Assessment of the Fuel Composition Impact on Black Carbon Mass

Particle number size distributions were measured using a TSI Engine Exhaust ... The two vehicles used in this study were a 2012 2.0 L Ford Focus flex-...
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Assessment of the Fuel Composition Impact on Black Carbon Mass, Particle Number Size Distributions, Solid Particle Number, Organic Materials, and Regulated Gaseous Emissions from a Light-Duty Gasoline Direct Injection Truck and Passenger Car Tak W. Chan,*,†,‡ David Lax,§ Garry C. Gunter,∥ Jill Hendren,† Joseph Kubsh,⊥ and Rasto Brezny⊥ †

Emissions Research and Measurement Section, Air Quality Research Division, Environment and Climate Change Canada, 335 River Road, Ottawa, Ontario, Canada K1A 0H3 ‡ Climate Chemistry Measurements and Research, Climate Research Division, Environment and Climate Change Canada, 4905 Dufferin Street, Toronto, Ontario, Canada M3H 5T4 § American Petroleum Institute, 1220 L Street, NW, Washington, District of Columbia 20005-4070, United States ∥ Phillips 66 Company, Highway 60 and 123, Bartlesville, Oklahoma 74003, United States ⊥ Manufacturers of Emission Controls Association, 2200 Wilson Boulevard, Suite 310, Arlington, Virginia 22201, United States S Supporting Information *

ABSTRACT: The influence of the aromatic hydrocarbons in gasoline on the fuel distillation parameter, as well as the particle number (PN), black carbon (BC), and other regulated gaseous emissions from a passenger car (PC) and light-duty truck (LDT), was assessed by operating two vehicles fueled with U.S. Environmental Protection Agency Tier 3 certification gasoline and two gasoline test fuels over two standard drive cycles. The two gasoline test fuels represent a range of commercial motor gasoline, with one containing less naphthalenes and lower heavy fraction volatility (T80, T90, and final boiling point) than the other. Observations showed that various gasolines have minor impact on both vehicles on regulated gaseous emissions and fuel consumption. Particulate emissions from both vehicles showed similar trends with fuel type, with lower naphthalene containing gasoline produced lower PN and BC emissions. In addition, the effect of fuel on particle emissions varied with vehicle type, drive cycle, and power to weight ratio. Results also showed that lowering the naphthalenes in gasoline produces smaller sized particles. The real-time particle emission time series from both vehicles suggested that the composition and volatility of the gasoline fuels are sensitive parameters in influencing particulate matter emissions. These results could support one possible explanation of the large variations in emission factors reported in the literature when using different gasolines in the same type of vehicle and driving conditions.

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INTRODUCTION Incomplete combustion of hydrocarbon fuels in internal combustion engines (ICEs) used in various transportation sectors generates fine particles. When emitted into the atmosphere, these particles could have direct and indirect influences on visibility and adverse health effects.1,2 Typically, a fraction of the particle mass is attributed to the refractory carbon which is often referred to as black carbon (BC). BC particles are made up by platelets of dehydrogenated carbon rings in a graphite-like configuration.3 BC particles generated from ICEs appear to be in aggregates and can be characterized using a fractal dimension and fractal prefactor.4−6 Fuels (e.g., gasoline, diesel, and jet fuel) are complex mixtures that consist of hundreds of different hydrocarbons, each of which has different influences on BC formation and thus impacts tailpipe emissions. Many researchers have shown how BC particle emissions from ICEs could change due to different fuel compositions. Early observations showed that different hydrocarbons have different tendencies to generate BC.7−11 Many studies often compared petroleum-based jet fuel to renewable jet fuel containing significantly less or even the absence of aromatic hydrocarbons. Observations suggested that © XXXX American Chemical Society

low fuel aromatic content, high hydrogen to carbon ratio, and low iso-to-normal paraffin ratio are factors to explain the lowered particulate matter mass (PMM) emissions from aviation turbine engines.12−15 Some studies reported that particle size distributions could shift to larger diameter with increasing fuel aromatic content and thus produce higher BC emissions.16−19 These studies have implied the possible existence of relationships between BC formation and fuel composition. Aikawa et al.20 suggested the use of a particulate matter index (PMI) to predict the change in particle mass (PM) and particle number (PN) emissions from vehicles operated on gasoline fuels of different compositions. The PMI of a fuel is calculated by summing the individual PMI values of all hydrocarbons present in the fuel. An individual PMI value is calculated by the ratio of the double bond equivalent + 1 (DBE + 1) value of a hydrocarbon to its vapor pressure at 443 K (170 °C) and then multiplying by the weight percentage of the corresponding Received: May 9, 2017 Revised: July 20, 2017 Published: August 24, 2017 A

DOI: 10.1021/acs.energyfuels.7b01345 Energy Fuels XXXX, XXX, XXX−XXX

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vehicles operating on medium and low ethanol blend level gasoline.33,34,37−40 In this study, particulate and gaseous emissions from a PC and a LDT were investigated through the use of the U.S. EPA Tier 3 certification gasoline and two additional test fuels. The latter were blended from standard refinery gasoline blending streams to meet approximately the same fuel specifications as those for the Tier 3 certification gasoline. The two test fuels were designed to study the effect of PMI and heavy fraction volatility on tailpipe particulate emissions and represent a large range of current commercial gasolines (Supporting Information). With the use of a PC and a LDT, this study provides an opportunity to evaluate the particle and BC emissions from two different vehicle platforms under the U.S. Federal Test Procedure 75 (FTP-75) and US06 Supplemental Federal Test Procedure (US06) drive cycles on the same set of fuels. TSI provides a direct physical correlation with BC formation, PMI correlations link the double bond equivalency and vapor pressure of individual components to BC formation, and heavy fraction volatility indicates the amount of heavy components in fuels. These three metrics were explored to further understand how different classes of hydrocarbons in gasoline could influence BC formation from ICEs. Previous studies showed that a gasoline particulate filter (GPF) can also influence tailpipe PM emissions.39,40,42,43 A second component of this study was to evaluate changes in the particle filtration efficiency and effectiveness of the GPF using the same set of test fuels described here. Results from the additional study are presented in a different manuscript (in preparation) and will not be discussed here.

