Micro tent stove design optimized for robust and easy combustion for winter backpackers comfort.

This post is about my micro tent stoves that are designed to provide heating for winter backpacking trekkers. They burn miserly quantities of bush sticks (~400g/h) whilst producing delightful heat for people and cooking in a small tent. However, they require careful design and operation to make the burn strong, stable and safe.

My micro tent stoves burns bush sticks that are often cold, wet and even frozen. They provide strong radiant heat, via a heat exchanger, for comfort and cooking in a small ultralight winter tent.

This post reports that a good micro tent stove design should:

• Provide limited but adequate combustion heat feedback into the incoming fuel sticks to prepare them for combustion.
• Produce surface temperatures between 350 and 550C, by limiting the heat exchanger size/area and use of a low conductivity construction metal. This ensures that there is strong radiant heat for comfort and cooking and that enough of the hot combustion gas reaches the flue pipe to sustain a strong stove draft.

Various other stove design evolution issues based on trial and error are discussed along with a definitive theory of combustion of wood and charcoal. Practical techniques to overcome design shortcomings and experiments with alternative designs to improve stoves are discussed.

Note: This has become a very long tome, so if you are not into learning about stove history, and the ‘how and why of stoves’ you may like to skip to the bottom addendum that links to stoves that optimise these design principles. They may inspire you to then read the whole post to understand, both the journey and the destination.

Background

Ancient. According to AC Scott, the earliest fossil record of fire on earth is some 420 million years ago while it was 1.5 million for human interaction with fire and 400,000 for evidence of habitual use of fire and 7,000 for regular widespread use.

Undoubtedly, we have used fire for a long time. However, I doubt the veracity of the ‘7,000’ claim because of our growing knowledge of Australia’s first people and their use of fire that stretching back to 45,000- 60,000 or more years (J Kohen).

If you are not a creationist, we could presume they arrived in Australia with fire skills learned by experience from somewhere else. If perchance they just materialized here without fire skill, our fire-prone continent would have quickly taught Australias highly intelligent first people of about fire use.

Modern. To more modern times, there is an early use of the term ‘stove’ (derived from Old Engish stofa) that means a heated room or space rather than the heating device itself.

This helps to make sense of René Descartes’ observation that he got “his greatest philosophical inspiration while sitting inside a stove”.

Maybe I can also say that: “I got my greatest stove ideas for this post while in a portable silnylon stove in the midst of a raging blizzard!”

Historic stove devices could be a simple as three rocks to support a pot. Shaped mud, stone or brick enclosures to contain and control the fire developed from this.

The open fire for room heating advanced to a closed chamber in 1735 using masonry construction. Ceramics were also used and towering ornate tile covered stoves were used to heat palaces.

The first enclosed metal stove, for room heating only, with a labyrinthine gas path was reportedly designed by Benjamin Franklin in 1742.

It was not until the turn of the 19th century that Benjamin Thompson developed the equivalent large stove for cooking and it was the forerunner to modern compact kitchen stoves that provide cooking and warmth for the room occupants in a domestic setting (Wikipedia).

Understanding how my micro tent stove works

The concept. I find it useful to think of my micro tent stove with inverted J-burners (defined later) as a tiny inverted smoke generator (or gasifier) that makes smoke from sticks then burns the smoke. Anderson and Reed are the ‘fathers’ of modern biogas stoves. “Now, I Don’t know if you can have two fathers but gene jockeys have produced baby mice with two mothers and no father.”

Nevertheless, according to them, my stove is not a true gasifier because there is an inadequate separation between the gasification zone and the gas combustion zone. So by their definition, my burner would be a ‘quasi-gasifier’.

Now, lighten up, it’s time for an ode;

Is it a tiny gas stove or just a fire?
Does wood burn in temperatures higher?
No, it just cooks the wood if it’s understood,
It’s the gas that burns in a quasi gasifier.

Long before I had a name for it, I stumbled across this issue of lack of separation of gasification with my tiny stove designs.

For my micro tent stove, this lack of separation is, on one hand, a design strength when burning crude ‘bush-collected’ damp/wet dense wood sticks in winter camps. It dries the sticks in preparation for pyrolysis.

On the other hand, it is a design weakness when burning drier and rotten wood as it can cause the pyrolysis zone to creep up the fuel tube. “Luckily on my adventures, I can usually find damp wood, or it finds me”.

A moving smoke combustion flame. The smoke from the hot bottom ends of the pyrolysing sticks is swept downwards by incoming fresh air (from the tent) and burns in a heat exchanger chamber.

The combustion of the smoke is assisted by passing over and through glowing hot charcoal. The speed of the incoming air creates turbulence and mixing of air with both the charcoal and smoke and is critical for their clean, fast and balanced combustion.

Feedback of radiant heat from static glowing charcoal. As well as providing strong radiant heat for warmth and cooking, some of the radiant heat from the charcoal in the quasi gasifier is also fed back into the lower end of the tips of the fuel sticks that are adjacent to the charcoal.

