Running 6500 watt Generator on Wood Gas

Hello, I’m in the process of building my first wood gasifier. I decided to follow the plans of Ben Peterson in his Wood Gasifier’s Builders Bible. I’m at the critical point of sizing.

At the moment, I only want to run a small generator but I want the ability to run a larger engine in the future. I’m substituting stainless for the majority of all the internal components in the reactor assembly ( so the cost of the build has increased substantially). So I don’t want to build two of them.

Does anyone know if I will have issues running a small generator on a unit sized for a lager engine ( 3.0L) ?

Any advice would be greatly appreciated!


You could make it have replaceable nozzles and choke plates to run the two engines, also use different fuel sizes. Would have to play it by ear.


Hello David!

Unfortunaly, this seems too big of a strech. There is abit of play when it comes to engine vs gasifier size but thats just too much in my opinion…

Stainless… Its not a cure for everything. As a material it isnt perfect for every occasion. Loads of heat cycles in a gasifier hearth and SS sucks at that. And at extreme heat it can act worse thain mild steel. I realy wuldnt bother with it. Keep in mind mild steel resists heat quite well as long as its in a non-oxidative athmosphere. In a gasifier its about the leaat oxidative as it can be…
In the end, its way easyer and cheaper to make the gasifier out of a bit thicker mild steel thain fearing a stress induced crack in a SS hearth.

Cooler parts like hoppers, gasifier housings etc, here stainless realy shines!


To add SS retains heat. When it retains it expands. The more heat it takes on the more it expands. That growth has to go somewhere and what will end up happening is it will displace in ways its not supposed too resulting in stress hardening and eventually failure. Mild steel is just fine in the hearth environment, the oxygen gets used up so it is impossible to oxidize the steel. Mild steel has a higher melting point than most SS as well. So if you melt mild steel you surely would melt the SS. Your chances of failures are more prone with SS than mild steel as the mild steel will dissipate that heat and it wont expand along with lower carbon content its less likely to stress harden.


Ok sounds like a good idea…thank you!

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Thanks for the info! Luckily I still haven’t purchased everything to complete the build

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You really should probably build a second unit for the smaller engine though. Ben reccomends different lengths of cooling tubes depending on the engine and gasifier size.


Thanks for the help! That will definitely save me some $$$!

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Ok… I was hoping for just one… I’ll make sure to size it for my generator. Thank you!

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Here is a big suggestion. Build Ben’s gasifier for your vehicle and build a charcoal unit or Gary Gilmores simple fire for your little generator.

This will greatly reduce your cost simplify things for you as you will not have to build two complex systems and charcoal will be much better suited for the smaller engine. You could even use the char ash from the larger unit to run your smaller charcoal unit.

This way you get your ears wet with both tech.


I have a question here , has there been anyone that has a vehicle that runs off of a Ben Peterson gasifer or is it just a Imbert type design?


Not an Imbert BobMac. Those as far as I know are direct contact metals walls flow shapers.
His BookSystem is more of a modified/evolved Scandinavian ash slope protected hearth.
Can’t say inverted V as it is mostly straight walled tube sections.
But . . . even then with it’s pyrolysis accelerator output gas heating section; and it’s unique gentle back and forth grate action is a beast of it’s own.

On the basic made from 100 pound capacity propane tanks Ben did size it capable of up to ~5.0L engines. IF you build with the full 10 air nozzles and greatly increase the cooling and down stream.
He/we had experiences with the big HD three phase, commercial Onan AC generators using the Ford industial 240 and 300 CID engines. These the guys who drove the capability.
David he only rates this down to 500cc engines. 5000cc to 500cc engines. 6 to 1 turn down ratio. Some of his own different designed; and built all SS units had a wider turn down ratio. Using the SS hold-in heat for advantages. But BenP insisted to only all hand-TIG welding those. Never MIG. Never stick.
His book system is for MIG’ing. Or a good carefull stick man building.
Your 6500watt electrical generator should be an under 400cc engine. These are usually 325-380-ish engines.

Take Matt Ryders advices.
He has the for real done-it experiences to know well.
Steve Unruh


Hi Matt,
Thanks for the advice…I do have an old Toyota 4Runner with a 4 cylinder engine that I’m not driving…might be a good fit.

Can you recommend any book or plans for a charcoal system that works and won’t ruin the engine?

Thanks for the help!


Hi Steve,
I don’t own a MIG welder, I have an Everlast TIG/Stick. I’m going to use TIG for everything that I can reach…On my build I currently have the choke mantle installed, which is 1/4” 304 stainless. I stick welded with 309L rod, then went over anything that looked a little sketchy with the TIG torch… it successfully passed a pressure test of about 40 psi without any leaks.

I guess I’ll have to change plans and build something different for the generator and size this unit up for my 1997 4Runner.

Thank you for your advice!


Simple Fire from Gary Gilmore.

He tested a lot of different systems. I get the same results as he does. So , from now on I just follow the things he recommends. Nice start and from there you can make the next steps.


Sounds great David.
Vesa Mikkonen (Finland) also only 100% fabricates with stainless steel. He has his own book with four different sized capacities plan sets in it.
[email protected]
His smallest is for four cylinder engines like Toyota’s (his car). Down to 4.1 kWel on wood gas electrical generators.
His designs have a lot of metals differential temperature expansion and contraction experienced features built into them.
As you are finding it is expensive to builds with all SS unless a lot of industrial scrap is locally available. Like from dairy, food, canning and the paper making industries.

A 500cc minimum hurdle may seem high and expensive. Really, it is not. That is the common 10,000 to 12,500 watt (on gasoline) portable generator engine sizes.
~$1300-1600 USA bought new. Sell off the smaller for $300-500 used. Then buy up. You will be at least 30% power derating on woodgas anyhow.
Steve Unruh


For the SimpleFire up draft charcoal gasifier info, Gary Gilmore’s Youtube videos are great. Also, there are SimpleFire threads on the DriveOnWood forum. Matt Ryder has tutorials for building a small down draft charcoal gasifier on his Thrive Off Grid DriveOnWood thread.


Thank you…I’ll check that out.

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Thanks for the info…I’ll check that out.

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That’s probably the most logical thing for me to do… truthfully I’m not ready to try running a vehicle with wood gas yet… but sizing up my generator would be an easier fix.

Thank you!