Diesel Particulate Filters (DPFs), also known as ‘particulate traps’ remove particulate matter from diesel engine exhausts. DPFs are becoming increasingly common in trucks, buses and cars owing to stringent international emissions regulations. DPFs are constructed from porous ceramic materials and consist of numerous small channels which are blocked at one end. The exhaust gas enters the DPF through the inlet channels at one end. It is forced through the porous walls of the channels and emerges from the outlet channels.
As it passes through the channel walls, the particulate matter is trapped. These will-flow filters can remove 99% of the particulate matter from the exhaust gases.
DPFs come in a variety of sizes and use a range of ceramic materials including
Truck DPFs are usually cylindrical in shape and the filter is clearly visible from either end. Car DPFs are often enclosed within a metal can, sometimes alongside a catalytic converter.
DPFs are designed to trap Particulate Matter (PM) which is produced during the normal operation of a diesel engine. PM is a complex mixture of small particles and droplets which are usually considered as 2 principal components – Soot and Ash.
Soot is produced by all diesel vehicles and consists of small particles of carbon which are trapped on the DPF. This soot can be removed by a process known as regeneration which burns off the carbon deposits and restores the flow through the filter. Most vehicles are fitted with regeneration systems that operate without any intervention from the driver. However, vehicles that operate in city centres with lots of stop-start driving do not regenerate their DPFs very effectively, which can lead to premature soot blockages.
Ash is a standard by-product of the combustion process that takes place in diesel engines. However, unlike soot, the ash that accumulates in filters cannot be burned off by regeneration processes (or by chemical cleaners added to the fuel tank) and requires periodic cleaning in order to maintain the efficient operation of the filter and the engine.
A clogged filter leads to an increase in exhaust back pressure. Many vehicles are fitted with a warning light that informs the driver when the DPF needs cleaning. Failure to clean the filter will produce a significant deterioration in engine performance and fuel consumption. In severe cases, the exhaust gas flow can become so restricted that the filter can be permanently damaged (cracked or burnt-out).