I’ve been goofing around with quadrajets for a couple of decades. In the quest for perfection I got a dual sensor AFR gauge from Innovate a couple of years back. Since then I’ve been tuning in AFR’s. California gas formulations contain an unknown amount and probably ever-varying amount of alcohol, so I’ve recently switched to Lambda from AFR’s.

My reasoning is that most of the documentation for AFR’s assumes your gasoline doesn’t contain alcohol and gives ranges from 11.5 - 17:1 AFR as acceptable with a stoichiometric target of 14.7:1. Unfortunately the alcohol in our gas changes the stoichiometric ratio down to 14.1:1. At first I just sort of ran it richer, adjusting AFR’s down by about .5 AFR to “account” for the unknown alcohol content. Generally my idle AFR’s were smoothest at about 13.8:1, just a tick rich. WOT AFR’s would touch 10.8:1 then settle out in the high 11’s or 12:1. Cruise seemed best at about 15.1- 15.3:1. My mileage went up and the engines ran better.

Eventually I read that AFR’s as reported from wideband 02 sensors are really just scaled lambda readings and that the lambda scale is fuel independent. Am I correct on these points? Meaning if I switch to lambda, since it’s reading exhaust oxygen content, it’s a better scale to tune on?

With AFR’s I feel like I was playing a guessing game on this week’s fuel. With lambda I think I don’t have to account for varying alcohol content in the fuel. Is that right?

So now in switching to the lambda scale without major changes to my qjets, I’m seeing smooth idle lambda values around .93. WOT seems strongest at .80 to .85, cruise lambda seems to run .98 to 1.07.

Directionally I think these are correct, since lambda <1 means rich and lambda >1 means lean, but I’m wanted see if any of you can confirm that my understanding is correct and that I’m on the right track.

Hope this is interesting to others as well.

Cheers
Chris

 

Yes, you are correct on your points regarding AFR & Lambda. And, yes lambda is independent of fuel. I followed your discussion on a previous thread & commented on that discussion. AFR is dependent on fuel while λ is not dependent on fuel. In an absolutely perfect stoichiometric equation the complete combustion (expiration) of all fuel source would equal 14.7:1. That would be the exact ratio at which enough air is consumed to expire (burn off=consume=combust) all fuel (at one atmosphere of air pressure). Bear in mind, this is a theoretic perfect mathematical model based upon the stoichiometric oxidation equation. Stoichiometric equations (reactions) are based upon stoichiometirc coefficients, of which is beyond this discussion. A perfect linear relationship would exist anywhere along a line of combustion if AFR = λ (which would be the equivalent of saying any point along that line (linear relationship) every point represents 14.7:1 [or simply zero; λ = 0]). I used to perform bomb calorimetry experiments against various compounds determining stoichiometric coefficients (in a bomb calorimeter). It's a very interesting form of experimental chemistry.

Simply stated, you're on the correct path thinking λ is a more exact scale than AFR.

https://classicoldsmobile.com/forums...-today-120989/

 

Norm
Many thanks for your guidance and confirmations.

I wasn’t sure I was getting it, but it’s great to know my knowledge is coming along.

Funny you mentioned bomb calorimetry, my Dad ran bomb blast effect simulations for the military in the early 50’s when they didn’t have enough to use in testing. Math was the next best way to figure out what the new-ish weapons would do, and therefore, how to use them. He always said math was a great way to begin learning about topics you know nothing about. Gone 20 plus years and he’s still right.

Cheers
Chris

 

Keep in mind since 14.7 is stoich, or a lambda of 1 for straight gasoline... then 12.5:1 equates to a .85 lambda for WOT, so you are well within reason.