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Author Topic: rear o2 sensor used in a/f calculation? audi aha 2.8 v6 (dbc)  (Read 4254 times)
k0mpresd
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there was some talk on audizine tonight about the rear o2 sensor being used in the a/f calculation as well as the front o2s on the 2.8 v6. specifically the aha engine code dbc motor.
ive read they are used on some motors to help with the a/f mix but would have never thought on an older drive by cable motor this was the case.
can anyone confirm either way? would just like to know 100%. i feel the answer is no, they are not used.  Smiley
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userpike
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« Reply #1 on: March 16, 2013, 08:13:48 AM »

there was some talk on audizine tonight about the rear o2 sensor being used in the a/f calculation as well as the front o2s on the 2.8 v6. specifically the aha engine code dbc motor.
ive read they are used on some motors to help with the a/f mix but would have never thought on an older drive by cable motor this was the case.
can anyone confirm either way? would just like to know 100%. i feel the answer is no, they are not used.  Smiley

I always thought that the rear 02 is for monitoring the cat, but if I was Bosch I think I would have it the way you are asking about because as the cat deteriorates, its efficiency would go down so why not have the ECU adjust for that?

Wish I knew the answer.
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phila_dot
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« Reply #2 on: March 16, 2013, 08:22:33 AM »

LRSHK
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k0mpresd
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« Reply #3 on: March 16, 2013, 08:24:04 AM »

LRSHK

thanks. maybe a bit more elaboration?
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phila_dot
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« Reply #4 on: March 16, 2013, 10:50:06 AM »

It's been awhile since I have looked at this, so I would have to look at my notes to give a solid answer.


Short answer, the rear O2's do influence fuel trims on ME7.1 and ME7.5. However, I have no experience with the application that you are referring to.
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k0mpresd
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« Reply #5 on: March 16, 2013, 01:43:57 PM »

It's been awhile since I have looked at this, so I would have to look at my notes to give a solid answer.


Short answer, the rear O2's do influence fuel trims on ME7.1 and ME7.5. However, I have no experience with the application that you are referring to.

thanks again for the reply. the ecu in question here is m5.9.2, so i really doubt they are used for anything but catalyst check.
ive read posts from robin and several others that the rear o2s do play a role in certain ecus but no one ever actually stated what those ecus were, so thank you again.
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dale116dot7
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« Reply #6 on: March 26, 2013, 01:36:58 PM »

Most ECMs with rear O2 sensors use them to 'trim up' the air-fuel ratio. The 'knee' on a precatalyst O2 sensor is relatively accurate but the tolerance is a lambda of about +/- 0.01 or so with age. If you look at the rear O2 sensor transfer function the lambda tolerance with age is about +/- 0.001 or so and also with the rear O2 sensor you can purge the oxygen from the catalyst after a deceleration event and restore NOx conversion rapidly. It improves emissions with age (substantially) so that you don't have to throw a MIL and replace the front O2 sensor every 15,000km - more like 150,000km.

Older controllers (1994 through maybe 2000) that had low-storage catalysts often would use an integral control only (perhaps with deadband) from rear O2 setpoint voltage - usually around 0.56 to 0.65 volts - to shift the front O2 setpoint or bias. In the case of a Bosch or Ford system with LSF or other switching-type front sensor it will be the bias time and direction; for GM it is a front averaged voltage setpoint. Newer catalysts with higher oxygen storage use either a PI controller or perhaps a predictor/corrector or observer algorithm because there may be a deadband in the oxygen stored to rear voltage transfer function; that depends on the blend of zirconia, ceria, and PGM in the catalyst. In addition, in order to see 'wiggles' in the postcat sensor to trigger P0420 you need the voltage 'held up' and not hanging out at 0.1 volts or so.

The control authority is maybe a lambda shift of 0.02 or 0.04 at most although sometimes during a catalyst recovery event there may be an enrichment to a lambda of maybe between 0.90 and 0.95.
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NOTORIOUS VR
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« Reply #7 on: March 26, 2013, 02:14:57 PM »

^^^ wow, that was pretty awesome!

Thanks for your input!
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SCHNELL ENGINEERING BLOG ·  STANDALONE ECUS · TUNING · DYNO · WIRING · PARTS · VEMS
Google Talk: NOTORIOUS.VR
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CoupedUp
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« Reply #8 on: March 26, 2013, 03:32:48 PM »

^^^ wow, that was pretty awesome!

Thanks for your input!

Haha, first post too.
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userpike
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« Reply #9 on: March 26, 2013, 10:16:30 PM »

Most ECMs with rear O2 sensors use them to 'trim up' the air-fuel ratio. The 'knee' on a precatalyst O2 sensor is relatively accurate but the tolerance is a lambda of about +/- 0.01 or so with age. If you look at the rear O2 sensor transfer function the lambda tolerance with age is about +/- 0.001 or so and also with the rear O2 sensor you can purge the oxygen from the catalyst after a deceleration event and restore NOx conversion rapidly. It improves emissions with age (substantially) so that you don't have to throw a MIL and replace the front O2 sensor every 15,000km - more like 150,000km.

Older controllers (1994 through maybe 2000) that had low-storage catalysts often would use an integral control only (perhaps with deadband) from rear O2 setpoint voltage - usually around 0.56 to 0.65 volts - to shift the front O2 setpoint or bias. In the case of a Bosch or Ford system with LSF or other switching-type front sensor it will be the bias time and direction; for GM it is a front averaged voltage setpoint. Newer catalysts with higher oxygen storage use either a PI controller or perhaps a predictor/corrector or observer algorithm because there may be a deadband in the oxygen stored to rear voltage transfer function; that depends on the blend of zirconia, ceria, and PGM in the catalyst. In addition, in order to see 'wiggles' in the postcat sensor to trigger P0420 you need the voltage 'held up' and not hanging out at 0.1 volts or so.

The control authority is maybe a lambda shift of 0.02 or 0.04 at most although sometimes during a catalyst recovery event there may be an enrichment to a lambda of maybe between 0.90 and 0.95.

Thank you sir!







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jibberjive
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« Reply #10 on: March 27, 2013, 10:55:05 PM »

Epic first post.  Thanks for contributing dale116dot7.
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