The petrol particulate filter: What a PPF does and why it can make cars quieter

If you have been browsing for a new or nearly new petrol-powered car, you may have seen passing mentions of petrol particulate filters.

These relatively new emissions control devices, also known as gasoline particulate filters – GPFs – are becoming increasingly common due to changes in technology and legislation.

If you’ve heard about the pitfalls of diesel particulate filters, or had a bad experience with one, the presence of a petrol particulate filter might justifiably give you pause for thought.

However, due to differences between the two, petrol particulate filters shouldn’t prove problematic – so you need not worry about buying a car equipped with one.

If you’d like to find out why, or if you’re just curious about how they work and what they’re designed to do, check out the YesAuto guide below for more details.

What is a petrol particulate filter (PPF)?

A petrol particulate filter is a type of exhaust aftertreatment system. This means that it’s designed to reduce harmful emissions produced by the engine.

In this instance, the filter – through which exhaust gas flows – captures soot and other particulate matter produced during combustion, which is harmful to the environment and poses a health risk.

The stored particulates are then burnt within the filter, during a process called regeneration, and the reaction that takes place converts them into less harmful carbon dioxide.

This negligible amount of carbon dioxide then exits the filter with the rest of the exhaust gases.

Do all petrol cars have a particulate filter?

No. The first mass-production PPF was introduced in 2014, in the S500 variant of the Mercedes-Benz S-Class, but it wasn’t until 2016 that they started being employed in a more significant number of cars.

Changing emissions laws and technology, however, are making PPFs far more commonplace – so your next new petrol car may well be equipped with one.

Why are PPFs becoming more common?

For starters, legislation and regulations regarding the size of particulate matter emitted by petrol engines, and the quantity of particles emitted, is becoming more stringent.

Consequently, manufacturers have to adopt new technologies – such as particulate filters – to meet legal requirements.

What’s further driving the adoption of petrol particulate filters is the increasingly widespread use of direct injection systems in petrol engines.

This technology, which is sometimes referred to as gasoline direct injection – GDI – delivers fuel directly into each cylinder in an engine.

This differs from conventional and common port injection, in which fuel is injected into the engine’s intake system.

Using direct injection allows for more precise fuel delivery and improved combustion, which can grant increased efficiency and power, particularly when coupled with other technologies.

However, due to differences such as how the fuel behaves when directly injected, these engines can produce – according to some reports – up to twice as much particulate matter as port-injected engines.

Direct-injected petrol engines are also reported to produce more ultra-fine particles, which are associated with negative health and environmental impacts, than diesel engines equipped with diesel particulate filters.

Using a particulate filter in engines with direct injection, as a result, is essential to minimise harmful particulate emissions.

How does a petrol particulate filter work?

Open up a petrol particulate filter and, at its core, you’ll find a honeycomb-like structure.

This is often made out of ceramic material and contains myriad channels, open at alternating ends, which have porous walls.

Exhaust gas flows into these channels but, being unable to exit easily at the other end, it has to flow out through the porous sections instead.

Particulate matter in the exhaust gas can’t fit through the tiny openings in the walls, so it instead gets trapped in the filter.

The high temperature in the filter, produced by exhaust gases during normal driving, subsequently burns the stored carbon soot off when conditions permit.

This prevents the filter from becoming clogged or completely blocked, avoiding performance-related problems while ensuring ongoing and proper filtration.

Petrol particulate filters may also be integrated with, or coated with, catalytic elements that help reduce other emissions and promote regeneration.

How effective are petrol particulate filters?

According to some manufacturers, direct-injection cars equipped with petrol particulate filters emit up to 90 per cent fewer particulates than comparable cars without a filter.

Are these like diesel particulate filters?

Yes, a petrol particulate filter is similar to a diesel particulate filter. One notable difference, however, is that PPFs do not store as much particulate matter as DPFs.

This is primarily because direct-injected petrol engines produce far less harmful material in total than diesel engines, which allows for reductions in filter capacity. 

Do PPFs have to regenerate, like DPFs?

Both petrol and diesel particulate filters use regeneration processes that burn off stored soot and prevent clogging.

However, there is a prominent difference between the type of regeneration generally employed by petrol and diesel particulate filters.

During any regeneration cycle, heat and oxygen are used to convert stored soot into carbon dioxide, which then exits the filter assembly along with other exhaust gases.

Passive regeneration takes place when the exhaust temperature is high enough, and conditions otherwise permit, allowing for soot to easily be burnt off.

Alternatively, active regeneration can be used. This relies on techniques such as altering the engine’s behaviour to increase the exhaust temperature to prompt regeneration.

Usefully, petrol engine exhaust temperatures are generally higher than those of diesel engines during normal driving.

This means that petrol particulate filters can often employ straightforward passive regeneration, so stored material is readily burnt away – which is another reason for the lower soot capacity of PPFs.

Active regeneration otherwise remains available for when driving and traffic conditions don’t permit passive regeneration.

Can a petrol particulate filter block up?

The way petrol particulate filters work, and the reduced amount of material they need to handle, means that they should be far less problematic than diesel particulate filters.

One manufacturer, for example, suggests that one 20-minute drive, at speeds over 44mph – between every refuelling stop – will be sufficient to prevent PPF clogging.

However, repeated short trips, cold starts or poor fuel could overload a petrol particulate filter with material and require an active regeneration cycle to clear.

These regeneration cycles may increase fuel consumption, cause increased engine bay temperatures, and a warning message may also be displayed.

As a result, if you have a car with a petrol particulate filter, it’s worth studying the relevant section in its manual to familiarise yourself with the potential messages, requirements and solutions.

Are there any other downsides to PPFs?

A common observation about petrol particulate filters is that cars fitted with them are quieter than cars without.

This isn’t a problem in a lot of cases – few are going to gripe about a regular hatchback being quieter, for instance – but some have found the noise of PPF-equipped high-performance cars disappointing.

A particulate filter is a slight energy-absorbing restriction in the exhaust system, after all, and can potentially change the way the car sounds.  

However, the quieter nature of new cars isn’t necessarily the fault of a PPF. Instead, increasingly strict noise restrictions – and the silencing required to meet them – are often why cars might sound more restrained than some would like.

Do I need to worry about petrol particulate filters?

PPFs use proven technology and don’t have to work as hard as DPFs, so major problems should be few and far between.

Consequently, potential noise-related gripes aside, there’s seemingly little to worry about.

As always, though, do research any potential purchase beforehand to see if there are any common issues.

Can I remove the petrol particulate filter?

Removing the particulate filter, which is a part of the factory emissions control system, will result in the car failing its MOT test. 

Similarly, gutting the PPF and welding its container closed – so it at least still appears present – will prompt an MOT failure, unless paperwork proving it was opened for repairs is available.

In any case, potential sound change aside, there is typically little benefit to removing a PPF. It's often the case that they do not restrict exhaust flow excessively, so any performance gains will be fractional at best.

And, yes, you can still tune a car equipped with a PPF – and many companies have already come up with modifications, including aftermarket ECU tuning and exhausts, that support and work with PPFs.

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