There has been some recent coverage in the news about Diesel Particulate Filters (DPF) and the effects that they have on vehicles. There is some confusion about how DPF works and how to drive the vehicle in order for the self-cleaning process to be initiated.
Vehicles spending a majority of time on short trips or in urban environments may not reach the speeds (and therefore temperature required) for the duration needed to allow the DPF to complete the cleaning process, consequently resulting in a warning light being displayed.
After which failure to then drive the vehicle in order to initiate the cleaning process could result in the filter or the engine being damaged, which is not covered by the manufacturer warranty.
If you’re buying a new car and plan to use it mainly for town-based, stop/start driving it may be wise to avoid a diesel car fitted with a DPF because of the possible consequences of incomplete ‘DPF regeneration’.
The science bit…
DPF catch bits of soot in the exhaust, and as with any filter they have to be emptied regularly to maintain performance. The collected soot is burnt off at high temperature to leave only a tiny ash residue, this is known as regeneration. Regeneration is passive, active or forced.
Passive regeneration takes place automatically on motorway or fast A-road runs when the exhaust temperature is high. Because many cars don’t get this sort of use vehicle manufacturers have had to design-in ‘Active’ regeneration where the engine management computer (ECU) takes control of the process.
Active regeneration occurs when the soot loading in the filter reaches a set limit (about 45%) the vehicle’s ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration. If the journey is too short while the regeneration is in progress, it may not complete and the warning light will come on to show that the filter is partially blocked.
It should be possible to complete a regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.
Forced regeneration is required where `Active` regeneration criteria have not been met or where soot levels have increased within the DPF to a point where normal regeneration cannot be performed: typically around 70% soot loading. At this point the vehicle will enter a ‘restricted performance’ mode to prevent further damage. If left the soot loading will keep rising.
At this level of soot loading a diagnostic tool must be used to force regeneration. Above around 85% soot loading regeneration can no longer be performed on the vehicle and the DPF will need removing to be cleaned or replaced. If you continue to ignore warnings and soot loading keeps increasing then the car won’t run properly and the most likely outcome will be that you will have to get a new DPF costing at least £1000 plus labour and diagnostic time.
What can you do you ensure that the vehicle does complete its regeneration either passively or actively?
• Ensure that you drive over 40 mph for more than 10 minutes regularly.
• Ensure you have the correct oil type -DPF equipped cars require low ash, low sulphur engine oils
• Keep your fuel levels above ¼ of a tank – low fuel levels prevent regeneration taking place.
• Keep your vehicle up to date with its service – a problem with the inlet, fuel or Exhaust Gas Recirculation (EGR) system causing incomplete combustion will increase soot loading.