Some thoughts on the 2.2 C5 EGR SYSTEM (Long)

[cachaciero]

Last weekend I did a run in my car which involved a lot of motorway miles, historically I have seen fuel consumption in the low forties but for this trip it has fallen to the mid thirties.
This coupled with trying to answer something on another thread made me realise that there was a lot I didn't know about EGR systems both generally and specifically so I went and did some research the following is what I have managed to establish generally and as regards the 2.2 C5 my speculation as to why things are the way they are.

First it would seem that the following is true :

A well designed and implemented EGR system can contribute to a slight improvement in fuel consumption maybe 1% while at the same time substantially reducing NOX pollution.

A poorly designed / maintained system will still reduce NOX pollution but will increase fuel consumption, and CO2 .

An EGR system is counter productive in reducing carbon particulates e.g. soot.

The optimum amount of Exhaust gases would appear to be about 40%.

Most small Turbo diesels only use EGR at low to mid engine speeds say to 2 /3K rpm but not at the higher rev ranges where most NOX is produced, for reasons which will become clear.

So from the above the first thing that stands out is that as an EGR system can produce more soot then the requirement for a FAP becomes even more neccessary.

The way that EGR is implemented varies but all systems depend on the the Exhaust gas pressure being higher than inlet pressure. At low to mid rev range this is not difficult and metering can be done with a valve between the exhaust manifold and the inlet manifold. As the engine revs increase so does inlet pressure while exhaust pressure stays relatively the same so it can be seen that with a simple system the amount of gas that can be recycled will fall with increasing revs. And the optimum amounts of gas recycling will not be achievable at the higher rev ranges.

If one considers that inlet tract pressure is a function of the amount of air pumped by the turbo and the resistance of the inlet valves it can be seen that if the inlet valve area was increased then the amount of air flowing into the cylinder would increase but the inlet tract pressure would fall, a fall in inlet pressure would enable more exhaust gas to enter the system. Of course rather than having one big inlet valve which would create other undesirable effects at low engine revs the same effect can be achieved by using two inlet valves.
Ah! Did I hear somebody in the audience say “swirl valves”?
And at what engine speed do the swirl valves open? 2.3K rpm, so my speculation is that the swirl valves have far less to do with performance and everything to do with getting better EGR at higher engine revs.

Another way of getting more exhaust gas into the inlet tract would be to increase exhaust gas pressure that on the face of it is difficult given that the pipe FAP silencer etc. will have a relatively constant resistance to gas flow, but there is the turbo and the turbo in the 2.2 is a variable geometry device which means that the exhaust resistance and hence exhaust manifold pressure can be controlled to some extent by increasing the effective pitch of the turbine blades. I suspect that control of the turbo is also an integral part of the EGR control system on the 2.2.

Another way of reducing the inlet pressure is to have an additional valve that shuts of the air from the intercooler and the 2.2 uses this technique, there is a butterfly valve on the main air inlet on the manifold this is modulated to control the proportion of fresh air entering the inlet manifold.
So in summary it is my belief that the 2.2 was a PSA experiment in trying to increase the rev range over which it was possible to get recycling of the exhaust gasses beyond the 2K range of previous generations. To do this they use modulation of the Turbo the EGR valve and the Air admittance valve plus the swirl valves.
NB This may have been a requirement of EURO4 I havn't studied the Euro3/4/5 requirements.

Did they get it right? Well the experience of most owners would appear to be no!.

However most people that contribute to forums such as this are second owners or first owners that have done high miles and are now having problems. There is substantial anecdotal evidence which indicates that by the time a car reaches 60K there is heavy coking in virtually every implementation of EGR in the mass market and that many cars are having problems with consumption, low engine pickup and other problems all of which appear to be resolved by cleaning / changing components in the EGR loop, e.g EGR Valve, Mass Flow Sensor.
How good the systems were when new is difficult to establish, although the published fuel consumption figures for the 2.2 were not brilliant by comparison with similar cars today so again that would tend to indicate that they didn't get it right.

