NOX storage

Bill from Rosetown Technician Posted   Latest  
Discussion
Emissions
2011 Chevrolet Silverado 2500 HD LTZ 6.6L (8 LML) 6-spd (1000)
P20EE - SCR NOx Catalyst Efficiency Below Threshold Bank 1
P207F - Reductant Quality Performance

I am wondering if anyone has experimented with idle loading of the SCR. I recently had a vehicle come in with NOX reduction codes that had passed prior to receiving the vehicle. The customer had stated that he let the truck idle for 5 hours and then his fluid quality poor light came on shortly there after while driving. While it was here it performed 75%+ NOX reduction during forced quality testing with the scan tool and during multiple test drives. I have an itch to reload the system with excessive idle times, but was hoping someone here has already experimented with a system. How long does NOX release from the SCR after having been excessively loaded and what concentration level increase should be expected ?

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Rusty from Oakham

 

Owner/Technician
 

I'd be inclined to drain and refill the DEF tank with some virgin fluid and run another quality test.

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Bill from Rosetown

 

Technician
 

Hello Rusty,

This system is working as it should. My intent with this post was to start a discussion on experiences with NOx storage in the SCR due to excessive idle time. I was hoping someone may have some hard data that they experience a 20% increase in NOx 2 readings for as long as 30 minutes afterwards, as an example .

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Rusty from Oakham

 

Owner/Technician
 

I'm unclear about what your suspicions or your findings are.

"The system is working as it should" you say. OK, if the system is fault-free, what do you believe caused the 2 DTCs, the prolonged idle?

Do we know if the DEF that was /is in the tank is old?

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Bill from Rosetown

 

Technician
 

It was not my intent to start this discussion in hopes of fixing this specific vehicle. My intent was to see if anyone has ran into NOx loading of the SCR due to excessive idle that has tripped any kind of NOx reduction issues, or has observed a measurable decrease in efficiency because of idling. As I stated in my original post It passed before it got here. It was operating at 75%+ NOx reduction both on multiple test drives and during a reductant quality test. During the test drives NOx1/NOx2 levels were 334/54 ppm and 224/27 ppm at the points collected which is a reduction of NOx through the SCR of 84-88% which is well above published literature of 70% reduction. During the forced fluid quality test the reduction was 83% at 135 ppm/23 ppm numbers. The customer had not added any DEF in between. So the DEF quality in my mind is not an issue. If the system is capable of reducing NOx levels more than 70% then the SCR and DEF are capable of performing properly. Without having a hard fault I would only be guessing at what the issue may be. My preference as far as testing goes is to first induce the issue and then determine the cause.

The DEF is approximately 1 month old ACdelco brand. The DEF was tested with a refractometer and the DEF injector was tested 5 times for flow, injector pattern, leaks, purging and fluid was visually inspected for any issues after flow was tested. Both NOx sensors drop to 0 during decel and from my experience look to be accurate. I could have done more comparisons perhaps during conditions where levels were higher and no DEF was being injected, but I was satisfied by the numbers in the low ranges to trust their accuracy.

I was hoping someone may have experimented with the idle loading at some point. I see vague mention of idle loading in literature, but have never done my own experiments or data acquisition.

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James from Bishop

 

Diagnostician
 

DEF quality poor . Fleet vehicle ? Some of those guys buy DEF in quantity and it goes bad on them . 

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Bill from Rosetown

 

Technician
 

Hello James,

Thank you. Good to know. I will keep that in mind.

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Chris from Commack

 

Technical Support Specialist
 

I might be looking at this the wrong way but, excessive NOx are created under high temperature combustion. It has been my experience that diesels idle typically below loaded or driving temperatures. There should be less NOx created under these conditions. Right?

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Rusty from Oakham

 

Owner/Technician
 

I agree based on the EGR command that I typically see on diesels at idle.

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Chris from Commack

 

Technical Support Specialist
 

I am beginning to think that a cooler exhaust may more adversely effect the scr system than the NOx build up while idling, Anyone know about temperature operating requirements of SCR catalysts?

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Bill from Rosetown

 

Technician
 

Off the top of my head I cannot remember the target operating temperature of the SCR. 

Would you mind elaborating what you are thinking with the cooler exhaust adversely affecting the SCR. Are you thinking it might become less effective at reducing NOx due to some other factor? 

