Patent Publication Number: US-6213761-B1

Title: Heating apparatus

Description:
FIELD OF THE INVENTION 
     The present application relates generally to heating devices, particularly to heating devices in which a highly efficient catalytic combustion apparatus is employed to generate heat from a vaporous fuel with reduced toxic emissions. 
     BACKGROUND INFORMATION 
     A typical catalytic combustion apparatus oxidizes a gaseous fuel, such as methane, butane or propane, at room temperature to generate heat. Generally, the fuel is introduced into a gas-tight housing where the fuel expands to completely fill the housing. As the fuel diffuses through a catalyst-containing support located at an outlet of the housing, ambient air mixes with the diffused fuel. The fuel-air mixture is then oxidized by a reaction promoted by the catalyst to produce heat. Such catalysts typically include noble metals such as platinum group metals or compounds containing the same. The substrates upon which the catalysts are supported are typically made from glass fibers, porous metals or ceramics such as ceramic wool or ceramic board and the like. 
     The products of the catalyst-enhanced oxidation reaction, such as carbon dioxide and water vapor, are discharged through the outside surface of the catalyst-containing substrate. Convection currents disperse the reaction products and draw in ambient air to provide oxygen to sustain the reaction. The reaction is normally started by igniting the reactants, by means of a flame (e.g. a pilot light) or a spark induced, for example, by an electrical ignition. 
     A drawback of such known combustion apparatuses is reliance on convection currents to circulate the reactants (air and fuel) and to remove combustion products from the catalyst-containing reaction zone. A factor which contributes to controlling the rate of oxygen available per unit area of catalyst is the rate of convection flow over the active catalytic surface. Convection currents often produce irregular and erratic flows of reactants over the active catalytic surface. Under such conditions, there is typically an uneven distribution of oxygen and/or fuel within the reaction zone containing the catalyst. Consequently, when oxygen is available in less than a stoichiometric amount relative to the fuel, incomplete combustion of the fuel occurs resulting in harmful by-products including carbon monoxide, unburned fuel and the like. As carbon monoxide and unreacted fuel accumulate, a dangerous health hazard arises that could result in serious injury or death to occupants of an enclosed space in which the heating apparatus is used. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a heating apparatus comprising a fuel source and an air inlet in communication with a mixing means for creating a uniform fuel-air mixture with a desirable ratio of air to fuel, before being combusted in a catalyst-containing combustion chamber for generating heat. Such pre-mixing of the fuel and air provides for a cleaner and more efficient heat generating combustion resulting in substantially reduced emissions of toxic substances such as carbon monoxide, unreacted fuel and the like, increased safety and an improved operating life of the apparatus. 
     In particular, one aspect of the present invention is directed to a heating apparatus which includes a combustion chamber having an inlet for receiving a uniform fuel-air mixture, and an outlet. The combustion chamber further includes a catalyst-containing substrate for initiating combustion of the fuel-air mixture. A fuel source and an air inlet are provided in communication with a mixing means for mixing the fuel and air under conditions which provide for a uniform fuel-air mixture, and for delivering the uniform fuel-air mixture through the inlet to the substrate in the combustion chamber to produce heat with minimal production of harmful by-products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side elevational view of an exemplary embodiment of a heating apparatus of the present invention. 
     FIG. 2 is a side cross-sectional view of the heating apparatus of FIG.  1 . 
     FIG. 3A shows an enlarged perspective view of an embodiment of a fuel-air mixing assembly for use in the present invention. 
     FIG. 3B shows an enlarged longitudinal cross-sectional view of the fuel-air mixing assembly shown in FIG.  3 A. 
     FIG. 3C is an enlarged top plan view of the fuel-air mixing assembly shown in FIG.  3 A. 
     FIG. 4A is an enlarged cross-sectional elevational view of one embodiment of a regulator valve assembly for use in the present application. 
     FIG. 4B is a graphical representation of a preferred profile of the fuel flow rate generated by the regulator valve assembly shown in FIG.  4 A. 
     FIG. 5 is an exploded view of an embodiment of the heating apparatus of the present invention. 