compound in the fuel (Supporting Information). Although the temperature 443 K could be arbitrary, this temperature is close to the typical in-cylinder temperature after injection in gasoline engines.21Thus, the choice of 443 K provides some relevance to fuel combustion in ICEs. Also, 443 K is close to the T90 value (i.e., temperature at which 90% of the gasoline is evaporated) of both the U.S. Environmental Protection Agency (U.S. EPA) Tier 2 and the new U.S. EPA Tier 3 certification gasoline. The suggested inversely correlated relationship between vapor pressure and PMI seems reasonable. PMI or aromatic content in gasoline has been observed to follow PM emission trends from several studies.20,21,23−25,27 The threshold soot index (TSI) has been used to relate the sooting tendency of fuels with different compositions.9 This index calculates sooting tendency based on the TSI values of the individual hydrocarbons that are represented in the fuel.10 The TSI value is calculated based on the molecular weight of a compound and its smoke point (SP) value26 relative to two reference compounds (Supporting Information). Similar to the PMI, fuels containing a higher aromatic content have higher TSI values as these compounds generate more BC during combustion when compared to aliphatic hydrocarbons. The higher emission rates of particles could lead to more coagulation resulting in particles with larger diameters, consistent with the particle number size distributions observed with high fuel aromatic content.16−19 One study suggested that operating an engine at the same conditions with fuels containing nearly identical fuel aromatic content but substituting the monoaromatics with naphthalenes (from 0.78% to 1.19% by volume) could increase BC and nonvolatile PM emissions by 40% and 30%, respectively.14 Multiple NASA studies also suggest that the fuel sulfur and aromatic content affects the volatile PM emissions and contribute to measurement variability. However, the amount of naphthalene present in the fuel has a larger influence on BC emissions.15 With the assumption that different hydrocarbons in gasoline have different influences on BC formation in ICE, reformulating gasoline and removing certain hydrocarbons could potentially result in changing the PM and even polycyclic aromatic hydrocarbons (PAHs) emissions.28 In North America, oxygen can be present in gasoline in the form of oxygenates, such as ethanol. In Canada, ethanol is present in gasoline up to 10% by volume due to the Canadian Federal regulations (except some premium gasolines). In the U.S., the Renewable Fuel Standard requires renewable fuels to be used and corn-based ethanol is the most widely used renewable fuel. Unlike hydrocarbons, oxygenates have complex impacts on BC formation. One study suggested that the SP for primary alcohols is similar to n-alkanes with the same carbon number while secondary alcohols have slightly higher sooting tendency. Aldehydes and ketones could have sooting tendencies that are comparable to those of n-alkanes with one less carbon atom.31 When ethanol is added to gasoline, it may displace aromatic compounds with high sooting tendency. Thus, tailpipe BC emissions could potentially be lowered.32 The combustion impacts of ethanol-containing gasoline also differ based on the following: (a) the fuel leaning effect,33 (b) variations in the fuel injection timing,34 and (c) the influence of the physical properties of gasoline.35,36 Furthermore, the amount of oxygen made available in the intake air during the different types of combustion affects the combustion temperature and also influences the BC formation.29,30 This may explain the mixed observations of particle emissions from

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EXPERIMENTAL SECTION

Emissions Exhaust Characterization Sampling. A simplified schematic and the complete description for the emission sampling system are given in Figure S1 (Supporting Information). Vehicle exhaust was directed into a full-flow constant volume sampling (CVS) and diluted with high-efficiency particulate air (HEPA) filtered room air. The diluted exhaust was measured by a suite of instruments to characterize both the gaseous and particle emissions from the vehicles (Supporting Information). Gaseous measurements were obtained using various Horiba and California Analytical instruments. Solid particle number (SPN) measurements for particles with diameters larger than 23 nm were obtained using a European Union Particle Measurement Programme (PMP) compliant system. SPN for particles larger than 3 nm were measured by an ultrafine condensation particle counter (CPC) downstream of the PMP system. Particle number size distributions were measured using a TSI Engine Exhaust Particle Sizer (EEPS) sampling downstream of a Dekati thermodenuder with an attempt to remove the majority of the volatile materials on the particles. BC measurements were measured using an Artium LII300 laser-induced incandescence (LII) instrument sampling directly from the CVS, and additional BC measurements were also obtained using a microAethalometer (mAeth) sampling downstream of an ejector diluter from the CVS. The real-time particle mass was measured using a Dekati mass monitor (DMM). Gravimetric PM measurements were made using an AVL Smart Sampler, and additional filter packs were set up to collect PM on quartz filters for thermal-optical analysis. Fuel consumption for the two vehicles (Tables 4 and 5) were calculated based on carbon mass balance using the gaseous emissions of carbon dioxide (CO2), carbon monoxide (CO), and total hydrocarbon (THC).40 Vehicle Setup and Fuel Exchange Procedure. The two vehicles used in this study were a 2012 2.0 L Ford Focus flex-fuel PC and a 2013 3.5 L turbo-charged V6 EcoBoost Ford F-150 LDT. Both vehicles were equipped with a wall-guided gasoline direct B

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Energy & Fuels Table 1. Specifications of the Two Test Vehicles model year displacement (L) engine configuration rated horsepower (hp) vehicle weight (kg) exhaust emission control mileage (km) certified emission standard

Ford Focus

Ford F150

2012 2.0 wall-guided GDI (naturally aspirated four cylinders) 160 1,474 three-way catalyst ∼40,000 Tier 2 bin 4

2013 3.5 wall-guided GDI (turbo-charged V6 EcoBoost) 365 2,722 three-way catalyst ∼4000 Tier 2 bin 5

injection (GDI) engine and a standard three-way catalytic converter (TWC). Specifications of the two vehicles are summarized in Table 1. A total of three or more emission test repeats were conducted in a randomized fashion for any given condition. This is to avoid any memory effect from the vehicles and to ensure any observed change in emissions was caused by the switch of test fuel. To facilitate the randomized fuel test sequence, three original equipment manufacturer (OEM) fuel tanks were installed externally on both vehicles allowing the fuel switch procedure to be conducted more efficiently. All fuel tanks have individual fuel lines feeding the target fuel to a common three-way valve before supplying fuel to the engine. In this study, fuel switch was done by adjusting the three-way valve position prior to the vehicle preparation (prep). As will be discussed in the next section, the long-term fuel trim measurements show that the setup and vehicle prep was enough to completely flush the previously tested fuel with the new fuel before the emission measurements. During the pre-emission and emissions study, standard engine related parameters, e.g., long-term and short-term fuel trim, coolant temperature, engine speed, oxygen concentration, catalyst temperature, throttle position, and air/fuel ratio, were recorded using the Autoenginuity software. Multiple tests were conducted on both vehicles before the study to determine whether the new configuration would generate error codes in the onboard diagnostic system. One error code was observed multiple times during the study on the PC. The error code was identified to be related to the evaporation system. Vehicle manufacturer experts were consulted, and they advised that the presence of this error code should have no effect on tailpipe emissions. Error bars in all figures and uncertainties reported in all tables represent the best estimate of a margin of error with 95% confidence interval assuming all the valid measurement repeats (N ≥ 3) represent a random subset selection of an infinitely large measurement group (Supporting Information). Drive Cycle and Test Procedure. The drive cycles followed in this study were the FTP-75 and US06 (Figure S2). These cycles provide a controllable and repeatable means to assess the change of emissions on moderate city and aggressive highway driving conditions.41−43 Prior to each emission test day, the target fuel was switched by selecting the appropriate valve position. The vehicle was set on a chassis dynamometer and a US06 drive cycle was first performed on the vehicle, followed by a standard Los Angeles Route 4 (LA4) drive cycle as the vehicle prep cycle before soaking at the standard temperature overnight. The emission test began on the next day with a cold-start FTP-75 drive cycle, followed by two back-to-back US06 drive cycles, in which the first cycle served as the prep cycle.42,43 In this study, the same driver was used to perform all the driving to ensure the consistency of the emission results. All gasoline vehicles typically operate in stoichiometric condition with close to the ideal 14.7:1 air/fuel ratio for the best combustion and to optimize the efficiency of the TWC. When a vehicle is operating on gasoline with very different fuel composition, the vehicle requires time to learn and adapt in order to reach the ideal combustion condition. Information gathered during closed-loop operation regarding fuel adjustment at various load points, i.e., the long-term fuel trim (LTFT), is being stored in the vehicle’s computer. The short-term fuel trim (STFT) reflects the immediate adjustment on fuel use but is not stored in the long-term memory of the vehicle. Therefore, LTFT

