Some thoughts on the 2.2 C5 EGR SYSTEM (Long)

[KevMayer]

This is an interesting thread. Great stuff cachaciero.

On the 2.2 Hdi, I was under the impression that the butterfly valves in the air inlet ducts are part of the FAP regeneration system which happens every 400km or so. The two valves work whereby the intercooler duct valve shuts and the intercooler bypass duct opens and is heated by a water jacket so that the inlet air is uncooled which helps to raise combustion temperature to the level needed for the Eolys to work. I bet the EGR is shut too to raise the temperature.

I also thought that the swirl valves were to overcome the problem of low torque on 16 valve engines at low revs. So, by shutting off one inlet valve you get 8 valve performance for better low speed torque. This was quoted as an issue on 16 valve petrol engines like the Escort XR3i and Audi 80 2.0 16valve etc.

Citronet describes the swirl system. The 'always open' duct being helical and producing a rotating vortex air flow (swirl) and the second duct having the swirl valve which is tangential and opens at 2100 rpm to supply an axial air flow at high revs. All this to optimise air flow into the combustion chamber. I believe that the Audi 2.8 V6 petrol engine of the early 1990s used a similar variable inlet duct system to optimise torque delivery.

This is the article I'm thinking of for the 2.2 Hdi:-


http://www.citroenet.org.uk/passenger-c ... tech3.html

[CitTone]

Ask yourself is there a MASS flow sensor on a traditional old style diesel engine system?. If not why would you need one on an HDI engine ?

Unfortunately, probably no reason other than meeting Euro emissions regs!

OTOH our C3 screws 92bhp out of 1.4 litres and does 63mpg.....which would all be fantasy numbers on a mechanical-injection system, and I can't believe that all comes out of actual physical engine design changes.

[cachaciero]

This is an interesting thread. Great stuff cachaciero.

On the 2.2 Hdi, I was under the impression that the butterfly valves in the air inlet ducts are part of the FAP regeneration system which happens every 400km or so. The two valves work whereby the intercooler duct valve shuts and the intercooler bypass duct opens and is heated by a water jacket so that the inlet air is uncooled which helps to raise combustion temperature to the level needed for the Eolys to work. I bet the EGR is shut too to raise the temperature.

Your impression is correct in that, the EGR system is disabled during a regeneration cycle precisely to ensure maximum combustion temperature for regeneration. I am not clear under exactly what conditions the inlet air from the inter cooler is bypassed and heated, I thought that it was only when the ambient temperature was low and the engine was cold but I could be wrong. However the Butterfly valve I am talking about is the "Air Doseur" valve which goes on the air inlet to the manifold.

As regards "swirl " valves well all the reasons you have stated may well be valid in part or total, and my description is supposition based upon what I had learn't about EGR systems. There can be no doubt that as revs increase so does turbo speed and inlet manifold pressure, now gas turbulence is required in the combustion chamber to promote air fuel mixing and flame front propagation so the gas flow into the chamber needs to be fast.This implies that the inlet tract needs ideally to be narrower at low speed and wider at higher speeds to ensure adequate cylinder filling. Now an EGR system relies on the exhaust pressure being higher than the inlet pressure but as inlet pressure increases due to turbo pressure this differential reduces, by a happy coincidence opening the second valve will cause the inlet pressure to drop, couple this with the ability to increase exhaust pressure by modulating the Turbo exhaust area will increase the rev range over which there is a positive exhaust pressure differential. I suspect that all these factors played a part in the original design.

Of course those responsible for " image" will describe these in such a way that the reader feels that he is getting better performance, they will gloss over the fact that some of the performance enhancing bits are dual purpose used in a system which actually reduces performance For example variable geometry turbo's, most of the things that I have read leave you thinking that they were invented to enhance performance when actually as far as I can see they were developed primarily to facilitate EGR design and operation. As regards the swirl valves I would suggest that most people can discern no performance degradation if the swirl valve is locked closed i.e effectively running on a 12 Valve head.

Interestingly the later version of the 2.2 does not as far as I can see have the swirl valve setup or a variable geometry turbo......

cachaciero

[pugsick]

Shhhesh this is some thread. Learnt a lot just by reading it (thank you).

