What is a diesel particulate diffuser and how does it work?

A diesel particulate diffuser, or DPD, is an emission control device used in diesel engines to reduce the amount of particulate matter (PM) emissions released into the atmosphere – they lower exhaust emissions. They’re also called DPF (diesel particulate filter). It works by trapping the particulate matter (‘soot’), which is a byproduct of incomplete combustion, and then diffusing it into the atmosphere in a less harmful form.

The DPD is part of the exhaust system and typically consists of a ceramic or silicon carbide honeycomb structure that is coated with a porous material. As the exhaust gases flow through the honeycomb structure, the particulate matter gets trapped in the porous material, while the exhaust gases pass through the honeycomb and exit the diffuser. In this way, it’s similar to a catalytic converter.

Some systems give the DPF level.

When the DPD is full, one or more warning lights will display on the dashboard.

How are the particulates destroyed?

Once the particulate matter is trapped in the porous material, it undergoes a process called regeneration or a ‘burn’. This involves raising the temperature of the trapped particles to a level where they can be burned off or oxidised, leaving behind only harmless ash. The regeneration process can occur either passively or actively.

Passive (‘auto’) regeneration occurs naturally as a result of the high temperatures generated during normal engine operation. Active (‘manual’) regeneration involves the use of a diesel particulate filter regeneration system, which raises the temperature of the particulate matter to the necessary level by injecting additional fuel into the engine or using an electric heating element; if this is happening, you must leave the engine running until it is finished. Diesel engines can develop issues with particulate matter levels if they are only ever used for short journeys where they don’t warm up enough.

In a manual burn, the driver initiates it by pushing the DPD button (or whatever process the manufacturer stipulates). Ignoring the warnings will eventually result in the filter needing to be completely replaced. Not doing the burn quick enough could mean that the vehicle enters a ‘limp mode’ to protect the engine, and could require a mechanic to fix it.

DPF burn process

If you are initiating a manual burn, pull over in a safe place and ensure you are not parked on flammable materials such as long, dry grass as the exhaust can reach as much as 800°C. Put the vehicle in neutral with the handbrake on and the engine running.

Press the DPD button. The engine idle will increase and it could take up to 20 minutes. Stay with the vehicle and don’t turn the engine off. Once the warning light is extinguished, or it says it’s OK on the vehicle’s readout, you can resume driving.

You should not do a DPF burn before it’s due because it degrades the oil earlier than is necessary.

How often should you service the diesel particulate filter?

Regular maintenance of the DPF is essential to ensure that it operates effectively and that the regeneration process is successful. If the DPF becomes clogged or damaged, it may not be able to regenerate properly, which can lead to reduced engine performance and increased emissions.

A DPF burn is only effective a limited number of times before it will need to be serviced.