measurements from various measurement repeats could be one indicator to identify insufficient preconditioning of a vehicle that is caused by switching between very different fuels.50,51 Typically the presence of ethanol could be one important factor in causing a significant change in LTFT due to the presence of oxygen in the fuel and also the fact that the addition of ethanol significantly changes the physical property of the resulting gasoline.52 In this study, vehicles were expected to learn quickly and adapt to the different fuel as all fuels have the same amount of ethanol (10% by volume) and similar physical properties (Table S1 and Table S2). Figure S3 and Figure S4 summarize the recorded LTFT of the two vehicles over the FTP-75 drive cycles on Tier 3 certification gasoline. These observations show that, despite the randomized test sequence, the vehicles have completely learned and adapted to the test fuel during the test and the LTFT values were comparable among various repeats. The LTFT value also did not change significantly even though the vehicles were operated on different fuels (Figure S5 to Figure S8). This shows that the vehicle preconditioning procedure in this study was sufficient. Test Fuel Properties. The test fuels used in this study were the U.S. EPA Tier 3 certification gasoline and two additional test fuels. The latter were blended from standard refinery gasoline blending streams to meet approximately the same octane number, total aromatics, olefins, ethanol, and fuel sulfur levels as those required for the Tier 3 certification gasoline. Prior to the study, fuel samples were sent to Southwest Research Institute (SwRI) for detailed analyses of hydrocarbon composition and the determination of the PMI values. A global gasoline survey suggested that the averaged PMI values of gasolines used around the world typically vary from 1.0 to 1.9.21,22 Gasoline used in Northeast Asia typically has the lowest PMI value while gasoline used in West Asia has the highest PMI values. North America gasoline has an average PMI value of about 1.4. In this study, the hydrocarbon compositions of the two test fuels were varied to cover a wide range of PMI of 1.1 to 2.5. For convenience, the two test fuels are hereafter labeled as “low PMI” and “high PMI”. In comparison, the PMI value for the U.S. EPA Tier 3 gasoline used in this study is 2.1. Selected fuel properties and compositions for all fuels are included in Table S1 (Supporting Information) and Table 2. The total aromatics levels of the three fuels vary within a range of about 1% by volume, but the low PMI fuel has a relatively higher amount of monoaromatics whereas the high PMI fuel has more diaromatics than the Tier 3 gasoline (Table 2). The variations in fuel composition among the test fuels have minor impacts on many of the physical fuel properties (Table S1) but mostly influence the higher boiling points hydrocarbons represented in the distillation curve (Figure 1 and Table S2). The distillation properties of the high PMI fuel are close to those for the Tier 3 certification gasoline. Replacing the naphthalenes in the low PMI fuel with monoaromatics and naphthenes resulted in lowering the heavy fraction of the distillation curve for the low PMI fuel. The distillation properties of the low and high PMI fuels are reasonably close to the allowed ranges for the certification fuel except that in both cases the T90 values are slightly beyond the expected range. All fuels met volatility limits of ASTM D4814 Volatility Class AA gasoline specification as shown in Figure 1. Fuel Properties and Soot Indices. The relationship between fuel composition and soot formation is complicated, and there is no universal indicator to correlate BC formation with combustion of such C

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all aromatics. This is consistent with the observations from early studies.7,8 Although oxygenates are included in the PMI calculation in Table 3, this is for information purposes only since the PMI correlation has not been demonstrated to be applicable to oxygenated fuels. Table S5 (Supporting Information) summarizes the PMI, TSI, and a number of other related parameters, such as DBE, molecular weight, number of carbon and hydrogen atoms in the molecule, carbon and hydrogen content, H/C ratio, vapor pressure at 170 °C (VP), boiling points, and SP for some commonly found constituents in gasoline. SP can be used as an indicator to infer BC formation from a specific hydrocarbon. The results in Table S5 generally suggest that paraffins and isoparaffins have low sooting tendency, followed by olefins, mononaphthenes, monoaromatics, and finally naphthalenes. But unlike PMI, SP does not differentiate various monoaromatics and naphthalenes well. Hydrogen content and H/C ratio also have similar resolution issues. Fuel VP is expected to relate to the vaporization rate, and higher VP is expected to result in complete combustion in an ICE and therefore is inversely proportional to BC formation. VP for paraffins covers a large range. For example, heavy paraffin, such as n-tridecane, has a low VP value, similar to that for a heavily sooting naphthalene compound such as 1-methylnaphthalene. But SP for n-tridecane is larger (i.e., lower sooting tendency) than that for 1-methylnaphthalene by more than 1 order of magnitude. A cyclic compound, such as ethylcyclohexane, has a VP comparable to an aromatic compound such as ethylbenzene. But SP for ethylcyclohexane is much larger than that for ethylbenzene. These observations suggest that VP and soot formation may not have a direct relationship. Results from Schug et al.30 also appeared to support this idea postulating that, for a diffusion flame, the tendency of a fuel to produce BC is dominated by the diffusion flame temperature and the fuel structure. They also suggested that fuel partial pressure was only a secondary minor effect contributing to BC formation. Since BC formation is a chemical process, it makes sense that a reaction pathway should be highly dependent on the molecular structure. Some studies have shown high correlation between BC emissions from gas turbines and TSI value of the fuel.44 Therefore, one possible reason for explaining a relationship between VP and BC formation in ICEs is that a compound with low VP may not be vaporized completely and participate in the gas phase combustion reaction and pyrolyzes to form BC instead. Interestingly, when excluding compounds with a benzene ring, correlation exists between VP and BC formation (as predicted by SP or TSI). This may imply

