Do Diesel particulate filters really work?

forgotmyname

sevenhills wrote: »

1985/95/05 what ever the year, they have had a long time to develop.


How is a regen forced, does the DPF heat up in some way?

You appear to have quoted Wikipedia, so just curious as to what car used a DPF in 1985.

Yes they have developed them for some years and they are better than they used to be, but what has that got to do with anything?

You force a regen by plugging in your diagnostic tool and telling the car to regen. It either dumps chemicals into the exhaust ot additional fuel etc.....

sevenhills

Ectophile wrote: »

In normal use, the "burning" in the DPF should produce a lot less soot than it has previously trapped. So overall, it's an improvement.

A friend has a 2011 car fitted with a DPF and when revved it certainly kicked out some smoke, I guess the soot was not blocking the DPF and just having a smokey exhaust. It just passed its MOT, I was surprised.

Gloomendoom

My car has a DPF and the inside of the tailpipes are spotless. I don't think it makes much smoke, if any at all.

Goudy

DPF's have changed a little over the years.
Modern, active filters trap the soot, which are fairly large particles and when the sensor detects the filter filling up it injects fuel, post combustion (exhaust stroke) which passes through an oxidation catalyst which causes the temps to rise and the soot to burn into ash, which are much smaller particles.

The ash will eventually need emptying, but as it's far smaller than soot the filter is capable of holding many many miles more ash than soot.

Trouble with DPF's are well noted.
Repeated short trips tend to produce more soot. An engine starting from cold will run a higher fuel to air mix until it's warmed up and create more soot, requiring more regenerations.

To burn soot, very high temps are needed, around 550c I believe, it also doesn't incinerate immediately, it takes some time.
This requires the engine to be working at a reasonable operating temp for a reasonable distance/time which short trips don't fulfill as the engine is often stopped before a regen even starts, let alone completes.
So now you have a faster build up of soot that require more regenerations that may or may not complete properly.

Often a dealer can force a regeneration through the engines ecu, but if the usage doesn't change the problem soon returns.

Some manufacturers like Citroen have been using a system that injects a fluid (PATS) into the catalyst. This lowers the temp the soot burns at by around 100c (or increases the temps by 100c, I can't remember which now!) and makes the process a bit more reliable.

The engine can suffer further issues with incomplete and too often repeated regeneration due to the post injected fuel.
Injecting fuel on the exhaust stroke far too often causes this fuel to leak past the piston rings into the sump which dilutes the engine oil.
Not only does the oil properties get compromised but the oil level can dangerously rise, sometimes enough to cause a diesel "runaway". This involves the oil finding it's way out of the breather and into the air intake and then into combustion, so it runs uncontrollable on it's own lubrication until it goes bang.
Modern diesels often have oil detection systems that monitor regenerations, thus knows how much fuel has been post injected and warns drivers the oil needs changing.


Adblue and exhaust gas recirculation (EGR) are nothing to do with soot emissions.
They are used to reduce harmful Nitrogen Oxides.

Modern engines, at certain times like on the over run and small throttle openings, inject no or so little fuel that the combustion temps rise massively as it is basically pumping air.
These high temp spikes causes the Nitrogen Oxides to proliferate.
Recirculating exhaust gases back around the engine reduces these combustion temps as these exhaust gases contain little oxygen, thus reducing the Nitrogen Oxides.

Adblue is also used these days. Very small amounts are injected into the exhaust gases when the engine is under the same hot/lean conditions and the fluid breaks down the Nitrogen Oxides into harmless Nitrogen and Oxygen.

I've probably made some mistakes above and I'd be pleased to be corrected, but this is how I understand the basics of these processes.

Gloomendoom

The fluid Citroen uses lowers the temperature at which the soot will burn.

Goudy

Gloomendoom wrote: »

The fluid Citroen uses lowers the temperature at which the soot will burn.

Thanks, I knew it was one or the other!

societys_child

Most garages I know, take it down the motorway for an "Italian Tune Up" before resorting to a re-gen . .

sevenhills

Goudy wrote: »

Modern, active filters trap the soot, which are fairly large particles and when the sensor detects the filter filling up it injects fuel, post combustion (exhaust stroke) which passes through an oxidation catalyst which causes the temps to rise and the soot to burn into ash, which are much smaller particles.

The ash will eventually need emptying, but as it's far smaller than soot the filter is capable of holding many many miles more ash than soot

How does the DPF know when its full, does it have a sensor inside it?


How does the PDF get hot enough to ignite the deisel fuel to clear the DPF? I assume the DPF is linked to the exhaust, once the deisel is burnt in the engine, it surely cannot reignite in the DPF/exhaust?

Goudy

Yes, it senses changes in pressure, the fuller the filter the higher back pressure of the exhaust gas.

The fuel to burn the soot is injected into the engine, but not on a normal compression stoke that would normally ignite in the chambers and push the piston down.
Instead it is injected on the exhaust stroke so it then passes out of the combustion chambers unburnt into the exhaust.

Due to this altered fuel/engine timing, the gases in the exhaust should be hot enough to ignite the fuel though there is an abundance on other harmful chemicals in the gas that don't burn or even aid a burn and at this point in the process, we're a bit short of oxygen in the mix to get the required temp.

So first it passes through the DOC (diesel oxidation catalyst) where magic happens.
Although I am not fully versed in the chemical formulas and reactions, I believe the DOC removes some of the harmful chemicals in the exhaust and turns others into useful chemicals by passing the unburnt fuel and hot gas over various metals and adds oxygen (oxidises other chemicals in there) that is stored in the DOC.

Remember the fire triangle? For a fire you need:
Fuel - we get that from fuel injected in to the exhaust stroke
Heat - we get that from the hot exhaust gas
Oxygen - we get that from the DOC that stores oxygen that is normally present in the exhaust in normal operating conditions.

These now aid the burn and increase the temp in the DPF behind it in the exhaust to burn the soot, well that's what I believe it does, I could be out a bit/lot!

forgotmyname

sevenhills wrote: »

How does the DPF know when its full, does it have a sensor inside it?


How does the PDF get hot enough to ignite the deisel fuel to clear the DPF? I assume the DPF is linked to the exhaust, once the deisel is burnt in the engine, it surely cannot reignite in the DPF/exhaust?

Tried wikipedia?

Because they do have sensors...

Cannot reignite in the exhaust? Why not? Adjust the timing enough you can get combustion flames from the exhaust.