Diesel Particulate Filter

A diesel particulate filter, universally abbreviated to DPF, is an exhaust after-treatment component that captures the fine soot produced by diesel combustion before it can leave the tailpipe. Diesel engines, because they burn fuel in a heterogeneous, locally rich flame, inevitably create particulate matter, the sooty carbon particles long associated with black smoke and with respiratory and cardiovascular harm. The DPF is the device that has made modern diesels visibly clean, trapping the vast majority of that particulate mass.

Inside its metal canister sits a honeycomb monolith, usually of cordierite or silicon carbide ceramic, formed into thousands of small parallel channels. In the common wall-flow design, alternate channels are plugged at opposite ends, forcing the exhaust gas to pass through the porous channel walls. The gas slips through, but the soot particles are too large and are deposited on the walls, where they accumulate over time. This is highly effective, removing well over ninety-five per cent of the particulate count, but the trapped soot steadily clogs the filter and raises exhaust back-pressure, so it must periodically be cleared.

Clearing the filter is called regeneration, and it works by burning the collected carbon off as carbon dioxide. Passive regeneration happens continuously when exhaust temperatures are high enough, as on a fast motorway run, often aided by nitrogen dioxide acting as an oxidiser. When the soot load grows too high without such conditions, the engine triggers active regeneration: it injects extra fuel, retards timing, or uses a downstream burner to raise exhaust temperatures to around 550 to 600 degrees Celsius and deliberately incinerate the deposit, a process lasting several minutes.

This reliance on heat is the source of the DPF's most common real-world difficulty. A car used only for short, cold, stop-start journeys may never reach the temperature needed to complete a regeneration; the filter slowly blocks, warning lights appear, and the vehicle may eventually refuse to regenerate without a workshop forced cycle or, in bad cases, a costly cleaned or replacement filter. Owners of such vehicles are usually advised to take an occasional sustained higher-speed drive. The ceramic also accumulates non-combustible ash from oil additives over its life, which is why low-ash diesel-specific engine oils are specified.

Fitted to virtually all diesel cars since the Euro 5 standard of 2009 and tightened further under Euro 6, the DPF is one of several devices working in concert. It deals specifically with particulates, while selective catalytic reduction, dosing AdBlue, tackles oxides of nitrogen, and an oxidation catalyst handles unburnt hydrocarbons and carbon monoxide. The petrol-engine equivalent, addressing the rise of particulates from direct-injection petrol units, is the gasoline particulate filter.