So, I understand that during a regen extra fuel is introduced into the engine increasing exhaust gas temperatures (EGT) to burn the soot out of the diesel particulate filter (DPF). My question is...how is this done exactly? Correct me if I'm wrong but the extra fuel is injected into the cylinders on the exhaust stroke. Based on that assumption and my understanding of the internal combustion engine the fuel would not ignite so how does the raw fuel increase the EGT's? Does it ignite somewhere in the exhaust system prior to the DPF?

Thanks for your help,
Al

 

Good question, I don't know the answer but my guess is that it is "burned" in the DOC.....

Diesel Oxidation Catalyst (DOC)
The DOC is a ceramic catalytic converter which oxidizes hydrocarbons in the exhaust and generates heat for the SCR and DPF to function properly.

 

 

Great info Darren, reps intended

 

Hydrocarbons in layman's terms is raw, unburned fuel when talking about exhaust emissions ( carbon monoxide is partially burned fuel). So yes the DOC makes sure it burns, making the exhaust good and hot to burn out the DPF. Excellent illustrations as well Darren, my rep gun empty as well.....

 

Guys, let me observe for a second....
Also let me speculate....
lol


I'm just piecing my opinion together from various things I've run across before.
I don't think the DOC is a "flame holder" type contraption device to promote combustion, I think it just reacts with the exhaust to scrub it.
The reason I think that, is that the future DPF systems have the DPF and DOC combined into a single material, so that wouldn't make sense if it operated like that.
Also, if fuel was simply sprayed down the exhaust, without burning it, we wouldn't see EGT's change when in regen, because all the heat would be in the exhaust.

I always assumed, that the exhaust heat alone would ignite the post injection, and the fuel would burn in the piston some, and in the manifolds some, as it is exiting.

Anyone ever check a EGT sensor on the DPF VS the engine one? If the EGT's are higher than the engine one, then it's definately burning more in the exhuast somewhere?!

 

Raw fuel is catalyzed (burned) in the DOC which thereby increases EGT going into the DPF. To help increase EGT's, the ECM also does something to maximize EGTA's further upstream. The remaining O2 in the exhaust oxidizes the soot in the DPF starting up around 800F. Passive regens work at lower temperatures using NO2 to oxidize soot starting around around 480F

When the DPF is cooking, my 6.4 controls the burn by slowly increasing the DOC outlet temperature to maintain about a 150-200F rise at the DPF outlet but no higher than 1200-1250F. I think the DPF is rated to 2000F before melting.

In general, the higher the temperature, the faster soot burns...so stop-and-go city driving really isn't conducive for efficient regens

 

The only places EGTs are measured in a stock 6.7 is the exhaust and EGR. The EGR measures it differently to know when to open the bypass is my understanding and there are 4 pyros in the stock exhaust.

I'm not sure I completely understand the part you said which I quoted?

Were you questioning the "fuel is burning" part or the "higher egt in the exhaust only" part?

 

DTS Articles - Diesel Particulate Filters
above article was written when DPF first used on 6.4 but the principle is still the same; lots of pictures/illustrations

First generation DPF on the GM Duramax utilized injecting the fuel for regen during exhaust stroke just like Ford; but later generation GM utilized an injector in the exhaust piping to get the fuel for regen to the DOC.

Bob

 

I was speculating, that if the DOC is actually reacting un-burned fuel, the EGT's should be able to be higher in the DPF, than on the engine, and I don't think thats the case.

"A Diesel Oxidation Catalyst or "DOC" is a device which utilizes a chemical process in order to break down pollutants from diesel engines in the exhaust stream, turning them into less harmful components. They are normally a honeycomb shaped configuration coated in a catalyst designed to trigger a chemical reaction to reduce particulate matter."

So, I think the reaction that takes place there is much similar to a catalytic converter, it reacts the exhaust gases with elements to break down the pollutants, but I don't think it "re-burns" any fuel or acts in that manner at all.

I believe it relies on heat to work, and that all the heat is generated from the engines post injections.

