Producer gas needs about 0.90-1.33 volumes of air per volume of gas for complete burning, so gas makes up roughly 43-53% of the mixture the engine inhales, far more than petrol vapour does. The mixing valve, a butterfly or sliding plate where the gas line meets the air intake, holds that ratio; too much gas gives misfires and too much air a slow, hot burn that backfires, and the right setting moves with load.
Part of Engine Connection
The mixing valve sets how much producer gas and how much air enter the engine; the stoichiometric ratio follows from the gas composition.
Work Out The Ratio From The Gas
Each combustible component needs oxygen: CO and H₂ need 0.5 mol O₂ per mol, CH₄ needs 2 mol. Air is about 21% oxygen, so air required per volume of gas is 0.5 ÷ 0.21 = 2.38 for CO or H₂ and 2.0 ÷ 0.21 = 9.52 for CH₄.
| Case | CO / H₂ / CH₄ | Air per volume of gas | Gas fraction of the mixture |
|---|---|---|---|
| Low ends | 18 / 12 / 2% | (0.18 + 0.12) × 2.38 + 0.02 × 9.52 = 0.90 | 1 ÷ 1.90 = 53% |
| Midpoints | 20 / 15 / 3% | 0.35 × 2.38 + 0.03 × 9.52 = 1.12 | 1 ÷ 2.12 = 47% |
| High ends | 22 / 18 / 4% | 0.40 × 2.38 + 0.04 × 9.52 = 1.33 | 1 ÷ 2.33 = 43% |
Composition ranges are from the parent and are explained in Syngas Composition. Because the composition drifts with load and fuel moisture, the correct valve setting drifts too.
What The Engine Sees
- Too rich (too much gas): the surplus gas does not burn completely, power falls and exhaust carbon monoxide rises. The parent says it causes misfires.
- Too lean (too much air): the mixture burns slowly and can still be burning when the intake valve next opens, so it backfires into the intake, and the engine runs hot, as the parent says (overheating). The intake system on a gas engine is full of a flammable mixture, so a backfire is a real hazard.
- Wet or dirty gas: shifts the effective ratio and adds misfires regardless of the setting.
Because the gas supply is a large part of the intake, the whole air path matters. Gas and air are mixed in the intake pipe, ahead of the throttle, so one valve controls power.
Method: Setting The Valve
- Start on petrol, bring the gasifier to good gas, then open the gas valve gradually while closing the petrol.
- With the engine on gas at a steady speed and light load, move the mixer toward more gas until the engine starts to misfire, then back off until it runs smoothly; then move toward more air until it begins to run hot or weak and come back.
- Choose the middle of that smooth zone. Re-check at higher load, where the correct point moves.
- A butterfly valve is simple; a sliding plate gives finer control. Fit a lock so vibration does not shift it.
- Keep the mixer on short, welded or well-sealed pipe.
How Much Gas Is Too Much To Leak
A gas and air mixture burns or explodes between lean and rich limits. Using the standard flammable limits in air (CO 12.5-74%, H₂ 4-75%, CH₄ 5-15%) and the midpoint gas, a Le Chatelier estimate:
- Combustible share of the gas: 20 + 15 + 3 = 38%, split CO 0.526, H₂ 0.395, CH₄ 0.079.
- Lean limit of the combustible part: 1 ÷ (0.526 ÷ 12.5 + 0.395 ÷ 4 + 0.079 ÷ 5) = 1 ÷ 0.1567 = 6.4% in air.
- As gas: 6.4 ÷ 0.38 = 17% gas in air.
So gas becomes flammable in air at about one part in six, a small fraction of the 47% the engine inhales. This is my estimate from standard limits, not a measured value for any gasifier; the nitrogen and carbon dioxide in the gas raise the real limit a little.
Common Mistakes
- Setting the mixer at idle only. Load changes the right point.
- Chasing a smooth idle with an air leak. An air leak on the gas side dilutes the gas and can fire inside the line.
- Using flame at a joint to find a leak. Find leaks with soapy water on a pressurised test, never a flame.
Safety
A flammable mixture is deliberately built in the intake
The gas is about 18-22% carbon monoxide by volume, and mixed with air it is flammable above about 17% gas, so any space where gas and air meet can burn or explode. Never run an engine on producer gas indoors, in a garage, in a shed or in an enclosed yard, and keep sparks and open flame away from the gas line. Leaks sound and smell like nothing: use soapy water on a low-pressure test, never a flame. See Carbon Monoxide Risks.
What’s Next
- Learn next: Engine Modifications — timing and compression for a slow-burning gas
- Combine with: Power Output Derating — what the mixture’s energy content means for power