We get quite a few enquiries about a dash warning light on some vehicles which are fitted with diesel particulate filters.
Several vehicle manufacturers are a bit concerned over the way some drivers are ignoring the warning light – for a long time.
As you will be well aware there are health issues over the sooty particles produced by diesel engines (and petrol engines, to some extent) which penetrate deep into the lungs. These particles are blamed for an increase in heart and respiratory disease.
As part of the increasingly tight exhaust emission regulations, particulate filters are being fitted to more and more new diesel cars to trap these sooty bits.
Two types of particulate filter are in use - those with additive and those without. To enable a vehicle to use a system without additive the particulate filter must be fitted close to the engine. Because the exhaust gases will not have travelled far from the engine they will still be hot enough to burn off the carbon soot particles. In these systems an oxidising catalytic converter will be integrated into the particulate filter.
In other systems the particulate filter is fitted some distance from the engine and as the exhaust gases travel along the exhaust they cool. The temperatures required for ignition of the exhaust gas can only be achieved by the use of an additive.
The particulate filter is made from a honeycomb ceramic monolith, like a catalytic converter, of silicon carbide. The monolith is contained in a metal casing and consists of many tiny channels which run parallel to each other and are interconnected.
Manufacturers of particulate filters favour silicon carbide because it is very thermally strong, has good conductivity and is resistant to wear.
The particulate traps do just that – they catch bits of soot – until they are full – and then they can’t trap any more. At this point the DPF needs to go through the ‘Regeneration Process’. During regeneration the soot is converted to a very small volume of ash which remains in the filter.
There are two types of regeneration – passive and active. Passive regeneration takes place, automatically, on motorway-type runs in which the exhaust temperature is high (somewhere between 350ºC and 500ºC). As the exhaust is so hot (hot enough to ignite the soot particles) the regeneration process can carry on continuously and steadily across the platinum coated catalytic converter.
A surprisingly large number of people don’t take their cars anywhere near a motorway so passive regeneration is only going to be possible once in a blue moon. Soot builds up in the DPF and intervention from the ECU is needed to shift it. In the case of a filter without additive when the soot loading reaches about 45 per cent the ECU switches off the EGR and increases the fuel injection period so there is a small injection after the main injection. These measures help to raise the engine exhaust temperature to over 600ºC which is high enough to burn off the soot particles.
This isn’t a very straight forward process if the journey is a bit stop/start. If the driver suddenly has to stop or slow down, the ECU can’t complete the regeneration and it has to temporarily abort the operation. When the vehicle’s speed reaches a neck-snapping 38mph, the ECU can have another go at regeneration. The ECU will keep trying for 15 minutes, but after two 15 minute attempts the soot levels (loading) in the DPF will have increased. At a soot loading of 50 per cent the ECU will have another go at clearing it. It raises the maximum exhaust temperature to somewhere between 600ºC and 650ºC to try and trigger a regeneration process. The system will try to create a regeneration for another 15 minutes and if this doesn’t work it will have one more go for 15 minutes. After this the DPF light on the dash will be illuminated.
If anyone opens the driver’s handbook to see what this light might mean, they will see that the DPF has been partially blocked with bits of soot because the car has only done short journeys. The driver then needs to drive at a speed of at least 40 mph for around 10 minutes to clear it. This type of drive should allow the temperature in the DPF to become high enough to burn off the soot.
If this doesn’t work then it’s a dealer job. | 
