the temperature continues to increase, the resistance rises to a level greater than 40Ω, and continues upward. This region of the resistance curve has been utilized in some conventional ignition systems as a safety feature, or fail-safe region, in that a gas valve is only actuated when the resistance falls within a certain range. The temperature value of about 1200 degrees is sufficient to ignite natural gas. 
     The ignition system according to exemplary embodiments of the invention includes an ignitor made from a material having a resistance versus temperature characteristic that typically does not exhibit a fail safe region such as that shown in curve  42 . Conventional ignition systems relying on a resistance fail-safe region are thus generally incompatible with ignitor materials having a metallic resistance characteristic. 
     According to one embodiment of the invention, the ignitor  20  comprises a composite material which may be formed by pressure sintering. Typically, the composite material includes an insulating ceramic, a semiconductive ceramic, and a metallic conductor. The insulating ceramic may comprise, for example, the nitride of a metal, e.g. AlN or Si 3 N 4 , or the oxide of a metal, e.g. Al 2 O 3 . Examples of suitable semiconductive ceramics include silicon carbide and boron carbide. Suitable metallic conductors include molybdenum disilicide and iron alloys, for example. The composite material typically has a metallic resistance characteristic. Examples of suitable pressure sintered composite materials include SiC—MoSi 2 —AlN and SiC—MoSi 2 —Al 2 O 3  composites, which are commercially available. 
     According to exemplary embodiments of the invention, SiC—MoSi 2 —AlN or SiC—MoSi 2 —Al 2 O 3  is utilized as the composite ignitor material. As shown in FIG. 4, SiC—MoSi 2 —Al 2 O 3  has a “metallic” resistance versus temperature characteristic in which the resistance of the material continues to increase with temperature, as shown by curve  44 . Other suitable ignitor compositions typically exhibit a metallic resistance versus temperature characteristic which may have a greater or lesser slope than that of curve  44 . 
     The composite ignitor can be made according to pressure sintering techniques that are well known to those skilled in the art. For example, the starting materials can be mixed in powder form to form large blocks of the composite ignitor material. The block is then sintered and hot-pressed. The block is cut into a conventional ignitor shape. Electrical leads and conductors are metalized onto the ends of the ignitor. Such composite ignitors are commercially available from Norton Ignitor Products, of Milford, N.H., for example. 
     The composite materials can be utilized in conventional ignitor designs such as shown in FIGS. 5 and 6. In FIG. 5, the composite material is constructed into a hair-pin or “U”-shaped ignitor  45 . A ceramic (or the like) holder  46  is filled with a high temperature insulating material and holds ignitor  45  in place in the gas stream. Leads  47  provide current to ignitor  45  in order to heat ignitor  45  to a desired temperature. Similarly, FIG. 6 shows an alternative shape ignitor  48  that is held by a ceramic (or the like) holder  49  and is heated via leads  50 . In addition, a metal shield assembly (not shown) and/or other conventional ignitor accessories can be utilized as would be apparent to one of skill in the art given the present description. 
     The ignitor, according to another embodiment of the invention, may comprise a resistive material disposed between two ceramic members. FIG. 7 shows an example of a suitable ignitor of this type. In FIG. 7, the leads  62  are electrically connected to the resistive material disposed between two ceramic plates  64 . The resistive material receives the current and generates heat, and may comprise, for example, molybdenum, tungsten, or a compound of tungsten such as tungsten carbide or tungsten silicide. The ceramic material, which may comprise silicon nitride for example, provides high temperature strength and thermal shock resistance to make the structure robust and isolates the resistive material from the ambient gases. The resistance characteristic of this type of heater is typically a metallic resistance characteristic in which resistance increases roughly linearly with temperature. Such heaters are commercially available from Kyocera Corporation, for example. 
     In another embodiment of the present invention shown in FIG. 8, ignition controller  16  includes a three-way (multi-position) solenoid relay  60 . Multi-position relay  60  has three possible positions. When the current (I) across relay  60  is less than a lower threshold current (I 1 ), relay  60  is in the open position (line  13  is not connected to gas valve  22 ). When the current (I) across relay  60  is greater than the lower threshold current (I 1 ), but less than an upper limit current (I 2 ), relay  60  is in the closed position (coupling line  13  to gas valve  22 ). When the current (I) across relay  60  is greater than the upper limit current (I 2 ), relay  60  is in the open position (line  13  is not connected to gas valve  22 ). This embodiment of ignition controller  16  can produce similar results to those achieved with a two relay circuit, such as the embodiment shown in FIG.  2 . 
     In another embodiment of the present invention shown in FIG. 9, ignition system  10  further comprises a timing controller  70 . Timing controller  70  is coupled to ignition controller  16  via line  71 . Timing controller  70  is adapted to block the flow of current to valve  22  and/or ignitor  20  in order to synchronize the ignitor and the valve operation. In this embodiment ignition controller  16  can be included as part of an electronic range controller  74 . Electronic range controllers are commonly used for controlling the operation of gas appliances and are well known in the art. 
     In the embodiment shown in FIG. 9, ignition controller  16  can further comprise a timing device  17 , such as a microprocessor, that is programmed to synchronize the opening of valve  22  corresponding to any time lag that may be present in ignitor  20  reaching a predetermined ignition temperature. For example, depending on the specific ignitor material used in ignitor  20 , a one to two second delay or a five to ten second delay may occur between the current (I) reaching a lower threshold current value (I 1 ) and when the ignitor actually reaches a suitable ignition temperature. After this delay, timing device  17  sends a control signal to timing controller  70  via line  71 . In this embodiment, timing controller  70  can comprise a switch (not shown) that is activated when timing device  17  sends the control signal to timing controller  70 . When the switch is activated, line  21  is coupled to valve  22  and valve  22  is actuated, releasing a flow of gas past ignitor  20 , which has reached a suitable ignition temperature. 
     In yet another embodiment of the present invention shown in FIG. 10, ignition system  10  includes a resistor  23 , that is connected in parallel with valve  22  along line  21 . Resistor  23  can be of a high resistance (e.g., about 1 meg-ohm (MΩ)). Resistor  23  acts to smooth current surges to valve  22 . 
     In another embodiment of the present invention shown in FIG. 11, valve  22  further comprises a valve actuation circuit  75  that includes a relay  76 . For example, in this embodiment, ignition controller  16  includes a first relay, such as relay  32  shown in FIG. 2, which is normally closed and opens when the current value I is greater than an upper threshold current I 2 . Relay  76  is normally open and closes when the current I is greater than a lower threshold current level I 1 . Thus, valve  22  is only opened when I 1 &lt;I&lt;I 2 . A conventional valve with an actuation circuit can be modified to incorporate relay  76  as would be apparent to one of skill in the art given the present description. 
     The present invention is particularly useful in a wide range of gas appliances and heating equipment, including gas ovens, furnaces, boilers, and water heaters. 
     The foregoing description of exemplary embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.