     FIG. 6A is an enlarged cross-sectional elevational view of another embodiment of a fuel-air mixing assembly with a helical structure disposed within the interior thereof. 
     FIG. 6B is an enlarged cross-sectional elevational view of a further embodiment of a fuel-air mixing assembly with a plurality of fin-like structures affixed to the interior side walls thereof. 
     FIG. 7A is a perspective view of a modified diffuser plate for use in the present invention. 
     FIG. 7B shows a top plan view of the diffuser plate shown in FIG.  7 A. 
     FIG. 7C shows a side elevational view of the diffuser plate shown in FIG.  7 A. 
     FIG. 8 is a partial cross-sectional view of an ignition assembly for use in the present invention. 
     FIG. 9A is top plan view of another embodiment of a diffuser plate for use in the present invention. 
     FIG. 9B is an elevational view of the diffuser plate shown in FIG. 9A in position relative to the ignition device. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides for a heating apparatus useful for generating heat through the catalytically accelerated reaction of a gaseous or vaporizable fuel and air mixture. The heating apparatus of the present invention can be used as a space heater for a variety of locations including tents, homes, factories, caravans, hatcheries, greenhouses, drying rooms and the like. The heating apparatus is constructed with the advantage of creating a uniform fuel-air mixture having a desirable air to fuel ratio. Such controlled pre-mixing of fuel and air provides for a cleaner and more efficient heat generating combustion resulting in substantially reduced emissions of toxic substances such as carbon monoxide, unreacted fuel and the like, increased safety, and improved operating life of the apparatus. With substantially reduced toxic emissions, the heating apparatus of the present invention provides for a safe, reliable and highly efficient direct heating system. 
     The heating apparatus of the present invention may be used with gaseous fuels having a vapor pressure greater than one atmosphere at room temperature (i.e. 20° C.), such as for example, methane, ethane, propane and butanes, and olefines such as propylene and butenes and mixtures thereof. Commercially available fuels such as natural gas, town gas, liquified natural gas, liquified petroleum gases and various waste hydrocarbon gases are suitable as well including mixtures thereof. The present invention is also applicable to vaporizable fuels (i.e. liquid fuels which may be formed into fine droplets) such as kerosene and other liquid hydrocarbon fuels which can be vaporized, or to permanent gas fuels such as hydrogen, which may be diluted with an inert gas such as nitrogen to control the temperature of combustion. 
     Referring now to the drawings, and particularly FIG. 1, an embodiment of a heating apparatus in accordance with the present invention is shown, in which a fuel such as propane is supplied under pressure from a fuel canister to a combustion chamber containing a catalyst substrate where the fuel reacts with oxygen in the presence of the catalyst under conditions and in a manner where heat is generated while eliminating or at least minimizing the presence of harmful by-products arising from incomplete combustion of fuel. 
     FIG. 1 illustrates a portable heating apparatus coupled to a fuel canister containing a fuel in a form suitable for efficient reacting with oxygen to generate heat. The term “fuel” shall include gaseous fuels (e.g. propane), vaporizable fuels such as kerosene, and mixtures thereof. The heating apparatus  10  generally includes a housing  8  defining a combustion chamber  16  therein (see FIG.  2 ). It should be noted that the housing  8  may be in a variety of shapes, including the shape of a cylinder, a rectangular solid and the like. Other shapes and sizes may be utilized depending on the application as will be recognized by one of ordinary skill in the art. 
     The combustion chamber  16  includes an outlet  17 , and an inlet in fluid communication with a fuel canister  12  through a regulator valve assembly and a fuel-air mixing assembly as will be described hereinafter. A substantially cylindrical collar  14  with a wider portion at the end opposite from the fuel canister  12 , is included for housing the regulator valve and fuel-air mixing assemblies. A knob  30  is connected to the regulator valve assembly for regulating the flow rate of fuel from the fuel canister  12  into the combustion chamber  16 , as will be described hereinafter. A heat shield  24  may be optionally provided between the collar  14  and the housing  8  to prevent the transmission of heat from the housing  8  to the other components of the apparatus  10 . 
