Some thoughts on the 2.2 C5 EGR SYSTEM (Long)

[MikeT]

Admittedly, I haven't studied the EGR theory but, if it's excess oxygen that cools the diesel combustion, how does displacing it with exhaust gases achieve the same?

You haven't understood that bit to well it's not excess oxygen that cools, it's the reduction of oxygen that cools, less than optimum oxygen means that the combustion is less complete and hence cooler, think of blowing on a fire, you blow the fire burns brighter and hotter because it's getting more oxygen, stop blowing and it still burns but duller and cooler.

I know what you mean and can't explain it any better, but trust me when I say, excess oxygen keeps diesel combustion safe, heatwise. It's something I've learned in my quest for tuning diesels, as you add more fuel you get more power and more heat, way more heat! Add more oxygen and the heat reduces.

From one article I've just read on a study into extending the rpm range of EGR's in diesels....

"It is generally known that there are two reasons to
reduce NOx by EGR. The first of them is the reduction
of combustion temperature. The addition of exhaust
gases to the intake air increases the amount of combus-
tion-accompanying gases (mainly CO2), which in turn
increases the heat capacity and lowers the combustion
temperature. The second effect is the reduction of oxy-
gen concentration in the intake air, which restrains the
generation of NOx."

This suggest cooling is accomplished by an excess of CO2 (mainly), while reducing the oxygen reduces the amount of NOx that can form.

[cachaciero]

You haven't understood that bit to well it's not excess oxygen that cools, it's the reduction of oxygen that cools, less than optimum oxygen means that the combustion is less complete and hence cooler, think of blowing on a fire, you blow the fire burns brighter and hotter because it's getting more oxygen, stop blowing and it still burns but duller and cooler.

I know what you mean and can't explain it any better, but trust me when I say, excess oxygen keeps diesel combustion safe, heatwise. It's something I've learned in my quest for tuning diesels, as you add more fuel you get more power and more heat, way more heat! Add more oxygen and the heat reduces.

I read this and thought "that flies in the face of all reason" but thinking it through maybe not.
Consider a fixed quantity of injected fuel combusted in a gas ranging from a gas with minimum quantity of oxygen required for combustion to a gas with and excess of oxygen.

I believe that the curve for temperature would show that at low o2 the temperature starts low because all the fuel is not being burnt and increases to the point at which all the fuel is completely combusted, after this adding more oxygen will have no effect because there is nothing to combust so if nothing else was happening the temperature would stay stable however once combustion is happening in an O2 rich mixture then the o2 will combine with Nitrogen if present to form oxides of Nitrogen, this is an endothermic reaction which will remove heat from the combustion process so the temperature will actually fall a little in an amount proportional to the Nox being produced

From one article I've just read on a study into extending the rpm range of EGR's in diesels....

"It is generally known that there are two reasons to
reduce NOx by EGR. The first of them is the reduction
of combustion temperature. The addition of exhaust
gases to the intake air increases the amount of combus-
tion-accompanying gases (mainly CO2), which in turn
increases the heat capacity and lowers the combustion
temperature. The second effect is the reduction of oxy-
gen concentration in the intake air, which restrains the
generation of NOx."

The only bit I don't understand is "increases the heat capacity" increases the heat capacity of what ? reduction of temperature will occur because there is insufficient oxygen to ensure complete combustion.

This suggest cooling is accomplished by an excess of CO2 (mainly), while reducing the oxygen reduces the amount of NOx that can form.

O.K lets think about something that we have glossed over...

The efficiency of a diesel is basically down to volumetric efficiency i.e the cylinder is always fully charged with gas prior to the compression stroke and combustion, unlike a petrol engine. It is important in a diesel that this gas charge is maintained otherwise the compression will be reduced and the gas temperature under compression will fall, fall too much and ignition will not happen.
The only way that the oxygen content can be reduced is by reducing the amount of air entering the engine, but this would reduce the gas charge in the cylinder and compression heat so the "lost" air has to be replaced with another gas i.e exhaust gas.

