This is an instructable I started about five years ago; it was a fail and only partially worked but here are some pictures and my intro from the time of the build. I thought some people might still find it interesting. I still remember people staring wide-eyed as they drove by my house seeing a mower with a "smokestack". LOL
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.