The impact from the direct injection and multi-port fuel injection technologies for gasoline vehicles on solid particle number and black carbon emissions

Applied Energy 226 (2018) 819–826

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

The impact from the direct injection and multi-port fuel injection technologies for gasoline vehicles on solid particle number and black carbon emissions ⁎

T

⁎

Liqiang Hea,b, Jingnan Hub,c, , Shaojun Zhangd, Ye Wua, , Rencheng Zhub, Lei Zub, Xiaofeng Baob, Yitu Laie, Sheng Sue a

School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China Chinese Research Academy of Environmental Sciences, Beijing 100012, China Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Nanjing 210044, China d Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA e Xiamen Environmental Protection Vehicle Emission Control Technology Center, Xiamen 361023, China b c

H I GH L IG H T S

PN and BC of light-duty GDI and MPFI vehicles are measured over the WLTC. • Solid general, GDI vehicles emit much more solid PN and BC than MPFI vehicles. • InEmissions MPFI vehicles are more sensitive to cold start, aggressive driving and air conditioner usage. • At −7 °C, ofemissions of MPFI vehicles are comparable or even higher than those of GDI vehicles. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Vehicle emissions Gasoline direction injection (GDI) Particle number (PN) Black carbon (BC) Worldwide Harmonized Light Vehicle Test Cycle (WLTC)

The gasoline direct injection (GDI) engine has substantially penetrated light-duty gasoline vehicles to help reduce fleet-wide fuel consumption across the world. However, increased particle emissions from GDI vehicles rather than the conventional multi-port fuel injection (MPFI) vehicles are of great concern. To investigate the particle emissions for these two categories of gasoline engines, we employed a dynamometer and measured the emissions of solid particle number (PN) and black carbon (BC) for four GDI and four MPFI vehicles under various testing cycles and conditions. Under the reference cycle (30 °C and cold-start WLTC), a strong correlation between solid PN and BC emissions is identified for both GDI and MPFI vehicles, although GDI vehicles without particle filters have significantly higher emissions of solid PN and BC than those of MPFI vehicles. Furthermore, varying the testing conditions by including cold start, low temperature, aggressive driving and air conditioning use all increase the emissions of solid PN and BC. These affecting factors pose more significant changes to particle emissions from MPFI vehicles than GDI vehicles. For example, at −7 °C, the solid PN and BC emissions of MPFI vehicles are increased by 4.17 times and 16.5 times relative to the results under 30 °C, and they are comparable to or higher than the emissions of GDI vehicles. Our results indicate that modern gasoline vehicles available in China’s market are likely to fail to comply with the upcoming PN emission limit (China 6), suggesting a serious need to adopt gasoline particle filters (GPF) for both GDI and MPFI vehicles. Advanced aftertreatment technologies and stringent regulations to control particle emissions from gasoline vehicles should fully consider varying real-world conditions to guarantee effective environmental benefits.

1. Introduction The growth of global transportation demand has posed substantial pressures regarding soaring energy consumption and greenhouse gas

(GHG) emissions [1], including short-lived climate forcing pollutants (e.g., black carbon, BC) [2]. Among all transportation sectors, on-road vehicle emissions are one important air pollution source resulting in significant health impacts [3–5]. Global policy makers have

⁎ Corresponding authors at: Chinese Research Academy of Environmental Sciences, Beijing 100012, China (J. Hu) and School of Environment, Tsinghua University, Beijing 100084, China (Y. Wu). E-mail addresses: [email protected] (J. Hu), [email protected] (Y. Wu).

https://doi.org/10.1016/j.apenergy.2018.06.050 Received 16 February 2018; Received in revised form 3 June 2018; Accepted 8 June 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

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Table 1 Summary of vehicle specifications. Model, manufacturer

Teana, Nissan Camry, Toyota GL8, Buick Cruze, Chevrolet Magotan, Volkswagen Passatc, Volkswagen Passatd, Volkswagen Malibu, Chevrolet a b c d e f