I have no 'electrical or mechanical' input (car numpty me) but owning a 407 2.0hdi FAP. i read this enthusiasticaly. All i can add is at my work we run 2 x 7.5t trucks. 1 x with the addblue system and the other with the 'normal' system. The adblue system actually does a lot less miles to the gallon than the other type. The biggest difference with the adblue apart from the mpg is the fact that as the engine is euro etc etc compliant you can enter the city of London. We have researched and the adblue system like others can be disabled quite simply.


I will now stand back and leave the experts to add their points. I know the stuff is cheap but is anyone wants any adblue for any trial or experimant they can let me know as i have drums of the stuff at work. (if exposed to the open air for long periods the stuff actually crystalises and turns into a salt like matter.

Thanks again.

[KevMayer]

The air piping on the 2.2 Hdi is as in the following diagram (well, it is on my C5 which is RP9362 ):-



Now, up to RP9151 (for some strange reason my C5 at RP9362 also has this arrangement)

Item 1 is 'Air doseur cold'
Item 2 is 'Air doseur hot'

Pipe 9 comes off the turbo and there's a T in pipe 11.

One leg of the T goes to the intercooler, the other leg bypasses the intercooler and the air goes through a water heated jacket (I'm assume it's for heating, could possily be for cooling but that wouldn't make sense).

Item 4 is fastened to the inlet manifold and the hot and cold air doseur valves are connected to item 4.

For normal operation 2 is shut (hot leg) and 1 is open (intercooled 'cold' leg).

For FAP regeneration they swap over and 2 is open and 1 shut.

For RP9152 and above then item 3 is fitted (and replaces items 1,2 and 4), also called Air doseur, and it sounds like this is fitted on your C5 (cachaciero's C5). Item 3 looks like it will give a hot/cold air mix (it may be a flip flop valve rather than a mixer).

Good stuff all this

[cachaciero]

Hi Kev

That's interesting, need to get my mind around the implications of the differences, but have to go and earn a crust tonight one thing I can clarify the bit of pipe between 7 and 7 on your drawing is a heat exchanger fed with hot water from the cooling system. So what you are saying is that during a regen the intercooler is bypassed andd the charge air into the manifold will be very hot. Now my understanding of that would be that the air density will be less and thus the mass of air entering the cylinder would be less and hence less oxygen available for combustion which could mean that the combustion process would not be so complete which is not what one would want. On the other hand there may be such an excess of oxygen anyway that it make little difference in this respect.
I think the valve change is probably just a way of saving money in production but I think it would also involve changes in software in the ECU. For EGR operation 2 would be closed and 1 modulated i.e partially closed to ensure adequate exhaust gas concentration in the manifold when the EGR valve was open.

cachaciero

[cachaciero]

Shhhesh this is some thread. Learnt a lot just by reading it (thank you).

I have no 'electrical or mechanical' input (car numpty me) but owning a 407 2.0hdi FAP. i read this enthusiasticaly. All i can add is at my work we run 2 x 7.5t trucks. 1 x with the addblue system and the other with the 'normal' system. The adblue system actually does a lot less miles to the gallon than the other type.

That's interesting I would have expected the AdBlue equipped vehicle to be the more economical, on the basis that the diesel is burnt more efficiently without being choked up with it's own waste product. I wonder if the AdBlue injection system increases the exhaust back pressure significantly, also do either of the trucks have a carbon filter (FAP) in the exhaust. Would be nice to get more feedback on this as I suspect that AdBlue will be coming to a car near you very soon. (Probably a Merc

How do you rate the 407?, interested as I keep toying with the idea of buying one.

cachaciero

[steelcityuk]

I'm not sure I agree that a well maintained EGR increases efficiency. That said I've skimmed the thread so could have missed a few points. But I would have thought that any pollution of a fresh charge with exhaust gases will reduce the amount of power generated which is reflected in the lower exhaust gas temperature. This is the reason why EGR can't be used at high revs/power, the relative excess of fuel and lack of oxygen would make the car smoke badly.

I think that the main problem with gumming up of the induction system is the mixing of cool oily air meeting hot dry exhaust gas. I can't speak for the 2.2 HDi but I think the 2.0 HDi's EGR system has a heat exchanger function with the cabin heater, this would make sense - if the engine is only under low load it won't heat up very quickly but it will be recycling a fair amount of exhaust gas which will not only heat up the cooling system but will also cool the exhaust gas which could help minimise the power drop from polluting the fresh charge.

This is just my thoughts not facts so please feel free to shred them.

Steve.