Table 2. Fuel Composition (by Volume) for All Test Fuels Based on the ASTM D6729 Method Tier 3 cert fuel

low PMI fuel

high PMI fuel

paraffins isoparaffins

19.95 27.72

11.36 39.73

11.95 41.49

total aromatics monoaromatics naphthalenes naphtheno-/ olefinobenzenes indenes

25.84 23.70 0.63 1.51

24.83 24.29 0.18 0.36

25.32 22.77 1.06 1.50

0.00

0.00

0.01

naphthenes mononaphthenes

7.88

8.43

6.01

total olefins n-olefins iso-olefins naphtheno-olefins

7.71 6.86 0.79 0.06

5.11 2.62 1.92 0.57

4.82 2.73 1.80 0.29

oxygenates unidentified

9.60 1.30

9.38 1.16

9.24 1.18

compd class by vol (%)

fuel in an ICE. Table 3 summarizes the fuel constituents in all fuels and the total contributions to the overall PMI values based on the information obtained from the detailed fuel analysis conducted by SwRI. The data suggest that monoaromatics and naphthenes are the main categories that significantly contribute to the overall PMI value of the fuel or potentially PM emissions. The ratio between the total PMI values and the contributing weights of different categories provides a relative comparison of the importance of each of the different hydrocarbon categories in relation to PM formation. The PMI correlation predicts that aromatics compounds have greater orders of magnitude contribution to PM formation than any other compounds. In addition, the PMI correlation predicts that, in an equal volume basis, diaromatics such as naphthalenes contribute the most in PM formation. A lower amount of PM is formed by simpler compounds, such as indenes and naphtheno-/olefino-benzenes, while monoaromatics compounds contribute the least during PM formation among

Figure 1. Distillation profiles of all test fuels by ASTM D86 method. The green marks at various percentage evaporated regions represent the allowable range for the Tier 3 certification gasoline specification. Orange marks represent ASTM D4814 Distillation Class AA commercial gasoline specification limits. D

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Table 3. Weight Contributions of the Fuel Constituents and Their Contribution to the PMI Based on the Original Methodology by Aikawa et al.20 low PMI fuel

a

high PMI fuel

Tier 3 gasoline

group

tot wt (%)

ΣPMI

ΣPMI/wt

tot wt (%)

ΣPMI

ΣPMI/wt

tot wt (%)

ΣPMI

ΣPMI/wt

n-paraffins isoparaffins monoaromatics naphthalenes naphtheno-/olefinobenzene indenes mononaphthenes n-olefins iso-olefins naphtheno-olefins oxygenatesa

9.9 36.3 28.5 0.2 0.5 0.0 8.7 2.4 1.8 0.6 10.0

0.018 0.077 0.716 0.128 0.078 0.002 0.040 0.005 0.009 0.005 0.006

0.002 0.002 0.025 0.516 0.172 0.344 0.005 0.002 0.005 0.009 0.001

10.4 37.9 26.7 1.4 1.8 0.0 6.2 2.5 1.7 0.3 9.8

0.020 0.081 1.246 0.822 0.301 0.003 0.031 0.006 0.008 0.002 0.006

0.002 0.002 0.047 0.573 0.164 0.344 0.005 0.002 0.005 0.008 0.001

17.0 25.7 27.9 0.9 1.8 0.0 8.2 6.3 0.8 0.1 10.2

0.019 0.058 1.163 0.514 0.277 0.000 0.042 0.011 0.007 0.001 0.006

0.001 0.002 0.042 0.596 0.151 0.000 0.005 0.002 0.009 0.013 0.001

sum

98.9

1.085

98.8

2.527

98.7

2.098

Oxygenates are included for information purposes only. The PMI correlation has not been demonstrated to be applicable to oxygenated gasoline.

Table 4. Gaseous Emissions, Fuel Consumption (FC), and Fuel Economy (FE) for the PC over the FTP-75 and US06 Drive Cycles on Different Fuelsa Ford Focus (Tier 3 certification gasoline)

a

Ford Focus (low PMI fuel)

Ford Focus (high PMI fuel)

drive cycle

FTP-75

US06

FTP-75

US06

FTP-75

US06

CO2 (g/km) CO (g/km) NOx (mg/km) THC (mg/km) FC (L/(100 km)) FE (MPG)

182.1 ± 0.9 0.38 ± 0.05 26.84 ± 2.39 39.74 ± 1.33 8.05 ± 0.05 29.22 ± 0.18

163.3 ± 1.3 0.89 ± 0.20 92.60 ± 21.18 37.63 ± 1.44 7.26 ± 0.06 32.40 ± 0.27

182.1 ± 1.1 0.42 ± 0.05 30.22 ± 2.82 36.45 ± 1.31 8.08 ± 0.05 29.11 ± 0.18

163.3 ± 1.6 0.81 ± 0.08 76.56 ± 5.72 36.85 ± 0.13 7.27 ± 0.07 32.35 ± 0.31

181.7 ± 1.2 0.43 ± 0.04 25.42 ± 1.13 42.68 ± 1.05 8.03 ± 0.05 29.29 ± 0.18

164.2 ± 1.4 1.02 ± 0.11 91.02 ± 12.44 43.10 ± 1.24 7.30 ± 0.07 32.22 ± 0.31

All uncertainties represent the margin of error with 95% confidence interval.

Table 5. Gaseous Emissions, Fuel Consumption (FC), and Fuel Economy (FE) for the LDT over the FTP-75 and US06 Drive Cycles on Different Fuelsa Ford F150 (Tier 3 certification gasoline)

a

Ford F150 (low PMI fuel)

Ford F150 (high PMI fuel)

drive cycle

FTP-75

US06

FTP-75

US06

FTP-75

US06

CO2 (g/km) CO (g/km) NOx (mg/km) THC (mg/km) FC (L/(100 km)) FE (MPG)

339.4 ± 4.32 0.27 ± 0.01 33.83 ± 5.81 26.91 ± 1.04 14.98 ± 0.19 15.70 ± 0.20

362.7 ± 0.36 3.12 ± 1.10 96.92 ± 10.67 40.03 ± 9.99 16.20 ± 0.41 14.52 ± 0.37

338.8 ± 0.15 0.29 ± 0.01 33.27 ± 0.76 24.61 ± 0.90 14.99 ± 0.21 15.69 ± 0.22

357.1 ± 0.59 1.42 ± 0.43 71.20 ± 5.23 32.86 ± 5.24 15.87 ± 0.17 14.82 ± 0.16

342.8 ± 0.03 0.26 ± 0.01 35.86 ± 4.28 29.10 ± 0.65 15.10 ± 0.15 15.58 ± 0.15

373.5 ± 0.29 1.47 ± 0.39 94.03 ± 4.23 31.83 ± 2.90 16.54 ± 0.07 14.22 ± 0.06

All uncertainties represent the margin of error with 95% confidence interval.

that there is something unique regarding compounds that possess a benzene ring during BC formation. It is accepted that one soot formation mechanism involves the addition of multiple benzene rings to an existing benzene molecule.3,45 The relationship between BC formation and the hydrocarbon structure can be explained by how feasible such a compound can chemically form naphthalene.46 Molecules containing a benzene ring or that resemble part of the benzene ring structure can form many stable radicals during an abstraction of hydrogen or methyl group. The stability of such a compound enables it to exist long enough to further combine with another existing benzene ring to form larger polycyclic aromatic hydrocarbons (PAHs), eventually leading to the formation of BC. Observations also suggest that structural rearrangement or further hydrogen abstraction can occur during combustion on a compound that contains a partial benzene structure and is often not the ratedetermining step, allowing the formation of a benzene-containing compound which can further react to form BC.47

Another issue to consider for BC formation in ICEs is the equilibrium between vapor and particle phase for a compound. One study suggested that despite the low vapor pressure of naphthalene, it is overwhelmingly present in gaseous form in vehicle exhaust.28 As a result, even if all fuel compounds can be vaporized completely, incomplete combustion will allow some of the naphthalene vapor molecules (if present in the fuel) to be available to combine with other benzene compounds to form BC.46

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RESULTS AND DISCUSSION Effect of Fuel Composition on Gaseous Emissions. Table 4 and Table 5 summarize the gaseous emissions from the two vehicles over different drive cycles when operating on different fuels. The expanded versions of the tables are given in Table S6 and Table S7 (Supporting Information). Overall, all regulated gaseous emissions for both vehicles on different fuels were close in value. However, when operating the PC on the E

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Figure 2. SPN emissions for all fuels over the US06 and various phases of the FTP-75 drive cycles for the (left) PC and (right) LDT. For clarity only 95% confidence interval error estimates for SPN (>23 nm) are shown.

