Rethinking NOx emission factors considering on-road driving with malfunctioning emission control systems: A case study of Korean Euro 4 light-duty diesel vehicles

Atmospheric Environment 202 (2019) 212–222

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Rethinking NOx emission factors considering on-road driving with malfunctioning emission control systems: A case study of Korean Euro 4 light-duty diesel vehicles

T

Taewoo Leea,∗, Myunghwan Shina, Beomho Leeb, Jaewoo Chungb, Deokjin Kimb, Jihoon Keela, Sangeun Leea, Ingu Kima, Youdeog Honga a b

National Institute of Environmental Research (NIER), 42 Hwangyeong Road, Seo, Incheon, 22689, Republic of Korea Korea Automotive Technology Institute (KATECH), 303 Pungse Road, Pungse, Cheonan, 31214, Republic of Korea

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Driving cycle Portable emissions measurement system (PEMS) Exhaust gas recirculation (EGR) valve Relative positive acceleration (RPA) Engine control unit (ECU) modification

This study aims to quantify the differences between the standard laboratory-based NOx emissions factor (EF) and on-road measurements, and to estimate how much NOx emissions exceed the current EF under typical Korean road traffic conditions. Four Euro 4 light-duty diesel vehicles (LDVs) manufactured in Korea were driven on a chassis dynamometer and on test routes that included urban and rural roads and a motorway. NOx emissions, average speed, and acceleration were measured and calculated for each 1 km trip. We focused in particular on the possibility of vehicle malfunctions causing increased NOx emissions, and compared NOx emissions before and after the repair of faulty exhaust gas recirculation (EGR) valves, which are a key NOx reduction system for LDVs. The current NOx EF for Euro 4 LDVs only estimates 27–31% of on their on-road NOx emissions, and low acceleration during the standard driving cycle is a part of the reason for this weak representativeness. Another is the lack of tools with which to monitor EGR valve degradation. Even with fully functioning EGR valve hardware, the capability of vehicles to control NOx emissions differ in laboratory and on-road conditions; it is weaker for the latter compared to the former because the software controls are different. The findings here provide insights for different regions where Euro 4 LDVs are still in use, depending on their age. For Korea and the EU, where the fleet is old, accurate assessments of the real risk of NOx emissions are beneficial in terms of increasing the effectiveness of control measures. In countries where the Euro 4 fleet is still young, comprehensive changes are recommended for the on-board diagnostic system and the subsequent maintenance of the hardware and software of EGR valve.

∗

Corresponding author. E-mail address: [email protected] (T. Lee).

https://doi.org/10.1016/j.atmosenv.2019.01.032 Received 17 July 2018; Received in revised form 14 January 2019; Accepted 19 January 2019 Available online 25 January 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved.

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1. Introduction The Seoul Metropolitan Area, the most populated region in Korea, is a non-attainment area for nitrogen dioxide (NO2). Almost 40% of the ambient air monitoring sites in this area reported an NO2 concentration higher than ambient air quality standards (0.06 ppm average over a 24 h period) (NIER, 2017a). This standard was exceeded for 30 days in 2016; the mean NO2 concentration for those days was 0.07 ppm. Furthermore, there was a seasonal trend, with 70% of the days occurring between November and February. Road transport contributes 49% of nitrogen oxide (NOx) emissions in the city of Seoul, of which heavy- and light-duty diesel vehicles (LDVs) account for 40% and 34%, respectively (NIER, 2017b). For Korean LDVs, the NOx limits decreased from 0.5 g/km (Euro 3, 2005) to 0.08 g/km (Euro 6, 2014) under the New European Driving Cycle (NEDC)-based type-approval test (EU, 2008; Korean MOE, 2011).1 The emissions limits for particulate matter (PM) were also decreased from 0.05 g/km (Euro 3, 2005) to 0.005 g/km (Euro 6, 2014). However, the effectiveness of emissions regulations are severely weakened because the on-road NOx emissions from LDVs often exceed the limits applicable to them several times over (Franco et al., 2014; Kadijk et al., 2015a,b, 2016; Kubota, 2016; Kwon et al., 2017; U.S. EPA, 2015; Weiss et al., 2011, 2012). Anenberg et al. (2017) reported that across 11 major vehicle markets, including the European Union, China, and the United States, over half of all on-road LDV NOx emissions exceeded certification limits by a factor of 3.2–5.7, depending on their applicable emission limits. In response, new LDV models registered from September 2017 onwards in Korea are subject to the Real Driving Emissions (RDE) test procedures (Korean MOE, 2016), which are similar to those used in the European Union (EU, 2016). The allowable NOx limit for on-road driving is 2.1 times the lab-based emission limit. The RDE test measures NOx, carbon dioxide (CO2), and carbon monoxide (CO) emissions using a portable emissions measurement system (PEMS) while a test vehicle runs on a real road. This will help to verify emission performance during on-road driving as well as in the laboratory. The Worldwide harmonized Light vehicle Test Cycle (WLTC) and the associated Worldwide harmonized Light vehicle Test Procedure (WLTP) replaced the NEDC-based test procedure for type-approval for LDVs in Korea and in the European Union (EU, 2017; Korean MOE, 2017a). The WLTC is considered more representative of real driving conditions than the NEDC (Tutuianu et al., 2015). For in-use LDVs, the Korean government decided to check tailpipe NOx concentrations and visible smoke during biannual Inspection & Maintenance (I/M) programs; currently, the latter is the only item checked in LDVs (Korean MOE, 2017b).2 However, the RDE, WLTP, and I/M NOx checks are all applicable to Euro 6 LDVs, and many LDVs on the road are still Euro 5 and older; their NOx emissions may be much higher than the associated limits. The Low Emission Zone (LEZ) is a policy measure intended to reduce the harmful impact of existing vehicles by restricting the entry of those that do not fulfill the minimum emissions standard (Ellison et al., 2013; Holman et al., 2015). The Voluntary Accelerated Retirement Program (VARP), which encourages vehicle owners to retire their older, more polluting vehicles through a financial incentive from the government, seeks to reduce emissions by replacing the vehicles (Antweiler and Gulati, 2015; Lavee et al., 2014; Lorentziadis and Vournas, 2011); it is also considered an option.

One of the key elements of the LEZ or VARP is setting the eligibility of vehicle fleets based on vehicle emissions performances.3 Euro 4 LDVs are allowed within the LEZ in Seoul, and are not covered by the VARP incentive (City of Seoul, 2017). Euro 4 LDVs were manufactured in Korea from 2006 to 2010, make up a fairly large share (26%) of the Korean LDV fleet (NIER, 2017b), and typically control NOx emissions using an exhaust gas recirculation (EGR) system (Bermudez et al., 2011; Millo et al., 2012). Some manufacturers have equipped diesel particulate filters (DPFs) in Euro 4 LDVs to reduce PM, although these are not always used as a primary emissions reduction strategy (ICCT, 2014). DPFs capture > 99% of diesel PM emissions by mass (Khalek et al., 2012). For PM, the Euro 4 LDVs equipped with DPFs are cleaner than Euro 3 and older vehicles and are acceptable within the Seoul LEZ. For NOx, however, Korean Euro 4 LDVs are quantified mainly through laboratory tests, but on-road performance is rarely validated. The lab-based NOx emissions factors (EF), which usually express the mass emissions of a pollutant per vehicle trip distance and are often used to quantify vehicle emissions performance, do not always capture real-world on-road vehicle emissions. Holman et al. (2015) indicated that the uncertainty regarding the real-world NOx EFs of diesel vehicles affect the prediction of the potential impacts of the LEZ. Underestimation of LDV NOx has been discovered and reported in previous studies through comparisons of on-road measurements with COPERT EFs (Ntziachristos and Samaras, 2016) in Europe as well as China (Kousoulidou et al., 2013; O'Driscoll et al., 2016; Shen et al., 2015). Low ambient temperature, e.g., around 0–5 °C, causes even greater NOx emissions owing to deactivation of the EGR valve for safety reasons (Department for Transport, 2016; Kadijk et al., 2016; Kwon et al., 2017). In terms of regulation, Ntziachristos et al. (2016) estimated the gradual reduction of the gap between real-world NOx emissions and EFs via RDE regulation, while Triantafyllopoulos et al. (2018) assessed the effectiveness of RDE regulation. In terms of the vehicle technologies, previous studies using PEMS measurements tended to focus on recent Euro 5 and 6 LDVs. Previous research conducted in China evaluated the on-road NOx characteristics of previous models, including old diesel taxis with model years ranging from 1998 to 2009 (Hu et al., 2012), Euro 0 to 3 LDVs (Huo et al., 2012), and Euro 3 and 4 LDVs (Yao et al., 2015). Additionally, Shen et al. (2015) summarized the results from more than 300 diesel vehicles that were tested in China, ranging from Euro 0 to Euro 4. In regard to maintenance, however, few data are available on possibly malfunctioning old Euro 4 vehicles. PEMS are among the most popular measurement units for on-road EFs. Additionally, the compact sensor-based on-board measurement system, Smart or Simplified Emissions Measurement System (SEMS), offers a cost-effective alternative to PEMS that makes large-scale monitoring possible (EC, 2017; Kadijk et al., 2016). Remote sensing is also used for on-road NOx measurements for monitoring (Bishop et al., 2001; Carslaw et al., 2011), for characterizing NOx degradation (Chen and Borken-Kleefeld, 2016), for developing distance-based mass EFs (Bernard et al., 2018), and for screening the emission performance of in-service fleets (EC, 2017). Euro 4 vehicles are relatively old in both Korea and the European Union, making it likely that malfunctions will occur in EGR valves, DOCs, or DPFs. Problems with DOCs or DPFs can be identified relatively easily from the visible smoke check during I/M, however faulty EGR

1 Korean emissions limits and test procedures for LDVs are similar to the European Euro standards. 2 The NOx check during LDV I/M will be subject to 2018 models and later Euro 6 LDVs in Korea. The first check will occur four years after from the registration of the first such vehicle. Thus, the first NOx check for LDVs under I/M will be conducted in 2022 in Korea. The NOx check is not applicable to Euro 5 and older LDVs.

3 For example, Transport for London (TfL) set Euro 4 as minimum standard for its LEZ in central London (TfL, 2017). From October 2017, TfL enforced a daily emissions surcharge of 11.5 GBP per day for pre-Euro 3 vehicles in the LEZ. The LEZ in Seoul now restricts Euro III medium- and heavy-duty diesel vehicles, and the city is considering expanding the restriction to Euro 3 LDVs (City of Seoul, 2017). The Korean government provides the VARP incentive for Euro 3/III and older diesel vehicles.

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valves are not detected because NOx is not checked during I/M for Euro 4 LDVs. The On-Board Diagnostic (OBD) system in a vehicle identifies malfunctions and deteriorations that cause emissions to exceed certain thresholds, and upon detection of these notifies the driver via the flashing malfunction indication lamp (MIL) on the dashboard (Cackette, 2016). However, the technical specifications of OBD systems in Euro 4 LDV monitors focus mostly on the state of the electrical circuits in the EGR systems and may not fully capture a gradual degradation in performance (Delphi Technologies, 2018). In-use compliance tests may not be applicable, because the legislated warranty period4 for emissions is likely to have passed for most Euro 4 LDVs. Remote sensing could be the most feasible way to identify LDVs with high NOx emissions, although few provisions have been implemented to enforce EGR repair or maintenance based on remote sensing data. Although Euro 4 regulation will fade out in Korea and the European Union, it is still active in some countries. In India, Bharat standards IV (based on Euro 4) applied to all new vehicles nationwide from April 2017. Vietnam and Thailand have followed Euro 4 emissions standards since 2017 and 2012, respectively. In Indonesia, Euro 4 emission standards are scheduled to be applied to diesel vehicles from April 2021 (Transport Policy). The objective of this paper is to evaluate the current NOx EFs for Euro 4 LDVs in Korea. We compare on-road NOx emissions data from four Euro 4 LDVs with the values derived from the corresponding labbased Korean EFs. This will quantify the degree of underestimation generated by the differences inherent in the methodology used to develop EFs. We also pay particular attention to the condition of the EGR valve, and the consequent NOx emissions, to estimate hidden or missing NOx levels caused by limited capabilities to identify and repair malfunctioning vehicles.

2.2. Test equipment We used chassis dynamometers and bench emission analyzers for the LE tests, PEMS for the ORE tests, and a rig tester to check the performance of the EGR valves. 2.2.1. Laboratory emissions (LE) test equipment The key instruments included a chassis dynamometer, a dilution tunnel, and an emissions analyzer. The vehicle emission tests over the driving cycles were conducted on a 48 in. (118 cm) chassis dynamometer.5 The second-by-second vehicle speed was calculated using the rotating speed of a dynamometer roller. Raw exhaust gas was diluted by a factor of approximately 10–40, depending on driving conditions, in a dilution tunnel. The bag-averaged NOx and CO2 in the diluted exhaust gas and ambient background sample were measured using an emissions analyzer and then combined with tunnel flow rates to obtain mass emissions.6 A chemiluminescence detector was used to detect NOx and a non-dispersive infra-red detector was used for CO2. Gravimetric filterbased PM emissions were measured in LE tests. 2.2.2. On-road emissions (ORE) test equipment We used two PEMS to measure OREs: one was the SEMTECH-LDV manufactured by Sensors Inc. and the other was the M.O.V.E manufactured by AVL LIST GmbH. A PEMS was installed in the cabins of the tested vehicles. A PEMS system consists of an exhaust gas volume flowmeter, an exhaust gas-sampling device, a gas analyzer, a power supplying system, a control system, and a data analysis system. Realtime 1 Hz exhaust concentrations were coupled with exhaust flow rates and converted to mass of exhaust gas data. The vehicle exhaust flow rates were measured using an exhaust flowmeter based on Pitot tube technology. Temperature and pressure sensors monitored the ambient temperature and barometric pressure, respectively, and a global positioning system device continuously logged the vehicle's speed, altitude, latitude, and longitude. The vehicle's operating variables, including engine rotating speed, percent of engine load, and EGR opening duty commands, were recorded via the OBD communication link. PM was not measured in the ORE tests.

2. Materials and methods We measured NOx and CO2 emissions from LDVs over pre-defined driving cycles in a laboratory and in real road tests. The purpose of the laboratory emissions (LE) test was to develop laboratory-based EFs for these selected vehicles using a conventional Korean methodology. The purpose of the on-road emissions (ORE) test was to quantify the characteristics of emissions over several 1 km trips. The results of the LE and ORE tests were compared over a range of vehicle speeds, accelerations, and EGR valve conditions.

2.2.3. Exhaust gas recirculation (EGR) valve rig tester We inspected the failed EGR system of v3F, once it was uninstalled using the part supplier's quality control facilities, by examining the gas flow through it as shown in the Supporting Information (SI). The rig tester applies a differential pressure of 500 mbar between the inlet and outlet of the EGR valve to simulate working conditions. The tester's engine control unit (ECU) simulator provided the command needed for the valve actuator to open the EGR valve. The test results showed the flow rate and flow specification that passed through the valve according to the command from the EGR duty.

2.1. Test vehicles We selected the four Euro 4 LDVs shown in Table 1; upon inspecting the EGR valve condition of each, we found that Vehicles 2, 3, and 4 had faulty EGR valves. A faulty EGR valve was identified by visual inspection and a gas flow rate test, as discussed in section 2.2.3 and 3.1. After the emissions tests were conducted on the vehicles in “as is” conditions, we replaced the EGR valve assembly (which consists of the EGR valve, an actuating solenoid, and an exhaust gas cooler) in Vehicles 3 and 4 and retested them. For clarity, we gave each vehicle an ID based on the condition of its EGR valve, with the letter “F” denoting the malfunctioning EGR. For example, “v3F” stands for Vehicle 3 with a faulty EGR valve (before maintenance) and “v3” stands for Vehicle 3 with a normal EGR valve (after maintenance). Vehicles 1, 2, and 3 were tested at the National Institute of Environmental Research (NIER) and Vehicle 4 was tested at the Korea Automotive Technology Institute (KATECH). The test fuel was ultra-low sulfur diesel, which has a sulfur content of less than 10 ppm.

4

2.3. Test driving cycles and routes A driving cycle is quantified based on second-by-second speed versus time. In the LE tests, vehicle emissions were measured while the test vehicle was operated on a chassis dynamometer according to each of selected driving cycles. We used the NIER suite, which is a set of 15 sub-cycles that have been used as the Korean standard for EF development since 2001 (NIER, 2001). Each of the sub-cycles has a different cycle-average vehicle speed, as shown in the SI. In Korea, the functional 5 The dynamometer used at NIER was an AVL LIST GmbH AN4652 (Graz, Austria), and the one used at KATECH was an AVL LIST GmbH RPL1220 (Graz, Austria). 6 The bench type emission analyzer used at NIER was a Horiba Ltd. MEXA 7200H (Kyoto, Japan), and the one at KATECH was an AVL LIST GmbH i60 (Graz, Austria).

Five years or 80,000 km, whichever comes first. 214

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Table 1 Key specifications for four selected light-duty diesel vehicles. Test vehicles a

Vehicle

Condition of the EGR valve

ID

Vehicle 1 Vehicle 2 Vehicle 3

Normal Failure Normal Failure Normal Failure

v1 v2F v3 v3F v4 v4F

Vehicle 4 a b c

Vehicle description

Engine capacity [L]

Model year

Accumulated mileage [km]

Emission control system

Van SUV SUV

2.0 2.0 2.0

2009 2010 2008

228,449 168,768 230,771

EGR, DOCb EGR, DOC, DPFc EGR, DOC

SUV

2.0

2008

201,575

EGR, DOC, DPF

EGR: exhaust gas recirculation. DOC: diesel oxidation catalyst. DPF: diesel particulate filter.

relationship between the emission result [g/km] and the average vehicle speed [km/h] of the NIER cycles–for example, a least-squared regression curve–is calculated as the EF of a given vehicle category. In practice, we tested seven to nine sub-cycles to build an EF instead of testing the whole package from the NIER suite. We also used type-approval test results measured under the WLTP, using emissions results and speed of four separate WLTC phases: Low, Middle, High, and Extrahigh. In this study, we used the WLTC instead of NIER cycles for v3F. The routes selected for the ORE tests comprised urban and rural roads and a motorway, as shown in the SI. The routes were designed based on RDE test requirements (EU, 2016). The two routes near NIER (R1, R2) are located in the Seoul Metropolitan Area and the other two routes, near KATECH (R3, R4), are located in the city of Cheonan. The nominal distances of the four test routes R1, R2, R3, and R4 were 71, 70, 100, and 77 km, respectively.

[g]. Then, EF [g/km] for each trip segment was calculated. There was no overlap among the trips. We were able to generate a sort of moving averaging window method based on RDE regulations. However, we employed the 1 km across which the vehicle travelled as the reference variable for averaging its windows, rather than using the reference value for CO2 emissions (Kousoulidou et al., 2013; O'Driscoll et al., 2016). For more details in regard to the data management methodology, please refer to Lee et al. (2013). Several 1 km trips from the ORE test allowed us to investigate the characteristics of the EFs for a varying range of vehicle speeds and accelerations; the shorter the averaging period, the larger the variability in the emissions data. We characterized the operating conditions of each vehicle by the average speed of each trip [km/h] and the Relative Positive Acceleration (RPA) [m/s2]. The RPA can be interpreted as the specific acceleration work of the trip (Demuynck et al., 2012) and is often used as a factor to compare different test cycles or trips (Barlow et al., 2009; Lee et al., 2013). It is calculated as the integral of the product of instantaneous speed and instantaneous positive acceleration over the time spent in each trip, as shown in Equation (1) below.

2.4. Collection of emissions data In the LE tests, we tested vehicles under the type-approval test protocol (EU, 2008) which, for example, controls the ambient temperature at 25 ± 5 °C. The hot engine start was one exception to this; before beginning every LE test, we preconditioned the vehicle by running it for 5 min at a constant speed of 70 km/h. The ORE tests were conducted under an ambient temperature ranging from 11 to 29 °C. All vehicles were warmed up before the PEMS measurements began. The cold start emissions were not included in the LE or ORE tests, because Korean lab-based EFs are intended for hot-running conditions. The PEMS analyzer was calibrated using zero- and span-gas before and after each ORE test. The duration of the trips ranged from 97 to 120 min. We validated the PEMS measurement by comparing the result of the bench emission analyzer over a WLTP on the chassis dynamometer for each PEMS installation. On average, the NOx and CO2 emissions measured by the PEMS were 6.8 and 1.3% higher, respectively, than the standard bag-averaged values; these are within the permissible tolerances in RDE regulation (EU, 2016).7

RPAj =

1 xj

tj

(vi, j × ai+, j ) dt 0

(1)

where: RPAj = relative positive acceleration of trip j [m/s2]; xj = travel distance of trip j [m], i.e., 1000 m; tj = time duration of trip j [s]; vi,j = instantaneous speed at time i in trip j [m/s]; 2 a+ i,j = instantaneous positive acceleration at time i in trip j [m/s ]. Additionally, vehicle specific power (VSP) is a powerful explanatory variable for this purpose (Jimenez-Palacios, 1998). We did not choose VSP because it is more suitable for a second-by-second based approach than for the 1 km trip segment basis approach. We calculated an activity-weighted average of the emissions of each EF function (for the LE) or the distribution of EFs (for the ORE) as shown in Equation (2) and denoted this value the combined EF.

2.5. Data analysis We calculated the cycle-averaged LE [g/km] using the bag-averaged emissions [g] and the total distance over the entire trips of the selected cycles [km]. Lab-based EFs and a speed dependent regression curve for the measurement results were calculated for each vehicle with both the normal and faulty EGRs. For the ORE tests, we divided the entire trip into 1 km segments based on 1 Hz vehicle speed data. Subsequently, we integrated secondby-second emissions over each 1 km segment to estimate mass emission

13

cEFj, k =

EFi, j × i=1

Ai, k Nk

(2)

where: cEFj,k = combined EF of pollutant j in road traffic conditions k [g/ km]; EFi,j = emissions value from the EF function at the center of each discrete speed bin i for pollutant j [g/km] (For deriving LE combined EF) OR mean value of the distribution of EFs at the center of each discrete

7 For NOx in [mg/km], ± 15 mg/km or 15% of the laboratory reference, whichever is larger. For CO2 in [g/km], ± 10 g/km or 10% of the laboratory reference, whichever is larger.

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speed bin i for pollutant j [g/km] (For deriving ORE combined EF); Ai,k = the number of trips in the combination of discrete speed bin i under road traffic conditions k contained in the reference activity dataset [-]; Nk = the total number of trips in road traffic conditions k contained in the reference activity dataset [-]; Subscript i = the discrete speed bin for average speed [km/h] of each 1 km trips. 1 = [0,5), 2 = [5,15), 3 = [15,25), 4 = [25,35), 5 = [35,45), … 12 = [105,115), 13 = [115,125); Subscript j = pollutants; 1 = NOx, 2 = CO2; Subscript k = traffic conditions; 1 = urban road, 2 = rural road, 3 = motorway. The combined EF contains traffic conditions, since the activity we choose for the activity-weighted average represents real-world Korean on-road driving situations.8 It is expressed as a form of the normalized number of 1 km trips at varying vehicle speeds in urban, rural, and motorway road traffic conditions, as shown in the SI. The distances travelled on urban and rural roads and a motorway in the Seoul area typically make up 50.6, 18.0, and 31.4%, respectively, of the share of trips taken by a vehicle (NIER, 2010).

Fig. 1. Laboratory-based emission factors for NOx and CO2 emissions, derived from the chassis dynamometer emissions measurement test, according to the exhaust gas recirculation (EGR) valve conditions. The “current EF” denotes the NOx emission factor for Euro 4 LDVs currently used in the official Korean emissions inventory system. We used the NOx emission factor function form for COPERT Euro 4 diesel passenger cars to generate a plotted fit curve of the data in this study (Ntziachristos and Samaras, 2016).

3. Results and discussion We compared the EFs derived from LE and ORE test in terms of driving conditions and EGR valve conditions, quantified the underestimated NOx, and evaluated the contributing factors.

increased. CO2 emissions were less affected by EGR valve failures; the emissions varied by less than 5% by EGR valve conditions, and when the EGR valve malfunctioned they decreased slightly under urban and rural traffic conditions (Table 3). PM emissions generally increased when EGR worked well, i.e., when more exhaust gas returned to combustion chamber. Based on results from the WLTP, gravimetric filter-based PM emissions decreased by 59% (0.044 g/km to 0.018 g/km) when the EGR failed in the non-DPF equipped Vehicle 3; this is indicative of the PMNOx tradeoff. We did not observe any significant PM difference in the DPF-equipped Vehicle 4.

3.1. Conventional emissions factors based on LE tests The lab-based NOx EFs for the normal EGR case was close to the current “official” Euro 4 NOx EF (Fig. 1). This similarity implied that selected vehicles would be a good representation of the NOx emission characteristics of the Euro 4 LDV fleet, whose emissions were quantified under the Korean inventory framework. The number of test vehicles was small, which has implications for the uncertainty of emission factor estimations. The uncertainty of NOx emission factors was estimated as 91.6, 61.8, and 38.8% for vehicle speeds of approximately 20, 30, and 60 km/h, respectively. The uncertainty is defined here as the statistical spread of the emission value (two standard deviations) divided by the expected value, i.e., the emission factor value (IPCC, 2001; Kraan et al., 2012). For CO2, the uncertainty was estimated as 27.0, 19.0, and 12.5% for vehicle speeds of approximately 20, 30, and 60 km/h, respectively. We use this lab-based empirical EF as the baseline in the rest of this study. NOx emissions increased by a factor of almost three when the EGR valve failed (Table 2). For faulty EGR cases, the uncertainty of the NOx emission factor was estimated as 29.7, 24.4, and 12.1% for vehicle speeds of approximately 20, 30, and 60 km/h, respectively. For CO2, the uncertainty was estimated as 3.0, 9.1, and 24.8% for vehicle speeds of approximately 20, 30, and 60 km/h, respectively. Gas flow rates through the failed EGR valve were 38–82% lower, depending on the EGR duty commands, than those specified by manufacturers of the part (Fig. 2). Rust and soot particles contaminated the moving parts of the valve and increased the friction resistance, as shown in the SI. The actuating power of the solenoid did not seem to be enough to open the contaminated valve. The EGR valve became stuck, the exhaust gas failed to recirculate into combustion chamber, and NOx emissions

3.2. On-road EFs from ORE tests The selected LDVs showed variable NOx emissions characteristics at varying vehicle speeds (Fig. 3). The mean emissions at each speed generally decreased in the low to medium speed ranges, or 10–40 km/h and 50–80 km/h, respectively. At high speeds (90 km/h and faster), v4 and v3F showed increasing NOx emissions trends while the others showed emissions levels similar to those seen at medium speed. Most 1 km trips showed higher NOx emissions than the LE EF, with their relative quantities varying according to speed and the condition of the EGR (Fig. 4). In other words, the effects of EGR valve failure on increasing NOx emissions are different for the LE and the ORE. In the normal EGR, the ratios of the NOx emissions for individual ORE trips to those of the lab-based EF were 3.3 ± 1.7, 3.4 ± 2.3, and 2.0 ± 1.8 at the low, medium, and high speed ranges, respectively. This excessive emission level is true when we consider the range of uncertainty of the LE EFs. Thus, we concluded that the uncertainty in regard to the LE EF does not significantly affect the comparison of LE and ORE EF, nor does it significantly affect the subsequent discussion in this paper. These ratios decreased by around 50% when the EGR valve failed, resulting in values of 1.7 ± 0.6, 1.6 ± 0.7, and 1.1 ± 0.6 for low, medium, and high speeds, respectively. These trends were similar for the combined EFs from the LE and ORE tests. In the LE test, the effects were so distinct that the LE NOx increased almost threefold. However, the overall NOx emissions increase by only around 20% in the ORE test (Table 2). The CO2 emissions for the trips were relatively less affected, as

8 The activity is a 1 Hz vehicle speed database of six light-duty vehicles for approximately 36,000 km of driving on urban and rural roads and a motorway in the Seoul area (NIER, 2010). These data were collected in 2010 under the suggested protocol of the sub-group on the Development of the Harmonized driving Cycle in the informal working group on Worldwide harmonized Light vehicles Test Procedure (Tutuianu et al., 2015).

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Table 2 Combined NOx emission factors under conditions of urban, rural, and motorway traffic, compared according to test type and exhaust gas recirculation (EGR) valve condition. Test type

Laboratory Emissions (LE) (g/km)a

On-Road Emissions (ORE) (g/km)

Ratio of ORE to LE (−)

Traffic condition

Ub

R

M

U

R

M

U

R

M

v1 v3 v4

1.020 0.994 0.994

0.541 0.516 0.516

0.493 0.525 0.525

2.585 2.451 4.595

1.489 1.184 2.776

0.992 1.027 2.613

2.53 2.47 4.62

2.75 2.30 5.38

2.01 1.96 4.98

Mean

1.003

0.524

0.514

3.210

1.816

1.544

3.20

3.47

3.00

v2F v3F v4F

2.478 2.478 2.478

1.406 1.406 1.406

1.504 1.504 1.504

3.053 3.718 4.164

2.077 2.007 2.454

1.929 1.665 2.309

1.23 1.50 1.68

1.48 1.43 1.75

1.28 1.11 1.54

Mean

2.478

1.406

1.504

3.645

2.179

1.967

1.47

1.55

1.31

2.47

2.68

2.93

1.14

1.20

1.27

Normal EGR

Faulty EGR

Ratio of Faulty EGR to Normal EGR (−) a b

The emission values in the LE and ORE sections were computed using Equation (2). “U,” “R,” and “M” denote “urban,” “rural,” and “motorway” traffic conditions, respectively.

the value of the underestimated NOx in the current Korean inventory framework. To evaluate this gap, we investigated why the LE tests were so unrepresentative and why the ORE results were not very sensitive to EGR valve conditions for these selected vehicles. 3.2.1. Representativeness of the LE tests The discrepancies observed between the vehicles in the LE and ORE tests revealed the weak point in the Korean EFs in terms of capturing the on-road NOx emissions characteristics of LDVs. The Korean EFs express emissions [g/km] using a function of vehicle speed [km/h]. However, the deviations in the NOx emissions vehicles travelling at the same speed ranges implies that another explanatory variable might be required to make more representative estimates. We found that the deviations between the emissions recorded in the ORE and LE tests showed a generally similar trend to RPA distribution: the distribution observed in ORE test was generally higher than in the LE test (Fig. 5).

Fig. 2. Flow rate through normal and faulty exhaust gas recirculation (EGR) valves and their specifications according to the EGR valve duty command. The specification indicates a lower limit for the flow rate at the given EGR duty command. The higher the EGR duty command, the wider EGR valve is open.

Table 3 Combined CO2 emission factors under urban, rural, and motorway traffic conditions, compared according to test type and exhaust gas recirculation (EGR) valve conditions. Test type

Laboratory Emissions (LE) (g/km)a

On-Road Emissions (ORE) (g/km)

Ratio of ORE to LE (−)

Traffic condition

Ub

R

M

U

R

M

U

R

M

v1 v3 v4

302.3 295.9 295.9

190.9 183.5 183.5

170.4 183.6 183.6

365.3 323.7 290.1

258.5 210.8 194.6

210.7 186.5 181.3

1.21 1.09 0.98

1.35 1.15 1.06

1.24 1.02 0.99

Mean

298.0

186.0

179.2

326.4

221.3

192.9

1.10

1.19

1.08

v2F v3F v4F

279.5 279.5 279.5

176.6 176.6 176.6

181.0 181.0 181.0

278.0 337.9 296.9

192.6 218.7 195.3

174.1 190.6 183.4

0.99 1.21 1.06

1.09 1.24 1.11

0.96 1.05 1.01

Mean

279.5

176.6

181.0

304.3

202.2

182.7

1.09

1.15

1.01

0.94

0.95

1.01

0.93

0.91

0.95

Normal EGR

Faulty EGR

Ratio of Faulty EGR to Normal EGR (−) a b

The emission values in the LE and ORE sections were computed using Equation (2). “U,” “R,” and “M” denote “urban,” “rural,” and “motorway” traffic conditions, respectively.

We collected data from 1 km trips at speeds of 40–60 km/h to examine the relationship between RPA and NOx emissions. The relationships in Fig. 6 show that NOx emissions have a positive dependent relationship with RPA. The regression curves explain that the underestimation of NOx is partially attributable to the low RPA used in the NIER cycles. The coefficients of determination (R2) range from 0.14

shown in Table 3 and in the SI. The ORE trips showed CO2 emissions that were on average 18% and 19% higher than those from the LE at low and medium speeds, respectively. In the high speed range, the CO2 emissions from the ORE were lower than those in the LE by 12%. These deviations were almost the same for both EGR conditions. The gap in the NOx emissions between the LE and ORE is considered 217

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Fig. 3. Variability of the NOx emission factors from 1 km trips taken during On-Road Emissions tests and comparisons to laboratory-based emission factors, according to vehicle speed. The three panels in upper row are for vehicles with a normal exhaust gas recirculation (EGR) valve; those in the lower row are for vehicles with a malfunctioning EGR valve.

Fig. 4. Relative variability of the NOx emission factors of 1 km trips taken during On-Road Emissions tests, normalized by the laboratorybased emission factors, according to vehicle speed. The three panels in upper row are for vehicles with a normal exhaust gas recirculation (EGR) valve; those in the lower row are for vehicles with a malfunctioning EGR valve.

weak R2 reveals the inherent limitations of the lab-based driving cycle, and suggests that developing EFs using ORE measurements would be a more effective solution to fully capture the real driving behavior of a vehicle. 3.2.2. Sensitivity of the ORE results Among the test vehicles, v4 showed some interesting behavior: the NOx emissions did not decrease after the EGR valve was repaired. This unexpected result implied that the hardware performance of the EGR valve was not the only factor governing the effectiveness of NOx emissions reduction through the EGR system. After the “hardware” is repaired, the valve is ready to carry out commands from the “software,” and the ECU commands the EGR valve to open or close through the EGR duty command. In the ORE, the ECU of v4 did not command the EGR valve to open as far as it did in the LE test (Fig. 7). In terms of the EGR command signal, the EGR operation of v4 was practically independent of the hardware conditions of the EGR valve, and the latter was closed under almost all operating conditions during the ORE test. This demonstrates that if repairs in hardware are not accompanied by corresponding improvements in software, the intended emissions reductions do not necessarily occur. This behavior causes quantitative differences in emissions. The EGR

Fig. 5. Variability of relative positive acceleration (RPA) of 1 km trips taken during the On-Road Emissions tests, compared to the cycle-specific RPA values of standard laboratory test cycles.

to 0.37, which are not very strong, because many factors affect the NOx emissions in the ORE compared to those from the RPA; for example, the ECU manages the electronic control schemes in the vehicle dynamically to cope with various on-road driving situations. The present assessment suggests that increasing the RPA level of the NIER cycle is one possible action to resolve this underestimation. At the same time, however, the 218

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however, they may not cover every aspect of EGR failure. For example, we believe that most reported claims concern “open” valve problems, not complaints that occur due to failures from “closed” valves.9 Drivers can hardly be aware when the EGR valve was always closed, because drivability of the vehicle may not have been negatively impacted in those conditions. Neither we nor the car owners could identify the EGR valve problem in v2F, v3F, and v4F, until we uninstalled the EGR valves and checked them again. There was no flashing OBD MIL on the dashboard, no sign of worsened drivability, no warning of the possibility of malfunction from the vehicle manufacturers or government authorities, and no notice of high emissions based on remote sensing surveillance. The lack of tools with which to screen for EGR failure can be interpreted as a weak point in management, since excessive NOx emissions from Euro 4 LDVs could increase as Euro 4 LDVs age. For example, if all cars such as v3 became like v3F, NOx emissions increase by 56%. On the other hand, Vehicle 4 did not show NOx degradation with EGR failure rates. As a composite result of two distinctive degradation trends for Vehicles 3 and 4, the fleet in this research showed only a 17% increase in NOx even when all vehicles’ EGR valve hardware was malfunctioning. This small amount of degradation correlates with previous research based on larger data sets. For example, Chen and Borken-Kleefeld (2016) analyzed about 44,000 remote sensing measurement records in Switzerland and reported that Euro 4 diesel cars had negligible NOx deterioration with increased mileage. Vehicles that had failures such as those in Vehicle 4 do not seem to deteriorate with increased usage; however, this is not because their hardware is not degraded; rather, their software commands are so limited that they are not strongly affected by hardware degradation. The negligible NOx degradation observed in a large fleet implies that the case of Vehicle 4 may not be extraordinary, but in fact may be prevalent in the Euro 4 LDV fleet.

Fig. 6. The effect of relative positive acceleration (RPA) on NOx emissions for individual 1 km trips taken at 40–60 km/h during the On-Road Emissions (ORE) tests (small red, orange, and yellow points), mean speed vs. mean NOx emissions of selected trips taken during the ORE tests (large red circle, upside-down orange triangle; large yellow square), and mean speed vs. corresponding NOx emissions of standard laboratory test cycles travelling at 50 km/h (large white triangle). The regression results are listed in the Supporting Information. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

performances varied in similar transient driving conditions and ambient temperature conditions in the LE and ORE tests (Fig. 8 and Fig. S12 in SI). In comparison to the LE test, the consequent NOx emissions were almost 10 times higher in the ORE test in this selected example. In the 10–50 km/h speed range, EGR duty commands in the ORE test were consistently lower than in the LE test (Fig. S13 in SI). In regard to regulation, this observation of v4 might be interpreted as the presence of a defeat device because the EGR operation is deactivated during on-road driving, even under the normal ambient temperatures. Additionally, the overall consequent NOx emissions (Table 2) exceeded the emission limit (0.25 g/km) by 10–18 times, which were higher than the recommended threshold to qualify as a defeat device, which is 2–5 times the emission limit (EC, 2017).

4. Conclusion The purpose of this case study was to estimate the level of NOx emissions that could be underestimated when the conventional EFs for standard vehicle and existing surveillance schemes are the only tools used to identify malfunctioning vehicles. We quantified the excessive NOx emissions from Euro 4 LDVs, in varying traffic conditions and with EGR valves operating at different conditions. The findings suggest that lab-based single function EFs are the main factors behind most of the underestimations of NOx emissions. It is expected that increasing the RPA level of standard driving cycles would be an intermediate step towards improving the EF estimates. An ORE-based EF or a “pool” of ORE data could be more of a substantial substitute for the current EFs. PEMS measurements for RDE regulation and SEMS or remote sensing measurement for in-service fleet screening (EC, 2017) would facilitate the building of a database of ORE EFs faster and more easily than before. The difference between NOx emissions calculated based on ORE tests and those based on LE NOx emissions tests can be regarded as a hidden risk that is not quantified under current schemes to estimate, control, and manage emissions in Korea. NOx emissions in excess of Euro 4 standards should be reevaluated, not left underestimated as they currently are. This may be leading policy makers to ignore the harmful impact of Euro 4 LDVs, which is important since these vehicles are regarded as borderline acceptable in terms of emissions performance. For example, Korea currently allows Euro 4 LDVs to enter the LEZ in Seoul area and excludes them from the VARP incentive program. These

3.3. Underestimated NOx emissions and their potential risk The results of the LE tests when the EGR valve was normal (v1, v3, and v4) represent the Korean EFs currently in use. If we select EGR valve condition as a key variable in determining the NOx emissions from for LDVs, the level of NOx underestimated would depend on the failure rate, as illustrated in Fig. 9. The ORE results and EGR failures demonstrate two real-world aspects of LDV operation: on-road driving and maintenance conditions. For the four selected vehicles, the current Euro 4 EF was only able to estimate 31% of the NOx emissions from a normal, well-maintained LDV in real driving conditions. The amount of NOx quantifiable could even be as low as 27% of the real emissions for vehicles in which the EGR malfunctions. We anticipate that maintenance demands for the old Euro 4 fleet will increase; however, we do not have many feasible ways to identify EGR failures in that fleet. Vehicle manufacturers report emissions-related field claims, which are related to the functioning of the EGR valve, every quarter; this is similar to a procedure implemented by the California Air Resources Board and the United States Environmental Protection Agency (Cackette, 2016). These reports have revealed that the failure rate of the EGR valve of the selected car models was 3.7%;

9 Drivers claim for sudden engine stops (34% of total claims), loss of power (28%), rattling or rumbling in the vehicle (20%), and excessive smoke (16%). These are typical behaviors that occur when the EGR valve is stuck at the “‘open” position.

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Fig. 7. Distribution of the exhaust gas recirculation (EGR) duty commands (acquired through the On-Board Diagnostic communications link in the 1 Hz frequency) from the engine control unit (ECU) to the EGR valve actuator observed in On-Road Emissions test and selected Laboratory Emissions test driving cycles for v4. Left-hand panel = KATECH Route 1 (mean speed 65.9 km/h, max speed 112 km/h); center panel = NIER cycle #11 (mean speed 53.7 km/ h, max speed 74.9 km/h); right-hand panel = NIER cycle #7 (mean speed 24.6 km/h, max speed 72.1 km/h).

findings suggest that the eligibility of these vehicles should be qualified based on a more realistic risk potential. The results of our study are beneficial for the development of EFs outside of Korea, because few on-road measurements are available for Euro 4 LDVs (Shen et al., 2015). In terms of reevaluating EFs, the results are quite relevant to the COPERT model (Ntziachristos and Samaras, 2016), because the lab-based Korean EF shows a comparable level to that in the COPERT model (shown in Fig. S14 in SI). Although the EGR design, durability, and control strategies differ between Korean and European LDVs, it might be worthwhile to note that similar underestimation could be applied to the estimation of NOx emissions for Euro 4 diesel vehicles operating in the EU area. Malfunctioning EGR valves increase the level of underestimation of these emissions because the fault has not been properly identified and corrected in real situations. For countries in which the Euro 4 fleet has aged, the demand for maintenance of Euro 4 LDVs might increase and gaining a tangible benefit would require the ECU software for the EGR control and mechanical repair processes for the valve hardware to be

modified. However, it is recommended that countries in which Euro 4 LDVs are being newly introduced should implement a more comprehensive OBD monitoring requirement for the EGR valve. This would be a feasible and practical control measure because the technology and legal requirements are already available. For Euro 5 and 6 vehicles, the monitoring area expanded to EGR system efficiency, EGR flow, and cooler monitoring, in addition to the EGR system's circuit continuity (Delphi Technologies, 2018). Manufacturers tend to incorporate valve opening position sensors in the EGR valve in order to detect the degree to which the EGR valve is open and to provide feedback to the ECU in order to compare the actual location with the commanded location. Additionally, the power actuating module for EGR valves has been shifting from a solenoid actuator to an electric motor. The latter is stronger than the former in terms of its actuating force. For example, the actuating force of a DC-type electric motor used in the Euro 5 and 6 version of v3 is 450 N, which is higher than the 30 N in the Euro 4 version of v3. We expect these improvements to reduce the likelihood of

Fig. 8. Time plots for the exhaust gas recirculation (EGR) valve duty commands, vehicle speed, instantaneous and accumulated NOx emissions of v4 in “start-runstop” short trips that have similar driving characteristics observed in the On-Road Emissions test and Laboratory Emissions test. 220

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Fig. 9. Illustration of the underestimated realworld on-road NOx emissions for selected Euro 4 LDVs according to the exhaust gas recirculation (EGR) valve failure rates. The bars marked with solid diamonds were quantified based on the measured data. The values are the weighted averages of the combined EFs of urban, rural, and motorway roads. Weighting factors are 50.6, 18.0, and 31.4% for urban, rural, and motorway roads, respectively. The bars without solid diamonds were quantified by the interpolation of the 0% and 100% bars. Panel (a) illustrates which vehicles had failures like those in Vehicle 3. Panel (b) indicates which vehicles had failures like those in Vehicle 4. Panel (c) shows the fleets composed of the four vehicles tested in this research. The EGR failure rate is defined here as the number of vehicles that have a faulty EGR valve in the fleet of interest divided by the number of vehicles in the fleet of interest.

EGR valve malfunctions, inability to detect functional degradation of the EGR valve, and subsequent excessive NOx emissions of more recent Euro 5 and 6 LDVs, compared to older vehicles. Nevertheless, similar exploration using the approach applied in this research needs to be carried out for Euro 5 and 6 vehicles in order to assure the effectiveness of OBD monitoring. For both new and aged Euro 4 fleets, remote sensing could function as a full-time surveillance tool for LDVs with high emissions, including those with faulty EGRs. Developing the threshold limit for this purpose would be an important future research topic. The I/M check for tailpipe NOx concentrations and visual smoke measurements, as used in Korea, can also be considered strong measures to control NOx emissions from LDVs.

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Funding sources This research was funded by a grant from the National Institute of Environmental Research (NIER) [grant number NIER-2017-01-01-077], which is an institute affiliated to the Ministry of Environment of the Republic of Korea (Korean MOE). The views expressed in this paper are those of the authors and do not necessarily represent the official position of the Korean MOE. All authors (T.L., M.S., J.K., S.L., I.K., and Y.H. from NIER; B.L., J.C., and D.K. from KATECH) were involved in the study design and handling (collecting, analyzing, and interpreting) data. T.L., M.S., and Y.H. wrote the report and made the decision to submit the article for publication. Acknowledgements We thank the dedicated personnel at the National Institute of Environmental Research (NIER) and the Korea Automotive Technology Institute (KATECH) testing laboratory for all their assistance. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.atmosenv.2019.01.032. 221

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