The Diesel Particulate Filter
What is it?
European Emissions legislation is becoming progressively more stringent with
regard to the level of emissions which vehicles are permitted to give off.
Diesel fuel when burnt gives off a range of exhaust gases including sooty
particles, or particulates, often visible on older diesel vehicles as black
exhaust smoke.
Many diesel engines now require the addition of a diesel particulate filter to
the exhaust system to comply with the emissions legislation.
A diesel particulate filter, or DPF, has a function similar to that of the
catalytic convertor fitted to most petrol-engined cars. It is a device designed
to remove diesel particulate matter or soot from the exhaust gas of a diesel
engine. Wall-flow diesel particulate filters usually remove 85% or more of the
soot, and can at times (heavily loaded condition) attain soot removal
efficiencies of close to 100%. A diesel-powered vehicle equipped with a
properly-functioning filter will emit no visible smoke from its exhaust pipe.
How does it work?
The DPF looks similar to a traditional exhaust silencer, but is mounted close to
the engine. Inside is a complex honeycomb ceramic structure designed to filter
the exhaust gases while minimising flow (which would otherwise limit
performance). By forcing the exhaust gases through the walls between the
channels of the DPF, the particulate matter is deposited on the walls, so
reducing the amount of air pollution.
Does it require maintenance?
Any filter has a finite capacity, so diesel particulate filters must be cleaned
out if they are not to block. This is most important, as an overfilled filter
can damage the engine through excessive exhaust back pressure and can itself be
damaged or destroyed.
When the DPF is becoming full, with around 80% load, it automatically cleans
itself by initiating a process called regeneration. This process increases the
temperature in the DPF and burns off the soot particles. To initiate
regeneration, the DPF needs to reach and maintain a high exhaust temperature.
This is normally achieved during motorway or higher speed driving. This "passive"
regeneration occurs continually, and will go unnoticed by the driver.
In certain circumstances (e.g. frequent short journeys or slow driving in
congested areas), automatic regeneration may not be triggered. In this case, an
amber warning light illuminates, with a message "soot filter full – see manual".
The manual explains how regeneration can be initiated. When regeneration is
complete, the message is cleared automatically.
It is important to take immediate action to regenerate the DPF. If regeneration
is not achieved, soot will continue to build up, and the amber warning light
will illuminate again, with a different message: "engine service required". The
vehicle should then be taken to a dealer immediately for forced regeneration to
be carried out. Failure to do so will cause difficulty in starting the engine,
and may require complete replacement of the DPF.
Because smaller engines run at lower temperatures, a fuel additive may be
required to promote regeneration. This additive, Eolys, is currently used on
Volvo's 1.6 and some 2.0 diesel engines where a DPF is fitted. The additive is a
cerium/iron based solution that is added to a special tank under the fuel tank,
which holds approximately 1.8 litres. Additive consumption is low and the tank
is topped up as part of the car's regular service schedule.
The fuel additive, whilst promoting regeneration to burn off soot particles,
leaves a small ash residue which cannot be burnt off. It is therefore necessary
to replace DPFs requiring the fuel additive at the 75,000 mile service interval.
Where the additive is not required, there is no need to change the DPF for the
lifetime of the car, provided that regeneration is carried out as required.
Do I need one?
The fitment of a particulate filter to diesel-engined cars is becoming
increasingly commonplace in order to meet emissions legislation and to achieve
cleaner exhaust gases. Nevertheless, driving constraints in such locations as
inner-city and islands (e.g. Channel Islands, Isle of Man) are unlikely to
facilitate passive regeneration. Please consider your driving style and normal
driving conditions to determine whether a diesel- or petrol-engined car is best
suited to meet your needs. Your Volvo dealer will be pleased to assist you