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4.8 oz. micro woodgas stove
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Michael Kackman
(smersh) - F
4.8 oz. micro woodgas stove on 05/14/2010 15:20:00 MDT Print View

Hi all,

I've lurked here for some time and got a lot of useful information, but decided to come out into the open to share a recent project that some here might appreciate.

I've been reading about the various gasification stove projects here and elsewhere with a lot of interest. I've got a larger wood gasification stove (a commercially produced Woodgas XL, which is about the size of a gallon can), and have been meaning to build a quart can stove of my own. In the back of my mind, though, I've continually thought there has to be a better way to do this....

My goals:

- more compact than a quart can

- lightweight: 8 oz or less

- sturdier than a can stove, without having to fit parts together

- integrated bottom plate, to catch ash

I started keeping an eye out for a suitable vessel to use as the platform. Every time I saw a double-walled stainless thermos, I would look at it and try to imagine it being transformed into a stove. The problem is that they're all too tall, or have too narrow of a top. Poking around, I found a little stainless tumbler online:


It's made by Interdesign, and the model is the Forma 21860 tumbler. I found one for ten bucks at Target in the bathroom supplies section. It looked promising, but once I got my hands on it, it seemed really heavy (probably 10 ounces or more), which was a double problem -- too heavy to be worth the trouble, and hard to work with.

Then the other day I was at the Container Store, and they had the same tumbler on the shelf. I looked more closely, and realized that there are two slightly different versions of it -- they had both, all mixed up together. They look the same from the outside, but one is half the weight, and clearly made of lighter weight stainless steel. Visually, they're almost impossible to tell apart -- the lighter one has a slightly crisper angle to the top edge, and the bottom inside surface has rippled concentric circles (you can see this in the pics below), while the heavier version has a completely smooth bottom.

Unfortunately, I don't know which version is newer, or more likely to be in stock at retailers.

4.8 ounces, 3-7/8" high, and 3-1/8" at the widest. Score.

The clerk wrapped it carefully in tissue paper, telling me she'd hate for me to get home with a scratched up cup. Heh, heh.

This is a pretty crude homebrew mockup, but I think the design has real merit. I initially thought I would use an Irwin Unibit, which is good for cutting into sheet metal. That tended to skid around too much, and it was a hassle to get inside to work. So instead, I used a 5/32" titanium double-cut bit, which worked pretty well. The pimples around the outside are from popping through with the drill, and bumping the outer wall. A little unsightly, but so it goes. Obviously, this would have worked a lot better with a drill press and a better clamping system. Even so, it turned out ok.

I taped up the sides and top edge to keep from marring the steel too much, and went to town. I just used a small punch to give the drill bit some purchase, and then simply drilled out each spot with a cordless drill:




I did the same thing on the inside, making a number of ventilation holes on the bottom of the combustion chamber:


Then, a cleaning slot so that any ash that gets down through can be poured out:


enlarged with some very crude dremel work:


Add a piece of gutter screen to lift up the fire to get a bit better airflow:


So how did it turn out?8

Edited by smersh on 05/14/2010 15:37:23 MDT.

Michael Kackman
(smersh) - F
pt. 2... on 05/14/2010 15:39:09 MDT Print View






It works. It clearly is gasifying, and it burns down to a small pile of ash.

The post-mortem:

As a proof of concept, this definitely works. It's a bit trickier than a quart can stove, since the combustion chamber is smaller, and the gap between the inner and outer walls is thinner, making air flow trickier. That makes it especially important to have good dry wood, in very small pieces. Even so, it works fine, and should easily boil up a pot of water very quickly. With some refinement it should make a good solo or daypack stove.

It would probably work well as a chamber for an alcohol stove, too. Worth trying, anyway.

Here's what I think it needs for round two:

- Better airflow at the bottom of the combustion chamber. I think I'll enlarge some of the holes in the bottom, and maybe see if I can drill some additional larger holes directly into the corner of the bottom, at an angle. I'll see how that works, and then maybe enlarge the exterior air intake holes.

If anyone is interested in trying one, please chime in with your results, mods, experiments. All I ask is naming rights.

Edited by smersh on 05/14/2010 15:42:13 MDT.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: 4.8 oz. micro woodgas stove on 05/14/2010 15:42:29 MDT Print View

I've never tried to fabricate something like this myself, so I won't make any claims. It does not seem as though you have much wood capacity. I have one ordinary wood stove, and its fire chamber is made out of a one-quart paint can. Now, I understand that yours is two layers.

Do you think that yours has good enough wood capacity? Have you tested it?


Michael Kackman
(smersh) - F
still experimenting on 05/14/2010 16:09:16 MDT Print View

I haven't built a pot stand, so I haven't tested it much yet. It's definitely small, but I would guess that boil times with a liter pot should be ok. The burn chamber probably isn't much smaller than a bushbuddy.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: still experimenting on 05/14/2010 16:17:46 MDT Print View

I've never seen a bushbuddy in the flesh, so I can't say. If I had to guess, I would guess that the design on a bushbuddy is close to some optimum dimension and material thickness.

Apparently you are trying to make something smaller or lighter.

Just to see if you can?

Would it be practical to start with some double-wall titanium vessel?


Michael Kackman
(smersh) - F
. on 05/14/2010 16:25:32 MDT Print View

I did it as an experiment as much as anything -- largely because the more I use gasification stoves, the more convinced I am of their usefulness. I usually use a Whisperlite when backpacking or camping, which I realize is far too heavy for most people's tastes here. If this works out, though, I'll definitely use it.