[Sid_the_Squid]

2.0 HDi's EGR system has a heat exchanger function with the cabin heater

Certainly does have a heat exchange with the car coolant system, either just before or just after the cabin heater. Can't say I've noticed my car heating the coolant up any slower or faster since disabling my EGR system.

[cachaciero]

I'm not sure I agree that a well maintained EGR increases efficiency. That said I've skimmed the thread so could have missed a few points. But I would have thought that any pollution of a fresh charge with exhaust gases will reduce the amount of power generated which is reflected in the lower exhaust gas temperature. This is the reason why EGR can't be used at high revs/power, the relative excess of fuel and lack of oxygen would make the car smoke badly.

I'm still not convinced about efficiency either but the article i got that from in fact several places all had that stance and apparently could prove it by all sorts of good technical arguments about charge mass, temperature transfer etc etc most of which was a little to complicated for my single brain cell so I took the summary to be correct

However at a more simple level.
In an ideal world in the combustion chamber you only need just enough oxygen to sustain the total combustion of the amount of fuel injected, any oxygen in excess of this is surplus to requirements and at high temperatures will combine with Nitrogen to form NOx.
In general terms there is far and away much more oxygen in a cylinder full of air than is required to combust the quantity of fuel injected over most of the usable power range of the engine.
So ideally to reduce NOx then two strategy's could be used, one reduce the amount of free oxygen available and / or reduce the combustion temperature to a level below which the endothermic reaction between Oxygen and Nitrogen can take place.
If the incoming air were to be limited to a quantity which had just the right quantity of oxygen and the balance made up with an inert gas say Nitrogen or better CO2 (but in our case exhaust gas then assuming perfect combustion the fuel would be converted to its maximum possible heat with resulting maximum cylinder gas pressure and hence torque.
Now it would seem that due to the vagries of gas / fuel mixing in the combustion chamber coupled with equally variable flame front propagation then the theoretical exact amount of oxygen would not get totally used thus the combustion temperature would be lower and some fuel would be unburnt so less torque.
So the conclusion from this is that to ensure that all of the injected fuel gets burnt more oxygen than the theoretical amount will need to be present, this then means that some of this free oxygen will if in a Nitrogen rich environment form NOx.
So it would seem that to reduce the amount of NOx produced the system has to be designed to run oxygen lean which will mean that not all the fuel will be burnt, thus temperatures will be lower than the temperature at which the endothermic reaction will take place BUT it also means that not all the potential calorific value in the fuel is being used to produce gas expansion and hence torque. So at a simplistic level like you I am not convinced that an EGR system can ever be efficient.

However your comment about not being able to use EGR at high revs / power don't stack up. Assuming an efficient cylinder gas charging system i.e manifolding inlet valve sizes the amount of fuel injected should never be more than enough to produce the required energy in the combustion process.
With modern electronically controlled engines there's no reason why an engine with an EGR system designed to work properly should smoke anymore at the high end than it does at the low end.
Historically early EGR systems have been restricted to low to medium power ranges simply because they relied on exhaust pressure being higher than manifold pressure, the problem with this is that with turbo engines with no other measures other than a simple EGR valve above about 2K revs the pressure differential ceases to be adequate to allow exhaust gas mixing to take place, hence some of the devices as used on the 2.2 to push the rev range at which mixing will take place to about 3 -3.5K revs at a guess.

I think that the main problem with gumming up of the induction system is the mixing of cool oily air meeting hot dry exhaust gas. I can't speak for the 2.2 HDi but I think the 2.0 HDi's EGR system has a heat exchanger function with the cabin heater, this would make sense - if the engine is only under low load it won't heat up very quickly but it will be recycling a fair amount of exhaust gas which will not only heat up the cooling system but will also cool the exhaust gas which could help minimise the power drop from polluting the fresh charge.

I would agree with you the 2.2 is no different in this respect and the cooler around the EGR supply to the manifold is, from the info I have primarily to reduce the temperature of the injected gas thus improving cylinder charge density's, something else that suffers with hot gas injection.

This is just my thoughts not facts so please feel free to shred them.

Steve.