     The housing  8  includes a gas permeable head screen  18  and a head pan  20 . The head screen  18  encloses the outlet  17  for safety purposes, i.e. preventing serious skin burns. In addition, the head screen  18  physically protects components within the combustion chamber  16 . The head screen  18  may comprise a mesh or a plurality of holes or other openings. In an exemplary embodiment, approximately 80% of the area of the head screen is open. The head screen  18  and the head pan  20  are securely fastened together by an annular clamp  22  which extends along the respective common peripheries for forming a gas-tight seal therebetween. A catalyst-containing substrate  32  is disposed in the combustion chamber  16  between the head screen  18  and the head pan  20 . 
     The introduction of the fuel-air mixture into the housing  8  under pressure and in a uniform fashion dramatically improves the operation of the heating apparatus  10  at various angles and orientations. With little effect on overall performance, the apparatus  10  can effectively operate in a vertical position resting on a base portion  28  of the fuel canister  12  or at a position such as 45° from vertical while supported by a leg stand  26 . 
     Referring to FIG. 2, the fuel is introduced into the lower portion of the combustion chamber  16  in the housing  8  via a regulator valve assembly  42  and a fuel-air mixing assembly  44 . An inlet port  46  of the regulator valve assembly  42  is configured for receiving and coupling with a fuel supply means such as the fuel canister  12  shown in FIG.  1 . The pressurized fuel is introduced into the inlet port  46  through an inlet tube  48 , which extends into the regulator valve assembly  42 . A fuel regulator assembly  249  is provided at the end of the inlet tube  48  for regulating the fuel flow through the regulator valve assembly  42  in accordance with the position of the adjust knob  30  attached thereon, as will be described hereinafter. As the regulated fuel passes through the fuel regulator assembly  249 , it exits the regulator valve assembly  42  through an outlet tube  52  in an outlet port  50  which extends into the fuel-air mixing assembly  44 . 
     The fuel-air mixing assembly  44  is defined by an elongated tubular member comprising upper and lower tubes  72  and  70 , respectively. It should be noted that the tubes  72 ,  70  of the fuel-air mixing assembly  44  may be of a unitary structure for ease of assembly. The elongated tubular member includes a bore  56  in communication with the outlet port  50  of the regulator valve assembly  42  at one end and the inlet  15  of the combustion chamber  16  at the other end. During operation, as fuel under pressure flows rapidly through the bore  56 , a low pressure gradient is produced therein relative to the ambient air pressure. An opening  58  in the lower tube  70 , proximate to the regulator valve assembly end of the fuel-air mixing assembly  44  admits ambient air into the fuel stream, i.e., air is drawn into the low pressure area of the fuel stream for mixing therewith. More than one opening  58  may be included. The size of the opening  58  and the diameter of the bore  56  are proportioned such that a specific amount of primary air is mixed with the fuel. Depending on the fuel consumption rate and the heat output of the heater, approximately 10 to 20% (preferably 10-15%) of the air required for proper stoichiometric fuel-air combustion is preferably introduced through the opening  58  into the fuel stream. The remaining 80-90% of the combustion air is drawn as secondary air at the head of the heater  10 . By limiting the amount of primary air intake to the above specified range, the risk of experiencing an explosive build-up of primary air in the lower portion of the combustion chamber  16  is eliminated or at least substantially minimized. In addition, the emission levels of harmful combustion by-products are further kept to a minimum. 
     Fuel and air entering the housing  8  into the lower portion of the combustion chamber  16  are radially diffused from the center by a diffuser plate  90 , which is preferably circular (see also FIG.  5 ), mounted above the inlet  15 . The purpose of the diffuser plate  90  is to ensure a thorough mixing and blending of the primary air with the fuel in the combustion chamber  16  and to facilitate a uniform flow of the fuel-air mixture through the catalyst-containing substrate  32  over the entire area thereof. A uniform fuel-air mixture flow ensures an efficient and complete combustion and prevents hot spots from developing on the surface of the catalyst-containing substrate  32 . Hot spots shorten the operating life of the catalyst-containing support  32 , degrade the catalyst, result in incomplete combustion, and decrease the overall efficiency of the apparatus  10 . 
