After about 2 months building & testing small woodstoves of various configurations, I stumbled on an easy to build, lightweight stove (3.5 - 4 oz) that will boil 1 quart of water and hold the boil for about 10 minutes, using only about 2 oz of wood as fuel. The stove is batch loaded, fun to use, and nearly smokeless when properly fired.

On the negative side, this stove requires a starter fluid and will blacken your pots. Also, the current version gets hot enough at the end of the burn to ignite newspaper under the burner, so the stove should not be used on flammable surfaces.

The design attempts to exploit the "batch-loaded, inverted down-draft gassifier" wood-burning technique and manages about 1/3 - 1/2 "blue flame" at peak output. As the gas-burning stage winds down and while the stove is still quite hot, the flame is mostly blue. Typically, the stove is burning wood gas shortly after ignition and has a stable yellow/blue flame within about 1 minute. After about 10 minutes the wood gas is depleted and the stove transitions to charcoal burning. Charcoal burning continues for about 20 minutes after this transition.

Description

The stove comprises a steel can, fire grate, stove windscreen, pot stand and pot windscreen. The steel can is the combustion chamber and is 3" in diameter and 4 1/2" tall. Primary air holes are punched at the bottom edge of the can, and secondary air slits are cut about 3" up from the bottom. A fire grate (wire screen) is fitted to the bottom of the can to allow even distribution of the primary air to the bottom of the fuel supply. The stove windscreen is made of light aluminum (disposable baking pan) about 3 1/2" in diameter and 4 1/2" tall. Slots are cut in the bottom of the stove windscreen to allow primary/secondary air to enter. The pot stand is fashioned by bending a coathanger into a clip that slips onto the rim of the steel can and holds the pot about 1 1/4" above the rim. The pot windscreen is a piece of doubled aluminum foil that goes from the ground to at least 1/2 way up the pot.

Development & Theory of Operation

As of the fall of 2003, I had been using an alcohol-burning, soda-can stove for several years. This stove had met my rather limited requirements for cooking on my annual AT section hikes; however, after reading about Rick's (aka geoflyfisher's) successful work building a simple, light, forced-draft stove, I inexplicably became inspired to try building a wood stove that would not require a battery and fan. I decided, more or less arbitrarily, that the stove would have to be very light (5 oz or less), would have to boil 1 quart of water, hold a simmer for 10 minutes, and be fun to use.

In researching what had already been done in this area, I was very suprised at the amount of information about wood burning stoves available on the internet. Soon, I had many weeks worth of reading to do. It turns out that some pivotal work was done on this as late as the mid 1990s. In May, 1996 T.B. Reed and Ronal Larson of the Biomass Energy Foundation in Golden, Colorado presented a paper titled "A WOOD-GAS STOVE FOR DEVELOPING COUNTRIES" at the “Developments in Thermochemical Biomass Conversion” Conference, in Banff, Canada. In this paper they describe a wood stove which uses a component they describe as the “inverted downdraft gasifier, which operated using only natural convection. I read this paper early on in my development process, but put it aside because it was "too complicated". Instead, I began countless iterations which employed the more intuitive Winarski Rocket style burner. Eventually, I became frustrated by the typical smokey & slow starting of the Rocket. (Note: for those 'Rocket' fans, my problems were mostly due to the very small scale of the stoves I was working on. These little stoves have almost no draft. Also note, the rocket burner worked well if the wood was very dry and the initial wood charge was small; however, this required too much baby-sitting and failed my "fun to use" design parameter.) I was about to call it quits, when I decided to just throw together a pure "inverted downdraft gasifier". I quickly fabricated a much simplified and smaller version of the Reed & Larson stove. The first lighting of this little stove was smoke free and delightfully easy. The metaphorical clouds dispersed, and I spent several days refining this design.

