DEF, Regen questions

[lakedaisy]

Unless I don't understand the system, DEF (SCR system) has nothing to do with regen (DPF).


6.7L Cummins Emissions System Explained

 

[kthaxton]

Unless I don't understand the system, DEF (SCR system) has nothing to do with regen (DPF).


6.7L Cummins Emissions System Explained

You are correct. I am assuming that Kirwin was referring to the fact that DEF usage is directly proportional to fuel usage. More fuel is used during regen, therefore more DEF as well.

It is a somewhat common misconception that DEF is used FOR regen however.

 

[Berrigan]

You are correct. I am assuming that Kirwin was referring to the fact that DEF usage is directly proportional to fuel usage. More fuel is used during regen, therefore more DEF as well.

It is a somewhat common misconception that DEF is used FOR regen however.

This was the reason for my original post. I assume (and maybe I am wrong) that higher ambient temps make for hotter engines and hotter exhaust. When the exhaust is hot enough the system is passively cleaning the DPT and active regens are not needed as often. Is this correct? Then the other question was DEF usage. Does the DEF get injected at all times the engine is running or periodically as needed (as conditions require more NOX control).

 

[kthaxton]

This was the reason for my original post. I assume (and maybe I am wrong) that higher ambient temps make for hotter engines and hotter exhaust. When the exhaust is hot enough the system is passively cleaning the DPT and active regens are not needed as often. Is this correct?

Yes, but if by higher "ambient temps" you mean outside temperature, I suspect that has little affect, if you mean in the general sense that the engine is running hotter (because it is working hard-ish), then yes. But I couldn't tell you what kind of work that takes. Perhaps the guys with EGT gauges could tell us if simply driving up a reasonable grade, unloaded, gets the exhaust temps up in the 950*+ range. Again, I doubt outside temps have much influence.

Then the other question was DEF usage. Does the DEF get injected at all times the engine is running or periodically as needed (as conditions require more NOX control).

That, I do not know. I suspect that when the engine is cold (not warmed up yet) that NOX production is low, therefore little to no DEF, but I am just speculating.

 

[Kirwin]

My thoughts are that NOX production is low at idle and higher the more load on the engine. I suspect that the system is constantly metering in DEF once the engine gets above a certain temperature. I'm speculating also. I will research what it says in the service manual and see if it sheds any light on the system operation.

 

[Newsa]

You are correct. I am assuming that Kirwin was referring to the fact that DEF usage is directly proportional to fuel usage. More fuel is used during regen, therefore more DEF as well.

It is a somewhat common misconception that DEF is used FOR regen however.

My understanding of the active regen process is that unburned/partly unburned fuel is sent down the exhaust to burn the soot out of the DPF. If the fuel is not burned in the cylinder (under high oressure), I think it does not produce NOx. High pressure (high compression cylinder) is required for NOx production. If the hot exhaust gasses ignite the excess fuel, no NOx should be created. NOx is formed from inert nitrogen in the air and requires both high temp and high pressure. Gotta have both. Not just high temp.

In previous rams I hear there was a fuel spray into the exhaust stream. These rams seem to inject fuel late into the cylinder?

 

[Newsa]

My thoughts are that NOX production is low at idle and higher the more load on the engine. .

True. The nitrogen and oxygen that form the NOx come from air. There is negligible Nitrogen/oxygen in ths fuel.

So most NOx production occurs when the VGT is pressuring the intake manifold with 20+ psi of air. More air + more oressure + more heat = more NOx.

I don't see much NOx being produced at idle even if it is undergoing an active regen.

I'm not saying the clicking isn't the def pump priming the injector. And I'm not saying the clicking isn't the injector getting the SCR sprayed down good foe future towing. Maybe it is. But the clicking might also not mean it is undergoing regen?
More fuel doesn't always mean more NOx. The NOx comes from the air, not the fuel

The burning fuel just provides the right environment for the NOx production (heat & pressure)

When the intake manifold is pressurized to 20+ psi and you have a 17:1 compression ratio and burning fuel further creating pressure/heat, those are ideal conditions for Nox production

 

[Hoefler]

You can hear the DEF injector opening (clicking) on my truck. I pull in from off the interstate, stand by the passengers side rear door, about every second or 2 you can hear it. Click click click.

