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Derek Goffin

Locale: North of England
fuel efficiency on 02/15/2008 09:07:46 MST Print View

WEll done for these tests I think you have shown that preheating primary air in a forced draft Zip stove reduces speed of boil. I would say the little fan is pushing more air through the less convoluted path of your modified stove and so you are getting a faster boil. Maybe this is what we all want, but I think efficiency is another concept.
To my mind the test of fuel efficiency is how much increase in temperature of standard large volume of water is produced by a standard load of wood in different stoves.
So with the same wood stove A gets 10 litres from 10-70 degree C, stove B 10-80 degree C before it goes out. Stove B is the most efficient.
I think first we need to decide on our criteria for success. Is it faster boil? the same task done with less fuel? minimum smoke? no scar on the ground?
I must say powered stoves are heavier than I am willing to carry backpacking.
If I was modifying the zip stove for efficiency. I would put back the double wall, drill holes around the top of the inner wall, (so that at least half the air came out that way) to let preheated secondary air out above the fuel and raise the pot 75mm with a bigger pot stand. I would use a windshield around all that in any wind at all.
I would predict that would be efficient, and relatively smokeless, but it might not win on speed of boil.

Bill Reynolds
(billreyn1) - M

Locale: North East Georgia Mountains
Has anyone tried this stove? on 02/15/2008 09:25:38 MST Print View

I have seen these on Ebay and they look pretty interesting. I wonder if anyone has experience with it?

Andrew Wilson
(andreww) - MLife

Locale: Vosges
passive downdraught gassifier woodstoves are not possible on 02/15/2008 09:28:50 MST Print View

You are all correct that these are not downdraft stoves. But you are incorrect that they are not gassifiers, and that they are some kind of urban legend. Whether the technology is best put to use for backpacking is another question.

Technically the Bushbuddy and its kin are inverted-downdraft-gassifiers. Gassification happens when you force air downward through a hopper toward a burn chamber. When the combustible material gets close to the combustion, it begins to smolder, i.e. gassify. What the forced air does is force this smoke downward and through the hot coals.

for a demonstration of how it works in a home heater.

Since this smoldering uses up the available oxygen as CO and H2 (et al.) are produced, more 0^2 must be introduced in order for the gases ignite. The ignition of these gases is secondary combustion.

That is what we are seeing as the flames exit the top holes of the Bushbuddy. Plug those upper holes and you would have a smudge pot. Contrary to what appears to be the popular wisdom, it is not the preheating of the secondary air that is most important (though it is not insignificant); it is the draft of fresh oxygen.

Inverted down-draft gassifiers are also known as top-lit-updraft-gassifiers, and have quite a following among third-world development workers, with improvements in efficiency over an open fire by three to four times. See the discussions at
These tinkerers are not just interested in efficiency, but in smoke and the other dangerous chemicals given off by non-complete combustion (ever gotten a headache from wood smoke? That's carbon-monoxide poisoning.)

see also

Another thing that is missed is that true "top-lit-updraft-gassifiers" must be lit from the top, like Garlington describes. The Bushbuddy can be a pure t-lud stove if it is treated like one, but it is typically a kind of hybrid which works on the same principles. If anyone fiddles around, they will find that the _cleanest_ burn overall will result from starting the fire with some alcohol soaked pieces of wood on top.

Perhaps the most important thing the tlud burners do is regulate primary and secondary air supply; hence regulating primary and secondary combustion. I found the Garlington burner most unsatisfactory. I know now that it is because there was too much primary air feeding the "smolder" stage, and not enough secondary air feeding the "combustion" stage. The Bushbuddy has this carefully regulated---at about four times more secondary air than primary air (measure the intake below the ash pan, which is very hard to see, and compare it to the combined diameter of the upper burn chamber holes). This keeps the smoldering happening at a good enough clip to boil our water, but not so fast as to provide more smoke than the oxygen in the secondary air can combust. Hence the "clean as a candle."

By the way, every kind of wood or biomass fire, even an open one, is a kind of "gassifier," because combustion always happens in two stages: gassification of volatile organic compounds (i.e. smoke), then combustion of these gasses (flames). A well-built teepee fire will be smokeless---but notice how high the flames climb. They climb in search of more oxygen. What all kinds of stoves do is concentrate the heat and regulate oxygen to create more complete combustion and more efficient heat-transfer.

Good technique and patience to create dry wood and find a sheltered spot can make any fire at least efficient enough for backpacking. But who wants to spend that much time (in our precious time in the wilderness, anyway: I like wasting days with fire while at home) fiddling with fire?

