A question of sizing

I am putting these all into general discussion as I am not sure where to post my questions just yet.
I appreciate all the help so far so a big thanks to all.
I am hoping someone can advise on sizing the hearth opening.
On the MEN build it’s a 5 inch hole and on the Australian one he recommends a 8 inch hole and His engine cu/in is closer to mine than the MEN build.
I am South African so working in metric. My hole size will be 200 (8") and I am using an old oxygen bottle for the oxidization and reduction zone and will be insulating the reduction zone tube with 1800 degC ceramic blanket.

The drawing is rough so doesn’t have all the details but serves as a rough idea on sizing so I can cut the respective parts.
So are my sizes as per drawing close to target?


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This is for building an Imbert like unit, pretty easy to build to begin with, it’s for wood, not coal


I believe your restriction will need to be tighter. 200mm is awfully big to be the tightest portion. 200mm would be more for a 6L engine, or a 5L with a lot of preheat.

The restriction is where the tar is cracked. I would go with the MEN’s suggestion of 115mm for your 3.5L.

If you design it so your restriction can be removed this will not be an issue. It’s fairly easy to design the hearth so your reduction and restriction are built into one piece. As the ashes build up it seals the system.

Edit:. I’ll send some photos to give an example. I just woke up so it might be a minute :rofl:


Thanks Jan. Can’t read it but do appreciate your sharing the link. I am assuming this one in the picture is an imbert design?


Kurt’s design is definitely one of his own, inspired by a few gasifiers he saw in Australia. There were some single nozzle Imberts but not with that much charcoal bed depth. I want to say his gasifier was made for a 318 cubic inch V8 for a station wagon, so about 5.2L. Kurt also used the refractory brick as a heat insulation to make the reaction more efficient for cracking tar, and the gasifier was triple walled so that single nozzle got a lot of preheating for the air.


Thanks Cody. It’s the one thing that’s kept me hesitant to start the build. I am busy doing a 2D cad drawing to get my head around what needs to be made and cut. Once I am sure about sizing everything to suit my engine I’ll start the build. I must have over a hundred books and articles and downloaded videos so far. Bit of a information overload at the moment.


Copy the text and translate with google, there you have all the measurements + how to calculate what fits your engine


Ahhh that makes more sense then. I assumed the air was from the bottom but see now that it was from the top and the air and water nozzle were one unit. I wondered why his hearth size was 8 inches but I see now that even though our CU/IN are similar bore? size isn’t. SO I’ll have to do a little more research on what size to make that opening and design a removable part for later modification.


Blond moment. Didn’t think of that. Thanks

Definitely crunch some numbers using the formulas that Jan sent. There’s also Dutch John’s website for calculating an Imbert hearth and I’m sure it’s the same formulas.

Once you know your dimensions you can draw it out.

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Timothy you have now reached a confusion-moment in understanding wood-for-shaft-power gasfier systems. From very small active volumes systems - Imberts. Too Very large active volumes systems - the WK. With most systems types somewhere in between these extremes. Many, often subtle differences to make each system work well. Where they vary the most is the needs for far different ranges of use forced internal flows velocities. The needed temperatures remain the same.

In any of my modern certified clean-burning bulk wood heating stoves I can, and daily do, operate them from a cold starting off range of using 3-4 different locations of inlet airs to make a roaring, open flaming fire to induce up quickly the rising heats chimney flow draft. Warm up to Hot all of the stoves internal materials.
THIS IS the now fashionable “newly discovered” Rocket stoveing mode. And that is where rocket stovers begin and end. They never know any other way. Macrobiotic thinking.

Now in any of my bulk wood heating stoves I MUST within 5 to less that 10 minutes cut back my airs allowed locations and volumes to not overheat the stove’s material to destruction.
Dramatically decrease the internal flow velocities. And this becomes normal typical wood stoving. Burn down. Reload. Do again. Mostly yellow bright flames. What the American and Canadian stove manufacturers like to sales picture as Ideal.

Not me anymore since ~1999. Then my game has evolved to continue moderate velocities flames combusting only then with now made gasses being heat driven from the still raw wood sticks core out past the outer sticks charred surfaces. Much less energetic flaming. Pretty evolving changing colored flames then. Wood Gasifing. What the better European stoves manufactures like to sales videos and sales picture up as Ideal.

Still not all-best heating use of the fuel wood for me. I impatiently wait for the wood splits and branch cuts to evolve down into a glowing HOT bed of intense radiating out red-glow char. Then almost no visible flaming. It is a clear blueish flame. Char burning. The Hottest. Needing almost the least air. And a slow best heat capturing flow.
Where the Charcoal gasifier guys start. Operate. And understandings, end at.