This sustains the pyrolysis of the wood, the heating of it in preparation for pyrolysis and lastly the drying (or thawing) of it as it moves closer to the pyrolysis zone.

Is that heat wasted up the flue pipe? Lastly, a significant portion of the heat of combustion of the smoke must be used (or wasted) as hot gases in a long flue pipe. This apparent inefficiency is essential to maintain a strong stove draft and robust combustion.

It also contributes to the prevention of inconvenient and messy tar and creosote deposits in the flue pipe. Such deposits can under rare circumstances become a hazard when they catch fire in a flue pipe.

“It is nice to have a clean flue pipe when camp pack-up time comes. Also, my latest pipe screw up method (shown in the video below) makes it fast clean and easy, even if a bit tricky.”

It is important to remember that the draft in the light flue pipes (150g) of these stoves substitutes for the draft created in a heavy heat riser (many kgs) in other stoves.

Draft failure. If for any reason the gas flame in an inverted J-burner is extinguished or the combustion zone is deprived of ‘woody’ fuel for pyrolysis then the strong flue draft will fail. This is because only the flaming and mobile gas that can sustain a strong draft in an inverted J-burner. Charcoal alone can not sustain a strong draft as there is too little gas evolved in its combustion. Instead, the heat from the charcoal can cause reverse burning up the fuel tube (As described later).

Stove design class. The micro tent stove has an inverted J-stick burner (more correctly a quasi-gasifier ) that makes it similar to a J-rocket stove or J-rocket mass heater (both described later) but luckily it lacks an insulated heat riser or any significant mass to fit in these categories.

My stoves are also designed to provide strong radiant heat. By contrast, rocket stoves directly heat cooking pots with flames and hot gas. On the other hand, a rocket mass heaters can provide for cooking but they largely heat air (and human bottoms directly) by thermal mass storage benches and slow release of heat. So neither are about strong radiant heat. “Neither would be welcome in my backpack or tent!”

The burning action of my stove is very similar to that in a wood pellet stove, and also the exemplar stoves described in Chapter 4 in aprovecho publications, designing improved wood burning heating stoves.

Such stove designs can create the most efficient and clean-burning wood heaters. They often have a fan that enables preheated air to be forced into the combustion and also for efficient air heat exchange.

The uniform sticks or wood pellets feed into a heavy hot burner crucible at a perfect rate to match the rate of gasification and charcoal depletion. All the stoves discussed so far must be quasi-gasifiers as the gasification and flame are not separated.

The perfection of the pellet stove is not possible in my backpacking tent stoves that must burn fuel of damp bush sticks. However, my micro tent stove designs can make my stoves achieve similar clean burning by limiting the amount of fuel that is pyrolysing at any time.

My designs require combustion heat to feedback into the usually damp fuel sticks to dry and then pyrolyse them. The stove has very little mass, insulation or heat storage to sustain the flame temperature and draft during process glitches.

I hasten to add that the delicately balanced wood gas flame, if extinguished, can be quickly reignited using a flameout recovery “This also demonstrates my stove is truly a smoke burner, quasi or otherwise.”

A new stove class. To avoid offending experts and other stove designers, I could define my micro tent stoves as ‘natural draft inverted J-burner quasi-gasifier with strong heat feedback to the fuel’. This is in preference to calling it a ‘J rocket mass-less heater’ or a ‘pellet stove that uses sticks instead of pellets’. I will shorten this to an inverted J- burner stove.

Note: For those who are racing ahead of me: I also tinker with tiny blower stoves for outdoor cooking with sticks and as you would expect the two threads have been merging over time.

Addendum: They now have fully merged into a tent stove that converts into an outside cooking stove with using wood or alcohol for fuel. So it is time for another ode:

Freely followed paths long enough will merge,
If map or GPS directions one resists the urge,
Let times imagination decide the destination,
And go long enough the paths will again diverge.

These transforming ten stoves use tiny USB fan-forced air blowers to achieve clean burning and high cooking temperatures with damp and wet sticks.

Curiosity has caused me to experiment with the application of the USB blower fan to inverted J burners. I briefly reference these experimental micro ceramic stick burners later in this post.

Note: For the impatient reader, the addendum at the bottom of this post links to a ‘three-in-one stove with both natural and blower functionality and more. However, I encourage you to read on, so that that you understand both the journey and the destination.

Importance of good stove draft

Gas flames for strong draft. For all heating stoves, a good draft is produced by hot gases in a flue pipe (conventional stoves) and/or flames and hot gases in a heat-riser (rocket stoves). It is essential for drawing in fresh air to sustain vigorous combustion. It can also be supplemented by the use of a fan.

Conventional stoves. The draft in conventional wood stoves is primarily produced by the flue pipe with a little bit of valuable draft derived by the rise of very hot flame and gas in the stove body (I define this as a de facto heat-riser).

