Gasifier sizeing

Hi guys, thod l make life easyer for new beginers in gasification wondering how big their gasifiers shuld be.

I spent hours and numerous sheets of paper calculating this things out when designing gasifiers. Imbert charts are one option but what if you need to size a different style gasifier? Flute? Simple fire? Even WK?

So, what l always did was take measurments from guys here that built good performing gasifiers, take their and mine engine size in to acount and try to proportion them.

The other day l was dusting my gasifier measurment memory department in the brain, compareing my sucsessfull builds and their sizes, and what l found was kinda groundbreaking for me, simplicity wise.

In most my gasifiers, the volume of active charcoal fits preety close to engine size! Talking about actual gasifier active volume, the volume taken by actual glowing charcoal. All insulations and ashcones taken away.

In real world this means a 2,5l engine will need about 2.5l of active charcoal in the hearth, that is calculated under the nozzles only. Preety easy now with online frustom andpiramid calculators to fit your engine.

Hope this is helpfull. Its not precise but shuld get you in the ballpark


I’m dumber than a box of rocks. If I understand you correctly for my 5.3l engine l would need 5.3 l between the nozzles and restriction and then a grate below that to allow ash and whatever to slip off to a clean out box below. I’m in a WK state of mind. Downdraft with cooling and cleaning to be addressed yet. Am I totally full of it?


Based on the measurements for my burn tube in the charcoal reactor I’m building that means I would have 8.368L of active charcoal, or at least that is the displacement of it.

Now I REALLY need to see if it will work in my Sierra.

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Hi Cody, the burning lobe of charcoal in not the size of the firetube. The lobe is the white hot charcoal in the center of the nozzles ring, it is ether a little above the nozzles or the nozzle in a up draft gasifer, or below in a down draft gasifer. Cross draft no brainer, the lobe moves the direction that the air is traveling. Outside thewhite lobe the charcoal is hot from heat but it is not burning because there is not enough oxygen to let it burn. So this charcoal becomes a insulation of protection in the firetube. The more oxygen you can get though the nozzle or nozzles the bigger the lobe. The engine at 2.5 L can only bring so much air into the nozzles at a time at a given rpm load. Big engines are also limited to your nozzles sizing holes. It all works on vaccum.


Mr Tom l seem to remember you not being dumb :smile:

What Bob sayd. I am talking ACTUAL live char area not gasifier volume. Its not hard to calculate with a to-scale skech and a calculator.

Lets say this is your clasical lmbert.

Then this is your live char zone l am talking about.

On a WK just do the same, calculate the frustom from nozzle holes to the choke plate. The grate on the wk is so high it can preety much be excluded…


Ofcorse let me point out again the measurement is not exact. You must adjust planing to your needs. If a lot of highway driveing is planed ofcorse the gasifier must be larger. If idlein is planed, probably smaller. But it gets you in the ballpark!


Hi Mr. kristijin,
First thanks for your knowledge,
Is it benchmark for roughly calculation of charcoal gasifier system or rule of thumb while making one


Good working concept KristijanL.
I think your new working in house multipurpose wood stove has contributed to your understanding now too. Open flame is just the necessary pre-curser to getting to the real energy release in the active charcoal pile. You can feel this. You can see this. It becomes visceral. It becomes intuitive. Prepped wood sizing to get to active charcoal becomes very real. Experiencing the quick, quick energy release of the sponge-like wood charcoals in conifer versus the much slower energy release in dense hardwood charcoals become real tangible too.

I am glad you add in the on grate active charcoal volume too.
I recall Don Mannes picture of his vertical and horizontal V-bar grate basket. Max Gasman inspired. That is a lot of active charcoal below his restriction.

Now since you are dealing with a volume.
A no math calculation way would be to measure out your engines volume in screened charcoal chunks.
Put those in a stretchy fabric bag. Sew it closed.
Now squish up the bag using twine or painters removable tape into your proposed char active zone shape.
Then seamstress/tailor measuring tape, do your measures.
As you say. You are only trying to get close.
Steve unruh

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Kristijan, I just wanted to add that nozzle air velocity has a lot to do with dimensions needed. A typical lazy nozzle, low turbulence WK has a close to 30 liter char pile to match a 5.2 liter Dakota.
Yeah, I had to check my calculations several times :smile:


I dont have the book on hand. Can anybody plese correct my measurements? If l remember right, nozzle circle of 12" for that WK and a choke of 6" and a height of 14" gives this

So thats 5.8 l

Again, l never gasified anything past 2.3l so those monster engine and gasifier sizes make my head hurt :smile: but its ballpark size isnt it?


