DPF nett effect on emissions? 2

[FunEng]

Regeneration of DPF’s seems to be quite popular and the system seems to be effective to control smoke. What is the nett effect on emissions and in particular Nox?

 

[BrianPetersen]

Normally the regeneration is done by providing a lean exhaust stream at a high enough exhaust temperature to let the catalyst in the DPF light up and burn off the soot.

This can happen during normal driving at higher speeds without the engine controls having to do anything, in which case, no effect on NOx.

If an active regen is needed, the VW system does it by injecting extra fuel late in the power stroke (long after the peak NOx formation) which raises the temperature in the oxidizing catalyst (upstream of DPF in same enclosure) and that raises the temperature enough to light up the DPF. This shouldn't appreciably affect NOx.

In any case, the VW system has the oxidizing catalyst and the DPF upstream of the de-NOx catalyst, which takes care of any NOx that might be there. The North American version of that engine also has two EGR systems (a high-pressure system that has a quick response time, and a low-pressure system that responds slower but draws exhaust from after the DPF so it doesn't clog and draws it through a cooler to reduce temperatures even further).

All of these DPF-equipped systems are on engines that meet current NOx emission standards.

Main effect of these emission controls from what I can tell, is higher fuel consumption under some operating conditions (when active regenerations are needed).

 

[FunEng]

Thanks Brian. Toyota systems the same? What happens to te emission levels DURING the regeneration process? How often and time duration do one expect this to take place?

 

[BrianPetersen]

I don't know how Toyota's systems work, because Toyota doesn't sell diesel engines in North America. I've read some of the VW technical papers on how their system works, so that's the one I'm explaining. But, from what I can tell, all of these systems are more or less similar in concept of operation. The main differences are the type of de-NOx catalyst (VW Jetta uses a lean de-NOx catalyst, others use SCR with AdBlue fluid.)

The temperature inside the DPF catalyst itself is well below the NOx formation threshold, even during regeneration, if *that* is what you are asking.

Raising the exhaust temperature to initiate DPF regeneration is normally done by a combination of retarding injection timing (tends towards less NOx) and injecting fuel late in the power stroke which ends up being oxidized in the oxidizing catalyst, raising its temperature - but again, the oxidizing catalyst operates well below the NOx formation threshold, even during regeneration. The predominant effect is increased fuel consumption during this mode, not necessarily higher NOx emissions.

The VW system has several different regeneration modes, because each of the DPF, the de-NOx catalyst, and the de-H2S catalyst all require regeneration by different methods and different frequencies. The de-NOx catalyst is regenerated every few minutes by running the engine at stoichiometric (unusually rich for a diesel engine) which feeds the de-NOx catalyst a small amount of hydrocarbons to reduce the stored NOx. (This is done by temporarily increasing the EGR rate through the low pressure EGR system.) The DPF is regenerated every few hundred kilometers by raising the exhaust temperature in lean-burn mode. I don't recall how the de-H2S catalyst is regenerated, but it is at an even lower frequency.

 

[dgallup]

Some of the heavy duty trucking & stationary diesel engine manufactures are using secondary injection systems in the exhaust to burn off the particular filter. I'm sure these also run cool enough to not affect NOx, obviously there is an increase in CO2. I don't know why they are taking this approach, perhaps their in-cylinder injection systems can not do a late secondary injection.

 

[70btdc]

DPF regeneration does increase NOx emissions. This is factored in at the end of the emissions test and is in the order of 1 to 1.1 * g/km test result. LNT is not popular in Europe Pascar applications yet.

http://www.dieselnet.com/tech/dpf_regen.html

 

[70btdc]

Extra fuel to heat filter = some extra NOx. You cant avoid it without LNT.

 

[bcs5274]

the whole dpf system in trucks was a waste, all trucks with these systems had a significant drop in fuel mileage,in some f series fords the mileage was half what the truck could do if the system was eliminated, so how can these systems be good for the enviroment if we burn twice the fuel???

thus now we have urea systems, so far, mileage is better but the head aches of another system to keep track of is yet to be seen

 

[drwebb]

DP regulation is intended to benefit public health- not the environment.

 

[BrianPetersen]

I do not know why the DPF system in the trucks was such a killer for fuel consumption. The VW DPF-equipped engines use a little more in city driving (because of the need to occasionally heat up the DPF) but in highway operation, they don't seem much different from the previous non-DPF engine (which had a different fuel injection system).

Only thing I can *maybe* see that could explain it, is that the VW system has the oxidizing catalyst and DPF extremely close-coupled to the engine and they are both in the same housing, so that the DPF is kept as hot as possible. The further the DPF is away from the engine, the more heat the engine will have to make in order to regenerate it.

 

[SomptingGuy]

How is a urea system a replacement for a DPF system? Surely you still need a DPF, even if you've switched from an EGR approach to a urea approach for NOx reduction?

- Steve