EGR and VGT

Keith Browning · March 11, 2006

I don't know if you saw my response to a post over at TDS asking why there was less boost with the EGR valve disconnected. I gave it a shot and I thought this might be a good topic for us to toss around. Here is what I posted:

Quote:

Since the actual "strategy" of electronic engine controls are typically a guarded secret by the manufacturers, they are never publicly published as far as I am aware. The following is "my personal interpretation" based on my knowledge and experience. If you know differently or agree with this, please speak up! Some of what I wrote is based on what is contained in the 6.0L OBD2 theory and operation publication which explains the mechanics of what happens, but, it's the REASON behind what's going on and why is what I am trying to explain, not only to you but to myself! /forums/images/%%GRAEMLIN_URL%%/crazy.gif

The turbocharger is actually controlled by adjusting the vanes to achieve a desired back pressure as measured by the EBP sensor. This back pressure is used to control manifold boost pressure. When the PCM opens the EGR valve, it controls it by reading the sensor that is inside the valve indicating it's position. But EGR is not as simple as that. There are several sensors that the PCM uses to calculate the amount of EGR FLOW and one of those sensors is... you guessed it, the EBP sensor! If the EGR valve fails to open (for whatever reason) the back pressure in the exhaust manifold remains unchanged indicating a low flow condition. The PCM is likely closing the vanes in the turbocharger in an attempt to raise the back pressure to increase EGR flow. In doing this, the turbocharger turbine loses the volume of expanding gasses it needs to turn at high speed and it slows down. This should help EGR flow by increasing back pressure in the exhaust but it also causes lower boost pressure in the intake manifold.

Now do you see just how important the EBP sensor is to the operation of this engine? The EGR and VGT perform a delicate but wicked dance balanced upon a tiny little sensor. In reality, the two systems actually fight each other. This is also the reason that making significant changes to the flow of air in and out of the engine by installing aftermarket intake and exhaust systems to your truck can cause performance issues.

HGM · March 12, 2006

Very good explanation Keith :cheers-jeers: .... This also explains why the EBP was phased out on the '03s /forums/images/%%GRAEMLIN_URL%%/wink.gif ..

Keith Browning · March 12, 2006

A similar question was raised about removing the EBP sensor via software. Considering what we have gone through with calibration changes and EBP sensor changes it seems risky to have designed the systems to depend on that sensor so heavily in the first place. Unless it is vital to determining EGR flow and OBD2 compliance why not have stuck a position sensor on the VGT and determined EGR flow using the other inputs... like it apparently can be done as with the '03s.

Cetane · March 13, 2006

My knowledge on the 6.0 strategy isnt very deep but doesnt the MAF also help determine the actual EGR flow? Say the engine needs "X" amount of air at a given power level and rpm. If is is flowing some amount less than "X" wouldnt this allow the PCM to infer the difference as the EGR flow. I am guessing that is what allowed the deletion of the EGBP sensor. Based on that I think the reason for the performance issues is that the PCM sees more airflow though the MAF and thinks the EGR valve is not flowing enough. This causes the valve to open more than it should making more smoke and HC emissions possibly even leading to misfire. Just a thought, again I dont know a lot about the differences between the 6.0 and other Ford engines but it sounds good to me /forums/images/%%GRAEMLIN_URL%%/smile.gif .

Keith Browning · March 13, 2006

My knowledge on the 6.0 strategy isn't very deep but doesn't the MAF also help determine the actual EGR flow? Say the engine needs "X" amount of air at a given power level and rpm. If is is flowing some amount less than "X" wouldn't this allow the PCM to infer the difference as the EGR flow.

Hey my knowledge is limited too but I am just trying to make sense of how this stuff is supposed to work. Once we determine that, we'll try to figure out how the EBP can be eliminated.

From what I know as fact, you are correct in your description however I think you are actually describing the Flow test which does in fact infer the actual EGR flow rate for a given position of the valve. I also understand that this flow test is only performed under certain conditions and is actually a part of the comprehensive component monitor. Yes?

Any way, once we know the volume of flow for a given valve position, EBP and MGP readings, I believe that MAF is not a part of the EGR closed loop operation. (please correct me if I am wrong) EGR is still controlled by managing the pressures in the intake and exhaust manifolds while the valve is open. Closing the VGT will raise EBP while lowering boost allowing flow.

Please read this carefully:

If the EGR valve is disconnected, stuck closed or clogged, does the PCM continue to close the VGT in an effort to raise EBP and increase EGR flow... causing lower boost pressures and a lack of power?

