Field of The Invention

This invention pertains to stoves for burning solid fuel, and more particularly to woodstoves having a plurality of combustion chambers for burning wood and the gases generated during combustion of the wood.

Background of the Invention

Woodstoves are known to emit harmful air pollutants, such as formaldehyde, acetic acid, and carbon monoxide, and other hydrocarbons, the latter including polycylic organic molecules ("POM's**), many of which are known or suspected carcinogens. These flammable gaseous and volatile liquid pollutants may be referred to collectively as "wood gas" and are generated during incomplete combustion of the wood, paper and other materials comprising the woodstove's fuel supply. Wood gas may be burned and thereby converted into carbon dioxide, water and other harmless substances.

Such combustion tends to be problematic in conventional o woodstoves since temperatures in excess of 1000 Farenheit are typically required to burn wood gas completely. These high temperatures are achievable, if at all, for only short periods of time in typical prior art woodstoves.

To ensure that emissions from woodstoves meet existing and proposed state and federal emissions standards, many woodstove manufacturers and users have installed catalytic converters in the exhaust gas airstream of their stoves.

Catalytic converters lower the temperature at which wood gas burns to temperatures that are readily achievable, o i.e., about 500 Farenheit, for extended periods of time,

in typical woodstoves. The use of catalytic converters in. solid fuel stoves is further advantageous in that the catalytic reaction occurring at the converter is exothermic. This exothermic reaction releases a higher percentage of the total heat energy of the wood than would be released if the woodstove was not equipped with a catalytic converter, and hence increases the combustion efficiency of the woodstove.

The use of catalytic converters as the sole means for reducing woodstove emissions is undesirable from several standpoints. First, under normal operating conditions, the efficiency of the catalytic converter decreases over time. Naturally-occuring substances present in small quantities in the wood burned in woodstoves react with the noble metals in the catalytic converter, reducing the ability of the metals to catalyze oxidation of wood gas. This low-level "poisoning" of the catalytic converter can easily reach catastrophic proportions when various substances such as colored newsprint, painted wood, or pressure-treated wood are burned in the woodstove. Poisoning of the catalytic converter increases the emissions level of the stove and decreases the thermal . efficiency of the stove. Second, the internal construction of catalytic converter-equipped stoves is frequently designed to encourage efficient operation of the catalytic converter. Should the user of a stove so-designed remove the catalytic converter, or should the converter become poisoned, the resultant emissions level could equal or even exceed the emissions level of older, conventionally-designed, non-catalytic converter-equipped stoves.

In an attempt to provide a woodstove the emissions from which will satisfy current and proposed emissions standards, while at the time avoiding the aforementioned problems typically associated with catalytic converter-equipped woodstoves, several "high-tech, non-cat" woodstoves have been developed. These woodstoves are designed to burn hot enough to consume most of the wood gas generated in the combustion of the firewood, and so do not require the use of a catalytic converter. Unfortunately, most of these woodstoves have relatively small fireboxes, with the result that the logs to be burned have to be cut a greater number of times to provide firewood that will fit into the woodstove. Additionally, while these stoves often extract a large percentage of the heat energy of the firewood, i.e., they have high "combustion efficiency", a majority of the heat energy of the firewood is often lost up the chimney in the exhaust gases, i.e., they have low "heat-transfer efficiency." Consequently the "overall thermal efficiency" of these woodstoves, i.e., combustion efficiency times heat transfer efficiency, is relatively low. Thus, because of the fast burn rate and small firebox size of these stoves, it tends to be difficult to maintain a fire in these stoves overnight.

To date, it has been difficult to achieve a woodstove that has high overall thermal efficiency, burns conventionally-sized firewood, and achieves relatively low levels of emissions without the use of a catalytic converter.

Objects of the Invention

A primary object of the present invention is to provide a woodstove that produces a relatively low level of emissions and achieves a relatively high overall thermal efficiency.

Another object of the present invention is to provide a woodstove having a primary combustion chamber for gasifying firewood and a secondary combustion chamber or chambers isolated from the primary chamber for burning the wood gas, with the secondary combustion chamber or chambers having a separate air inlet for supplying sufficient air to support nearly complete combustion of the wood gas.

Yet another object of the present invention is to provide a woodstove having a primary combustion chamber for gasifying firewood , a secondary combustion chamber for thoroughly mixing oxygen with the wood gas produced in the primary combustion chamber, and a tertiary combustion chamber having a catalytic converter disposed therein for burning any residual wood gas/oxygen mixture.

A further object of the present invention is to provide a woodstove that will accept firewood of up to twenty-one inches in length and will burn relatively cleanly, for relatively long periods of time, without the use of a catalytic converter.

Still another object of the present invention is to provide a woodstove having an air inlet system that moves preheated air past windows in the stove so as to prevent creosote from condensing on the windows.

