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DPF Burn-off, Overrun and Instantaneous MPG


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If I were to:

  • Display Instantaneous MPG permanently in the dash
  • Lift off throttle well before ending a journey
  • Check Inst MPG

Would it show something less than 99 mpg during burn-off and, therefore, allow me to extend the journey until completed?

Sam :beardy:   RR Sport HSE Dynamic towing Swift Elegance Grande 845

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During a regen when overrunning mine shows mpg as around 100 to 300, as opposed to 999mpg when not mid-regen.

The only way I know it is doing a regen until I switch off.

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Mine obviously must do a regen, but i have no inkling whatsoever when it is happening, I wish that I did.

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Coachman VIP 575. Mercedes ML 250 auto, One wife, three kids, five Grandkids.

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I thought the burn off was due to the very high heat of an engine that has been running for quite a while, not anything to do with mpg??????

Edited by daveat92
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2 minutes ago, daveat92 said:

I thought the burn off was due to the very high heat of an engine that has been running for quite a while, not anything to do with mpg??????

 

It needs additional fuel to burn off

48 minutes ago, Stevan said:

During a regen when overrunning mine shows mpg as around 100 to 300, as opposed to 999mpg when not mid-regen.

The only way I know it is doing a regen until I switch off.

 

That's interesting,  So you are able to continue a journey until completed?

Sam :beardy:   RR Sport HSE Dynamic towing Swift Elegance Grande 845

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Passive regeneration can happen during the course of a journey using engine produced heat in the exhaust system.

 

Active regeneration is ecu controlled and works with extra fuel being injected into the cylinders to really increase the exhaust gas temperature.

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I have noticed the only time my wife's Golf Mk 7, 2.0 litre, cooling fan cuts  in is during a  managed DPF burn out.

We can hear that, during say 30 mph zones when the overall noise is less. I can also notice it if I am paying attention to the trip mpg, as  that becomes typically  higher that normal, I can also detect that way and by the fan when the process is over.

In that vehicles case piggybacking an LED indicator lamp off the fan would be a simpler way of being more aware  of DPF cleaning activity,  than my hearing the fan.

 

With our Disco 4 no such fan activity can be observed, its mechanically driven, ,so there is no access to  a  power feed involved in that.

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I expect it all depends on he vehicle.

On my Citroen, I just have to make sure I do a 40+ miles drive, non stop about once a month. Then the heat in the exhaust from the really hot engine does the cleaning (burning?).

There is no additional hardware, or software involved.

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9 minutes ago, daveat92 said:

I expect it all depends on he vehicle.

On my Citroen, I just have to make sure I do a 40+ miles drive, non stop about once a month. Then the heat in the exhaust from the really hot engine does the cleaning (burning?).

There is no additional hardware, or software involved.

 

Dave, I suspect there is some further involvement going on, unless that 40 mile blast is all the vehicle does each month.  Citroen certainly do it on  other diesel engines, but it can all happen with the driver blissfully unaware.

 

Passive only regeneration IMO, is the domain of the  seriously fast long distance driver, if that.

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1 hour ago, bob25 said:

Passive regeneration can happen during the course of a journey using engine produced heat in the exhaust system.

 

Active regeneration is ecu controlled and works with extra fuel being injected into the cylinders to really increase the exhaust gas temperature.

Not quite right, in active re-gen, the derv is injected into the exhaust stream just before the Diesel Oxidation Catalysts  ( cat ), as it passes through the cat, a chemical reaction raises the exhaust heat passing out to up to 600 deg c, that is needed to remove the soot from insufficient passive re-gen's, the derv doesn't actually burn.

 

The active system is also triggered when doing a forced re-gen by utilising the specific ECU command, this is needed when successive active re-gen's haven't been completed due to the engine being stopped before completion.

