Gasoline Particulate Filters

W. Addy Majewski

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Abstract: Gasoline particulate filters (GPF) have been introduced to reduce particle number emissions from GDI vehicles. The filters utilize wall-flow substrates first developed for diesel particulate filters. The GPF regenerates passively, but an active regeneration assist is needed to prevent filter plugging during low temperature duty cycles. Ash has an impact on GPF performance and—if the GPF is coated with a three-way catalyst—can be a source of catalyst poisoning.

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 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 been also 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 GDI vehicles—passenger cars and light commercial vehicles—adopted in both jurisdictions. The Euro 6 and China 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:

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:

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]. Daimler and other carmakers (including Volkswagen and Peugeot) have announced their plans to introduce GPFs on more vehicle models. The number of GPF applications is expected to grow after PN RDE testing requirements become effective at the Euro 6d-TEMP stage. GPFs may be also adopted for some port fuel injected (PFI) engines, even though PFI vehicles are not subject to PN/PM emission standards.

Most early GPF applications include an uncoated GPF positioned downstream of a TWC catalyst. As the technology matures, GPFs may be coated with a three-way catalyst. This catalyst coated GPF configuration is sometimes referred to as the 4-way catalyst.

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.

Acknowledgements

We are grateful to Christine Lambert of Ford Motor Company and Carl Justin Kamp of MIT who provided ash images (Figure 9, Figure 10, Figure 11) 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.

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