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Diesel particulate filters (DPFs) and AdBlue diesel exhaust fluid (DEF) explained

What are DPFs and DEFs?

In order to meet strict European emissions legislation, all modern diesel engine vehicles will be fitted either with a Diesel Particulate Filter (DPF) or an AdBlue Diesel Exhaust Fluid (DEF) system OR both depending on manufacturer or vehicle model.

Both systems are designed to significantly reduce harmful emissions before they can be released into the atmosphere causing respiratory problems if people are exposed to high concentrations over time – especially in large towns and cities.

In summary, Diesel Particulate Filters (DPF) are designed to trap soot particles produced during the combustion process whilst AdBlue Diesel Exhaust Fluid (DEF) systems are designed to reduce harmful nitrogen oxide emissions by turning them into harmless nitrogen. To achieve Euro 6 regulations (which is mandatory in order to meet London’s Ultra Low Emission Zone plans for 2020) most manufacturers are now incorporating this technology into their diesel powered vehicles.

Diesel Particulate Filters (DPF)

Diesel particulate filters need to "regenerate"

Whilst the DPF filter can hold a certain amount of soot, when it gets full it needs to clear out the soot through a process known as “regeneration”, otherwise the vehicle cannot operate properly. Regeneration occurs when the filter reaches a sufficiently high temperature, allowing the soot to be converted into a much smaller amount of ash. On most systems to allow the filter to automatically regenerate, the engine must be driven at sufficient speed, to ensure a high enough temperature of the exhaust gas is reached. Typically a vehicle must be driven at 50mph or above for at least 20 minutes to effect this process.

What happens if the filter does not regenerate?

If the vehicle is not driven in a way that enables automatic regeneration, an excessive amount of soot will build up, which, if not resolved will reduce the performance of the vehicle and potentially damage the filter.

Once the filter is full, a warning light will appear on the dashboard to warn the driver. The problem can usually be resolved by allowing the filter to regenerate until the warning light goes out – i.e. by driving at 50mph or above for at least 20 minutes.

If traffic conditions or speed limits do not allow the vehicle to be driven so the filter regenerates, it will need to be taken to a franchised dealer for a forced regeneration.

If the warning light is ignored, it will cause damage to the vehicle which will NOT be covered under the manufacturer’s warranty or a maintenance agreement. A replacement DPF filter can cost as much as £1,500 to replace, or much higher for some brands.

Do all DPFs require regeneration?

The vast majority of modern diesel vehicles have diesel particulate filters that require the vehicle to be driven in order to activate the regeneration cycle, however, not all require this method. Some have systems that can heat up the particulate filter and regenerate it without the need for a higher speed drive cycle. Some inject fuel into the filter, which burns thereby increasing the temperature in the filter, while others have heaters built into the filter. You can find out what type of DPF filter is fitted to your vehicle via the manufacturer’s website or by checking the vehicle’s handbook.

Is a diesel vehicle suitable for my needs?

If you are mainly driving in town or in urban areas, the DPF is likely to reach capacity much sooner. At this point, you will need to regenerate the filter by driving the vehicle at a higher speed (typically 50mph or above) for at least 20 minutes. If you don’t think your driving style is going to achieve this, a petrol, petrol hybrid or electrically fuelled vehicle may be more suitable for you.

"AdBlue" Diesel Exhaust Fluid Systems (DEF)

DEF systems must be topped-up immediately when alerted

AdBlue is non-flammable, high purity urea solution injected into the exhaust system just ahead of the catalytic converter and is used to clean up diesel exhaust emissions. If your vehicle is fitted with DEF technology, it is important that you ensure the AdBlue fluid is topped up regularly, according to the manufacturer’s guidelines. Low level warnings must not be ignored as this causes the vehicle to go into reduced power mode, also known as “limp mode”.

On average, AdBlue fluid is used at a rate of around 5 litres per 100 litres of diesel and reduces nitrogen oxide emissions by more than 90%. Most manufacturers have chosen AdBlue technology to comply with Euro 6 emissions legislation.

Will the AdBlue tank require top ups between services?

The AdBlue tank will be refilled at every service, but the size of the tank will vary from model to model – therefore top ups between services may be required, as indicated by dashboard warnings. Also other factors such as mileage covered, journey types, driving style and environmental conditions will affect the rate of consumption.

How do I find and fill the AdBlue tank?

The AdBlue storage tank and filler is positioned differently depending on model. Typically, the filler cap is located next to the fuel filler or under the rear floor in the boot, although it can be in the engine bay on a van. Please check the vehicle’s handbook to find out the location of the AdBlue tank on your vehicle.

AdBlue needs to be handled with care!

When handling AdBlue it is important to wear protective clothing such as rubber gloves and glasses as the fluid can easily damage surfaces such as vehicle parts, plastic, items of clothing etc.

Where can AdBlue be purchased?

AdBlue fluid can be purchased via most franchised dealers, fuel stations, online or from high street motor retailers such as Unipart or Halfords.

What will happen if the AdBlue tank is not refilled when advised?

If a driver fails to respond to the dashboard warnings eventually the vehicle will simply fail to start. Once again, it is important to top-up the AdBlue fluid before the low level warning light comes on and the vehicle goes into “limp mode” as this may cause the vehicle to fail its MOT as it will not be able to maintain the required emissions level.

Is a diesel vehicle with AdBlue suitable for my needs?

With London’s Ultra Emission Zone plans for 2020, diesel Euro 6 compliance will be essential to avoid the £12.50 daily surcharge. Many diesel vehicles will also be fitted with a Diesel Particulate Filter (DPF) which requires regeneration, as well as AdBlue technology so it is important to consider your driving style before deciding whether a diesel vehicle is suitable for your needs.


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