Table 6. PN and Particulate Matter Emissions for the PC over the FTP-75 and US06 Drive Cycles on Different Fuelsa Ford Focus (Tier 3 certification gasoline) drive cycle SPN (>3 nm) (1012 particles/km) SPN (>23 nm) (1012 particles/km) EEPSb PN (1012 particles/km) DMM PM (mg/km) mAeth BC (mg/km) LII BC (mg/km) AVL 1065 TPM (mg/km) NIOSH 5040 EC (mg/km)c NIOSH 5040 OC (mg/km)c

FTP-75 3.18 2.36 3.64 0.99 0.88 0.68 1.07 1.08 0.17

± ± ± ± ± ± ± ± ±

0.52 0.39 0.60 0.32 0.10 0.10 0.04 0.06 0.02

Ford Focus (low PMI fuel)

US06 4.66 3.46 5.19 1.22 1.19 0.90 1.50 1.25 0.25

± ± ± ± ± ± ± ± ±

FTP-75 comp

0.94 0.74 0.99 0.35 0.30 0.21 0.25 0.11 0.06

1.46 1.02 1.61 0.24 0.25 0.17 0.50 0.34 0.07

± ± ± ± ± ± ± ± ±

0.32 0.24 0.24 0.03 0.04 0.03 0.06 0.02 0.02

US06 2.00 1.39 1.78 0.35 0.27 0.20 0.43 0.34 0.07

± ± ± ± ± ± ± ± ±

1.13 0.84 0.56 0.17 0.10 0.08 0.05 0.06 0.02

Ford Focus (high PMI fuel) FTP-75 comp 3.48 2.65 4.09 0.80 1.00 0.77 1.36 1.39 0.22

± ± ± ± ± ± ± ± ±

0.12 0.01 0.16 0.21 0.10 0.03 0.08 0.07 0.02

US06 4.53 3.37 5.04 0.93 1.05 0.80 1.04 1.19 0.20

± ± ± ± ± ± ± ± ±

0.24 0.20 0.33 0.30 0.15 0.09 0.08 0.06 0.03

a

All uncertainties represent the margin of error with 95% confidence interval. bValues are integrated over the entire number size distribution (∼5 to 550 nm) from the EEPS measurements. cNIOSH 5040 results are integrated results over the entire drive cycle.

Table 7. PN and PM Emissions for the LDT over the FTP-75 and US06 Drive Cycles on Different Fuelsa Ford F150 (Tier 3 certification gasoline) drive cycle SPN (>3 nm) (1012 particles/km) SPN (>23 nm) (1012 particles/km) EEPSb PN (1012 particles/km) DMM PM (mg/km) mAeth BC (mg/km) LII BC (mg/km) AVL 1065 TPM (mg/km) NIOSH 5040 EC (mg/km)c NIOSH 5040 OC (mg/km)c

FTP-75 6.50 4.93 7.86 2.13 2.56 2.06 3.69 4.00 0.76

± ± ± ± ± ± ± ± ±

0.79 0.58 0.70 0.33 0.14 0.12 0.20 0.21 0.04

Ford F150 (low PMI fuel)

US06 5.79 3.64 6.13 1.24 1.42 1.11 3.02 1.67 2.16

± ± ± ± ± ± ± ± ±

FTP-75

1.29 0.58 1.01 0.38 0.15 0.14 0.36 0.14 0.71

3.74 2.71 4.40 1.32 1.15 0.84 2.12 1.78 0.35

± ± ± ± ± ± ± ± ±

0.43 0.28 0.48 0.59 0.19 0.10 0.15 0.16 0.05

US06 2.17 1.54 2.31 0.60 0.56 0.40 1.80 0.69 1.69

± ± ± ± ± ± ± ± ±

0.72 0.44 0.74 0.30 0.10 0.10 0.59 0.04 1.26

Ford F150 (high PMI fuel) FTP-75 8.21 6.30 9.25 2.65 3.37 2.69 4.55 5.77 0.86

± ± ± ± ± ± ± ± ±

0.47 0.35 0.59 0.75 0.38 0.33 0.03 0.18 0.10

US06 5.78 3.52 5.90 1.74 1.54 0.99 3.27 1.61 2.26

± ± ± ± ± ± ± ± ±

0.62 0.22 0.56 0.42 0.63 0.02 0.29 0.11 0.73

a

All uncertainties represent the margin of error with 95% confidence interval. bValues are integrated over the entire number size distribution (∼5 to 550 nm) from the EEPS measurements. cNIOSH 5040 results are integrated results over the entire drive cycle.

high PMI fuel, the averaged THC emissions were consistently higher for all driving conditions. For the LDT, increased THC emissions only occurred during phase 1 of the FTP-75 drive cycle (Table S7). Inspections of the test fuels indicated that

they all have comparable energy content (Table S1). Therefore, it is not expected that the fuel consumption (FC) measured on both vehicles would dramatically change as a function of test fuel. Interestingly, the PC FC on the high PMI fuel was not F

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Figure 3. Averaged particle number size distributions (from 5 to 550 nm) over various phases of the FTP-75 and US06 drive cycle for the (a−d) PC and the (e−h) LDT on different fuels. Uncertainties are error estimates with 95% confidence interval.

emissions when fueled with high PMI fuel compared to the low PMI fuel. This is consistent with the order based on prediction from the PMI. But the SPN emissions for Tier 3 and high PMI fuels are often within statistical uncertainty. Another interesting observation is that the LDT emissions during the US06 drive cycle were comparable to the PC emissions. Using the low PMI fuel as an example, over the US06, the PC SPN (>23 nm) emissions were 1.4 × 1012 vs 1.5 × 1012 particles/km for the LDT (Table 6 and Table 7). When switching to the high PMI fuels, the emissions for the PC were 3.4 × 1012 vs 3.5 × 1012 particles/km from the LDT. But the cold-start SPN emissions were much higher for the LDT than for the PC (Table S8 and Table S9).

