A/F Ratio?

[bjtheman1]

If you take your short term trim and multiply by 14.64 technically that's the a/f ratio command by the ecu right?

 

[BruceH]

If you take your short term trim and multiply by 14.64 technically that's the a/f ratio command by the ecu right?

No. The trims are corrections made by the o2 sensors to bring the a/f to 1 lambda in closed loop.

Long term trims are based off of trends from the short term corrections. They are predictive corrections. Short terms are fine tune corrections made on the spot.

 

[dysan]

I don't know if it changes when you are boosted but at WOT my STFT's are showing what is commanded. Mine is set to be .780 at WOT and that is what shows when logging at the track in the STFT fields. At part throttle, it is exactly what Bruce is stating.

 

[Swarzkopf]

If you take your short term trim and multiply by 14.64 technically that's the a/f ratio command by the ecu right?

No - the trim value is a % deviation of 'actual' AFR from commanded AFR.

So, if you're at idle and your STFT is 1.01:
- The PCM is commanding an AFR of 14.64
- The upstream O2 sensor is reporting an actual AFR that's 1% leaner, hence the STFT value of 1.01
- The acual AFR is 14.78 - 1% leaner then commanded 14.64
- The PCM attempts to add 1% fuel to get AFR back to commanded

Multiplying the short term trim by 14.64 at idle, part throttle, or any time the PCM is commanding 14.64 AFR will give you the ACTUAL AFR that upstream is O2 seeing, NOT the commanded AFR. As stated, the fuel trim is the deviation in AFR observed by the O2 sensors.

 

[bjtheman1]

Great info guys, really appreciate it.

 

[lito]

Well, the short term trims, as ford use them in these cars are actual commanded lambda, always. So, in a way, the initial statement is true. The long term is just a learned value that is used as a base correction, if the adaptive is not disabled, the initial statement is still true, over the learned correction.

At the end both statements (OP and Bruce's) are somehow basically true.

The thing is maybe what you define as commanded, and I think that is where the interpretation gets mistaken.

When you are at closed loop, your are not commanding lambda 1, you are targetting lambda 1 instead, the process commands different values to achieve this target. You only command an specific value at open loop, that is where you can actually talk about it commanding 1.000, 0.900, .820 or whatever.

When your tune is off, and adaptive is or either disabled or hasn't learned anything yet, PCM will command any value within the allowed (0.6-1.4 in stock form) to achieve lambda 1.000 target switching, so it can be commanding .820 to do it, for example, so effectively it is commanding 12AFR(gas) to see lambda 1.

Hope these clears the concepts a little further.