All - DPF regen and low EGTs | Motoring discussion | Back Room Forum

Wed 21 Dec 2011 03:15

All - DPF regen and low EGTs - unthrottled

With DPF now effectively mandatory on all Euro V compliant diesel engines, the problem of DPF regeneration on vehicles used predominantly for short, low speed journeys is going not going to disappear any time soon.

The fundamental problem is low Exhaust Gas Temperatures (EGTs) inherent with the low engine load. With low EGTs, the ECU has to spray disel into the exhaust gas stream to raise the EGTs to a sufficiently hightemperature to burn off the soot. Clearly this is waste of fuel since diesel burned in the exhaust does not contribute to useful work. But the more worrying problem is unburned diesel running into the sump and contaminating the oil.

There is a very simple way to raise EGTs-raise the intake air temperature. Modern cars are designed with Cold Air Induction which draws in cold air from the front of the engine bay. A simple assembly of plumbers' PVC pipework can be arranged to draw air from over the exhaust manifold instead of from the front grille. So instead of taking air at 5C (in winter), you take air at, say, 25C. A hotter intake temperature will increase the temperatures throughout the engine cycle-including the exhaust-which will make the task of regenerating the DPF much easier.

Of course, there is no free lunch. Hot air is less dense than cold air-so the available power will be reduced. However, if DPF regen is a problem, you are not using very much power. The other problem is increased NOx emissions (which is why the technique is not employed by OEMs.) However the MOT emission test is very lax and does not measure NOx emissions at all...

Perhaps worth a try.

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Wed 21 Dec 2011 07:36

All - DPF regen and low EGTs - RT

There's an even easier solution to the DPF regen problem on diesels used predominantly for short, low speed journeys - only buy a diesel if it's used in the right circumstances.

Too many people are jumping on the diesel bandwagon when their usage pattern just doesn't suit the nature of diesels.

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Thu 22 Dec 2011 16:21

All - DPF regen and low EGTs - Roly93

There's an even easier solution to the DPF regen problem on diesels used predominantly for short, low speed journeys - only buy a diesel if it's used in the right circumstances.

Too many people are jumping on the diesel bandwagon when their usage pattern just doesn't suit the nature of diesels.

Exactly right, thats why my wife has a petrol car for shoppign and schoold run and I have a 2,0 TDI to do on average at least two 150 mile+ motorway runs per week.

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Wed 28 Dec 2011 14:40

All - DPF regen and low EGTs - jamie745

only buy a diesel if it's used in the right circumstances.

Too many people are jumping on the diesel bandwagon when their usage pattern just doesn't suit the nature of diesels.

The thing youre missing is 'usage patterns' only started to become an issue when they started needlessly putting all this emissions savings rubbish onto cars long before they were fit for purpose. 10 years ago you could have a diesel on a low mileage, they were generally the same price as their petrol counterparts and the fuel was cheaper to encourage us all to use it.

Ok in the past running an old diesel primarily for short trips wasnt the best thing for it but at least it didnt lumber you with a £1000 bill when a part breaks which doesnt need to be there. You talk about a 'bandwagon' but can you blame people? People are switching to supposed super economical diesels primarily due to fuel prices, if petrol was still 55p a litre - which it is, without tax - then we wouldnt be having this conversation. Most people are buying these things totally unaware that the EU has decided to make diesels unreliable and get unhelpfully fobbed off by dealers at the same time.

Rather than blame the customers who buy diesels in good faith for the problems, i'd rather blame the idiots who made them an unwise purchase in the first place.

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Wed 21 Dec 2011 09:19

All - DPF regen and low EGTs - craig-pd130

My old Mondeo IV and current Volvo V60 both use a butterfly flap in the mouth of the intake manifold to radically throttle air intake into the engine during DPF regeneration. This helps to keep the temperatures in the DPF canister within optimum range for soot burn-off, without allowing runaway temperatures ... this would counteract your suggestion, unfortunately.

