Woodgas, biogasification, gasification, are all descriptions of this project and the spell check doesn't like any of those words. I've been reading in Farm Show Magazine about people converting their trucks and tractors to run on firewood; and I said "what the @#%?". But I checked things out a little more online a found quite a bit of information on this, especially a FEMA guide released in the late 1980's.
This is a simple process and you don't have to understand the chemistry for it to work. Basically you burn dry wood, grass, coal, etc in a tube at very low oxygen levels which gives off hydrogen and carbon monoxide, both flammable gases, and suck that into the engine to burn like propane. Actually its supposed to burn about as clean a propane too but with less power.
Warning: This project involves combustible gasses, poisonous gasses, hot surfaces and fire! If you plan to build something like this, don't burn yourself, don't do this indoors, and do this sober! Don't add any liquid starter fuel or anything similar; I learned the hard way with a fire ball shooting out of the burn chamber; I lost a good two months growth on my beard and one eyebrow. It's kinda funny since I didn't really get hurt, but it could have been worse.
Step 1: Remove Gas Tank and Carb
Actually I later found out these things would have been nice to keep to make start-up easier.
Step 2: Build a Platform
Step 3: Build a Burn Chamber
A stainless steel bowl was used for the burn pot attached at the bottom of the fuel chute with some heavy copper wire.
Step 4: Cooling Tube
Step 5: Build a New Carburetor
Carburetors meter the flow of air and fuel to the engine. I used a combination of plumbing, gas, and electrical conduit fittings to make this work. 1/2 inch copper pipe fit perfectly into the engine intake tube; to that I soldered a pipe thread fitting. To the fitting I attached a brass gas valve as the throttle control.
Step 6: Filter
Step 7: Notes and Issues
In the end I found multiple fail points, imagine that :)
1. The cooling tube wasn't long enough to cool the hot gasses before they reached the tar filter, thus burning the wood chips and sucking them into the motor.
2. The pull start up sucked, I had many blisters to prove it. If I were to do it again I would leave on the original carb and gas tank, start the engine with gas, then convert over to wood gas with a running engine as the vacuum.
3. The best fuel seemed to be little twigs, although my dream was to run this off of the lawn clippings.