Edited by Silversurf

Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it.  :rolleyes:

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19 minutes ago, Silversurf said:

Not quite right, in active re-gen, the derv is injected into the exhaust stream just before the Diesel Oxidation Catalysts  ( cat ), as it passes through the cat, a chemical reaction raises the exhaust heat passing out to up to 600 deg c, that is needed to remove the soot from insufficient passive re-gen's, the derv doesn't actually burn,

 

Are you sure? If so it begs the question why DPF regeneration over fuelling is the claimed cause of engine oil dilution.

That  over fuel can well end up burning in the oxidation cat stage.

 

Edit: Wiki, clears it up, either technique could be used depending on system design.

 

https://en.wikipedia.org/wiki/Diesel_particulate_filter#:~:text=DPF filters go through a,and lowers the filter pressure.&text=Passive regeneration takes place normally,buildup on the DPF walls.

Edited by JTQ
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1 minute ago, JTQ said:

 

Are you sure? If so it begs the question why DPF regeneration over fuelling is the claimed cause of engine oil dilution.

That  over fuel can well end up burning in the oxidation cat stage.

Yes you are correct, thanks, and I should have mentioned that engines that use post injection fuel delivery  ( extra fuel delivered after the initial burn so doesn't ignite ) suffer from this, with high engine speeds and long distances this is a problem,  but not as much as with slow engine speeds and short journeys, the commuter / school run where the problem is greater.

 

The reason the problem is not quite as great on the long journey engines, is that under prolonged high engine temps most of the derv  will boil off and the vapours be removed by engine sump scavenging.

 

The system I described, now prevents the dilution, and consequent shorter life of engines.

 

The company fleet my eldest son works for don't do oil changes on a time and mileage basis, they dip the oil and have it analysed, only changing it when chemically required, he has mentioned in the past that there was a marked difference in dilution between identical engines the short journey ones needing more frequent changes, but as the new vehicles have been coming on stream there is a marked reduction in the oil bill, the short journey ones still requiring more frequent changes, simply due to the effect of short journeys, lots of cold starts as with any type of engine. 

Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it.  :rolleyes:

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From the same Wikipedia article mentioned by JTQ above:-

 

Regeneration is the process of burning off (oxidizing) the accumulated soot from the filter. This is done either passively (from the engine's exhaust heat in normal operation or by adding a catalyst to the filter) or actively introducing very high heat into the exhaust system. On-board active filter management can use a variety of strategies:[9]

 

As there is nothing in the manual or maintenance schedule about doing anything, I assume my Citroen 1.6ltr Picasso C3 vehicle is the passive type in bold above. That's why I am told to do a long drive fairly frequently (once a month).

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5 minutes ago, daveat92 said:

From the same Wikipedia article mentioned by JTQ above:-

 

Regeneration is the process of burning off (oxidizing) the accumulated soot from the filter. This is done either passively (from the engine's exhaust heat in normal operation or by adding a catalyst to the filter) or actively introducing very high heat into the exhaust system. On-board active filter management can use a variety of strategies:[9]

 

As there is nothing in the manual or maintenance schedule about doing anything, I assume my Citroen 1.6ltr Picasso C3 vehicle is the passive type in bold above. That's why I am told to do a long drive fairly frequently (once a month).

 

As already said I doubt very much your engine only  does passive regeneration and can't do managed regeneration,.

Such a car without managed regeneration would only be viable for a minute fraction of the buyers.

 

The real life situation with at least our domestic cars  are the engines are asked to develop so little of their capability in nearly every metre they drive, [we don't sit at 100mph or accelerate continuously really working the engine ] so  exhaust temperatures are way down. Down to below the levels needed to initiate a self burn in the DPF let alone sustain it till completed. Therefore,  other techniques are needed on our engines to develop those required high temperature , other than as a natural side effect from the power they develop.

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Dave

 

Just about every DPF equipped vehicle does both passive and active regen, they have to as not all cars run for long enough on every journey to get the DPF  hot enough to rely on passive regens only. 

 

Just because you are not aware of an active regen taking place doesn’t mean they are not happening (‘cos  they most certainly are!)  

