Solid particle number emissions for all four GDI and PFI vehicles over the three different driving patterns. Credit: ACS, Chan et al. Click to enlarge.

Black carbon has a negative impact on the environment and on human health. Studies suggest that atmospheric BC particles have a direct radiative forcing second to CO₂. Exposure assessment studies have linked BC particle exposure to various human health issues, such as lung function and blood pressure. (Earlier post.)

In their study, the Environment Canada/MECA team used an aethalometer—an instrument that provides a real-time readout of the concentration of black carbon aerosol particles in an air stream—to measure real-time BC emissions; solid particles were measured using a European Union Particle Measurement Program compliant system.

They also presented the performance of a prototype GPF on removing BC particles from a selected GDI vehicle at various conditions.

The major difference between the PFI and GDI engines lies in the fuel injection method and mixture preparation. This leads to the potential for wetting the cylinder wall in the GDI engines. In addition, the inhomogeneity of the fuel mixture under stratified operation in GDI engines during the compression stroke tends to contribute to higher particulate emissions compared to the exhaust emissions from traditional PFI engines.

—Chan et al.

A number of studies have established some general trends re: particulate emissions:

• Cold-start at low ambient temperature significantly increases particle number emissions from a PFI vehicle compared to a GDI vehicle primarily due to the difference in fuel injection and mixture preparation strategies between GDI and PFI engines.

• Nanoparticle emissions from gasoline spark ignition vehicles can be strongly dependent on vehicle speed and engine load.

• The role of ethanol on BC emissions is not clear. The authors point to a detailed chemical kinetic model suggesting that oxygenated compounds, such as ethanol, could reduce BC formation. However, observations from flame studies suggest that ethanol can both increase or decrease BC formation depending on whether a non-premixed or premixed flame is involved during the combustion process. Splash blending further complicates the situation.

For their study, the Environment Canada/MECA team used two midsize sedans and two compact vehicles. The midsize sedans were a 2011 2.4L Hyundai Sonata wall- guided, stoichiometric GDI vehicle (GDI#1) and a 2010 2.4L Volvo S40 PFI vehicle (PFI#1). The two compact vehicles were a 2012 2.0L Ford Focus wall-guided, stoichiometric GDI vehicle (GDI#2) and a 2013 2.0L Ford Transit Connect PFI vehicle (PFI#2).

All vehicles accumulated mileage on-road prior to the emission measurements; all vehicles were equipped with three-way catalytic converters (TWCs). Two fuels were used, a Tier 2 certification gasoline (E0) and a splash blended 10% by volume blend of ethanol with certification gasoline (E10).

Drive cycles used in this study were the US Federal Test Procedure (FTP-75) and US06 Supplemental Federal Test Procedure (US06).

The GPF used on the GDI#1 vehicle was a custom design device provided by the Manufacturers of Emission Controls Association (MECA).

Among their findings were:

• BC solid particles were emitted mostly during cold-start from all GDI and PFI vehicles. Cold-start BC mass emissions from the two GDI vehicles represented 65−84% of the total BC mass emitted over the entire FTP-75 drive cycle. For the two PFI vehicles, cold-start BC mass attributed to 86−100% of the total BC mass emissions over the entire FTP-75 drive cycle.

• Hot-start emission results were lower by orders of magnitude than those from cold starts. In general, both the stock GDI#1 and GDI#2 BC mass emissions were comparable, varying from about 1.6−2.5 mg/mi at standard temperature to 4.3−4.5 mg/mi at 0 °F. Although much lower than the cold-start results, these values were still higher compared to both the PFI#1 and PFI#2 BC mass emissions, which varied from non-detectable to 0.6 mg/mi at standard temperature to 0.01−0.5 mg/mi at cold ambient temperatures.

• Both GDI vehicles had either similar or much higher emissions during the less aggressive FTP-75 drive cycles than for the US06 drive cycle, compared to their corresponding PFI counterparts.

• The use of a gasoline particulate filter (GPF) reduced BC emissions from the selected GDI vehicle by 73–88% at various ambient temperatures over the FTP-75 phase 1 drive cycle. The ambient temperature had less of an impact on particle emissions for a warmed-up engine. Over the US06 drive cycle, the GPF reduced BC mass emissions from the GDI vehicle by 59–80% at various temperatures.

• E10 had limited impact on BC emissions from the selected GDI and PFI vehicles during hot-starts. E10 was found to reduce BC emissions from the GDI vehicle by 15% at standard temperature and by 75% at 19 °F (−7 °C).

Observations from this study revealed several important implications regarding BC emissions obtained at real-world, on-road ambient conditions. First of all, both BC mass and solid particle number emissions increased considerably with decreasing ambient temperature, particularly evident during cold-starts from both the GDI and PFI vehicles. In addition, this study also showed that driving conditions have different impacts on BC emissions between GDI and PFI vehicles. The two warmed-up PFI vehicles had very low emissions over the phases 2 and 3 portions of the FTP-75 drive cycle compared to emissions observed during the US06 drive cycle. However, the two warmed-up GDI vehicles still emitted considerable amounts of BC mass during phases 2 and 3 portions of the FTP-75 and US06 drive cycles. Although the change in BC mass emissions at less demanding driving conditions varies from one GDI vehicle to another, these emission levels were still considerably higher than their corresponding PFI counterparts.

… Particles that are emitted from GDI engines are primarily solid in nature; therefore, a GPF with a similar operating principle as a diesel particulate filter was expected to be useful in removing these emission particles. The assessment of the BC mass emissions from the prototype GPF used in this study showed that an optimum designed GPF can provide one alternative to reduce BC mass and solid particle number emissions from future GDI vehicles without compromising fuel economy. Over the FTP-75 phase 1 drive cycle, the GPF reduced BC emissions by 73−88% at various ambient temperatures. Over the US06 drive cycle, the BC emissions were reduced by 59−80%.

—Chan et al.

Resources

• Tak W. Chan, Eric Meloche, Joseph Kubsh, and Rasto Brezny (2014) “Black Carbon Emissions in Gasoline Exhaust and a Reduction Alternative with a Gasoline Particulate Filter,” Environmental Science & Technology 48 (10), 6027-6034 doi: 10.1021/es501791b