Sample TEM images of soot particles produced by the stock-GDI (left) and the GPF-equipped (right) vehicle. The stock GDI PM shows solid carbonaceous particles; the GPF PM shows an amorphous aggregate, coated with semivolatile material, and nucleation-mode particles. Credit: ACS, Saffaripour et al. Click to enlarge.

GDI engines help improve fuel economy and therefore reduce CO₂ emissions; however, they can have higher particulate emissions due to shorter fuel/air mixing times in the cylinder compared to multiport fuel injection engines. (Earlier post.) Advanced fuel injection strategies are currently used to control gasoline particulate emissions in-cylinder (earlier post) but they are designed for a particular emission test cycle and may be less effective under real driving conditions according to some.

Gasoline particulate filters—similar to diesel particulate filters—can control particulate emissions under all operating conditions.

Gasoline-direct-injection (GDI) vehicles are emerging in the market but their particulate-matter (PM) emission properties, in particular PM size and morphology, and the effects of after-treatment technologies developed for these vehicles on PM emission have not yet been well researched. A detailed knowledge of the size and structure of vehicle-emitted PM improves the assessment of its health-related impacts, and is essential for accurate PM measurement using optical diagnostic techniques.

… The objectives of the present work are (1) to investigate the role of drive cycle on the size and morphology of PM emitted by a GDI vehicle, (2) to study and understand how the use of a catalyzed gasoline particulate filter (GPF) affects the size and morphology of emitted particles, and (3) to gain a better understanding of semivolatile-material emission and the formation of ultrafine nucleation-mode particles in a GDI-vehicle exhaust.

—Saffaripour et al.

The test vehicle was equipped with a three-way catalytic converter (TWC) and exhaust-gas recirculation. The custom-designed GPF featured a filter substrate made of cordierite ceramic and had a wall-flow design.

The gradual accumulation of soot in the GPF could lead to filter clogging, which increases the engine-exhaust back pressure. Therefore,

To oxidize accumulated PM—which can lead to filter clogging and increased engine-exhaust back pressure—the filter is regenerated passively when the temperature is sufficiently high and excess oxygen is available in the exhaust. To enhance the oxidation of particles, the GPF substrate was coated with a Pd−Rh catalyst.

Among their findings:

• The stock-GDI vehicle emitted graphitized fractal-like aggregates over all driving conditions. The mean projected area-equivalent diameter of these aggregates was in the 78.4–88.4 nm range and the mean diameter of primary particles varies between 24.6 and 26.6 nm.

• Post-GPF particles emitted over the US06 cycle appeared to have an amorphous structure, and a large number of nucleation-mode particles. This indicates the emission of a substantial amount of semivolatile material during the US06 cycle, most likely generated by the incomplete combustion of accumulated soot in the GPF during regeneration, the team suggested.

• The size of primary particles and soot aggregates did not vary significantly by implementing the GPF over the FTP-75 cycle; however, particles emitted by the GPF-equipped vehicle over the US06 cycle were about 20% larger than those emitted by the stock-GDI vehicle. The researchers said this may be attributed to condensation of large amounts of organic material on soot aggregates.

• High-contrast spots, most likely solid nonvolatile cores, were observed within many of the nucleation-mode particles emitted over the US06 cycle by the GPF-equipped vehicle. These cores were either generated inside the engine or depicted incipient soot particles which were partially carbonized in the exhaust line.

• The effect of drive cycle and the GPF on the fractal parameters of particles, such as fractal dimension and fractal prefactor, was insignificant.

Resources

• Meghdad Saffaripour, Tak W. Chan, Fengshan Liu, Kevin A. Thomson, Gregory J. Smallwood, Joseph Kubsh, and Rasto Brezny (2015) “Effect of Drive Cycle and Gasoline Particulate Filter on the Size and Morphology of Soot Particles Emitted from a Gasoline-Direct-Injection Vehicle” Environmental Science & Technology doi: 10.1021/acs.est.5b02185