Making sure my understanding of STFT is right.

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redtopracer
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Making sure my understanding of STFT is right.

Post by redtopracer » Tue Dec 17, 2019 10:59 am

Ok, say my lambse is .859 and my STFT1 is showing -10%.
That would mean the PCM believes i am running 0.9449 lambda on bank 1 right?



Lambse*STFT%+lambse=lambda?


If not what would be the right formula?


Lambse refers to requested lambda

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cgrey8
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Re: Making sure my understanding of STFT is right.

Post by cgrey8 » Tue Dec 17, 2019 11:53 am

It depends.

At its most basic, LAMBSE is the value that the EEC believes should be the actual AFR.

Now in practice, how you think of LAMBSE changes based on whether the EEC is in Open Loop or Closed Loop fueling mode.

When in Open Loop, LAMBSE is thought of as the commanded AFR. This IS the lambda the EEC is trying to attain. And being this is Open Loop, there will be no adjustment or verification based on actual results. The code is flying blind.

This is where you, as the tuner, come in with datalogs of your EEC and Wideband O2 sensor(s). Running conditions that show a significant error between LAMBSE (commanded) and Wideband (actual) require that the tune be adjusted to get the actual closer to the intended/commanded.

Typical candidates are:
  • Injector Slope/BP/Offset parameters
  • MAF curve
  • Startup and Stabilized Open Loop Fueling Tables
  • Accel/Transient enrichment
  • Adaptive Learning (i.e. KAMRF) controls
A deeper analysis into why commanded and actual are different has to be made to determine which of these parameters (and possibly others) that need to be modified, and by how much, to reduce the error without inadvertently increasing the error at other running conditions.


When in Closed Loop, the EEC is using the HEGO(s) as feedback as to how accurate it's LAMBSE value(s) are. The EEC is constantly adjusting the LAMBSE value(s), within limits, to attain the HEGO switching behavior. So under stabilized Closed Loop operation, LAMBSE values give you an indication as to how far off the EEC is from it's target. In the case of Closed Loop, the target is always somewhere very near 1.00 lambda...the AFR HEGOs switch at, so you don't really think of LAMBSE as a commanded AFR like you do with Open Loop condition. Also note, it doesn't matter what your WB is reporting. What matters is what the HEGOs are reporting. This is why the way you tune Closed Loop and Open Loop conditions is just different. And thus the way you think about LAMBSE is different even though the actual meaning of the LAMBSE value is exactly the same regardless of the mode.


As for the formula to convert LAMBSE to STFT, I think it's:
(1.0 - LAMBSE) * 100

So if you have an Open Loop LAMBSE value of .90, you have STFT as follows:
(1.0 - .9) * 100 = 10%
The EEC is intending on a burn condition with 10% more fuel than there is oxygen.
...Always Somethin'

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redtopracer
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Re: Making sure my understanding of STFT is right.

Post by redtopracer » Tue Dec 17, 2019 1:14 pm

GAHHHHHH that didnt help!!!!!!!lol


ok this is for tuning in forced CL with narrowband only and widened adaptives. stock injectors and MAF so my hardware datapoints are known and verified.system is setup to pull all commanded fuel from base fuel fn1362/fn1307



If my commanded lambse in base fuel is .859 and I were showing -10% STFT1 in CL would that be -10% off of a lambda of 1 or the commanded .859?

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cgrey8
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Re: Making sure my understanding of STFT is right.

Post by cgrey8 » Tue Dec 17, 2019 1:48 pm

If you are correct that this is forced Closed Loop, then the Base Fuel tables are irrelevant. Those are Open Loop tables. Closed Loop has an implicit target of 1.00 lambda all the time. You can veer slightly from that, on average, by altering HEGO Bias values. But Closed Loop wants to see HEGOs switch. So unless you have also altered the HEGO switchpoint voltage, the Closed Loop action will have the HEGOs running actual conditions slight rich and slightly lean. The Bias just alters how aggressive the Closed Loop algorithm is at adjusting rich-when-lean & lean-when-rich.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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