It is clicking from hot exhaust cooling.

 

[Newsa]

It is clicking from hot exhaust cooling.

I wondered the same.

But i haven't heard the clicking so i don't know if it is different

 

[RustyJC]

High pressure (high compression cylinder) is required for NOx production.

NOx production is time and temperature dependent. How hot is the combustion zone in which the fuel is being burned, and how long do the temperatures conducive for NOx production last during the combustion cycle? That was one reason for the heavy EGR strategy in the pre-DEF trucks - the recirculated exhaust gases were an inert heat absorber that reduced combustion temperatures, and thus NOx production. Pre-DEF, the strategy to minimize NOx production in the combustion zone was to burn the fuel as cool and as quickly (thus, retarded injection timing) as possible - minimizing both time and temperature.

The use of DEF allows the engine designer to program the engine for best power and fuel economy (which, unfortunately, promotes NOx production) and then clean up the resulting higher levels of NOx afterwards in the exhaust system with DEF and SCR which reduces NOx to N2 and H20.

Very little NOx (relatively speaking) is produced at idle where combustion zone temperatures are lower due to the tremendous amount of excess air in the combustion zone which absorbs the heat of combustion.

Rusty

 

[Newsa]

http://www.sciencedirect.com/science/article/pii/S0010218007001137

NOx emissions significantly increases with increasing pressure.

 

[RustyJC]

http://www.sciencedirect.com/science/article/pii/S0010218007001137

We're talking about reciprocating Diesel-cycle engines, not gas turbines. I retired 12/31/15 after 42 years of service with an engine manufacturer. I'm very aware of the R&D work that we did to understand NOx formation and NOx reduction strategies for spark gas (Otto cycle) and diesel engines. I've provided the insight we gained - take it or leave it.

https://www.dieselnet.com/tech/diesel_emiform.php

Rusty

 

[Newsa]

We're talking about reciprocating Diesel-cycle engines, not gas turbines. I retired 12/31/15 after 42 years of service with an engine manufacturer. I'm very aware of the R&D work that we did to understand NOx formation and NOx reduction strategies for spark gas (Otto cycle) and diesel engines. I've provided the insight we gained - take it or leave it.

https://www.dieselnet.com/tech/diesel_emiform.php

Rusty

From a chemistry perspective. The chamber doesn't matter that much. They are both combustion reactions with hydrocarbons. The air is what creates the NOx. The fuel and chamber only create the conditions for the NOx production

 

[RustyJC]

From a chemistry perspective. The chamber doesn't matter that much. They are both combustion reactions with hydrocarbons. The air is what creates the NOx. The fuel and chamber only create the conditions for the NOx production

The fundamental difference is steady-state (gas turbine) versus cyclic (reciprocating engine) combustion processes.

Rusty

 

[Newsa]

From a chemistry perspective. The chamber doesn't matter that much. They are both combustion reactions with hydrocarbons. The air is what creates the NOx. The fuel and chamber only create the conditions for the NOx production

Your article assumes high pressure. It doesn't discount it.

"Diesel fuel is injected into a cylinder filled with high temperature compressed air."

 

[RustyJC]

Your article assumes high pressure. It doesn't discount it.

"Diesel fuel is injected into a cylinder filled with high temperature compressed air."

I can compress air to 5000 psig. If it's at ambient temperature, no NOx will form. Pressure is merely the device used to bring the air in the combustion zone to the ignition temperature of the injected diesel fuel. The NOx is formed DURING the combustion process. How much NOx is formed depends on (1.) how hot the combustion process is - temperature and (2.) how long it lasts - time. The hotter the combustion process and the longer it lasts equals more NOx production.

