Lambda Sensors

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Lambda Sensors

Postby Peter704070 » Wed Apr 14, 2010 8:54 pm

Hello

My question this week is about Lambda Sensors, I know how they both work and what patterns you get and how to interpret them.

What I would like to know is why a lambda sensor fluctuates between rich and lean. I know it might seem a daft question but really cannot understand if an engine is at idle shouldn't it read the same.

Thanks Pete
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Re: Lambda Sensors

Postby Autonerdz » Thu Apr 15, 2010 5:08 pm

Pete,

A basic Zirconia sensor can only show rich or lean and it switches very fast at stoic. It has no way of indicating how rich or lean. So, PCM feedback is constantly adjusting to keep the O2 switching back and forth. It cannot hold mixture at any level. It averages.

Later wide band type air fuel sensors (differing designs) can indicate how rich or lean. This allows the PCM to target a specific fuel mix and go there. This permits more precise fuel control over wide mixture ranges. These sensors do not toggle rich and lean.

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Re: Lambda Sensors

Postby Orevin » Fri Apr 16, 2010 11:40 am

The O2 sensors react to the fuel trim strategy of the PCM, which reacts to the O2 sensor output...

Goggle for "fuel trim explained".

There are some good videos on Youtube as well, like this one:
https://www.youtube.com/watch?v=JxnZNRwFjmY
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Re: Lambda Sensors

Postby Peter704070 » Fri Apr 16, 2010 5:59 pm

Hello

I've just watched this video and wanted to ask a question.

https://www.youtube.com/watch?v=mzsOY_Ah ... re=related

What I want to know is why is the engine trying to balance the mixture why cant it just keep the bar in the middle. (I.e the bar sits in the middle on the video)

Any help would be great.

Also just imagine an engine running fine and the lambda sensor fluctuating every second. I then disconnect the injector, in theory now i have too much air coming out of one of the cylinders, my scope pattern should go up to 0.8v shouldn't it?

Im sorry if the questions I ask seem simple but Im a teacher and only 27 so really keen to be good at using a Picoscope.

Pete
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Re: Lambda Sensors

Postby Autonerdz » Fri Apr 16, 2010 9:46 pm

Peter,

If you are dealing with a later model engine with an Air Fuel sensor, you can keep the bar in the middle, or anywhere else the PCM wants because the sensor can tell the PCM exactly what the fuel/air mix is. The PCM could target a 20:1 lean mix and the Air Fuel sensor can feedback that information and tell the PCM when it got there. In this way, the PCM can achieve any desired fuel ratio for any driving condition.

Before these advanced sensors, the technology just wasn't there. As I mentioned before, the older style Zirconia sensors didn't have the capability to tell the PCM where the mixture was other than it was rich or lean. All the PCM can do with that information is cut fuel when rich and add fuel when lean, bouncing back and forth.

If you had a disconnected injector with an older style O2, then this would tend to lower the voltage the O2 reports (lean indication). The PCM may then respond to what it believes is a general lean condition and add fuel..this would make the remaining cylinders too rich.

Hope this helps.

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Re: Lambda Sensors

Postby Paul Trickett » Fri Jan 07, 2011 9:51 pm

Peter,

The catalyst needs a rich and then lean mixture to ensure correct operation, The oxygen sensor is only reporting back what the pcm is requesting.
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Re: Lambda Sensors

Postby rehtnap » Sat Apr 23, 2011 12:42 am

hi
im just relearning diagnostics from been a mechanic with rough knowlage of system function to retraing to be a diagnostic technician.( had enough of oily engines and clutches lol)
do i understand correctly that theoretically if a basic zirc sensor was placed in a fixed perfect non changing mixture flow it would produce a fixed voltage.
therefore the fluctuating mentioned by steve at a set rpm is actually a sort of inertia effect in the system ie the sensor read rich so the ecu adjusts the sytem to bring it weaker but cant do it spot on so it over runs and reads too weak so the system reacts to send it rich and the cycle begines again. so if so the time scale of a cycle is the time taken for the system to recieve the signal, adjust mixture and that mixture to reach the sensor(having first been fired)
roughly speaking its flipflopping the mixture to obtain the nearest to correct as the output from the sensor is not accurate and the system cant react quickly enough enough to hold it at constant ideal.
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Re: Lambda Sensors

Postby fisher4x4 » Tue May 31, 2011 11:28 pm

In my opinion Paul Trickett is 100% correct. The fluctuation between rich and lean is there by design. In order for the catalytic converter to work properly it needs a rich/lean switching mixture. It has to do with the oxidization and reduction processes within the converter. I have not had much chance to deal with lamda sensors but i am betting that these vehicles still switch between rich and lean in order to allow the converter to work. I can post the exact oxidization and reduction principles and theory of operation of the converter if anybody is interested, just my converter course book is at work now and i don`t want to misinform anybody.
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Re: Lambda Sensors

Postby Autonerdz » Wed Jun 01, 2011 3:43 pm

fisher4x4,

The reason the old Zirconia O2 cycles is because that is all it can do. It has nothing to do with cycling to 'feed the cat'.

Modern catalysts do not need cycling.

The real reason these cycled is because that was the best fuel control technology available at the time. Cycling was not a desired effect, it was an unavoidable side effect. Now that we have wide band and air fuel ratio sensors that do not need to cycle, they don't. And the cats appreciate it. They never really liked cycling anyway. For one thing, there are less pollutants to catalyze. :shock:
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Re: Lambda Sensors

Postby fisher4x4 » Tue Jun 07, 2011 12:05 am

Thanks Tom for straightening me out on that. I just took a converter course a few weeks ago and either i misunderstood or i was misinformed. I was told the cerium stores oxygen during the lean cycles and releases it when it cycles rich to oxidize the HC and CO. Sorry if i mislead anybody. I just wish there was a more standardized learning structure in place for high tech driveability diagnostics.
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