The 'inverted downdraft gasifier' is NOT intuitive and goes against the way most of us learned to build a fire. It works because the fire is started ON TOP of the wood supply. The flames consume the top layer of fuel and drive gas from the wood. As sufficient heat is generated, the layer below the current layer is consumed in a similar manner. This flaming region is called the flaming pyrolysis zone, and proceeds downward through the stack of fuel at a speed controlled roughly by the amount of air made available to it. The gas that is driven off from the flaming pyrolysis zone flows up through the charcoal stack where additional air is added resulting in nearly complete combustion and no smoke generation. Well, OK, it is more complicated than that, but you get the idea. It works.

Operation

Find a supply of dry sticks about the diameter of a #2 pencil and smaller. The amount of fuel required is approximately 2 oz by weight. Dump out the ash from the previous firing, and position the stove windscreen and pot stand on the stove body. Break up the sticks into about 1" lengths and throw them into the burner. As the stove fills, periodically shake/tap the stove to settle the fuel. When you get close to the secondary air slits, use only the smallest of the sticks you have collected. Shake/tap down the stove once again.

Spray a small amount of starter fluid (charcoal starter fluid, kerosene, alcohol, etc.) on the top surface of the wood supply. Do not use too much, because you only want to ignite the top layer of wood. (Igniting the lower layers will result in a smokey mess.) Light the starter fluid. After about 1 minute you should have a good flame going. Postion the pot on the stand and place the pot windscreen around the pot.

Caution: If the stove fails to ignite, do not spray additional starter fluid. There will be hot embers present from your previous attempt which are capable of igniting the fluid as you spray it on. This could result in SEVERE injury to you and others. The safe thing to do is unload the stove, reload it, and try again.

Note on Starter Fluids: I have had the most reliable results with charcoal starter and kerosene. They seem to soak into the wood somewhat. Alcohol works ok, but you have to use a little more for a reliable start. (However, alcohol trumps charcoal starter & kerosene in that it has good secondary uses on the trail.) For some reason, I have not had consistent, reliable starts with white gas or gasolene. Rick suggested using lamp oil which apparently works well, but I have not tried that yet.

Pictures

	The combustion chamber is a 3 inch steel can 4 1/2 inches high. Four holes are punched at the lower edge and a screen is inserted into the can to assure the even distribution of primary combusion air across the bottom of the fuel charge. The secondary air inlets are 4 slots about 3 inches up the can. The stove windscreen is a piece of heavy aluminum from a disposable baking tray that has been paperclipped into a cylinder.
	The fuel must be broken up into small pieces and loaded into the combustion chamber.
	Top view of the stove ready for lighting. The fuel has been added and shaken down tightly and the stove windscreen placed around the combustion chamber.
	The pot stand has been fitted, the fuel ignited, and the pot windscreen positioned.
	Here, the wood gas generation is in full swing. There is little to no smoke being produced.
	At the end of the burn, all that remains is a small pile of fine ash.
	In it's current configuration, the bottom of the stove gets too hot to place directly on combustible materials. Here, newspaper placed under the stove has smoldered.
	If you can deal with black pots, you will probably like this little 4 oz. stove

