What Are Petrol Particulate Filters, And Why Do They Make Cars Sound Worse?
We've been talking more and more about petrol particulate filters in recent years, but how do they actually work?
A particular term has been creeping into more and more car reviews in recent years - ‘petrol particulate filter’. These aren’t as well known as diesel particulate filters (DPFs), which have been around a lot longer, becoming ubiquitous from 2009 onwards.
Patrol particulate filters, also referred to as OPFs (otto partikelfilter) and GPFs (gasoline particulate filters), are a much more recent phenomenon. They’ve become necessary due to European Union rules surrounding the amount of particulate matter petrol engines are allowed to emit.
Particulates from petrol engines have been tightly controlled since the days of Euro 5. However, the replacement of the old NEDC (New European Driving Cycle) lab test cycle with the tougher WLTP (Worldwide Harmonised Light Vehicles Test Procedure) regime from Euro 6c onwards (joined by RDE, Real Driving Emissions, from Euro 6d) has made it much harder to comply. For a lot of cars, particulate matter emissions needed to be reduced drastically, leading to the rise of the OPF.
Particulate filters usually sit downstream of the catalytic converters, typically located inside a metal box that can look a lot like an extra silencer. Open it up, and you’ll find a ceramic honeycomb structure, much as you do with a DPF.
When exhaust gasses make their way through the device, the honeycomb filter traps excess nitrous oxide, carbon monoxide and hydrocarbon particulates. Due to the high temperature of the filter, these are all burned off, giving off a little water, nitrogen and carbon dioxide in the process. Hey presto - a decrease in particulates of around 90 to 95 per cent.
Diesel particulate filters have a reputation for getting clogged over time, particularly on cars that are used mostly for shorter, lower-speed journeys. With OPFs, this isn’t as much of an issue. As a kick-off, higher petrol engine (and therefore exhaust) temperatures make them more effective at burning stuff off, and secondly, petrol engines usually emit lower volumes of particulates.
Research has, however, shown that petrol engines can be just as bad as diesel units when it comes to nanoparticles, which are 100 times small than the diesel particulates (10 microns across) we started worrying about first. As they’re so much smaller, it’s thought they can go deeper into your lungs. Not fun.
But we digress - to sum up, no, a petrol particulate filter shouldn’t need any maintenance. Unless you’re bothered by our next point, you’ll likely forget it’s there in time. Or never have known in the first place.
Widespread adoption of OPFs is good news from an environmental perspective - we can’t deny that. But it’s hard not to be sad about the side effect of quieter engines. The reason for the noise reduction is simple - the particulate filter is an additional restriction, and restrictions in an exhaust system mean a more muffled sound coming out of the tailpipe.
As noted by then-Aston Martin chief engineer Matt Becker when speaking to CT a couple of years back, OPFs make it especially difficult to add all the artificial pops and bangs we grew accustomed to in the 2010s. In some cases, this isn’t such a bad thing. The Jaguar F-Type R, for instance, arguably sounds better post-OPFs, with its once excessive machine-gun-like exhaust character toned down considerably. Now, it’s all about the lovely V8 rumble, which remains plenty audible.
For a lot of cars, though, the news is less good. OPF-equipped second-generation Audi R8s (facelift onwards) are noticeably more muted than their predecessors. To make matters worse, the closely related Huracan Evo can use the same engine without filters, with Lamborghini opting instead for a combination of port and direction injection to reduce emissions and comply with the rules.
Before long, it’ll be easier to reframe this question as “what cars don’t have particulate filters?” Starting in late 2017, PSA Group installed OPFs on all its GDI (gasoline direct injection) petrol engines. Volkswagen starting fitting OPFs to all its cars in 2018, as did BMW. Mercedes, Volvo, Seat, Audi Skoda are among others using them.
Supercars and luxury sports cars aren’t immune, either. Aston Martin models powered by twin-turbo V8s, which are sourced from Mercedes-AMG, have used OPFs for a little while now. Porsche 911 models, including the GT3, are OPF users. The Audi R8 we’ve already discussed, and even Ferrari is in on the act, starting with the 3.9-litre twin-turbo V8 in the F8 Tributo.
From a technical standpoint, yes you can. Quite easily, in fact. Myriad companies offer OPF delete pipes for stacks of different cars. However, there are a couple of caveats. Firstly, any such modification would need to be accompanied by some ECU fiddling to prevent the missing OPF sensors from giving a fault code or even forcing the car into limp mode. Plus, you might have noticed a warning in the fine print on some websites, usually saying something along the lines of ‘track use only’.
The OPF is part of your car’s emissions control system, so as with fitting a de-cat pipe to ditch a catalytic converter (or ditching a DPF), an OPF delete has can leave you in legal hot water if discovered. VAG tuning company Revo has spoken out against OPF deletes. “We anticipate it being as illegal to remove as DPF,” the British firm’s website states, adding, “Therefore removal of the GPF is not a process supported by Revo.”
See also: The Pros And Cons Of Different Exhaust Materials
That’s not to say the solution is hopeless if you want to retain road legality. If you focus on removing the restrictions you’re actually able to, there are still aural improvements to be made. We’ve heard plenty of decent results from OPF-back or axle-back exhaust setups, for instance, and in time aftermarket companies will only get better at working with OPFs, both for exhausts and other modifications.