Registration year

2014 2015 2012 2015 2014 2015 2015 2012

Mileage (km)

1660 9539 33,397 687 5657 4828 4600 10,633

GVWa (kg)

Engine

1870 2000 2380 1778 2000 2060 2060 1977

Power rating (kW)

Displacement (L)

100 108 120 80 118 118 118 146

2.0 2.0 2.4 1.5 1.8 1.8 1.8 2.5

Engine type

Emission standard

PFI PFI PFI PFI GDIb GDIb GDIb GDIe

China 4 China 4 China 4 China 4 China 4 China 4 China 4 Tier 2/ULEVf

Gross vehicle weight. Wall guided GDI engine with turbo-charged four cylinders. Passat 1 in the following sections. Passat 2. Center mounted GDI engine with naturally aspirated four cylinders. Approximate to China 4/China 5 standards.

environmental conditions, such as low ambient temperature, would greatly increase particle emissions from both GDI and MPFI vehicles [26]. Third, the instability of semi-volatile particles and poor repeatability of particle mass weighting would increase the measurement variability for ultra-low emission vehicles [34]. A particulate measurement program (PMP) methodology has been applied in the regulations (e.g., Euro 6 and China 6) to only measure PM and PN of solid particles with diameters greater than 23 nm. Furthermore, BC is one important component of primary solid particles that has significant adverse climate and health effects [1]. BC measurement could improve the understanding of solid particle emissions from gasoline vehicles [35,36]. In this study, we focus on particle emissions from modern gasoline cars and consider all the technical progress as noted above. We recruited eight in-use gasoline LDPVs in China and measured their particle emissions, including solid PN and BC over a dynamometer. We also examined the impacts from low temperature conditions and usage of on-board air conditioner (AC) in the laboratory. The findings in this paper can provide researchers and policy makers with useful first-hand information to improve environmental assessment tools (e.g., China’s National Emission Inventory Guidebooks) [37,38] and future policies [39].

implemented increasingly stringent regulations for the sake of our environment and climate systems. Taking gasoline cars as an example, the current European limits of gaseous pollutant emissions have been lowered by 50–60% compared with the 2000 levels (Euro 6 vs. Euro 3) [6]. Meanwhile, European passenger cars in 2021 shall comply with an average carbon dioxide (CO2) emission target of 95 g/km under typeapproval procedures [7], representing a reduction up to 40% compared to the early 2000 s. Worldwide progresses on emission standards of light-duty passenger vehicles (LDPVs) are also remarkable in other major vehicle markets (e.g., the U.S. [8], China [9], Japan [10]). These stringent regulations have posed substantial influences on the global automotive industry. Motivated by the progressive fuel economy standards, downsized vehicles [11] and engines [12], turbocharged engines [13], gasoline direct injection (GDI) [14,15], lightweight materials and technologies [16–18], and electrified powertrain systems [19–21] have been applied into vehicle products to mitigate CO2 emissions. For example, turbocharged GDI engines adopted by gasoline cars would mitigate energy consumption and CO2 emissions by approximately 15% compared with the conventional multi-port fuel injection (MPFI) engines. The market share of GDI engines has impressively climbed to 49% of global gasoline car sales in 2016 [22]. Nevertheless, the shift from MPFI to GDI may cause challenges to eliminate air pollutant emissions. There are serious concerns of particulate matter (PM) emissions from GDI vehicles, as GDI engines emitted higher than MPFI engines [23,24] and diesel engines with diesel particulate filters (DPF) [25]. However, recent studies report that the particle emissions from MPFI vehicles under adverse conditions (e.g., cold start [26], low temperature [27]) are compared to GDI vehicles. Some measurements have revealed that particle emissions from gasoline vehicles are strongly associated with driving conditions (e.g., vehicle speed and engine load) [28]. To control particle emissions from gasoline vehicles, a particle mass (PM) limit has been added for GDI vehicles in Europe since the Euro 5 stage [6], which is also included in China by the China 5 standard [29]. The Euro 6 standard has further regulated particle number (PN) emissions [6], requiring the usage of gasoline particulate filters (GPF). Given the considerable presence of GDI vehicles, it is fundamental to characterize their particle emissions and importance affecting factors. Notably, a few recent advancements related to emission measurement should be considered by researchers. First, the Worldwide Harmonized Light Vehicle Test Cycle (WLTC) has been adopted by the Euro 6 standard since September 2017 [30]. The WLTC is improved over the previous New European Driving Cycle (NEDC) by adding more real-featured and transient traffic patterns, which is expected to address the road-to-lab discrepancy issues of nitrogen oxides (NOX) [31] and CO2 emissions [32] as well as energy consumption [33]. Second, harsh

