desoot denox desox

[greasybob]

I have a 2012 Dodge 2500 6.7 with code P21b2 "Excessive time to enter closed loop Nox absorber control". Sounds very technical don't it ? There is a theory of operation in the codes definition but I'm looking for something that describes the chemical processes involved here, at least in layman's terms. From looking at the possible causes it seems EGR is heavily involved, cooling the cylinder I would assume. Because this is a 2012 RAM there is no DEF injected to the process and I thought that the DEF was more for the desoot process anyways. If someone has a link to a good read on all this diesel emissions stuff and how it works and how the 02 is involved, please post it.

[Witsend]

DeSoot, DeNox, DeSox?

The Sox never made it to the post season . Need to DeCoke , DeChoke, DeCubs

[Joe Rappa]

Here's the Reader's Digest Version:

Nox Adsobers (Yes. Spelled Correctly) perform a couple of different reactions depending on temperature and available fuel mixture.

Under lean conditions NO is converted to NO2 in the oxidation cat, then the NO2 is stored on an alkaline metal inside the adorber. Basically it creates a nitrate, turning gaseous NO2 into a solid nitrate.
Later, when the exhaust is hot and rich, it converts the nitrate into N2 and CO2.

The SOx is about the same thing. SO2 is converted into SO3, stored in the adsorber. The only problem with SO3 conversion is that it takes a lot more heat. Rich exhaust and 1600F temps can be required to DeSOx the adsorber, but the Adsorber regens very quickly...like in 15 seconds or so.

The Wide Band O2 Sensor monitors air fuel ratio so it can monitor the process. It also watches the EGTs.

Attached is a pic of a Cumins regenning in DeSOx/DeNOx. Note the short burst to 1600F.

Joe

[Joe Rappa]

By the way, the SOx adsorbtion does not happen on purpose. SOx is not a regulated emission. It is captured by mistake on the NOx adsorber.
DeNOx reactions happen regularly on the Adsorber, but as the SOx gets deposited there is starts taking up room on the adsorber. Eventually the vehicle does a DeSOx event, like seen above, during a regen to clean the SOx out of the adsorber.

I've never seen any info on the PCM strategy for determining SOx poisoning of the NOx adsorber. I don't know if it tracks NOx adsorbtion capability or if it is just on a time cycle...or maybe just tied to mobile regens. If you come across this info in your search for info, please point me to it.

Lastly, DEF has almost nothing to do with soot. It is used to turn NOx into N2 and CO2.

Here's the chemistry:
DEF is a blend of 1/3 urea and 2/3 water.
When heated, DEF turns into ammonia gas and isocyanic acid.
Isocyanic acid react with the water and forms more ammonia gas and CO2
Ammonia reacts easily with NOx, making N2 and H2O, and a lot of heat.

Ammonia is really what we want in the exhaust to convert NOx emissions into something safe. Urea is just a safe way to transport and inject it.
because it is so easy and inexpensive to convert NOx on vehicles running DEF, you can advance the timing and kill the EGR when it is operating. Doing so makes less soot.

Joe

[greasybob]

Here is some more reading. Nothing on Sox though. The reason I thought the DEF was for desoot is that the fluid is injected mid way into the exhaust system. Now I see that in 2013 and later models the Particulate Filter is moved from the rear of the system to a position in front of the SCR Catalyst. Also did not know that the DEF fluid is more or less constantly injected into the system at about 2% of the diesel usage. Thanks for the Info and all your help. http://www.cumminshub.com/emissions.html http://www.discoverdef.com/def-overview/faq/

[Joe Rappa]

In most systems used today, the DPF comes before the SCR. I had learned that if the SCR became contaminated with soot that it had to be replaced. Then I saw some Duramaxes and the Ford 6.7L put the SCR first. I don't know how they work with soot in them, but it's good that it can. The extra heat from the NOx reduction adds heat to the DPF.

Thanks for the links. Interesting that Cummins says DeSOx happens frequently. That's not my understanding of it.

Joe

[greasybob]

A few things here. The vehicle in question has an after market cold air intake. The cold air intake relocates the MAF sensor and in my experience this will usually miss up the calibration. I'm thinking that this is what's setting the code P21b2 as most closed loop calibrations look for MAF as part of the algorithm.
The second thing is the lack of data pids to see what the 02 sensors are seeing and the lack of any closed loop data. The 2 sensors report to it's own module and this module reports to the ECM. At least that's what it looks like. But there is no 02 module supported in the scanner menu. The 02 module is shown communicating on a CAN J1939 data line in the ECM schematic but this isn't shown in the communications schematic. The CAN J1939 is what is usually found in large trucks and off road equipment diesels.

[Joe Rappa]

The lack of O2 data is a pain, but it's only used for exhaust regeneration. It has nothing to do with fuel control. There's also no loop status to report. Diesels don't use it because there's no fuel ratios to worry about. At idle, a pilot injected diesel might be as lean as 200:1. Maybe 20:1 at the smoke limit. Anywhere between there when at cruise.
As you noticed, NOx and O2 sensors are J1939 devices on diesels. I don't recall seeing O2 data on any diesel, big or small. NOx and Corrected NOx are common though.

Joe

[greasybob]

So the "closed loop" term here is used as part of the NOX adsorber function ? The first cause listed for the code is MAF Calibration. So the ECM must still use MAF and add fuel accordingly to get the desired result ?

[GM guy]

MAF

MAF sensors on diesels are generally used for EGR diagnostic strategy

[Joe Rappa]

So the "closed loop" term here is used as part of the NOX adsorber function ? The first cause listed for the code is MAF Calibration. So the ECM must still use MAF and add fuel accordingly to get the desired result ?

Closed Loop on a diesel running a NOx Adsorber makes no sense to me. If it has an SCR Cat and an NH3 sensor, then it has a closed loop NOx monitoring system, but that's all I'm aware of.
SCR systems have 3 types of NOx control.

1) Open Loop. Uses open loop DEF Injection Mapping, and uses NOx Sensors to see if it is working correctly. If it isn't, a trouble code gets set, but makes no attempt to change the DEF injection rate based on NOx readings.

2) Closed Loop. Same as above, except the PCM will trim the DEF injection amount based on the NOx conversion efficiency.

3) Closed Loop with an Ammonia Sensor. (also has NOx Sensors) This system is the best. It monitors unreacted ammonia that makes it out of the SCR. NOx sensors also detect ammonia by mistake. Ammonia sensors don't have that problem. So if you have a system with only NOx sensors, it could mistakenly see high NOx instead of high ammonia. One problem could be caused by not enough DEF. The other by too much. So, they err on the side of not enough to avoid confusion.

Joe

Joe