Position of oxygen sensor in exhaust

[patprimmer]

I have become involve in tuning older Hondas by replacing a chip in the ECU with a rewritable chip or with a real time emulator where fuel and ignitions maps and quite a few other parameters can be altered to re-tune to optimise a different set of priorities and/or for significant changes to the engine.

One of the main feedbacks is reading a wide band oxygen sensor as a basis for altering fuel maps.

Some oxygen sensors have very good life, but others consistently fail within a few months, first sign being erratic or inaccurate readings.

I am suspecting that they are either running to cold and loading up with carbon or in some cases running to hot and being damaged by heat. All the cars involved have had turbochargers added and most are used exclusively for competition, but some are dual purpose or even various types of competition from autocross to hill climb to drag racing to circuit racing. One guy in Sweden even races on ice at times.

My questions.

Does an oxygen sensor need to be held in a specified temperature range to be accurate and/or durable.

If so what happens when to cold.

What happens when to hot.

Is moving the sensor closer or further from the turbo a legitimate way to control operating temperature of the oxygen sensor?

Regards
Pat
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[MacGyverS2000]

Consider that O2 sensors will often have heating elements in them... what does that tell you about cold temps? The sensor doesn't work well when the engine is cold... what does that tell you about cold temps?

There's more leeway on the hot side, but they're not bullet-proof. If you're running FI, my guess is you're also running pretty rich (typical tuning)... that leads to carbon build-up.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Yep, a:f in the region of 11:1.
I would expect enough heat would burn the carbon off. I just wonder how much they will take and what indicates to much. Do you look to get heat marks like the metal body of a spark plug? If not, do you move it closer to the turbo.

Regards
Pat
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[MikeHalloran]

According to my very limited understanding, O2 sensors are heated and/or located close to the exhaust port so they will heat up quickly and give data good enough for closed loop operation within seconds of engine start.

Anything that increases the sustained EGT at the O2 sensor, e.g. rich mixture and/or adding a turbo, has to shorten its life.

I'd start by moving the 02 sensor farther downstream, by many diameters for a competition engine, by a few diameters for a hotted up street engine.

... but all of that is just a WAG.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Mike. Your pretty much where I'm at, but I realised I was guessing.

I know they can get covered with carbon and stop working and can then be carefully cleaned with an oxy torch, just like a spark plug fouled with carbon. I have to admit that I only did that to an old single wire narrow band unheated one.

Regards
Pat
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[bradrs]

Do you have access to the datasheets for the wideband sensors in question? I believe I have seen datasheets for many of the ones that are being used in the aftermarket boxes. That would be a good starting point.

Are the ones used in competition, or even the street ones, using a fuel other than regular unleaded pump gas? Running any additives? There are quite a few things that can foul a sensor besides just running rich.

 

[MacGyverS2000]

Typical sensor locations are right on the header (usually a diameter or two downstream of where all of the primaries come together) and just after the cat. At low AF ratios, not only are you creating extra carbon build-up material, you're cooling the exhaust charge, exacerbating the issue (though not cooling it enough to affect the sensor's readings).

How much power are we talking about here? I would shoot more for 12-12.5 AF under heavy load.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Dan. These are all turbo charged and i.6 litre D series engines making between 250 and 500hp.

It is the 500hp engine that is getting real short life, like 15 or 20 1/4 mile runs.

Regards
Pat
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[evelrod]

I dunno, Pat. I think if I were doing it I'd just move it and see what happened...kinda "range it in" approach cause the whole idea of an O₂ sensor has me wondering what value it has in a drag race engine unless it's also a street engine, too.

I'm kinda old and don't know much about all this new fangled stuff, give me a set of Webers or SU's and "I'm there!"

Rod-----File this under WAG, too.

 

[patprimmer]

Rod

I found a recommended temperature range and I will try a few runs with some EGT measurements in a few spots and aim for the hottest spot that never exceeds spec with a small safety margin for variations in ambient.

