Duramax EGT temperatures from the exhaust manifold to the back of the DPF

Wayne

Wayne

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I snapped this picture of my EGTs while pulling this weekend. The truck is a Chevy 2021 L5P Duramax.

This is the sequence of sensors:

Manifold => EGT => Turbo => EGT1 => DOC => EGT2 => SCR => EGT3 => EGT4 => DPF => EGT 1/5

The EGT of the stream almost always loses 200+ degrees while passing through the turbo. I have seen a temperature drop of That's part of the reason passive regens don't work on these trucks. The EGTs are way too low to burn off soot after expanding and cooling at the turbo. Soot burns off at about 1100°F and the DPF inlet pulling is 622°F, which is way to cool to burn soot.

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This is the same screen, except during a regen. GM adds fuel in the combustion chamber and at least one place in the exhaust stream, presumably before EGT2, which is the just before the SCR.

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Interesting. I thought GM just used a secondary injector downstream of the turbo for the regen. I know Ford cooked a lot of 6.4' pumping the fuel through the rear two jugs in that motor. Always wondered why they thought that was a good idea. I can see using it to get temps up quickly, but constantly pouring fuel through there just seemed like a bad idea.
 
Wayne, from what I understand an exhaust temp inside the DPF or SCR catalyst above 600F is enough for passive regen to accomplish something in the DPF ( I think it is actually closer to 400 F on the SCR), so you forgot to factor in the idea that the purpose of the Diesel Oxidation catalyst that is in front of the DPF is to raise exhaust temps inside the DPF..

alot of the self cleaning process is load dependent and time dependent.. ie the people who short trip daily drive these trucks are far more prone to DPF fouling that someone who goes out and runs the thing under load for long time periods. Also worth saying the newer the vehicle, the more tweaked the programming has went into the emissions system...

You could say some of the original stuff from the early years where they might not have initiated a active regen until the soot loading was higher, but the time interval between active regens was longer had more issues than a newer truck where they do more frequent regens on a shorter time interval or lower amount of soot loading..

Now if you had to perform a stationary regen, the engine management system is going to do whatever tricks it can to get that DPF above about 1000 F... and it can take 30 to 45 minutes at that temp to do the job, but the process itself while under load is accomplished at lower temps than you think.

Ctech mentioned a secondary injector in the exhaust system, but you would only find something like that on a DPF only engine that used a NOx adsorber catalyst ( say like from 2007 to 2010 ish) before all the manufacturers went to a SCR system and started throwing DEF at the Nox emissions instead of trying to dump unburnt fuel into the DOC to get enough heat for the NOx Reduction Cat to do its thing...

FWIW they can still inject trace amounts of fuel into the exhaust system , they can inect small amounts of fuel on the exhaust stroke, which acts like a catalyst in the DOC to raise system temps on a active regen..
 
On the Duramax, the first catalyst is the DOC (diesel oxidation catalyst) that serves the purpose of converting carbon monoxide into carbon dioxide and to catalyze hydrocarbons into carbon dioxide and water. The DOC is about two inches downstream of the turbo. On the gauge EGT1 is the temperature of the exhaust gas entering the DOC. EGT2 is the temperature of the exhaust entering the SCR, so you can see that the DOC doesn't add much if any heat to the stream. The DOC is passive and requires no additional heat to do its job.


MSCH said:
Wayne, from what I understand an exhaust temp inside the DPF or SCR catalyst above 600F is enough for passive regen to accomplish something in the DPF
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Some of the unburned hydrocarbons yes, others no.

Propylene burns off at 600°F so virtually all burns off passively. Other unburned hydrocarbons need higher temperatures, up to 1100°F to completely burn off.


MSCH said:
so you forgot to factor in the idea that the purpose of the Diesel Oxidation catalyst that is in front of the DPF is to raise exhaust temps inside the DPF
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The SCR (Selective Catalyst Reduction) is in front of the DPF. The purpose of the SCR is to convert nitrogen oxides into nitrogen and water vapor. The DEF (ammonia) is injected upstream of the SCR. The catalyst in the SCR promotes the chemical reaction between the ammonia and nitrogen oxides. DEF is injected all the time into the SCR in small amounts.

Sensor EGT2 is in front of the SCR and EGT3 is downstream of the SCR. The chemical reaction in the SCR doesn't add net heat to the stream. The reaction is exothermic, but not enough to overcome the heat lost to the air surrounding the SCR.


