Diesel Particulate Filters (DPFs) are designed to trap harmful soot particles downstream in the exhaust system. This prevents the particulates reaching the atmosphere where they pose a risk to human health.
However, the DPF will rapidly block up unless a process called regeneration happens to clear it out. Soot or carbon particles will burn completely away – like charcoal on a BBQ – provided they reach a minimum temperature of around 600°C. DPFs either rely upon exhaust gas temperatures becoming hot enough for this to happen, or else have active regeneration strategies whereby extra fuel is injected into the exhaust system, when soot build up is detected, to burn it off.
Nevertheless, under certain circumstances soot can build up in the DPF to the point where it causes problems for the vehicle, triggering warning lights to inform the driver to take action, or restricting the power of the vehicle. Prolonged periods of slow-speed, stop-start driving or idling will make DPF problems more likely. Hence, vehicles mostly driven in urban city-centres, or in rural areas where faster driving speeds are not generally possible, are most likely to experience DPF problems. Also at risk are vehicles confined to speed-limited sites, such as petrochemical works, where the exhaust temperature is never likely to get hot enough to regenerate the filter.
Ash Build Up
Over time, the DPF will also collect ‘ash’ due to small amounts of engine oil being burned. This ash will not burn away like soot, so stays in the DPF. Eventually (after 100,000+ miles) ash will build up to the point that the DPF needs clearing out (requiring an external filter clean) or replacing. However, the lifetime of the DPF may be reduced if the wrong kind of oil is used. Vehicles fitted with DPFs require ‘low SAPS’ (Sulphated Ash, Phosphorous, Sulphur) oils, which produce minimal ash. Using a lower quality oil may produce more ash, leading to premature DPF blockage. Similarly, a fault with the oil system of the vehicle which causes the engine to burn a lot of oil unnecessarily may also affect the DPF.