Summary of the Invention

These and other objects are achieved by a woodstove

comprising a primary combustion chamber for burning and gasifying firewood with primary supply of air. A hollow floor defines the bottom of the primary combustion chamber and an aperture is formed in the hollow floor for pneumatically coupling the primary combustion chamber with a secondary combustion chamber positioned directly beneath the hollow floor. The wood gas produced in the primary combustion chamber is drawn through the aperture into the secondary combustion chamber where the wood gas is mixed with a secondary supply of air and is burned. A tertiary combustion chamber is coupled with the secondary combustion chamber and a catalytic converter may optionally be disposed in the tertiary combustion chamber. In the event a catalytic converter is so disposed, unburned oxygenated wood gas that passes from the secondary combustion chamber into the tertiary combustion chamber, is burned as it passes through the catalytic converter.

A primary air inlet system is provided for delivering preheated atmospheric air to the primary combustion chamber. A secondary air inlet system is provided for delivering preheated air to the secondary and tertiary combustion chambers. The primary and secondary air inlet systems are designed to direct preheated air onto and past windows provided for viewing the interior of the woodstove so as to prevent creosote from condensing on the windows.

Description of the Drawings

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the

acco panying drawings wherein:

Fig. 1 is a front elevational view of the stove of the present invention; Fig. 2 is a side elevational view of the stove illustrate in Fig. 1 _? Fig. 3 is a plan view of the stove illustrated in Fig. 1; Fig» 4 is vertical, cross-sectional view taken along line 4-4 in Fig. 1; Fig. 5 is a horizontal, cross-sectional view taken along line 5-5 in Fig. 4; Fig. 6 is a horizontal, cross-sectional view taken along line 6-6 in Fig. 4y Fig. 7 is a horizontal, cross-sectional view taken along line 7-7 in Fig. 4; Fig. 8 is a horizontal, cross-sectional view taken along line 8-8 in Fig. 4; Fig. 9 is a horizontal, cross-sectional view taken along line 9-9 in Fig. 4; Fig. 10 is vertical, cross-sectional view taken along line 10-10 in Fig. 1; Fig. 11 is a vertical cross-sectional view taken along line 11-11 in Fig. 10; and Fig. 12 is a plan view of the top wall of the tertiary combustion chamber.

Detailed Description of the Invention

Referring to Figs. 1 and 4, the woodstove of the present invention comprises a primary combustion chamber 20, a ^" secondary combustion chamber 120, and a tertiary combustion chamber 320. Except as noted below, the

woodstove, including the walls defining the combustion chambers, is preferably made from cast iron having suitable metallurgical properties. Alternatively, sheet metal or a combination of cast iron and sheet metal may be satisfactorily employed in the fabrication of the present stove.

Turning now to Figs. 4 and 9, primary combustion chamber 20 is positioned above secondary combustion chamber 120, and is defined by opposing side walls 22 and 24, front wall 26, rear wall 28, floor 30 and top wall 32. Primary combustion chamber 20 is provided for burning and gasifying firewood. Primary combustion chamber is preferably sized to receive up to six logs each having a diameter of about 6" and a length of up to about 21", or a corresponding volume of split or otherwise-shaped firewood having a length of up to about 21". Aperture 34 extends entirely through floor 30 and is approximately centered between side walls 22 and 24 and between front wall 26 and rear wall 28.

Upper window 36 is supported in frame 37 so as to form an air-tight seal with the frame. The latter is disposed in opening 38 in front wall 26 so as to form an air-tight seal with the front wall. Frame 37 and opening 38 may designed in a known manner to allow window 36 and frame 37 to be removed from the stove for cleaning or replacement. Window 36 is preferably contructed from two panes of high temperature glass having a low coefficient of thermal expansion, with the outer window preferably being made from an annealed boro-silicate and the inner window preferably being made from a glass ceramic. Suitable glass for window 36 is readily commercially

available and is manufactured, for instance, by Schott of Germany. The two panes are spaced apart from one another a distance sufficient to maintain the inner window at an elevated temperature so as to prevent the condensation of creosote on the inner window.

Referring next to Figs. 4, 9 and 11, by-pass lock 46 is provided in rear wall 28 for pneumatically coupling primary combustion chamber 20 with the exhaust gas outlet, as described hereinafter. By-pass lock 46 comprises a plurality of identically-sized and vertically-extending slots 48. Each slot 48 is spaced a substantially uniform distance from adjacent slots, and extends entirely through rear wall 28.

By-pass lock 46 also comprises sliding plate 50 mounted for slidable movement along ways 52 and 54 (Fig. 11) . The latter elements are mounted on the outside surface of rear wall 28. Sliding plate 50 comprises a plurality of identically-sized, vertically-extending fingers 56 (Fig. 11) secured together by horizontally-extending ribs 58. Ways 52 and 54 are constructed and fingers 56 are sized and spaced apart from one another so that when sliding plate 50 is positioned _, along ways 52 and 54 so that fingers 56 overlie slots 48, the slots will be pneumatically blocked thereby occluding primary combustion chamber 20 from the exhaust gas airstreaπ Primary combustion chamber 20 may be pneumatically coupled with the exhaust gas airstream of the stove by moving sliding plate 50 along way 52 and 54 so that fingers 56 lie between adjacent ones of slots 48.