Cetane · March 13, 2006

Well, I had this big long explanation typed out then we lost power and my whole explanation went black. Since it is so late I will just hit on a couple things. I think there is a misconception about the relationship between boost pressure, EGR and backpressure. Backpressure is kind of like a measurement of how much work the turbo is doing. If you raise the VGT percentage both MAP and backpressure should go up until you get out of the efficiency range of the turbo then the MAP will go down but backpressure will continue to rise. EGR is simply a release of some of this backpressure and should almost always be at a higher pressure than the intake. I am not sure in what other instance you would see the EGBP go up and boost go down. In your example if the EGR is disconnected and PCM does compensate you should see the VGT percentage go up(closing????) to the max value at which point the turbo would likely be out of its efficiency range. This would account for the lower MAP and the higher EGBP, the higher cylinder temps, as well as the lack of power. At this point the turbo would actually be acting as a kind of exhaust brake. If you had the truck in front of you I would expect to see the VGT percentage as high as possible with the MAP lower than expected and the EGBP way too high. As for the MAF stuff, I am not sure how it works for this engine but it definitely is a major player in the EGR determination in other engines. Again if this is based on other diesel engines and may not be the way the 6.0 is designed but it sure sounds like it. If you have a specific scenario I would be happy to try it tomorrow and let you know the results since I know this may not be too clear.

GregH · March 13, 2006

The EGR flow is calculated by the temperature difference between IAT1 and IAT2. The backpressure is calculated by taking MAF and subtracting EGR flow. Backpressure can still be calculated without a MAF by taking RPM and MGP and subtracting out EGR.

HGM · March 13, 2006

Well,

This topic allows for allot of speculation.... The latest info I have, shows that the PCM will command the VGT to cycle(like a monitor test) with the EGR closed, while closely monitoring the MAF. While the MAF and MAP change with the VGT cycle, the PCM can calculate VGT operating range(see if its working).. Basicly, we are now using MAF and speed density on the same truck.. The PCM knows what it commanded, now its watching the MAP and MAF to verify it, rather than the EBP..

When the EGR opens, we have no idea what the flow is.. IAT 1 & IAT 2 are indicators for diagnostics, not nearly accurate enough for the PCM to verify flow %...

Honestly guys, I think we're giving the PCM too much credit.. The VGT and EGR are separate systems working off the same input(EBP).. This has caused troubles since day 1.. The VGT see's EBP drop, so it wants to make more(it has no idea why it dropped).. If EBP goes up, it opens to bleed it off.. The EGR does what it wants to by monitoring the EBP, if it doesnt see EBP change when opening, it opens more.. When it finaly snaps open, its flooded with EGR and the VGT must compensate for the EBP loss.. This is how the old system works, not nearly as sophisticated as we would like.. The two are like an old married coulple who live in the same house, but have no idea(or care) what the other is doing..

Keith Browning · March 14, 2006

Honestly guys, I think we're giving the PCM too much credit..

Perhaps we are. I think I am giving my theory too much credit as well. There are a lot of good explanations and summaries here and I love these kinds of threads but it's apparent we can't come to a conclusion. Before the original thread over on TDS got deleted I was getting some fairly confident reports that boost pressures were lower with an inoperative EGR valve.

Duh! It just hit me! I will get out my VDR and make a couple recordings. One with the EGR operating and another with it disconnected. (yes, you can say that in these Forums) I will post the screen shots and we can pick 'em apart! Sound good?

HGM · March 14, 2006

Sounds good... My gut says, boost will remain the same... I dont have a problem being proven wrong.. When you stop learning, you might as well find another career..Keep us posted..

Jim Warman · March 14, 2006

I have a feeling that boost will be lower, if for no other reason than to keep combustioon chamber temps within reason.

Doc, if you get a chance to play with the VDR, can you use some jumpers and monitor EGR_VP under some high boost or backpressure conditions? I'm wondering if there is enough pressure differential to unseat the valve without it being commanded.

I'd like to try it myself but I'm still up to my a** in alligators.

Cetane · March 14, 2006

I would guess if the PCM doesnt go into some kind of limp home strategy that the boost will be similar(edited... hey its late). As for the the EGR valve being pushed open shouldnt it be the other way around? There have been problems with the EGR being stuck closed when the backpressure is too high on "other" engines, since the 6.0 is a dual poppet you shouldnt be able to generate enough backpressure to hold it closed.

