O2 Sensors 8

[Fmangas]

What is the advantage of doing fuel trim with both post an pre cat O2 sensors? Everything I've seen and read the precat O2 does 90% of the trim and Post does about 10%. I've ran into a system that will allow either O2 to do 100% percent of trim.

 

[ivymike]

it lets you gage catalyst condition?

 

[BrianPetersen]

As far as I know, the front sensor is the primary trim sensor, and the way the rear sensor responds is compared to the front sensor to determine if the catalytic converter is working properly.

 

[mattsooty]

Basically a three way catalyst has two operating modes during which two distinct types of reaction occurs within it: -

Reduction - With an abscence of O2, achieved with a feed gas of lambda <1 (deals with NOx)
Oxidization - With an excess of O2, achieved with a feed gas of lambda >1 (deals with HC & CO)

To enable each reaction to occur the fuelling has to be modulated rich/lean in a sort of sine wave. This is called lambda dither and, the average over a given time is lambda=1. This is achieved by the front sensor, which ECU uses to control the amplitude of the rich/lean excursions.

During lean operation the catalyst behaves like an O2 store and, upon the introduction of rich feedgas there is a delay before this oxygen us used up and reduction reactions can begin. This is not ideal for the efficient reduction of all 3 pollutants. To effectively reduce all 3 pollutants the exit gas of the catalyst needs to be lambda=1.

A rear sensor is thus employed to give fuelling control for the exit gas, however, the type of control is slightly different. Instead of affecting the amplitude the ECU uses the rear sensor to effect a bias by holding the time that the fuelling remains rich or lean. This is called bias.

So basically the front sensor controls to lambda = 1 and the rear sensor controls the switchover/hold time for the rich/lean excursions.

MS

 

[Fmangas]

Thanks, this explains a lot.

 

[patprimmer]

Matty

That as an extremely informative post. You would think the OP would give it a star.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[mattsooty]

Why, thank you Pat, but don't worry about me, I'm used to not receiving praise!

MS

 

[Lou Scannon]

I'm always happy to settle for "faint damns" myself...
Good post Matt!

 

[BrianPetersen]

matt's post DEFINITELY gets a star from me!

 

[Bribyk]

Very informative...

 

[patprimmer]

Matty

I sometime receive a rebuff for my best answers, especially when they are in reply to a stupid question. That does NOT imply that this question was stupid. One about head gasket installation springs to mind where my advice was read the instructions.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[ivymike]

that one was a classic.

 

[rickjones]

Thanks for the great explanation mattsooty!. On a system with both narrowband primary and secondary O2 sensors as on the earlier models the secondary O2 is strictly for converter efficiency, monitoring the converters ability to store oxygen. It has no role in fuel trim adjustment, no?

 

[mattsooty]

Rick

"the secondary O2 is strictly for converter efficiency, monitoring the converters ability to store oxygen"

No, I dont believe that to be the case; the delayed switch time/trim, as calculated using the rear sensor, is adjusted as the catalysts ability to store O2 diminishes with time and/or poisoning.

The fact that the front sensor is two step or wide band matters not, the same rationale applies - a lambda dither/modulation is used to invoke both oxidization/reduction reactions within the cat.

Furthermore and as mentioned by others, the secondary function of the rear sensor is also the diagnosis of the state of the catalyst.

MS