And beyond all that, I love my Bushbuddy not because its the best possible solution, but because it is a well-thought, handsomely produced design. If this weren't what caused us to buy stuff in the first place, we'd all be sleeping dry and happy and rich under plastic tarps in dressed in thrift-store synthetics. I do that too, but even in my thriftyness, every once and a while a thing of beauty comes along whose concept and construction are good enough to arrest me.

Andrew "Cheapweight" Wilson

Dan Yeruski
(zelph) - MLife

Where the name came from on 02/15/2008 11:10:50 MST Print View

These two guys are the culprits that came up with the name "Inverted downdraft Gasifiers" :

T. B. Reed and Ronal Larson
The Biomass Energy Foundation, Golden, CO., USA

I quote a small amount of what they said:

(quote) In 1985 we developed the “inverted downdraft gasifier” (also called “upside downdraft,
or pyrolysing gasifier) operating on natural draft. The name comes from the fact that the fuel
charge is lit ON THE TOP, and forms a layer of charcoal there; the flaming pyrolysis zone is
below that; the unburned fuel is on the bottom of the pile, and primary air for pyrolytic
gasification enters at the bottom and moves UP, forming gas in the flaming pyrolysis zone, as
shown in Fig. 2.
At that time we built a clean, efficient stove using a jet of compressed air to mix
secondary air with the gas and a venturi burner to hold the flame. However, developing country
households typically do not have compressed air, so we began development of a natural draft,
close coupled cooking gasifier. In 1991 we described a cooking stove based on the inverted
downdraft gasifier with natural draft secondary air entering the gasifier above the charcoal zone.
The combustion in this stove was relatively clean, but the poor air-gas mixing resulted in a
unstable, partly yellow flame. The stove is marketed under the name “GAS-I-FIRE”. [4](end of quote)

This is a diagram of the stove of which they refer to as figure 2:




Edited by zelph on 02/15/2008 11:31:10 MST.

Derek Goffin

Locale: North of England
inverted downdraught lets call it up draught on 02/15/2008 13:11:06 MST Print View

I think Andrew is agreeing with me but hasnt understood me.
Surely we dont need to call an updraught stove an inverted down draught stove. On a bushbuddy level I have tried lighting at the bottom by dropping a flaming firelighter down a hole and lighting at the top.Within a minute the whole firebox is alight so itop lit or bottom lit makes little difference.
The only thing I am sure I disagree with Andrew about is he puts too little emphasis on the preheating of the secondary air in the bushbuddy. Before the bushbuddy gets warm enough to preheat the secondary air it smokes large quatities of white smoke and then when the secondary air temperature rises suddenly we have flames as the preheated air mixes with the smoke and burns. To repeat my father's saying "secondary combustion needs all 3 T's including temperature".

Dan Yeruski
(zelph) - MLife

Re: inverted downdraught lets call it up draught on 02/15/2008 14:01:10 MST Print View

Andrew has alot of stuff all mixed up in there. Lots that I disagree with and some that I agree with.

Derek, it's not the preheated air that ignites the SMOKE, it's flame from under the smoke that makes it's to the top edge of the stove. Once there, all three elements are present OXYGEN, HEAT(Flame that made it's way to the top, FUEL(In the form of volital gases in the smoke)

Next time you use your stove, put a match to the top edge of the stove where the smoke is coming out. It will ignite the smoke/gasses. Same thing occurs when you blow out a candle, it starts to smoke, draw a lit match close to the smoke and watch the flame of the match jump kinda jump towards the smoke and ignite it. That's what is occuring at the top of the stove. Warmed air is not igniting the gasses. Just a side note: Cold air(condensed) contains more oxygen molecules than (expanded) warm air.

Ask your father about what is necessary to acheive fire.

Early on as a trained fire fighter we learned never to break a window of a burning structure until told to do so. To do so would allow oxygen in and fuel the smouldering fire within into a raging fire. Only when the firefighters are in position with charged lines would the windows be broken to allow the water to be discharged into the structure.



ZMART ZTOVE and also the Sierra Zip



By F.W. Hottenroth

The ZMART ZTOVE is a stove with a very high efficiency rating. The School of Energy of Bharathidasan University in India has carefully tested it in accordance with the percent heat utilised procedure outlined by VITA. The efficiency in bringing 10 kg of water from ambient temperature to boiling measured better than 55% on the average of four tests run on consecutive days. The DNES in India requires an efficiency of only 20% for a stove to be deemed acceptable as an "Improved Stove".

The high efficiency of the ZMART ZTOVE has been achieved by combining high combustion efficiency with careful focusing of most of the generated heat on the surface of the cooking vessel.