Some kind of double-walled titanium would probably work great. The only things I've seen, though, are Snow Peak-style double-walled cups, which are pretty expensive to hack up, and the gap between the two walls is likely too thin to work efficiently.

The engineering of stoves is pretty complicated -- hole sizes and placement, thickness of the gap between the walls, air flow, burn chamber size and dimensions, etc. It's all pretty well beyond me, so I'm just experimenting to see what I can come up with. I was just trying to come up with a one-piece design that was built from something off the shelf.

Edited by smersh on 05/14/2010 16:27:48 MDT.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: . on 05/14/2010 17:02:30 MDT Print View

Where you operate outdoors, what do you expect to find for fuel wood?


Keith Selbo
(herman666) - F - M

Locale: Northern Virginia
Re: Re: 4.8 oz. micro woodgas stove on 05/16/2010 13:50:43 MDT Print View

"It does not seem as though you have much wood capacity

I don't follow. Can't you just add wood as the charge burns?

George Geist
(geist) - M

Locale: Smoky Mountains
Re: 4.8 oz. micro woodgas stove on 05/16/2010 14:48:44 MDT Print View

> I don't follow. Can't you just add wood as the charge burns?

Hi Keith,

You can, but the theory of the woodgas stove is
to have it burn down from top to bottom. Adding
fuel on top of the burn layer disrupts the gasification
(in theory).

I have made a whole series of these micro woodgas stoves and have found they just don't put out enough heat on one fuel load to boil a liter of water. They burn with real pretty blue jets of flame, but the water doesn't reach a rolling boil. I found I had to use a bigger stove (wood volume) like the bushbuddy and "Penny Wood stove" or I had to boil much smaller quantities of water (pint).

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: 4.8 oz. micro woodgas stove on 05/16/2010 15:16:57 MDT Print View

If you compare the wood capacity of a Bushbuddy, Ti-Tri Caldera, Compact Wood Stove, and any others, I will guess that there is a magic minimum capacity for wood, and if you drop below that, you'll have your hands full of problems. Just for round numbers, the firebox ought to be about one quart.

You might try firing it with wood pellets or something less natural, but that would destroy the usefulness of a wood stove in a wilderness setting.


David Franzen
(dfranzen) - F

Locale: Germany
Re: Re: Re: 4.8 oz. micro woodgas stove on 05/16/2010 17:23:28 MDT Print View

what is the advantage of a gassification stove over a standard wood burning stove?

Keith Selbo
(herman666) - F - M

Locale: Northern Virginia
advantages of gasification on 05/17/2010 06:59:25 MDT Print View

What are the advantages of gassification? For this application, probably none. If you have time to search these threads, there's a post by a guy who showed that the holes at the tops of these so called gassifyers were supplying heated air which allowed for more efficient combustion of the gasses coming off the wood. That counters what the proponents of this style of stove suggest.

There's chemical energy in the wood and getting as much of it into your pot as possible by whatever means possible is what counts. Toward that end, I use a 5 oz titanium stove with a wind screen that keeps the hot gasses close to the pot and a sub one ounce fan powered by my headlamp batteries. It cooks ok without the fan. With it, it rivals canister stoves in cooking speed.


Note: fan shown has been replaced by one half the size & weight.

Edited by herman666 on 05/17/2010 07:00:48 MDT.

Michael Kackman
(smersh) - F
re. on 05/17/2010 09:23:11 MDT Print View

I did another couple of burns with this -- it's definitely still an experiment. Realistically, it's better suited to 16 oz. of water than a liter. A liter was overly optimistic. I would agree that there's a point of diminishing returns -- if it's too small, you can't get enough fuel load, and adding more fuel midway complicates the burn cycle. I'll still mess around with it a bit, if only to better understand how these stoves work.

I'm not sure a quart volume is really necessary, though. The Bushbuddy and other small stoves are far less than that, and the popular DIY version uses a quart can as the outer chamber, with a much smaller soup can inside (18 oz.?).

Kevin Beeden
(captain_paranoia) - F

Locale: UK
re: Just to see if you can? on 05/17/2010 11:06:02 MDT Print View

> It's all pretty well beyond me, so I'm just experimenting to see what I can come up with. I was just trying to come up with a one-piece design that was built from something off the shelf.

Some of us like to experiment with stuff, and if that means taking an idea, hacking at a bit of metal, seeing what happens, and then publishing the results for others, then that's reason enough.

Some of understand this desire, and appreciate your efforts, Michael. Thanks for posting the sequence of pictures.

Look forward to hearing results of burn and boil times when you've got a pan support rigged up. My 'standard load' for testing alcohol burners is 500ml; I rarely boil more than 500ml at once in the field, and my pans are mostly smaller than 1 litre. 2 US cups seems to be a common US 'standard load' for such measurements.

Keith Selbo
(herman666) - F - M

Locale: Northern Virginia
Re: re. re-fueling on 05/17/2010 11:53:13 MDT Print View

Lifting the pot and throwing in another hand full of wood just doesn't sound that complicated to me.

Lawson Kline
(Mountainfitter) - M

Looks Nice! on 05/17/2010 15:27:32 MDT Print View

Looks Nice! How long does the wood last with the wood capacity in question?