Hopper size compared to fire tube

Hello everyone.

How if at all does hopper size affect how a gasifier operates? (Other then fuel capapacity of course ) I would like to use 55 gallon drums out of conveience but they would be much bigger then my fire tube for the small generator Im wanting to run as a learning project.

Also are there guides for fire tube dimensions that are free or in premium content? I am waiting to be aproved for premium and am wondering.

Thank you


As long as you make ramps for the fuel to flow smoothly there isn’t much problem.

Only downside to having a giant hopper relative to the hearth is when there’s an issue and you need to get to the nozzles you’d have to dig a lot of fuel out, or dive into the barrel to get to the hearth.


How small of a generator are we talking about?

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Hi tuna thanks for reply

I have a 2000w(estimate) brigs generator and 7250w predator would like to power both of them with the same unit interchangbly.

The predator is 13 hp. I think the brigs is 5 but the horsperpower rating is worn of its been used hard.

What dimensions would be practical?

I would like to go as large as i can without going over size. Since my material is larger and i wouldnt have to worry about running out of horsepower so much.


I’m biased, but if you want something that can run both I would suggest a charcoal gasifier. If you live in an area where you can burn wood then it would be ideal and not hard. If the charcoal is made correctly then you also run no risk of tar.

I ran a 2 Liter sized Mazda B2000 using a 55 gallon drum as the gasifier body and a heavy steel pipe as the nozzle, I welded it in near the base and had 5 holes drilled into the pipe. At the top of the barrel was a 2 inch exit pipe for gas and a tall ammo can to hold my bag filter.

Koen van looken has excellent threads covering charcoal gasifiers, also Kristijan Leitinger.


Welcome to the DOW TaylorK.

For your intended engines-generators’ loads click open the top bar stacked papers tool icon.
Then open up the many, many systems projects for on Small Engines. Read away man.

Now specifically on a raw woods input gasifier hopper volume versus hearth capacity question . . .
I have learned a contradictory lesson on that one.
Much of the historic and even current advice is, ( . . . then make the hopper as large as you please.")
I found this to be bullshit.
Make the hopper capacity for 1-2 hours loaded running time and then you will have minimal problems with normal 15% (by weight) moisture wood.
The input raw wood must cook down to wood charcoal. Large loads of wood will smoke/volatiles and steam thermally crash the system. Physically clog the downward flow of the fuel materials.

I say this from experiences from the 1st GEK II I operated back in 2009; to some very large elaborate systems.

Much of the elaborations adds to systems are to counter the effects of fuel batch overloading.
You can stretch fuel hopper capacity to ~4 hours loaded running times with pre-dried, and pre-warmed woods using the throwaway waste heats off of the engine-generator and the gasifer cooling and filtering systems.

Pre-converting the wood, to wood charcoal is relieving the engine fuel making gasifier system from the thermal loading for converting raw woods volatiles and wood moistures.

Steve Unruh


Cody, l must disagree here. But the real answer to the question is that generic, “it depends”.

What kind of fuel? Hopper design? Insulation? Water extraction?

Even with a downdraft charcoal gasifier l noticed moisture migrating upward, wich can be a problem in some cases. Short drives per instance, will make the charcoal too dry over the top of the firetube, and the moisture acumulates on the top layer. Then, at a longer drive, you hit the moist spot…
But charcoalgasifiersare extremely forgiveing, as you sayd. And at worst, the gas will be a bit weaker. With wood, its a whole other thing. This phenomena can glue together the top layrs of chunks with tar, and make a bridge. Not to mention lessened fuel efficiency as the water in the pyrolisis zone rises up and meets the cold wood, recondenses, soaks them dripping wet. This water will need to be reboiled later, wich requires energy.
Just as wood boilers need to operate at a minimum temperature, same stands for the hopper.

Edit Steve beet me to it :grin:


I’ll always concede to the more experienced.

I’m running under the assumption he didn’t intend to use the whole drum as a hopper. Shortening and splicing down to maybe 20 gallons.

Taylor if you’re looking for more literature, I’d suggest to also get the Wood Gasifier Builder’s Bible by Ben Peterson. That and Have Wood Will Travel are the two books I bought.
WGBB is 20 bucks on Amazon, or it was when I bought it.


Mulling all this over and awaiting my books membership.

Im glad for the charcoal suggestion. While having drawbacks i think it would be good for my small generator where easy start up is imporant and i have alot of extrawood that would otherwise go to waist so the energy needed to make charcoal is of less a concern.

Thank you everyone.


Taylor, Matt Ryder demonstrated his M-1 unit at Argos last year. It was five minutes from light up to generator running. No constantly adjusting the air-fuel ratio as one might have on wood. It started and ran well. Matt has to make his commercial gasifiers as close to idiot proof as possible. That’s what it takes in the commercial arena. He has an ammo box unit that he shared with us. It’s based on his larger units he sells. Charcoal is a great place to start and get your feet wet. Beware the woodgas bug will bite you when the generator comes to life on wood.


Ok intersting will look that up. Also I only live 1.5 hours from Argos and rented pasture there to graze cattle there. Hopefully i can make it.


Whatever you end up doing make sure to start another thread and take lots of pictures, bonus if you do some videos. If you end up using the WK design make sure it’s posted Premium-side.


Hi Taylor,
Gary here has something you can make out of a 5 gallon bucket. Probably the easiest way to get started that I’ve seen.


Go to the posting “Small Engine Portable Fuel Station”. I think it is exactly what you would need. TomC