Sliding plate linkage 58 (Fig. 11) is provided for causing sliding plate 50 to move along ways 52 and 54.

Sliding plate linkage 58 comprises tabs 60 and 62 secured to adjacent ones of fingers 56, and pressure plate 63 (Fig. 9) attached to adjacent ones of fingers 56 so as to extend between tabs 60 and 62. Linkage 58 also comprises driver 64 mounted for rotational movement in support bracket 66. The latter is secured to rear wall 28 above sliding plate 50. Tabs 60 and 62 extend normally away from rear wall 28 a selected distance- (see Fig. 9) . .Driver 64 comprises shaft 68, adjustment head 70 secured to the top end of the shaft 68, and rod 72 secured to the bottom end of shaft 68. An eccentric surface (not shown) is provided on a portion of the outer surface of rod 72. Shaft 68 is positioned above tab 62 (see Fig. 9) and rod 72 extends downwardly between tabs 60 and 62 (see Figs. 4 and 9) when sliding plate 50 is positioned to pneumatically block slots 48.

Turning next to Figs. 3, 4, and 11, opening 86 (Fig. -3) is provided in one half of top wall 32 and in upper portion 96 (Fig. 3) of side wall 22 adjacent top wall 32 for loading wood into primary combustion chamber 20. A corresponding opening 88 is provided in the other half of top wall 32 and in upper portion 100 (Fig.3) of side wall 24 adjacent wall 32, also for loading wood into primary combustion chamber 20.

Gull wing doors 90 and 92 are provided for pneumatically sealing opening 86 and 88, respectively. Gull wing door 90 comprises top portion 94 to which upper portion .96 of side wall 22 is attached, and gull wing door 92 comprises top portion 98 to which upper portion 100 of side wall 24 is attached.

Trunnions 102 (Fig. 3) are attached to doors 90 and 92

and extend into correspondingly-sized apertures 104 (Fig. 3) provided in blocks 106 (Fig. 3) secured to the inside ^" surfaces of front wall 26 and rear wall 28 adjacent the middle of top wall 32. Trunnions 102 and apertures 104 are positioned so as to pivotally support doors 90 and 92 so that the doors may be pivoted about axes that extend in parallel with one another and extend substantially normally"to front wall 26 and rear wall 28 adjacent the middle of top wall 32. Suitable gasket material (not shown) is secured to doors 90 and 92, and/or the portions of the woodstove the doors contact when closed, so as to provide a substantially air-tight seal between the exterior environment and primary combustion chamber 20 when the doors are closed.

Turning next to Figs. 4 and 6, secondary combustion chamber 120 is positioned below primary combustion chamber 20 and is defined by side walls 22 and 24, front wall 26, rear wall 28, top wall 122 and floor 124. Secondary combustion chamber 120 is provided for burning wood gas generated in primary combustion chamber 20 and for mixing the wood gas with air.

Lower window 126 is disposed in door 128 so as to form an air-tight seal with the door. Lower window 126 is substantially identical to upper window 36, i.e., it is constructed from two panes of high temperature glass having a low coefficient of thermal expansion, except that the lower window is somewhat smaller than the upper window. Door 128 is disposed in opening 130 in front wall 26 and is mounted to pivot about shaft 132. The latter is secured to front wall 26 below window 126 so that the axis of the shaft extends normally between the planes along

which side walls 22 and 24 extend.

Lip 134 (Fig. 4) is provided at the top edge of door

128 and fastener 136 (Fig. 4) is rotatably mounted in front wall 26 above lip 134. When fastener 136 is positioned as illustrated in Fig. 4, the head of the fastener engages lip 134 holding door 128 in air-tight o contact with front wall 26. By rotating fastener 136 90 in either a clockwise or counterclockwise direction, fastener 136 is decoupled from lip 134 and door 128 is free to pivot on shaft 132, whereby the door may be swung down to provide access to the interior of second combustion chamber 120. Suitable gasket material (not shown) is attached to door 128 and/or opening 130 so that an air-tight seal may be achieved between front wall 26 and door 128 when. the door is closed.

Optionally, an ash lip 148 may be secured to door 128 extending normally away from the door. By this

-connection, when door 128 opened ash lip 148 will pivot downwardly with the door to a position beneath the door.

Referring next to Figs. 4, 6 and 11, inner walls 150 and 152 (Fig. 6) are provided in secondary combustion chamber 20 extending in parallel with and spaced from side walls 22 and 24. Inner walls 150 and 152 extend between top wall 122 and floor 124 and are attached to and extend forwardly from rear wall 28, but terminate before reaching front wall 26. In secondary combustion chamber 120, rear wall 28 extends only between inner walls 150 and 152, and thus terminates before reaching side walls 22 and 24.

Circular aperture 158 (Fig. 4) is formed in top wall

122 extending entirely through the top wall 122. Aperture

158 is positioned directly beneath circular aperture 34

and has the same inside diameter as aperture 34.

