Nominal AFR

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p4ndiamond
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Nominal AFR

Post by p4ndiamond » Wed May 01, 2019 8:47 pm

How exactly does nominal AFR work in the tune ? If i swap fuels from E10 to E85 do i just place the correction in this scalar while leaving the tables such as base fuel etc. the same ?

I got a good explanation about this , but curious of its details and inner workings. i typically scaled the fuel injectors to swap fuels but never realized this was in the tunes. On BE they just call it "AFR

I tune based on the gasoline AFR scale no matter what fuel i use

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cgrey8
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Re: Nominal AFR

Post by cgrey8 » Thu May 02, 2019 5:14 am

The answer is very similar to what's explained in this tech doc:
Widebands measure lambda (λ), not AFR.

Once you've read that over, see if this explanation makes sense. The tune is using lambda in the background. It's the display tool (BE in this case), that is assuming the fuel's stoich AFR to be 14.64 in order to display a value in terms of AFR.

This is why most tuners don't view LAMBSE and WB feedback in terms of AFR, and prefer to see the value in its native form of lambda, so there's no conversion needed. It completely avoids the confusion.
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sailorbob
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Re: Nominal AFR

Post by sailorbob » Thu May 02, 2019 11:19 am

The 'Nominal_AFR' parameter (regardless of whether it is defined as a calibrateable parameter in a strategy or exposed as a hack) is the value the ECU uses to calculate the amount of fuel needed to achieve a stoichiometric A/F ratio for gasoline (I cannot recall seeing any stock calibration using an A/F ratio for a different fuel).

IMHO tuning in lambda has no advantage over tuning in A/F ratio as you still need to change both the 'Nominal_AFR' parameter and your desired A/F ratio when changing the fuel. For example, when tuning normally aspirated cars you would typically start off aiming for an WOT A/F ratio of 7.0:1 for E85 (a lambda of 0.72) but for E10 you would aim for an A/F ratio of 12.0:1 (a lambda of 0.85).

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cgrey8
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Re: Nominal AFR

Post by cgrey8 » Thu May 02, 2019 12:06 pm

Ok, so Nominal_AFR is similar to GUFx's Z_AFR scalar? If that's the case, then yeah some of what I said probably doesn't apply.

In GUFx, you set the Z_AFR to compensate for differences in actual AFR between different fuel being used. Prior to this setting, you change the injector slope values. But with the Z_AFR scalar, you can leave the injector parameters alone and adjust how the EEC converts the mass of air measured to the mass of fuel needed.

But once that value is set, I personally prefer to view LAMBSE and WB in terms of lambda, so I don't have to think about which fuel the AFR is relative to. Take a Wideband reading of 12.5. If the software collecting this value from the WB is assuming gasoline, then that's a number that's rich-of-stoich. For E85, it's lean-of-stoich. Knowing the AFR is 12.5 doesn't tell me much unless I know what AFR value would represent stoich. I am left with either making assumptions OR querying further for more information which might confuse the person further. But because of the math involved, the actual exhaust condition is either a little off or it's a LOT off particularly if the person has already said they are burning E85.

Compare this to if the AFR is expressed as a lambda value. There's far less confusion. I don't need to know or care whether the software is setup to believe the fuel is gasoline, E10, E85, or C16 race fuel. I know .85 is a "rich" mix in all cases. Now it may not be quite as rich as is required for the condition (i.e. target might should be .80). But relative to stoich, .85 is rich regardless of software settings. Thus I can more confidently assess the actual condition and advise accordingly.
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p4ndiamond
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Re: Nominal AFR

Post by p4ndiamond » Thu May 02, 2019 3:30 pm

Makes sense. so my thing is in the tune since i am utilizing AFR on the tables. do i have to change the tables to match this scalar or is it done automatically in the background per the conversion .

While i preference to use AFR across the board for ease of translation , i can certainly utilize LAMBDA

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