Chevy Colorado & GMC Canyon banner
  • Hey Everyone! Enter your ride HERE to be a part of this months Ride of the Month Challenge!

1 - 14 of 14 Posts

·
Registered
Joined
·
8 Posts
Discussion Starter #1
Reading through the Regen Interval thread got me to thinking. If our diesels passively regenerate when exhaust temps are elevated wouldn't it help to wrap the exhaust with header wrap to hold the heat in as long as possible?
 

·
Registered
2021 Colorado 2.8L Diesel Z71
Joined
·
286 Posts
I doubt the LWN DPF does much passive regeneration due to the system design, with the SCR catalyst before the DPF.

Straight oxidation of carbon soot with O2 won't happen at any appreciable rate below 500C, and doesn't really get going until 550C. The system achieves those kinds of temperatures during an active regeneration.

Passive soot regeneration is driven by the reaction C + NO2 => CO + NO. This reaction occurs around 300C to 450C. Problem is that the SCR catalyst is reducing almost all the NO and NO2 upstream of the DPF, so even when the temperature is high enough for the passive reaction, the NO2 concentration will be very low.

Whatever you do, never place insulation over the catalysts themselves. That will increase the metal temperatures, increase expansion, and cause the fiber mats that are squeezed between the ceramic catalysts and the metal shell to loose their grip. Very expensive failure for you.
 

·
Registered
Joined
·
679 Posts

Radiated energy of the exhaust is extremely low compared to what is shot out the tail pipe. The engine on its own is not capable of safely generating the needed heat. So any passive regen is just out of the question.

As for why the Italian tune up sometimes works... It doesn't really. But some of the soot in the dpf are still somewhat volatile. Meaning more C-H bonds. Less aromaticity. These will burn readily at lower temps. In other words they are not the problem.
 

·
Registered
Joined
·
510 Posts
I'm still not convinced it wouldn't be somewhat helpful. EGT temps while on the highway are higher than those at slower speeds and the DPF fills up slower as well. It would be nice if GM didn't dork up the emissions on this truck, but we have what we have. If someone had the money it would be worth it to Jet hot coat everything, but that seems like a lot of work, lol.
 

·
Registered
2021 Colorado 2.8L Diesel Z71
Joined
·
286 Posts
I'm still not convinced it wouldn't be somewhat helpful. EGT temps while on the highway are higher than those at slower speeds and the DPF fills up slower as well. It would be nice if GM didn't dork up the emissions on this truck, but we have what we have. If someone had the money it would be worth it to Jet hot coat everything, but that seems like a lot of work, lol.
Seriously, at the kinds of temperatures available in normal operation downstream of the turbocharger you can't drive any soot oxidation directly with oxygen. It's too cold at the 300C - 450C you can reach in the DPF.

The 'passive' reaction relies upon the much more reactive NO2 molecule. But SCR catalysts are 95% to 99% effective when they are in their happy temperature, which is the same temperature range where the passive reaction will happen. So.....the molecule you need in the DPF for passive regeneration was already reduced in the SCR. All you got left to work with is N2 (useless) and O2 (which needs more than 550C to oxidize soot). So a little more temperature doesn't matter.

HD diesel engines place the SCR downstream of the DPF. They can, on long highly loaded highway drives typical of Class 8, keep a filter soot level under control under 100% passive regeneration.

Passenger cars have long placed the SCR in front of the DPF (because passcar duty cycle is cold anyway, they need the heat for the SCR). In any case, this means that almost all of the soot collected has to be actively regenerated. Your regeneration interval will be a function of how much soot the engine is making, which is a function of your duty cycle. My estimate is your active regeneration interval is like, 95% set by incoming soot rate, and 5% set by passive regeneration.

Put another way, if you want longer regeneration intervals, drive on the highway. And when you are in town, drive like you have an egg under the accelerator pedal. I've been getting around 500 miles between regenerations with my 2021, but the truck is rather new.
 

Attachments

·
Registered
Joined
·
510 Posts
I get what you're saying, but anecdotally I noticed slower DPF fill rates at faster speeds on the highway than at slower which corresponds to higher EGT temps and a larger soot load. Right now my ZR2 emission system is a total crap shoot since it regens every 100 miles, but in the past it was much better and I had increased intervals driving at 75 vs 65...
 

·
Registered
2021 Colorado 2.8L Diesel Z71
Joined
·
286 Posts
I get what you're saying, but anecdotally I noticed slower DPF fill rates at faster speeds on the highway than at slower which corresponds to higher EGT temps and a larger soot load. Right now my ZR2 emission system is a total crap shoot since it regens every 100 miles, but in the past it was much better and I had increased intervals driving at 75 vs 65...
I have different findings with my CCSB Z71 4x4. My truck is at its happiest cruising 65 mph at ~ 1650 rpm. The DPF filling rate is very low. If I speed up to 75 mph at ~ 1930 rpm the rate goes up a lot (By rate I mean DPF full % increase per mile travel, not per unit time).

What you and I do not have available to us is a map of the soot mass rate coming out of the engine as a function of speed and load. Is the engine a lot smokier at 1900 rpm than at 1600? My truck suggests it is. But also keep in mind that 75mph demands a lot more power from the engine than 65 mph, so both rpm and load are changing. Your truck maybe acted differently, but is different again now with short intervals. Shortening intervals suggests a problem.