Personally, I think manufacturers should adopt the system that is used in some large-capacity diesels in trucks: simply have a glow plug in the DPF canister. This means that less fuel needs to be injected post-combustion as the glow plug gets the temps up faster and you should get more efficient coking of the soot.

Surely the unit cost of an extra glow plug can't be THAT much higher ...?

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Wed 21 Dec 2011 09:44

All - DPF regen and low EGTs - Andy P

I wonder if the "problem" DPFs are those that use oxygen-based regeneration (which requires temperatures of around 600°C to regenerate) - those that use NO2-catalysed regeneration only need to reach 250°C, which is normal diesel exhaust temperatures.

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Wed 21 Dec 2011 10:01

All - DPF regen and low EGTs - madf

Just drive up a steep hill about 1 mile long in low gear at 4,000rpm..

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Wed 21 Dec 2011 11:50

All - DPF regen and low EGTs - OldSkoOL

Just drive up a steep hill about 1 mile long in low gear at 4,000rpm..

Why?

You'd have to be pulling a caravan into a head wind up a very very steep hill to get any benefit.

Typically the engine temps drop the higher the RPM, the less work it has to do. Hills are easy work at 4,000rpm, all you are doing is adding to the problem butu using too lower gear for the job, using more fuel.

The temperature will raise going uphill in a high gear at about 1,800 - 2,000 rpm. And providing your car can take it, cruising around at 1,400rpm.

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Wed 21 Dec 2011 20:56

All - DPF regen and low EGTs - unthrottled

Yes. I don't know why the advice that revving a diesel engine raises the EGTs because, for a given power level, it doesn't. As you raise the RPM, the air/fuel ratio leans out which drops the temperature.

This is data from a real industrial engine:

RPM Power Exhaust Temp (post turbine)

1750 255hp 380C

1900 " 350C

2100 " 335C

This is at rated output. You can see why pottering around at 30mph, EGTs are going to be a problem.

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Wed 21 Dec 2011 10:16

All - DPF regen and low EGTs - unthrottled

NO2 is a ferocious oxidising agent-buit with Euro V , you can't have a very high concentration of it in the exhaust strean!

re intake throttling-light throttling is used to facilitate EGR (eitherwise the exhaust gas wouldn'ty recirculate, would it?!). Unfortunately, exhaust gas promotes soot formation, not exactly what we are looking for.

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Wed 21 Dec 2011 12:47

All - DPF regen and low EGTs - craig-pd130

EGR is also disabled during DPF regeneration because it reduces EGTs (and produces soot too, as you say).

This is the most detailed explanation I've yet seen about how latest-generation DPFs work: http://www.honestjohn.co.uk/forum/post/index.htm?t=91443

Here's an extract:

Active regeneration is controlled to achieve a target temperature of 600*C at the inlet of the particulate filter without exceeding the temperature limits of the turbochargers and close-coupled catalysts. During the active regeneration period:

-- The turbochargers are maintained in the fully open position to minimize heat transmission from the exhaust gas to the turbochargers and to reduce the rate of gas flow through the particulate filter. This enables optimum heating of the particulate filter. If the driver demands a higher torque the turbochargers will respond by closing the vanes as required.

-- The throttle is closed as this assists in increasing the exhaust gas temperature and reducing the rate of exhaust gas flow, both of which increase the speed at which particulate filter is heated.

-- The exhaust gas recirculation (EGR) valve is closed as the use of EGR lowers exhaust gas temperatures and therefore makes it difficult to achieve the regeneration temperature in the particulate filter.

-- The glow plugs are sometimes activated to provide additional heat in raising the temperature of the particulate filter. To maintain glow plug serviceability the activation period of the glow plugs is restricted to 40 seconds.

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Thu 22 Dec 2011 07:49

All - DPF regen and low EGTs - thunderbird

Got 2 diesels with DPF and how/when the regen takes place differs greatly, before you ask neither has an aditive tank.