statistically different than that for the other two fuels. For the LDT, FC over US06 operated on the high PMI fuel was statistically higher than the other fuel. The presence of more diaromatics in the high PMI fuel could have an influence on the heavy fraction volatility (T80, T90) and could result in less complete evaporation during the cold-start causing higher THC emissions. More tests are needed to verify this hypothesis. Effect of Fuel Composition on SPN Emissions and Number Size Distributions. Figure 2 shows the solid particle number (SPN) emissions for the PC and LDT over the different drive cycles on all fuels. The figure also include the solid particle fraction, estimated by the ratio of the SPN (>23 nm) to SPN (>3 nm). Both vehicles produced higher SPN G

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Energy & Fuels Figure 2 also shows that the fraction of SPN larger than 23 nm (measured as the ratio of SPN (>23 nm) to SPN (>3 nm)) tends to be lowest for the low PMI fuel. This trend holds true for the measurements conducted when operating the PC on both the FTP-75 and US06 drive cycles. Typically, 70−71% of the solid particles were larger than 23 nm when operating on the low PMI fuel compared to 74−78% for the other two fuels. For the LDT, combusting the high PMI and Tier 3 fuel over the FTP-75 resulted in 74−78% solid particle with diameter larger than 23 nm compared to 71−73% for the low PMI fuel. However, over the US06 the low PMI fuel generated 71% solid particles larger than 23 nm compared to only 61−63% for the other two fuels, even though these ratios are still considered close within statistical uncertainty. Figure 3 summarizes the averaged particle number size distributions obtained from all valid measurement repeats (N ≥ 3) measured over the US06 and various phases of the FTP-75 drive cycles for the two vehicles on different fuels. Uncertainties are the error estimates with 95% confidence interval (Supporting Information). The size distribution measurements were obtained downstream of a thermodenuder which removed a significant amount of volatile materials. The trends observed on the particle number size distributions are consistent with the SPN (>3 nm) measurements (Table 6 and Table 7). Figure 3 shows that the shapes of the particle number size distributions for both vehicles are similar, although the absolute concentration varies depending on the driving conditions. In all cases, operating on low PMI fuel resulted in statistically lower PN emissions than the other fuels. For both vehicles, the particles emitted during all drive cycles fell in the accumulation mode within the range of 50−70 nm. Particles emitted from both vehicles when operating on the low PMI fuel were generally lower in number, smaller in diameter and peaked at about 50 nm. Particle number emissions from both vehicles were at the lowest level when operating on the low PMI fuel, which is expected. However, operating on the high PMI (PMI = 2.53) fuel did not always translate to PN emissions that were higher than those from using the Tier 3 certification fuel (PMI = 2.10). When considering the EEPS and SPN (>3 and >23 nm) emissions for individual phases of the FTP-75 drive cycle, operating both vehicles on the high PMI fuel always resulted in higher averaged PN emissions in general (Table S8 and Table S9). However, only the difference in emissions observed during the cold-start FTP phase 1 drive cycle is statistically significant in 95% confidence interval. For the US06 drive cycle, operating both vehicles on Tier 3 fuels always resulted in higher PN emissions but the differences are not statistically significant. Effect of Fuel Composition on PM, BC, EC, and OC Emissions. Figure 4 shows how the real-time BC measurements, collected using the micro-Aethalometer and the laserinduced incandescence instrument, are compared. The two sets of measurements were linearly correlated over the entire measurement range regardless of the vehicle, fuel, and drive cycles. This is consistent with the previous observations which showed that the artifact corrected mAeth BC measurements were consistent with the LII BC measurements.42 In this study, the mAeth BC data were generally larger than the LII BC by about 33%. At the lowest concentration range, the mAeth BC data were higher than the LII BC data by up to 56%. Some studies have reported higher mAeth BC mass due to an artifact.48 At very low BC concentration, both the multiple scattering and filter loading effects, which have opposite effects

Figure 4. Relationship of the BC mass measurements measured between the mAeth and the LII over US06 and different phases of the FTP-75 drive cycles.

to the light absorption, can complicate the artifact correction. Previous study suggested that the LII measurements could potentially be capable of removing some of the interferences caused by the presence of the semivolatile materials on the particles without actively conditioning the particles such as using a thermodenuder.49 Relationships between the BC (collected using the mAeth and LII) and the elemental carbon (EC) measurements (collected over quartz filters) are shown in Figure 5. Generally,

Figure 5. Comparison of the BC mass measurements measured between mAeth or LII and the thermal−optical filter EC over the US06 and FTP-75 drive cycles. Filter FTP-75 EC measurements are unweighted integrated measurements over the whole drive cycle whereas the mAeth or LII BC FTP-75 BC measurements are composite averages.

BC emissions from both vehicles for various drive cycles and fuels can be adequately fitted by a linear regression line. Overall, mAeth BC mass was about 62% of the filter EC while LII BC mass was about 50% of the filter EC. These observations were different than the previous observations which showed both mAeth and LII BC have a close to 1:1 relationship with the filter EC when operating the vehicle on Tier 2 certification gasoline.42 The use of a new certification gasoline could have impact on the BC emissions from the two test vehicles in this study and could change the relationship between BC and EC. In this study, the real-time measurements suggest that the unweighted FTP-75 BC (or PM) mass for the PC would be H

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Energy & Fuels higher than the FTP composite average by 1−6% for various fuels. For the LDT, the unweighted average could be higher than the FTP composite average by 7−9%, causing the BC measurements to compare less than the EC measurements in absolute value. EC collected on filters can potentially include contribution of particles that are too large (>1 μm) to be measured by the real-time instrument causing the BC data to be lower. Figure 6 summarizes the comparison of the organic and elemental carbonaceous mass (i.e., OC + EC) or real-time PM

Figure 7. Comparison of the DMM PM against filter-based 1065 PM over the US06 and different phases of the FTP-75 drive cycles.

emission standards for these vehicles were set at 10 mg/mi (∼6.2 mg/km). For the PC, the 1065 PM emissions were 0.5, 1.4, and 1.1 mg/km for the low PMI, high PMI, and Tier 3 fuels, respectively. The corresponding 1065 PM emissions for the LDT were 2.1, 4.6, and 3.7 mg/km, respectively. Although neither vehicle is certified to Tier 3 emission standards, these observations provide some interesting insights. For example, operating of each of these two vehicles on two very similar fuels (Tier 3 and the high PMI fuel) can result in a difference in PM emissions of 20−30%. Carrying this further, it is evident that altering the hydrocarbon composition of a fuel to lower its PMI could change the PM emissions from this PC by up to a factor of 2.7 (i.e., 1.36/0.50) without making any hardware modification or vehicle engine tuning. For the LDT, the range of emissions can also vary by a factor of 2.1. The DMM PM and LII BC emissions, as well as the relative fraction of BC in the particles for both vehicles are given in Figure 8. Overall, emissions from the LDT were typically higher than for the PC and most of the emissions from the LDT were emitted during cold-start. PM and BC emissions from the LDT when operating on the high PMI fuel were generally higher than for the Tier 3 emissions, but often were about the same within statistical certainty. For the PC, results were mixed. For example, while the high PMI fuel BC was either higher than or within measurement uncertainty with the Tier 3 BC, the high PMI fuel PM was always lower than the Tier 3 PM. Mixed results also occur for the filter-based measurements; for example, FTP-75 OC, EC, and 1065 PM for the high PMI fuel were also higher than the Tier 3 measurements, but the opposite is true for the US06 results. The BC mass fraction in particle shown in Figure 8 was estimated by the ratio of the LII BC to DMM PM. Both instruments were likely to underestimate particles with diameters larger than 1 μm, and therefore these estimated percentages likely reflect fine PM trends. In some cases, such as during phases 2 and 3 of the FTP-75 drive cycle when emission levels were low, both measurements were very close to the detection limit of the instrument and resulted in BC fractions over 100% in several instances. For the PC, results suggest that the use of both the low PMI fuel and Tier 3 fuel produced particles with a lower BC fraction (based on the ratio of LII BC/DMM PM), which varied from about 57% to 81%. But when using the high PMI fuel, BC mass fractions in the particles were typically larger than 86%. Operating the LDT on the low PMI fuel resulted in particles with 51% to 67% BC