MSCH said:
alot of the self cleaning process is load dependent and time dependent.. ie the people who short trip daily drive these trucks are far more prone to DPF fouling that someone who goes out and runs the thing under load for long time periods.
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When I see people daily driving a modern diesel around town, it makes me cring. When driving around town, I never see the DPF inlet over about 350°F. This isn't hot enough to passively burn anything off and soot accumulates quickly.

MSCH said:
Also worth saying the newer the vehicle, the more tweaked the programming has went into the emissions system...
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MSCH said:
You could say some of the original stuff from the early years where they might not have initiated a active regen until the soot loading was higher, but the time interval between active regens was longer had more issues than a newer truck where they do more frequent regens on a shorter time interval or lower amount of soot loading..
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My 2018 Duramax would regen base off the pressure differential between the front and rear of the DPF. If towing with high EGTs, it might only regen every 1,200 miles, because there was a lot of passive burning off of the trapped hydrocarbons that burn at a lower temperature. My 2021 Duramax regens at a maximum of 800 miles period. Evidently GM learned not to let the hard to burn hydrocarbons accumulate like they did in the 2018. Much reprogramming.


MSCH said:
Ctech mentioned a secondary injector in the exhaust system, but you would only find something like that on a DPF only engine that used a NOx adsorber catalyst ( say like from 2007 to 2010 ish) before all the manufacturers went to a SCR system and started throwing DEF at the Nox emissions instead of trying to dump unburnt fuel into the DOC to get enough heat for the NOx Reduction Cat to do its thing...
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The 2010 L5P injects fuel into the cylinder on the exhaust stoke and via the 9th injector in front of the DPF.
 
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Wayne said:
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The 2010 L5P injects fuel into the cylinder on the exhaust stoke and via the 9th injector in front of the SCR.
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I think you mean the 9th injector is before DPF... the SCR shouldn't need any additional heat , it just there for NOx emissions..
I cannot say I am up on all the difference between all the equipment in the world, but traditionally, that exhaust injector is just used during an active regen, to help bring up exhaust temps in the DOC/DPF . the fuel acts as an oxidizer, doesn't acutally burn.. but it increases DPF temps.
 
Wayne, when I get time to do a better job of photo work, I'll take some better pics... but this should be enough to get you started, it is straight from Cummins..

IMG_0516.JPG
 
MSCH said:
I think you mean the 9th injector is before DPF
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Corrected, thanks.

MSCH said:
that exhaust injector is just used during an active regen, to help bring up exhaust temps in the DOC/DPF
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Yes, the 9th injector in front of the DPF is only used during the active regen.

MSCH said:
the fuel acts as an oxidizer, doesn't acutally burn.. but it increases DPF temps.
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Cummins refers to using fuel to be "oxidized" in the DOC. Chemists refer to burning as a "reduction oxidation reaction" (Redox). The technical name for Redox reactions are often shorted to "oxidation" or "oxidized" or "burned".

It is interesting that Cummins doesn't mention an injector in front of the DPF, only the DOC. GM absolutely injects fuel into the exhaust stroke, which is burned in the DOC AND has an injector in front of the DPF. My guess is that GM uses the heat from the fuel burning in the DOC to make sure they can ignite the fuel injected in front of the DPF. It looks like Cummins and GM do things a bit differently.
 
Wayne, Cummins is writing in a generic manner to the masses that deal with these engines, so the process is understood by anyone with a 6th grade or higher reading level and a reasonable amount of intelligence. :) ..

You can bet if they wanted to explain the chemistry, they would use one of their chemical engineers to explain it so no one would understand it but a chemist.. ( FWIW I had access to some of their more technical explanations in the past, but reading it just make's most people's head's swim and doesn't actually make any difference..) aka a chemist can make it out to be magic but all the end user really needs to know is the temps the thing operates at and the basic process.

reality is it doesn't matter if you know how it exactly occurs, just the basics as the thing is self contained and ran by a computer.

Cummins has used a extra injector on earlier systems that still used older type injection, but my guess is they realized that with HPCR they really don't need to do it that way... I would be willing to bet GM stayed with the extra injector simply because they were already doing it that way and then did away with later as it is redundant and could be accomplished with the HPCR system.. You also know they have to re certify the system if they change it, so that probably came into play too...

I did look up the GM exhaust system, and it is true on a Dmax that the DPF is after the SCR cat... which leads me to wonder, why did GM put the SCR up front instead of dealing with the soot first?
I'm sure they had a reason.
 