     As the uniform fuel-air mixture permeates throughout the combustion chamber  16 , the mixture flows through an inner screen  34 . The inner screen  34  provides a base support for the porous catalyst-containing substrate  32  to preserve its substantially planar shape and prevent any distortions, e.g., center portion bowing downwardly. A deformed catalyst-containing support  32  creates hot spots in the surface thereof which can degrade overall performance and operating life of the apparatus  10 . A centrally-located, generally circular depression  35  in the inner screen  34  extends downwardly towards the top surface of the diffuser plate  90 . During operation, the circular depression  35  acts as a thermal conductor for transmitting heat from the catalyst-containing substrate  32  to the diffuser plate  90 . The heated diffuser plate  90  in turn acts as a heatexchanger for heating the incoming fuel-air mixture, which slightly raises the pressure gradient in the combustion chamber  16  for total saturation of the catalyst-containing substrate  32 . An increase in gas pressure further facilitates the uniform distribution and flow of the fuel-air mixture along the entire length of the catalyst-containing substrate  32 , even when the apparatus is oriented at an angle such as 45° from vertical. 
     The catalyst-containing substrate  32  is a woven fabric-like ceramic pad composed of materials such as aluminum silicon, zirconia, titania, silica and alumina and mixtures thereof that is porous for facilitating gas diffusion and refractory for resisting the heat accompanying combustion. The catalyst-containing substrate  32  further includes a catalyst material composed of a noble metal such as platinum and compounds there of which facilitates the oxidation of the fuel-air mixture to generate a flameless combustion. 
     As the uniform fuel-air mixture flows into the catalyst-containing substrate  32 , the balance of the ambient air, or secondary air, required for complete combustion circulates throughout the surface of the catalyst-containing substrate  32 . There the uniform fuel-air mixture is oxidized by the catalyst-containing substrate  32  for a clean and efficient reaction. 
     A more detailed view of the fuel-air mixing assembly  44  is shown in FIGS. 3A through 3C. As described above, the fuel-air mixing assembly  44  is a tubular member, preferably having a circular cross-section, comprised of a lower tube  70  connected to an upper tube  72 , such as by threaded engagement. The bore  56  is cylindrically shaped for permitting fuel to flow therethrough. Radially directed openings  58  are provided in the surface of the lower tube  70  for drawing ambient air therein for mixing with the fuel stream. The length of the fuel-air mixing assembly  44  should be such that thorough mixing and blending of the fuel and the air can be achieved. 
     Referring to FIG. 4A, an enlarged side cross-sectional view of the regulator valve assembly  42  is shown in greater detail. The regulator valve assembly  42  provides a means for regulating the fuel flow into the heating apparatus  10 . As mentioned above, the regulator valve assembly  42  primarily includes an inlet port  46 , an outlet port  50 , and the fuel regulator assembly  249  attached to the knob  30 . The inlet port  46  is configured for receiving fuel from the fuel canister  12  as described above, and the outlet port  50  is fluidly coupled to the fuel-air mixing assembly  44  for discharging the fuel therein. 
     The fuel regulator assembly  249  includes a valve core  254  coupled with the inlet tube  48 . The valve core  254  opens and shuts the regulator valve assembly  42  for regulating the fuel flow rate. A stem  253  slidably connected to the valve core  254  triggers the opening and shutting of the valve core  254 . The fuel regulator assembly  249  further includes a diaphragm assembly  250  disposed within the regulator valve assembly  42  proximate the valve core  254 . The diaphragm assembly  250  is composed of a flexible material customarily employed for diaphragms such as rubber, elastomer, latex, polypropylene, and the like that permits back and forth movement in relation to the stem  253  and the valve core  254 . A member  252  typically in the shape of a button is disposed in the diaphragm assembly  250  for engagement with the valve core stem  253  when a pressure is applied against the diaphragm assembly  250 . An actuator  43  connected to the knob  30 , radially engages a cam collar  244 . As the actuator  43  is radially turned, the actuator  43  slides back and forth in response to the cam profile (see FIG.  4 B and the description hereinafter) of the cam collar  244 . This sliding movement of the actuator  43  causes the tension in a spring  251  to vary as it presses against the diaphragm assembly  250  for operative engagement with the valve core  254 . The higher the spring tension, the greater the rate of fuel flow through the valve core  254 . 