But......at the end of the process the combustion temperature will be lower because there is less oxygen available than is required to totally combust the injected fuel quantity, the fuel, will burn but because of the low oxygen content it will produce more soot. The exhaust / CO2 is used to maintain charge volume.

[MikeT]

Sorry, I should have chosen my words more carefully but if I replace "oxygen" with "air", my comments will be more accurate.

The second main component of air, nitrogen, also has a role to play in the combustion process. From my understanding, any excess gases that does not burn will act as a heat buffer.

In that paragragh I copied/pasted, I take it to mean "increase the heat capacity" of the gases which "buffer" the combustion process. As these gases therefore have a "cooling effect" it makes sense that if the ratio of such gases are increased, the peak temperatures will decrease.

In a standard tuned diesel, there's always a surplus of oxygen that does not find any fuel left to combust with (that's not to say all fuel molecules are completely combusted because that doesn't happen in reality). I therefore think it's reasonable to assume some of that surplus oxygen can be displaced by exhaust gas without increasing the rate of incomplete combustion, at the same time reducing the opportunity for NOx to form?

Of course, I could be completely wrong but until I learn different, it seems to work for me.

[cachaciero]

Sorry, I should have chosen my words more carefully but if I replace "oxygen" with "air", my comments will be more accurate.

The second main component of air, nitrogen, also has a role to play in the combustion process. From my understanding, any excess gases that does not burn will act as a heat buffer.

O.K could go with that, applies of course to CO2 and other exhaust gases.

In that paragragh I copied/pasted, I take it to mean "increase the heat capacity" of the gases which "buffer" the combustion process. As these gases therefore have a "cooling effect" it makes sense that if the ratio of such gases are increased, the peak temperatures will decrease.

I understand, possibly another way of thinking about this is to consider that for an internal combustion to work gases have to expand, to expand they need to be heated.
The combustion process produces a finite amount of heat the more gas there is the more it will "soak" away the heat until the temperature and pressure are in equilibrium and thus the lower the temperature will be.

In a standard tuned diesel, there's always a surplus of oxygen that does not find any fuel left to combust with (that's not to say all fuel molecules are completely combusted because that doesn't happen in reality). I therefore think it's reasonable to assume some of that surplus oxygen can be displaced by exhaust gas without increasing the rate of incomplete combustion, at the same time reducing the opportunity for NOx to form?

That's what the proponents of EGR systems would argue
However if as you say that not all fuel combusts even in an oxygen rich environment which is true I would submit that even less will combust in an environment from which oxygen has been removed EVEN if there is still a net excess of oxygen and that must be true practically otherwise there would not be the increase in soot which is the result of incomplete combustion.
In an ideal theoretical world if one could guarantee that every molecule of fuel would combine with any available oxygen molecule in the combustion process then as a process EGR would work well but unfortunately in practice it doesn't happen. The reasons why this does not happen are as I understand it to do with gas turbulence, stratification etc.
Designers apparently go to great lengths to get inlet systems that maximise benefits that can be obtained by tailoring gas flows, look at the way the two inlet tracts have been crafted on the 2.2.
However even if they were to design the ultimate in terms of gas flow within a couple of months of use an EGR system will have coked up the inlet tract back of valves etc to completely nullify any benefit, in that sense an EGR system is self defeating .

cachaciero

[MikeT]

I would submit that even less will combust in an environment from which oxygen has been removed EVEN if there is still a net excess of oxygen and that must be true practically otherwise there would not be the increase in soot which is the result of incomplete combustion.

That is a fair submission IMO. As I mentioned, a leaner AFR will produce less heat (and soot) than a richer AFR would. I would be most interested to learn of the combustion process more intently, especially the order of how components act and react but I have trouble studying all the scientific terms and math formulae.

I can only surmise that the buffering exhaust gases can absorb far more heat than the reduced oxygen will create. That's a far cry from the oft repeated claim "reducing oxygen content reduces diesel combustion temperature" which is what baffled me.

