Combustion Lambda and Air-Fuel Ratio
The lambda and air-fuel ratio a modern analyzer shows beside excess air, from the same flue oxygen.
Example
You enter
- Fuel natural_gas
- Flue-gas O2 (%, dry, air-free) 3
You get
- Lambda 1.1676
- Excess air 16.8%
- Actual air-fuel ratio (by mass) 20.0827
Details, formula, and sources
lambda = 20.9 / (20.9 - O2), excess air = (lambda - 1) x 100, AFR_actual = lambda x AFR_stoich (17.2 natural gas, 15.5 propane, 14.5 #2 oil, by mass). A gas appliance at 3% O2 is lambda 1.17, 16.8% excess air, 20.1:1 AFR against the 17.2:1 stoichiometric - the same tune three ways. Lambda = 1 + excess_air/100, the number the instrument happens to display. A tuning aid, not a certified combustion test.
lambda = 20.9 / (20.9 - flue_o2_pct); excess_air_pct = (lambda - 1) x 100; afr_actual = lambda x afr_stoich(fuel).
Combustion-analysis practice lambda and air-fuel ratio, by name.
The lambda and air-fuel-ratio relations are standard combustion-analysis practice; the analyzer and appliance manufacturer instructions govern the actual tune.
Estimate. AHJ and licensed professional govern.
Field names used by the API: fuel, flue_o2_pct, lambda, excess_air_pct, afr_actual
- Lambda from O2 lambda = 20.9 / (20.9 - O2) = 1 + excess_air/100; the same physics excess-air-o2 reportscombustion analysis practice
- Stoichiometric AFR by mass, 17.2 natural gas, 15.5 propane, 14.5 #2 oilfuel stoichiometry