Rocket type stoves. Rocket stoves have heat-risers, that ideally should be well insulated. The heat risers are often not very tall, but the very high temperature of the gas and flame in them creates a very strong draft.

Rocket mass heater. Rocket mass heaters (RMH) also have exemplary formal heat-risers, that ideally should be well insulated. The heat riser vents into an inverted metal drum where the fast stream of hot combustion gas strike the base of the drum and heats a hot plate for cooking. The strong riser draft forces the less hot gas down the drum. It then exits to pipework that passes through a large thermals mass bench (Usually made of cob and masonry.) where heat is stored. Finally, the riser draft can force the warm exhaust out through the flue pipe that does not need to create a draft of its own. The thermal mass bench slowly releases its heat over many hours and are usually formed as delightful bum warming benches.

“It is important to appreciate the unique feature of the two above burners is that the hot combustion flame and gas, within the heat riser, drives an unrelentingly strong stove draft. This happens before any heat is used for cooking, room heating or bum warming. By contrast with my stoves, the draft is created largely by the heat that escapes the cooking and warming functions.”

The hot flame in a riser. The photo below gives some idea of the heat and gas speed that is involved in a heat riser.

All the above stoves have a substantial thermal mass to sustain the draft and temperature during glitches in the combustion process.

Micro tent stove with an inverted J-burner. In contrast to the wonderful rocket stoves, my little tent stoves can not (yet) practically have a heat riser because of the weight and volume considerations for ultralight backpacking.

To provide an effective cooking surface, and generous radiant heat, my stoves have horizontal and shallow heat exchanger boxes or bodies. These have internal baffles that force the flame/hot gas to take a long and turbulent path through the labyrinth then on to the flue pipe. “There is no heat riser here and hot gas/air must first be sucked downwards within the hot fuel stick/burner tube”

A tortured gas pathway. The ‘flatish’ heat exchanger labyrinth improves the heat exchange to provide generous radiant heat and an effective cooking surface. It is also an effective spark arrestor. However, this long tortured path has some undesirable consequences:

• It impedes the gas flow through the stove,
• It takes much of the heat from the gas so that there is a much lower gas temperature when the gas finally enters the flue pipe to create a draft,
• It eliminates the de facto heat riser effect as found in a conventional batch fed box stoves.
• It eliminates the natural venting path, for combustion gases, up the flue pipe, as for other stoves, when the wood gas flame fails and the stove is only burning charcoal. In this situation the fuel tube becomes a ‘de facto flue pipe” and the sticks may burn in the upward direction. “Not good, especially in a tent!”

Consequently, my stoves depend entirely upon flue pipe draw to maintain combustion draft. They also have a small and delicately balanced burner in a stove that has no significant thermal mass to smooth out burner glitches as other stoves may do. “If the flame stops the normal forward draft also stops.”

This all means that, with my stove designs, I must be prepared to waste some heat in the flue pipe to sustain strong and stable combustion.

However, this waste is of little consequence when the fuel is dead sticks and the maximum fuel consumption is an ‘unbelievable’ 400g/h. “Some of that ‘waste’ can also be recovered from the surface of the flue pipe as useful tent heating.”

Primary and secondary air ports

Primary air port. This port has a slide valve fitted so that it can be closed or constricted so that all air or more air will be drawn in through the secondary air port. This prevents the smoke release into the tent while re-fueling. The valve is also designed to prevent sparks from exploding charcoal being ejected out through the port and into the tent. The main purpose of the port is to direct air, at speed, into the charcoal bed at the bottom of the burn chamber.

Secondary air port. This is the top end of the fuel tube and air flows in through the fuel sticks to support their drying and pyrolysis. The flow through this port can be stopped by covering it with a ‘snuffer can’ that can be made from a soft drink can.

This cover can be used to restricted airflow to control excessive pyrolysis and what I later call ‘charcoal choking’ and ‘reverse-burning’. Closing of this port also increases the flow through the primary port. “It can also be used as a snow/rain cap for the flue pipe when the stove is not in use.”

When using damp fuel sticks, the secondary port can be left open so that there can be continuous access to the fuel tube for easy refuelling where reverse burning is not likely and more heat for pyrolysis and wood drying is probably required. This mode of operation can be a little less efficient because of excess air entering and cooling the heat exchanger a little. However, this is of no consequence when there is still abundant heat output.

Also, the effect of opening the port can be largely offset by constantly ‘over-filling’ the long fuel tube with fuel sticks. They can be conveniently ‘stacked’ beyond the fuel tube limits, as shown in the second last photo below. “This is a rather lazy and very easy way to run the stove when the fuel sticks are cold and damp.” (The later described ‘snuffer-can’ by design can also cover the protruding sticks when required.)

Clarification of air port terminology. Unfortunately, my definition of primary and secondary are the opposite of those used by the gasification experts Anderson and Reed and most others. However, I will stick with my terminology. “I also excuse myself for this fault because it seems logical to me that if there is a singular airflow port into a quasi-gasifier then it must be ‘primary air’.”

