On-road gaseous and particulate emissions from GDI vehicles with and without gasoline particulate filters (GPFs) using portable emissions measurement systems (PEMS)

https://doi.org/10.1016/j.scitotenv.2019.136366Get rights and content

Highlights

  • Catalyzed GPFs showed significant reductions in real-world PM emissions.

  • Urban and high-altitude driving showed elevated PM emissions.

  • Highest particle number concentrations were seen for low speeds and positive accelerations.

  • Real-world NOx emissions showed reductions with the catalyzed GPFs.

Abstract

This study assessed the on-road gaseous and particulate emissions from three current technology gasoline direct injection (GDI) vehicles using portable emissions measurement systems (PEMS). Two vehicles were also retrofitted with catalyzed gasoline particulate filters (GPFs). All vehicles were exercised over four routes with different topological and environmental characteristics, representing urban, rural, highway, and high-altitude driving conditions. The results showed strong reductions in particulate mass (PM), soot mass, and particle number emissions with the use of GPFs. Particle emissions were found to be highest during urban and high-altitude driving compared to highway driving. The reduction efficiency of the GPFs ranged from 44% to 99% for overall soot mass emissions. Similar efficiencies were found for particle number and PM mass emissions. In most cases, nitrogen oxide (NOx) emissions showed improvements with the catalyzed GPFs in the underfloor position with the additional catalytic volume. No significant differences were seen in carbon dioxide (CO2) and carbon monoxide (CO) emissions with the vehicles retrofitted with GPFs.

Introduction

Road transport is a major source of nitrogen oxides (NOx) and particulate matter (PM), impacting air quality throughout the world. Elevated concentrations of mobile source emissions are responsible for adverse health impacts, including respiratory and cardiovascular diseases, or even premature mortality (Kampa and Castanas, 2008; Bates et al., 2015). Mobile source emissions have been significantly changed over the years as a result of stricter vehicle emission standards and efforts to reduce greenhouse gas (GHG) emissions. In the United States (US), Corporate Average Fuel Economy (CAFE) standards are pushing automotive manufacturers to meet fuel economy levels for passenger cars. Similarly, carbon dioxide (CO2) emissions from newly registered cars in the European Union (EU) must decrease to about 95 g per kilometer by 2021.

The share of gasoline direct injection (GDI) engines has grown rapidly in both the US and the EU. GDI technology enables both an increase in specific power and a better fuel economy (with simultaneous reduction in CO2 emissions), compared to traditional port fuel injection (PFI) engines (Alkidas, 2007). However, GDI engines are known to produce higher PM mass, black carbon, and particle number emissions than PFI engines and modern technology diesel engines equipped with diesel particulate filters (DPFs) (Karavalakis et al., 2015; Saliba et al., 2017; Zinola et al., 2016). PM formation in GDI engines is due to partially evaporated liquid fuel leading to fuel rich regions in the combustion chamber that promote the generation of PM (Karlsson and Heywood, 2001; Piock et al., 2011). Studies have shown that most GDI PM emissions are formed during the cold-start phase and during highly transient operations (Chen et al., 2017; Koczak et al., 2016). The dynamic market penetration of GDI engines along with their elevated PM emissions create a growing public health concern in terms of PM exposures in urban areas.

Concerns about the real-world performance of vehicles and the lack of real-world operation represented of chassis dynamometer tests are now being addressed with test protocols capable of characterizing real-world vehicle emissions. Portable emissions measurement systems (PEMS) have been widely used to measure vehicle gaseous and particulate emissions under real-world conditions (Weiss et al., 2011; Gallus et al., 2016; Kwon et al., 2017; Yang et al., 2018a). PEMS have been proved to be an important tool for emission inventories because they enable testing under a wide variety of driving conditions, including road gradients, altitude and environmental conditions variations, and strong accelerations (Zhang et al., 2019; Bishop et al., 2019; O'Driscoll et al., 2018). In the US, PEMS measurements are required for in-use compliance testing of heavy-duty diesel vehicles, while the EU has implemented PEMS-based type-approval testing for light-duty vehicles starting from the Euro 6 standards. Overall, previous work has shown that there are substantial differences in emissions measured on-road using PEMS compared to laboratory testing (May et al., 2014; Chossière et al., 2018; Fontaras et al., 2017; Andersson et al., 2014). A number of studies have been conducted on different types of vehicles using PEMS, including heavy-duty trucks (Mendoza-Villafuerte et al., 2017; Johnson et al., 2009) and light-duty diesel and gasoline cars (Valverde et al., 2019; Khan and Frey, 2018), and off-road equipment (Cao et al., 2016; Cao et al., 2018). Gallus et al. (2017) found CO2 and nitrogen oxides (NOx) emissions were strongly correlated with driving parameters, showing increases with road grade. Wang et al. (2018) reported increases in carbon monoxide (CO), NOx, and particle number emissions at elevated altitude. Other PEMS studies have shown that real-world NOx and particulate emissions are affected by fuel type, after-treatment control, and engine power (Quiros et al., 2016; Huang et al., 2013; Demuynck et al., 2017).

