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Old 05-07-2012, 07:57 PM   #182
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Originally Posted by ChrisRo View Post
The stoich itself is a set value in the PCM at 14.079, but the car itself won't actually know what fuel it has in the tank. Since ethanol is a regulated deal, it just makes sense to include that in at least US calibrations.

The only thing it knows is the quality of the fuel in terms of adaptive spark and knock sensors. If it's 87 octane, it's not going to be aggressive as if you fill with 93 octane. The knock sensor strategy is intelligent enough to add/subtract based on audible feedback.

It also will know the commanded and actual lambdas. So, the car just needs a proper lambda, and needs knock sensors that can add/remove timing based on fuel quality. In those terms, yes - it will know the fuel quality.

The proper lambda for E85 and the proper lambda for 100% gasoline, for a given car/mods, will be the same. If that lambda too lean or too rich for gasoline, it's going to be too lean or too rich for E85. The only difference is when you speak about it in terms of air/fuel ratio. So really, the car doesn't need to know what fuel is in it. Make sense?

Hope this helps!
Here is some interesting reading on this subject:

In order to maintain proper fuel control, the PCM strategy needs to know the stoichiometric Air/Fuel Ratio for use in the fuel pulse width equation. On pre-2000 MY flex fuel vehicles, the percent alcohol in the fuel was determined by reading the output of the Flex fuel Sensor. The percent alcohol was stored in a register called Percent Methanol (PM). Although current alcohol-blended fuels only include ethanol, the percent methanol nomenclature has persisted. On 2000 MY and later vehicles, the Flex Fuel Sensor has been deleted and PM is inferred. The strategy to infer the correct A/F Ratio (AFR) relies on the oxygen sensor input to maintain stoichiometry after vehicle refueling occurs.
(see pages 62-63)
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