Fuel tube design considerations and the Holy Grail of tent stove design.

No fuel tube. With a very short fuel tube or no tube at all, short sticks and wood chips can be fed in easily, but somewhat tediously for a very satisfactory burn. However, running this way, there is a small risk of sparks jumping out into the tent, so a longer fuel tube is better in this regard. “I have found that in desperation almost any solid fuel will burn in the no-tube mode if it can be broken up into small enough pieces and the sparks can be safely managed.”

Short fuel tubes. All my micro tent stoves can be run with a very short fuel tube such as a ‘burner glass’ or short custom made metal tube, made out of titanium or stainless steel foils. The foils feel quite flimsy. However, once welded into a tubular or conical shape they become amazingly strong and ‘feisty.’

The conical stove tube adaptors, that I use to mount the tall fuel tube in, can also be used as a short burner tube. In practice this is the way I prefer to start the stove where it is easy to precisely add the; some charcoal chunks, wood splinters & chips, leaves, bark etc and ‘waxies’ and ‘flame drizzlers to kindle the delicate infant blaze before fitting the long fuel tube and dropping in more solid fuel for robust carefree combustion.

Lazy long fuel tubes. The main attraction of a longer fuel tube is the increased fuel load that is possible and the time that the stove will burn for without needing refuelling.

For anyone who has followed my posts over the years, you may have noticed that I have evolved taller fuel tubes and taller snuffer cans to allow increased fule stick length and laziness.

It also makes it easier to prepare fuel sticks as I find it is much easier to split long fuel sticks (than short ones). There are less of them, they last longer in the burner. Also, their greater individual weight and length means that they feed in better and they more easily collapse the charcoal on their bottom ends.

I find it is easier to use a simple strong knife (and a bush stick used as a mallet) to split a thick 500+mm long billot of wood into small sticks. I then break the sticks down into suitable lengths for the fuel tube. The alternative of cross-cutting the billot requires a saw and considerable effort and then there is still a lot more splitting to do.

Me

Very long fuel sticks. Longer and longer fuel sticks will introduce the idea of an all-night stove that is the Holy Grail of tent stovers die-hard ‘stofa-people’. Experiments on all-night-stoves are reported in simple micro barrel stove.

Long fuel tubes and limits to their length. It is tempting, without practical experience, to think that the Holy Grail could be reached by simply making longer fuel tubes. It could be a roll-up design like the flue pipe or telescopic.

Yes and yes, it could be, and yes, I tried it, and I can hear stovies saying it could be a ‘dirtybig’ fuel tube that is as tall as the flue pipe and you (or preferably a companion) could go outside into the falling snow to put the fuel in from above the tent canopy.

Me

The reality is that long fuel tubes introduce their own complexities and problems that I discuss later.

Snuffer can extension. The snuffer cans primary function is to limit air entry via the top of the fuel tube which functions as the secondary air port (This is discussed later under ‘Causes of wood gas starvation’). Consequently, it can be quite short. On the other hand, if it is very long it can greatly extend the usable fuel stick length, potential fuel load and the period of time that the stove will run for on a load of sticks.

Stove component size matters. For a backpacking micro tent stove, the size of supplementary components matters, especially if they can not fit within the stove body. Consequently, tall aluminium soft drink cans make excellent lightweight snuffer cans that can be ‘nested efficiently with the roll up flue pipe and other components.

Causes of wood gas starvation in a micro tent stove

My micro tent stove design relies on a steady flow of pyrolysed wood gas to sustain the big expanding flame and the flue pipe draft. There are three ways in which the failure of the downward flow of fuel sticks can contribute to depriving the flame of its gas supply.

Charcoal choking. This can happen when charcoal builds up and prevents the entry of fresh wood for gasification and is a particular problem when the fuel sticks are thick, dry and rotten.

Charcoal is a wonderful hot burning clean fuel when in the right place and in the right proportion.

Mothy The Elder

Directing most of the incoming air through the primary air port and into the coal-burning zone is the best way of preventing or fixing this problem. This is done by covering the secondary air port with a ‘snuffer can’ (see above) and opening the primary port. “Use of forced air is another promising technique that I am experimenting with, but would best be avoided in a simple ultralight tent stove.”

Fuel stick pre-ignition. ‘Gas starvation’ can also come about if the wood gas ‘combustion zone’ is allowed to work it’s way too far up the fuel stick tube. This usually happens in combination with ‘charcoal choking’. “The smoke exits up the fuel tube and deprives the stove of the fuel for the proper draft.”

The longer the fuel tube is the greater the propensity for pre-ignition. Both choking and pre-ignition can be reduced or prevented by a combination of the following actions:

• Depriving the pyrolysis zone of oxygen via the top of the fuel stick tube (the secondary air port) by covering it with a  snuffer-can made from an aluminium soft drink can in combination with, 
• Preferentially direction of the incoming air toward the charcoal bed by fully opening the primary air port valve and capping the top of the fuel tube with the ‘snuffer-can’,
• Addition of damp sticks to the dry fuel mix.

