The following link gives a very detailed explanation of Lambda sensor operation.
www.picotech.com/auto/lambda_sensor.html
However it does not explain why a leak in the exhaust system can cause the measured Lambda value at the Tail pipe to rise to an unacceptable level ( marginally) for emissions testing at MoT time. Blocking the leak caused the Lambda to drop sufficiently to be acceptable as a pass.
I have been thro various the various options of reduced backpressure or possibly air being sucked into pipe by exhaust gas flow but cannot see this as viable explanation, since the 'positive' pressure is forcing exhaust gases out of the leak.
Anybody got a definitive answer? Meanwhile I will order a new box and clean out the pit!
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pmh (was peter)
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Yep I agree with your principle but however due to the nature of pulses in the exhaust and the fact that, as the gases cool they contract , combined with engine overun etc air is drawn into the blow however small & effects the lambda dramatically. Some cars more susceptable than others, usually a minor leak at manifold or front pipe is the worst place however small
Doc
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i d thanks for the reply,
I can understand this if the leak is before the Lambda sensor, but in this case it was the middle box ie after the CAT and after the sensor. Sorry I did not make this clear in the OP.
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pmh (was peter)
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>>but in this case it was the middle box ie after the CAT and after the sensor
Yes, and any air getting in - even here - will change the composition of the gases as measured by the four gas analyser. The analyser calculates lambda from the proportion of each of the measured gases, it doesn't (can't!) measure lambda directly.
Number_Cruncher
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pmh
It's a common misconception that there's a mean positive pressure in an exhaust. Screwing in an exhaust pressure gauge into a Lambda sensor hole shows that, overall, a small vacuum exists.
Exhaust leaks pulse. The [often supersonic] pressure waves drive some gas outwards all right- but the following negative wave sucks air in.
As a good tailpipe reading is only about 0.25% oxygen; it doesn't need much to push it outside limits.
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>>often supersonic
Really?
Number_Cruncher
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NC
So I've read/been told - many years ago..... "The exhaust shock waves may be travelling at supersonic speeds as they leave the head..."
Can't say that I've ever bothered to measure them myself...
Go on then... I know that you're dying to tell me that was all wrong....
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>>Go on then...
Ha ha!
No, I'm just curious, and interested to hear where you found the info.
I can well imagine that the local speed of sound in the exhaust is higher than the speed of sound in the ambient air, but in the main, I would expect to find pulses propagating at that, local, speed of sound. I think that if there are shocks (in the sense of there being a jump discontinuity in the pressure), then these may travel supersonically, for a short length downstream of the exhaust valve seats. I can't imagine a shock passing un-hindered through the cat though!
Number_Cruncher
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NC
I was told that it's a fairly localized phenomenon just around the exhaust valve. [Combustion dynamics course at Champion? Around '73? Long time ago....] One of those "sit-and-listen" courses.
It's going to depend a lot on the CSA, surface finish and profile of the port and the manifold - I can't see a supersonic shockwave getting round some of the cast manifold bends commonly fitted in that era. [Maybe that's why they cracked so much...?]
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>>Around '73?
You have a good memory!
I was a bit surprised when you mentioned it - but, exhaust dynamics isn't my strong suit, although I have done a little bit of acoustics, which is why I thought the idea of supersonic waves travelling along the exhaust seemed a bit odd.
Last night I had a quick look in my reference books which cover the subject, and none of the mentioned the existence of supersonic waves - this is probably more of a comment on the paucity of my technical library rather than the truth of the situation!
However, I do know that the sudden admittance of gas at pressure can cause a shock to form, (the stark discontinuity between ambient gas, and gas at high pressure effectively propagates) and can believe that as the exhaust valve opens, and blowdown begins a shock might travel down the manifold.
What I don't know is how quickly the shock front degenerates. Once the waves are strong enough to be considered as shocks, the wave speed is actually amplitude dependent. This means that different parts of the wave travel at different speeds, and the wave actually changes its shape as it propagates. (The nearest thing to this that you can visualise is when a wave which has been travelling for thousands of miles across the ocean without changing shape comes into shallow water - it gains height, and the top begins to overtake the lower part of the wave - it breaks)
Number_Cruncher
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NC
I used to have a very good memory - almost total recall of any fact or figure that I'd ever bothered to take note of. Now I struggle to remember my own mobile number.... It's like living in a fog.
I'd think that cam timing would have a big part to play too. Pressure disparity across the exhaust valve at initial opening must be the defining factor. The expanding amplitude and interference effects from such a random initiation point as a round valve, probably means that the supersonic distance is very limited indeed.
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