By my real gripe is the expense. An inefficient engine and all the costs associated with it, the increase in soot and breathing obstructions... doesn't make the EGR as beneficial to me as it's made out to be IMO.
Ditto for the CAT's and FAP's.

In my case, the EGR and CAT design is a direct obstruction to the careful designs of the intake and exhaust you mentioned earlier.

[cachaciero]

That is a fair submission IMO. As I mentioned, a leaner AFR will produce less heat (and soot) than a richer AFR would. I would be most interested to learn of the combustion process more intently, especially the order of how components act and react but I have trouble studying all the scientific terms and math formulae.

Yes know what you mean I have the same problem

I can only surmise that the buffering exhaust gases can absorb far more heat than the reduced oxygen will create. That's a far cry from the oft repeated claim "reducing oxygen content reduces diesel combustion temperature" which is what baffled me.

Well that depends on exactly what you define as combustion temperature and WHEN you measure it.
If you define combustion temperature as the temperature in the middle of the gas charge some microseconds after ignition then that is one temperature and the amount of oxygen will largely be the arbitter of what that temperature will be all else being equal.
If however you are using exhaust gas temp as a measure of combustion temp then there will be other factors involved in this such as the quantity of gas inducted the temperature of this gas at induction, how much heat has been conducted away in the surrounding structure etc etc.
Something else that you might like to consider is the differences between Co2 and N2 both the specific heat and gas constant are quite different, my brain went tilt trying to work out what effect that would have.

By my real gripe is the expense. An inefficient engine and all the costs associated with it, the increase in soot and breathing obstructions... doesn't make the EGR as beneficial to me as it's made out to be IMO.
Ditto for the CAT's and FAP's.

In my case, the EGR and CAT design is a direct obstruction to the careful designs of the intake and exhaust you mentioned earlier.

Couldn't agree more and the manufacturers don't even have the honesty to put cleaning of the EGR system as a minimum maintenance item in their maintenance schedules, the cost of which if done might scare the original purchaser.

cachaciero

[KevMayer]

Earlier in this thread you mentioned Wynns EGR3 spray for cleaning the inlet mainfold and EGR valve. So, I got 2 cans from GSF and i've squirted them into my inlet ducting pipework. I took the pipe off the top of the intercooler on my 2.2 Hdi and sprayed up the pipe towards the inlet manifold.

I applied one can last Friday and the second can on Tuesday night. My trial hasn't been running long, but, my immediate impression is that the tickover seems a lot smoother, the low down pick up feels more punchy and the fuel economy looks promising.

The cans cost just over £8 each.

Here's a link to Wynns product description which includes some interesting information, especially about how a healthy EGR system can reduce pinking:-

http://www.wynnoil.co.uk/products-wynns ... d-egr3.htm

I'll update some more feedack when I've done a few more miles.

[cachaciero]

Hi Kev

With ref to EGR3 that's good to know, your experience seems to chime with the experience of others on other fori.
Interesting also to know that GSF carry it. I certainly intend to give it a go. I was thinking of disconnecting the EGR pipe at the back of the engine and injecting some down there with a view to removing some of the crap that must accumulate around the EGR valve itself, don't care if it gets stuck shut but would be a bummer if it stuck open . With the pipe off and vac supply removed to EGR valve I was then planning on injecting into the manifold through the EGR connection point. Of course this may be over kill......

cachaciero

[MikeT]

I tried encouraging pugsick to de-coke his engine this way, good to know there's a product that might actually be of some benefit. I've used carb cleaner in a similar way to good effect on petrol engines. I've also learnt that even water (which is turned to steam in the hot engine) can have a beneficial effect. I've actually got a water injection system fitted to mine so expect the head, turbo and exhaust to be pretty clean by now.

On another note, I've almost finished reading some more info on Peugeot's FAP system and it struck me, what's to stop someone just gutting the filter so it cannot get restricted with soot in the first place? I know there's a pressure differential sensor but wouldn't it just see an unblocked reading within spec and not throw a MIL (and not attempt a regen) or is the reality different?