Tests

Discussion

1. This file contains some early, off topic, discussions that occured in the Hammockcamping group concerning the stove. The discussions were later moved to: http://groups.yahoo.com/group/BackpackingStoves/
2. This link shows some current work on a slightly larger stove plus some on-going refinements to the stove described here.
3. Q:What does the fire look like?
   A: Typical Flame Characteristics throughout the burn.
4. Q: I was trying to imagine where the fire grate (screen) goes.
   A: You need about 1/8" to 1/4" of air space at the bottom of the can. I just used multiple layers of folded chicken wire and pushed it down to the very bottom of the can. You have enough space if you can look through one of the primary air holes and see light through the opposite one.
5. Q: I am impressed that it will burn so long on one 2 oz load of fuel.
   A: Yes, it surprises me too. Also, no smoke! which is another nice surprise.
6. Q: I was expecting it to need reloading like i hear the Zip and other similar stoves need.
   A: If you do reload this stove as it burns down, it will smoke. It is probably better to size the burner to the amount of heat you will need rather than try to reload one that is too small.
7. Q: What can are you using?
   A: I'm using a small can that vegetables came in. It is 3 inches in diameter and 4 1/2 inches tall.
8. Q: Have you tried lighting it with pine needles or paper as some suggest, instead of the liquid?
   A: I want to try lighting with tender, but have not done so yet. I was also thinking of some sort of fabric dipped in wax that could be layed on top of the fuel charge.
9. Q: Effects of wood that is not quite dry?
   A: The more wet and the more green the wood the more difficult starting is (more fluid must be used). Also, when the stove transitions to charcoal burning, wet/green wood tends to smoke. So far, blowing down on the coals has added enough air to hasten the transition back to smokelessness.
10. Q: I assume the stove windscreen is causing some of the air/smoke from the secondary slits to be pulled under the stove in a preheated way and to burn the wood gas.
    A: The stove windscreen acts as insulation for the combustion chamber and heats the primary/secondary air. Concerning the secondary air inlets, all that happens there is that additional, slightly pre- heated fresh air is drawn into the combustion chamber. If you look carefully on the picture of the stove body, you will notice that the bottom of the secondary-air slit has been pushed toward the center of the can. This creates a small venturi that draws in more air (this seems to help). On my current stove, I have cut four additional secondary slits, two ridges higher & halfway in between the slits shown in the picture. Don't know if they help much (or any for that matter).
11. Q: --- In BackpackingLight@yahoogroups.com, "E. Burch" wrote: I remember seeing a commercially available version a couple years ago made out of stainless but have been unable to locate it since. I think it was called the "Bushbuddy". Did you, by any chance, stumble across this SS stove while doing your research?
    A: Yes. You can see the patent for Bushbuddy if you go to www.uspto.gov and search for patent #5,842,463. This patent was issued in 1998 and is a knockoff of the Reed/Larson stove. I suppose it is possible that there is a relationship between the patent holder and the ones that first described the idd gassifier stove. The bushbuddy is also very similar to the WoodGas camp stove currently being sold at http://www.woodgasllc.com ; however, the camp stove has a fan where the bushbuddy did not. I did purchase the WoodGas Camp Stove to see how it works. It is a cute little thing, but a little heavy for backpacking. It works very well.
12. Q: --- In BackpackingStoves@yahoogroups.com, David Anderson wrote: have you tried using a small plastic pencil sharpener to make shavings out of those pencil sized pieces, instead of using lighter fluid? It seems to me that a nice little pile of shavings with the last couple of sticks tossed over them would work well for this, without having to carry the starter.
    A: David, I like your idea and will give it a try. If it works as it seems it should, it would certainly not take long to pay for itself in terms of weight of starter fuel not carried.
    A: From Rick: I tried spinning a stick in a pencil sharpener last night. It needs to be a nice dry stick without rot. If it is beginning to rot, it just becomes more like peat moss than fire starter.
13. Q: --- In BackpackingStoves@yahoogroups.com, Rick wrote: What do you use to spray the starter fluid? I tried a tuberculin syringe today, but the lamp oil froze the rubber stopper in the syringe tube.
    A: I'm just using a 4oz bottle that camp suds came in. (The valve is similar to the one found on lighter fluid cans.) That was my entire fuel supply for my last 8 day section hike (one, half ounce firing of my pepsican stove per day)! Anyhow, it doesn't spray very well and tends to use too much fuel; however, I estimate that I can fire the stove between 40 & 50 times if I use slight care.
14. Tip From: mikeb@p....

    I noticed in the plans you mentioned somewhere about using a little fabric dipped in wax as a fire starter. Here is what I do. I save lint from our clothes drier and collect old candles. Ask friends or even buy some parafin from the grocery store. Melt wax over low heat. Mix drier lint and wax into a paste. I put it in an old plastic ice cube tray then let harden, then pop out. Makes great fire starters cubes. You could shave off a little to start the wood in your stove. I also use pine shaveings(son had some left over from when he had a pet gerbil) I think if you used clean parafin(canning type) you could cook over it because a cube burns atleast 15 minutes... MIKE