Stoich VE tables vs. Commanded AFR

[cscott]

If the VE tables are set to stoich, and commanded AFR goes to ~13 under PE... how is a non-stoich AFR achieved?

My understanding is that the OEM O2 sensers/ECM are narrow band, and only work close to 14.7 AFR. So I don't see how a PE enriched AFR happens. Is the idea that the primary VE table is stoich and the secondary VE table tuned to the desired PE AFR?

[SiriusC1024]

When PE occurs fueling is based off VE table for the V component of PV=nRT where n is solved for. n moles of air is calculated then n/stoichiometric moles fuel is added. PE is OL. If you disable closed loop operation then fueling is based entirely off this without being adjusted by O2 sensor feedback.

CL fuel trims correct for errors in the calculation.

The secondary VE table is referenced when the MAF is failed.

[cscott]

That makes sense... thank you. However that didn't answer my question. I'm sure I asked the question poorly being I'm misunderstanding something fundamental. I'll ask another way...

Why does there seem to be a capability of fueling to an AFR other than stoich and how does it work? e.g. PE criteria is met (TPS/RPM/temp). PE EQ ratio evaluates to an AFR 13.0. Commanded AFR becomes 13.0. How does that happen? What/how is fueling adjusted to become 13.0? How does the ECM know that 13.0 has been achieved?

Based on your answer... maybe it works like this?
VE table is calibrated is whatever PE/WOL fueling should be. When in CL, MAF updates fueling from STFT -> LTFT to become stoich. When PE is called for, fueling is based solely of VE. The actual AFR generated by evaluating for the PE EQ ratio doesn't actually impact fueling itself.

Asked even simpler. Why does the Commanded AFR exist?

[07GTS]

ve table should be calibrated to be stoich in a perfect world, PE is a multiplier of stoich so add fuel, the 14.68 afr divide by PE 1.125 is 13.04 afr (pe is inverse of lambda) u need a richer afr to make power safely

[cscott]

I think I get it now. Seems to be much simpler than I initially thought.

PE ratio is used to adjust the fueling based on the VE table value (and any LTFT adjustments). The fuel value in a given VE table cell is 10. PE ratio is 1.1. When that cell is used to fuel in a PE state, the fuel value becomes (10 * 1.1).

Thanks to @07GTS and @SiriusC1024

[SiriusC1024]

You're asking how PE works since the narrowbands are confined to the stoichiometric ratio? The answer is PE is OL operation.

An easier way to look at it is MAF. The MAF curve is Hz vs Airmass directly (the n=PV/RT for VE). OL operation follows the MAF curve to calculate the stoichiometric ratio. PE is a modifier to the stoichiometric value.

[gtstorey]

Another way of looking at it is the ECM has to assume everything is working like it should when in PE because the narrowband can’t correct it. A certain amount of fuel is added for a certain amount of air. It doesn’t have anything to tell it that it isn’t correct if something is off. This is why a wideband is necessary when tuning WOT.

[cscott]

Makes perfect sense now. Thanks everyone.