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Simple hot air engine

Posts: 75
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Imagine: A wood fire - (rocket??) with two chimneys, each chimney has a rocking "piston" - and pistons are connected so that as one rises the other falls. Pistons are attached to a flywheel to eventually do work.

Yes - I'm a dreamer - "but I'm not the only one"

First a human gives the flywheel a bit of a spin - its own inertia will be too much for the hot air.

The hot air rising in one chimney 1 pushes up on Piston 1. Piston 2 is falling.

The two chimneys are connected by a pivoting flap contrivance. The falling piston 2 pushes air down and the flap makes that air go to chimney 1 helping the Piston 1 to rise. At the same time the clever little flap system (maybe there are 2 flaps that are linked) must stop that air from blowing back down its own chimney to the fire.

There will be a slight build up of hot air pressure in Chimney 2 under its flap so that now as piston one falls and piston 2 rises that hot air is released pushing up Piston 2. Of course the flap separating Chim 1 and Chim 2 opens allowing the falling Piston 1's air to join the hot air pushing up Piston 2.

Chim Chiminey??? Perhaps it is all Mary Poppinsey???

A very old bloke in Australia told me his dad operated a sawmill with just a wood fire somehow powering it. But he didn't know how it was done. I'm wondering if there is a tinkerer out there who could think this thru and maybe start a discussion.

Chances are this has been tried before - but my searches have not uncovered anything other than the stirling engine.

As I am keen to BBQ bamboo into biochar and that process calls for cut lengths of feedstock - I'm hoping the hot air that the process generates can be used to good effect to drive the sawmill. A nice little outcome if it could work. Seems to me a heavy flywheel would be necessary as the energy in the hot air would be too low to do much. The flywheel acts like a battery - slowly spinning up to speed thru some gearing system and then having its speed slowed as the cutting action introduces friction

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Location: Richmond, Utah
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I think this could work much like the old Lennox Pulse furnaces, but would need to be driven by the combustion of pyrolytic gasses in order to have enough effect to make it worthwhile.
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You will be faced with a dilemma with this kind of heat driven machine.

1. If your stove is efficient (like a rocket stove), there will not be much energy available in the draft going up the chimney. If you try to further use the remaining energy, it will likely cause draft problems with the stove.

2. If your stove in inefficient, with a lot of heat going up the chimney, your idea of harnessing the moving gasses is probably less efficient than say, a steam boiler and a steam engine. Just as a data point, overall efficiency in a small piston engine steam rig, of home scale, is less than 10%, and probably less than 5%.

For a back of the napkin calculation, tell me about how much wood you would burn (pounds per day for example) and I can tell you what the likely useful energy you could harvest.

Finest regards,

Garry Hoddinott
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Bill & Troy - Thanks for the feedback.

The purpose was to use the heat resulting from pyrolysis of bamboo - I'm using bamboo as a stack function on 60 acres. Why bamboo? 1. I'm hoping that the Climate dudes internationally decide on a global price for carbon so I get an income. 2. In the meantime I sell some species shoots as vegetables, 3. Although bamboo does not give such wonderful charcoal, the charcoal is not so important as it is being ground to powder and sold as soil amendment. 4 Bamboo processes CO2 better than wood - up to 10 times - so I read. 5 It grows in consistent easy to work with diameters and that is important in pyrolysis. 6 I can sell poles if climate stuff is a fizzer. 7. My cattle get the green ends 8. the leaf drop is mounded on plastic sheets a few times a year and my African night crawlers turn it into saleable / useable vermicast rich compost. Oh did I mention it is easy to harvest with simple tools

You have to love permaculture!! And bamboo!

Bill - yes the pyrolysis gases are burned as part of the process. Heat (hot air) is captured within the double skinned system and there is basically only one way out. I'm using a rocket stove approach to generate initial heat with the chimney directed through the middle of the bamboo feedstock retort. Stuff I have read indicates the rocket design - ie 90' bend promoting a turbulent burn and insulation, in this case given by the large feedstock retort surrounding the chimney should mean that only a little wood fuel is used throughout the process. Gasses given off during pyrolysis can only get out near the base of the retort surrounding the chimney and rejoin the rocket fire zone to be burned providing the majority of the fuel for the pyrolysis process.

Troy perhaps it is the very efficiency of the rocket stove, that might help this. That rocket sound we hear is air moving. I'm trying to harness that moving air. Perhaps you are referring to rocket MASS heaters where the great volume of heat extracted through the burning of all available fuel is given off to a thermal mass and exits as cooler air.

In a single chimney a fan would certainly be turned by the hot air escaping with such force. But I'm not sure it could capture enough energy to do any useful work. I was hoping that the rocking motion of 2 pistons might actually capture some energy and transfer it to a flywheel that could store the energy for bursts of woodcutting. It seems a shame to have all that energy stuff happening about and then having to use gasoline to run a saw!!! I have read that even after using the wood gas to power the pyrolytic process 20 % is available after scrubbing and filtering to power an internal combustion generator which I guess is the ultimate capture of hot air - the hot air from each explosion is what powers the pistons.

Bill I am not sure about the wood quantity burnt per day as most of the fuel is in fact the gas.
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Just put a copper spiral around the outside of the pipe then a layer of insulation. Place an insulated tank above the spiral connect midway and bottom to top and bottom of spiral then fill with thermal oil and you have a simple economiser. Then the clever bit, Attach a thermal oil steam evaporator and you then have steam on demand to turn steam engines, steam pumps, steam generators, steam sawmills etc.

I would not advocate replacing the thermal oil with water unless you are confident of the safety aspects of steam however of the shelf solutions are available with the safety built in.

We can fix it! We just need some baling wire, some WD-40, a bit of duct tape and this tiny ad:
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