On wood gas a vehicle keeps 40-60% of its rated power, which for a car costs far less top speed than power, because aerodynamic drag rises as the cube of speed, but costs a great deal on hills, where the power needed rises with speed and weight. In the illustrative case below, a 1,750 kg car with a 60 kW engine falls from about 166 km/h to 115-136 km/h on the level and from 112 km/h to 55-77 km/h on a 6% grade, and a loaded 8-tonne truck falls to 17-25 km/h on a 6% grade.
Part of Wood Gas Vehicles
Speed and power reduction on wood gas: the road-load arithmetic that turns a lower engine power into a lower speed on the level and on hills.
The Road-Load Equation
Power at the engine to hold a steady speed v on a grade G is:
P = [ m × g × (G + Crr) + ½ × ρ × CdA × v² ] × v ÷ η
where m is mass, Crr the rolling-resistance coefficient, ρ the air density (1.2 kg/m³), CdA the drag area, and η the transmission efficiency. The values below are assumptions for a typical car and a typical truck, not data for any vehicle; change them to your own.
Car: 1,500 kg plus a 250 kg gasifier system = 1,750 kg; Crr 0.012; CdA 0.7 m²; η 0.85; rated engine 60 kW.
Level Road
- At 100 km/h (27.8 m/s): rolling 1,750 × 9.81 × 0.012 = 206 N; drag 0.5 × 1.2 × 0.7 × 27.8² = 324 N; total 530 N × 27.8 = 14.7 kW at the wheels, 14.7 ÷ 0.85 = 17.3 kW at the engine.
- At 120 km/h the engine needs 26.4 kW; at 140 km/h 38.5 kW; at 166 km/h about 60 kW.
- So the petrol car’s top speed is about 166 km/h. On gas at 40% (24 kW) the speed where need equals supply is about 115 km/h, and at 60% (36 kW) about 136 km/h.
- Ratios: 115 ÷ 166 = 69% and 136 ÷ 166 = 82%.
The carrier says top speed on wood gas is 60-70% of petrol’s. The arithmetic supports that only at the poor end of the power range (40%); at 60% power the car holds 82%. A truck with a tall gear ratio, or an engine limited by revs, can fall further.
Hills
Steady speed the power allows on a grade (solving the equation for v):
| Grade | Petrol, 60 kW | Gas at 60%, 36 kW | Gas at 40%, 24 kW |
|---|---|---|---|
| 0% | 166 km/h | 136 km/h | 115 km/h |
| 6% | 112 km/h | 77 km/h | 55 km/h |
| 10% | 85 km/h | 55 km/h | 37 km/h |
At 50 km/h (13.9 m/s) on 6%: climb 1,750 × 9.81 × 0.06 × 13.9 = 14.3 kW, plus rolling 2.9 kW and drag 1.1 kW, 18.3 kW at the wheels, 21.5 kW at the engine. That is within a 24-36 kW gas engine, but a 10% grade at 50 km/h needs 32.7 kW, at the top of the gas range. The grade scales with weight: the 250 kg unit adds about 17% to the climb power (1,750 ÷ 1,500 = 1.17).
A Loaded Truck
Truck: 8,000 kg including cargo and unit; Crr 0.010; CdA 5 m²; η 0.80; rated engine 80 kW (assumed).
| Grade | Petrol, 80 kW | Gas at 60%, 48 kW | Gas at 40%, 32 kW |
|---|---|---|---|
| 0% | 89 km/h | 71 km/h | 59 km/h |
| 6% | 39 km/h | 25 km/h | 17 km/h |
| 8% | 32 km/h | 19 km/h | 13 km/h |
On a long 6% climb the truck crawls at 17-25 km/h, and the throat then runs hard for long stretches, which is fine for tar but demands gas. This is the “significantly reduced” hill climbing of the carrier’s table. Gearing helps only if the engine can reach the revs where its gas supply is highest (Driving Technique).
Method: Check A Plan
- Take the vehicle’s mass with unit, fuel and cargo, and the steepest long grade on the route.
- Compute the power for a speed you can accept using the equation; divide by 0.4 for the rated engine you need.
- If the result exceeds the engine’s rating, plan to switch to petrol on the climb (dual-fuel) or reduce the load.
Common Mistakes
- Using the rated power. On gas the engine delivers 40-60% of it.
- Forgetting the unit’s weight. It adds to every climb.
- Taking the steady speed as a guarantee. Wind, surface and gear changes shift it.
Safety
Slow climbs mean long exposure, and a failing engine is no time to open the gasifier
Producer gas is about 18-22% carbon monoxide by volume. A truck crawling uphill with a cab downwind of the exhaust and a leaking joint is the setting for slow poisoning. Keep the cab ventilated, stop and get out upwind at the first headache, drowsiness or nausea, and never refuel or open the lid of a hot unit on the hill. Never park with it lit in a shed, garage or enclosed yard. See Carbon Monoxide Risks.
What’s Next
- Learn next: Fuel Preparation & Carrying — the last practical matter before a trip
- Combine with: Power Output Derating — why the power is lower