2. Methods 2.1. Experimental descriptions Vehicle information. Emission measurements were conducted during 2014–2015 by using a 48-in., 4-wheel-drive light-duty chassis dynamometer (Model 1ASM150K-8, AVL List GmbH, Germany) in the Xiamen Environmental Protection Vehicle Emission Control Technology Center (VETC), where the type-approval testing capability has been qualified by the Ministry of Environmental Protection (MEP) of China. The LDPV candidates included both GDI and MPFI engines and represented typical models in China’s automotive market. They all declared compliance with the most stringent emission standards at that time (e.g., China 4 for seven LDPVs and Tier 2/ULEV for one LDPV, see Table 1). All the vehicles used an automatic transmissions (AT) and were equipped with three-way catalytic (TWC) converters operating in closed-loop controls. None of the GDI vehicles tested were equipped with GPF because GPF is a future technology option in China [39]. In this study, the gasoline fuel was specifically provided by oil companies for type-approval testing and the sulfur content was below 10 ppm. The fuel property information is summarized in Table S1. Test cycles and conditions. In terms of the driving cycles, all the vehicles were tested under the WLTC developed by the United Nations 820

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Finland) were used to measure the particle size distributions. The technical details of pollutant analyzers including the accuracy and range information are provided in the Supplementary Information (Table S3).

Economic Commission for Europe (UNECE)’s Working Party on Pollution and Energy (GRPE) [40]. We selected the Class 3 cycle (Version 5) of the WLTC according to vehicle properties (engine power, curb mass). This WTLC version consists of low-speed (average speed of 18.9 km/h), medium-speed (39.5 km/h), high-speed (56.6 km/h) and extra-high-speed (92.0 km/h) phases. The duration of the entire cycle is 1800 s, and the total distance is 23.3 km. In addition to the WLTC, three GDI and two MPFI vehicles were tested additionally under NEDC and Federal Test Procedure-75 (FTP-75, one regulatory cycle in the U.S.). Fig. S1 provides a comparison among the three cycles regarding the instantaneous speed information. An environment chamber (Model EC45192327, Imtech Deutschland GmbH & Co. KG, Germany) was used to control uniform, accurate and stable conditions regarding ambient temperature and humidity during the testing and soaking periods. In addition to a reference condition (ambient temperature of 30 °C, with cold start), we opted to use a series of varying conditions such as hot start (ambient temperature of 30 °C, without cold start), low ambient temperature (−7 °C) and AC on. It is noted that we selected 30 °C as a regular ambient temperature for all cycles (e.g., NEDC, WLTC, FTP-75) to keep the consistency, which is an upper level of the ambient temperature range applied in China 5 emission standard (i.e., NEDC-based). We acknowledge that the future China 6 will specify the testing temperature of 23 ± 5 °C for the WLTC-based protocol, which is lower than the condition used in this study. Table S2 presents a detailed experimental matrix defined by pollutant category (solid PN and BC), testing cycle and measurement condition for each vehicle. Emission measurement system. Fig. 1 presents a schematic diagram of the emission measurement system. The measurement system of solid PN following the PMP procedure could be categorized into three components: (1) a particle pre-classifier (PCF) with a cutoff size of 3 μm; (2) a volatile particle remover (VPR, Model 379020A-30, TSI, USA) to eliminate volatile and semi-volatile species that included one primarily PN diluter (PND1, dilution and heating up to 150 °C), one evaporation tube (ET, over 300 °C), and one secondary PN diluter (PND2, dilution and cooling to below 35 °C); and (3) a PN counter (Model CPC 3790, TSI, USA) with 50% instantaneous counting efficiency (i.e., second-by-second) for 23-nm. An aethalometer (Model AE51, Magee Scientific Company, USA) was employed to measure realtime BC concentrations in the flow. In this study, the sample flow rate of the aethalometer was 150 mL/min, and the temporal resolution was selected at 1 Hz to keep in line with the solid PN measurement. In addition, a real-time particle spectrometer (Model ELPI ®+, Dekati Ltd.,