Rusty

 

[Kirwin]

No, the engine is still running and you can hear the injector "firing", its a rhythmicaly spaced, very distinct click. Several have reported in on here wondering what it was. Its just most noticeable under the above conditions. Pull in to a Flying J after pulling my trailer for a while, park and leave the engine running, go over to the passengers side and you'll hear it.

It is clicking from hot exhaust cooling.

 

[Kirwin]

I think the discerning factor is that in Diesel combustion there is excess oxygen. There is always excess oxygen by the nature of the combustion in a diesel powered engine. Thus more NOx compared to a spark or gas turbine combustion process. In a diesel combustion occurs until the fuel is gone. In spark ignition and gas turbines, combustion occurs until the fuel is gone but the oxygen is closely metered and it is also gone. Hence NOx production is severely limited by the nature of the combustion. My $0.02 for what its worth. Ken Irwin

 

[RustyJC]

I think the discerning factor is that in Diesel combustion there is excess oxygen. There is always excess oxygen by the nature of the combustion in a diesel powered engine. Thus more NOx compared to a spark or gas turbine combustion process. In a diesel combustion occurs until the fuel is gone. In spark ignition and gas turbines, combustion occurs until the fuel is gone but the oxygen is closely metered and it is also gone. Hence NOx production is severely limited by the nature of the combustion. My $0.02 for what its worth. Ken Irwin

NOx formation is actually highest at the point of stoichiometry - a perfect air/fuel mixture where all the fuel and all the oxygen is consumed in the combustion process. That's where the combustion temperature is the highest. Because of this characteristic, two different NOx reduction strategies have been employed with Otto-cycle engines - rich burn and lean burn.

As the mixture gets richer and moves away from the stoichiometric point, combustion temperatures drop. Most automotive engines are rich-burn engines. That's why, if you lean them out [moving closer to the stoichiometric point], they'll melt a piston or spark plug electrode as combustion temperatures rise. These rich burn engines require exhaust aftertreatment (catalytic reduction) to address the high levels of unburned hydrocarbons they produce.

Lean burn engines have cooler combustion because of the excess air, but in the case of spark-fueled engines, the lean mixtures are harder to ignite. That's why they will normally use a small, rich burning precombustion chamber that issues a flame torch to ignite the lean mixture in the main combustion chamber - Honda's CVCC engines were an automotive example of a lean burn engine. Many large industrial spark gas engines are lean burn engines and still use precombustion chambers (aka Jet Cells where I worked). Our lean burn engines used the trademarked name CleanBurn.

Diesel engines are, by design, lean burn engines. At idle, there's an overabundance of excess air, so combustion temperatures are low and very little NOx is formed. As loads go up, the amount of excess air decreases (they move closer to the stoichiometric point), so combustion temperatures and NOx formation increase. That's why, as you add more fuel to a diesel engine, combustion temperatures will rise (they will drop on a rich burn engine). They can actually be overfueled enough to move over to the rich burn side of stoichiometry when more fuel is available than oxygen - that's the case with those diesel engines "rolling smoke".

Rusty

 

[Newsa]

I can compress air to 5000 psig. If it's at ambient temperature, no NOx will form. Pressure is merely the device used to bring the air in the combustion zone to the ignition temperature of the injected diesel fuel. The NOx is formed DURING the combustion process. How much NOx is formed depends on (1.) how hot the combustion process is - temperature and (2.) how long it lasts - time. The hotter the combustion process and the longer it lasts equals more NOx production.

Rusty

You are correct, you need BOTH high temp and high pressure. I never said high pressure alone would form NOx. The dinitrogen bond in nitrogen gas is too strong to be broken even with a 2000 degree+ oxy-acetylene torch at atmospheric pressure.

The engines you studied and researched were compression ignition engines. So by default the pressure was more than 1 atm at the "top" of their compeessions stroke. If you created NOx with just a flame (no compression or pressure) and you published your research, i would be interested in reading that