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Reading AFR with CNG
Posted: Wed Dec 19, 2007 11:25 pm
by neongreen
Hi guys, I'm new to this forum. I have been thinking a lot lately about making my ranger bi-fuel. I have the general concept worked out, but I still have a lot of questions. The one I would like to address here is about WBO2 sensors.
If I understand correctly, O2 sensors read the amount of O2 in the exhaust and interpolate that into a voltage which and then be read as an AFR. Will an O2 sensor read fuels besides gasoline accurately? Are AFRs and O2 content in the exhaust consistently converse regardless of the fuel type?
Posted: Thu Dec 20, 2007 1:49 am
by devojet
WBO2's will work with most fuels. You will just need to know the stoichiomic ratio of the fuel you are using or work in lambda. I have used mine with both petrol and diesel.
Posted: Wed Dec 26, 2007 2:09 pm
by neongreen
I know that stoichiometric for CNG is 17.2:1. Did your display work correctly with diesel or did you have to modify something?
Posted: Thu Dec 27, 2007 2:17 am
by devojet
Yes my display worked fine with diesel. You will just need to check your manual for your wideband to check if there are any settings for different fuel types.
Posted: Thu Dec 27, 2007 5:38 am
by efahl
This seems like a good time to get on my podium and rant (again) about relative units versus absolute...
All of you are probably accustomed to tuning using kPa absolute for MAP by now, as opposed to inches/Hg and psig. Well, I've always thought (and said several times in the past) that we should all stop using AFR to measure mixtures and use the unitless lambda. What is the stoichiometric mixture for gasoline? "14.7 AFR" I hear shouted out by the hordes, but indeed that's pretty rare. Usually it's closer to 14.2, but it is different for every batch of fuel that the oil companies mix up. What's stoich with E15, E85, CNG, et cetera? Puzzlement ensues.
I can assure you, though, that when you read lambda = 1.0, you have a stoichiometric mix. Ditch your AFR gauge and replace it with a lambda gauge and you'll never have to worry about AFR again (I jest, of course, you will need to know an approximate mass ratio for calibrating required fuel). The "LED bar" on the front page of MegaTune is a lambda gauge of sorts, with the center being 1.0, so you all are probably fairly familiar with it already. If everyone starts using lambda numerically, the world will be a better place.
Eric