2.2. Calculation of emissions Prior to calculating emissions, we verified the time alignments of second-by-second profiles for solid PN and BC along with other instantaneous parameters (e.g., speed and acceleration) by determining the highest correlation coefficient between the two series of values. Based on the instantaneous concentration profiles, the time-specific emission rates of solid PN (#/s) and BC (mg/s) were calculated with Eqs. (1) and (2).

ERPN =

ERBC =

V × fr × Cs × 105 6

V × BCc 6 × 107

(1)

(2)

where ERPN and ERBC are the instantaneous emission rate of solid PN (#/s) and BC (mg/s), respectively; V is the instantaneous volume of the diluted exhaust gas, m3/min; fr is the instantaneous dilution factor in the VPR; Cs is the instantaneous concentration of solid PN measured by the CPC, #/cm3; and BCc is the corrected instantaneous BC concentration (see Eq. (3)), ng/m3. The volume and concentration data were all normalized to the standard ambient temperature and pressure condition (0 °C and 1 atm). As the aethalometer filter darkens during the collection of lightabsorbing particles, we applied Eq. (3) to correct the raw data [41,42]. This correction enables us to improve the agreement of aethalometermeasured BC concentration against that measured by filter weighting and thermal-optical method.

BCc =

BC0 0.88exp(−ATN /100) + 0.12

(3)

where BCc and BC0 are the corrected and raw BC concentrations, respectively, ng/m3; and ATN is the light attenuation measured by the aethalometer. Distance-specific emissions of air pollutants by cycle phase were calculated for each vehicle based on the instantaneous emission rates (see Eq. (4)).

Fig. 1. Schematic diagram of the emission measurement system. 821

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3600 × ∑t2= t1 ERi, j, t t ∑t2= t1

vi, t

than the emission variability between different driving cycles (see Fig. 2). A possible reason is that various aspects of inter-cycle distinctions could lead to different effects on particle emissions. For example, WLTC has higher proportions of transient and aggressive driving conditions than NEDC, which is expected to increase particle emissions. On the other hand, the longer driving distance of WLTC could lower distance-specific emissions contributed by cold start compared with NEDC. From a future perspective, the China 6 standard will be technologyneutral, which will include a unified emission limit of solid PN (6.0 × 1011 #/km) applicable to both MPFI and GDI engines [9]. This is unlike the Euro 6 standard that specifies that PN emissions limit is only applicable to GDI engines in the gasoline fleet [6]. Fig. 2 shows that only one vehicle (MPFI-Teana) could meet the future China 6 PN emission limit. Serious concerns remain for the current MPFI vehicles at risk of exceeding the future PN limit. We suggest that the GPF should also be used by MPFI vehicles to effectively deliver regulation attainment in the future. Thus, the technology-neutral feature of future emission standards may favor further penetration of GDI vehicles in China. For BC, the emissions of GDI vehicles range from 1.62 mg/km to 4.08 mg/km over various cycles, notably higher than those of MPFI vehicles (0.24–0.67 mg/km). Similar comparison results have also been reported by emission measurement studies for vehicles in the world [26,44,45]. BC emissions present larger deviations than those of PN, which might be attributed to a great effect of background level at CVS on BC measurement than that on PN [46]. Strong correlations (R2 = ∼0.9) are identified between solid PN and BC emissions for both GDI and MPFI vehicles (see Fig. 3). Although the GDI vehicles have higher emissions than the MPFI vehicles, there is no significant difference between two vehicle categories concerning emission ratios (EFPN / EFBC ). Chan et al. [26] also found reported emission ratios (EFPN / EFBC ) typically ranging from 1 × 1012 to 2 × 1012 #/mg BC for four vehicles varying in engine and after-treatment technologies (e.g., MPFI, GDI without GPF, GDI with GPF).