High combustion efficiency and a hot fire generally go hand in hand. To assure a hot fire, the ZMART ZTOVE contains the burning fuel within a well insulated cylinder. Air for combustion is preheated so it will not chill the fire and slow down combustion. When the stove is lit, brisk tongues of flame are seen moving almost vertically across the normal upward path of the fire. The high combustion efficiency of the ZMART ZTOVE is evident by the small amount of smoke generated in normal usage.

Focusing of generated heat on the cooking vessel by the ZMART ZTOVE is far from perfect but it is reasonably effective. The fire is contained within a cylinder of good insulating material such as vermiculite of diatomaceous silica, materials which have five times the insulation value of sand. This cylinder effectively limits the amount of heat moving radially out from the fire.

A radiant reflector under the grate stops downward heat loss. Open fires and most stoves waste a considerable amount of their heat energy in heating the ground. The ground under the ZMART ZTOVE remains cool.

Most of the generated heat energy from the wood is directed upward against the bottom of the cooking vessel. According to the tests of Bharathudasan University it took less that 0.5 Kg of wood to boil 10 Kg of water and there was more than 0.1 Kg of charcoal remaining from the initial 0.5 Kg of wood.

The high efficiency of the ZMART ZTOVE has been achieved with a low cost product. It is composed of a few simple parts which can be readily produced using elementary sheet metal equipment. It should be produced under proper supervision to assure good quality. Factory type production is mandatory to provide the quantities needed so every wood- burning family will be equipped with an efficient stove by the turn of the century.

It has been estimated that it can be built in India to sell in India at a retail price which can be paid back out of savings in fuel wood expenditures in six weeks. Similar figures may be anticipated for other countries.

Women who have tried the ZMART ZTOVE like it because it starts so quickly and has such little smoke. It cooks fast and requires little attention. It has been designed to upgrade their status from cooking over a three stone open fire to cooking on a modern stove in a comfortable posture. Most women will want to advance quickly from a single stove to a two burner or three burner stove. The ZMART ZTOVE lends itself to this programme. Additional burners can be added as desired.

The ZMART ZTOVE is designed for long, trouble free life. The housing, legs and pot supports will last indefinitely. The burner bowl should give many months of service in spite of the extremely hot fire within. Life tests indicate that burners operating at maximum output for six month equivalent usage show no serious deterioration. If a burner bowl does burn out after hard usage, it can be simply lifted out and a new, low cost burner bowl put in to take its place.

Z Z Corp is now seeking licensees to make and market ZMART ZTOVES in other countries where wood shortages are causing problems. Z Z can furnish detailed drawings and technical help to licensees plus continuing engineering assistance.


Z Z CORP (end quote)

The Bush Buddy burns well because it allows all the oxygen it wants to ignite a well prepared fire bed. No alcohol necessary if you know how to build the fire. Proper tinder, stacking of the DRY fuel no restrictions around the stove etc. Look at all the holes that let in air for the bb.

The 2 stoves on the bottom are the "Globe Stove". Made of stainless steel mesh, weighs 45 grams, costs $4.00 to those that MYOG. My design. Gear Heads buy their stoves, Stovies make their own.



Edited by zelph on 02/15/2008 14:33:15 MST.

Derek Goffin

Locale: North of England
flame too on 02/15/2008 15:09:46 MST Print View

Sorry Dan, I was just assumimg that we had ignition. As well as the 3 T's we need the oxygen in the secondary air and a means of ignition. The bush buddy if fed steadily should always, after startup, have flame. I totally agree with you and was using an unreasonable shorthand. Non of this is new knowledge. My father was taught this in the late 1940's and he is now long dead. It is very difficult with a small stove to get the right mix of primary and secondary air with uneven fuel supply. Pure pyrolosis stage needs more than twice the ratio of secondary to primary air of pure charcoal burning. This is why a reasonably steady feed giving a mix of pyrolysis and charcoal combustion, is better than big batches.
I agree wholeheartedly with Andrew that the Bushbuddy is well designed, I think it could have a lighter design for the outside skin, but it is inspirational.
My stove overheats the ground it stands on so I like to put it on a rock. Carrying an insulated disk, if it had minimal weight, would be an improvement, and also as Dan points out with the Zmart stove will increase efficiency. Does anyone know the lightest design for such a thing.
For me this is a good thread

Casey Cardwell
(Niles) - MLife

Locale: On the Dirt in Oregon
Re: flame too on 02/15/2008 17:01:38 MST Print View

I've been doing my best to try and follow this thread, but a lot of it is over my head. So I have a quick question that might help me understand something.

