The new Euro 5 standard requires changes to new car government emissions policy which will make particulate filters routine in diesel car exhausts as catalytic converters are within petrol cars.
The goal is to reduce the diesel particulate (soot) emissions by 80% however it seems that the technology does have some problems. Since the introduction of the DPF in 2006, most Diesel powered motor vehicles now have this component fitted as standard. Adjustments to peoples driving style may also be required to gain maximum benefit from the DPF systems.
What are they?
In basic terms, the Diesel Particulate Filters are part of the exhaust system responsible for cleaning the engine exhaust gasses before they enter the atmosphere. The DPF effectively traps bits of soot in the exhaust.
How do they work?
As with any filter, they have to be emptied regularly to uphold performance. For a DPF this procedure is called regeneration – the accumulated soot is burnt off at high temperature to leave only a tiny ash residue. Regeneration may be either passive or active.
Passive regeneration
Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Many cars particularly small cars do not spend adequate time or achieve routine speeds to passively regenerate; therefore manufacturers have designed active regeneration where the engine management computer (ECU) manages the process.
Active regeneration
When the filter attains a soot limit of approximately 45%, the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. The regeneration may not be completed if the journey is a little stop/ start and hence the warning light will illuminate to show that the DPF is partially blocked.
It should be possible to begin a complete regeneration and clear the warning light simply by driving for 10 minutes or so at speeds greater than 40mph.
If you ignore the light and continue to drive in somewhat slow, stop/start pattern soot loading build up until around 75% when you can expect to see other dashboard warning lights illuminate too. At this point driving at speed alone will not be sufficient and the car will have to go to a dealer for regeneration.
Expensive repairs
If warnings are still ignored and soot loading continues to increase then the most probable result will be a new DPF costing around £1000.
Mainly town based driving
If your own car use is mainly town-based, stop/start driving it would be wise to choose petrol rather than risk the problem of incomplete DPF regeneration.
DPF additives
The most common type of DPF features an integrated oxidizing catalytic converter and is located very close to the engine where exhaust gases will still be reasonably hot so that passive regeneration is practicable.
In various cars, there is not always the space close to the engine – which has resulted in manufacturers using a different type of DPF which relies on a fuel additive. The additive lowers the ignition temperature of the soot particles. It is stored in a separate tank and is routinely automatically mixed with the fuel whenever you fill up. Tiny quantities are required - a litre of additive should treat around 2800 litres of fuel, enough to cover 25,000 miles at 40mpg.
In this case, this type of DPF regeneration will be initiated by the ECU every 300 miles or so depending on vehicle use and will take only 5 to 10 minutes to complete. You shouldn't notice anything other than perhaps a puff of white smoke from the exhaust when the procedure is finalized.
The AA for example have evidence that some of these DPF systems are failing to regenerate, even on cars used mainly on motorways. It would appear that on cars with a very high sixth gear, normal engine revs are too low to generate a sufficiently high exhaust temperature. In these cases occasional harder driving in lower gears should be sufficient to burn off the soot.
Check the handbook
If you buy a car with a DPF fitted it is important to read the relevant section of the vehicle handbook so that you understand exactly what measures to take if the warning light illuminates and how, if at all, your driving style may need to be adjusted to ensure maximum DPF efficiency and life.