Regards
Pat
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[MacGyverS2000]

Can you verify it's actually carbon build-up causing the issue? That's an extremely short amount of time for carbon to build up any appreciable amount, so I would definitely lean towards temp being the cause.

Does the sensor recover on its own after it has had a chance to cool down, or is it essentially fooked at that point? A good high-temp sensor should get you into the 400-450C range, but with high-HP, FI apps you can reach that limit pretty quick. Watching the turbo housing turn cherry red tells you something about the possible EGTs :-/

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

In the cases with very short life, they are pretty well done and do not recover. The first symptom is they start reading richer with each gear change. No there is no sensor to detect which gear is selected, so it's to do with the changes in conditions that occur with a gear change, like an a:f spike maybe.

I am not sure how they handle a:f when the blow off valve operates, however these are MAP sensor not MAF sensor based systems.

I have not had them in my hand. My involvement to date has been at an advice level.

The owner has found a set of instructions and specs that give max and minimum temperatures.

He decided they are cheap and will just keep replacing them short term. I have advised him to collect some EGT data at different spots and compare to min max specks for the sensor.

Until I hear back, as far as I am concerned this is now closed, at least until I am installing my own.

I certainly would like to see their data before I install my own.

Regards
Pat
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[NickJ67]

Pat,

You may have seen this before but seems to be some good info regarding installation of the sensor (pages 6 - 8) and includes a warning about temperatures on page 8 (not to exceed 500ºC at the hex) even a little sketch of a possible heatsink.

http://www.innovatemotorsports.com/support/manual/LC-1_Manual.pdf

These LC-1 kits use a Bosch sensor.

Regards

Nick

 

[patprimmer]

Thanks Nick.

I should have taken the advice I often hand out. RTFM. I did not have a manual, but should have thought to search for one.

Regards
Pat
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[mattsooty]

Pat,

Been a long time chap...

I would venture that your problem is not one of heat, per se, more so moisture.

Basically, after a cold start, most OEMs delay providing the O2 sensor with full heating power. This delay is so that there is no risk of condensation forming on the 'relatively' hotter ceramic contained within the sensor - its called the 'Dewpoint Delay'. If it were to occur then cracking & failure is highly likely

My recommendation would be to delay switching on the sensor heater for a couple of minutes to ensure that things are nicely warmed through.

Hope that's of some help...

MS

 

[patprimmer]

Matty

Thanks for the input. Original problem was solved by surprise surprise, reading the manual,however your advice sounds very interesting as ome have a very short life and I expect with hot rodded ECUs they may deliberately or accidentally disabled a sensor warm up dry out mode. We have a similar problem with some heater elements on injection moulding machines and a two phase warm up greatly extends heater element life.

Regards
Pat
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[dgallup]

Quote: "Basically, after a cold start, most OEMs delay providing the O2 sensor with full heating power. This delay is so that there is no risk of condensation forming on the 'relatively' hotter ceramic contained within the sensor - its called the 'Dewpoint Delay'. If it were to occur then cracking & failure is highly likely"

???? I don't doubt the delay, just the logic. Condensation happens when warm gasses encounter colder surfaces.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[mattsooty]

Ha Ha, no, you're quite right - my choice of wording is decidedly ambiguous!

The condensation doesn't form due to the ceramic element being hotter - the presence of liquid phase water on the cold exhaust walls puts the element at risk of thermal shock, should any of it find its way on to the element. A risk that isn't present when everything is up to temp.

MS

 

[NormPeterson]

In other words, you let whatever moisture land on the sensor that's going to, and when there shouldn't be any more condensation going on, that's when you turn the heaters on to boil off what did land and not evaporate?


Norm

 

[patprimmer]

Certainly with heater bands on moulding machines, you initially turn the heaters on to low power to reduce thermal shock while you dry out the element and to allow some more gradual and uniform expansion, then when dry and already partly expanded all over, you turn them up.

Regards
Pat
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