     The regulator valve assembly  42  further provides a pulsating fuel flow to the heating apparatus  10 . With the diaphragm assembly  250  spring-biased against the stem  253  during operation, the valve core  254  opens for permitting fuel to flow therethrough. The flowing fuel overcomes the spring pressure and the diaphragm assembly  250  is temporarily displaced off of the valve core stem  253 , causing the valve core to close. With the valve core  254  thus closed, the pressure in the chamber  45  subsides and the diaphragm assembly  250 , biased by the spring  251 , presses against the valve core stem  253  once again, causing it to open. This pattern produces a continuous oscillation of the diaphragm assembly  250  which results in the pulsing of the fuel flow. The amount of fuel associated with each oscillation is dependent at least in part on the tension of the spring  251 . The pulsing action provides an added benefit of aiding in the mixing of the fuel-air mixture primarily in the fuel-air mixing assembly  44 . 
     Referring to FIG. 4B, a graphical representation of fuel flow rate relative to the cam profile of the cam collar  244  is shown. The regulator valve assembly  42  operates in three principal stages, one of which is transitory for providing maximal fuel flow during ignition, and referred to as the IGNITE stage. The other two positions include OFF and ON. At 0°, the regulator valve assembly  42  is in the OFF position which is characterized by the closure of the valve core  254  and the absence of fuel flow. In this position, the actuator  43  is at its furthest point away from the diaphragm assembly  250  and the tension of the spring  251  is thus at its lowest. 
     As the actuator  43  is rotated along the cam collar  244  from 0° to a first activation position  201  (e.g. about 74°), the actuator moves inward linearly, as shown by a ramp segment  200 , with the tension of the spring  251  increasing accordingly and the valve core  254  opening accordingly. Beyond the first position  201 , the fuel flow rate remains substantially constant as the actuator is rotated (through the horizontal ramp segment  202 ) to a second position  203 . The ramp segment  202 , between about 74° and about 162°, represents the ON stage in which the valve core  254  is open sufficiently (e.g., 75% of capacity) for providing normal operating fuel flow. 
     To advance to the IGNITE stage, the actuator  43  is turned beyond the ON stage to a third activation position  206  (e.g. about 182°). At the position  206 , the valve core  254  is open at an increased capacity for permitting additional fuel flow therethrough. 
     The increased fuel flow rate facilitates the ignition of the combustion reaction in the heating apparatus  10 . Once the apparatus  10  is ignited, the user releases the knob  30  and the actuator  43  being biased by the spring  251 , rotates back to the ON stage for restoring the fuel flow to the normal operating rate. 
     Stopping features (not shown) arranged on the actuator  43  and collar  244 , in a known way, prevent the actuator from advancing beyond the position  206 . As shown in FIG. 4B, position  206  is located along an inclined ramp  205 . By thus locating the position  206  along an incline, it is ensured that the actuator  43 , under the biasing force of the spring  251 , will rotate back down to the ON position when the knob  30  is released. 
     A hump  209  is provided on the cam profile of the collar  244  at a position on the ramp segment  202  (e.g. about 152°) to prevent the actuator  43  from turning beyond the ON stage without user intervention. As such, once the heater has been lit and the user releases the knob  30  from the IGNITE position, the actuator  43  rotates back to the ON stage and is captured between the hump  209  and the position  203 . To turn the heater off, the user rotates the actuator  43  over the hump  203 , across the ramp segment  202  and down the ramp segment  200 , back to the OFF position. 