The introduction of more challenging test procedures, such as real-driving emissions (RDE) for type approval in the EU, as well as stricter emission standards, such as the California LEV III PM mass limit of 1 mg/mile beginning in 2025 and the Euro 6a particle number limit of 6 × 1011 particles/km, make the reductions in target pollutants more difficult to be met with engine improvements alone. While stricter solid particle number regulations in the EU may have led to the introduction of gasoline particulate filters (GPFs) in the passenger car fleet there, at the time it is not expected that GPFs will be widely adopted in the US. Several studies have reported that the use of GPFs resulted in dramatic reductions in PM mass, number, and black carbon emissions from GDI vehicles (Yang et al., 2018b; Araji and Stokes, 2019). A recent study even showed that the use of catalyzed GPFs can reduce secondary organic aerosol formation (Roth et al., 2019). In addition, studies have shown reductions in particulate emissions and improved conversion efficiencies for CO and NOx emissions with the use of catalyzed GPFs under real-world conditions with minimal impact on CO2 emissions (Schoenhaber et al., 2017; Yoshioka et al., 2019). Demuynck et al. (2017) investigated the deployment of GPFs on GDI vehicles using PEMS and found significant reductions in particle number emissions under RDE conditions. A similar study also showed reductions in particle number emissions with the use of GPFs, without any detectable increase in CO2 emissions (Ogata et al., 2017).

The primary objective of this study was to improve our understanding of the particulate emissions from three current technology GDI light-duty vehicles under different driving conditions mimicking urban, rural, and highway driving patterns, and included changes in altitude, road grade, and environmental conditions. Emissions testing were conducted on two vehicles in the stock configuration as well as after replacing the OEM underfloor three-way catalyst (TWC) with a catalyzed GPF. The catalyst formulation on the GPF was typical of an underfloor catalyst on vehicles of the same class, however, no attempt was made to exactly match the GPF catalyst formulation with that on the stock underfloor converter. Furthermore, the mileage accumulated on the GPF was not matched with the mileage of the TWC that it replaced. Therefore, the gaseous emissions are provided as observations for the purpose of relative comparison and are not intended to draw absolute conclusions. The results of this study will be useful in understanding real-world emissions from GDI vehicles and their contribution to air pollution in the Los Angeles Basin and other urban areas.

Section snippets

Vehicles and GPFs

Three 2017 and 2018 model year GDI vehicles, referred to as GDI1, GDI2, and GDI3, were tested on-road for gaseous and particulate emissions. Detailed descriptions of the test vehicles are shown in Table 1. GDI1 and GDI3 were equipped with naturally aspirated engines and wall-guided fuel injection systems, whereas GDI2 was equipped with a turbocharged engine and a centrally-mounted fuel injection system. All vehicles were operated with overall stoichiometric air-fuel ratios and certified to meet

Particulate emissions

Fig. 1(a–b) show the soot mass or black carbon emissions and gravimetric PM mass, respectively. For all vehicles on all test routes, PM mass emissions were below the Tier 3 PM mass standard of 3 mg/mile. Consistent with previous studies, the use of catalyzed GPFs resulted in important reductions in PM mass and black carbon emissions (Yang et al., 2018b; Chan et al., 2014). The decreases in PM emissions with the GPFs ranged from 12%–49% for GDI1 and 60%–96% for GDI2. Similar filtration

Conclusions

A reduction in real-world emissions from GDI vehicles is essential for air quality and health in populated areas and megacities. This study investigated on-road gaseous and particle emissions from three current technology GDI vehicles using PEMS. Two vehicles were also retrofitted with catalyzed GPFs to evaluate whether this technology is able to reduce on-road ultrafine particles and black carbon emissions and ultimately improve air quality. Testing was conducted on four test routes in the

Declaration of competing interest

I, Georgios Karavalakis on behalf of the co-authors of this manuscript, certify that we have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge

Acknowledgements

We acknowledge funding from the South Coast Air Quality Management District (SCAQMD), United States under contract 17331 and the Manufacturers of Emission Controls Association (MECA), United States under contract 15040420. The authors thank MECA for providing the catalyzed GPFs for this program and also for their technical support and guidance.

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