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Old 01-20-2009, 03:10 PM   #2 (permalink)
some_other_dave
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Nope, that's not really it.

The engine management figures out how much air the engine is taking in by using the MAP/MAF/whatever, the throttle position, intake air temp, and other sensors. It crunches those numbers to get an overall "how much air is going in" figure, and calculates a basic injector opening time. Other sensors, primarily the O2 sensor, then provide fine-tuning for that time value.

I do not know how much "authority" the O2 sensor feedback loop has, but it is definitely not the primary metering device. For one thing, most O2 sensors respond relatively slowly compared to the engine revolutions.

There are plenty of "special cases" built in, however. When the throttle goes from closed to more open, the engine adds extra "acceleration enrichment" so the engine doesn't bog or stumble when the throttle position changes. When the throttle is wide open, it adds "full throttle enrichment" for more power (after all, that's why most people push the pedal all the way down!). When the engine is cold, the mixture is also enriched because cold engines don't burn fuel as well as warmed-up ones.

In all of those cases, the engine goes "open loop" and ignores the O2 sensor feedback entirely. It just relies on the basic metering and temperature/RPM/whatever adjustments.

-soD
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