An insulative layer 160 (Fig. 4) is provided beneath substantially the entire secondary combustion chamber 120. Firebrick, or other material having a similar thermal conductivity, may be satisfactorily employed as insulative layer 160.

Optionally, an ash tray 162 (Fig. 4) may be provided for collecting wood ashes that pass through opening 244 during combustion of the firewood in primary combustion 20 or that are brushed through opening 244 during cleaning of the woodstove. Wall 164 (Fig. 4) may be provided in secondary combustion chamber 120 extending between floor 30 and floor 124 adjacent rear wall 28 for limiting the distance ash tray 162 (Fig. 4) may be inserted into the secondary combustion chamber.

Turning now to Figs. 4-10, a primary air inlet system is provided for introducing fresh air into primary combustion chamber 20. The inlet system comprises bottom tube 186 (Fig. 5) that runs from the back of the stove toward the front of the stove under insulative layer 160. Fresh air enters tube 186 through opening 188 (Fig. 5) formed in the end of the tube adjacent the back of the stove. Midway along the length of tube 186, the latter splits into legs 186A and 186B giving the tube a Y-shaped configuration, as best seen in Fig. 5. Leg 186A intersects and is pneumatically coupled with vertical tube 192 (Figs. 5 and 10) and leg 186B intersects and is pneumatically coupled with vertical tube 194 (Fig. 5) . Vertical tube 192 is provided inside the stove at the intersection of front wall 26 and side wall 22 and vertical tube 194 is provided inside the stove at the intersection of front

wall 26 and side wall 24. Vertical tubes 192 and 194 extend upwardly from the tube legs 186A and 186B, respectively, and intersect and are pneumatically coupled with opposite ends of upper tube 196 (Fig. 9) . Upper tube 196 extends between vertical tubes 190 and 192 and is positioned directly above upper window 36. Slot 198 is provided in bottom wall 200 of upper tube 196 extending across the entire length of the bottom wall so as to pnueumatically couple upper tube 196 with primary combustion chamber 20.

Referring next to Figs. 4, 5 and 10, a damper system may be optionally provided for controlling the flow of air into bottom tube 186. The damper system comprises shutter plate 220 that is pivotally mounted to the rear-most portion of floor 124 (see Fig. 4) so that the shutter plate may be pivoted between closed and open positions. In the closed position, illustrated in phantom -view in Fig. 4, shutter plate 220 substantially prevents air from entering bottom tube 186. In the open position, illustrated in solid view in Fig. 4, air is permitted to flow past shutter plate 220 into bottom tube 186. Of course, intermediate positions exist between the open and closed positions in which less air is allowed to enter tube 186 than will enter the tube when shutter plate 220 is in the open position.

A linkage assembly is provided for causing shutter 220 to pivot between open and closed positions comprising arm 230, pull chain 232, and lever 234. Arm 230 is connected to shutter plate 220 so as to extend away from the plate toward side wall 22. By this connection, shutter 220 may be pivoted between open and closed positions by

manipulating arm 230 up and down so that the shutter moves toward and away from opening 188.

One end of chain 232 is attached to shutter plate 220 and extends upwardly along the outer surface of rear wall 28. The other end of pull chain 232 is attached to lever 234. The latter is pivotally mounted to rear wall 28 of the rear wall so that lever 234 may be readily manipulated. By pulling up on lever 234, shutter plate 220 is moved to the open position, and by pulling down on lever 234 shutter plate 220 will move to the closed position under the pull of gravity.

Referring next to Figs. 4 , 7 and 10, floor 30 and top wall 122 are spaced apart from one another so as to form cavity 240 (Fig. 4) . Cylinder 242 is disposed in cavity 240 extending between floor 30 and top wall 122. The inside diameter of cylinder 242 is substantially identical to the inside diameters of circular apertures 34 and 158 so as to provide a single opening 244 of substantially uniform inside diameter pneumatically coupling primary combustion chamber 20 with secondary combustion chamber 120. While it is preferred that opening 244 be of circular cross-section, it is to be appreciated that the opening may also be of square, oval, rectangular or other cross-section. To maximize efficiency of the present woodstove, opening 244 is preferably not a slot, i.e., its length should not exceed its width by more than a factor of approximately 2 to 1. Inlets 246 (Fig. 7) in cylinder 242 pneumatically couple cavity 240 with opening 244.

As best seen in Figs. 4 and 7, a plurality of baffles 260 and 262 are provided in cavity 240 surrounding opening

244. Baffles 260 (Fig. 4) are secured to the bottom surface of floor 30 and extend toward but terminate before reaching top wall 122. Baffles 262 (Fig. 4) are secured to the top surface of top wall 122 and extend toward but terminate before reaching floor 30. Gaps 264 (Fig. 7) are provided in baffles 260 and 262 separating the baffles into a plurality of contiguous segments.

