The effect of ethanol and iso-butanol blends on polycyclic aromatic hydrocarbon (PAH) emissions from PFI and GDI vehicles
Introduction
Alcohol fuels have been used primarily in spark ignition (SI) engines. Almost all gasoline sold in the United States (US) contains 10% (by volume) ethanol (E10). The US Environmental Protection Agency (US EPA) has also approved the use of 15% ethanol (E15) for model year 2001 and later vehicles. Higher ethanol blends are also commercially available for use in flexible fuel vehicles (FFVs). For FFVs, gasoline is allowed to contain as much as 83% (E83) and as little as 51% (E51) ethanol. Ethanol as an automotive fuel has different properties than those of gasoline, including a higher octane number, which enables higher compression ratio engines leading to greater thermal efficiencies, and a lower C/H ratio that can potentially reduce greenhouse gas (GHG) emissions, such as carbon dioxide (CO2) (Shirazi et al., 2020; Zaharin et al., 2018). Besides ethanol, the use of butanol as an automotive fuel has shown good potential in spark ignition (SI) engines (Deng et al., 2013; Irimescu, 2012). Butanol has a four-carbon structure, with physicochemical properties closer to gasoline than ethanol. It exists as different isomers depending on the location of the hydroxyl group and carbon chain structure, with n-butanol and iso-butanol being popular candidate fuels for SI combustion (Shirazi et al., 2020).
Recent studies have primarily focused on gasoline direct injection (GDI) engines, since they are being widely adopted in the US and European markets due to their ability to increase fuel economy and reduce GHG emissions (Alkidas, 2007; Duronio et al., 2020). The main issue of GDI engines is broadly recognized as being their higher production of particulate matter (PM) relative to port-fuel injection (PFI) engines, attributed to their internal mixture formation and the fuel impingement in the cylinder (Piock et al., 2011; Stevens and Steeper, 2001). Several studies have investigated the emissions performance of both ethanol and butanol blends in PFI and GDI engines (Suarez-Bertoa et al., 2015; Hubbard et al., 2014; Wallner and Frazee, 2010; Karavalakis et al., 2014a; Dernotte et al., 2010). Storey et al. (2010) showed lower nitrogen oxide (NOx), carbon monoxide (CO), and PM emissions when they tested E10 and E20 blends on a GDI vehicle operated on the Federal Test Procedure (FTP) cycle. Maricq et al. (2012) reported small benefits in PM emissions for ethanol blends up to 20%, but significant PM reductions with ethanol blends higher than 30% using a GDI vehicle over the FTP. Similar findings were reported in a recent work with higher ethanol blends on a GDI-FFV over the LA92 cycle (Yang et al., 2019a). The authors also showed reductions in the emissions of particle number, black carbon, benzene, toluene, ethylbenzene, and xylenes (commonly known as BTEX), as well as 1,3-butadiene with higher ethanol blending. However, the use of ethanol blends resulted in considerably higher acetaldehyde emissions, a phenomenon that has been shown in the majority of the published literature. Butanol blends have been shown to reduce the emissions of total hydrocarbons (THC), CO, and NOx (Yusoff et al., 2017; Broustail et al., 2012; Karavalakis et al., 2014b; Irimescu, 2012). Karavalakis and co-workers (2014b) tested a PFI-FFV and a GDI-FFV over the FTP and LA92 cycles fueled with ethanol and iso-butanol blends and found CO, THC, PM mass and particle number emission reductions with the alcohol fuels, whereas acetaldehyde showed sharp increases. They also found higher formaldehyde and butyraldehyde emissions for the iso-butanol fuels similar to other vehicle studies (Ratcliff et al., 2013; Aakko-Saksa et al., 2014).