Could the systems be improved to prevent coking? Well I believe the answer to this is yes but it would cost more. On a car with a FAP system the logical place to get exhaust gas would be downstream of the FAP at this point the exhaust gas will be filtered, “clean” in a carbon and oil vapour sense and so if injected into the inlet system there would be no carbon products to cause coking. However a piece of high temp pipe work from the back of the exhaust system to the inlet manifold would cost significantly more than the little itsy bit of pipe currently used from the exhaust manifold . Further at this point gas pressure would be too low to inject into the manifold so it would need to be injected and metered before the turbo this would mean high temp components and pipe work where currently low temp components and rubber pipe suffice, however it does seem to me the best way of getting optimum mixing over all rev ranges without the build up of coke and gunge which currently beset all current EGR systems.
NB Not all the “gunge” that builds up in the inlet manifold / inlet tract comes from the EGR system a lot, possibly the largest part will come from various oil breathers all of which seem to end up in the inlet piping system somewhere.

Can the EGR system be removed? Well according to the Bosch data I have a failure of the EGR system WILL put the car into LIMP mode, but it doesn't explain how it detects failure I suspect that the MAF is probably key to this.

If the EGR system can be disabled it should in theory reduce the clogging rate of the FAP as the higher combustion temperatures should produce less carbon particulates. And more favourable conditions for carbon to be burnt off in the FAP without having to do a regeneration cycle.

cachaciero

[CitroJim]

Most interesting cachaciero, I never realised the system was so complex in the 2.2 HDi. No wonder it gives problems!!!

The amount of coking seen in the 1.9 and 2.1TD IDI enegines with EGR in operation has to be seen to be believed so i can well understand how coking in the HDi will cause absolute havoc...

So, disabling the EGR on these engines is by no means straightforward then and not a simple case of closing off the EGR valve ports.

Sounds like a software whizz is needed to render the system inoperative by rewriting some code in the ECU as has seemingly been done to render the FAP inoperative.

This all harks back to the early days of the American petrol "emissions" engines when they played with such techniques as AIR and ended up with hideously complex systems that neither worked well and strangled performance and economy. It took a few generations to sort the issues and I guess this is where we are with the 2.2HDi, a flawed first-generation attempt to make a clean diesel...

[old'uns]

all seems perfectly logical to me

i know next to nothing about these systems but HGV's are now swinging more to the AdBlue way than mechanical EGR's.

coincidence?, costs? or the most effective way to get to euro5 & beyond?

[cachaciero]

Most interesting cachaciero, I never realised the system was so complex in the 2.2 HDi. No wonder it gives problems!!!

The amount of coking seen in the 1.9 and 2.1TD IDI enegines with EGR in operation has to be seen to be believed so i can well understand how coking in the HDi will cause absolute havoc...

So, disabling the EGR on these engines is by no means straightforward then and not a simple case of closing off the EGR valve ports.

Well I don't really know until it's tried, as I said in a previous post the Butterfly Air Admitance valve could be disabled in the fully open position just by removing the vacuum supply, the question in my mind is will the ECU throw a wobbly because the airflow as measured by the MAF is outside the map.

Sounds like a software whizz is needed to render the system inoperative by rewriting some code in the ECU as has seemingly been done to render the FAP inoperative.

That would be my guess at the moment I certainly feel that to get the best out of the engine with no EGR various MAPS might benefit from being modified, lets wait and see if somebody "sticks their hand up" and says I've done it it's easy

Off one thing I am sure, regular de-coking of the EGR system is probably a worthwhile exercise. The problem with that is that on the face of it it requires extensive breakdown to do it, However Wynns do an EGR cleaner in a spray can which those that have used speak positively about so I think that I will give that a go first.

cachaciero

[Citroenmad]

Ill have a more in depths read of that later, as ive just skimmed through it.

Ive not looking into how an EGR should be disabled correctly, however our C5 SX has had the vacuum pipe removed for a long time, well before i got the car 20K miles ago. No problems have been caused due to this and it has not stored an error code. This is a 2.0HDi 110 though.

I did the same with our old C5, also a 110 HDi, and it made performance better, with more low down pull.

I regularly clean out the intake system and EGR with air intake cleaner, the cars go noticeably better and with increased mpg.

[corsehf]

Bloody Hell cachaciero!,
I now know more about the EGR system - not sure i understand any of it!!

If i have this right, are you saying that the 2.2 has a poorly designed system which if removed, would make the car run better??
You also mention, that if it was removed, it may bring up an error code.
Ok, as it is now possible to remove the DPF and Eolys system by telling the ECU it is no longer fitted, can the same be done for the EGR system???

