While at anything but PE mode, how does the PCM know how far off the mixture it is to adjust the LTrims?

 

Because the NBO2 sensors know where 14.7 is. When the AFR is at 14.7, the O2 sensors report a voltage around .45~.55 mV. If the PCM sees a voltage higher than that, it knows the fueling is rich. If it's lower, the PCM knows the AFR is lean. Short Term Fuel Trims fluctuate the AFR several times per second slightly richer and slightly leaner than a 14.7 AFR. If the fueling calculation is wrong, the PCM will see that in the average of the STFT's over a period of time. Those averages are then converted to LTFT's to help re-center the STFT's around a 14.7 AFR. This works over and over again in a continuous loop - hence why we call it closed loop operation.

 

I thought under normal driving the O2s just fluxuate in voltage from high to low or is that only at idle?

 

the purpose of the narrowband o2's is to keep the stoich point. They switch up and down, both to know where stoich is, and for proper cat operation.

Ryan

 

The O2's are fluctuating because the AFR is fluctuating as a result of the STFT's.

 

yup, the narrowband O2s don't infact know what 14.7:1 is... they know what stoich is which for 100% gasoline cars is 14.7 ... the PCM corrects fueling reactively not proactively... so it sees a reading that's too high or too low and it'll change fueling appropriate so the average is stoich (this is why the stock narrobands can also work with E85 situations... stoich is different, but the PCM just cares about "do I add more fuel or take fuel away")

 

totally reminds me of this http://www.micom.net/oops/USAF%20Training.wav

 

The stoich mixture value is a data value in the tune file.
NBO2s cross threshold at an exhaust oxygen level that
is stoich or close enough. The threshold is another field
of values w/ airflow mode etc. The realism of O2 sensor
switching is affected by other exhaust oxygen sources
like leaks and shoot-through, by temperature.

The PCM looks for crossings in time and figures the
average short term. Error updates STFTs and out-of-
bounds long enough will push the LTFTs. Sensor cold
will aggravate trimming, if the update is faster than
the sensor response then it's going to see-saw all
the time. PCM tries to create crossings by proportional
fuel add/subtract. Overdriven makes fewer and sloppier
crossings.

Accumulated trims for the trim cell active are applied to
the fuel shot calculation in closed loop and are sticky
if positive, into WOT open loop.

 

It's really not that complicated...at least not to me. Maybe that's because I use EFI Live. j/k

 

its not complicated...just your quote " it knows the fueling is rich. If it's lower, the PCM knows the AFR is lean." made me think of that audio clip.

"The o2 sensor knows its at stoich because it knows what isnt stoich, and because it knows that it knows what stoich isnt it must know what it is" or something to that effect

 

 

That leads toooooooooo....

How does the sensor know what stoich is?

 

http://www.google.com/search?hl=en&q...=Google+Search

 

http://en.wikipedia.org/wiki/Oxygen_sensor

This was a good write-up...

 

That is a nice write-up, but I don't see the answer.

The sensor measures how much oxygen is in the sample as it passes through the sensor. So then the sensor knows a) how much o2 is in unused in combustion, and b) how much o2 was used. Without measuring for a fuel residue, and/or without knowing how much fuel was burned, how can the sensor know that the burn mixture was at the stoich value for the fuel?

 

Why are you questioning this again? It works....it's been working for many years....what does it matter?

There's a theory that I think many people (GM engineers included) tend to forget - especially when designing PCM operating systems:

K.I.S.S. - Keep It Simple, Stupid

 

But how do you KNOW it works.

Just playing devil's advocate and questioning. Call it the pursuit of knowledge. If we all know tool A does job Y just fine, but we don't know how, how are we going to get job Y done when tool A breaks or can't function?

 

 

I'm not sure 100% on how it works... but the key thing to remember is that it is NOT measuring 14.7:1 AFR... it is measuring the amount of oxygen left over based on the amount of oxygen in outside air (20%)

For a given fuel source (hydrocarbon based), there is an ideal amount of oxygen for combustion, however after combustion oxygen is also a by-product... stoich is when the byproduct is what is expected for an ideal mixture, anything above or below is non stoich

As for how we know it works... I personally don't know, but I know i'm not qualified enough or educated enough on the subject to invent my own sensor to compare to the narrowband... so I trust the experts and the fact that over time the average AFR of a car that's properly metering incoming air, matches stoich (14.7:1 for 100% gasoline) based on results of several companies producing wideband controllers , some of which use different sensors than others

I'm sure if you search enough or if someone here is much more well-versed in chemistry than I could explain the scientific reason

but I think it's basically something like:

xHC + yO2 = zO2 (this value has to be constant for a stoich burn no matter what kind of fuel is used)+ aC + bH20 (water is a result of combustion) + cHC (unused hydrocarbons) (xyza and b are variables, I don't know the values)
(so i guess it'd be aHC + cO2 + bN = dO2 + eNO + fHC + gH20 in this formula the the O2 after combustion is a constant for stoich, HC is left over hydrocarbons, hydrogen fuses with oxygen to form water, there are obviously other gases that are formed, CO (carbon monoxide) etc... but in the end we know that at stoich there should be a set percentage of the resultant exhaust that is pure oxygen not combined with nitrogen, carbon, or hydrogen)


the end output you want is a known value for z a and b, in order to have this a (amount of O2) must be the ideal value

This ideal value is based on the chemical makeup of the hydrocarbon used (also has to account for the large amount of nitrogen in the air and other gases)

so for example, pure gasoline (no ethanol) to obtain the value of oxygen after combustion that would be stoich... we know that it has to be:

1HC + 14.7O2 + other gases = Constant O2 + Other Gases

If this constant O2 varies, then the O2 sensor provides an electrical output in that direction

Now take E10 with a 14.13 stoich
1HC + 14.13O2 + other gases = Constant O2 + other gases

The amount of starting O2 is different but the Constant O2 is the same as pure gasoline when stoich

E85:
1HC + 9.x O2 = Constant O2 + other gases

But if we add more fuel (HC) to these examples, the constant O2 changes because now there's more Hydrogen and Carbon molecules for it to bind to, thus decreasing the Constant... if there's not enough fuel, there's too much O2 and not enough binding occurs thus O2 constant goes up

Note... any of the above may be wrong... I've edited a few times adding more things I've thought ot... seems to make sense to me but I may have mistakes

so back to the original question (well not original but azzhauler original)

The O2 sensor knows what stoich is because it knows what that Constant O2 value is... the engineers that developed it developed the electrical circuit so that it would output 450mv when this constant was reached and vary if the amount of oxygen deviates in either direction (but their main focus is to provide accurate results at or close to stoich, they didn't design it to measure the amount of oxygen, only to measure it in relation to the ideal constant)

 

All the PCM cares about is that it needs to switch the O2 sensor voltage around .450mV average, by adding or subtracting fuel. It couldn't careless about AFR. The further from the 450mV average drives the fuel trims.