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Technical notes

Outdoorsy stuff > DIY woodburning backpack stove

Power output data
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Power output of outdoor stoves is usually measured by testing how fast a stove can bring 1 liter of water to a boil under standardized conditions.

Testing under standard conditions shows that this stove will bring 1 liter of water to a boil, starting at lighting, in 6.3 minutes. If considering just the maximum-output portion of the graph, that time shrinks to 3.7 minutes. Stove output is an estimated 11,000 BTUs per hour. Fan output and stove power in this stove are little affected by wind, altitude, or low temperatures.

Weight savings
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Weights of conventional stoves plus their fuel start high on the first day of a trip and decrease as fuel is used up. The longer the trip, the higher the starting weight. Also note that cartridge stoves, which run on butane and related mixures, perform poorly in cool temperatures and tend to become nonfunctional in ambient temperatures somewhere between 0 and 32 °F. With alcohol, double the fuel weight.

Weight of this stove is a constant 18 ounces: no fuel to carry and no budgeting of fuel, carrying excess fuel throughout a trip, or running out at the end.


How it works
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For fuel, the stove uses natural bits of wood you find at your campsite destinations. Obviously, this is how the stove system eliminates the weight and other problems of artificial chemical fuels. Typical fuel will be twigs, broken sticks, small pine cones, and bits of bark from the ground. At a typical campsite, using just your feet, you can accumulate wood scraps for 2 days of cooking in about 2 minutes.

Sometimes, wilderness deadwood will be moistened by rain. No problem: it will still be burnable and useable. Once wood has naturally died, dried, and shrunk, it won’t reabsorb enough rain to return to its “green” condition as when it was alive.

Wood fire in a chamber, even if the chamber were thoroughly Swiss-cheesed with air holes, tends to be starved for oxygen. In chemistry, the relevant term is stoichiometry: the proportions of chemicals needed to drive and balance a chemical reaction. With wood fire, oxygen tends to be the critical (missing) component. Damp wood especially needs the help of abundant oxygen to start quickly and then burn well.

This goal is a stove that starts dependably and quickly, blasts away as needed, and makes minimal smoke. A supercharger satisfies all these concerns and delivers surprisingly good fire performance and convenience. The supercharger’s fan normally forces a steady 3 cubic feet of air per minute into the fire. A natural air draft from the heating of air is also always present whether the supercharger is on or off.

Air from the supercharger (and/or passive air from the environment) enters the stove’s preheating chamber, where the flow of air is briefly held, baffled, swirled, and preheated before being forced into the fire. This preheating aids combustion.

This preheating is positioned low where (1) heat is most concentrated and (2) where we’d most like it to be for environmental safety reasons. The hot stove embers, or “coals,” are held above the preheating chamber, which serves as insulating air space between the fire and the ground below (which may hold minor combustible materials on the forest floor). The stove’s combustion chamber consists of a (mostly) solid shell. Thus, fuel and heat are tightly managed and focused upward toward the cookware. Wind screening is part of the design.

Some wood stoves, whether small and portable or large and industrial, emphasize a pyrolitic mode of combustion. In pyrolysis, most of the air draft is injected not to the burning wood but higher, in the zone of wood gas (flame). Such a design tends to be complex and heavy, and burning may be slow and finicky, particularly when involving moist fuel.

This design emphasizes direct, rapid oxidation low in the stove in the zone of the fuel itself. Still, the stove design allows some forced air draft to bypass the fuel, be channeled upward along internal chamber corners and behind baffles, and be injected higher in the combustion chamber and  into the wood gas.

Green factor
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This stove releases no fossil-fuel carbon. In contrast, conventional wilderness stoves release fossil-fuel (petrochemical) carbon that was sequestered underground millions of years ago. Conventional camp stove fuel (“whitegas,” Coleman® fuel, kerosene, butane, propane) is refined from petroleum sucked from underground. This abnormal pumping of millions-of-years-old fossil carbon into Earth’s atmosphere is what drives the current issue of climate change.

The carbon dioxide this stove vents to the atmosphere was removed from the air only a few years earlier by trees and converted to wood. This atmospheric carbon would otherwise, inevitably, be re-released back into theatmosphere a few years in the future by metabolism of insects, fungi, and other microorganisms or by forest fire. Forest fires, started by lightning, area natural part of forest ecology, and for thousands of years Native Americans intentionally lit forest fires and played an important role in maintaining a healthy forest ecology.

Your small wood cooking fire in the wilderness, then, is just part of a CO
2 cycle that occurs naturally on a scale of a few years or decades. Indeed, where natural forest fires are nowadays suppressed by improper forest management, wood cooking fires may reduce excess fuel that can lead to nasty, massive forest-fire conflagrations.

Carbon dioxide, which the stove releases, has 1/8 (one eighth) the greenhouse effect of methane. Methane (CH
4) is the main gas product when termites and bacteria metabolize forest deadwood. So, when wood is burned and thus denied to these forest organisms, greenhouse gas emissions are effectively reduced by a large factor.

Supercharger and grill
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All electrical components are housed inside a single module that forces air into the side of the stove. Benefits of this modular, side-mounted arrangement are numerous:

The stove becomes extremely stable, by shrinking stove height some 2 inches from what it would be using an undermount fan arrangement. The stove body also becomes able to handle huge weight loads.

  • No wires are exposed. All are protected from breakage and melting.

  • Everything electrical is protected from cooking spills, precipitation, heat, and dirt.

  • The supercharger module can be used as a handle to move the stove.


The grill opens fully for initial fuel loading. The grill then swings down in either a raised or lowered position, depending on your needs or the size of your cookware. It provides a continuous surface as good as or better than those on the fanciest home grills. And yet you’re looking at an 18-ounce system good for long hikes on the Pacific Crest Trail (this stove has been on it), Appalachian Trail, etc.


 
                        
                  


Miscellaneous
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This stove uses a single common 9-volt battery. It will drive the supercharger for several hours, good for bringing several dozen quarts/liters of water to a boil. A good commercial alkaline battery (Duracell®, Energizer®, Rayovac®), inserted fresh, will power the supercharger a total of about 4 hours. A lithium 9-volt battery (Ultralife®, Rayovac®) has been found to last 12 hours in the supercharger.

A battery at a temperature of 0 ºF or even lower will power the fan. Anyway, when faced with extreme conditions, it is easy to warm a battery up to nearly 98º F in a shirt pocket using body heat. A battery weighs 1 ounce (lithium) or 1½ ounces (alkaline).

One needn’t be “a natural” or expert at making fire to use this stove. One can place a compact wad of natural tinder (pine needles, tiny twigs, brown grass or weeds, moss, or lichen) or paper in the middle of the fuel pile, or apply a small amount of commercial fire starter (charcoal starter fluid kept in a plastic vial). Rain-moistened wood, if it’s the only supply available, will burn well in the stove with its oxygen supercharging.

Cooking options are numerous. Low heat for simmering or grilling can be achieved by letting the fire reduce to hot embers and/or using the supercharger little or not at all.

Black compounds will naturally coat the bottom of cookware when used over a wood flame. Yet, a black coating will actually increase the heating efficiency of cookware to your benefit. Indeed, some experts recommend, and some cookware makers apply, a black coating for just this purpose. Soot is easily dealt with by packing cookware in a plastic bag. As for the stove itself, soot only forms on the inside of the stove and on the grill.

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