Note 1: Although this is not currently possible with my slim micro tent stove designs, I have experimented with having a greater separation of the hot charcoal combustion chamber from the pyrolysis zone so that it becomes a gasifier rather than a quasi-gasifier, as differentiated by Anderson and Reed. My experimental stick burner also directs the primary air flow downward more directly into the base of the charcoal bed. These changes appear to totally prevent the above problems. “I just need to be able to design it into a tiny ultralight stove. These experiments are discussed in experimental ceramic stick burner.”

Note 2: I am now happy to report that I have at last designed a sweet ultralight and compact little KISS stove that avoids all these issues and many more without requiring any complexity. Here is an example in this little video of Nick’s stove at nighttime.

Failure of fuel stick feed-in in a micro tent stove. Hastily-prepared thick, wet or frozen fuel sticks can burn so that they form ‘long pencil points’ so that the centre of the stick is not burnt enough to let it collapse. These points prevent the wood from moving down into the pyrolysis zone to sustain pyrolysis.

This situation can easily be fixed by splitting such wood into finer sticks that let the radiant heat from the charcoal bed penetrate deeper into the stick. A simple ultralight stick splitting technique is described in fuel stick preparation for appropriate burning.

Such split sticks, from core wood, make efficient fuel and often contain more energy-dense fuel than smaller ‘whole’ sticks do.

Mothy The Elder

Alternatively, other thinner dry sticks can be loaded into the fuel tube alongside or preferably, in front of the problem stick/s. This can sustain the supply of wood gas and combust the problematic stick/s in the wash of the flames.

Ironically, once this issue is understood, it can be used to advantage as an effective way of regulating the stove heat output and extending burn duration when the cooking is over.

Removing the smoking problem stick/s in a small tent is not a good option. However, slipping some accelerant such as my ultralight my fire starters, waxies or fire-drizzlers, can quickly restore the flame, while some thin dry sticks are organized.

These tiny accelerant strips simply make an instant, but short-lived, source of combustible gas as they fall into the charcoal bed. A few drops of olive oil or crumbs of candle wax, ‘chainsaw crud’ or sugar will do the same. “If nothing else, it demonstrates that the stoves are truly quasi-gasifiers.” 

Difference between wood and charcoal combustion in a micro tent stove

Wood combustion. To burn wood it first must be heated to 280-500C (wood pyrolysis temperature) to cause it to make smoke quickly (pyrolysis). It is this smoke or wood gas that may burn if it is hot enough (est 350-750C) Browne and there is enough oxygen mixed with the smoke. Otherwise, it is just smoke. “It may be good for pacifying bees, but not for keeping you warm”

The smoke burning process makes extra gas that expands and, moves away from the wood as a moving tongue of flame. To quote Browne; “It can go forth and find oxygen”. In my particular stove design, this is essential for the transportation of heat to the heat exchanger and eventually to the flue pipe to maintain the stove draft.

Charcoal combustion. In contrast to wood, charcoal can ignite at temperatures as low as 250C and even as low as 100C Browne and that is why it is so good for starting a fire (and making gunpowder). It accounts for about 1/3 of the total combustion energy in wood and is a very good fuel.

In my micro tent stoves, in particular, charcoal must be burnt away quickly to make way for new incoming wood, because charcoal alone, can not provide enough hot gases to sustain stove draft.

When charcoal is effectively burnt it can reach a temperature of 1000C. It becomes a powerful source of radiant heat that, unlike the absconding smoke flames, it stays in place and consistently sheds this intense radiant heat back into the:

• Surrounding stove body to provide it with a store of heat,
• Wood above to cause more pyrolysis and smoke,
• Fuel tube to provide heat to dry the incoming fuel sticks and
• Wood gas to provide heat to promote its clean and complete high-temperature combustion.

Wood gas protecting charcoal from burning? It is also interesting to note that the charcoal that is being made during the pyrolysis of the wood is largely protected from burning by a shroud of gas distilling from the wood. The flame will preferentially consume most oxygen before it can reach the charcoal.

Consequently, without the intervention of good stove design, the charcoal must wait for the depletion of gas before it can naturally burn.

Even then, without managed drafting, it must wait for oxygen from the air to slowly diffuse to the charcoal surface to sustain a slow combustion Browne. “Such combustion is about as exciting as watching paint dry.”

This is a naturally slow process and that is why we use gentle glowing charcoal to ‘keep a fire going’. However, we blow into the same coals to rapidly accelerate the combustion process.

At home, at bed-time, I bury a good chunk of charcoal deep in the stove ash, so that it can be recovered in the morning to rekindle the fire for the day. Under the ash, only a small amount of air can diffuse through to reach the charcoal. This means that the combustion rate is continuous but very slow. It can be instantly blasted into an inferno with the air jet from my USB fire blower when required.

