Gang,

When I replaced my both AFR and 02 Sensor sometime ago, I took a boroscope peek at the 3 way CAT from the oxygen(02) sensor hole.



I then realized there are 2 honeycombs or monolith inside and the 02 sensor is in the middle and not at the end of 2nd honeycomb.
Below image is boroscope using 02 sensor hole.




So I been googling the last 2 days, trying to understand in better detail chemically how a 3 way CAT works and found out that the 2 honeycombs have different duty,
also different precious metals.

The information I took most notice is, 1st honeycomb is a REDUCTION CAT and the 2nd honeycomb is OXIDATION CAT


These 2 video explain well the chemical reaction taking place for those 2 honeycomb/monolith.


.



My question to those with decent chemistry 101 ( I hate chemistry ), why would MB and some other car brand too, placed the 02 sensor ( downstream) between the 2 honeycombs ?
Many car brand will place the downstream 02 sensor after the CAT or after the 2nd honeycombs.


.
Example of typical 02 sensor after CAT








Supposedly this is the chemical reaction at first ( reduction ) and second ( oxidation ) honeycomb.
http://www.cchem.berkeley.edu/molsim.../kinetics.html


So my question is :
What advantage or why is the oxygen sensor placed between the 2 honeycombs and not downwstream of both honeycombs?


This question research also made me understand about oxygen storage capacity of a CAT, which is how the expensive DTC P0420 is calculated from :
A common emissions code seen in the field, a P0420 trouble code indicates that the catalytic converter isn't functioning efficiently.
https://www.walkerexhaust.com/suppor...ely%20elevated.


Thank you in advance Guys...........

 

I like chemistry and did well in it in school. However...

Here is a non-chemistry explanation. It is just a guess. The downstream sensor is used to check catalyst health, therefore its installation must be downstream of at least a portion of the catalyst. Could it be possible that installing the sensor in the middle of the catalyst unit is the most practical (effective and low cost) location? It can't be installed upstream of the catalyst. If it was installed downstream of the second catalyst cell matrix, it would receive cooler exhaust gas, might be exposed to underbody damage, or it might result in a higher component cost because of installation design details (added material)?

Maybe the mid-cat installation location results in a catalyst health signal that is "good enough"?

In short, I don't know for sure but could envision possible reasons.

 

Thanks Chassis.
I been scouring the internet and could not find the answer, so far.
If I understand better, that way I can troubleshoot better, because indeed the 02 downstream sensor is only to monitor CAT health and nothing more, but also able to corrupt ECM decision if
the 02 sensor goes bad....unfortunately.

The upstream AFR is the one deciding good 14.7 but also will do rich & lean swing for the purpose of CAT to work its magic.
Why I am so keen to find out the reasoning of why MB choose that mid spot, is because the remarkable throttle response improvement and hence tranny shift becomes
so good-easy-precise too by replacing both AFR and 02 sensors, which mine is only 39,000KM / 24,200 miles today but 9 years.

I been looking at the heating strategy of these sensors heater and there is a guard against damage from condensation, which is also the one responsible for slow re-heat speed of these 2 types of sensors.
Example like this :

01. Cold engine start. The heater will work VERY FAST, under 10 seconds ECM will heat the sensors to 400C / 752F

02. If engine is rather warm, like say 2-3 hours after engine shut down, or I run the engine and being stationary in my garage for say 20 minutes ( test purpose ) , or I do some
test where I need engine start/running for very short 1 minute, the heater strategy will take into account the condensation and it will not heat up the sensors anymore at next engine start and that
means fueling is in open loop state much longer than usual. Open loop gives "bad" or rich exhaust smell, I don't like it.

According to Bosch AFR users, most aftermarket AFR controller which does not have proper software for Bosch specific AFR , usually made AFR short lived due to a poor condensation-damage prevention
strategy. https://www.nzefi.com/bosch-lsu-wide...-applications/

I also think that engine running often but super short duration for parking space management in my house , where my parking is front to back and not side by side, have also made my AFR and 02 sensor
much shorter lived from mileage perspective.

 

The mid bed location for the catalyst efficiency monitor (after oxidation and before reduction catalysts) allows the sensor to do what its name entails - sense oxygen. When an O2 sensor is used to trim air/fuel ratio, there is rapid switching rich-lean of the O2 sensor. When used as a cat efficiency monitor, a far more gradual switching “rich-lean” (residual unburned or unoxidized HC being the “rich” signal) is the expected signal since there is always an ebbing/flowing of the residual HC to O2 balance due to cat temps, engine load etc.

Now, immediately after the oxidation cat, is the reduction cat. It’s job is to remove oxygen from the oxides of nitrogen present in the exhaust, resulting in elemental nitrogen N2 and oxygen O2 as the product.

If the cat efficiency sensor were placed at the very end of the reduction catalyst, it could not accurately assess what is occurring in the oxidation catalyst since extra oxygen is being introduced into the exhaust stream, courtesy of the reduction reaction of nitrogen oxides, mentioned above.

 

Thank you Bill

 

I would say having the O2 sensor in the middle of a 3-way would exist to monitor the efficiency of the first section, then be used as a baseline for the last O2 to check the second section. So in my mind, any car with an O2 in the middle would also need one before and one after in order to work. #1 for fuel ratio, an obvious must have on any emission car, and I assume a baseline value for comparo. O2 #2 compare to #1 to be sure O2 is up xx%. Then O2 #3 to compare to #2 to be sure O2 has dropped xx% to check the second half of the cat. So, for example, on a good cat; ten O2 molecules go in, fifteen are found in the middle, then nine out the end. On a dying cat; ten go in, eleven in the middle, nine out the back. As long as you don't measure the middle, it's good to go, but it's actually converting just a fraction of pollutants.
I have no idea on the % of molecules at each step, it was just an example of the math checking out when it's not actually working right. I've also wondered for some time; what if they could separate the carbon from CO2? Would the carbon build up and quickly clog the exhaust, or would it exit just fine? And, what would the city as a whole look like? Would we not notice or would everything be covered in black dust, like London in the 19th century.

Fyi, I don't recall ever seeing a cat with an O2 in the middle. Maybe it's a rather new thing? I dunno, I just see it as one more thing that costs more $, and will cost more $ and hassle down the road. Cats for my car are so expensive that you can get a loan for them. Not really kidding. The service manager at the MB dealer near me said repairs are so expensive these days that they offer financing.

I like that Engineering Explained guy a lot, he's really sharp and often has parts cut open to better explain. He only gets a C+ on that Cat video, but overall he rocks. He has the best explanation I've seen on how a torque converter works. Another good one was how various limited slip/posi/torsion etc differentials work, which, despite being rather simple, surprisingly few people really understand. Some believe an open diff only drives one wheel, but I can't dumb myself down far enough to grasp the thought process there. On the other end of the spectrum, my open diff works so well that I would not bet $ it's open because it's a better posi than many posiz. Many, maybe most, believe a posi etc actually senses wheel slippage and actively transfers power to the other wheel. This is how people selling posi's etc pitch the product so I don't blame anyone, but yeah, no.