 

Good info.

 

The DOC is exactly the same as a CAT. It oxidizes any hydrocarbons (unburnt fuel) whether from incomplete combustion or deliberately sprayed into the exhaust.

It does indeed need heat to work. The DOC only starts working above the 'light-off' temperature which averages between 400-600F, hence the engine has to increase its EGTA's to heat it up before spraying fuel out the exhaust to do any good.

My EGTA's go through the roof when regen starts even though CAT temperature is down in the 300-400F range. Once the CAT heats up to about 800F, the DPF outlet temperature starts rising quickly on its own up to 1100-1200 as soot starts burning.

Here's a complete regen cycle on my 6.4 in mixed city/freeway showing EGT11 (pre-DOC, 2nd from bottom), EGT12 (post-DOC, middle), and EGT13 (post-DPF, top plot). DPF pressure is the bottom plot. I didn't chart EGTA since it's all over the place. Notice pre-DOC always runs 400-600F. It's more than likely fuel is being pumped into the DOC that whole time. But the neat thing is to watch the temperatures of each sensor and how they correlate with each other as regen starts and completes. Notice DPF pressure is easily pegged the first half of the regen... that's due to the additional exhaust gas pressure created from the DOC /DPF burning fuel and soot. Every time DPF pressure drops to the bottom is when I'm idling or coasting. Regen is complete when DOC outlet temp drops suddenly at the end. Note how the lowest DPF pressure reading continues to drop as regen progresses.

Temperature in F is on left axis, DPF pressure is on the right axis in PSI. Bottom units is time of day in the evening. This whole regen lasted about 25minutes but notice the DPF burn only happens after it has been heated up to 800F 5 minutes into the regen. Stopping a regen before then is basically a complete waste of fuel

 

Very interesting post, thank you for adding that !!!!!

 

This is a complicated problem. So a bottom line question I have with respect to the exhaust system operation AND fuel economy, what are the best overall strategies for operating the truck? Would you try to change the strategy when the exhaust system is in the regen mode? Since exhaust system regeneration is totally dependent on high temperatures, it would seem, for example, that pulling a load uphill and generating high exhaust temperatures for a half hour would reduce forced regen cycles .... but mileage sucks. Or pull at 75 mph vs: 65 mph for 20 minutes every couple hours. I see that short start/stop cycles, city traffic, extended idling, etc. would increase the regen cycle need. Do fuel additives change the regen cycle frequency? Did I just hijack this thread?

 

Since I have a 6.4, I don't have a feel for how passive regens work. There isn't much of any passive regen that ever happens on my rig since it runs so much EGR. It seems I get about 300-325miles between regens if the weather is hot and my DPF temperatures can sit above 450F for extended periods; otherwise I regen every 225-250miles in mixed driving while empty.

In my observations, I get the least impact on economy when I'm driving straight freeway or expressway with a constant throttle such that the DPF doesn't cool off during the regen. It seems the regen process is analogous to keeping a pile of BBQ briquettes hot enough to smoulder, but limited not to get so hot to melt the DPF.

Supposedly the closer the DPF is to 1100-1200F, the quicker soot burns. Anything cooler and the soot burns at an exponentially slower rate. Running the AC on a warm day seems to give me an extra 30-40F at the CAT and DPF.

I've run many tankfuls with and without Diesel Kleen. It doesn't appear to help any in terms of regen frequency, although engine does sound quieter and smoother. I always fill up at the name brand stations like Shell and Chevron and pass on the no-name stations, so I think that has something to do with it

I find myself planning the longer trips that I need to make in my truck and not doing any short city drives less than half-hour when I'm expecting a regen because regen will start just as I'm near my destination and I'll have to invariably shut it down before it had a chance to do anything. The other thing to keep in mind is regen won't start until engine coolant is >160F. So starting off on a cold start in the morning means I need a 40minute drive to do a full regen

I hate this whole thing...it's probably best to just drive, forget about the whole nonsense, and enjoy your rig :-)