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Bill from Rosetown

 

Technician
 

Chris,

I agree with your statement. I am no expert and have no equipment to measure NOx besides the sensors that the truck is equipped with. I would say I have not really observed NOx readings at an idle with these and they do not register for long at an idle if memory serves me correctly.

My concern is that when the SCR is at lower temperatures during extended idle it absorbs (if I am defining that correctly) NOx into the SCR and then releases it once the SCR heats up. 

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Martin from Burleson

   

Diagnostician
   

I took a class on dpf's and scr's, not brand specific, and it was told to us that the exhaust temps needed to be at least 500 degrees for the exhaust to "do it's thing". Sorry, I'd have to find and look over the material to give a better response. But that seemed to be stressed heavily. As I understood it, at lower temps the exhaust will load up and cause problems. My truck (18 ram 2500) at a long idle, at operating temp, once stabilized, my egts are only at about 300-350. Maybe lower. Perhaps this scr is near the end of it's life? Surely this isn't the first time the truck has had prolonged idle times. High mileage truck? Or just high engine hrs? I'm no expert either so I'm interested...

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Chris from Commack

   

Technical Support Specialist
   

A typical Diesel Oxidation Catalyst or DOC stores oxygen to combine with CO and HC. The SCR doesn't store anything, it consumes DEF for its ammonia content to be used in the chemical reaction to reduce NOx. I do not believe that the SCR setup stores NOx for future reaction.

Exhaust temp monitoring should look for a trend to increase after each component you could monitor EGTs while driving. I would not look for efficiency in this type of system after a long idle as I believe most diesels run lean enough at idle that there is not always enough heat ( especially in cold weather) to maintain full operating temp. These newer generations of performance diesels also could compensate for this and increase fueling at idle but this would increase fuel consumption and may increase emissions.

The factory description below describes the process more thoroughly.

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Bill from Rosetown

 

Technician
 

Hello Chris,

In GM PIP 5007F Bullet point 6 it states during long periods of idle the SCR stores NOx and briefly releases it when it warms up. Not saying the General is always right, but It sure would be nice to have some hard data/recordings. I just don't feel like experimenting when it may put something at risk. You could well be right about it not storing NOx. It has not been my experience to see increases in temperatures from substrate to substrate under normal driving conditions unless it is in regeneration. Our colder temps will really play havoc even in regen in very cold weather while driving

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Chris from Commack

   

Technical Support Specialist
   

I'll have to see if I can find some more information on this. I was under the impression that NOx was not stored. The second link describes a NOx absorber used to store NOx. I am not sure of its application though.

en​.​wikipedia​.​org/wiki/Selective… en​.​wikipedia​.​org/wiki/NOx_adsor…

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Bill from Rosetown

   

Technician
   

In the LML ATG Book it briefly mentions the SCR storing NOx as well. The bosch diesel engine management book mentions NOx storage catalysts as well as denoxtronics systems, but nothing clear on scr NOx storage. The owner of this truck was cool with me idling to load it up and then record data out, but I didn't want to poison the system.

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Robert from Kennett Square

   

Owner
   

Bill and Chris,

There is a paper available at Diesel​.​net that gives a pretty simple explanation on NOx Adsorbtion (not ABsorbtion, btw . . .) Sadly, it is necessary to be a subscriber to the site to access the full paper. However, a summary of the paper is available free ( dieselnet​.​com/tech/cat_nox-t…​.​php ). The first paragraph starts with: "In active NOx adsorbers, stored NOx is periodically released—with a typical frequency of about once per minute—during a short period of rich air-to-fuel ratio operation, called NOx adsorber regeneration. HTH . . .Bob

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Bill from Rosetown

 

Technician
 

Thank you Bob,

Figured I likely had my terminology muddled.

Are you a member of diesel​.​net Looks like it may have some nice content.

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Robert from Kennett Square

 

Owner
 

Bill, your terminology was fine - just wanted to point out that absorption is where something mixes with something else whereas adsorption involves something (in this case, NOx) coating something else (the catalyst). And yes, I do a bit of "light diesel" training (Dodge/Ram Cummins / GM Duramax / Ford Power Stroke) and the website has helped me tremendously. Regards, Bob 

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Bill from Rosetown

   

Technician
   

Hello Martin,

During the failure record for this truck EGT3 was 466F and EGT2 was 505F. Service information states optimal SCR reaction takes place at 390F-750F. It also states that temps below 480F poisoning of the SCR can happen. To me thats a bit of a muddled way to put out service info. I am afraid I did not document exhaust temperature during correct operation (even though I should have)

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