How it works: Particulate Filter System

Diesel Particulate Filter – How it works

In 2000, Peugeot made their debut with their innovative FAP (Filtre of Aparticules) in the Peugeot 607, their flagship executive saloon.

The FAP system has since been included on almost all their HDI engines and if you go and buy a new diesel vehicle today from other manufacturers 90% of the time there will be some form of Particulate filter system in place.

Peugeot paved the way with FAP and as it’s such an innovative, ecological and emission busting idea, it’s become widely used in slightly different variations by other manufacturers. It’s often abbreviated as DPF (Diesel Particulate Filter).

Why the need for it?

A diesel engine by design produces some very nasty emissions. They produce very sooty, black, stick to the back of your throat, exhaust fumes. This is mainly do to the inefficiency of the engine as it can’t fully burn the fuel which means that unburnt hydrocarbons get released from the exhaust pipe. Although diesels have a lower CO2 output, these hydrocarbons are very nasty and are second on the list after Carbon Monoxide for causing serious health problems.

With emissions regulations becoming ever more stringent, the need for the motoring industry to concentrate on ecological matters and for human safety something needed to be done to drastically cut these emissions.

What does it do?

The particulate filter does exactly as its name implies. It filters/catches those sooty particles. Now you’re probably thinking, “Well if there’s a filter, surely it needs to be changed every time it gets blocks and surely in a car that’ll be practically  every week?” Well this is where the system gets clever, up to a certain extent, it cleans itself out, this is called regeneration.

So then, how does it actually work. (We’re basing this on the Peugeot system, after all they brought it to the car industry first)

There are 3 main stages to this whole system:

1.) There is a hidden reservoir just beside the fuel tank (you ‘might’ be surprised to find this out) that you cannot and don’t have access too. The fluid in this secret tank is highly toxic so should only be handled by professionals.

When you put fuel in the car it works out exactly how much additive is needed and squirts it directly into the fuel tank. (In normal use at about 40mpg, the fluid should last for around 25,000 miles, this is just an estimate as each car will use it more or less depending on lots of things)

The additive lowers the ignition temperature of the trapped particles. It basically, through chemical reaction, makes it easier later on for the car to ‘burn off’ the particles.

2.) The second stage of the system is the car actually catching the particles in the special filter. This filter can be housed along with the catalytic converter or just before it, depending on the type of DPF system and car manufacturer.

3.) This is the very clever bit, the regeneration. When the car measures the amount of oxygen flowing through the car using sensors, it can tell if the gases flowing through the filter aren’t at the correct levels, indicating that the filter is becoming full/blocked.

When this happens the car enters into regeneration mode. This will typically happen every 300 miles and you shouldn’t even be able to tell the car is doing it.

Regeneration mode is there to burn off the caught particles for good.

This is how it’s done. When the engine is running normally, the injectors squirt once into the cylinder which then ignites to push the piston down to create movement. In regeneration mode, the car does the same thing except just as the initial squirt ignites, the car squirts in a second/extra bit of fuel to create a bang with a much higher temperature.

The incredible extra heat in the exhaust gas burns away the caught particles. Once it’s clear, regeneration stops and the car resumes service as usual.

Other systems, terminology and warnings you may come across when researching FAP/DPF:

Just to note that some manufacturers don’t use the additive and instead place an oxidising catalytic converter very close to the engine. Being so close to the engine, the exhaust gasses are hot enough to burn the particles away without the need for the additive to lower the temperature.

Passive Regeneration: Takes place in all types of DPF system, this happens when the car is driven hard or on the motorway for a long time, basically meaning the car ‘naturally’ burns off the caught particles.

Manufacturers realise this causes a problem with cars that only do short journeys so they engineered into the system a process specifically designed to clear the particle filter/trap.

Active Regeneration: This is the method described above, this is when the car ‘Actively’ changes the ‘burn’ to clear out the filter.

Passive regeneration is good but it can cause problems with cars that only do short journeys and aren’t revved/driven hard. When the car senses a partially blocked filter (around 45%), a warning light will be displayed, at this point a drive of over 40mph for a sustained period of over 10 minutes in a lower than normal gear is sufficient to clear out the filter.

If the initial warning is ignored and the light is not cleared/gone off the car will at approx 75% blocked display a different light or message on the dashboard computer to tell you it’s blocked and you will need to visit a dealer where they can hook the car up and have it regenerate.

Remember that if you continue to ignore the warning lights and the system malfunctions, the car can go into ‘limp home mode’ which cuts the power of the engine to practically crawling pace and the car will force you to take it to the dealer to sort it out.

It is best to try and sort this type of issue out straight away as you run the risk of the filter becoming damaged. Which for a new one at a main dealer costs around £1000.

On that bombshell, that’s all we can tell you about this innovative and ecological masterpiece.


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