Gasoline Particulate Filters

W. Addy Majewski

This is a preview of the paper, limited to some initial content. Full access requires DieselNet subscription.
Please log in to view the complete version of this paper.

• Introduction
• Materials and Configuration
• Emissions and Engine Performance
• GPF Operation and Control

Introduction

Gasoline particulate filters (GPF) are an emission aftertreatment technology based on diesel particulate filters (DPF), developed to control particulate emissions from gasoline direct injection (GDI) engines. The technology is also referred to as petrol particulate filter (PPF) and, in some German literature, as Otto particle filter (Ottopartikelfilter in German), abbreviated OPF.

The population of GDI vehicles has been increasing, driven by CO₂ and/or fuel economy requirements. In 2016, an estimated 2/3 of new gasoline cars in Europe were GDI [3615]. The proportion of GDI vehicles has also been rapidly increasing in North America—within nine years after its first significant use in the market, GDI penetration has climbed to 48.5% of new light vehicle sales in the United States [3616]. Emissions from the growing GDI vehicle fleet are a public health concern and a potential major source of ambient particle pollution in highly populated urban areas.

GPFs are expected to be used primarily in the European Union and in China, to meet the particle number (PN) emission standards for gasoline passenger cars and light commercial vehicles adopted in both jurisdictions. The Euro 6 regulations set PN (as well as PM) limits for GDI vehicles that are equivalent to those for diesels. The European PN standards, both effective for new types of GDI cars from September 2017, are:

• A PN limit of 6.0×10¹¹ km^-1 over the NEDC / WLTC test cycle (Euro 6c)
• RDE testing for PN emissions with a conformity factor of 1.5, i.e., PN_RDE = 9.0×10¹¹ km^-1 (Euro 6d-TEMP)

China 6 regulations also include a WLTC PN emission standard of 6.0×10¹¹ km^-1, effective from July 2020, as well as RDE PN requirements from July 2023. The Chinese PN standards are not limited to GDI, but apply to all gasoline vehicles.

The above standards could also be met—at least in certain types of vehicles—via in-cylinder controls such as fuel injection strategies, without particulate filters. However, the GPF has several advantages compared to in-cylinder controls:

• Effectiveness under all operating conditions—While in-cylinder strategies tend to be more effective under certain modes of operation, the GPF provides PN emission control under all engine operating conditions—an advantage that is especially important in RDE testing.
• Control of emissions from engine faults—Increased PN emissions can occur as a result of engine faults and malfunction, such as increased lube oil consumption. These emissions can be effectively controlled by particulate filters [3632].
• Control of unregulated emissions—The GPF can control certain unregulated emissions, including polynuclear aromatic hydrocarbons (PAH). GDI engines, even though equipped with three-way catalysts (TWC), may produce significant levels of toxic PAH emissions [3618].

Gasoline particulate filters are not expected to be widely adopted in North America, where particle emissions are regulated through mass-based PM limits only. The US Tier 3 PM limit of 3 mg/mi, as well as the 2025 California LEV III limit of 1 mg/mi will likely be met through in-cylinder control technologies.

Commercial Status. Gasoline particle filters were first launched in a mass production application by Daimler, who introduced a GPF on their Mercedes-Benz S500 luxury sedan in early 2014 [3617]. The number of GPF applications has increased rapidly since 2017, as a result of the PN RDE testing requirements that became effective at the Euro 6d-TEMP stage. Filters were introduced on additional models by Daimler, as well as by Volkswagen, BMW, Peugeot and other manufacturers. By mid-2018, one GPF manufacturer—Corning—supplied one million GPFs for the European market, which indicates that within about one year the technology reached a market penetration of at least 10% of gasoline vehicles. GPFs may also be adopted for some port fuel injected (PFI) engines, even though PFI vehicles are not subject to European PN/PM emission standards.

Most early GPF applications included an uncoated GPF positioned downstream of a TWC catalyst. As the technology matured, GPFs have also been coated with a three-way catalyst. This catalyst coated GPF configuration is sometimes referred to as the 4-way catalyst. One of the early applications of a coated GPF was the 1.0 TSI engine used in the 2018 VW up! GTI city car.

While the GPF and DPF technologies are closely related, there are a number of differences in the filter configuration, operation and control strategy, which are due to the differences in the operating conditions, and the particulate emission rates and composition between gasoline and diesel engines. These aspects of GPF technology are discussed in the balance of this paper. An in-depth discussion of strategies and technologies to reduce PM emissions from gasoline engines can be found in the literature [4573].

Acknowledgements

We are grateful to Christine Lambert of Ford Motor Company and Carl Justin Kamp of MIT who provided ash images (Figure 13, Figure 14, Figure 16) as well as valuable comments on this paper. The section on hydrated ash is based in part on the communication with Dr. Kamp, and includes some observations from his research on the morphology of DPF/GPF ash that was not yet published at the time of writing.

###