Diesel Particulate Filters (DPFs)

15th April 2017

My Golf Mk 7 1.6 TDI is my first diesel-engined car fitted with a diesel particulate filter (DPF). I've been keen to learn more about how DPFs work so that I can look after the one fitted to my car. Here is what I've discovered along with an analysis of DPF related parameters logged by VCDS during one particular journey.

What is a DPF?

DPF's capture the black soot particulates that many older (pre-2006) diesel-engined cars expel through their exhaust pipe. I used to regularly give my 2004 Golf Mk 5 1.9 TDI's engine a blast up a steep hill in 2nd gear at high revs just to blow-out any soot which had accumulated in the exhaust system. I would usually see a cloud of smoke in the rear-view mirror after doing this. This doesn't happen with my Mk7. In fact, the first time I took my car for an MOT I queried the emissions result because the smoke test showed zero!

DPFs capture the black soot particulates produced by the engine and a reaction with the DPF's catalytic coating at temperatures in excess of 350°C converts them into carbon dioxide (CO₂). This process is called regeneration and there are two types; Passive and Active.

Passive Regeneration

Passive regeneration occurs during normal use of the vehicle whenever the temperature of the exhaust gases entering the DPF exceeds 350°C. The main factors affecting exhaust gas temperature appears to be engine speed and engine load. My experience is that during city driving, exhaust gas temperatures rarely exceed 300°C, and even when cruising on a motorway at 70mph (just over 2,000rpm), exhaust gas temperatures don't increase much more because of the relatively low engine load. Consequently, active regeneration is most often relied upon.

Active Regeneration

Active regeneration is triggered by the car's ECU when it senses a 45% loading of particulates in the DPF. It quickly increases the temperature of the exhaust gases entering the DPF into the region of 650-700°C by:

Turning off Exhaust Gas Recirculation (Another emission-reducing process which causes lower exhaust gas temperatures), and
Triggering a second injection of fuel into each cylinder after the main injection, specifically to increase exhaust gas temperature.

This usually allows the accumulated particulates to be incinerated within approximately 10 minutes.

Both forms of regeneration occur largely unknown to the driver. Possible giveaway signs that active regeneration is in progress are:

a slightly raised idle speed (around 1,100rpm rather than nearer 800rpm),
the Auto Start-stop system not kicking in, to enable active regeneration to continue (and a confusing message appears on the Auto start-stop status screen), and
a slight increase in instantaneous fuel consumption

Shutting down the engine during active regeneration is usually nothing to be worried about. A giveaway sign that this has happened is the radiator cooling fan continuing to run at high speed to dissipate residual heat from the DPF.

Diesel Particulate Filter
dash warning light.

If the level of particulates in the DPF continues to rise, the Diesel Particulate Filter warning light will appear on the dash. This is to prompt the driver to drive sufficiently for active regeneration to complete. If the driver does not do this and particulate levels continue to increase, more warning lights will appear on the dash and/or the performance of the engine will be reduced. If the driver continues to ignore these warnings, it may become impossible for regeneration to occur and may result in the DPF needing to be replaced.

VCDS Logging

I used VCDS to log a number of DPF-related parameters during a single journey lasting 58 minutes. The journey consisted of urban driving (30mph speed limit) for the first 26minutes followed by 27 minutes of motorway driving (mainly cruising at 56mph*) with a final 5 minutes of rural driving (60mph speed limit).

Click to view full size.

* Whilst this motorway cruise was at 56 MPH, a subsequent log of exhaust temperatures whilst cruising at 70 MPH (In 5th/top gear at just over 2,000 RPM) didn't result in any significant rise in exhaust gas temperatures. Certainly not enough to faciliate passive regeneration.

Observations

No passive regeneration appears to be taking place initially as particulate levels are seen to be increasing. The rate of increase is 1.73g/hr. Also, exhaust gas temperatures are observed to never exceed 350°C which is required for passive regeneration to occur.
Exhaust gas recirculation (not shown on the graph) drops permanently to zero between 37minutes and 50minutes which is thought to indicate Active Regeneration is taking place. There is a corresponding steady drop in DPF Soot Mass during this period.
The Soot Mass Calculated figure was 21.43g at the time Active Regeneration was triggered and 2.84g when Active Regeneration finished. Since Active Regeneration is triggered when particulate loading reaches 45%, this equates to a DPF capacity of approximately 48g.
Exhaust gas temperatures entering the DPF are seen to rise quickly to 650°C during active regeneration
Particulate levels remain above 350°C for 2 minutes after Active Regeneration finishes allowing some passive regeneration to occur. The minimum Soot Mass Calculated figure achieved was 2.36g.

Conclusions

I am relieved that the DPF in my car appears to be functioning correctly. I have confirmed that my relatively sedate driving style is not conducive to passive regeneration being sufficient to deal with the collected particulates. However, by observing the relatively low exhaust gas temperatures during typical driving, I don't think many drivers would be able to rely on passive regeneration alone.

I am relieved that there appears to be only 2.36g of residual soot in my car's DPF which it is unable to dispose of. Volkswagen state that the DPF should be inspected at 90,000 miles and that cars will go through regeneration cycles more frequently as they get older. My car currently has approximately 67,000 miles on the clock and so I am not confident that there is plenty of life left in the DPF yet.

My observations come mainly from logging this single journey. Just for my own piece of mind and curiosity, I plan to log a few more journeys to see, amongst other things, whether passive regeneration occurs when cruising at 70mph on a motorway, is there any significant difference in the rate of particulate build up between urban and motorway driving, and to quantify the increased fuel consumption caused by active regeneration.

As always, I welcome any comments you may have on this topic.