You say your truck regens every 100 miles. Are you looking at an OBDII reader to watch the regenerations happen? What soot filling level are you seeing reported when a regen finishes?

My guess is that the GM regeneration strategy is mostly driven from the DPF pressure sensors, since this approach is pretty reliable in DPFs that are primarily actively regenerated. If your regeneration intervals are shortening, there are a few possible causes:
1) It is possible that your DPF is filling with ash or is suffering from ash-bridging which makes it act like it is full of ash. This means the 'starting' pressure drop is high, so each regeneration can't reset you to empty, but just some fraction of empty. Basically.....your DPF is smaller now because it is full of ash and so it fills up faster.
2) Your engine is making more soot because of 'problems' and this soot is filling the DPF faster.
3) There is a problem with the pressure sensor, possibly drift or a downstream leak resulting in erroneous high pressure drop readings.
4) You are driving more aggressively, or have changed your use pattern of the truck without considering it.
 

·
Registered
2020 ZR2 Diesel Extended Cab
Joined
·
74 Posts
[/QUOTE]

My soot readings are rarely upward to regen. I have seen them drop from say 92 to 85 and sometimes even more before rising up to 100% for an active regen; this occurs during driving. Sometimes they will jump 15% over a course of 15 miles and enter regen. My distance between regens is currently 420 miles.

Over time as the DPF fills with ash the regens will become more frequent but will the after regen soot readings increase or do they set to base level and just rise more quickly? At 40K miles the readings still drop to 3% after regen.

Does the software have a maximum interval before it will perform a regen regardless what the sensors tell it?
 

·
Registered
Joined
·
8 Posts
Discussion Starter #12
Interesting conversation. Lots on knowledgeable people on this forum.
 

·
Registered
2020 ZR2 Diesel Extended Cab
Joined
·
74 Posts
If the DPF temperatures are monitored by exhaust sensor #5 so as to control the fifth injector and thus the DPF temperatures, I don't know why wrapping the DPF and exhaust system from just past the 5th injector to exhaust sensor 5 would be bad. It would keep the DPF warmer and heat up faster and more uniform during regen thus requiring less fuel to do so.

Someone wrapped a DPF on Sprinter;

 

·
Registered
Joined
·
510 Posts
I have different findings with my CCSB Z71 4x4. My truck is at its happiest cruising 65 mph at ~ 1650 rpm. The DPF filling rate is very low. If I speed up to 75 mph at ~ 1930 rpm the rate goes up a lot (By rate I mean DPF full % increase per mile travel, not per unit time).

What you and I do not have available to us is a map of the soot mass rate coming out of the engine as a function of speed and load. Is the engine a lot smokier at 1900 rpm than at 1600? My truck suggests it is. But also keep in mind that 75mph demands a lot more power from the engine than 65 mph, so both rpm and load are changing. Your truck maybe acted differently, but is different again now with short intervals. Shortening intervals suggests a problem.

You say your truck regens every 100 miles. Are you looking at an OBDII reader to watch the regenerations happen? What soot filling level are you seeing reported when a regen finishes?

My guess is that the GM regeneration strategy is mostly driven from the DPF pressure sensors, since this approach is pretty reliable in DPFs that are primarily actively regenerated. If your regeneration intervals are shortening, there are a few possible causes:
1) It is possible that your DPF is filling with ash or is suffering from ash-bridging which makes it act like it is full of ash. This means the 'starting' pressure drop is high, so each regeneration can't reset you to empty, but just some fraction of empty. Basically.....your DPF is smaller now because it is full of ash and so it fills up faster.
2) Your engine is making more soot because of 'problems' and this soot is filling the DPF faster.
3) There is a problem with the pressure sensor, possibly drift or a downstream leak resulting in erroneous high pressure drop readings.
4) You are driving more aggressively, or have changed your use pattern of the truck without considering it.
I have an ultra gauge that I watch the soot level on and what I base my previous posts on. When it's finished regenerating it is anywhere from 14-20% full still but usually drops a couple of percentage points in the first couple of miles after it's done.

I would believe that I have less functional DPF capacity, thankfully it seems to be getting worse (some days it regens twice on my 100 mile round trip) and I'm eventually hoping it will throw a CEL so GM will actually do something about it. I don't know why the engine would be making more soot for any particular reason, I haven't switched oil brands or anything. I would be curious to know if a catch can would help, but I want this problem fixed before I band-aid it... I did buy a new pressure sensor but I haven't installed it yet, but that was another thought that I had. I haven't changed my driving pattern though, in fact it's lots more highway driving confined to 65mph because of construction. I do know this will be my last diesel even though I love the truck, I've never had so much crap gone wrong on a vehicle before.

I don't mean to de-rail this thread with my issues though. I still think that having hotter exhaust entering the DPF is likely a good thing. It may not throw it into passive regenerations but it may slow soot build up and it may allow for less diesel to be wasted heating the thing up. I don't know how much warmer the exhaust would be, but even if it's only 10% it may be worth it.
 
1 - 14 of 14 Posts
Top