Partitions 263 and 265 (Fig. 7) are provided in cavity 240 extending between floor 30 and top wall 122. Partition 263 intersects and extends forwardly from rear wall 28 in parallel with side wall 24, but terminates before reaching front wall 26. Partition 263 is positioned adjacent and is spaced a selected distance from side wall 24. Partition 265 extends in parallel with side wall 24 and is positioned between opening 244 and partition 263. Partition 265 intersects partition 266 and terminates before reaching rear wall 28.

Cavity 240 is divided into two pneumatically isolated sections 240A (Fig. 7) and 240B (Fig. 7) by a first partition 266. The latter extends between floor 30 and top wall 122 and is attached to and extends forwardly from rear wall 28, approximately normally to the rear wall, to a point forward of opening 244 between the opening and front wall 26. At this point, first partition 266 extends toward side wall 24, approximately normally to the side wall. Before reaching side wall 24, however, first partition 266 extends forwardly again toward front wall 26 and intersects the front wall adjacent lower window 126. A second partition 268 is provided extending between floor 30 and top wall 122 in parallel with side wall 22 and spaced a selected distance from the side wall. The

length of second partition 268 is selected so that the latter intersects rear wall 28 and is spaced from front wall 26.

A third partition 270 is provided extending between floor 30 and top wall 122 in parallel with side wall 22 between first partition 266 and second partition 268. -Third partition 270 is attached to front wall 26 adjacent upper window 36 and extends rearwardly toward rear wall 28, but terminates before reaching the rear wall.

Slot 272 is provided in top wall 122 extending in parallel with and spaced from front wall 28. Slot 272 is positioned directly above lower window 126 and pneumatically couples- cavity 240A with secondary combustion chamber 120.

Turning now to Figs. 2, 4 and 7, secondary air inlets 300 and 302 are provided in side walls 22 and 24, respectively, adjacent back wall 28. Inlet 300 is positioned in side wall 22 so as to couple cavity 240A with atmospheric air. Inlet 300 is positioned to confront approximately the midpoint of the length of second partition 268. Inlet 302 is positioned in sidewall 24 so as to couple cavity 240B with atmospheric air.

Teardrop plate 304 (Fig. 2) is provided for selectively blocking off secondary air inlet 302. Plate 304 is pivotally mounted to side wall 24 above inlet 302 and is sized so that when the plate is pivoted to block inlet 302, substantially no air will pass through the inlet. Reference marks 306 in side wall 24 are provided as a guide for determining what portion of inlet 300 is blocked by plate 304. For instance, when plate 304 is pivoted to intersect a first one of reference marks 306, a

predetermined portion of inlet 302 will be blocked. A corresponding plate (not shown) and set of reference marks (not shown) are provided adjacent secondary air inlet 300. Referring to Figs. 4, 6, 8 and 12, a tertiary chamber 320 is provided for burning and exhausting gases vented from secondary combustion chamber 120. Tertiary combustion chamber 320 is defined by rear wall 28, side walls 322 and 324, back wall 328 and top wall 330. A pair of apertures 331A and 331B (Fig. 12) are provided in top wall 330 for exhausting gases from tertiary combustion chamber 320. Secondary combustion chamber 120 is coupled with tertiary combustion chamber 320 through openings 332 and 334 (Fig. 11) formed in rear wall 28 beneath the point of intersection of floor 30 with rear wall 28.

An opening 336 is provided in back wall 328 beneath the point of intersection of top wall 330 with the back wall. Cover plate 338 is releasably securable by -conventional means to back wall 328 so as to pneumatically block opening 336. Horizontal wall 340 is disposed in tertiary combustion chamber 320 so as to extend along a plane that is slightly above the plane along which floor 30 extends. An opening 342 is provided in horizontal wall 340 pneumatically coupling the lower portion of tertiary combustion chamber 320 with the upper portion of the tertiary combustion chamber.

Catalytic converter 344 is disposed in tertiary combustion chamber 320 so as to rest on horizontal wall 340. Sleeve 346 surrounds catalytic converter 344 and provides an air-tight seal between the converter and the walls of the tertiary combustion chamber, including horizontal wall 340. By this placement of catalytic

converter 344, all wood gases exiting secondary combustion chamber 120 must pass through catalytic converter 344 before exiting tertiary combustion chamber 320.

Preferably, catalytic converter 344 comprises two discrete catalytic converters positioned next to one another, with each converter having a width of about two inches, a length of about seven inches and a thickness of about three inches. Catalytic converter 344 comprises a ceramic honeycomb having a plurality of cells extending through the thickness of the honeycomb. Each cell preferably has cross-sectional area of about 0.20 inch and has exposed surfaces coated with a noble metal such as platinum. Preferably, the cells are formed in the honeycomb with a density of about 16 cells per square inch. Of course, skilled practitioners will appreciate that catalytic converters having other dimensions and cell size and density may also be satisfactorily employed as catalytic converter 344. A satisfactory catalytic converter is manufactured by Applied Ceramic of Atlanta, Georgia under the product number PN-2731-2.

