Over one million vehicles in Europe ran on wood gas during World War II, and a downdraft gasifier carried on or towed behind a vehicle can turn any gasoline vehicle into one that runs on wood chunks. Expect significantly reduced performance, a startup of 20-30 minutes and constant attention, and weld every joint rather than bolting it, because a gas leak is lethal from carbon monoxide.
During World War II, petroleum shortages forced over one million vehicles across Europe to run on wood gas — from delivery trucks in Sweden to buses in Germany to farm tractors in France. The technology works. A gasifier mounted on or towed behind a vehicle converts wood chunks into combustible gas that feeds directly into the engine’s intake. Performance drops significantly, startup takes 20-30 minutes, and the system demands constant attention, but it turns any gasoline vehicle into one that runs on sticks.
Vehicle Gasifier Design
Sizing
A vehicle gasifier must produce enough gas for the engine’s demand at driving speeds. The key parameter is gas flow rate:
- A 2-liter gasoline engine at highway load consumes approximately 80-120 m³/hour of producer gas
- A smaller 1.3-liter engine needs roughly 50-80 m³/hour
- The gasifier’s hearth constriction (throat) diameter determines maximum gas production
| Engine Size | Throat Diameter | Firetube Diameter | Fuel Consumption |
|---|---|---|---|
| 1.0-1.5 L | 60-75 mm | 200-250 mm | 15-25 kg wood/100 km |
| 1.5-2.5 L | 75-100 mm | 250-300 mm | 25-40 kg wood/100 km |
| 2.5-4.0 L | 100-130 mm | 300-400 mm | 40-60 kg wood/100 km |
Refer to biomass-gasification for detailed gasifier construction. A vehicle gasifier uses the same downdraft (Imbert) design but is built more compactly and with vibration resistance.
Weight & Space
A complete vehicle gasifier system weighs 150-300 kg including:
- Gasifier reactor: 50-100 kg
- Cyclone separator: 15-25 kg
- Gas cooler: 20-40 kg
- Filter: 10-20 kg
- Piping, frame, fuel hopper: 30-50 kg
- Fuel load (50-80 kg of wood chunks)
This weight reduces vehicle payload capacity significantly. Trucks and vans are better candidates than small cars.
Material Selection
- Reactor: Thick steel (3-5 mm). A decommissioned fire extinguisher or small pressure vessel makes an excellent inner reactor
- Outer shell: Thinner steel (1.5-2 mm). A 55-gallon drum works for larger vehicles
- Gas piping: 40-50 mm steel or thick aluminum tubing. Must withstand vibration without cracking at joints
- All joints: Welded, not bolted — vibration loosens bolts and gas leaks are lethal (CO)
Vehicle Mounting
Mounting Options
Truck bed mount (most common): The gasifier sits upright in the truck bed or on a frame behind the cab. Gas piping runs along the chassis to the engine intake. This is the WWII standard for trucks.
Trailer mount: The entire gasifier system on a small trailer towed behind the vehicle. Advantages: any vehicle can use it; easy to disconnect. Disadvantages: adds length, complicates reversing, trailer must be sturdy.
Rear-mounted frame (cars): A steel frame bolted to the rear of the vehicle, extending behind the trunk/boot. The gasifier hangs on this frame. Works for passenger cars but affects handling (rear-heavy) and visibility.
Vibration Isolation
Road vibration shakes loose everything:
- Mount the gasifier on rubber isolators (cut from old engine mounts or tire sections)
- Use flexible connections in the gas piping (a 15 cm section of heat-resistant silicone hose or a bellows joint)
- Secure all components with multiple attachment points — a gasifier that breaks loose at speed is catastrophic
Heat Shielding
The gasifier reactor reaches 800-1,400°C internally:
- Wrap the reactor in ceramic fiber blanket (salvaged from industrial insulation) or multiple layers of sheet metal with air gaps
- Keep all flammable materials (wood fuel, rubber, plastic) at least 15 cm from the reactor surface
- Shield the vehicle body from radiant heat with a metal barrier
Gas Cleaning for Mobile Use
Gas cleaning is the same as a stationary gasifier (see biomass-gasification) but must be more compact and vibration-resistant.
