Why we don't display EQ ratio values as AFR

[Chris@HPTuners]

Hi all,

After a couple of posts on LS1tech in regards to this and seeming unlikeliness that the detail would be explained in public here's what i was talking about, and why we don't display the various tables such as PE, PE Adders, Openloop EQ etc. in AFR.

Displaying EQ ratio adders accurately is impossible
Various tables that modify the current EQ ratio by adding or subtracting to/from it can never be represented accurately. The best way to illustrate this is via a simple example.

Lets assume we are tuning the PE table along with the Adder vs ECT. This value adds to whatever PE vs RPM value is looked up giving the final PE EQ ratio.

PE table lookup = 1.1 (now you could display this as 14.7/1.1 = 13.4 AFR for gasoline fuel.) Now also say the ECT Adder is 0.03. The PCM takes both these numbers and adds them giving a final value of 1.1 + 0.03 = 1.13 (or 14.7 / 1.13 = 13.0 AFR). In this case for gasoline fuel the adder gives us a difference of -0.4 AFR from the base PE lookup.

Now lets repeat the example with the same 0.03 adder, but with a PE lookup of 1.3. So, the PE lookup gives us 14.7 / 1.3 = 11.3 AFR. And with the adder we get 1.3 + 0.03 = 1.33 giving 14.7/1.33 = 11.1 AFR. In this case the exact same adder gives us a difference of -0.2 AFR.

Which one should we display?

In the case of the PE table if the IAT Adder is used it gets even worse becuase you have 3 values adding together.

Why...
The reason for this is the inverse relationship between EQ ratio and AFR. When you add two eq ratio's and express them as AFR you need to do this:

AFRfinal = 14.7 / (X + Y) where X & Y are the two eq ratio's.

For those also familar with the process of working out the combined resitance of two resistor in parallel this is obvious.

Now to display the change in AFR due to the Y eq ratio adder it gets a little complex but it is:

AFRchange = 14.7/X - [ 14.7 / (X + Y)]

which is the same as

AFRchange = 14.7 * [ 1/X + 1/(X+Y) ]

There is no way to calculate the AFRchange due to Eq Ratio Y without knowing Eq Ratio X. You have to assume X is something. You really don't want to see the equation for when there is two adders...

The alternate approach is that you display the 0.03 adder in absolute terms as 14.7/0.03 or 490 AFR. Which is technically correct, but how many even advanced users can understand that 11.3 + 490 = 11.1?

We think for new users, it is far better to learn once the way the PCM does fueling, understand it, and then not have to learn it when you can't understand why your fueling isn't adding up.

The other big issue with displaying in AFR is for flex fuel vehicles where not just the lookup values for openloop, PE etc. are changing the EQ ratio (AFR). But, also the value of stoich itself can change as well! No doubt anyone trying to tune using AFR in these circumstances is in for some hard going...

Anyway, that's the reason we didn't implement this feature a loooong time ago and why we think that it's better to learn the right way.



Chris...

[jimmyblue]

I'm not sure I understand that logic but the
bottom line is, the PCM does all of its "thinking"
based on fuel air multipliers (EQ); fuel delivery
is the calculated product of airflow and other
sensors' input. AFR is only a human notation of
choice. If you want to understand what's being
done and why, stick to the computer's basis and
you'll both be happier. Fuel Air Multiplier PID is
a must-log.

[S2H]

not only that but some things have adders...and it can be confusing when a multiplier is actually a divider in most cases
Stoich/# = total ratio

if it was labeled 13.0AFR and you had a multiplier labeled ECT add 1.4AFR
you can see how many people would mistakenly add that to be 14.4AFR instead of being 11.6AFR
where as If its 1.12538 + 0.13582 = 1.2612
its not likely to get messed up by simple human error