Catalytic converter 344 may be installed in tertiary combustion chamber 320 by removing cover plate 338 and inserting the catalytic converter through opening 336 so as to contact and be supported on horizontal wall 340. By reinstalling cover plate 338, opening 336 is pneumatically blocked and all-gases exiting secondary combustion chamber 120 will pass through catalytic converter 344, as noted above.

The use of catalytic converter 344 in the woodstove of the present invention is optional. Preferably, catalytic converter 344 will be used only when emissions regulations

and/or the desire to maintain a fire in the woodstove for relatively long periods of time without reloading the stove require the use of a catalytic converter.

Referring to Figs. 4 and 12, reversible exhaust outlet

350 is attachable to top wall 330, as illustrated in solid view in Fig. 4. Exhaust outlet 350 comprises a cylindrical housing 352 sized to receive a conventionally-dimensioned stove pipe. Exhaust outlet 350 also comprises walls 354 and 356 attached to cylindrical o housing 352 so that each wall forms a 45 angle with a plane that lies along the longitudinal axis of cylindrical o housing 352. Wall 354 forms a 90 angle with wall 356.

Aperture 358 (Fig. 4) is provided in wall 356.

Plate 362 is attachable between back wall 328 and rear wall 28 when exhaust outlet 350 is disposed so that outlet wall 356 is positioned immediately adjacent and extends in parallel with top wall 330, as illustrated in solid view in Fig. 4. When plate 362 is so attached, the plate extends substantially in parallel with outlet wall 354 and pneumatically seals the spaced formed between rear wall

28, back wall 328, the plate 362 and side walls 322 and

324. An opening (not shown) is provided in rear wall 328 through which outlet housing 352 extends when outlet 350 is positioned as illustrated in Fig. 4.

When outlet 356 is positioned as illustrated in solid view in Fig. 4, the upper-most surface of outlet cylinder 352 will be approximately 22" above the surface on which the woodstove is positioned. Since fireplace lintels, such as the one illustrated representationally at

364, are typically positioned at least 24" above the hearth floor of the fireplace, the present woodstove may

be positioned partially within a fireplace since outlet cylinder 352 will fit beneath a lintel 364. Similarly, when outlet 350 is vertically positioned as illustrated in phantom view in Fig. 4, outlet wall 356 does not extend rearwardly of plate 362 or back wall 328. This arrangement minimizes the distance the woodstove must be spaced from combustible objects such as a wall or fireplace mantel.

Reversible outlet 350 and plate 362 may also be positioned as illustrated in phantom view in Fig. 4. In one of the phantom views, reversible outlet 350 is - positioned so that the longitudinal axis of cylindrical housing 352 extends in parallel with top wall 32. In the other of the phantom views, reversible outlet 350 is positioned so that the longitudinal axis of cylindrical housing 352 extends normally to top wall 32. In both of the phantom views, plate 362 extends between top wall 330 and the back wall 328 and is disposed so as to block the space defined by top wall 330, rear wall 28, back wall 328 and side walls 322 and 324.

Referring to Figs. 1 and 2, the woodstove of the present invention is supported on legs 370 positioned at each of the four corners of floor 124. Legs 370 are sized so that the woodstove is positioned above the surface on which it rests a distance sufficient to prevent conduction of excessive heat to the surface. ^•

OPERATION

Referring now to Figs. 2-11, to operate the woodstove of the present invention, teardrop plate 304 is pivoted so that secondary air inlet 302 is either partially or

completely unblocked. The corresponding teardrop plate (not show) adjacent secondary air inlet 300 is pivoted so that inlet 300 is either partially or completely unblocked. Typically, the teardrop plates will be fully opened only when it is desired to maintain a very hot, fast-burning fire in the woodstove and will be fully closed only when it is desired to stop the combustion process in the woodstove, e.g., when there is a chimney fire. Under normal operating conditions the teardrop plates will be adjusted so that openings 300 and 302 are partially blocked.

Shutter plate 220 is pivoted to the position illustrated in solid view in Fig. 4 by moving lever 234 upwardly to the position illustrated in solid view in Fig. 4. In this position, atmospheric gases are free to enter the primary air inlet system through opening 188. Lever 234 is coupled by pull chain 232 and arm 230 to shutter plate 220 so that the shutter plate may be conveniently adjusted near the top of the woodstove.

By-pass lock 46 is opened to couple primary combustion chamber 20 with exhaust outlet 350 by rotating adjustment head 70 in a counterclockwise direction. This rotation of adjustment head 70 is transmitted via shaft 68 to rod 72 causing the rod to rotate in a counterclockwise direction in a circular arc about shaft 68.

As rod 72 moves through this circular arc, the rod contacts tab 62 and urges the tab, and sliding plate 50 attached to the tab, toward side wall 24. As sliding plate 50 moves along ways 52 and 54 toward side wall 24, slots 48 are exposed pneumatically coupling primary combustion chamber 20 with exhaust outlet 350.