One carries out regen as and when it needs to, I either hardly notice it or it rarely happens, occationally the engine note becomes harder and more "diesel" like when pulling away but that's about it. During the summer on a holiday trip we got a bad smell in the car, thought it was the ancient Rover we were following, overtook it but smell was still there. It was still smelling when we arrived about 20 miles later but by then I knew what it was. When we set off home it restarted once it met all the conditions, regen lasted another 20 miles or so, in 4 years that is the only really noticable regen.

Our other car does a regen every 300 miles exactly regardless of how you use it. They last about 12 miles or so and are virtually undetectable (during the very first one it smelt like the car was on fire), only the huge drop on the fuel confuser corfirms your suspicions. Both cars get the same type of use which is about 50/50 main road/town.

Both handbooks say the same thing, no need to rev the nuts off the car, provided the revs are above 1600 the regen will complete.

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Tue 27 Dec 2011 20:41

All - DPF regen and low EGTs - Hamsafar

I had a double DPF failure on the Audi A8.

I had a small garage remove them and smash the internals out with a crowbar via one opening. It smashed up like a crunchie bar. They are a double affair, the first third was a catalyst to catalyse the unburned fuel to increase a temp and the rest was soot and brittle honeycomb. We didn't have to cut them open to hollow it out.

The ECU also needed a reflash which included remap, but this wasn't neccessary for normal operation to resume.

Result = £2280 at Audi saved. 10% better mpg and MUCH better performance driveability (although this would be remap. I wouln't worry about buying an old car with the DPF.

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Tue 27 Dec 2011 21:00

All - DPF regen and low EGTs - unthrottled

If you had bought an engine which matched your actual use as opposed to your aspirational use the problem would not have occured. DPF problems occur in engines that seldom see more than 15% of their rated output-making one wonder why people spend so much on the other 85%...

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Tue 27 Dec 2011 21:48

All - DPF regen and low EGTs - oldroverboy

If you had bought an engine which matched your actual use as opposed to your aspirational use the problem would not have occured.

100% correct!

as someone said recently on these pages, the triple whammy of BIK, VED and the silly idea of pursuing the highest mpg car with the draconian anti-pollution requirements have conspired to create so many extra problems, and it is not going to get better!

Edited by OldRoverboy on 27/12/2011 at 21:48

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Thu 29 Dec 2011 10:11

All - DPF regen and low EGTs - xtrailman

I had a Honda Accord back in the 80's that had a automatic pick up off the exhast manifold, when the engine temperature increased the air intake switched over to the normal inlet.

I assumed that all engines would have this or simular system?

Regaurding the PDF, i have only done 18K miles in three years, and never even had a regen light on, so as far as i'm concerned short journeys with a diesel arnt a problem.

I do know the 2008 model had regen lights come on, according to reports, but my 2009 model must have had a mod, because as i say its never been a problem.

In fact i'll stck my neck out and say i think most of the PDF problems, were due to "teething" problems?

Nissan do advise running over 2K revs minimum, if the PDF light comes on.

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Thu 29 Dec 2011 10:49

All - DPF regen and low EGTs - RT

I had a Honda Accord back in the 80's that had a automatic pick up off the exhast manifold, when the engine temperature increased the air intake switched over to the normal inlet.

I assumed that all engines would have this or simular system?

Certainly the 1980 Nova had a crude temperature-sensitive flap to use cold air once the engine started warming up.

My recollection is that cold-only intakes were used once fuel injection became standard because fuel vaporisation is better than old carburettor engines.

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Thu 29 Dec 2011 17:02

All - DPF regen and low EGTs - unthrottled

My recollection is that cold-only intakes were used once fuel injection became standard because fuel vaporisation is better than old carburettor engines.

Correct. This doesn't stop snake oil salesmen from peddling cold air intakes (cones on sticks which are actually warm air intakes) when the factory fit a perfectly good CAI as standard.

I would only advocate a warm air intake on an engine that is operated almost exclusively at low load below normal operating temperature.

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Thu 29 Dec 2011 18:08

All - DPF regen and low EGTs - 659FBE

Hmm - we're drifting off topic here and I'm a diesel afficionado, but I can't really let this cold air comment go unchallenged...