When you make charcloth you burn the cloth inside a sealed container, except for some sort of vent hole (in this example, I'm thinking of using an alcohol stove to heat the container). When The cloth starts to combust (?) you get a lot of smoke coming out of the vent hole. Ever so often that smoke will ignite and fire like the jet of an alcohol stove without having been exposed to any direct flame. I always assumed that that smoke was getting so hot that it hit its ignition point and burst into flame as soon as it got enough oxygen. Is that true? Is that what some folks are saying is happening with the preheated air in the double wall designs?

In any case, I'm inspired to try and make a wood burning stove now. Now I just have to figure out (1) which type of stove I should be making and (2) how in the world to make it.

Edited by Niles on 02/15/2008 17:06:28 MST.

Derek Goffin

Locale: North of England
ignition on 02/16/2008 04:03:39 MST Print View

Casey, yes and yes, except hot air is entering smoke rather than smoke jetting out into air. If it is hot enough it ignites by itself but apply a spark or flame and it happens earlier.

Joy Menze
(catamountain) - M
general wood and woodgas stove overview link on 02/16/2008 09:13:55 MST Print View

Bill Reynolds, your ebay collapsible wood stove is a fancy CAD cut-out of the Nimblewill Nomad stove. An overview of that stove and other woodburners, including woodgas, can be seen at Zen Stoves.

Thru-Hiker sells titanium sheets but is out of stock now, link on sidebar. But as seen above, it looks like stainless steel mesh is another good alternative.

Edited by catamountain on 02/16/2008 09:26:53 MST.

Dan Yeruski
(zelph) - MLife

Re: Re: flame too on 02/16/2008 10:23:08 MST Print View

I would speculate that the flame of your stove ignited the gases that had accumilated on top of and around the container containing the cloth.

I made a device to visually help me determin the correct distance from stove to bottom of pot. It's adjustable, raises and lowers the pot in relation to the stove top. I was using windshield washer solvent in my glass pot because of freezing temps in my garage(too lazy to get fresh water from house)I didn't realize there was enough alcohol in the fluid to be concerned about. As the wndsld solvent heated up the alcohol vapors ignited by the stoves flames coming up the side of the pot without me knowing it.
The flames inside the pot being invisible during the testing. I made ready my camera to make a movie of the testing under dark conditions and then shut the lights off. to my surprise, I could then see the flames inside the pot. I took some photos of the flames to be posted with test results on my website

This is why I speculate flames came up along side the container being used to bake the cloth and ignited the gasses coming out the hole.

First two photos are a steel mesh cone shaped wood burner that worked very well.(MYOG)

The second two are of the device used to determine correct distance burner to pot. The burners flame is visible inside the glass pot. The flames changing color and pattern can be seen clearly through the pyrex type glass.



Edited by zelph on 02/16/2008 10:30:14 MST.

Joshua Mitchell
(jdmitch) - F

Locale: Kansas
Zmart Ztove on 02/16/2008 14:15:01 MST Print View


Where'd you come up with that Zmart Ztove? Is that a new offering from the Sierra Zip guys?

Dan Yeruski
(zelph) - MLife

Re: Zmart Ztove on 02/16/2008 14:33:20 MST Print View

Vintage Stoves!!!! As far as I can tell it's their first model put on the market many years ago. I have other photo on my website, I'll drag them over.

Dan Yeruski
(zelph) - MLife

more photos on 02/16/2008 21:19:44 MST Print View



Dan Yeruski
(zelph) - MLife

clean as a candle on 02/18/2008 20:24:27 MST Print View

This one is for Andrew Wilson and others when they say their stoves burn clean as a candle. :)

It shows just how "Clean as a Candle" is when we talk about wood burning stoves, even the Bushbuddy.

Turbulence is what causes a candle to burn dirty/sooty, try it at home, see for yourself.

Dan Yeruski
(zelph) - MLife

garlington stove on 02/20/2008 11:09:45 MST Print View

This will give you an idea of what the Garlinton stove looks like when burning from the top down. I used low odor mineral spirits to moisten the top twigs to get it started. I made an additional 3 test burns that failed to burn as good as this first one. Up to this point it is a finicky stove and is difficult to light evenly without a lot of liquid starter fuel. I will resume my testing of it when it gets a little warmer.

Clickable movie



James Ennis
(JimEnnis) - F

Locale: South
Different designed wood stove on 04/04/2008 23:29:11 MDT Print View

I built one of these a couple of weeks ago and had the opportunity to use it on an overnight hike last weekend. Had a lot of rain, so fuel was damp, but was able to boil 12 oz. of water in approximately 6 minutes. I was very happy with the results, considering the damp fuel situation.