Next, gull-wing door 90 and/or door 92 are/is opened and a load of fuel, such as wood and newspaper, is added to primary combustion chamber 20 through opening(s) 86 or/and 88. The fuel is ignited and doors 90 and/or 92 are closed. Atmospheric air enters bottom tube 186 through opening 188 and travels through the bottom tube until reaching vertical tubes 192 and 194. The atmospheric air then travels upwardly through vertical tubes 192 and 194 until reaching upper tube 196. The atmospheric air then travels along the length of upper tube 196 and spills out of the upper tube through slot 198 onto upper window 36.

After a satisfactory fire has been established in the woodstove, the atmospheric air passing through the above-identified tubes of the primary air inlet system will be preheated by thermal energy conducted from primary combustion chamber 20 and secondary combustion chamber 120 through the walls of tubes 192, 194 and 196. By preheating the atmospheric air delivered to priamry combustion chamber 20, combustion is encouraged in the primary combustion chamber. The preheated air is delivered into primary combustion chamber 20 through slot 198 positioned above upper window 36 to (1) heat the upper window and (2) flush gaseous creosote away from the upper window, so as to prevent creosote from condensing on the upper window.

During start-up of the woodstove, most of the wood gases generated by the combustion of the firewood exit primary combustion chamber 20 through slots 48. The wood gases then pass through aperture 358 in exhaust outlet 350, and pass out of the exhaust outlet through a conventional stove pipe (not shown) coupled to the exhaust

outlet. Some of the exhaust gases generated in primary combustion chamber 20 during the start-up of the woodstove will pass downwardly through opening 244, as described in detail below.

After a satisfactory fire is established in primary combustion chamber 20, by-pass lock 46 is closed by rotating adjustment head 70 in a clockwise direction. This rotation of head 70 is transmitted, via shaft 68, to rod 72 causing the rod to drive tab 64, and sliding plate 50 attached to the tab, toward side wall 22 until fingers 56 are positioned to block slots 48. A cam surface (not shown ) is positioned on rod 72 so that after fingers 56 are moved to block slots 48, further clockwise rotation of adjustment head 70 urges the can surface against pressure plate 63. The cam surface drives pressure plate 63, and sliding plate 50 attached thereto, against rear wall 28 so as to pneumatically seal slots 48. By this sealing of slots 48, wood gas can exit primary combustion chamber 20 only through opening 244.

During start-up as well as steady-state operation, of the woodstove, atmospheric air enters cavity 240A through secondary air inlet 300 and cavity 240B through secondary air inlet and 302. Cavities 240A and 240B are heated by conduction of the thermal energy in the hot gases present inside primary combustion chamber 20 and secondary combustion chamber 120. This thermal energy passes through floor 30 and top wall 122 into cavities 240A and 240B. Additionally, after time, hot coals will accumulate on floor 30 introducing additional thermal energy into cavity 240 by conduction through floor 30.

Atmospheric air entering inlet 302 travels through

cavity 240B toward inlets 246 and passes through the inlets into opening 244. Baffles 260 and 262 and partitions 263 and 265, are provided to increase the surface area of heated metal that atmospheric air flowing through cavity 24OB will contact before reaching inlets 246, so as to ensure the atmospheric air is sufficiently preheated before passing through the apertures into opening 244. Paritions 263 and 265 are also provided-to increase the length of the path that atmospheric air moving through cavity 240B to inlets 246 must follow, so as to ensure the atmospheric air remains in cavity 24OB a period of time sufficient to effect adequate preheating of the air. Gaps 264 are provided to facilitate the passage of atmospheric air through baffles 260 and 262.

Atmospheric air entering inlet 300 travels through cavity 240A toward slot 272 and passes through slot 272 into secondary combustion chamber 120. Partitions 268 and 270 are provided to increase the surface area of heated metal the atmospheric air contacts so as to ensure the air is sufficiently preheated before reaching slot 272. The latter is positioned above lower window 126 so that a supply of preheated air is continously directed on and moved past the lower window. By this delivery of preheated air, gaseous creosote present in the secondary combustion chamber is flushed away from the lower window preventing the creosote from condensing on the lower window and visual access to the secondary combustion chamber is maintained.

During steady-state operation of the woodstove, preheated atmospheric air delivered through slot 198 in upper tube 196 supports combustion of the solid fuel in

primary combustion chamber 20. During combustion of the fuel, wood gas is released. As noted above, wood gas is a mixture of flammable gases and volatile liquids consisting of hydrogen, carbon monoxide, methane, acetic acid, formaldehyde and other hydrocarbons. The wood gas is drawn downwardly through opening 244 into secondary combustion chamber 120 by the draft or negative pressure in the secondary combution chamber generated by the chimney to which the woodstove is attached. In the secondary combustion chamber, the wood gas is thoroughly mixed with secondary air introduced into the secondary combustion chamber through opening 244 and slot 272, as noted below.

Secondary combustion chamber 120 is insulated by hollow cavity 240, by insulative layer 160 and by tertiary combustion chamber 320, with the result that relatively o o high temperatures, e.g. 1000 -1600 F, are achievable and maintainable in the secondary combustion chamber. These high temperatures, together with the introduction of secondary air, support the combustion of a significant portion of the wood gas drawn downwardly into second combustion chamber 120.