While the impact of alcohol fuels on regulated, PM, and gaseous toxic pollutants has been well characterized, there is more limited information on the polycyclic aromatic hydrocarbon (PAH) emissions. PAHs are generated by combustion processes of carbon-based fuels and they are known for their high toxicity, and mutagenic and carcinogenic effects to humans, as classified by the International Agency for Research on Cancer (IARC, 2010). PAH emissions from SI engines are generally associated from unburned fuel that survives the combustion process or from the pyrosynthesis of low molecular weight aromatic hydrocarbons that, act as molecular precursors for soot formation and growth (Richter and Howard, 2000). The formation of PAHs involves a complex series of reactions, which often includes the formation of radicals (i.e., H, OH, CH3, C2H4, C2H2) that further oxidize to form the aromatic rings (Slavinskaya et al., 2012; Richter and Howard, 2000). The formation of fused aromatic ring structures and PAHs also takes place through various reaction pathways, such as surface growth reactions (hydrogen abstraction-acetylene-addition, the HACA mechanism) (Frenklach, 2002). Previous studies have shown that gasoline vehicles are an important source of PAH emissions in urban centers, especially high molecular weight PAHs (Jiang et al., 2005; Fujita et al., 2007; Perrone et al., 2014). Other studies have shown that GDI vehicles are an important source of PAH emissions compared to PFI vehicles (Aakko-Saksa et al., 2014; Yang et al., 2018; Zheng et al., 2018). Yang et al. (2018) showed that GDI exhaust contained heavier 5–6 rings PAH species, when they tested two GDI vehicles over the LA92 cycle. Munoz et al. (2018) found elevated emissions of genotoxic PAHs in GDI exhaust, with their concentrations being significantly higher than those of a diesel vehicle equipped with a diesel particulate filter (DPF). PAH emissions are also dependent on the fuel type used (Pedersen et al., 1980). Munoz et al. (2016) investigated the formation of PAH and nitrated PAH emissions from a GDI-FFV when operated on gasoline, E10, and E85 under transient and steady-state conditions. They found significant reductions in PAH emissions for the E10 and E85 blends compared to gasoline. Seggiani et al. (2012) also reported PAH reductions for E15 compared to gasoline, but for E20 PAH emissions were comparable to gasoline. De Abrantes and co-workers (2009) observed reductions in PAH emissions from gasoline vehicles burning various ethanol blends. Aakko-Saksa et al. (2014) showed reductions in PAH emissions from a GDI vehicle fueled with iso-butanol and ethanol blends relative to gasoline.
In this study, vapor- and particulate-phase PAH emissions from one GDI-FFV and one PFI-FFV over the FTP cycle using a chassis dynamometer are reported and evaluated. The vehicles were operated on various low, midlevel, and high ethanol blends and an iso-butanol blend. Emissions of organic and elemental carbon fractions and metals were also measured and quantified for all vehicle/fuel combinations. The emission results are discussed in the context of engine technology and fuel type.
Section snippets
Vehicles and test cycle
This study included two test vehicles, equipped with stoichiometric engines with three-way catalysts (TWCs). One vehicle was a 2014 GDI-FFV pickup truck. This vehicle was equipped with a 5.3 L V8 wall-guided direct injection engine with a horsepower rating of 355 hp (at 6500 rpm) and a mileage of 2649 miles. The other vehicle was a 2013 PFI-FFV pickup truck. This vehicle had a 3.7 L V6 PFI engine with a horsepower rating of 302 hp (at 6500 rpm) and a mileage of 13,700 miles. The vehicles were
PM mass and EC/OC emissions
Fig. 1 shows the gravimetric PM mass, and the EC and OC fractions for all vehicles and fuels over the FTP. The GDI-FFV exhibited considerable higher PM emissions compared to the PFI-FFV. Previous studies have also demonstrated that PM mass emissions are significantly higher for GDI engines compared to PFI engines (Saliba et al., 2017; Chen et al., 2017; Karavalakis et al., 2014a, 2014b). The elevated PM emissions with GDI engines are due to insufficient air/fuel mixing and evaporation, which
Conclusions
This study investigated the chemical composition of PM emissions from two FFVs equipped with PFI and GDI engines when operated on ethanol blends (E10, E51, and E83) and an iso-butanol blend (iBut55). Both vehicles were exercised over duplicate FTP cycles on each fuel using a chassis dynamometer. Our results showed elevated PM mass emissions for the GDI-FFV compared to the PFI-FFV. The use of alcohol fuels led to lower PM emissions for the GDI-FFV due to more complete combustion as a result of
Credit author statement
Cavan McCaffery: Formal analysis, Validation, Investigation Thomas D. Durbin: Writing – Review and editing, Validation Kent C. Johnson: Resources, Data Curation Georgios Karavalakis: Writing – Original draft, Writing – Review and editing, Data Curation, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge funding from the South Coast Air Quality Management District (SCAQMD) under contract 12208 and the California Energy Commission (CEC) under contract 500-09-051. The authors thank Mr. Kurt Bumiller and Mr. Mark Villela of the University of California, Riverside for their contribution in conducting the emissions testing for this program.
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Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.