 

Ask your Citroen dealer when you are next passing. 

Silversurf

 

Are you saying that some Diesel engines have an injector in the exhaust manifold/turbo as well as in the combustion chambers??? 

Experience is an awful teacher who ends up sending you simply horrifying bills

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4 hours ago, daveat92 said:

 

On my Citroen There is no additional hardware, or software involved.

 

There is!   And it's been used on some Citroen & Peugeot engines since 2000 and on every PSA engine since 2009.   On every Citroen or Peugeot, alongside the diesel tank there's a 3ltr tank (sometimes a bag) of PAT fluid.   Every time the car is refueled, a measured amount is injected into the diesel.   (So don't fill with small quantities)   Roughly a litre of Eolys will be used every 25000 miles, so warning messages will appear when the engine has covered 70-75K miles.   The tank then must be topped up with fluid and the computer told that it's been done.   The message warns of filter blockage.   Ignore the message and the engine goes to limp-mode.   The particle filter is fitted with a sensor at each end and when the computer detects a preset difference between the two, regeneration takes place.   It happens every 600 or so miles.        A year ago, at 70000 miles, the Eolys tank on my C5 was refilled in preparation before going to Spain.   

Eolys or Pat fluid shouldn't be confused with Adblue.    That is an additional fluid that has been used for the past few years.    PSA vehicles now require both.

Edited by Jaydug
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Citroen C5-X7 Tourer+Avondale Rialto 480/2
https://jondogoescaravanning.com

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My Jeep brings a message up on the dash when it's doing an active regen. I've only ever seen it once, just after I collected it from the dealers, where presumably it had done a number of short trips. 

 

Never seen it since, so my driving style and journeys are obviously enough for regular passive regens. Mixed use, in winter I use it for the 7 mile round trip to the gym, 12 miles each way to work, and longer trips maybe only once a month or so. Summertime, I mostly use the motorbike, so the Jeep rarely does the short trips, mostly being used for towing and longer journeys. 

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6 hours ago, Mr Plodd said:

Dave

 

Just about every DPF equipped vehicle does both passive and active regen, they have to as not all cars run for long enough on every journey to get the DPF  hot enough to rely on passive regens only. 

 

Just because you are not aware of an active regen taking place doesn’t mean they are not happening (‘cos  they most certainly are!)  

 

Ask your Citroen dealer when you are next passing. 

Silversurf

 

Are you saying that some Diesel engines have an injector in the exhaust manifold/turbo as well as in the combustion chambers??? 

 

Yes ............but not in the manifold or turbo, though depending on the construction of the manifold / down pipe without a turbo, one could be fitted there in theory.

 

There are many permutations, and have many names, some inject raw derv under a high pressure, some derv and air, some a derv vapour, some have heaters and so on, but they will be downstream of the turbo and upstream of the DOC, in many cases close to the DOC inlet, their only purpose is to raise the heat of exhaust gasses from around 300 - 400 c, which is ample for a passive re-gen, as long as the car is driven till the re-gen is complete, to around 600c for an active re-gen, which again must be allowed to take its course to completion, if not a forced re-gen must be done at a cost.

 

They introduce derv into the exhaust as fine droplets  or vapour, as this passes through the DOC, the catalytic action produces the heat required to more completely burn off the soot deposits in the DPF.

 

As mentioned above in the thread, a light informing the driver that either re-gen is taking place and not to stop or shut down the engine till it goes out would be ideal, easy to accomplish with a signal from the ECU when either re-gen is taking place................... but how many drivers take notice of warning lights anyway, even the red ones.

 

No 2 son does quite well out of folk who ignore all colours of warning lights, especially the red ones.

 

Here's a photo of one used by the Citroen Peugeot group.

 

image.png.800bb66b1181f544b689e222251d2bb3.png

 

 

Edited by Silversurf

Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it.  :rolleyes:

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:Thankyou:

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Experience is an awful teacher who ends up sending you simply horrifying bills

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I feel that saying 300 to 400 C  DPF entry temperature is" ample for passive regeneration" will for many of us with DPFs be somewhat misleading.