As the wood gas passes downwardly through opening 244, preheated secondary air introduced through inlets 246 is mixed with the wood gas. This secondary air supports combustion of the wood gas in secondary combustion chamber

120 and in opening 244 and is believed to be almost totally consumed during this combustion process. Any air vented through openings 246 that is not consumed in the combustion of the wood gas in secondary combustion chamber

120 and in opening 244 mixes with the wood gas present in secondary combustion chamber 120.

Additional preheated secondary air is introduced into secondary combustion chamber 120 through slot 272. When sufficient temperatures are achieved in secondary o combustion chamber 120, i.e., temperatures of 1000 o -1600 F, the majority of the preheated secondary air introduced through slot 272 is consumed in the combustion of the wood gas in the secondary combustion chamber. When lower temperatures are present in secondary combustion o chamber 120, i.e., temperatures under 1000 F, the majority of the secondary air introduced through slot 272 is mixed with the wood gas in the secondary combustion chamber so as to create a substantially homogeneous wood/gas air mixture.

Strong convection currents are generated by the passage of primary and secondary air through and out of the woodstove through the primary secondary and tertiary combustion chambers. These convection currents cause secondary air to flow through inlets 246 and slot 272 and into and out of secondary combustion chamber 120 with relatively great velocity. This relatively high velocity flow of the secondary air creates turbulence in secondary combustion chamber 120 that causes the wood gas to mix with the secondary air so as to achieve the aforementioned homogeneous wood gas/air mixture.

The wood gas/air mixture in the secondary combustion chamber is drawn toward front wall 26, passes around inner walls 150 and 152, and is drawn toward openings 332 and 334 in rear wall 28 by the aforementioned draft or negative pressure generated by the chimney to which the woodstove is attached. The wood gas/air mixture then passes through openings 332 and 334 and enters tertiary

combustion chamber 320. The tortuous path created by inner walls 150 and 152 that the wood gas must follow before reaching openings 332 and 334 further ensures the wood gas will be thoroughly mixed with secondary air by the time the wood gas passes through the openings.

The wood gas/air mixture then passes upwardly through opening 342, catalytic converter 344, apertures 331A and

331B in top wall 330, opening 358 in outlet wall 356 and out of the woodstove through outlet cylinder 352. The majority of the wood gas entering catalytic converter 344 is burned in the catalytic converter and the by-products of that combustion, i.e. carbon dioxide and water, comprise the primary constituents of the exhaust gases released by the woodstove. The use of catalytic converter

344 is advantageous in that it supports, the combustion of wood gas at temperatures readily achievable in the present o woodstove, e.g. temperatures of 500 F and above. Catalytic converter 344 is positioned in tertiary combustion chamber

320 just above the plane along which floor 30 extends and directly behind primary combustion chamber 20. By this placement, the catalytic converter is heated by conduction of thermal energy from the primary and secondary combustion chambers and from the hot wood gas/air mixture passing through the catalytic converter to the ideal o o temperature range, i.e. 700 -1000 F, for supporting catalytic combustion of the wood gas. Additionally, by this placement, the catalytic converter will not be subject to flame impingement.

By thoroughly mixing the wood gas with preheated secondary air in secondary combustion chamber 120 prior to its introduction into tertiary combustion chamber 320, a

substantially homogeneous mixture of gases is introduced into catalytic converter 344. By introducing gases of know composition into catalytic converter 344, the efficiency of the catalytic oxidation of the wood gas is maximized resulting in relatively complete combustion of the wood gas. Resultantly, the emissions level of the woodstove is minimized.

Since a sizeable percentage of the wood gas created in primary combustion chamber 20 is consumed in opening 244 and secondary combustion chamber 120, the present woodstove may be operated without catalytic converter 344 and still achieve a relatively low emissions level. When the woodstove is operated without catalytic converter 344, primary and secondary air will enter and pass through the woodstove with greater velocity than when the catalytic converter is installed in the teritary combustion chamber. Catalytic converter 344 thus acts as a barrier to airflow through the woodstove and thereby slows the rate of combustion in the woodstove. Nevertheless, satisfactory burn times, e.g„ up to five hours, and relatively low emissions levels are achievable in the present woodstove without the use of catalytic converter 344.

Opening 336 and cover plate 338 are provided -so that catalytic converter 344 may be readily installed in or removed from tertiary combustion chamber 320. To install catalytic converter 344, cover plate 338 is removed and the catalytic converter is inserted through opening 336 and is positioned to rest on horizontal wall 340. To remove catalytic converter 344, the foregoing process is followed in reverse.

While the stove of the present invention is designed

primary for burning wood, other solid fuels such as coal or peat may also be satisfactorily burned in the stove. As skilled practitioners will appreciate, it may be desirable to modify the present stove in known ways to maximize the efficiency of the stove for burning solid fuels other than wood. For burning most solid fuels other wood, catalytic converter 344 is preferably not used.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.