What is a diesel particulate filter (DPF)?

Most modern diesel cars have a diesel particulate filter, or DPF. But what is it and what does it mean for your car? Here we explain exactly what a DPF is, how it works and why your diesel car needs one.

DPF stands for diesel particulate filter. Diesel engines work by burning a mixture of diesel fuel and air to generate the power that drives the car. The burning process generates a lot of waste byproducts like carbon dioxide gas and soot particles that pass through the car’s exhaust pipe and get blown out into the atmosphere.

Those waste byproducts aren’t good for the environment, so cars have various emissions control systems that ‘clean’ the gases and the particles that pass through the exhaust. A DPF filters out the soot and other solid particles from the exhaust.

The exhaust gases produced by burning fuel in a car’s engine can harm the environment. Carbon dioxide emissions, for instance, contribute to climate change.

Other waste byproducts, known as particulate emissions, contribute to poor air quality in areas that suffer regular traffic congestion. Particulate emissions are tiny pieces of solid matter, like soot, which you can see in the form of black smoke coming out of some older diesel vehicles. Some of those particles are made up of really nasty stuff, like nitrogen dioxide, that has been linked to asthma and other respiratory illnesses.

Even without a DPF, an individual car produces a very small quantity of particulate emissions. But the cumulative effect of thousands of diesel vehicles packed together in a relatively small area like a city can create a serious problem. It’s vital to reduce those emissions as much as possible so that’s why your car needs a DPF – it greatly reduces the particulates coming out of the tailpipe.

If this is making diesel cars sound like an environmental disaster, it’s worth bearing in mind that the latest models meet very strict limits on particulate emissions. Indeed, they produce them in such small amounts that they’re no worse in that regard than petrol cars, emitting as little as 0.001g per kilometre of driving. It’s worth remembering, too, that diesel cars produce less carbon dioxide than petrol cars and they give better fuel economy.

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Every diesel car that meets the current Euro 6 emissions standards has a DPF – it’s impossible to meet those standards without one. Euro 6 came into force in 2015, though many older diesel cars also have a DPF. (Peugeot was the first carmaker to fit its diesels with a DPF, way back in 2004.)

A DPF just looks like a metal tube, but there’s clever stuff going on inside it, which we’ll come to in a moment. A DPF is often the first part of a car’s exhaust system. You can actually see the DPF under the bonnet of some cars.

The DPF contains a fine mesh that collects the soot and other particulate emissions that come from the engine. It then periodically uses heat to burn off the accumulated soot and particulates. The burning process breaks them down into gases that pass through the exhaust pipe and disperse into the atmosphere.

Burning off the soot and particulates is known as ‘regeneration’. There are several ways DPFs can do it. Most of the time, they use heat accumulated from the engine. But if the exhaust system isn’t hot enough, the engine can use a bit of extra fuel to generate more heat.

There’s a perception that DPFs are prone to failure, but they’re actually no more likely to fail than any other part of a car. They just need proper maintenance, which some people don’t realise is required.

Most car journeys are only a few miles long, which isn’t enough for the car’s engine to reach its ideal operating temperature. A cold engine works less efficiently, producing more soot and the exhaust system doesn’t get hot enough for the DPF to burn it off. A few thousand miles of short journeys, which can easily accumulate if you rarely drive outside your local area, could cause the DPF to clog and fail.

The solution is actually simple – just go for a long drive. Every 1,000 miles or so, drive at least 50 miles at a reasonably high speed. That’ll generate enough exhaust heat for the DPF to go through its regeneration cycle. Dual carriageways, 60mph A-roads, and motorways are best for such drives. If you can make a day out of it, so much the better!

Alternatively, DPF-cleaner fluids are available, but they can be expensive and their effectiveness is questionable.

If you regularly do long journeys, you’re unlikely to have problems with your car’s DPF.

A DPF is most likely to fail if it clogs up as a result of repeated short journeys. You’ll see a warning light on your car’s dashboard if the DPF is in danger of clogging, in which case, your first step should be to go for a long, high-speed drive. That should generate the exhaust heat the DPF needs to go through its regeneration cycle and unclog itself. If that works, the warning light will go out. If not, take the car to a garage which can use other techniques to unclog the DPF.

If a DPF clogs completely and starts to fail, black smoke will come out of the exhaust and the car’s acceleration will become sluggish. Exhaust fumes may even get into the car’s interior, which is dangerous. At that point, the DPF needs to be replaced, which is a very expensive job. In most cases, you’ll be looking at a bill for at least £1,000. By comparison, those long, fast drives seem like a bargain.

Petrol engines also produce soot and particulates as they burn fuel, albeit in much smaller quantities than many diesel engines. However, the latest legal standards on soot and particulate emissions are so strict that the newest petrol cars need a ‘petrol particulate filter’, or PPF, to meet them. A PPF works in exactly the same way as a DPF.

Contrary to what you may think, particulate filters don’t affect a car’s performance or its fuel economy.

In theory, a particulate filter can reduce the amount of power an engine produces because it restricts the flow of the exhaust gases. That could ‘choke’ the engine and cause it to produce less power. In reality, though, the amount of power a modern engine produces is governed by its computer, which changes how the engine works to compensate for the filter.

The engine’s computer also makes sure the filter doesn’t reduce fuel economy, although it may worsen if the filter starts to clog.

The only effect a particulate filter has that you might actually notice relates to the exhaust noise, and in a good way. It’ll be quieter than that of a car without a filter.

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