     Referring to FIG. 5, an exploded view of an exemplary embodiment of the heating apparatus  10  of the present invention is shown. The regulator valve assembly  42 , including the internally threaded inlet port  46 , the externally threaded outlet port  50  and the actuator  43 , is coupled with a regulator plate  60  through a centrally located hole  62  and secured therewith by a nut  64  threadedly engaged to the outlet port  50 . The regulator plate  60  and regulator valve assembly  42  are inserted through a bottom opening  68  of the collar  14  and mounted to a partition  13  with a centrally located partition hole  12  (see FIG.  2 ). The regulator valve assembly  42  is oriented with the end of the actuator  43  visible through an opening  66  in the collar  14 . The knob  30  is then securely affixed to end of the actuator  43  such as with a screw  63 . The outlet port  50  of the regulator valve assembly  42  partially extends through the partition hole  12 . 
     A lower tube  70  having a internally threaded top end  74  with a nut-shaped exterior, and a bottom end  76 , is inserted through a top opening  69  in the collar  14 . The bottom end  76  of the tube  70  is fluidly coupled with the outlet port  50  of the regulator valve assembly  42 . A cylindrical bracket  78  extending from the bottom of the heat shield  24  is inserted through the top opening  69  and mounted with the collar partition  13 . Three screws  63  are inserted through the bottom opening  68  in the collar  14  to tightly secure the regulator plate  60  and the bracket  78  to the partition  13 . 
     As shown in FIGS. 3A and 3B, an upper tube  72  having a flange  85  extending around the opening at the top end  86  and an externally threaded bottom end  88 , is inserted through a centrally located hole  84  in the head pan  20  (see FIG.  2 ). The flange  85  abuts against an edge portion  87  of the hole  84  for retaining the upper tube  72  therein and for forming a gas tight seal therebetween. The head pan  20  is placed on top of a plurality of support tabs  82  extending along the periphery of the heat shield  24 . The bottom end  88  of the upper tube  72  extends through the hole  80  in the heat shield  24 . The bottom end  88  is then screwed into the top end  74  of the lower tube  70  for fluid communication therebetween and for securely retaining the head pan  20  to the top of the heat shield  24 . A lock tab  21  is provided under the head pan  20  for fitting engagement with a slot (not shown) in the heat shield  24 . The lock tab  21  serves to immobilize the assembled housing  8 , heat shield  24 , and the upper tube  72  together for secure engagement. For increasing the support and stability of the heating apparatus  10 , the triangular leg stand  26  is pivotally attached to the bottom portion of the heat shield  24  by the ends thereof. 
     Referring again to FIG. 5, the diffuser plate  90  is mounted to a top surface of the head pan  20  at a distance over the opening in the top end  86  of the upper tube  72 . The head pan  20  further includes a flange portion  38  extending along the rim thereof. A permeable inner screen  34  is placed in the head pan  20  supported along the flange portion  38  thereof. A woven fabric-like catalyst-containing substrate  32  is placed on top of the inner screen  34 . A holder  40  is placed transversely across the top of the catalyst-containing substrate  32  for secure retainment. The head screen  18  having a flange portion  36  extending along the rim thereof, is then placed on top of the head pan  20  for containing the holder  40 , the catalyst-containing substrate  32 , and the inner screen  34 . The annular clamp  22  secures the respective flange portions  38  and  36  of the head pan  20  and head screen  18  together for secure retainment and gas-tight seal therebetween. Accordingly, the retained head screen  18  and head pan  20  in combination, hold the inner screen  34 , catalyst-containing substrate  32  and support holder  40  in position within the combustion chamber  16 . 
     A thermal indicator disc  92  may be provided in the center of a concave portion  94  of the head screen  18 . Due to the low temperature combustion and flameless nature of the apparatus  10 , there is no visual indication of heat when the apparatus is operating. A user who touches the head screen  18 , not knowing whether the apparatus  10  is operating or not, could potentially incur severe burns. The thermal indicator disc  92  alerts such users to the operating status of the apparatus  10  and may thereby prevent potential injury. The thermal indicator disc  92  performs such a function by changing colors as the head screen  18  heats up to the operating temperature. Common for thermochromatic materials, the color scheme may be coordinated with various specific temperature gradients. For example, at ambient temperature the thermal indicator disc may be black. At 160-170° F., the color changes from black to red, and at the operating temperature of 200-300° F., a white sunburst mark appears in the red field. It should be noted that a wide variety of colors or words (e.g. Caution Hot) may be utilized to provide the user with a warning as to the operating status of the apparatus  10 . In addition, other forms of thermal indicators employing mechanisms such as bimetallic material, for example, may be utilized for the purposes described above. 