The 400 C [barely a hint of red in the dark] is just about on the very lowest threshold  at which  a passive burn might be obtained with the likes of some Citroen & Peugeot engines that specifically employ a fuel carried catalyst like cerium oxide, the active component of Eoyls. However, the temperature needed   even then with a fuel carried catalyst is  typically considered as 500 C.

 For those systems without the likes of Eoyls, then there needs to be an entry temperature of about 600C for a DPF burn passive or otherwise

 

An important function of fuel carrier catalyst like Eoyls is simply to reduce the  DPF burn temperature so the better chances of passive regeneration. These days it is not so commonly adopted in our cars, many simply rely on active regeneration where use of excess fuel provides the temperature lift. If your driving gets a passive burn that's a bonus, in reality, I doubt with our cars many do.

 

That occasional motorway thrash for most of us is not to get a passive burn more to get a full cycle active burn,  things up hot enough for the ECU to initiate it and long enough to complete it.

 

 

Edited by JTQ
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So is all this confined to modern diesels or are petrol engines starting to become involved?

 

Also I've  had my diesel for five years and never noticed any of this business.  But I don't  know if all is fine or if I'm  building up a big problem for the future.

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28 minutes ago, JTQ said:

That occasional motorway thrash for most of us is not to get a passive burn more to get a full cycle active burn,  things up hot enough for the ECU to initiate it and long enough to complete it.

 

 

How does towing a fair distance compare with an M-way thrash?

Sam :beardy:   RR Sport HSE Dynamic towing Swift Elegance Grande 845

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21 minutes ago, rovinmad said:

So is all this confined to modern diesels or are petrol engines starting to become involved?

 

Also I've  had my diesel for five years and never noticed any of this business.  But I don't  know if all is fine or if I'm  building up a big problem for the future.

 

As you have not had a problem, then it must be working not building up issues.

 

Unless totally superseded by EVs, IMO, if not in production yet, petrols will need particulate filters as they make even finer more damaging particulates.

They will also need NOx mitigation eg Adblue, as burning the air's nitrogen content, its source, is all about burn temperature, and in seeking greater fuel efficiency, burn temperatures tend to rise.

I suspect for domestic transport the pressures will be on the adoption of EVs rather than the somewhat lost cause of developing IC engines.

 

19 minutes ago, SamD said:

 

How does towing a fair distance compare with an M-way thrash?

 

Towing is great, it requires much more power from the engine, like speed to like speed, hill to hill, so turbo exit temperatures rise and condition improve for getting a burn, either passive or active.

Climb the Alps steady on the legal speed limit, it might even be happy getting a passive burn!

Towing is IMO good news for diesels generally, those hotter temperatures cleaning soot  from everywhere, turbo vanes , valves, injectors etc.

 

 

 

 

 

Edited by JTQ
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I've  only done 25000 in five years with this car.  How quickly do the DPFs fill up?  I do try to avoid short journeys but haven't  done any towing since last year.

Edited by rovinmad
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19 minutes ago, rovinmad said:

I've  only done 25000 in five years with this car.  How quickly do the DPFs fill up?  I do try to avoid short journeys but haven't  done any towing since last year.

 How long is a bit of string question?

Typically with our domestic car's DPFs they tend to block with soot by 20% in something like  300 to 500 miles, and this figure triggers an active burn.

 

The DPF blocks with ash even after decent soot burns; many makers aim for an ash filling DPF life of 100,000 plus kilometres. {our Golf Mk7, I was quoted 150,000 km life]

But the variables are massive, oil type, oil consumption,  fuel type mineral vs synthetic, use of the wrong fuel additives, [some owner then even use two stroke oil to quieten combustion noises, mad  re DPFs] and of course driving styles.

 

 

Edited by JTQ
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