An engine running on producer gas makes 40-60% of its rated petrol power, and the main cause is arithmetic: a stoichiometric mixture of producer gas and air holds about 2.46 MJ per cubic metre against about 3.80 for petrol vapour and air, so at the same air throughput and efficiency the ceiling is about 65% (59-69% across the composition range). The remaining gap to the observed 40-60% comes from suction lost in the gas train, hot gas, slower burning and wet gas, so a conversion is never transparent.
Part of Engine Connection
Derating is the loss of engine power when petrol is replaced by producer gas, which can be bounded from the energy per volume of the mixture the engine inhales.
An Engine Is A Volume Pump With A Fixed Energy Per Cubic Metre
At a given speed an engine inhales a fixed volume of mixture per minute. Its power is that volume times the energy in each cubic metre of mixture times its efficiency. If the efficiency and the volume of mixture are unchanged, power scales with the energy per cubic metre of mixture, so compare the two mixtures.
Producer gas and air. Use the heating values worked out in Energy Content of Wood Gas (CO 12.63, H₂ 10.79, CH₄ 35.80 MJ/Nm³, from standard enthalpies of combustion) and the stoichiometric air in Mixing Valve & Air-Fuel Ratio:
| Case | Gas LHV, MJ/Nm³ | Air per volume of gas | Energy per Nm³ of mixture |
|---|---|---|---|
| Low ends | 4.28 | 0.90 | 4.28 ÷ 1.90 = 2.25 |
| Midpoints | 5.22 | 1.12 | 5.22 ÷ 2.12 = 2.46 |
| High ends | 6.15 | 1.33 | 6.15 ÷ 2.33 = 2.64 |
Petrol vapour and air. Lower heating value 44 MJ/kg and stoichiometric air-fuel ratio 14.7 kg/kg (standard values for petrol).
- Air per kg of petrol: 14.7 ÷ 1.293 kg/m³ (air at 0 °C) = 11.37 m³.
- Petrol vapour per kg, taking a molar mass of about 100 g/mol: 10 mol × 22.414 L = 0.224 m³.
- Mixture per kg of petrol: 11.37 + 0.224 = 11.59 m³.
- Energy per m³: 44 ÷ 11.59 = 3.80 MJ/Nm³.
Ratio, gas to petrol: 2.25 ÷ 3.80 = 59%, 2.46 ÷ 3.80 = 65%, 2.64 ÷ 3.80 = 69%.
This is a ceiling at equal throughput and efficiency, not a prediction. Petrol engines at full power run slightly rich, which raises the petrol figure and widens the gap a little, and a petrol engine’s evaporating fuel chills the charge so more mass enters; both are second-order and I have not quantified them.
Why Real Engines Fall Below The Ceiling
The parent gives 40-60% of rated power, so the observed value is about 62-92% of the 65% ceiling (40 ÷ 65 and 60 ÷ 65). The further loss comes from:
- Suction spent in the gas train. Each 1 kPa of suction used costs the engine about 1% of its intake charge (1 kPa ÷ 101 kPa). A cyclone, cooler, filter and mixer can add up to several kPa, and a loading filter adds more.
- Hot gas. Warm charge is less dense. Gas cooled to 40 °C instead of 20 °C is 313 ÷ 293 = 1.07 times as large per unit mass, which costs about 6-7% of the charge.
- Steam. Gas at 40 °C is 7.3% steam by volume (Moisture Separation), displacing fuel and air.
- Slower burn. The compromise in timing and compression (Engine Modifications) recovers only part of it.
Raising compression to 10-12:1 gains at most 7-12% in the ideal cycle, which would lift a 65% ceiling to about 70-73%.
What It Means For The Job
- A 10 kW petrol generator gives 4-6 kW on producer gas (the parent’s figure, consistent with the 40-60% range).
- To deliver a load of 5 kW, the rated engine must be 5 ÷ 0.6 = 8.3 to 5 ÷ 0.4 = 12.5 kW.
- For a vehicle, the loss is felt on hills and at speed; the road-load arithmetic is in Speed & Power Reduction.
A diesel in dual-fuel mode derates differently, because its air intake is not throttled and the gas replaces a fraction of its diesel; I have no sourced figure for it.
Common Mistakes
- Sizing the engine for the load. The engine must be 1.7-2.5 times the load.
- Quoting the ceiling as the result. 65% is an upper bound; plan on 40-60%.
- Blaming derating on a bad gasifier. Some of it is built into the mixture’s energy density.
Safety
The extra engine you need still runs on carbon monoxide
Producer gas is about 18-22% carbon monoxide by volume. A bigger engine on a bigger gasifier makes more of it. Never run a producer-gas engine in a garage, shed, lean-to or enclosed yard, even briefly, and never idle one in a vehicle cab or beside a sleeping area. See Carbon Monoxide Risks.
What’s Next
- Learn next: Speed & Power Reduction — the road-load consequence for a vehicle
- Combine with: Engine Modifications — recovering part of the loss