We recently featured an article about an Iowa couple who beat the high price of gasoline by adapting a woodburning fuel-gas generator to their pickup truck. In the piece we mentioned that MOTHER EARTH NEWS’ research crew was “hot” to build a gasification unit of their own design and that we’d be reporting in a future issue on what we hoped would be an improved wood-fueled powerplant.
Well, the future is now, and–as sometimes happens–we have some good news and some not-so-good news. The good tidings, however, couldn’t be better: Dennis Burkholder and Robyn Bryan (our resident experts on wood gas) have indeed designed and built a producer-gas generator and used it to turn our “old” alcohol-powered Chevy truck into a wood-powered truck. The “cooker” itself seems to be working perfectly, and the fellows have even designed a dual-fuel carburetion unit which–as far as we know–is just flat unique. To date, the pickup has logged about 1,000 wood-gas miles with no problems and can hold its own in stop-and-go traffic, on hills, and along interstate highways.
And, as you’ll probably be glad to know, the bad news isn’t all that bad: You see, because we really don’t feel that a 1,000-mile test period is adequate to determine the long-term effects of producer gas on an engine, we won’t release any detailed construction information–in the magazine or as a set of plans–until we’ve had a chance to put some more highway behind us, checked the oil for contamination, looked for unwanted deposits in the fuel distribution system, and inspected the valves and seats for wear.
Furthermore, although we’ve tried several different filter mediums (all of which did a fine job of scrubbing solid particles from the gaseous fuel for a limited time), we’re still looking for an inexpensive straining material that  is waterproof, so that it can maintain its integrity in a moist environment,  doesn’t break down under prolonged use, and  is capable of serving as a “flame trap” in the unlikely event that a back flash should occur through the system.
So, at present–even though we feel we’ve taken a giant step toward developing a practical and affordable alternative to petroleum-powered transportation–we still believe that our design has room for improvement in several specific areas. And when those refinements are made, be assured that you’ll be among the first to know about them!
How Wood Gasification Works
Our converted pickup’s fuel–scraps of wood usually no larger than a 6″ length of 2 X 4–rests on a cone-shaped hearth that is, itself, contained within an airtight drum. A limited amount of oxygen is allowed to enter the vessel through an inlet valve, which also incorporates the forced-air blower used to create a draft when starting the fire (once the engine is running, its vacuum provides all the draw necessary to keep the fuel burning).
As air enters the generator, it’s directed through a series of ducts into the combustion zone just below the hearth. The wood chunks burning in this vicinity then consume most of the oxygen in the air–creating carbon dioxide and water vapor–and drop red hot embers onto a grate near the base of the cooker. At the same time, the heat from this reaction helps to drive moisture and convertible gases from the solid fuel stored above the “ignition” area.
The CO2 and water vapor, together with some wood tar, are then pulled through a restrictor throat just above the charcoal grate. This venturi speeds the flow of gases, then forces them through the glowing coal bed. It’s during this “trial by fire” that the fumes are converted into the burnable vapors carbon monoxide, hydrogen, and–to a lesser extent–methane. (The final product also contains a good amount of nitrogen, some tar and ash, and a bit of unconverted carbon dioxide and moisture.)
The noncombustible nitrogen and CO2 can remain in the fuel without causing appreciable problems, but most of the tar, ash, and water vapor must be removed from the gas to prevent accumulation and potential engine damage. To accomplish this, the vapors are first routed through a condensing unit which precipitates the moisture (and increases the density of the fuel charge), then are passed into a filter, where the finer particles are separated.
From there, the gas is piped to a manifold, which allows regulation of both the air/fuel mixture and the speed of the engine. (In addition, this distribution system allows the operator to switch back and forth between gasoline and producer gas at the pull of a lever while still providing the engine with filtered air in either fuel mode.)