Compact Cyclone
A 15-20 cm diameter steel cylinder with tangential gas inlet:
- Gas enters tangentially, spins, heavy particles hit the wall and fall into a collection chamber
- Clean gas exits from the top center
- Mount vertically for gravity-assisted particle collection
- Empty the collection chamber every 100-200 km
Cooler
The gas must be cooled from ~300°C to below 40°C before entering the engine:
- Use a section of finned radiator (salvaged car heater core or small radiator)
- Mount in the airflow path — behind the vehicle’s grille, or with a dedicated fan
- Condensation trap at the lowest point — drain regularly
Filter
Final stage before the engine:
- A canister packed with cotton cloth or dry sawdust
- Replaceable element — carry spare filter material
- Change every 200-500 km depending on gas quality
Operating Procedures
Startup Sequence (20-30 Minutes)
- Load fuel: Fill the hopper with dry wood chunks (3-5 cm cubes). Charcoal is even better for faster startup
- Light the gasifier: Open the air inlet, insert a lit torch or burning paper through the lighting port (a small door near the air nozzles)
- Wait for gas production: Run the blower fan for 10-15 minutes to pull air through the gasifier. Use the blower, never the engine. An earlier version of this step offered “engine vacuum by cranking without starting” as an alternative — that pumps uncombusted producer gas straight out of the exhaust, at ground level, beside the person cranking, during the phase when the gas is richest in carbon monoxide and not yet burnable. A blower does the same work and vents where you put the outlet.
- Test gas quality at a flare, not at the pipe mouth. Fit a short flare stack to the gas outlet — a length of pipe standing up and away from the vehicle — let the gas flow, stand upwind, and light it from the side with a torch on a stick. Clean gas burns with a near-invisible to pale blue flame; a smoky yellow flame means tar and the bed is not hot enough yet.
Do not hold a match to the gas outlet
Lighting the gas is the right test. Doing it at the pipe mouth is not. Your face sits in the gas stream for however long it takes to strike and hold a light, breathing gas that is about a fifth carbon monoxide at its coolest and richest — and a flame at the mouth of a pipe full of combustible gas can flash back down it. An earlier version of this step said to hold a lit match to the gas pipe.
The flare costs a scrap of pipe and puts the flame two feet from your hands and well away from your face. Light it from the side, never from above or directly in front, and never lean over a flare to see whether it has caught.
- Start engine: With the gas valve open and the air-gas mixer adjusted, start the engine on gasoline, then gradually switch to wood gas by closing the gasoline supply and opening the gas valve
- Warm-up: Run at idle on wood gas for 2-3 minutes before driving
Driving Technique
- Expect reduced power: 40-60% of gasoline power. Accelerate gently, accept slower hill climbing
- Keep the RPMs up: Wood gas engines run better at higher RPM with lighter load. Downshift earlier on hills
- Monitor the gas temperature: If the gas gets too hot (above 50°C), the engine loses power. Increase cooling
- Fuel the hopper: Check fuel level every 30-45 minutes. Refuel from your onboard wood supply
- Typical range: 60-100 km per full hopper load (50-80 kg of wood)
Shutdown
- Switch back to gasoline for the last 2 minutes of driving (clears wood gas from the engine)
- Close the air inlet to the gasifier (starves the fire)
- Leave a vent path open to the flare — do not seal the unit. Disconnect the gas line from the engine and leave it open to the flare stack, or crack the gas valve to atmosphere pointing up and away from any building or doorway
- Let the gasifier cool naturally — do not open it while hot (hot charcoal exposed to air ignites violently)
Closing every valve on a hot bed builds pressure and fills the vessel with gas
An earlier version of this sequence closed the air inlet and the gas valve and left it at that. A bed of glowing charcoal under a hopper full of wood does not stop when you close the valves: residual heat keeps pyrolysing the fuel above it for a long time afterwards, and the gas it makes has nowhere to go. It pressurises, then leaves through whichever seal is weakest — which is usually upward, through the hopper lid, exactly where someone stands to check the unit. And the vessel you open the next morning is then full of combustible gas sitting over a bed that may still be hot.
Vent it until it is cold. This is reasoning from the physics rather than a procedure quoted from a wartime manual, so treat it as the argument it is — but the argument is hard to get around, and sealing a reacting vessel is a bad habit in any case.
