When you get your vehicle hooked up to a gas analser, you'll see the 4 or 5 kinds of emissions you're interested in, and a lambda or AFR value derived from those emission values. I was wondering how the AFR is derived from those emissions. Surely it takes more in account than just the oxygen concentration like a oxygen sensor; the AFR doesn't change dramatically when only the oxygen concentration rises.

Is it just a mathematical function with the emission values as its variables, or is it more complicated? Assume my CO is at 5%, my HC is at 150ppm, my CO2 is at 11%, and my O2 is at 2%. Can I calculate my AFR?


Lambda is calculated using the Brettshneider equation. Brettshneider. It takes into account the mass balance of the constituents of the gasses in the exhaust from a combustion process. Oxygen concentration is only one of the variables. Oxygen sensors do not measure oxygen concentration, they work by switching from high to low voltage at the stoichiometric point.

There are numerous lambda calculators online.

  • Thanks for reacting. The link seems to be dead, but the brettshneider equation brought me a little further. But it seems to be a common geometrical formula, like bilinear interpolation. I can't see how that translates to calculating lambda. Care to elaborate a bit? By the way, there are online calculators indeed, but they don't show the equation they use, which is what i'm after. – Bart Aug 27 '17 at 21:01
  • @Bart I fixed the link. It is a white paper done by Bridge Analyzer Company that explains the equation better than I can. I know of no other formula in use today that is used to calculate Lambda. – Fred Wilson Aug 27 '17 at 23:34
  • Awesome, that was exactly what I was looking for! It indeed outputs the same answer as the online calculators i found, do. My question is essentially answered, but do also you know if petrol in different countries, or with different RON/MON ratings make a difference? Thanks again for the link! – Bart Aug 28 '17 at 9:05

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