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Originally Posted by Christ
How can you be sure that engine is running at or approximately at stoich? How do you know it's not running lean or rich, or burning a different fuel, or any of a few other variables?
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Because any modern engine has an ECU and sensors, that keep it in the proper range. If it was running far rich or lean, it would fail to meet its emissions standards.
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You still need a reference point to quantify a PPM count, no matter what it's in reference to.
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No you don't need a reference point. You know what goes in - air and fuel in a particular ratio - you know what comes out - exhaust gasses containing a certain percentage of unburned hydrocarbons - and therefore you know, from simple math, how much of the fuel is unburned. Remember conservation of energy? Well, this is just the compliment, conservation of mass.