Still not good enough. Once I do have that ~8-10 pounds; 3-4 kilograms of hot glowing charcoal producing the heat energy, THEN add on 3 splits of fresh raw wood on top of it. Have to actually cut back the air a touch more then. The hot-hot char then does split some the the woods moistures into combustible hydrogen and oxygen.
Then I can heats producing cruise along and do for 10-16 hours a day. I try for this for at least 3000 of my annually needed 5000 hours of heating a year to be this in this cycle. Neighbors now since about 2000-03 do not even know anymore when I am wood stoveing or not.

See the points?
A wood gasifer is a system that can make it own needed char internally. A well operated gasifer system will the use that char and the woods out gasses to make a settled, and one pass filtered clear blend of motor grade fuel gases.
Using wood as your fuel input sources you will need to think in full cycles of from cold starting; warming up to optimal; then shutting down for the best next from cold starting experiences.
You will need to think and operate, and eventually understand across all of the operating spectrums.

Ha! And this why some systems will use relatively small restriction throat areas with relatively small volumes of below the restriction char-reduction volumes. Velocity, speed demon systems.

Why the WK and KurtJ’s system call for huge 8" 300mm restrictions choker plates. And large volume below that and above the grate char-bed reduction volume cavities. They relatively slow internal velocity systems, balanced with larger active nozzles volumes and below restriction volumes.

You can compare and contrast, alright. But mix systems and you’ll end up with a poor performing mythical creature. Who tars. Who burns through. Either tars or pulled though abrasive ashs kills engines.

Ha! Ha! I’ve worded carefully to not allow the fellows who insist on acronym’ing to four letters secret-squirrel talk tagging these different systems.
Me, German named Unruh, I prefer to keep my short-cuts predication classifications inside my head.
Lables hung will limit. True Life is in spectrums ranges. Not snap-shot frozen ideals. Wedding cake moments.
Steve Unruh


Thanks Steve. So the trick is to find the balance. As a blacksmith I understand fire and air and managing the two for a controlled burn. Having built a few engines and carburetors I understand how fuel/air ratios. So basically what you are saying is find the happy middle ground that will produce enough gas to keep the engine happy over all ranges from cold start, idle, acceleration and cruising. And to do this one has to make sure that where the magic happens matches the engine requirements.
Makes sense so I think I’ll stick to the MEN build for now. I have all the info I need for that one and I can make it modular with standardized units for easy assembly and repair or flexible arrangement and use.
Appreciate the input.There is so much info out there and differing variations of downdraft systems that it gets confusing to try understand how each individual changed things to suit their own needs.


The MEN is a very good build, proven design. Some people forget they worked on that design through a few different types before settling on the one we see nowadays. And I think your 3.5 is in a good size, it’s in the center of their measurement charts basically.

Mother Earth News originally made that gasifier for a Chevrolet C10 pickup, with an inline 6 cylinder engine. I can’t find what the displacement was but it was either a 250 or 292 cubic inch. Not terribly big back in the 70s and 80s.

Shouldn’t be too hard to make it modular serviceable.

@paul and @Ron_L run MEN designs or have in the past. They could give you more learned experience information.


I don’t want to talk out of turn or blow my own horn but below is what MEN had to say after they log some miles with me and paid for a trip for me to go to the Utah Salt Flats

Many years ago, when I managed the MOTHER EARTH NEWS research facilities in North Carolina, we built wood-powered vehicles for the same reasons Keith does today. But Keith has taken wood gasification well beyond what I could’ve imagined.


There’s no questioning that Wayne. You’ve come up with a blue ribbon gasifier.


There’s no doubt Wayne. I just can’t justify the cost with my exchange rate for something that will never arrive otherwise I would buy the book in a heart beat. Our government run systems are so corrupt and broken that my local post office had to shut doors because they stole all the money and didn’t pay rent for over a year.
Courier is just too expensive.
So please don’t think I am being a freeloader. I have to think of the most viable option available to me with the resources I have in my country. Already the help I have received here freely has been way more than I expected and I am most grateful to those that have assisted.


I truly understand Tim.

My partner migrated from South Africa and went back a few years ago to visit family and was robbed and beaten. No one was charged :confused:


That is sad yet often the case.

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Ok guys, let’s get Tim a premium membership, even if he can’t get a book. I am willing to pay for it if Chris will give me a price for membership minus book.


Thanks Al. The gesture is greatly appreciated but really not required. I will pay for a 6 month subscription at the end of April. I have been researching this for over a year now so a few more months won’t kill me.
Thanks though.

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