Please kindly explain oxygen sensor 1

[knowlittle]

I am a materials engineer and curious about automotive oxygen sensor. I understand the principle of oxygen sensor. Actually, I was involved in research on oxygen pumping using zirconia. What I don't understand is signal fluctuation. According to internet, it cycles up and down about once a second. Why?

 

[Warmington]

Narrowband O2 sensors have avery limited range of measurement, and they are non-linear with respect to O2 concentration in the exhaust. They really only send two signals to the ECU, rich and lean. The ECU will then adjust the fuelling to send the AFR back the other way. Due to the lack of resolution, this inevitably leads to the cycling you mention. To compound this problem, some ECU's will actually take an average over say a seconds worth of samples and adjust using that value. You will see the steps in the signal about once a second for that reason.

 

[cheapcharlie]

o2 sensors are chemical generators capable of about 1.2vts. they gen voltage by passing o2 when rich mixtures inside exhaust cause an inbalance from outside the exhaust,for example a lean mixture would cause no o2 to flow so voltage is low.the slow change you see with a scan tool is a funtion of the scan tool not the o2 funtion the p.c.m sees.also since .45 is considered a stoke mixture most systems use this as a ck point to see if sensor is working properly. a properly working sys will spend no time to speak of at this voltage.
hope this helps

 

[rickjones]

The reason the 02 signal cycles up and down is because of fuel trim in closed loop operation. When the input signal is high (rich) to the pcm, the injector on time is decreased therefore the 02 signal falls back down and this ongoing cycle continues. The fuel trim is indirectly controlling the 02 wave signal you speak of.

 

[franzh]

I have seen O2 sensors that react fast enough to show an individual cylinder firing (or actually, misfiring). We had to use a labscope, not the forementioned scanner, they are not fast enough. A speed buffer is placed in scanners to prevent readouts that move too fast to be accurate, sort of an averaging feature.

By the way, these O2 sensors are OEM Rich/Lean sensors, when mounted closer to the exhaust valve flange.

Franz

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[mattsooty]

knowlittle,

The real reason this cyclic behaviour occurs and is desirable is, as follows: -

As rightly said above the controller determines the position of Lambda=1 by correcting the rich/lean signal of the 2 point lambda sensor (adding fuel when the sensor reports lean and removing when it reports rich). However, it is not a sinusoidal behaviour.

The operating band of a three-way catalyst is quite narrow and located between a little richer than stoic and a little leaner.

Since the catalyst has two reactions occurring in it, oxidization (to remove HC & CO) & reduction (to remove NOx) both an excess of O2 and a lack of O2 are required – which can be achieved by the cycling the AFR.

To ensure that these two conditions occur, and emissions are efficiently reduced, a closed loop fuelling controller introduces a delay time in the switch over from rich/lean & lean/rich to bias toward one or other of the reactions (hence the behaviour is not sinusoidal). The aim is to efficiently remove all pollutants from the tailpipe gases, to aid this the cat is also designed to store O2 to better aid oxidization reactions.

The rear lambda sensor determines the amount of O2 that is stored in the cat by reporting lean when the cat is full of O2 and rich when it is empty and adjusts the amount of time bias that the fuelling will remain rich or lean.

Since O2 storage is a measure of the cats effectiveness rich & lean excursions can be used for catalyst diagnostic purposes - but that’s another story altogether.

MS

 

[knowlittle]

Thank you for your replies. I understand much better now.