Emergency Procedures
- Gas leak (headache, dizziness, clumsiness, confusion — CO symptoms): stop the vehicle and get out into fresh air. That is the whole of the first step. Everything else — shutting the gasifier down, opening the cab up, ventilating, finding the leak — happens once you are outside and breathing. An earlier version of this line put three tasks ahead of getting out, which inverts the priority at the worst moment: the symptoms that trigger this entry are themselves the impairment, so a reader working down a list is doing it with the judgement already going. Emergency Procedures works the timing through — a 1% leak into a sealed 3 m³ cab reaches 6,700 ppm in 7.5-12 minutes, against an immediately-dangerous level of 1,200 ppm. And do not get back in to move it, fetch anything or switch it off. Going back into the cab is how a survivable exposure becomes a fatal one, and it is how a second person becomes a casualty
- Gasifier fire (external — fuel hopper or filter ignites): Close all air inlets. Smother with sand or dirt. Do NOT use water on a hot gasifier (steam explosion risk)
- Tar clogging (engine misfires, loses power): The gasifier is running too cold. Stop, let it heat up with the blower on before resuming
Performance & Limitations
Realistic Expectations
| Parameter | Gasoline | Wood Gas |
|---|---|---|
| Power output | 100% | 40-60% |
| Top speed | 100% | 60-70% |
| Hill climbing | Normal | Significantly reduced |
| Startup time | Seconds | 20-30 minutes |
| Range per “tank” | 400-600 km | 60-100 km per hopper |
| Fuel cost | Gasoline price | Free (wood) |
| Maintenance | Low | High (daily cleaning) |
Fuel Preparation
Wood gas vehicles consume 1-2 kg of wood per kilometer. For a 100 km trip, you need 100-200 kg of prepared fuel:
- Wood must be cut into uniform 3-5 cm chunks (too small clogs the grate, too large bridges in the hopper)
- Dry — below 20% moisture. Wet wood produces excess tar that clogs everything
- Hardwood preferred — denser, more energy per volume. Softwood works but produces more tar
- Carry extra fuel for the return trip or plan resupply points
See Also
- biomass-gasification — Stationary gasifier design and theory
- biodiesel-production — Liquid fuel alternative for diesel vehicles
- ethanol-fuel-production — Liquid fuel for gasoline vehicles
Historical Context & Lessons
Wood gas vehicles were not a curiosity — they were a necessity. Key facts from the WWII era:
- Sweden: 73,000 vehicles converted by 1945 (nearly 40% of all vehicles). The Swedish government published detailed conversion manuals for every common engine
- Germany: Over 500,000 wood gas vehicles operated during the war. Public refueling stations sold pre-cut wood blocks
- Finland: The Finnish military operated wood gas trucks and even tanks in extreme cold (-30°C), proving the technology works in harsh conditions
- Post-war: Nearly all wood gas vehicles were converted back to gasoline within months of petroleum becoming available again. This demonstrates both the technology’s viability and its inconvenience compared to liquid fuels
Key lessons from history:
- Standardized fuel preparation is critical — Sweden set national standards for wood block sizes
- Dedicated fuel depots along routes solved the range problem
- Tar fires in the gasifier or filter were the most common breakdown. Carry spare filter material
- Engine life was reduced by approximately 30-50% due to abrasive particles in imperfectly filtered gas
- Dual-fuel capability (switch to gasoline for starting and hills) dramatically improved practicality
Choosing Between Wood Gas, Biodiesel, and Ethanol
| Factor | Wood Gas | Biodiesel | Ethanol |
|---|---|---|---|
| Fuel source | Any dry wood | Vegetable oil / animal fat | Sugar or starch crops |
| Engine type | Gasoline | Diesel | Gasoline |
| Startup time | 20-30 min | Instant | Instant |
| Range per load | 60-100 km | 400-600 km | 300-500 km |
| Power retention | 40-60% | 95-100% | 70-75% |
| Fuel prep complexity | Cut & dry wood | Chemical processing | Ferment & distill |
| Ongoing fuel cost | Lowest (wood) | Moderate (oil + chemicals) | Moderate (crops + energy) |
| Best for | Local/farm vehicles | Trucks, generators | Cars, small engines |