Just sittin around the campground thinking that I may have figured something out in my effort to understand the 6.4L truck. We have talked a couple times on here about the truck having a need to regen immediately after a hard pull. I got my one and only "DRIVE TO CLEAN" message after a long hard pull up the mountain into Cheyenne. So, I'm thinking that the truck wanted to regen all the way up that mountain but could not because the load (17,000 lbs) and the climb already had the EGTs so high that the computer would not allow the regen to occur because the regen would increase the EGTs beyond a safe level. I may or may not have gotten the "Cleaning" message as I could have missed it. But, I'm guessing it didn't send a message because the computer intervened and stopped it from even trying. Now that's just a country boy's guess. Does anybody think I'm right?

 

I don't think so, Stanley.

Higher EGTs would make the regen process easier for the engine to complete. I've had times where my truck would enter regen while pulling. I remember Scott had his enter regen in the middle of a pull during our meet in OH in January, and this was at full thottle.

I only saw the "drive to clean" message once on my '08, and that was during a prior meet we had in Millersburg, it happened when I stopped on the hill to take some pictures. Got in, put it in drive, and put the pedal on the floor and it entered regen.

 

Seems to me that those higher EGT's on a hard climb would do the cleaning for you...but it does not. I think the 6.7 does this to a great extent.

What I think is going on is you are producing more soot then can be handled passively or active then the normal regen process.

So because is it so much then I think it enters the drive to clean mode.

More than in can handle.

 

Got me thinking.... Now, I'm not an exhaust expert nor am I the Word, but here's what I get after adding up all the numbers.

I think active regen is not allowed if EGT's are too high to avoid cracking the DPF.

Also, at heavy load, there's likely more soot being generated than can be burned off passively at the same time, even if EGT's are sufficient. More importantly, there's insufficient O2 left in the exhaust for an active or passive regen to work properly. If conditions continue long enough to preclude a regen from starting normally, soot levels accumulate beyond the normal trip-point and a drive-to-clean shows up.

It would be nice to have some sort of indicator (like those mpg economy lights) that lights up to say when we're driving in some optimum range where soot output is minimized and the DPF is emptying rather than filling.

I think I remember seeing some DPF loading chart showing when active regens start. I don't remember the exact numbers, but it was something like, at 50% DPF loading, active regen starts. If by 75% regen hasn't started, there's a drive-to-clean message. Beyond that, it goes into limp mode or shuts down the truck to prevent catastrophic damage

Two fundamental problems we have are, 1: our regens require O2 in the exhaust for oxidation, so near-WOT driving doesn't do anything for passive regens. 2: EGR is used for NOx reduction and that results in more soot production. I suspect that simply plugging up the EGR path is a cheap way to reduce the regens and gain some power back (at the expense of NOx emissions)

The 6.7 uses SCR for NOx reduction in the tailpipe rather than in the engine and therefore it can afford to run with less EGR and higher combustion temperatures. Therefore it generates less soot, gets better power, and better everything by design.

Consider our emissions equipment 'rev1.0'. The 6.7 is 'rev2.0'. Let's see what rev3.0 will look like... maybe by then, our engines will require a soot generator to run properly

 

Hopefully things will improve but I am doubtful. I'm still a firm believer that the EPA is simply a puppet controlled by big business and big oil. Do you think we are the only ones that realize this emissions crap actually leads to worse MPG and the consumption of more oil, thus creating more pollution? Prior to 2008 diesels were (for the most part) getting far superior MPG than gassers with less problems and a far longer running life. It didn't take them long to realize this meant fewer vehicle sales and millions less in fuel sales. I guess maybe we're all crazy for swallowing the blue pill and believing what we hear on tv and see in the news.

 

Good post, Aquaman. But I don't think that engine loading and EGTs have anything to do with whether or not the truck will enter regen.

When the active regen starts, the truck enters an alternate running mode. the throttle plate becomes active limiting air intake, injection timing changes, and power becomes somewhat limited for some reason. The ultimate purpose of this is to get the exhaust to a condition conducive to burning off soot. Meaning ~1,200°F EGTs and some oxygen. Why would the engine not be able to switch to this mode while under load?

Oxygen may be necessary for the oxidation, but there will always be plenty of oxygen in the exhaust, as these engines run perpetually lean. It doesn't take 41 PSI of boost to create 350 HP, and therefore the turbos nearly always push more boost than is necessary to the cylinders. Remember that the 6.7L engine only makes 30 PSI of boost and is capable of regenerating with no reduction of power.

The other problem with this theory is that the 6.7L completes passive regens best when it's heavily loaded and frequently seeing WOT. To me this implies that oxygen isn't the problem here.

And if it were, why can't the engine switch to active regen strategy to provide the necessary O2?

 

I was wondering about that too.... why not just bump up the boost a bit. The only thing I can think of is it might actually cool off the EGT's for passive regens. Maybe there's a balance of paramters to avoid DPF thermal runaway at high EGT's also.

I'm sure the Ford engineers have learned what not to do on rev1 and applied all that wisdom to the 6.7

Well, as I said... I don't necessarily know what I'm talking about

 

Well, OK lot's of ideas. I like my theory though, truck at WOT, loaded to the max, a maybe 40 or 50 mile climb from 600 MSL to 5000 MSL, in July. What are the EGTs? 1200 or more, making plenty of soot and loading the DPF up. No way ( I think) the computer lets an active regen start and pour fuel into the exhaust thus raising the EGTs to what? 1400, 1500? So, as soon as I clear the Interstate and let the EGTs cool I'm in dire need of a cleaning. So much so that the truck gives me the "Drive to Clean" so I don't shut down and miss another regen opportunity. That actually makes sense, which probably means it's completely wrong.

 

Missed this one... one thing I do know is NOx oxidizes soot but at much lower temperatures than O2. Since we have so much EGR, there's not much NOx coming out of ours, and by the time it leaves the CAT, there's hardly anything left other than soot for DPF.

6.7 relies on SCR and therefore can afford to have higher NOx content leaving the exhaust before the CAT. Half the soot is probably gone in the 6.7 before it even reaches the DPF. So heavy loading and WOT and correspondingly high EGT's make that all happen a lot easier than ours would... makes sense...

 

Very interesting theory, and unfortunately I don't know enough to really form an opinion on it. Makes sense though...

I just can't imagine how 30-40 PSI of boost won't leave enough oxygen in the exhaust to regenerate. Ever notice how during regen boost levels actually decrease feeding the engine less oxygen?

 

Ya know? I think it's all about NOx emissions. The whole reason we have all this mess was to meet new NOx limits. Dealing with soot is just a secondary problem.

I'll bet whatever is happening always keeps NOx within limits, regardless of whether it's efficient for regens. For us, that seems to include messing with EGR and boost

 

I don't think so. The way I read it the EPA went to great lengths to make soot (particulate matter) a carcenigen. That allowed them to mandate the DPF technology. I read their studies, they know all about DPF clogings, fuel in the oil, excessive engine wear and poor MPG. But they don't care, not their problem. If it was just nox the Cat would handle it. I continue to think they wanted to get rid of the soot because it looked bad to the gasser drivers who then pointed to diesels and asked why they needed a cat while diesels were making all that smoke and didn't. Just MHO.