Figure 6. Comparison of the total carbonaceous mass and real-time DMM PM against filter-based 1065 PM over the FTP-75 and US06. All FTP-75 measurements are composite averages.

(from a Dekati Mass Monitor (DMM)) against the PM mass measured using the U.S. EPA 40-1065 regulation protocol. In this case, both the real-time DMM PM and 1065 PM for the FTP-75 drive cycle were composite averages whereas the carbonaceous mass (OC + EC) was an integrated mass collected over the entire FTP-75 drive cycle. Results showed that in general the OC + EC mass was higher than the 1065 PM by 41% whereas the DMM PM was lower than the 1065 PM by 50%. A number of reasons can contribute to these observations. As previously discussed, the unweighted filter OC + EC mass typically overestimates by 1−6% or 7−9% for the PC and LDT, respectively. Also, the collection of particles larger than 2.5 μm in diameter is limited for the 1065 PM filter pack system due to the presence of a cyclone. The absence of a cyclone on the quartz filter pack system could potentially allow some larger particles to be collected if they should be present. Finally, even though OC measurements were corrected for positive artifact, it is possible that not all of the positive artifact was accounted for. In the 1065 system, all connections and the filter pack were heated to 52 °C to minimize the contribution of the positive artifact as warmed diluted exhaust encountered a room temperature filter surface. In the DMM system, particles larger than 1 μm in diameter are not expected to be accurately measured in number concentration, therefore leading to a slight underestimation when converted back to mass. When the FTP75 results were broken down by individual phases, the correlation between the DMM PM and the 1065 PM improved significantly (Figure 7), showing that the DMM PM represents 91% and 70% of the 1065 PM for the PC and LDT measurements, respectively. The Tier 3 certification PM emission standard for model year 2017 and later PCs and LDTs is currently set at 3 mg/mi (or ∼1.9 mg/km) over the FTP-75 drive cycle. The Tier 2 PM I

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Figure 8. DMM PM and LII BC for all fuels over the US06 and various phases of the FTP-75 drive cycles for the (left) PC and (right) LDT. For clarity only 95% confidence interval error estimates for BC are shown.

than the maximum exhaust temperature reached over the FTP75 drive cycle (600 °C for PC and 500 °C for LDT). The PC and LDT exhibited relatively similar emission patterns for the FTP-75 drive cycle. However, the emissions from the LDT were higher than those from the PC, especially during periods of acceleration which usually resulted in elevated emissions. In those events, both vehicles exhibited lower SPN and BC emissions when operating on the low PMI fuel in comparison to the high PMI fuel. The PC showed qualitatively similar SPN and BC emission patterns on the low PMI and the high PMI fuels except that the magnitude of the emissions for the low PMI fuel was lower than that for the high PMI fuel over most portions of the FTP-75 driving cycle. For the LDT, there was significant elevation of both SPN and BC emissions during periods of aggressive acceleration of the FTP-75 drive cycle but, qualitatively, the emission patterns for the low PMI and high PMI fuels were similar. It is hypothesized that the presence of greater concentrations of high boiling naphthylenic aromatic hydrocarbons in both the Tier 3 and high PMI fuels would require a longer time to vaporize, resulting in less complete combustion and higher OC and EC emissions when operating on the high PMI and Tier 3 fuels. When operatng over the US06 drive cycle, the effect of vehicle technology/performance (as represented by the two vehicles) and fuel composition on the emission patterns was somewhat confounded. For both the PC and LDT, operating on the high PMI fuel over the US06 drive cycle typically resulted in higher emissions in general than for the low PMI fuel. For the LDT, operating on the high PMI fuel over the US06 drive cycle resulted in more frequent emission spikes, and some occurred even during periods with steady high speed and no rapid acceleration (e.g., ∼200−300 and ∼350−450 s during the US06). The higher demanding driving condition of the US06 cycle requires a larger fuel volume and longer fuel injection duration and increases the chance for wetting the piston surface and cylinder wall. Fuel that does not evaporate from the piston surface will be ignited by the premixed flame and burn by diffusion forming BC particles.53,54 In addition,

fraction. When switching to the other two fuels the emitted particles were primarily BC (i.e., >90%), except for the US06 case on the high PMI fuel when the BC fraction in the particle was about 57%. Comparing the OC and EC emissions from the two vehicles shows that, for the PC, EC contributed roughly 83−86% of the total carbonaceous mass regardless of the type of fuel or drive cycle. For the LDT, roughly 84−87% of the total carbonaceous mass collected over the FTP-75 drive cycle was EC in nature regardless of the type of fuel used. However, over the US06 drive cycle, EC represented 29% of the total carbonaceous mass when the low PMI fuel was used. For the high PMI and Tier 3 fuels, EC represented 42−44% of the total carbonaceous mass. Effect of Fuel Composition on Emission Time Series. The real-time SPN (>23 nm) and LII BC emissions generated while operating the vehicles on the low and high PMI fuels over the different drive cycles were compared in order to illustrate and understand the relative impact of fuel composition and vehicle operation. In each panel of both Figure 9 and Figure 10, the two shaded regions represent the minimum to maximum emission levels at each vehicle operating point obtained from all available measurement repeats from the same instrument. Also included in the figures are the vehicle speed and acceleration. Operating both vehicles on the low PMI fuel always resulted in lower SPN or BC emissions regardless of drive cycle especially during cold-start. For example, the FTP phase 1 LII BC emission ratios between the high PMI fuel and the low PMI fuel for the PC and LDT are 5.9 and 2.8, respectively (Table S8 and Table S9). For DMM PM, the ratios were 3.7 and 2.0 for the PC and LDT, respectively. Also, the LDT cold-start emissions were significantly higher and lasted over a much longer time than for the PC despite the higher horsepower output of the LDT. This is because the moderate driving condition over the FTP-75 drive cycle did not generate enough heat to warm up the LDT engine as fast as the PC (Figure S9). Over the US06 drive cycle, the exhaust temperatures for both vehicles were comparable to each other, reaching a maximum exhaust temperature of 700 °C (Figure S10). This is higher J