Shred them moi? far be if for me to shred anybodys input on a subject for which my own understanding is somewhat imperfect
I started this thread in part to explain my understanding of what is a quite complicated and controversial part of the car in the hope and belief that it would make others think and contribute bits of their own understanding / knowledge that way we may all end up knowing a bit or a lot more than we did, if nothing else it will hopefully make people think harder before fitting a new MASS Flow sensor because they believe that it is a prime sensor as regards fuel supply On that note it does seem to me that minor failures / differences in components could make quite a difference to the oxygen available for combustion with noticeable changes in perceived performance. In that respect it is interesting to note that the 178BHP 2.2 has a Lambda sensor in the exhaust manifold, that would allow far greater control of exhaust gas oxygen content.

cahcaciero

[MikeT]

I started this thread in part to explain my understanding of what is a quite complicated and controversial part of the car in the hope and belief that it would make others think and contribute bits of their own understanding / knowledge that way we may all end up knowing a bit or a lot more than we did

This is a great thread, helping me try and understand these modern diesels, thanks everyone for their input!

Here's my tuppence worth, if I may...

I don't doubt EGR's and FAP's etc, do their thing when looked at in isolation but looking at the whole picture, I suspect they cause an increase in pollutant output through reduced engine efficiency and added costs.

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?

And secondly, regarding the EGR only being possible at low/medium load, isn't it a matter of physics that (within the efficiency island/map of the turbo) exhaust flow is always greater than compressor flow?

[CitTone]

isn't it a matter of physics that (within the efficiency island/map of the turbo) exhaust flow is always greater than compressor flow?

You'd hope so, with all that heat being added!

[pugsick]

cachaciero

Sorry for my delay in replying ref the 407. Great car i would say if working and maintained correctly (i guess the same as anyother vehicle). Very echonomical when not in limp mode and as far as looks and cabin go i would say its good.

That the said i am having a long term affair with a 'bitch' of a car that 1 day (soon) i hope to tame and enjoy a 'normal' relationship.

Im sure MikeT would agree after his inspection last weekend..

[cachaciero]

This is a great thread, helping me try and understand these modern diesels, thanks everyone for their input!

Here's my tuppence worth, if I may...

I don't doubt EGR's and FAP's etc, do their thing when looked at in isolation but looking at the whole picture, I suspect they cause an increase in pollutant output through reduced engine efficiency and added costs.

Yes Mike that's my feeling as well but then theres pollution and then theres........pollution. It seems to me that the first target handed on to the auto industry was to reduce NOx which was laudable and I do agree with.
However the measures taken to achieve this have reduced NOx but increased other pollutants such as particulates which have also now become a target. Those targets have been achieved but at the cost of a relative increase in fuel burn which in turn means more carbon which has also become a target. The one good thing that has come out of it is that the corresponding developments of the modern HDi engine has resulted in significant reductiond in terms of consumption and even with the "fuel cost" of the pollution control methods it is still more economical than it's predeccesors

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.

And secondly, regarding the EGR only being possible at low/medium load, isn't it a matter of physics that (within the efficiency island/map of the turbo) exhaust flow is always greater than compressor flow?

It's not just flow you need it's pressure, but of course the two are inter- related. Simplistically if the Turbo was a 100% efficient then with no restriction on the outlet or inlet then you could get an outlet flow which would be higher than the exhaust flow, the work being done to achieve this being a pressure and temperature drop across the exhaust turbine.
However flow without pressure is not a lot of use so once you start to increase the pressure of the outlet so you will need to do more work on the input side so the pressure and temperature drop across the exhaust turbine will be even greater.
The dynamics of this get complicated even for a simple turbo / blowing system simply because you need power in the exhaust pipe to drive the turbo and the turbo output directly effects that power availability and it's not at an accelerator pedal level a simple relationship, acceleration can be quite flat and then once there is sufficient power in the pipe it comes in with a surge, when it comes in is not just a question of how much pedal you give, it depends on revs and engine load as well i.e how much energy there is available in the exhaust pipe.
Once you start playing with variable geometry exhaust turbines and restrictor valves on inlet manifolds it gets real complicated and way beyond my limited ability to visualize.
cachaciero

[cachaciero]

cachaciero

Sorry for my delay in replying ref the 407. Great car i would say if working and maintained correctly (i guess the same as anyother vehicle). Very echonomical when not in limp mode and as far as looks and cabin go i would say its good.

That the said i am having a long term affair with a 'bitch' of a car that 1 day (soon) i hope to tame and enjoy a 'normal' relationship.

Im sure MikeT would agree after his inspection last weekend..

Ah! needs a good shagging then I get the impression that quite a few of them are like that maybe best to steer clear then, can't afford the sub-contractors fees

cachaciero