Petrol engines use a volatile fuel - diesels don't. Petrol will only burn usefully in an engine as a vapour which means that it has to effectively evaporate in air. This requires latent heat of vaporisation.

Rub petrol on your skin and it feels cold. This is because the hotter molecules within the petrol leave the bulk liquid first, leaving the lower energy (colder) molecules behind in the bulk liquid. This is the principle of refrigeration.

In a petrol engine with a carburettor, latent heat of fuel vaporisation will cool the main discharge orifice which will give rise to icing from atmospherically held moisture at low air temperatures. This has to be avoided in order to maintain fuelling calibration and one solution is to heat the incoming air fron the exhaust manifold. A thermostatic "switch" is fitted to better engines so that the air temperature at the carburettor can be regulated.

Multipoint petrol injection does not suffer from this lmiitation as the fuel spray is directed onto the back of the (hot) inlet valves in an area where there is easily enough ambient engine heat to avoid icing. This means that the incoming charge can be as cold as you like.

For either a petrol or a diesel, cold charge air is an advantage - cold air is more dense and contains more oxygen per cylinder fill. Turbocharged diesels usually have a charge cooler (intercooler) to increase the air density and hence the engine power.

In the case of petrol engines, the maximisation of charge density is often a less well appreciated advantage of multipoint fuel injection.

Sorry about a non-seasonal text book - I'll get back to my G&T.

659.

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Thu 29 Dec 2011 20:15

All - DPF regen and low EGTs - unthrottled

659 My textbooks are well thumbed too!

For either a petrol or a diesel, cold charge air is an advantage

Nobody disputes the advantage of cold air at WOT. However, people experiencing DPF problems are not using WOT very much-it at all.

Petrol engines use a volatile fuel - diesels don't. Petrol will only burn usefully in an engine as a vapour which means that it has to effectively evaporate in air. This requires latent heat of vaporisation.

Irrelevant. All non oxygenated fuels must be vaporised as a pre requisite to combustion. A warmer intake charge yields higher temperatures during the compression stroke and hence better vaporisation when the diesel is injected.

In the case of petrol engines, the maximisation of charge density is often a less well appreciated advantage of multipoint fuel injection.

Not true. A carburettor/single point injection can actually achierve higher charge densities than port injection-in spite of the pressure drop across the venturi. This is because the fuel is largely vaporised by the intake charge-thus cooling the intake charge and increasing its density. In port injection, the fuel is largely vaporised by the hot intake valve-thus losing most of the the charge cooling effect.

The advantage of MPI with respect to charge density comes from the ability to design the runners and plenum for dry flow, rather than compromising the design to cope with wet flow. Cylinder-to-cylinder variation is also dramatically reduced as are the lean/rich spikes occuring during transients.

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Fri 30 Dec 2011 11:52

All - DPF regen and low EGTs - Bobbin Threadbare

No apologising!

I enjoyed reading both of the above posts :-)

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Fri 30 Dec 2011 16:55

All - DPF regen and low EGTs - corax

In the case of petrol engines, the maximisation of charge density is often a less well appreciated advantage of multipoint fuel injection.

Not true. A carburettor/single point injection can actually achierve higher charge densities than port injection-in spite of the pressure drop across the venturi. This is because the fuel is largely vaporised by the intake charge-thus cooling the intake charge and increasing its density. In port injection, the fuel is largely vaporised by the hot intake valve-thus losing most of the the charge cooling effect.

I certainly appreciate charge density on a cold damp morning with multipoint fuel injection - the difference is hugely noticeable. Surely if a single point injection has better charge density you wouldn't notice it over an engine with multipoint fuel injection because it would produce less power for the same size engine, due to less accurate fuel metering and having to rely on a uniform manifold vacuum for equal distribution of atomised fuel which doesn't happen.