Me

Within a miro stove, the charcoal combustion rate can be greatly increased by designing the stove to causes a stream of incoming air to be focused predominantly on charcoal.

It greatly increases the rate of combustion of both charcoal and the nearby wood.

Most importantly, it shifts the balance in favour of the charcoal combustion by disrupting the wood gas shroud. This in turn reduces the ‘charcoal choking’ of the burner.

Note: I discuss the focused natural draft and fan-forced draft burner options in my post on a series of experiments with an experimental ceramic stick burner. This little video reveals the micro conflagration of such focused charcoal burning in a glass charcoal trap with a little wood present.

The combustion cycle

The wood gas flame drives a powerful flue pipe draft. In contrast, charcoal combustion provides a much weaker draft.

The draft sucks air in to vigorously burn the charcoal. The radiant heat from the charcoal dries the fuel sticks, pyrolyses the sticks and produces more wood gas. The gas is heated by the charcoal and burns completely as the flame moves away to be able to provide heat for flue pipe draft……  “The initiation of this cycle can be easily achieved with a few chips of dry wood, leaves, lichen, bark, acorns, gum-nuts, olive oil & paper etc and some waxie fire starters.”   

Wood and charcoal combustion- partners in a virtuous cycle of their own destruction mediated by a perpetual flame. By now, I think you would appreciate that a combination of wood, charcoal and fast air are critical for raging success in my little stoves. The last factor is the flame to keep the cycle going.

Flameout. When my stoves are still ‘stinking hot’ but have no flame they will turn into a generous smoke generator and may just stay that way without intervention. I call this ‘flameout’.

During ‘flameout recovery’ of a hot stove, I will simply slip a waxie fire starter up the primary air port and light the ‘tail-end’, like a fuse (Much safer than alcohol or shellite!). When the flame reaches the super-hot smoke it ignites the wood gas flame.

A gas lighter with a long snout can also be used, but the following photo of ‘dead’ lighters indicates that is not a good strategy. It is too tempting to put it inside the burner and this wrecks them quickly. Consequently, I only use cheap simple little lighters to light the tail of the waxie and mother nature does the rest. I must be a slow learner!

Me

It is still a delight to see it to see a flameout recovery happen. On rare and special occasions the gas will explode multiple times before re-establishing the steady wood gas flame.

“Just like our ‘old bomb’ student cars used to do when the fuel mix was a bit lean.”

When this happens a series of evenly spaced perfect smoke rings ‘sail out’ from the primary air port across the tent, expanding as they sail on. Sorry I have never caught this as a video. This video of flameout recovery with a waxie will have to suffice.

Slow temperature oscillations. During the evolution of my stove design, I became aware of the macro oscillation (periodicity of minutes) of the stove temperature. Other larger tent stoves show similar oscillations even though they have not to my knowledge been reported.

When charcoal is dominant in the fuel mix, the outside of the stove closest to the charcoal deposition zone gets noticeably hotter and there is less heat further away.

On the other hand, when there is more wood in the mix, the temperature is a little lower but the heat is more dispersed.

Even my little continuously fed inverted J burner micro tent stoves, with apparently steady fuel feed, exhibit small oscillations as ‘charcoaled’ sticks collapse.

Fast burner oscillations or chuffing. All my good little stoves chuff to me to tell me that they are working well all by themselves. Fading of the chuff is a reminder that they need attention. There is plenty of time to prevent a stove stall or flameout. Usually, a jiggle of the loaded sticks or the addition of more sticks is all that is needed to quickly restore the gentle babble.

I expect that larger stoves with considerable mass may chuff, but the reserve of heat in the stove mass and fuel makes it so slight that it would be inaudible.

I think the fast oscillation cycle goes like this: An increase in flue pipe gas temperature produces an increased flue pipe draft. The extra draft produces strong fanning of the charcoal. It makes more radiant heat from the charcoal, but it also cools the gas flame a little and reduces the flue pipe draft. The pulse of increased radiant heat produces more wood gas from the wood. The gas flame then grows and moves off to provide heat for cooking and comfort. Some of the increased heat from the flame is ‘wasted’ up the flue pipe to increase the flue pipe draft once more and starts the cycle again.

Me

Extreme chuffing. Slower and more violent chuffing (sound and flames) can be heard and seen while viewing the primary air port in my experimental stick burner video. This variant of the burner behaved this way without the use of the blower and the secondary air port was blocked off.

I’ll admit that this chuffing was provoked a little with some extra ‘fire drizzlers’ that enriched the combustible. Also bear in mind that this burner is not a stove, it is just an inverted J-burner that is using much of its energy to make angry noise, and flashing flames. It is generally showing off, like a teenage boy, while it is doing no useful heating for me.

