DPF (Diesel Particulate Filter) – How it works

A Brief History

Particulate filters in one form or other, have been included on some/various non-road vehicles since the 1980s, heavy duty machinery, generators etc.

In 2000, Peugeot made their debut with the innovative FAP (Filtre of Aparticules) in the Peugeot 607, their flagship executive saloon. They were the first passenger vehicle manufacturer to include a DPF on mass market vehicles. That system used a solution in a separate secret tank that when you refuelled, would add itself into your fuel tank at the correct ratio, this lowers the combustion temperature of the particulates to help them be burnt off during regeneration.

There have been variations of DPF/SCR (Diesel Particulate Filer/Selective Catalytic Reduction) since then, join us in 2018 and there is one method in particular that dominates. AdBlue.

I’m going to cover the various DPFs and how they work as there are literally millions of cars out there running all the older variations, all very similar.

Why the need for it?

A diesel engine by design produces some very nasty emissions, Particulate Matter/Nitrogen Oxide/NOx. They produce very sooty, black particulates that research has shown causes serious damage when inhaled. Heart Disease, Lung Cancer are just 2 of the worst things linked, as smaller particulates can cross the barrier and get into your blood stream. It’s also responsible for Smog and Acid rain.

This is mainly due to the inefficiency of the diesel engine as it can’t fully burn the fuel, which means that these gases, particulates and unburnt hydrocarbons get released from the exhaust pipe into the atmosphere.  Due to their low CO2 output, the EU/UK pushed for Diesel sales to try and reduce CO2 emissions to limit climate change, however, this had an adverse affect on the air quality we breathe due to the increased particulate matter. Now leaning on the manufacturers, The latest Euro 6 regulations have become ever more stringent, companies and manufacturers are doing their best to reduce NOx levels. Euro 5 vehicles have a permissible level of 180mg/km of NOx whilst Euro 6 vehicles have a permissible level of 80mg/km, as you can see that’s a hefty drop.

What does a DPF do?

The particulate filter does exactly as its name implies. It filters/catches those sooty particles.

The particulate filters are situated in the exhaust system, some are located closer to the engine, some are located further away. This effects how they are regenerated which we’ll explain.

On an early Peugeot right through to a new Euro 6, the actual DPF works the same, physically catching and trapping particulate matter. When the filter gets to a certain percentage full, the oxygen sensors on the vehicle will tell the car to go into regeneration. 

This is where the locations of the DPF come into play as there are a few variations on regeneration.

On the vehicles where the DPF is very close to the engine, the temperatures are naturally much hotter there, allowing for better passive regeneration meaning there should be less need to have active regeneration. On vehicles where the DPF is further away, active generation would be needed on a more regular basis.

Passive Regeneration: This happens on day to day driving, when the vehicle is up to temperature, it allows the DPF to reach a temperature high enough to burn off the trapped soot. If the DPF is further away, it will take longer to reach these temperatures, if at all.

Active Regeneration: This, is clever. The vehicle will raise the temperature of exhaust gases in order to burn off the soot itself. As the fuel and air mixture in the cylinder ignite, the car will time and then inject a 2nd spray of fuel at the right moment to ignite the exhaust gases leaving the cylinder. This raises the exhaust gas temperature massively, burning off the soot. 

Whilst this is happening, most don’t realise it, however you may notice slightly lower MPG and your radiator fan spinning up more often.

Forced Regeneration: If the vehicle has only been used for short runs and doesn’t get up to temperature for a decent period, the DPF will get blocked. Once it gets to a certain percentage, the vehicle will go into limp mode as back pressure will be too high, potentially causing engine damage. Forced regeneration is when the car is hooked up to diagnostic software at a dealer or specialist and ‘Forced’ to do a regeneration.

SCR and AdBlue: As mentioned, Euro 6 required a massive drop in these harmful emissions, so to meet the requirements manufacturers have also included SCR (Selective Catalytic Reduction). Usually placed after the DPF, this is a further catalytic chamber in the exhaust where AdBlue is injected into the exhaust stream, causing a reaction that splits NOx into it’s harmless elements, Nitrogen, Water and a little bit of CO2. This means the gases exiting the tailpipe are essentially up to 95% clean in simplest terms.

Most of the big manufacturers now use AdBlue. Volkswagon, Mercedes, Audi, BMW, Jaguar, Landrover, Peugeot, Citroen to name a few and are all usually badged with a name with blue in it, such as Bluetech or BlueHDi to mark this.

AdBlue is made up of Urea (Yes, what’s in our Urine) and De-ionised water. The Urea is man made, it doesn’t come from humans or animals, this when put into the catalyst chamber turns into ammonia that causes the NOx to split into its elements.

AdBlue is available now in bottles or even at pumps in petrol stations and at time of writing, can be as little as 60p per litre from the pump.

If you have a vehicle fitted with the AdBlue SCR system, there is usually a cap placed next to the fuel filler for you to add this. The vehicle will tell you when it’s getting low and you should fill it at you earliest convenience, if you allow it to run out, it will go into limp mode for environmental reasons until it is added with some requiring a a diagnostic hookup to get it out of limp mode, so don’t let it run out.

If you accidentally put AdBlue in your fuel or your fuel into your AdBlue tank, call your roadside assistance or a misfuelling company right away as to avoid damaging either systems, don’t try and run your car like normal.

2018 MOT Changes for Diesels in the United Kingdom

During 2018 in the UK the MOT regulations became more strict, focusing more on Environmental factors, such as making sure a diesel vehicle that had a DPF when new, still has one fitted and hasn’t been removed obviously (removing the DPF and replacing with some exhaust tube) or discreetly (some will cut open the casing, remove the filter and weld the casing back up to make it appear as if the DPF is still there), both methods are obvious to an MOT tester. A visual inspection of where the DPF is/should be and the amount of smoke exiting the exhaust is now taken into account and the vehicle would fail if the tester believes the DPF has been tampered with and there is a visibly higher level of black smoke exiting the exhaust.

Cars pre-DPF or ones sold when new without a DPF won’t be penalised with the tighter test, they will still however be subject to the other items, such as fluid leaks, if your car has an fluid leak and there is no dripping (I.E there is a coating on your engine or pipes, it will be a minor defect (advisory) if there is an accumulation that looks as if it can drip or is dripping, that is a Major Defect and will fail the car.

In short DPF and SCR catch and transform dirty emissions into ones that are much less harmful for us and the environment.