Gasoline Particulate Filter

A gasoline particulate filter, abbreviated to GPF and sometimes called a petrol particulate filter (PPF), is an exhaust component that traps the microscopic soot particles produced by modern petrol engines. It is the petrol equivalent of the diesel particulate filter that has been fitted to diesel cars for years. Its emergence reflects an awkward truth about contemporary petrol engines: although petrol combustion was long considered clean of soot, the move to direct fuel injection reintroduced a particulate problem that now has to be controlled.

The need arose specifically because of gasoline direct injection. When fuel is sprayed straight into the combustion chamber rather than into the intake port, there is less time for it to mix evenly with air, and some droplets burn in locally fuel-rich pockets. This incomplete combustion produces fine soot, including very small ultrafine particles of concern to health regulators. As direct injection became widespread for its efficiency and performance benefits, petrol engines began emitting particle numbers that earlier port-injected engines did not, prompting the regulatory response.

The filter itself is a honeycomb structure, typically of cordierite or silicon carbide, with a large number of small parallel channels. Alternate channels are blocked at opposite ends, forcing the exhaust gas to pass through the porous channel walls. The walls let the gas through but capture the solid soot particles, often achieving better than 90 per cent reduction in particle number. The trapped soot accumulates over time and must periodically be burnt off in a process called regeneration.

A significant advantage of the GPF over its diesel counterpart concerns regeneration. Petrol engines run much hotter, with exhaust temperatures frequently high enough to burn off accumulated soot continuously and passively during normal driving, so the filter cleans itself without special intervention in most conditions. This avoids many of the active-regeneration headaches associated with diesel filters, where short, cold journeys can prevent the filter from reaching the temperature needed to clear, leading to blockages. The GPF is also frequently integrated into the same housing as the catalytic converter to save space and exploit the heat already present there.

Gasoline particulate filters became common from the Euro 6d stage of European emissions regulation, when stricter particle-number limits were applied to petrol cars to bring them into line with diesels. In everyday use they are largely maintenance-free, though like any exhaust restriction they impose a very small flow penalty, and a damaged or contaminated filter can eventually require replacement.

The GPF belongs to a family of after-treatment devices and is best understood by comparison and association. It is the direct analogue of the diesel particulate filter, exists chiefly to manage the soot from direct injection, works alongside the catalytic converter that handles gaseous pollutants, and was driven into widespread use by the Euro 6 emissions standard.