(4)

where EFdist . i, j is the average distance-specific emission for cycle phase i of pollutant j , unit in #/km for solid PN, and mg/km for BC respectively; t1 and t2 are the start and end time of cycle phase i , s; ERi, j, t is the instantaneous emission rate of pollutant j at time t during cycle phase i , unit in #/s for solid PN, and mg/s for BC; and vi, t is the instantaneous vehicle speed at time t for cycle phase i , km/h. We further calculated the average emissions over each entire cycle by weighting all sub-cycles (see Eq. (5)). The detailed weighting factors of NEDC, WLTC and FTP-75 are listed in Table S4, which are determined according to the emission regulations in China (applicable to NEDC and WLTC) and U.S. (applicable to FTP-75), respectively. n

EFdist . c, j =

∑

(WFc, i × EFdist . c, i, j )

(5)

i=1

where EFdist . c, j is the average-distance specific emission of pollutant j for the entire cycle c , unit in #/km for solid PN, and mg/km for BC respectively; n is the total of the phases for each test cycle, WFc, i is the weighting factor of cycle phase i , and EFdist . c, i, j is the average distancespecific emission of pollutant j for cycle phase i of cycle c , unit in #/km for solid PN, and mg/km for BC respectively. 3. Results and discussion 3.1. Emissions of solid PN and BC under the reference condition (30 °C with cold start) Fig. 2 presents the emissions of solid PN and BC measured under the reference condition (30 °C with cold start). The GDI vehicles have higher emissions of both solid PN and BC than those of the MPFI vehicles, which are consistent for all the three test cycles (WTLC, NEDC and FTP-75). For example, the emissions of solid PN for the GDI vehicles (2.50 × 1012 #/km to 3.91 × 1012 #/km) are higher than those for the MPFI vehicles (3.62 × 1011 #/km to 1.13 × 1012 #/km) by approximately 2–11 times. The emission discrepancies between GDI and MPFI vehicles could be attributed to the differences in fuel injection systems and mixture preparation. The wetting wall effect in the cylinder and inhomogeneous fuel and air mixture under stratified combustion of GDI vehicles are apt to higher particle emissions relative to tailpipe emissions from MPFI vehicles [26,43]. Obviously, the discrepancy of particle emissions between GDI and MPFI vehicles is more significant

Solid PN emissions (#/km)

10

3.2. The impacts from cold start and ambient temperature conditions For MPFI vehicles, Zheng et al. [36] reported that the BC emissions could be much higher due to cold start during the first 100 s. Our measurement results further confirm that the significant effect of cold start occurs on GDI vehicles (see Figs. 4 and S2). All the vehicles have higher solid PN emissions under cold-start WLTC than those under hotstart WLTC, representing dramatic increases of 102% ± 100% for the GDI vehicles (N = 4, the number of tested vehicles and hereinafter) and

13

10

12

10

11

10

WLTC

NEDC

FTP-75

WLTC

NEDC

FTP-75

1

BC emissions (mg/km)

EFdist . i, j =

10

0

10

-1

MPFI-Teana MPFI-Camry MPFI-GL8 MPFI-Cruze GDI-Magotan GDI-Passat 1 GDI-Passat 2 GDI-Malibu

Test cycles Fig. 2. Emissions of solid PN and BC under the regular condition (30 °C with cold start). Note: The error bars represent the standard deviation of repeat measurements, and the plots without error bar indicate only one test was carried out. The measurement repeats for each vehicle are summarized in Table S2. 822

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13

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12

As Fig. S2 shows, our results reveal 1.75 to 49.9-time increases (N = 8) for solid PN emissions due to the cold start, and the elevation is even greater for BC emissions (2.58 to 60.5-time increases, N = 6). Furthermore, solid PN and BC emissions of the MPFI vehicles are both more sensitive to the cold start compared with the GDI vehicles. The cold start effect would decrease rapidly as engines warm up, typically after the first 200–300 s. For example, under the medium, high, and extra-high-speed phases of WLTC, we could hardly see significant differences of solid PN and BC emissions between the cold-start and hotstart conditions (see Fig. S3). To explore the impact from low ambient temperatures, two GDI and two MPFI vehicles were further tested at a low temperature condition (−7 °C). The emissions of solid PN and BC become notably increased as the ambient temperature decreases from 30 °C to −7 °C. The mechanisms that affect the particle emissions of GDI and MPFI vehicles are quite complex. The higher particle emissions at low temperature is because a relatively poor mixture of fuel and air due to the low temperature might trigger the incomplete combustion and more particle formation [27]. Under this condition, the engines also tend to inject excess combustion fuel to promote good drivability [47]. Our real-time lambda values representing the air-fuel ratio show that the excess fuel injection was significant during the cold-start phase for one MPFI vehicle (MPFI-Camry, see Fig. S4). At −7 °C, the solid PN emissions over cold-start WLTC are increased by 0.84 ± 0.13 times for GDI vehicles (N = 2) and 4.17 ± 1.02 times for MPFI vehicles (N = 2) compared with the results at 30 °C. For BC, the emissions are increased by 1.94 ± 0.18 times for GDI vehicles (N = 2) and 16.50 ± 9.28 times for MPFI vehicles (N = 2) due to the low temperature. Similar to the cold-start effect, particle emissions from the MPFI vehicles are more sensitive to low temperature than the GDI vehicles. Notably, at −7 °C, one MPFI vehicle (MPFI-Camry) has higher solid PN and BC emissions than two GDI vehicles in our study. Detailed measurement results under different temperature conditions (30 °C and −7 °C) are summarized in Tables S5 and S6. The particle size distributions of two vehicles (MPFI-

2

Overall: y=1.04 10 x, R =0.89 12 2 MPFI: y=1.69 10 x, R =0.93 12 2 GDI: y=1.03 10 x, R =0.89

10

MPFI-Teana MPFI-Camry MPFI-GL8 GDI-Magotan GDI-Passat 1 GDI-Malibu NEDC FTP-75

WLTC -1

0

10 BC emissions (mg/km)

10

1

Fig. 3. The correlation of between solid PN and BC emissions under the reference condition (30 °C with cold start).

192% ± 129% for the MPFI vehicles (N = 4). Meanwhile, the BC emissions are increased by 186% ± 168% for the GDI vehicles (N = 3) and 321% ± 255% for the MPFI vehicles (N = 3) (see Fig. 4). The cold start effect is further quantified in terms of extra emissions per cold-start trip, which are estimated according to the discrepancy between two test conditions (e.g., cold start vs. hot start) for each vehicle. The results indicate that the extra cold-start emissions of solid PN are 3.18 ± 1.37 × 1013 #/trip for the GDI vehicles (N = 4) and 1.08 ± 0.56 × 1013 #/trip for the MPFI vehicles (N = 4). For BC, the extra cold-start BC emissions are 32.7 ± 6.63 mg/trip (N = 3) and 6.46 ± 3.39 mg/trip (N = 3) for the GDI and MPFI vehicles, respectively. In addition, the first low-speed phase dominates the cold start effect, accounting for over 80% of extra cold-start PN and BC emissions.

Solid PN emissions (#/km)

WLTC : 30 C 10

14

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Cold start Hot start

Te

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ana amry C

GL

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-2

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Cr

Te

ana

y mr Ca

GL

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t1 tan go ssa Pa Ma

u lib Ma

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WLTC : -7 C MPFI

y mr Ca

uze gotan ssat 1 ssat 2 alibu M Pa Pa Ma GDI BC emissions (mg/km)

BC emissions (mg/km)

GDI

MPFI

Solid PN emissions (#/km)

Solid PN emissions (#/km)

L. He et al.

10

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10

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GL

GDI

8

t1

ssa

Pa

MPFI

10

-1

10

-2

y mr Ca

GL

GDI

8

t1

ssa

Pa

Fig. 4. Emissions of solid PN and BC under different start and ambient temperature conditions. 823

u lib Ma

u lib Ma

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Solid PN emissions (#/km)

(a) Solid PN 10

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other phases of the WLTC, NEDC and FTP-75 (see Fig. S1). It is noted that the average and maximum speeds are both higher than those of US06 cycle, which is considered as one aggressive driving cycle in the U.S. [48]. To evaluate the impact from aggressive driving behaviors, we compare the emissions of solid PN and BC in four phases of hot-start WLTC (30 °C) (see Fig. 5). Under the extra-high-speed phase, the emissions for the MPFI and GDI vehicles are both increased unlike under the other three phases, and the trend for MPFI vehicles is more significant, which is also reflected by the lambda profiles (see Fig. S4). The average extra-high-speed emission of solid PN is 2.27 ± 1.06 × 1012 #/km for GDI vehicles (N = 4) increased by 12–74% higher than that of the high-speed phase. For MPFI vehicles (N = 4), the solid PN emissions under the extra-high-speed phase are increased by 7.52 times to 17.0 times than the high-speed emission results. The BC emissions under the extra-high-speed phase also become greater compared with the emissions under the other three phases, ranging from 0.71 mg/km to 2.53 mg/km for the GDI vehicles (N = 3) and 0.09 mg/km to 0.36 mg/km for the MPFI vehicles (N = 3), respectively. The impact from AC usage on vehicle emissions is also concerned, which is an important aspect of environmental conditions during summer [49]. Four tested vehicles were measured under hot-start WLTC (30 °C) with their air conditioners on and off, respectively. With the AC on, the average engine loads of two vehicle examples (MPFICamry and GDI-Passat 1) were increased by 25–30% (see Fig. S6). Meanwhile, the average emissions are increased by 132% for solid PN and 144% for BC compared with the results with the AC off (see Fig. 6). The solid PN emissions are increased by 38–61% for GDI vehicles and 195–232% for MPFI vehicles. The BC emissions are increased by 28–55% for GDI vehicles and 91–401% for MPFI vehicles. Therefore, the particle emissions from MPFI vehicles are also more sensitive to AC usage than GDI vehicles. A sub-cycle observation indicates that the increase of solid PN and BC emissions for MPFI vehicles (Camry and GL8) due to AC usage is more significant in the extra-high-speed phase than in the other phases (see Table S7). For example, in the extra-highspeed phase AC usage would increase solid PN and BC emissions by 403% and 742% for the MPFI-GL8 vehicle. By contrast, the emission increased due to AC usage for GDI vehicles (Passat 1 and Malibu) are less varying between various sub-cycles. Similarly, the impact from AC usage on the particle size distributions for MPFI-Camry is more significant than the GDI-Passat 1 for high-speed and extra-high-speed subcycles, as shown in Fig. S7.

GL

Cru

u ze otan sat 1 sat 2 alib g s s a a a M P P M

BC emissions (mg/km)

(b) BC 10

1

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MPFI

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-2

Te

ana

Ca

Low

mr

8

y

GL

go Ma

Medium

tan

1

sat

Pas

High

Ma

lib

u

Extra-high

Fig. 5. Phase-specific emissions of solid PN and BC under the hot-start WLTC (30 °C).

Camry and GDI-Passat 1) are measured by using one Electrical Low Pressure Impactor ®+ (ELPI+) (see Figs. S5 and S7). The results confirm that the PN emissions of the MPFI vehicle at −7 °C are comparable or even higher than the GDI vehicle for various speed phases (see Fig. S5).

3.3. The impacts from aggressive driving and air conditioning (AC) use The extra-high-speed phase of the WLTC could represent a typical aggressive highway driving profile because its average speed (92.0 km/ h) and the maximum speed (131.3 km/h) are far higher than that of

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ss

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ibu

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mr

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t1

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lib

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Air conditioning operating at their maximum level

Fig. 6. Emissions of solid PN and BC with air conditioning operating at the maximum level and air conditioning off, respectively. 824

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4. Conclusions [2]

Progressive fuel economy standards for passenger vehicles have spanned a rapid penetration of GDI engines due to the higher fuel economy that have rapidly squeezed the market share owned by the MPFI engine counterparts. To address the concern regarding particle emissions from GDI vehicles, we employ a dynamometer to measure solid PN and BC emissions from four GDI and four MPFI vehicles in China. The measurement results under the regular conditions (30 °C, with cold start) indicate that the GDI vehicles have much higher emissions of solid PN and BC compared with those of the MPFI vehicles, which are consistent over different testing cycles (WTLC, NEDC, FTP75). Strong correlations between solid PN and BC emissions are identified for both GDI and MPFI vehicles. For example, the solid PN emissions under the WLTC are measured 2.50 × 1012 #/km to 3.91 × 1012 #/km for GDI vehicles and 3.62 × 1011 #/km to 1.13 × 1012 #/km for MPFI vehicles, respectively. The released China 6 will include a stringent PN emission limit of 6 × 1011 #/km applicable to both GDI and MPFI vehicles. Therefore, both GDI and MPFI vehicles available in the present market will be confronted with high non-attainment risks of the China 6 PN limit. We suggest both GDI and MPFI vehicles should be equipped with particle filters in the future. To further explore affecting factors in solid PN and BC emissions, we opt to include additional testing conditions varying by start status (hot start vs. cold start), ambient temperature (−7°C vs. 30 °C), cycle phase, and AC use (AC on at the maximum level vs. AC off) for each vehicle. Our measurement results indicate that emissions of the MPFI vehicles, for both solid PN and BC, are more sensitive to all the considered testing conditions compared with those of the GDI vehicles. For example, the emissions of solid PN are increased by 102% for the GDI vehicles and 192% for the MPFI vehicles due to the effect of cold start. Low ambient temperature is a common harsh operating condition in winter, which has been identified as posing one of the most significant impacts among all the affecting factors. Our results show that the solid PN emissions are increased by 0.84 times and 4.17 times for the GDI and MPFI vehicles, respectively, and the increases of BC emissions are even greater than those of solid PN. Therefore, we note that real-world emissions of solid PN and BC from gasoline vehicles have great uncertainty due to the operating conditions. Advanced emission control technologies (e.g., GPF) and regulation designs should fully consider these complicated affecting factors to deliver effective emission mitigation.

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Acknowledgments [19]

This study was sponsored by the National Key Research and Development Program of China (2017YFC0212100); the National Natural Science Foundation of China (NSFC) (No. 21577135 and No. 91544222); and the Special Research Projects for Public Welfare of Environmental Protection (No. 201409013). Dr. Shaojun Zhang is supported by Cornell University's David R. Atkinson Center for a Sustainable Future (ACSF). The authors would like to acknowledge Mr. Yongming Lin, Mr. Jiajun Xu, Mr. Jianyi Ji, Mr. Yingnan Liu and Mr. Jian Lin of Xiamen Environmental Protection Vehicle Emission Control Technology Center (VETC) for their contributions in experimental measurements for this study. The contents of this paper are solely the responsibility of the authors and do not necessarily represent official views of the sponsors.

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Appendix A. Supplementary material

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2018.06.050.

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