Top: Gravimetric PM mass, PM mass calculated based on the IPSD method, black carbon, and EC/OC emissions over the LA92 cycle. Bottom: Total particle-phase PAH emissions, expressed in ng/mile, for both test vehicles over the LA92 cycle. Yang et al. Click to enlarge.

The found that using a GPF did not show any fuel economy and CO₂ emission penalties, while the emissions of total hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx) were generally reduced. Polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs were quantified in both the vapor and particle phases of the PM, with the GPF-equipped vehicles practically eliminating most of these species in the exhaust. For the stock vehicles, 2–3 ring compounds and heavier 5–6 ring compounds were observed in the PM, whereas the vapor phase was dominated mostly by 2–3 ring aromatic compounds.

Although GDI vehicles offer the potential of improved fuel economy, less fuel pumping, and charge air cooling, they tend to produce higher particulate matter (PM) emissions when compared with the traditional port fuel injection engines. In GDI engines, fuel is sprayed directly into the combustion chamber, which leads to incomplete fuel evaporation due to the limited time available for fuel and air mixing, resulting in pockets with high temperatures but insufficient oxygen, leading to pyrolysis reactions and soot formation. Additionally, as the fuel comes directly into contact with the cold cylinder walls and piston, a small amount of fuel may impinge on the piston, which during evaporation may lead to diffusion combustion and PM formation.

The rapid market penetration of GDI vehicles has led governments to impose stricter standards to control PM emissions. California LEVIII and US Tier 3 regulations will begin a four year phase-in starting in 2015 and 2017, respectively, to a PM maximum of 3 mg/mile from the current 10 mg/mile LEVII limit. LEVIII will begin a four year phase-in of a tighter 1 mg/mile starting in 2025. In the EU, the Euro 6a particle number (PN) standard for GDI vehicles was reduced from 6x10¹² particles/km to 6x10¹1 particles/km in September 2017.

… It is important to better understand the toxicity of the particles being formed in GDI combustion. Today, the literature is scarce about the toxic properties of PM emissions from GDI vehicles, such as those of polycyclic aromatic hydrocarbons (PAHs), their oxygenated (oxy-PAHs), and nitrated derivatives (nitro-PAHs). PAHs have long been recognized as one of the major soot precursors for soot particles, while they are also classified as carcinogenic and 85 mutagenic compounds adsorbed onto the PM or partition in the semivolatile PM phase.

This study aims to better characterize the toxicity of PM from GDI vehicles and the potential for catalyzed GPFs to reduce this toxicity. Additionally, some oxy-PAH and nitro-PAH species have been recognized as similarly or more toxic than their parent PAHs.

—Yang et al.

In the study, the team used two MY 2016 passenger cars. The first was equipped with a 2.0-liter all-guided direct injection SI Atkinson cycle engine; the second was equipped with a 1.5-liter downsized turbocharged centrally-mounted direct injection engine. Both vehicles were operated stoichiometrically, and were equipped with three-way catalysts (TWCs). Both were certified to meet LEV III SULEV30 (PZEV) and LEV II emissions standards and had 14,780 and 24,600 miles at the start of the campaign, respectively.

After measuring the baseline emissions, the researchers retrofitted both vehicles with a catalyzed GPF installed in place of the underfloor TWC. The original close-coupled catalysts were retained in their stock location. The catalyzed GPFs were provided by MECA. The GPFs were sized based on the engine displacement of each vehicle and they were catalyzed with precious metal loadings typical of underfloor catalysts matching the certification levels of the two vehicles.

Both vehicles were tested over duplicate LA92s and US06 cycles using California E10 fuel. The LA92 test cycle or the California Unified Cycle (UC) is a dynamometer driving schedule for light-duty vehicles developed by the California Air Resources Board (CARB). LA92 consists of three phases (i.e., cold-start, urban, and hot-start phases) and has a three-bag structure similar to the FTP cycle. LA92 is characterized by higher speeds, higher accelerations, fewer stops per mile, and less idle time than the FTP. US06 was developed to reflect aggressive, high speed, and high acceleration driving behavior. Unlike the LA92, it is a hot-start test typically run with a prep cycle to ensure the vehicle is warmed up.

The results showed that current technology GDI vehicles could indeed be an important source for on-road ultrafine particles and black carbon emissions and ultimately a contributor to urban air pollution. This study also showed that that catalyzed GPFs can improve the conversion efficiency for NO_x, THC, and CO emissions and have no measurable impact on CO₂ emissions and fuel economy.

This is one of the few studies revealing that GDI vehicles could significantly contribute to PAH and nitrated PAH emissions and to our knowledge, the only one that looked at remediation of these toxics using a catalyzed gasoline particulate filter. We found that the use of catalyzed GPFs could significantly reduce the PM mass and black carbon emissions, as well as total and solid particle number emissions without having a measurable impact on the vehicle’s GHG emissions and fuel economy. The catalyzed GPF significantly reduced the particle-phase PAHs and nitro-PAHs emissions, especially the less volatile or highly reactive PAH species. On the other hand, the vapor-phase PAHs did not show the same filtration efficiency as the PM-bound compounds.

This study showed that GDI vehicle exhaust is characterized by diverse PAH distribution profile, ranging from 3-6 ring species. The projected increased penetration of GDI vehicles in the US market, suggests that future health studies aimed at characterizing the toxicity of GDI emissions are needed to understand the health risk associated with non-GPF-equipped GDI PM emissions.

The fact that GPF adoption from US vehicle manufacturers is not as dynamic as in the EU, due to the more stringent European PN standard especially over real-driving emissions (RDE) testing, should raise concerns about the lack of societal and air quality benefits from the GDI fleet.

—Yang et al.

Resources

• Jiacheng Yang, Patrick Roth, Thomas D. Durbin, Kent C. Johnson, David R. Cocker, III, Akua Asa-Awuku, Rasto Brezny, Michael Geller, and Georgios Karavalakis (2018) “Gasoline Particulate Filters as an Effective Tool to Reduce Particulate and Polycyclic Aromatic Hydrocarbon Emissions from Gasoline Direct Injection (GDI) Vehicles: A Case Study with Two GDI Vehicles” Environmental Science & Technology doi: 10.1021/acs.est.7b05641