Me

I thought for some time that these fast oscillations were probably unique to my little backpacking stoves that have minimal mass, thermal or otherwise (opposite to a rocket stove that just roars). However, the above tiny, but heavy experimental ceramic burner still chuffs, so maybe thermal mass is not part of the reason.

Good micro tent stove design

” Now all the above prattle was just the background about combustion and general stove design. It is important stuff to know to be able to have a good shot at stove design specifically for ultralight winter tent camping.”

My early micro tent stove designs, with inverted burners, were about making them ultralight with a volume that was as small as possible, to enhance radiant heat output.

They had a flat pot bearing surface on top and a stove volume was designed to contain ALL the stove components while backpacking. This meant that the titanium or stainless steel heat exchanger box (~100g) needed to be quite long and require a sealable opening that could be used to stow the components. “This was not easy.”  

Effective stove heat transfer to cold trekkers. I have described my preference (endorsed by my camping companions) for strong radiant heat from a micro tent stove to directly heat campers bodies in a small ultralight tent.

It is indeed a strange thing that strong radiant heat that is received on part of our bodies rapidly spreads to our whole body, I guess that it is our strong blood circulation systems that achieve this.

Me

I remember when I first lit up the ‘Stanley stove’, my long-suffering camping companion (of many trips) on the other side of the stove said: “Tim, that is amazing, within seconds I can feel that lovely warmth coming from that little stove.”

Unfortunately at that stage, it was not easy to harness some of that heat for cooking or water boiling. However, I did manage to melt a neat round frying pan shaped hole through an insulation pad and down through my polytarp groundsheet. “I should have sat the stove in a pan of water, but then it would have been difficult to use the boiling water from there to make a cup of coffee”.

Later, the sheet was cut in two, through the hole, and the halves are still in use. The hole provides a little extra safety margin around the stove and a reminder about the intense heat that is involved.

Despite the little setbacks, from this moment onward, I would strive to make my stoves little ‘heat radiators’ rather than big ‘air heaters’. It was also obvious to me that a centrally located stove would give the best distribution of radiant heat and other benefits that are described in ultralight tent stove- part 3 and more recently in stove and flue pipe location.

Good micro tent stove design for radiant heat and stable combustion. My simple rule is to make the stove body/heat exchanger small enough so that it reaches about 300-500C on the outside for nice radiant heat. These temperatures also encourage high internal temperatures for clean and efficient combustion of damp and wet fuel sticks. They also ensure the maintenance of a strong stove draft. My smallest micro tent stoves (with burners with similar power) will reach 450-550C. While this temperature will be very good for radiant heat (and impressive to look at), it may not be so good for the longevity of some stove parts.

I should also mention that although the stove is primarily designed to be a radiator, it does heat the air in the tent. This provides a different welcome warmth in winter and is good for drying clothes up in the top of a bell or tipi tent.

It also eliminates any ice or condensation from the inside of the tent. However, it is relatively ineffective for body warmth in an ultralight backpacking tent. This is because the hot air rises away from our bodies, does not benefit cold feet and is quickly lost through the thin canopy of an ultralight tent and through rooftop vents that are needed to vent moisture from breathing, wet clothing and cooking.

How do the flames/hot-gases travel in a micro tent stove with an inverted burner? The gas flow in my box and oval micro tent stoves starts with the inverted burning or pyrolysis of the bottom ends of a bunch of fuel sticks. This means that the stove draft must first pull or suck hot wood gas and air down from the hot fuel tube, which is to some extent is trying to be another flue pipe that is ‘pulling’ in the opposite direction.

Next, the gas is heated and ignited as it is turbulently pulled through a glowing bed of charcoal before its combustion is completed in the horizontal labyrinth of the heat exchanger box and sometimes in the base of the flue pipe.

These stoves (photos below) have vertical dividing baffles down their length to force the combustion flame and gas to take a long and turbulent  ‘doubled-back’ path toward the flue pipe exit hole. This is done to improve the heat exchange. However, the flames and hot gases, after leaving the charcoal bed combustion zone rapidly rises to the top of the heat exchanger chamber ceiling and make it considerably hotter on the top than on the other surfaces.

This heat concentration is ideal for cooking. Typical maximum temperatures would be 350-550C on top, 200C on sides and 150C on the bottom. There are notable exceptions to this rule where the gas must rapidly change directions and causes turbulence. The consequence of this can be seen as hot spots in the following series of photos.

Even at the entry to the flue pipe, there is great turbulence and high temperatures on the outside of the flue pipe reflect this. These temperatures rapidly decline further up the pipe. It happens to such an extent that with a bottom temperature of 250C, the metal at the top may only reach a ‘comfortable-touch-temperature’ of 50C where the pipe passes through the protective micro tent stove flue gland in the tent canopy.

Flue pipe ‘turbulators’ in a micro tent stove? It was tempting to think that I might extract a little more heat from the flue pipe by hanging a small square/s of stainless steel (~1/3 of the cross-sectional area of the pipe) down in the pipe, on a wire, to create a little extra turbulence.