HGM · March 14, 2006

We've had issues with it being blown open when its slightly open already.. This is why the spring pressure is so high.. If you monitor the EGRVP and DC, you'll see that the valve will barely move(or not at all) before 30%.. I have a hard time saying its impossible, but I douubt it will blow open from BP.

Keith Browning · March 15, 2006

Okay, I got lucky today. A 2005 with 10,825 on the odometer... 22,817 if calculated by engine hours. No wonder the EGR valve was clogged!

I made three recordings, all identical runs and trigger points on a VERY steep hill in 1st gear, full throttle. The test variables are:

New EGR valve & freshly de-coked intake and EBP tube /forums/images/%%GRAEMLIN_URL%%/mad.gif
With the EGR valve disconnected
With the EGR valve disconnected and engine harness plugged into a dummy valve

I would like to narrow the PID list down to a minimum. I know which ones I want to look at but I would like to know your choices. Just give me a few days to get this up. First, I left the floppy at work and I will be taking the screen shots at home with my WDS desktop version... aside from that I'm doing 12 hour days lately and I have two little girls at home that distract me from getting anything else done. /forums/images/%%GRAEMLIN_URL%%/eek.gif

HGM · March 15, 2006

I would think:

APP,ECT,EOT,load,EGRDC,EGRVP,VGTDC,MAP,BARO,EBPV,EBP,& MGP..

Those should give a good idea what the conditions are..

Keith Browning · March 16, 2006

GRAPHS 189kb

You can click on the graphs link above and open the image in a browser window. A late version of Internet explorer will allow you to zoom in on the image. To download and view in the program of your choice, right click your mouse and select "Save As."

I noticed the biggest difference between recordings 1 & 3. Recording 1 showed slightly higher boost and a quicker RPM acceleration.

Recording 3 in contrast had less boost, higher EBP readings and look what the VGT is doing.

If you want to see different PIDS or something speak up. It won't take too long to produce new graphs.

Cetane · March 16, 2006

I have a some questions. Is there something like a desired setpoint for both MAP, and EGBP? I am curious how it was following the requested values. Are those values for MAP and EGBP typical for a full throttle launch? I wonder if there is some drivetrain protection active. Those values seem very low. What is the definition of EGRDC? I am assuming it is the commanded EGR percentage. Since run three doesnt have one, did it trip a limp home or MIL?

Jim Warman · March 16, 2006

Did you clear codes or anything before run three? I see we aren't seeing any duty cycle at all and the valve position is stagnant at 1.2 volts.... I would have expected to see the PCM manipulating the valve, even it is is "stuck closed".

Without seeing even an attempt at EGR manipulation, I have to wonder what we are looking at strategy-wise...

Keith Browning · March 16, 2006

This truck came in with an EGR that was fully clogged and was stuck slightly open causing the 1.22V reading on EGRVP. I had to de carbon the intake and EBP tube. My intention was to graph an event with a failed or inoperative valve so I used it as a dummy valve later on. Perhaps I should have used a working valve instead? The MIL was not on but it did set a P1335.

This truck came in running fine, just a check engine light on. At the time of these recordings was running like a raped ape! I am sure the 1/3 bottle of Ford's cetane booster in 1/2 tank of fuel helped that. /forums/images/%%GRAEMLIN_URL%%/smirk.gif

I would like to run this test again on a 2003 or 2004 engine. If there are any necessary changes I should make or try, list them please. Next time I will use a working valve as a dummy to more accurately simulate someone disabling the EGR.

When I was placing the graphs on in the image I was able to superimpose them over the original. The one thing that was obvious is the rate of acceleration was better with everything plugged in and working. So I proved at least that screwing with the EGR wont increase POWER or boost. The question now is why?

HGM · March 16, 2006

Cetane, EGRDC-Duty Cycle..

Keith,

Arent we looking for max boost and EBP? Its hard to tell with the pointer where it is.. Which one showed max?

I'm more curious about the tight 17% on #2.. It seems logical to be that we would have less VGT needed to retain EBP with no EGR flow..On the other hand, isnt 1.2v a little high for a closed EGR? I'm thinking .9v should be completely closed.. Have you got IAT 1 & 2?

Good job on the recordings, looks like you hit the button at the same time each time..

Is there a way you can post the live recordings on here and we can pull them down ourselves and look at the whole file?

Keith Browning · March 16, 2006

Quote:

Is there a way you can post the live recordings on here and we can pull them down ourselves and look at the whole file?

My server apparently does not like the filename of the WDS archive file. If you want the recordings I can e-mail them to anyone who wants them. Send the e-mail address you want me to send it to using the Private Message function.