Under normal operating conditions where the upstraem sensor is at lambda, is the downstream sensor also at lambda? Unfortunately my car is no longer stock, so I can't log the data and see for myself; additionally most of the gas stations around here have labels on the pumps stating that the gas may contain up to 10% ethanol, which would throw the stoich value off.
I dont think the down stream sensor is at lambda. I mean the fuel is going through a catalyst which should burn off extra fuel and other "pollutants". It should be leaner than the upstream sensor.
1994 Mazda Miata turbo, aero, hoosiers Class=SSM
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Lambda is Lambda regardless of the fuel. Lambda = 1.0 is stoich whether gasoline or ethanol or any blend of the two.
Don't confuse Lambda with AFR.
'12 Camaro T3 2SS/RS LS3 M6, SLP TVS 2300, Flex Fuel
Originally Posted by Iam Broke
What Broke said. Lambda is Lambda (the point as which fuel combustion is most "efficient") regardless of what the AFR is.
Your after cat o2 sensor should always read leaner than the front o2 unless the cat is not performing its job of converting 2NO into O2 and N2, 2CO into O2 and CO2, and hydrocarbons into water.
Well its not really most efficient its where the math works out in the chemistry for what should be a complete burn. As in there is exactly enough oxygen to combust 100% of the fuel. It also happens to be the most happy medium between emissions (N02, CO2, CO, etc), fuel economy, and power
1994 Mazda Miata turbo, aero, hoosiers Class=SSM
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Don't know what motor you're talking about but most post-cat sensors are O2 sensors. The LNF has a Lambda sensor pre-cat and O2 sensor post-cat. They don't really need to see exact mixtures post-cat, mostly just rate of change.
And it isn't necessarily "leaner" than the front, it can go high and low depending on what's going into the cat. What you want to see with the post cat O2 is slow, steady voltage swings, not fast switching like a pre-cat O2 would do.
I think by lambda he meant narrowband. Ie a sensor that senses if you're at lambda or above or below.
1994 Mazda Miata turbo, aero, hoosiers Class=SSM
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Lambda sensor=wideband sensor.
O2 sensor=narrowband sensor.
Yeah I guess I'm not really sure what he's asking.
02 sensors are all lambda sensors
narrowbands only report yes and no (rich or lean) not how far rich or lean you are from lambda 1
widebands show how rich or lean or how far off from lambda 1 ( stoich) the exhaust is
PB's 1/4 mi 12.21 117.75 trap ,1/8 mi. 7.779 93.99trap , 1.949 short time (FWD W body)
Well, not really.
All O2 sensors are not Lambda sensors, unless you mean Lambda for gasoline only. If you were running something other than gasoline the O2 sensor (narrowband) would not be usable to control mixture to "Lambda".
Narrowbands do not report "yes and no", they have a range from roughly 0 to 1 volt, with .450v being the crosspoint for Stoich. They do in fact read rich or lean, but are only accurate in a narrow range around stoich for gasoline. Changing PE for instance will most definitely show different O2 values at the high end of the voltage range, but wouldn't be accurate enough to really do anything more than give you an idea of how rich it is. It will absolutely tell you more than "yes or no" though.
Last one is somewhat correct... Widebands, or Lambda sensors, read from roughly 0 to 2 volts, with 1 volt being "Lambda" for gasoline AND other fuels with different stoich values. It doesn't really tell you how "far off" of stoich you are, unless you do the math and subtract the reading from 1.
I know some of this is semantics and what people are used to calling things, but it seemed like you were correcting me in your comments. The state I live in has instructed and licensed me for the last few decades to know this stuff. I don't believe the OP's question was ever answered because people were getting confused with terminology. That's why I said I wasn't sure what he was actually asking.
I'm looking for an easy way to determine the stoich value of an ethanol blend; but there is too much conflicting info out there.
For example a lot of gas stations around here have signs stating it may contain up to 10% ethanol; well I seen stoich values for E10 on different forums to be any where from 13.8 to 14.3 and the best answer I've seen is 14.1.
Another forum posting states that stoich is the point where there is no oxygen left in the exhaust, but somewhere says it is where there is the least amount of by products in the exhaust, and the max power point is where there is no oxygen left.
What rnjmur and Leafy say agrees with the service manual that the cat does convert byproducts to O2; however it seems to make sense that the more byproducts there are the leaner the downstream sensor will read, meaning that you are not at stoich.
So I was wondering what kind of voltage others are seeing on an ecotec engine on the downstream sensor at idle?
Ok the easy way to determine stoich? Get the car into closed loop. At the point that the narrow band is at the cross over point IE reading stoich, look at your fuel flow value and your air flow value and figure it out. Dont forget to account for the changing density of fuel with heat (and take said temp measurement at the fuel rail).
1994 Mazda Miata turbo, aero, hoosiers Class=SSM
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I have also seen variations on stoich for particular e blends when searching the internet.
