What does a diesel particulate filter do?
The DPF is fitted within the exhaust system and is designed to catch soot particles and nitrous oxide (NOx) from the combustion process that would otherwise be released into the atmosphere. The DPF is highly effective and traps around 80 per cent of these harmful particulates.
Why do modern diesel engines need a particulate filter?
European legislation places increasingly strict exhaust emissions targets on all vehicle manufacturers. From September 2009, the ‘Euro 5’ legislative standard came into effect, part of which required all diesel cars registered from that point on to be equipped with a diesel particulate filter, or DPF.
These filters are used throughout the industry in all diesel-powered passenger vehicles, and the efficiency of this system is becoming increasingly important now that tougher ‘Euro 6’ emissions standards came into effect in September 2014.
Do I need to replace a diesel particulate filter during normal servicing?
Diesel particulate filters do have a capacity limit and can become full. But unlike traditional air, oil or pollen filters that need to be exchanged manually at regular intervals, the DPF has a much longer service life and is designed to be regenerated to restore its performance. The car’s electronic control unit (ECU) is programmed to do this automatically, neutralising the soot by burning it off at high temperature within the exhaust system while the vehicle is running. All modern cars with diesel engines follow this procedure.
How does this regeneration occur?
The regeneration occurs automatically, usually without the driver being aware that it is taking place. In the majority of cases, the process is carried out when the engine and exhaust system is up to normal operating temperature and the vehicle is travelling at over 40mph.
However, if your driving is limited to inside the city, the low speeds and frequent stops mean the conditions for normal regeneration do not exist. In these instances, the ECU monitors the accumulation of soot and instructs an alternative regeneration programme to begin well before the filter becomes saturated.
This programme injects small quantities of fuel into the engine after combustion, thereby increasing the temperature within the exhaust system and creating an environment where it is possible to safely burn off the soot.
This latter method is very successful within the small percentage of vehicles where normal regeneration is not possible. However, due to the nature of city traffic, the regeneration process can be interrupted when the vehicle completes its short journey and is turned off. If this occurs, the ECU is programmed to recommence the process when the engine is restarted and back up to temperature again.
How can I ensure that the DPF regeneration process completes successfully?
By creating frequent opportunities for the ECU to complete its normal regeneration programme. Quite simply, this means regularly driving the car at a sustained speed above 40mph for a period of at least ten minutes.
Will I be warned if all DPF regeneration processes have been unsuccessful?
Very occasionally, use of the vehicle might be such that it will have been impossible for the ECU to complete the administration of any regeneration procedure. At this point, the DPF will have become overloaded and stopped functioning, so the vehicle will illuminate a warning light in the dashboard. It will then be necessary for the soot to be neutralised and the filter regenerated manually at a main dealer.
It is extremely unwise to ignore this visual warning as the DPF can become irreparably damaged. What’s more, it is illegal to remove the filter entirely.