You opinions on a single nozzle?

What are you thoughts on a single nozzle in a downdraft wood gasifier?

Central coming from top or from bottom or from the side?

I’m thinking of making one with the nozzle coming from the bottom through the grate.


This would place the nozzle material in the hottest possible place in the gasifier at all times, thus maximum air preheating. Perhaps use silicone carbide or tungsten carbide. this would also keep the reaction zone closer to the same place all of the time. I’m not clear on WHY you want to use just one nozzle.

Simple routing of the of the air intake instead a nozzle ring which may get uneven distribution anyway.
It may also make the gasifier smaller in diameter as I don’t need double walls.

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Hello Jim .

I would give a lot of thought to fuel bridging .


If you come up through the grate, this tube will oxidize into nothing. It would cause all kinds of flow issues as well.

When you input your intake air, you generally do not want to do this at the jet ring level. Design so that the air is incoming lower on the unit. This will allow the air to preheat and even out the flow with out biasing the jets.

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It’s maybe better to have to nozzle coming down from the lid then, it will help against bridging and save it from oxidize?

I could also mount it on the side, but I may get a uneven oxidation zone then?

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Look up Flash001USA on youtube. This is pretty much how he built his unit.

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If you go from the top then you now this piping in the way of your fuel. Bridging will already be a challenge and this will simply add to this problem. Yes if you flow into the side, then yeah your basically going to have an unbalanced oxidation zone with raw fuels bypassing into the reduction causing lots and lots of tars and week gas.

The key to this; is to have balance and the physics will fight you every step of the way to obtain this balance. We are dealing with solid fuels that are subjected to thermal process liberating the molecule chains. These chains can be complex or simplified. The goal for us is to simplify these chains as much as possible. The complex chains carry the un-cracked tars. We are also releasing chemically bound water. This combined with the tars will migrate into the hopper, and absorb into your fuel. You now have sticky wet fuel. Sticky wet fuel does not flow very well and wet sticky fuel does not burn all that great as well. We now begin a viscous cycle of bride and release creating oscillating flows and tar production.

This can not be done simple, if was easy all of us here on this forum would already be doing it. There is a reason for a fixed oxidation zone, to create as much heat as we possibly can in a local zone to effectively break down these hydrocarbon chains to the simplest form we can before entering the carbon bed / reduction zone. Once here we also need to be as hot as possible in order for the reactions to take place. This is where we strip the oxygen out of the chains creating and leaving CO from CO2 and making CO and leaving H2 from chemically bound water. The hotter the more thorough the reactions. Again physics tries to kick our butts here as this is an endothermic process and sheds our valuable heat as these processes take place.


If the nozzles are mounted on the side, what are the minimum number of nozzles to get an even oxidation zone without raw fuels bypassing the reduction?

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Depends on the size of your unit. The Imbert spec sheet tells you this based on what size unit you require for your application. If you build an imbert follow those specs as close as you can. Once you build it, you can then learn from it. Dont try to re invent it, you will be destined for failure and waste your time and money. Build this first learn from it and then move on to more developed designs.

Ive built probably over 300 machines larger and small; Im still learning.


Hi Jim,
I chronicled an imbert build at my site, " My first small engine run "
in the Small engine section on this site. Check it out before you
start, for a whole bunch of heads up presentations. Yes, it’s long, but
it’s thorough and it works great.


I have a few ideas on how to produce even distribution of air to the nozzles.

  1. I made my cooling fins in spirals. This will cause rotation of air inside the housing and thus even distribution. I think.
  2. I added what I call a fresh air spreader.

*Note: These are just ideas. They have not been tested yet.


Single nozzle designs have been used, going back to WWII. But, as far as I understand, these were only for charcoal gasifiers. I believe the Kalle design used a nozzle from above. It would be imaginable to have a star type diffuser on the nozzle.

But as mentioned by others, in a wood system it seems impractical because it would cause troubles with downward wood movement, tending to cause a cavity to form in the charcoal combustion zone. More importantly it would lead to unburnt tar gases sneaking down along the outside edges of the firetube.


Ok you guys that are looking for the easiest possible solution to create an engine grade gasifier:

So lets say we build the easy solution, you put many hours into your build, money out of pocket. May have spent a months time working weekends to build it. Now the day has finally come and you light this machine. Yes! victory!! its lit!!. You now move to adapting an engine, more time and money. Now the day comes and you are successful at getting your engine running. Yes! Victory again!!! You succeeded in your endeavor! You may have even documented all of your success on Youtube. We have never seen this before right?

Well not so fast, the next day or at least after engine cool down is the real test. If you did succeed, then you should be able to re start your engine. If not then time to pull the valve covers and check the valves are not stuck from tar.

Will you succeed at building a engine grade gasifier in the simplest possible form?

Perhaps, Im not going to say you cant, but I will say the odds are heavily stacked against you. If you fail at the “simple / easy approach” then its no longer the simpler easier approach. Because you have wasted time, energy and money into something you need to either scrap / throw away or continue on trying to make it work.

If you build an Imbert or Simple fire and can follow the specs, run the specified fuels that is dried, your chances of success will be much greater. You can then add your innovation to address any short comings. Building an Imbert is really not all that complicated if you can build the alternatives; then you have the skills to build an Imbert. Don’t over complicate it. If you can NOT make an Imbert work then you sure would not have succeeded at making the simpler alternatives work.

We are trying to show you the easiest road to take.


Is there any way to see if the gasifier works without passing through tar, without switching on an engine?
Is there any filter or other I can run the gas through to test

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I think Mike LaRose used white paper towels.

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But do you see the difference between tar and soot mixed water?

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There really is not a 100% way to do this as the engine will drive the gasifier to parameters differently than your blower. The end result maybe different than running on blower. Beside this tars may not be ready to drop and bypass what ever you use to detect them.

On my engine adapter, it has a chamber where the gas/air mix enters into before entering the carburetor. If your gas air mix is cool enough generally this is where tars like to drop. In my adapter at its bottom there is a very small hole drilled into it. This is specifically for expelling moisture build up. But if you are making tar, you will also see it expelled here. If you see any at all even a drop, stop the gasifier and get the engine running on petrol and run for at least 15 minutes. If you think there is excessive tar getting in then run longer. As long as you are not letting this happen too much no harm will be done. You will want to change your oil as well if you are getting tar in you engine. It dont go away and can lead to other mysteries later. lol


Thanks for your advice, tar is my horror.


If you get a valve stuck on a small engine its really not the end of the world. To free it, all you need to do is remove the fuel tank, carburetor and spark plug. Remove the intake rocker arm and carefully push the intake valve all the way open. You will most likely need a hammer and brass punch. But be careful not to hit the piston. Then you simply apply heat in thru the intake of the head and heat it up. Once you get enough heat in there you will see the valve raise up and close. You will then want open the valve back up and keep applying the heat. Just dont over do it, we dont want to melt the seals or anything. You keep doing this until the valve will no longer stick. You then re assemble the rocker arm and turn the engine over carefully so that the intake valve is once again open. Add more heat, now you crank the engine over observing the valve, you can now spray in a solvent to help clean and free the valve. If the valve can keep up with the rocker arm movement you can now let it stand for a few minutes and cool. After cooling remove the rocker arm and depress the valve and see if it returns. If it does all is good you can put it back together. If not repeat the process.

On a small engine this can be done in less than 15 minutes and its fairly easy to do.