In the US they call devices with a similar purpose stack robbers (photo of stack robber).

They are usually used on much bigger batch fed stoves that often have a lot of waste heat going up the flue pipe. They also call them a ‘spark-box’ on account of the dual function of arresting sparks.

Unfortunately, in my simple tests, the disturbance of the flue pipe draft was just too much and the stove stopped working well. Nevertheless, it was worth a try and it showed me how ‘borderline’ my stove draft actually was.

Examples of my micro tent stoves with inverted stick burners

Evolution of the slim oval micro tent stove. For my oval stoves, I gave up on storing most of the components inside the stove. Instead, I made the stove very slim light and strong. It greatly increased the cooking area on top.  As a consequence, the heat exchanger/cooktop makes a very slim pack-friendly load when it is slipped down a pack closest to ones back. The other components are designed to fit concentrically and compactly into a small cylindrical bundle that is strong and can be packed efficiently in and around the roll-up flue pipe.

“If you ponder this design, you can see that it has evolved into big flat stack robber/ spark arrestor.”

Best stove metals to get strong radiant heat from a micro tent stove

It is tempting to think that a highly conductive metal such as mild steel would make the most effective heat exchanger. This may be the case where the stove is designed to heat the air in a tent. It is indeed used in many rocket mass heaters, but these largely use cheap steel drums etc for heat exchanging to air and other storage materials like cob.

If the tent stove material is highly conductive then more heat will be dissipated through conduction to the air and less will be available for radiation that is most wanted from stoves of my design.

Both stainless steel and titanium are terrible conductors of heat. This allows them to get much hotter than mild steel and make them much stronger radiators. “Luckily, they are also more heat resistant to withstand this higher temperature.”

Refractory coatings. I coat some stoves with a DIY protective refractory coating. Luckily, the ugly dark colour of the ‘burnt-in’ coating greatly improves the stoves emissivity.

Conclusion

I have grown to accept the obvious (now), that for these very efficient inverted J-burner micro tent stoves with a limited and steady rate of combustion of wood, they should:

• Have a heat exchanger with a limited volume or surface area so that it maintains a minimum cooktop temperature of ~350C to provide an adequate surface temperature for cooking, strong heating comfort for the tent occupants and a strong flue pipe draft for sustained and robust fuel combustion under adverse conditions. 
• Have a large enough heat exchanger to limit the maximum cooktop temperature to about 500C to improve stove longevity.
• The shape of the cooktop should be optimized for fitting one or more pots for snow melting, boiling water and meal simmering,
• Have heat exchangers made of titanium or stainless steel to enhance radiant heat emissivity while sparing more heat to sustain clean combustion and strong flue pipe draft,
• Provide a steady output of heat for warming and cooking without the wild oscillations in temperature, as experienced with other small tent stoves that are batch fed fuel. 

Gone are the day when I expected to turn a rectangular 4L olive oil tin into a large efficient and effective heat exchanger with a cooking surface. Yes, it could boil water but it took forever. The heating for comfort was modest when compared with that from my smaller stoves with a burner of similar size.

Me

This is the last ode for this post;

Is it the stove design so simple, boring and elegant?
Or is it the operator so sharp, smart and intelligent?
To burn so well, hot as hell, with hardly a flue smell,
The answer’s in the room like the proverbial elephant.

Addendum 1

Since preparing this post, I have put the design theory into practice in a new stove design series. I call them dome stoves. They are made from a tough fire resistant but light circular stainless steel fire bowl. When inverted it becomes what I call a ‘fire dome’. It fire dome fits into an equally tough and light custom cooking pot. I call the most sophisticated stove in the series the miniature dome stove- a three-in-one stove.

It also converts into a powerful blower stove for fast outdoor cooking with long sticks or indoor cooking with an alcohol burner.

Miniature Dome Stove- A three-in-one-stove.

The simplest stove in the series is my KISS STOVE. (Keep-It- Simple-Stupid). It is lighter and extremely compact. The innovative roll up flue pipe and flue guard adaptor tube enables the flue pipe to fit within the stove fire dome that is nested in a custom cooking pot while backpacking.

The bad news is, that the KISS Stove runs much hotter (100-150C higher) than most of my previous stoves. This is great for cooking and comfort while winter camping. “That’s what this little 350g bundle of joy is all about!” However, it causes more rapid metal decay to some parts as described above.

The really good news is that, with the KISS Stove, the decayed parts can be easily removed and replaced. They can be DIY parts made inexpensively from spare stainless steel foil or waste sheet metal using simple hand tools.

Tent stove- a tiny backpacking stove

Tim

Other related posts:

Experimental barrel stoves from coffee tins. Searching for the Holy grail of an overnight tent stove.

Pudding bowl stove

Dual purpose tent heater/outdoor cooking stove 

Ultralight tent stove- Overview

Rollup flue pipe for tent stoves and hot tenting

Ultralight fire starters