By the way, i know nothing about what i'm saying!!!

[cachaciero]

all seems perfectly logical to me

i know next to nothing about these systems but HGV's are now swinging more to the AdBlue way than mechanical EGR's.

coincidence?, costs? or the most effective way to get to euro5 & beyond?

Don't know, AdBlue is not that expensive but requires yet another catalyser in the exhaust plus liquid injection and metering and space for another tank, on the other hand because combustion temperatures will be higher and less soot will be produced may simplify the FAP part, it's a complicated subject

It would appear that S.C.R systems of which AdBlue is but one although the most popular one is capable of getting to Euro6 for Heavy Duty Engines.

cachaciero

[cachaciero]

Bloody Hell cachaciero!,
I now know more about the EGR system - not sure i understand any of it!!

You are not alone I have done a lot of reading and x checking and the write up is a top level summary as I understand it, when you get down into the real technical stuff of heat transfer conversions etc I get lost too

If i have this right, are you saying that the 2.2 has a poorly designed system which if removed, would make the car run better??

Lets just say that my understanding is that the 2.2 was PSA's first attempt at pushing to Euro4 requirements and that it was still work in progress at the time it went into production, that some of their design choices wer'nt the best but they were stuck with them and only got them right on the later 173BHP 2.2 which is basically the same engine with more power and better economy and meets and even exceeds Euro4.

You also mention, that if it was removed, it may bring up an error code.

True I believe that is a strong probability if it was designed to Euro4 that would be a requirement (my understanding) and the Bosh Data says that it will.

Ok, as it is now possible to remove the DPF and Eolys system by telling the ECU it is no longer fitted, can the same be done for the EGR system???

Probably,... only requires someone with the code knowledge

cachaciero

[xantia_v6]

Where is the MAF sensor plumbed in? Just thinking...

[cachaciero]

Where is the MAF sensor plumbed in? Just thinking...

Just downstream of the Aircleaner i.e it see's the total fresh air flow into the engine.

cachaciero

[Deanxm]

Where is the MAF sensor plumbed in? Just thinking...

Just downstream of the Aircleaner i.e it see's the total fresh air flow into the engine.

cachaciero

So how does the engine meter recycled exhaust gas? it has to allow for this surely?

D

[corsehf]

cachaciero, - see what you have done now!!!

This is going to be one of those threads!!!

[cachaciero]

Finally found a reference to something which I had suspected, EGR systems require a FAP to mop up the increased soot produced by EGR system, so there we have it one anti pollution system produces more pollution of a different kind which itself then requires a solution. Anybody for croquet ? or maybe a Tea Party?

http://www.fwi.co.uk/Articles/2009/04/0 ... nology.htm

cachaciero

[Deanxm]

Because this gas is high in water vapour, it cools the combustion process and reduces NOx, saysPerkins.

Surely what it really does is displace good clean oxygen rich combustion air for burnt gas to reduce the intensity of the burn and lowing combustion temps?

I know when i disconnected the egr on my other vehicles low down torque was improved, egr on these systems doesnt activate at higher revs as has been said so it would seem these systems reduce performance considerably.
Newer systems with variable pitch Turbochargers may have sidesteped this performance issue but surely its just robbing from Peter to pay Paul?

D

[cachaciero]

Where is the MAF sensor plumbed in? Just thinking...

Just downstream of the Aircleaner i.e it see's the total fresh air flow into the engine.

cachaciero

So how does the engine meter recycled exhaust gas? it has to allow for this surely?

D

The following is my guestimate of how it probably works.

The ECU will be running a software model of the engine EGR system, this model will consist in part of MAPS which will define the relationship of various valve positions, to other devices and inputs such as temperature revs etc.

The position of all the various valves will be computed from this model based upon such input parameters as revs throttle position temperature etc Tthere is no feedback from the EGR valve itself, the same is true for the turbo modulation.but Turbo pressure is measured and could be used as a feedback parameter.
The fresh air flow into the inlet manifold is measured by the MAF. now the model contained within the ECU will calculate what the fresh air flow into the engine should be for any given EGR valve and Turbo condition it then only needs to modulate the Air Admittance Valve until that value as measured by the MAF is obtained, if the value is not reached it would flag that as an error.

Hope that that makes sense.

cachaciero