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Fri 30 Dec 2011 18:13

All - DPF regen and low EGTs - unthrottled

Surely if a single point injection has better charge density you wouldn't notice it over an engine with multipoint fuel injection because it would produce less power for the same size engine, due to less accurate fuel metering and having to rely on a uniform manifold vacuum for equal distribution of atomised fuel which doesn't happen.

I'm not sure what you mean. Charge density and fuel metering are two different things. Single point injection systems make better use of evaporative cooling, but this is compromised by the need to design the manifold for wet flow ie the runners need to be short, (which eliminates resonance tuning) and hot, (which negates the charge cooling).

As you note, singlepoint systems cause uneven distribution of atomised fuel between the cylinders which hurts power.

The reason multipoint systems are easier to cold start is not due to charge density, but due to the fact that more of the fuel enters the cylinders as a vapour. When fuel droplets hit a cold metal surface, they tend to coalesce and form puddles which trickle into the cylinders. Liquid fuel is unburnable. The closer the injectors are situated to the combustion chamber, the easier the cold start.

Both Formula 1 and Nascar engines introduce the fuel a long way from the valves in order to maximise the charge cooling to gain maximum power. The former use MPI, the latter carburetion.

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Fri 30 Dec 2011 18:28

All - DPF regen and low EGTs - Collos25

lt aways amazes me how fast Nascar vehicles are when you are stood at the side of track and they come past at full speed quite some experience.

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Sat 31 Dec 2011 10:13

All - DPF regen and low EGTs - Andy P

Correct me if I'm wrong, but I was always under the impression that fuel was injected into the cylinder as a dispersion of micron-sized droplets, not a vapour (two distinctly different forms of matter), and that the reason MPFI is better is that the particle size of these droplets was smaller (7-20 microns) compared to SPFI, where they are much larger (70-300 microns).

Since combustion efficiency is directly related to the size of the fuel droplets, the smallere the better.

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Sat 31 Dec 2011 11:34

All - DPF regen and low EGTs - 659FBE

Petrol is a volatile fuel which vaporises in air. Optimum combustion is obtained when the fuel is fully vaporised at the correct ratio with air for the engine in question. Many petrol engines make good use of a variable mixture strength for instance, providing a richer mixture around the spark plug to aid ignition. Latent heat of vaporisation has to be supplied from somewhere, however you do it.

Diesel fuel is non volatile and does not vaporise - latent heat requirement is negligible. As rapid reaction with oxygen is required during combustion for effient engine operation, the trick is to atomise the fuel into as many tiny droplets per charge as is possible. This exposes the maximum possible surface area of fuel to the charge air and promotes more complete combustion.

Of course, a diesel engine has no spark plug and so the instant of ignition is primarily (but not exclusively) dependent upon the precise timing of injection. It will now be evident that a diesel fuelling system has to be capable of developing extremely high pressures to give good atomisation, precise timing and accurate metering.

A tall order - which evaded Rudolph Diesel until Robert Bosch came to the rescue.

We have reached the rather unsatisfactory stage in diesel engine development where high injection pressures of the order of 2k Bar are needed and which, for real longevity and fuel lubricity tolerance ought to be furnished from an oil lubricated fuel system. The VAG PD was such a system - as are all of the other unit injector systems which are very common on a wide spectrum of larger diesel engines - chosen of course for their longevity.

A unit injector system cannot furnish fuel for DPF afterburn and so oil lubricated diesel fuel systems are now dead in the water for small diesels due to current particulate legislative requirements. This is a great pity, since a reliable and long-lived small diesel is not now generally achievable in a relatively simple engine with a high tolerance of fuel lubricity variations.

The politicians have thrown out the thermodynamic advantages of the diesel baby with the DPF bathwater.

659.

Edited by 659FBE on 31/12/2011 at 11:40

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Sat 31 Dec 2011 13:24

All - DPF regen and low EGTs - craig-pd130

I still don't understand why manufacturers don't use a glow plug in the DPF canister to initiate coking. Glow plugs are robust, reliable, and often last the useful life of the car they're installed in. What's more, this approach would surely eradicate the current uncertainty over how much fuel needs to be post-injected, etc, as you'd get predictable temperatures quickly.

