A wood gas car uses roughly 2.3-3.1 kg of dry wood for each litre of petrol it replaces, which is about 16-31 kg per 100 km for a car using 7-10 litres, in line with the carrier’s table of 15-60 kg per 100 km and far below its text claims of 1-2 kg per kilometre. Range is the hopper mass divided by consumption, from 83 to 533 km on a 50-80 kg load depending on the engine class, and 10,000 km a year takes about 0.52-1.4 hectares of managed coppice.
Part of Wood Gas Vehicles
Range and fuel consumption for a vehicle on producer gas: how much wood a kilometre takes, how far a hopper goes and how much land the fuel needs.
The Carrier Gives Four Different Consumptions
| Source in the carriers | Statement | Dry wood per 100 km |
|---|---|---|
| Sizing table, wood-gas vehicles | 15-25 kg (1.0-1.5 L), 25-40 kg (1.5-2.5 L), 40-60 kg (2.5-4.0 L) | 15-60 |
| Fuel preparation, wood-gas vehicles | ”1-2 kg of wood per kilometre”, 100-200 kg for 100 km | 100-200 |
| Energy balance, biomass-gasification | 1 kg of wood drives 1-1.5 km | 67-100 |
| Performance, wood-gas vehicles | 60-100 km on a 50-80 kg hopper | 50-133 |
These cannot all be true. The test is energy.
Derive It From Energy
- Dry wood is 18.5 MJ/kg (BTU Content Compared to Wood). The parent says 70-75% of this ends up in the gas: 12.95-13.9 MJ per kg.
- Petrol holds about 32 MJ per litre (typical lower heating value, recalled).
- If the engine is as efficient on gas as on petrol: 32 ÷ 13.9 = 2.3 kg of wood per litre of petrol replaced. If it is only 80% as efficient: 32 ÷ (12.95 × 0.8) = 3.1 kg per litre.
- A car using 7-10 L/100 km (an assumption) therefore needs 7 × 2.3 = 16 to 10 × 3.1 = 31 kg per 100 km.
That sits inside the table’s 15-60 kg. The table’s rows correspond to petrol-equivalents of 15 ÷ 3.1 = 4.8 up to 60 ÷ 2.3 = 26 L/100 km, a small car to a heavy truck. The text claim of 100-200 kg per 100 km would be 150 kg × 13 MJ = 1,950 MJ, or 61 litres of petrol per 100 km, which is a heavy truck’s consumption and not a car’s. So the 1-2 kg per km and 1-1.5 km per kg figures are too high by roughly 3-13 times for a car; the table is the one consistent with the physics. The extra weight of the gasifier raises consumption somewhat, which I have not quantified.
Range Is Hopper Mass Over Consumption
| Engine class | Consumption, kg/100 km | Range on 50 kg | Range on 80 kg |
|---|---|---|---|
| 1.0-1.5 L | 15-25 | 200-333 km | 320-533 km |
| 1.5-2.5 L | 25-40 | 125-200 km | 200-320 km |
| 2.5-4.0 L | 40-60 | 83-125 km | 133-200 km |
Each cell is hopper ÷ consumption × 100, taking the lowest consumption with the largest range. The carrier’s “60-100 km per full hopper” only fits the heaviest class on the smaller load, so it is a truck figure, not a general one. The longest-range cell, 533 km, is for a 1.0 L engine with the biggest hopper, which a small car cannot carry (Weight & Space Constraints).
What To Carry
Fuel is quoted dry. A kilogram of wood at 15% moisture on a wet basis holds 0.85 kg of dry matter (20% wet basis is 25% on a dry basis; 15% wet basis is 17.6% dry basis). To cover 100 km at 25-40 kg of dry wood, carry 25 ÷ 0.85 = 29 to 40 ÷ 0.85 = 47 kg of the wood as stacked. Wood that has been rained on may be at 25% or more and its dry content falls accordingly.
What The Land Cannot Do Meanwhile
Take 10,000 km a year for a car in the 1.5-2.5 L row (an assumption):
- Dry wood: 10,000 ÷ 100 × 25-40 kg = 2.5-4.0 t.
- As air-dried wood at 20% wet basis: 2.5 ÷ 0.8 = 3.1 to 4.0 ÷ 0.8 = 5.0 t.
- Managed coppice yields 3.5-6 t/ha/yr of air-dried wood (mixed ash and hazel, Coppicing & Fuel Management): 3.1 ÷ 6 = 0.52 ha to 5.0 ÷ 3.5 = 1.4 ha.
That land is not growing grain, vegetables or grazing for the whole year. The same wood is also 31% (3.1 ÷ 10) to 100% (5.0 ÷ 5) of one household’s annual heating wood at the coppice article’s 5-10 t a year, so the vehicle competes with warmth.
Method: Measure Your Own
- Weigh the fuel loaded and what remains at the end of a run.
- Record the distance and the load.
- Divide to get kg per 100 km, and plan from that, not from either text.
Common Mistakes
- Quoting range without the engine class. The range changes fourfold.
- Planning a trip from the 1-2 kg per km figure. It overstates fuel by 3-13 times for a car.
- Counting wet wood as fuel. Water carried is range lost.
Safety
A long trip is more start-ups, refuels and exposure to carbon monoxide
Producer gas is about 18-22% carbon monoxide by volume. Each refuelling opens the hopper to gas, and each stop-start adds a cold start with cool, rich, almost all-carbon-monoxide gas. Refuel outdoors, upwind, and never in a garage, shed or enclosed yard; never sleep in the vehicle with the unit lit. See Carbon Monoxide Risks.
What’s Next
- Learn next: Fuel Preparation & Carrying — preparing the wood for the trip
- Combine with: Speed & Power Reduction — the other limit on a trip