MSCH said:
Wayne, Cummins is writing in a generic manner to the masses that deal with these engines, so the process is understood by anyone with a 6th grade or higher reading level and a reasonable amount of intelligence. :) ..

You can bet if they wanted to explain the chemistry, they would use one of their chemical engineers to explain it so no one would understand it but a chemist.. ( FWIW I had access to some of their more technical explanations in the past, but reading it just make's most people's head's swim and doesn't actually make any difference..) aka a chemist can make it out to be magic but all the end user really needs to know is the temps the thing operates at and the basic process.

reality is it doesn't matter if you know how it exactly occurs, just the basics as the thing is self contained and ran by a computer.

Cummins has used a extra injector on earlier systems that still used older type injection, but my guess is they realized that with HPCR they really don't need to do it that way... I would be willing to bet GM stayed with the extra injector simply because they were already doing it that way and then did away with later as it is redundant and could be accomplished with the HPCR system.. You also know they have to re certify the system if they change it, so that probably came into play too...

I did look up the GM exhaust system, and it is true on a Dmax that the DPF is after the SCR cat... which leads me to wonder, why did GM put the SCR up front instead of dealing with the soot first?
I'm sure they had a reason.
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All good points and I too wonder why the DPF is placed before in some designs and after in others.
 
Wayne said:
All good points and I too wonder why the DPF is placed before in some designs and after in others.
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the DPF predates SCR, would be my guess.. so it was there first..
I would think it is better to have the DPF first as it is more heat dependent than th eSCR cat and the SCR Cat wouldn't have to deal with so much soot. I don't know what effect soot has on a SCR Cat but you would think cleaner is better than dirtier..

you'll also notice on most of the most modern class 7 and 8 trucks they have gone away from the inline DPF and SCr Cat, and went to what they call the " One Box System" where one lays on top of the other,usually where the step is into the cab.. I think the idea behind that is to minimize heat loss between the two devices
 
MSCH said:
I would think it is better to have the DPF first as it is more heat dependent than th eSCR cat and the SCR Cat wouldn't have to deal with so much soot. I don't know what effect soot has on a SCR Cat but you would think cleaner is better than dirtier..
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My thoughts exactly and I don't know why the DPF is last in my system.
 
@MSCH The video and Cummins material is very good and helps to explain what's coming. Now if we can just figure out how to meet the emissions requirements that were just passed.
 
Wayne said:
@MSCH The video and Cummins material is very good and helps to explain what's coming. Now if we can just figure out how to meet the emissions requirements that were just passed.
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I think they already cleaned em up about as far as they need to and now they are just trying to make the OEM's jump thru decreasingly smaller hoops just to see if they can. I'm certainly glad that I live where I don't face tailpipe tests and all the other nonsense associated with living a clean green lifestyle.
 
BoiseRules said:
and... this is why I'm thankful to own a 2007 "Classic" LBZ...

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it probably depended on exactly when your truck was manufactured because I am pretty sure that all trucks came equipped with a DPF at a certain point in 2007..... or maybe the change is 2007.5 you find a similar situation with Dodge Rams in 2007, some are old school 5.9's and others are sort of considered to be 2007.5 and have the 6.7... but 2007 was when diesel emissions laws really came into effect and every new diesel on the road ended up with a DPF and everybody began using ULSD as on road fuel.
 
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My 2007 “classic” was the last model year for the GMT800 platform and they are considered “unicorns” in the Dmax community. You are correct that the new GMT900 platform started mid-year as a 2007.5 and had the LMM Duramax.
Wayne, feel free to join DuramaxForum. I’m an admin for the site under the username BoiseRob.
 
I always thought it was DOC, DPF and then SCR. I think it would be counter productive to lower the temp before the DPF and I thought that’s what the SCR did. Was lower temp and convert the NOX to ammonia and water. I don’t really know for sure exactly how it works. I know it’s a pain when it doesn’t.
Knock on wood but I can drive mine around in town for quite a bit before it shows anything on the DPF gauge. I just put a little over 700 miles on my Cummins grocery getting and the gauge went up to 25% but I hooked up to the TT and 20 miles in it was back to zero. I can run mine 75-80 mph for quite a while and it not drop down. I’ve been getting the 24 hour active regens for a while but I did change where I get my fuel. Towing it doesn’t really matter what fuel is in the tank.
 
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