Surely the unit cost of an extra glow plug, wiring and a bit of ECU coding can't be THAT high?

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Sat 31 Dec 2011 16:09

All - DPF regen and low EGTs - unthrottled

Because the glow plug will only initiate soot burn off in a localised area-the entire trap needs to be burned off.

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Sat 31 Dec 2011 16:21

All - DPF regen and low EGTs - craig-pd130

Yes, however I'm talking about using the glow-plug to initiate regeneration. Once the soot has started coking, the process can be maintained to completion by some of the engine management strategies mentioned earlier.

My point is that having a (relatively) reliable -- and controllable -- heat source in the DPF canister to initiate the regeneration process has to be a better approach than simply injecting extra fuel and hoping it will light off.

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Sat 31 Dec 2011 16:26

All - DPF regen and low EGTs - unthrottled

Diesel fuel is non volatile and does not vaporise

At the in-cylinder temperatures experienced during injection (0-20 degrees BTDC), diesel fuel certainly is volatile. If it wasn't, it wouldn't mix with air and would not burn.

latent heat requirement is negligible.

Not so. Remember the fuel spray is highly stratified, not equally distributed as in port injected engines. The air in the path of the fuel droplet is cooled considerably-and the rate of evaporation is dependant on the temperature of the air-which is in turn dependant on on intake temperature. With low compression ratios and high amounts of EGR, modern diesel engines are now experiencing relatively high HC and CO engine out emissions at low load-previously unheard of in diesels. The problem is not lack of oxygen-the problem is lack of heat for a complete burn.This can be ameliorated by using a hot air intake. Modern diesels use cooled and uncooled EGR where appropriate to try to keep HC and CO emissions in check.

Engines with DPF problems are averaging very low load, full load performance is not a major consideration in this case.

The politicians have thrown out the thermodynamic advantages of the diesel baby with the DPF bathwater.

Well they did the same for petrols back in '92. Why should diesels get an easier ride? There's no law mandating DPF. A lot of lost efficiency could be regained by using Urea injection to reduce NOx in the exhaust-and hence reduce eGR. This would greatly reduce soot production and the problem of frequent DPF regens.

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Sat 31 Dec 2011 17:31

All - DPF regen and low EGTs - xtrailman

Do Euro 6 engines still have a dpf?

Edited by xtrailman on 31/12/2011 at 17:34

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Sat 31 Dec 2011 18:06

All - DPF regen and low EGTs - unthrottled

Sincesubsequent regulations are invariably getting more stringent-yes.

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Sun 1 Jan 2012 09:36

All - DPF regen and low EGTs - xtrailman

Dam, i was hoping otherwise.

Now all cars are getting stop start, so thats an expensive starter motor, and battery, come replacement.

It appears that motoring is just getting more complicated and expensive.

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Sun 1 Jan 2012 13:19

All - DPF regen and low EGTs - Bobbin Threadbare

It appears that motoring is just getting more complicated and expensive.

You hit the nail on the head there..!!

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Sun 1 Jan 2012 16:31

All - DPF regen and low EGTs - bazza

Euro 6 by all accounts might be a turning point away from diesel in mainstream cars. It will require very significant lowering of nitrogen oxides, the technology to do it exists in the form of catalytic reduction but the complexity and add-on cost to produce looks daunting, notwithstanding the likely maintenance, fault-finding and service life of all the plumbing and electronics. Plenty on google, type in Adblue and HGV.

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Mon 2 Jan 2012 17:58

All - DPF regen and low EGTs - xtrailman

The mazda CX-5 diesel is euro 6.

So i dont think youre right there?

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Mon 2 Jan 2012 18:08

All - DPF regen and low EGTs - unthrottled

It'll use a lean nox trap which doesn't require urea injection.

Lean Nox traps do not have as high NOx conversion efficiencies as Selective Catalytic Reduction-AND you have to regenerate the trap about every minute or so compared to about every 200-300 miles for a DPF. Guess how that's done...?

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