A vehicle gasifier is sized by the gas the engine can swallow at full throttle: a 2-litre engine at 3,000 rpm takes in at most about 68-85 normal cubic metres of gas an hour, and the carrier’s 75-100 mm throat for that engine is 44-79 cm², loaded 3-10 times harder per square centimetre than the 5 kW stationary example. The carrier’s claim of 80-120 m³/h at highway load exceeds what a 2-litre engine can ingest at 3,000 rpm, so it describes a higher speed or a flow the engine cannot reach.
Part of Vehicle Gasifier Design
Sizing a vehicle gasifier means finding the engine’s maximum gas demand and choosing a throat and firetube that serve it without running cold at light load.
The Engine Sets The Maximum Gas Flow
A four-stroke engine inhales its displacement once every two revolutions. At n rpm and displacement D litres the intake is D × n ÷ 2 litres a minute, which is 0.03 × D × n cubic metres an hour. Gas is a fraction f of the mixture, 47% at the midpoint composition (Mixing Valve & Air-Fuel Ratio), and the volumetric efficiency VE (the fraction of the displacement actually filled) of a gas engine at full throttle is taken as 0.8 (an assumption). So:
Gas at full throttle = 0.47 × 0.8 × 0.03 × D × n.
- 2.0 L at 3,000 rpm: 0.03 × 2.0 × 3,000 = 180 m³/h of mixture; gas = 0.47 × 0.8 × 180 = 68 m³/h (85 if VE = 1).
- 2.0 L at 4,000 rpm: 240 m³/h of mixture; gas = 90 m³/h (113 if VE = 1).
- 1.25 L at 3,000 rpm: 42 m³/h. 3.25 L: 110 m³/h. 4.0 L: 135 m³/h.
These are volumes at intake conditions; at normal conditions they are roughly 10-15% lower. The carrier says a 2-litre engine “at highway load” consumes 80-120 m³/h; that needs about 3,500-5,000 rpm at full throttle, not a cruise, and the same carrier’s own fuel table implies far less (see below).
Hearth Load: The Vehicle Table Against The Stationary Example
Hearth load is gas flow divided by throat area. From the first carrier, the 5 kW stationary unit is a 9-10 cm throat (63.6-78.5 cm²) at 12.5-20 Nm³/h, a load of 0.16-0.31 Nm³/h per cm². For the vehicle table:
| Engine | Throat | Throat area | Full-throttle gas at 3,000 rpm | Hearth load |
|---|---|---|---|---|
| 1.0-1.5 L | 60-75 mm | 28-44 cm² | 34-51 m³/h | about 0.8-1.8 |
| 1.5-2.5 L | 75-100 mm | 44-79 cm² | 51-85 m³/h | about 0.7-1.9 |
| 2.5-4.0 L | 100-130 mm | 79-133 cm² | 85-135 m³/h | about 0.6-1.7 |
At cruise the carrier’s wood use (25-40 kg per 100 km at about 80 km/h, 20-32 kg/h of dry wood at 2.5 Nm³/kg) gives 50-80 Nm³/h through 44-79 cm², a load of 0.6-1.8. For the 2.0 L case the full-throttle load is 68 ÷ 78.5 = 0.86 to 68 ÷ 44.2 = 1.5, which is 2.8 times the top of the stationary range (0.86 ÷ 0.31) to 9.6 times the bottom (1.5 ÷ 0.16), so 3-10 times; across the whole table the ratio is about 2-12. Either way the vehicle throat carries several times the stationary example’s load. Both cannot be a single “correct” loading for the same hearth design. I could not find a sourced maximum hearth load, so I cannot say which is right; the practical reading is that vehicle units were built to the higher figure and accepted a narrower range of load, while the stationary example is conservative. Check any build for tar at the loads it will actually run.
The Idle Problem
At idle (about 800 rpm, throttle closed), a 2-litre engine fills only about 30% of its displacement (an assumption):
- Mixture: 0.03 × 2.0 × 800 × 0.3 = 14.4 m³/h; gas = 0.47 × 14.4 = 6.8 m³/h.
- Against the full-throttle 68 m³/h, that is a turndown of about 10 to 1, and a load of 6.8 ÷ 60 cm² = 0.11 per cm², below even the stationary example.
A throat sized for the peak runs cold at idle and makes tar, which is why the second carrier says to keep the revs up, and why an engine left idling on gas is a tar source.
Method: Sizing A Unit
- Take the engine’s displacement and the highest rpm you will hold under load.
- Compute its maximum gas intake with the formula; this is the most the throat ever has to supply.
- Pick the carrier’s table row and check the throat area gives a hearth load in the range the table implies, about 0.6-1.9.
- Keep the throat one third of the firetube.
- Build, run outdoors, flare, and tune the nozzles (Air Nozzle (Tuyere) Design).
Common Mistakes
- Sizing for a petrol engine’s top rpm. The gas supply caps power long before the engine’s limit.
- Using the stationary hearth load for a vehicle. The throat comes out several times too big and runs cold.
- Ignoring turndown. Idle and downhill stretches are where tar forms.
Safety
A bigger gasifier is a bigger vessel of carbon monoxide
The gas is about 18-22% carbon monoxide by volume. Never light, test or idle a vehicle gasifier in a garage, shed or enclosed yard, and never leave one idling beside a cab or sleeping area. A throat that runs cold at idle makes tar, and tar fires in the filter or gasifier were the most common breakdown in the carrier’s history. See Carbon Monoxide Risks.
What’s Next
- Learn next: Weight & Space Constraints — what the sized unit weighs and takes up
- Combine with: Hearth Constriction — the throat design itself