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Discussion Starter · #1 · (Edited)
Hello everybody,

As most of you know, diesel engine equipped vehicles following model year 2007 have been equipped with emissions equipment that many of us do not like, or necessarily understand. I have been lurking around here for quite awhile and keep seeing relatively similar questions asked with respect to these emissions systems, so I am going to take some time out of my (slow) day today to put together an outline of the emissions systems commonly found in diesel vehicles today, and how they work.

The emissions systems which are made up of the physical components located in the exhaust stream downstream of the engine are referred to aftertreatment systems. I will be focusing on these aftertreatment systems, rather than other emissions controlling equipment like fuel injection events/patterns, EGR, etc.

Diesel Oxidation Catalyst (DOC)

The diesel oxidation catalyst is commonly the first component of the vehicle’s aftertreatment system. The DOC is a flow-through type of catalyst, similar to the three way catalyst found in every gasoline engine equipped vehicle within the last 20 years. The exhaust simply flows through channels in the DOC, contacting the precious metal wash-coat which is applied to the catalyst substrate during manufacturing. These wash-coats are typically a mixture of Platinum and Palladium, with the mixture varying between manufacturers or applications.

Here is a schematic of a DOC.


The primary function of the DOC is to oxidize hydrocarbons (fuel) in the exhaust during an active regeneration event. Outside of regeneration events, the DOC is constantly converting gaseous constituents to aid in the passive regeneration of the DPF (I will get into this again later) as well as converting the exhaust to less harmful emissions along the way.

Diesel Particulate Filter (DPF)

Next down the line is the diesel particulate filter (DPF). The DPF is called a wall flow type of catalyst, where the overall construction of the DPF is such that every-other inlet channel is blocked at the outlet side. The exhaust enters the DPF through the inlet channels, and is forced through the channel wall into the outlet channel. The channel walls are what is filtering the PM (soot) out of your exhaust.

As the walls get filled with PM, the wall PM layer is formed. After the wall is full, a PM cake layer begins to form on top of the wall where PM has already been captured. After this point, the PM cake layer is the primary filtering mechanism in the DPF. DPF filtration efficiency is commonly in the range of 97-99%, meaning that only 1-3% of the engine out PM emissions actually make it out the tailpipe, which is why if you see black smoke coming out of the tailpipe of your DPF equipped truck, you have issues which should be investigated to avoid further damage to your costly aftertreatment systems.

Here is a schematic of DPF and how the PM filtration occurs.


As the DPF filters PM out of the exhaust, the PM soot load increases over time. With this increasing soot load, the back pressure exerted on the engine also increases. To reduce this back pressure and increase engine efficiency, the DPF undergoes periodic regeneration events to prepare the DPF to filter more PM. DPF regeneration occurs in one of two ways; passive and active regeneration.

Passive regeneration is when the engine-out emissions work with the DOC to form NO2, which is the primary oxidizer of the PM within the DPF. This is a slow process and is most effective when an SCR system is used in conjunction with a DOC/DPF to tune the engine using what is called the NOx vs PM tradeoff. When an engine is tuned to emit less NOx, it WILL produce MORE PM. The opposite is also true, where the engine is tuned to emit less PM, NOx emissions are increased. Using an SCR system, higher NOx reduction is possible, thereby allowing a cleaner engine, reducing the need for DPF regeneration and ultimately saving fuel.

The other type of regeneration event is called active regeneration. During active regeneration, raw fuel is injected into the exhaust stream via an auxiliary fuel injector located downstream of the turbo, or by using a very late fuel injection event in-cylinder to push raw fuel out of the engine with the combusted gases.

During active regeneration, the raw fuel passes over the DOC, and the DOC oxidizes this fuel to produce O2 and heat. The DPF is also catalyzed typically, so any excess fuel which makes it past the DOC is oxidized within the DPF to further increase exhaust temperature and O2 concentration. Typical DPF inlet temperatures during active regeneration can range from 450-650°C/850-1250°F. The primary oxidizer of the PM within the DPF during active regeneration is the increased O2 concentration in the exhaust. Chrysler wants you to never know when active regeneration is happening, so the only way you’ll know about it is by a drop in your instantaneous fuel mileage indicated in the vehicle as you’re driving.

Active regeneration can’t happen all the time, however. The DOC must be hot enough that its light-off temperature is reached. Light-off refers to the point where the DOC is able to covert the majority of the hydrocarbons which enter it, which is usually around 300°C. If active regeneration is attempted below the light-off temperature, excess fuel will be consumed, regeneration will take longer, and your overall MPG will significantly suffer.

As the DPF is regenerated, the PM within it is oxidized and is turned to ash, which slowly builds up in the DPF. I do not know much about how long it should normally take to reach the point of needing to clean the DPF out of the built up ash, but I’m sure that information is out there somewhere. As the PM load within the DPF is reduced, the back pressure on the engine increases, thereby returning engine efficiency to its clean-DPF state, in preparation to do it all over again.

Selective Catalytic Reduction (SCR)

For awhile, diesel engine equipped vehicles following model year 2007 were only equipped with DOC and DPF aftertreatment systems. With the most recent EPA legislation requiring another reduction in tailpipe NOx emissions, SCR systems have added to effectively reduce NOx emissions.