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Figure 9. Minimum to maximum SPN (>23 nm) emission ranges from the PC and LDT, derived from all valid measurement repeats, over the different drive cycles operating on the low and high PMI fuels.

configuration and calibration between the two vehicles, which is independent from the fuel volatility, also play roles. Interestingly, during the steady speed periods of the US06 drive cycle, the LDT instantaneous values of the SPN and BC emissions on the low PMI fuel could often lower than emissions from the PC. The LDT has a vehicle power to weight ratio of 0.100 kW/kg compared to a ratio of 0.081 kW/kg for the PC suggesting that the PC has 19% less power (compared to the LDT) to overcome the weight of the vehicle to accelerate or maintain at high speed condition during the US06 drive cycle. Therefore, when switching from the low PMI to high PMI fuel, there was a much larger increase in emissions (particularly OC) for the PC than for the LDT.

vehicles typically run slightly rich at high load condition. The high speed condition also reduces the time available for the soot to mature in engines.55 It is hypothesized that the emission spikes observed for the LDT could have occurred during a period when some of the higher boiling aromatic hydrocarbons could not be vaporized completely to participate in the gas phase combustion reaction, leading to higher emissions. This is consistent with the increased OC and EC emissions during the US06 drive cycle when operating on the high PMI fuel than for the low PMI fuel despite the fuel consumption remaining the same. However, comparing between the two vehicles, the change in emissions was not proportional to the change in fuel consumption, which also suggested that the difference in engine K

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Figure 10. Minimum to maximum BC emission ranges from the PC and LDT, derived from all valid measurement repeats, over the different drive cycles operating on the low and high PMI fuels.

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CONCLUSIONS

Observations demonstrated that replacing the naphthylenic aromatic hydrocarbons (diaromatics) in the fuel (i.e., high PMI fuel) with monoaromatics and naphthenes (cycloparaffins) (i.e., low PMI fuel) reduced PN and BC emissions from both the PC and LDT, which is consistent with observations from many studies.14,15,27 However, this study also showed that the effect of fuel consumption on vehicle emissions was different on vehicles depending on many other factors, including the vehicle types, power to weight ratio, and driving conditions. Operating both vehicles on high PMI or Tier 3 fuels typically resulted in higher (PN, BC, OC, and EC) emissions. In addition, it is also hypothesized that the emission spikes observed from the LDT on high PMI fuel during the US06 drive cycle could be caused

A study was conducted on a gasoline powered PC and LDT, both of which were equipped with a wall-guided GDI engine and were from the same manufacturer. Particle, BC, and gaseous emissions were measured on the two vehicles operated over two standard drive cycles and fueled with two test gasolines blended to represent high and low values of the range of PMI for current commercial motor gasolines. These measurements were compared with emission measurements when operating the same vehicles on EPA Tier 3 certification gasoline. L

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ACKNOWLEDGMENTS We acknowledge the contribution of the Emission Research and Measurement Section (ERMS) staff for their assistance in conducting this vehicle emissions research project. Financial support was provided by the Government of Canada’s Program of Energy Research and Development (PERD) Projects AFTER10, AFTER11, and PERD 3B03.002 and Transport Canada. The test vehicles were provided by Transport Canada through the ecoTECHNOLOGY for Vehicles (eTV) program. The test fuels were provided by the American Petroleum Institute (API). The prototype GPFs were provided by Manufacturers of Emission Controls Association (MECA). We thank Jeff Jetter of Honda R&D Americas for providing assistance to understand various technical questions regarding the PMI and the staff and members of the API for their helpful reviews and comments made during the preparation of this work.

by the presence of the high boiling points of the naphthylenic aromatic hydrocarbons that could not be vaporized completely during demanding driving conditions. Overall, particles emitted from both vehicles have diameters from 50 to 70 nm. Operating on the low PMI fuel typically led to formation of smaller particles with diameters of about 50 nm. Quartz filter PM measurements suggest that the EC represent 83−86% of the emitted particles from the PC for all fuels and drive cycles. However, this is only true for the LDT for the FTP-75 drive cycle. Over the US06, the particles emitted from the LDT for Tier 3 and high PMI fuels contained 42−44% EC and, when operated on low PMI fuel, contained as little as 29% EC. In this study when examining each of the two test vehicles individually, changes in fuel composition were observed to influence emissions measured on all of the drive cycles evaluated. Among various components, the diaromatic hydrocarbons were observed to have a significant influence on particle formation even present in a small amount. When comparing the results across the two vehicles, differences in vehicle technology and performance characteristics can influence the fuel composition impact and shall not be ignored. The LDT had higher PM emissions than the PC in all cases for all drive cycles tested. This is especially true for cold-start emissions. For this reason, only limited conclusions may be drawn from this study since only two vehicles from one manufacturer were tested. Results could be different for other vehicles, manufacturers, or a fleet. Nonetheless, this study provides useful insights regarding tailpipe emissions for different types of vehicles under different driving cycles and provides a possible explanation that the difference in fuel composition could potentially be an important factor for the large variations in emission factors reported in the literature for similar operating conditions. Thus, fuel composition and the distillation properties of fuels are two important pieces of information for linking fuel effects to PM emissions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01345. Further details on the soot indices, fuel compositions, and distillation profiles, test procedure, sampling setup, margin of error calculations, drive cycles, long-term fuel trim measurements, exhaust temperature, and extended tables of gaseous and particle emissions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (416) 739-4419. ORCID

Tak W. Chan: 0000-0003-4706-0556 Notes

Disclosure: The analysis, results, and conclusions presented are those of the authors alone. The conclusions and views expressed do not necessarily, and should not be taken to, reflect those of API. The authors declare no competing financial interest. M

NOMENCLATURE BC = black carbon CO = carbon monoxide CO2 = carbon dioxide CPC = condensation particle counter CVS = constant volume sampling DBE = double bond equivalent DMM = Dekati mass monitor EC = elemental carbon EEPS = engine exhaust particle sizer EPA = U.S. Environmental Protection Agency FBP = final boiling point FC = fuel consumption FE = fuel economy FTP-75 = U.S. Federal Test Procedure 75 GDI = gasoline direct injection GPF = gasoline particulate filter HEPA = high efficiency particulate air ICE = internal combustion engine LA4 = Los Angeles Route 4 LDT = light-duty truck LII = laser-induced incandescence LTFT = long-term fuel trim mAeth = micro-Aethalometer MW = molecular weight OC = organic carbon OEM = original equipment manufacturer PAH = polycyclic aromatic hydrocarbons PC = passenger car PEI = particulate evaluation index PM = particle mass PMI = particulate matter index PMM = particulate matter mass PMP = particle measurement programme PN = particle number SP = smoke point SPN = solid particle number SwRI = Southwest Research Institute T90 = temperature at 90% distillation THC = total hydrocarbon TSI = threshold soot index TWC = three-way catalyst US06 = US06 Supplemental Federal Test Procedure VP = vapor pressure DOI: 10.1021/acs.energyfuels.7b01345 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b01345 Energy Fuels XXXX, XXX, XXX−XXX