[cachaciero]

On another note, I've almost finished reading some more info on Peugeot's FAP system and it struck me, what's to stop someone just gutting the filter so it cannot get restricted with soot in the first place? I know there's a pressure differential sensor but wouldn't it just see an unblocked reading within spec and not throw a MIL (and not attempt a regen) or is the reality different?

Mike
I think this has been covered by myself and others before, basically not a lot is the quick answer
Again I am assuming that all PSA FAP systems work in a similar way. From such info that I have the system is intelligent enough to "know" when a filter is not there or when there is a split in the pipe or some other leak prior to the FAP i.e it expects to see a pressure drop of some magnitude believed to be about 1 millibar. There have been a few posts of late of people that have removed the FAP on 2.2's and had the ECU modified to remove the code related to the FAP.

Personally I believe that it should be possible to replace the differential pressure sensor with a resistor network that will make the ECU "see" 1 millibar and that should be enough. It won't do a regen unless the pressure across the FAP reaches a certain value higher than 1 mB.
On the 2.2 I feel pretty sure that the Eolys injection bit can be disabled in the BSI menu.

I seem remember that wheeler actually quoted the pressure for a new FAP and somebody maybe wheeler actually related sensor voltages to pressure so a trawl through the Citroen section on FAP should find the data.

cachaciero

[cachaciero]

Well today was "interesting" for a variety of reasons, it's true you do have to take the front apart to change a headlight lamp at least you do if you have Xenons.':oops:' Any way I digress as part of todays exercise of trying to successfuly fix something today a can of Wynns EGR cleaner went in, then I noticed the split in one of the vacuum pipe that connect to the swirl valve EV so I guess that that has not been working correctly, fixed that so now I will have to see if there is any improvement in fuel consumption.
Failed to work out if my failed Xenon is the lamp or the ballast unit, fitted LED sidelight bulbs and then discovered the rear nearside light cluster is out........this car is really getting to me.

cachaciero

[KevMayer]

Have you been getting the ESP/ASR Antipollution etc. warning while the Swirl valve vacuum pipe has been split?

[Paul-R]

Have you been getting the ESP/ASR Antipollution etc. warning while the Swirl valve vacuum pipe has been split?

Or has it thrown a fault code for the Lexia to read.

BTW, I suspect not.

[cachaciero]

Swirl Valve warnings and ESP/ASR Interestingly no except once, back in November. I was crawling along in heavy nearly snowbound traffic and "boing" Anti-Pollution ESP/ASR, oh "sh*t thinks I, but time and conditions didn't allow for much thought, never got to a position where I could find out if the engine had gone into Limp mode though I suspect it had. Following day Service light but car was normal and after a day or so the Service light went out so thought no more about it. Not quite true there was some idle speculation as to what combinations of cold -10 that night and moisture in the inlet might do but that was about it.

The split was a very small split on the swirl valve side of the EV I doubt that the leakage was enough to cause the turbo to go overpressure which is what normally happens with leakage in this system,
Mind I got the Anti Pollution warning for the first couple of starts after dumping a can of EGR3 cleaner in the system, not sure what that was about but all seems to be well at the moment.

cachaciero

[MikeT]

Well that depends on exactly what you define as combustion temperature and WHEN you measure it.

If you define combustion temperature as the temperature in the middle of the gas charge some microseconds after ignition then that is one temperature and the amount of oxygen will largely be the arbitter of what that temperature will be all else being equal.
If however you are using exhaust gas temp as a measure of combustion temp then there will be other factors involved in this such as.....my brain went tilt trying to work out what effect that would have.

Ok, I see what you mean but at the same time, bringing in more variables is known headache territory for me so I'm a bit lost now.

Except to say I think I mean peak combustion temperature though your time/space-specific example means I need to go study more so I'm off to watch a film or something now I guess the formation of NOx is somewhere around that point?

Something else that you might like to consider is the differences between Co2 and N2 both the specific heat and gas constant are quite different, my brain went tilt trying to work out what effect that would have.

Go on, spill the beans. All I could find was N2 specific heat ratio? stays constant while CO2 increases with temperature? Oooh me head.