SCR systems are composed of a set of catalysts (where wash-coats are typically composed of a mixture of Copper or Iron and Zeolites, a diesel exhaust fluid (DEF) tank/pump, and a DEF injector located between the DPF and SCR catalysts.

The SCR catalysts are made up of several different types of flow-through devices. The first is a decomposition catalyst which breaks down the urea in the DEF into ammonia (NH3). Depending on the operating conditions, NH3 can be stored in the catalyst for higher NOx concentration operation.

Following the decomposition catalyst is the main reaction catalyst, where NOx reduction actually occurs. The NH3 reacts with the exhaust gases to form less harmful, or non-regulated constituents like N2, H2O, and CO2.

Depending on the operating conditions when DEF dosing is active, there may be an excess of NH3 in the exhaust, where the third set of catalysts come in; the clean up catalyst. The main purpose of these catalysts are to reduce the excess NH3 emissions before exiting the tailpipe.

Here is a schematic of an SCR system.


Similarly to active regeneration, DEF injection into the SCR system does not happen all the time. Similarly to the DOC, there is a minimum temperature threshold where the SCR reactions effectively occur, which is generally above 250°C. If DEF injection occurs much below these temperatures, the DEF can begin to crystallize on the injector tip, or inside the exhaust pipe, which can eventually lead to other issues.

Along with the major components, there are also a number of sensors in the system to monitor the overall performance.

Temperature sensors upstream and downstream of the DOC monitor the conversion of hydrocarbons across the DOC during active regeneration events.

Temperature sensors upstream and downstream of the DPF monitor the DPF temperature during regeneration events to control the fuel injection strategies. Delta-pressure sensors are also used on the DPF to minor the DPF PM loading, in order to determine whether a regeneration event is required or not. There is also a PM sensor located downstream of the DPF which monitors the functionality of the DPF and to detect when a malfunction with the DPF has occurred.

Temperature sensors upstream and downstream of the SCR system monitor the SCR temperatures to determine the DEF dosing strategy to be used. NOx sensors upstream and downstream of the SCR system monitor the SCR system’s ability to reduce NOx emissions by comparing the upstream and downstream readings. There is also an NH3 sensor downstream of the SCR system to monitor NH3 slip through the system, and to modify the DEF dosing strategy accordingly.

I hope this (long) write up has given a decent, albeit brief, overview of the emissions equipment found under all of your trucks these days. In case you are wondering where I got my information, I studied the effects of biodiesel blends on DPF active regeneration for my Master’s in Mechanical Engineering. I pulled the schematics off of Google, so nothing is my work there.

Feel free to PM me if you have more in-depth questions or anything of that nature. Perhaps this thread will be stickied if enough people find the information useful :)
 
G

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:rolleyes:

Not that I don't agree with you, but it is what it is, and your diesel trucks come with this stuff now whether you like it or not, and as such, I thought this place could use a little education on the subject rather than the "BLAH BLAH BLAH HOW DO I DELETE THIS CRAP?" threads.
:agree2: Might as well educate ourselves on what we actually drive. But it still does suck:zot:
 

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Thanks for the explanation! These systems are here to stay and it is nice knowing how they work. I just hope they last without having costly replacement/wear parts in the future.
 

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Appreciate you taking the time to post all that info. It's a good read.:thumbsup:
 

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Discussion Starter · #13 ·
I have no idea. I just know that there aren't any regen indicators, at least that I know of. Just up to preferences during development I guess.

I think it's a good idea for an owner to know, so that when a regen is happening you can continue driving until it's finished to avoid thermally shocking the aftertreatment systems.

Shutting the truck down in sub zero temps while the exhaust is soaked to 1200F can't be easy on the materials, at least IMO
 

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Just remember, if we all lined our trucks up on the coast of California and ran the engines, what comes out of the tailpipes is cleaner than the air/smog China sends to us across the ocean.

Don't we all feel good now about making our emissions standards so high that China can emit 10X the emissions needed to produce the product we used to make here. (Sarcasm intended)
 

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Discussion Starter · #18 ·
Just remember, if we all lined our trucks up on the coast of California and ran the engines, what comes out of the tailpipes is cleaner than the air/smog China sends to us across the ocean.

Don't we all feel good now about making our emissions standards so high that China can emit 10X the emissions needed to produce the product we used to make here. (Sarcasm intended)
The air coming out is probably cleaner than the air going in, in a LOT of places around the country. It is utterly surprising how little PM comes out of the tailpipe of these trucks.

In my research, it took sampling exhaust gases downstream of the DPF for a full hour to collect a measurable amount of soot on a filter for determining the PM concentration in the exhaust. We would sample something like 50 cubic feet of exhaust and would only be left with only .0012g of PM on the filter. Its amazingly clean what comes out of the tailpipe of these trucks.
 

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Discussion Starter · #19 ·
Oh, I almost forgot. I read somewhere last year that flame broiling a single hamburger at BK emits more particulates into the atmosphere than a modern diesel semi truck driving down the highway for 100 miles, or something like that.
 

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Thank you for the useful info! Although I didn't read it all the way through I will when I get more time! Very useful
 
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