I trust the values in GM flex fuel tunes and you can do the math for your blend from there...
here is a copy & paste of the data in the stoich cells of a flex fuel vehicle in the repository
stoich value e%
14.68100 (0%)
14.32168 (6.25%)
13.97952 (12.50%)
13.60797 (18.75%)
13.25566 (25%)
12.92114 (31.25)
12.56442 (37%)
12.19048 (43.75%)
11.83815 (50%)
11.47339 (56.25%)
11.13043 (62.50%)
10.77895 (68.75%)
10.42239 (75%)
10.06388 (81.25%)
9.70616 (87.50%)
9.35160 (93.75%)
9.00220 (100%)
PB's 1/4 mi 12.21 117.75 trap ,1/8 mi. 7.779 93.99trap , 1.949 short time (FWD W body)
Thanks Murphinator; that wasn't exactly what I was asking for, but it was what I really needed.
A true E10 mixture would be stoich at 14.15 AFR. More than likely they use different mixes of ethyl alcohol in the different seasons as well as each station possibly using different mixes, i.e. so in summer you maybe at E10 and winter you maybe E5. Unfortunately there is no easy way to get the ethyl alcohol ratio without being able to actually to test the fuel in a laboratory. Witha wideband O2 you maybe able to guestimate ethyl alcohol mixes if you change your stoich and look at your average fuel trims, but that would be not be extremely accurate.
The downstream O2 should always read lean if the catalytic converter is functioning properly. It is not necessarily a higher concentration of byproducts, but a ratio of O2 to the other gasses found in the exhaust. I would guess that the actual voltage of the downstream sensor should be around .2V and never higher than .4V at idle. Anything higher than .4V would indicate the catalytic converter was not functioning properly (lambda of 1 or lower/AFR of 14.7 or richer) and the car would produce a code.
Murphinator; you wouldn't happen to know how a flex-fuel vehicle determines the ethanol blend it is running?
The older ones used a sensor in the gas tank. The newer ones use the narrowband o2 sensor and the math I suggested you do.
1994 Mazda Miata turbo, aero, hoosiers Class=SSM
Take my drop box referral and we both get an extra 250 Mb free!
Come on people, I don't know where you're getting this info, but it's wrong.
First, there's plenty of ways to check ethanol or alcohol content of any fuel. The cheapest is getting a graduated beaker (sp) and adding water to the fuel, mix it up and wait for the water (and alcohol) and gas to separate. There are also electronic sensors that are available, I use a Kent Moore product that you pour a sample of fuel in, look at the frequency it reads with a DVOM and it will tell you the exact percent of alcohol. Takes 2 minutes and can be done at the gas pump.
Post cat O2 readings... As I said in my post above (sometimes I wonder if anyone reads what I post. I often think people's opinions of the average dealer tech is so bad I should have never used "gmtech" as my screen name. I'm NOT your average dealer tech and I know a lot of them are dumb#sses.) post cat O2 readings can be ANYWHERE between 0 and 1 volt and be perfectly normal, and does NOT mean the cat is no good. What the ECM is looking at to determine if the cat is good is the frequency of rich/lean transitions. Think about it, the pre cat O2 sensors switch between rich/lean very fast and constantly. If the cat weren't there or working, the post cat O2 would also switch between rich/lean very quickly and constantly. Actually, having the post cat O2 sitting at 0 or .1 or .2 volts isn't that common, even at idle. It's completely normal for the post cat O2 to be reading .6 or .7v at idle or cruise. The only times you can really say when it's "supposed" to be reading a certain value is at WOT/PE, where between .85 and 1 volt is normal, and when you're in DFCO, where it will go full lean. (Usually.)
So, to answer the OP's specific question, which I don't think anyone did...
It could be anything from .2 or so to .7volts. Rarely 0 or 1 volt. If I had to say what I've seen most of the time at idle I'd say sometimes .25 and sometimes .65.So I was wondering what kind of voltage others are seeing on an ecotec engine on the downstream sensor at idle?
Sorry if I came off as a bit of a d!ck, just saying if you're not 100% sure you have the correct answer, you probably shouldn't post. Misinformation seems to spread faster than truth.
this question has allready been answered but it may be worth mentioning that in tandem with an ethanol sensor from a flex fuel vehicle there is a company called zeitronix that provides constant e % monitoring with a digital guage
here is a link
http://www.zeitronix.com/Products/ECA/ECA.shtml
PB's 1/4 mi 12.21 117.75 trap ,1/8 mi. 7.779 93.99trap , 1.949 short time (FWD W body)
OK, here's a question for any Cobalt/ecotec owner and/or tuner; the PCM will test your cat's efficiency (P0420 CEL) whenever at idle with clutch depressed for several seconds(manual) or idle car in gear (automatic), by first making the AFR richer by about 1 unit; i.e. from 14.7 to 13.7, and then lean by about the same and monitoring the downstream sensor output. If you log the data at this time you'll see it; so what kind of voltage are you seeing on the downstream sensor during this test?
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