     In an exemplary embodiment of the present invention, the heater apparatus  10  provides about 3,000 BTU of heat with a fuel flow rate range of about 75 to 82 cubic centimeters per minute. The air opening  58  in the fuel air-mixing assembly  44  is about 0.14 inches in diameter and the cross-sectional diameter of the fuel-air mixing assembly  44  is about 0.5 inches. The housing  8  has a cross-sectional diameter of about 8 inches with the surface of the head screen  18  radiating about 72 BTU per square inch. The carbon monoxide emission characteristic of this embodiment of the heating apparatus  10  is about 15 parts per million, even in a reduced oxygen environment. This is compared to 80 to 150 parts per million of carbon monoxide generated by a prior art catalytic combustion apparatus and 30 to 35 parts per million of carbon monoxide generated by a typical lit cigarette. It is noted that the measurements provided herein are not meant to be limiting and provide only one example of a preferred embodiment of the invention. 
     A modified fuel-air mixing assembly is shown in FIG.  6 A. The modified fuel-air mixing assembly  100  is comprised of a unitary tubular member with a helical structure  102  disposed within the interior. Other components and functional aspects of the fuel-air mixing assembly  100  are essentially the same as the fuel-air mixing assembly  44  previously described. The fuel-air mixing assembly  100  includes projections formed from the helical structure  102  within the bore  56  of the fuel-air mixing assembly  100 , which create an improved mixing effect therein by imparting a turbulent flow of the air and fuel throughout the bore  56 . 
     Another embodiment of a fuel-air mixing assembly  104  is shown in FIG. 6B which includes fin-like projections  106  adapted for the purpose of inducing turbulent flow of the fuel and air for providing a desirable uniform fuel-air mixture. 
     As shown in FIGS. 7A through 7C, respectively, a modified diffuser plate  110  may be employed. The modified diffuser plate  110  includes a circular radially corrugated body  112  mounted to the head pan  20  by a pair of braces  91  over the chamber inlet  15 . The corrugated body  112  includes a plurality of radially directed channels  114  along the lower surface thereof. The channels  114  provide for an improved uniform distribution of the fuel-air mixture within the combustion chamber  16  toward the catalyst-containing substrate  32 . 
     In a further embodiment of the present invention as shown in FIG. 8, the combustion chamber  16  includes an electrical ignition device  120  with one end mounted to the head pan  20  and the other end having a spark emitting electrode tip  122 , extending through the inner screen  34  and the catalyst-containing substrate  32 . The ignition device  120  provides the user with a simple method of initiating a self-sustaining combustion reaction for generating heat. As the actuator  43  of the regulator valve assembly  42  is turned to the IGNITE position (see FIG.  4 C), an initial large quantity of the fuel-air mixture is introduced into the combustion chamber  16  and thereby diffuses quickly through the catalyst-containing substrate  32 . As the ignition device  122  is activated, a spark or series of sparks is created at the electrode tip  122  to ignite the denser than usual fuel-air mixture. It should be noted that the ignition device is not limited to the form described above and may include other forms such as electric, flame, and the like as known by one of ordinary skill in the art. 
     In addition to the use of the ignition device  120 , a modified diffuser plate  130  may be optionally included in the combustion chamber  16  as shown in FIGS. 9A and 9B. The diffuser plate  130  is similar in design to the diffuser plate  90  shown in FIGS. 2 and 8. However, the diffuser plate  130  includes an upwardly sloping ridge extending from the center to the edge of the diffuser plate  130  for forming a trough  134  in the undersurface portion thereof as shown in FIG.  9 A. In mounting the diffuser plate  130  on the heat pan  20 , it is preferable to orient the trough  134  towards the electrode tip  122  of the ignition device  120  as shown in FIG.  9 A. Initially, during ignition, the trough  134  provides a more focused fuel-air mixture flow towards the ignition device  120  for a faster and safer ignition. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages.