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10 Dec 2014 
Diesel Particulate Filters (DPF) and the Environment

Reducing diesel soot emissions by 80%:

The goal is an 80% reduction in diesel particulate (soot) emissions, but the technology's not without troubles; roadside assistance patrols are already being called to automobiles with the particulate filter warning light illuminated, which generally indicates a partial blockage of the DPF filter.

Certainly, changes to driving fashions may be required for maximum gain from these emission-reducing systems.

How do the filters function?:

Diesel Particulate filters (DPF) or 'snares' do only that, they get bits of soot in the exhaust.

As with any filter (think of the bag in your vacuum cleaner) they need to be emptied regularly to keep operation. For a DPF this diesel filter blocked procedure is called 'regeneration'; the collected soot is burnt off at high temperature to leave only a tiny ash deposit. Regeneration might be either passive or active.

Passive regeneration

Passive regeneration occurs automatically on motorway-type runs when the exhaust temperature is high. Many autos do not get this form of use though so manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Energetic regeneration

When the soot load in the filter reaches a set limit (about 45%) the ECU can make modest adjustments to the fuel injection time start regeneration and to improve the exhaust temperature.

It must be possible to start an entire regeneration and clear the warning light by simply driving for 10 minutes or so at speeds greater than 40mph.

If you keep driving in a comparatively slow and ignore the light, stop/start design soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights illuminate also. At this stage driving at speed alone won't be sufficient and also the auto might have to go to a dealer for regeneration.

Expensive repairs:

If warnings are still dismissed and soot loading continues to increase then the most likely result will be a new DPF.

Mostly town driving that is based:

If your own automobile use or lease automobile use is chiefly town-based, stop/start driving it would be wise to decide on petrol as opposed to risk the hassle of incomplete DPF regeneration.

DPF additives:

The most common kind of DPF features an incorporated oxidising catalytic converter and is located quite close to the engine so that passive regeneration is potential, where exhaust gases will still be comparatively hot.

There's not consistently space near the engine though some producers use an alternate type of DPF which relies on a fuel additive to lower the ignition temperature of the soot particles in order that the DPF can be found further from the engine.

The additive is stored in another tank and is automatically mixed with the fuel whenever you fill up. Miniature amounts are required though a litre of additive should treat around 2800 litres of fuel.

With this specific kind of DPF regeneration will be commenced by the ECU every 300 miles or so depending on vehicle use and will take 5 to 10 minutes to complete. You should not see anything other than maybe a puff of white smoke from the exhaust when the process is finished.

AA experience:

The AA has seen evidence of DPF systems neglecting to regenerate - even on cars - which are used largely on motorways. Their decision is the fact that on cars with a very high sixth gear engine revs are excessively low to generate adequate exhaust temperature, but occasional harder driving in lower gears should be sufficient to bum off the soot in such instances.

Check the handbook:

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