What happens if you go too big? Will it run too cool and make tar and weak gas?

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Kristijan, I only calculated a 17" tall, 12" wide WK firetube with a minimal ash cone and came up with 30 liter.
I had no intention to dismiss your formula. I just wanted to point out to anyone starting from scratch that nozzle size can compensate for process volume and vice versa.

Edit: Oh, and your calculator says radius, right? You punched in diameters, didn’t you? (58 L :smile: )

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You said height of 14"…??? Most firetubes are 18" or 22" the Nozzles are 15" above restriction.

Again we are talking about the white hot active charcoal burning lobe here. Once the oxygen is used up the charcoal can not burn.
The reduction opening is where the gases are moving at the highest speed after leaving the nozzles or nozzle going what ever direction the vaccum is pulling the air. The stronger the vaccum pull the more oxygen can come out of the nozzles to a point of fiction vaccum of inches of water. The charcoal bed can be this point of fiction too. We call the charcoal bed constipated or to tight. All these things can and will effect the shape of the active charcoal burning white hot lobe.
The ash cone shape also will determine the lobe shape too the restiction opening if it has one or not.
Lots of variables here to look at even moisture passing through uncunverted to Hydrogen.
The lobe is not a fixed shape or size. Only a constant vaccum pull will establish this for what type of nozzle configuration you have and hope sizing.
Every gasifer has slightly different effects on this this.
Tune it with fuel type and size to make it work for your needs is important too.


Just one thought, if we look at how charcoal gasification works, it can run smoothly from the bottom up or from the top down, or even crosswise, just that the process is littered with charcoal. In my opinion, the wood gasifier under the nozzle never has too much charcoal, just as Kristjan wrote, I think this is the minimum volume for a certain engine.

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If you have a lot of charcoal under the nozzles, will it not be easier too dense with ash then?

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What Bob sayd Jakob. I am only talking about the frustum starting at the nozzle circle and goind down to the restriction. Only l wasnt sure of the height so l went for 14 instead of 15".
I think the charcoal touching the firetube anywhere might be too cold to do any real work thats why l went with the straight walled frustum. Only where charcoal is actualy white hot to do any work.

But JO is right too! Forgive my head computer, it freezes sometimes :smile: on the above screenshot l punched in DIAMETER instead of radius and then skipped one decimal at the resault. Its what happens if you dont get to it right, take your time and concentrate on what you do.

Ok with the 12" firetube, 15" nozzles to the restriction the resault is 16.5l of active char. More thain double the motor volume. JO is correct, it seems the turbolence plays such a huge role here.

You are right. Only problem is drag and ash plugging. An updraft charcoal gasifier realy has no other option for ash cleaning but to empty it periondicly.
The downdraft gasifier grate needs to be adjusted just right, otherwise it plugs and kills power. @JO_Olsson had prooven this great with his Rabbit gasifier and screw adjustible grate height.


And one more time this illustrates to me how all small internal active capacity, fast velocity, high turbulence calculation systems drive you into making an Imbert.

As if this was the only way any gasification should ever be done.
I see this same myopia in the Rocket-stovers.
And now the going across the board factory engine turbo charger systems.

Different strokes are what makes for a broader, safer, more flexible capability base. Whether you are water swimming, or Living.

So Kristijan . . .
Velocity manic systems: then 1X engine volume.
A volume-active system like the WK: then 2X of engine volume.
I still like this better than long-strings of math’s first to calculate engine gas usage volumes at maximum power. Then a cold gas flow through the gasifier.
Sheez. Look at all of the real-in-usage multiple factors that leaves out!!
And then one factor miscalculation and you results goes, Splat. Opps.
Time to build a new system.

When in worry. When in doubt. Go bigger, and in-use, tune it down.