Can Euro V heavy-duty diesel engines, diesel hybrid and alternative fuel technologies mitigate NOX emissions? New evidence from on-road tests of buses in China

Applied Energy 132 (2014) 118–126

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Can Euro V heavy-duty diesel engines, diesel hybrid and alternative fuel technologies mitigate NOX emissions? New evidence from on-road tests of buses in China Shaojun Zhang a,b, Ye Wu a,b,c,⇑, Jingnan Hu d, Ruikun Huang a,b, Yu Zhou a,b, Xiaofeng Bao d, Lixin Fu a,b,c, Jiming Hao a,b,c a

School of Environment, Tsinghua University, Beijing 100084, China State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China c State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China d Chinese Research Academy of Environmental Sciences, Beijing 100012, China b

h i g h l i g h t s  Euro V diesel buses reduce NOX emissions over Euro IV but exceed the limit by 180%.  Diesel hybrid and NG buses are better options to control NOX than Euro V diesel buses.  LNG engines plus SCR provide superior NOX control due to higher exhaust temperature.  Diesel hybrid is the only technology not significantly sensitive to average speed.  Air conditioning reduced NOX emissions of hybrid buses although increased fuel use.

a r t i c l e

i n f o

Article history: Received 10 December 2013 Received in revised form 10 April 2014 Accepted 4 July 2014

Keywords: Diesel hybrid bus Natural gas bus NOX Selective catalyst reduction Operating condition

a b s t r a c t Nitrogen oxides (NOX) emissions are creating significant air quality challenges in China’s megacities. Since Euro IV diesel buses equipped with selective catalyst reduction (SCR) systems failed to mitigate on-road NOX emissions as expected, real-world NOX emissions from newly introduced Euro V diesel buses, diesel hybrid and alternative fuel (e.g., natural gas) buses are of great concern to policymakers in China. In this study, NOX emissions from two Euro V diesel, two Euro IV diesel hybrid, nine compressed natural gas (CNG) and two liquefied natural gas (LNG) buses were measured on-road by using portable emission measurement systems (PEMS). The average NOX emission factor of the Euro V diesel buses was 7.5 ± 0.1 g km1 for a typical driving cycle, 37% lower than the Euro IV diesel buses. However, the average brake-specific emission factor still exceeded the Euro V standard by 180%. The diesel hybrid buses had an average NOX emission factor of 4.4 ± 1.1 g km1, much lower than their conventional diesel counterparts. CNG and LNG buses also had lower NOX emission factors. The average NOX emission factor of the LNG buses was 3.2 ± 0.7 g km1, due to the performance of the SCR systems under higher exhaust temperatures. Furthermore, real-world NOX emission factors for all tested vehicle categories except diesel hybrids were significantly sensitive to changes of average speed. Operation of air conditioning in the bus reduced average NOX emissions by 38% for diesel hybrid buses although fuel consumption increased. These results suggest hybrid and CNG/LNG technologies are better options than the Euro V diesel engines to mitigate NOX emissions from urban buses. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In Feb 2012, China’s State Council approved the national ambient air quality standard (NAAQS) amendment, establishing ambient ⇑ Corresponding author at: School of Environment, Tsinghua University, Beijing 100084, China. Tel.: +86 010 62796947; fax: +86 010 62773597. E-mail address: [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.apenergy.2014.07.008 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

limits for fine particle matter (i.e., PM2.5) and ozone (O3) concentrations as well tightening the limit for annual nitrogen dioxide (NO2) concentrations [1]. Many of China’s megacities face significant challenges to meet these NAAQS [2–4], Beijing, for example, is a densely populated metropolis with the largest registered vehicle population (i.e., 5.2 million in 2012) in China [5,6]. The remarkable growth of Beijing’s vehicle population has transformed the pollution pattern

S. Zhang et al. / Applied Energy 132 (2014) 118–126

from coal-based air pollution to a mix of coal- and vehicle-based air pollution over the past decade [2,7–10]. This shift requires a greater focus on stringent emission controls on motor vehicles to further mitigate emissions in China’s megacities (e.g., Beijing, Guangzhou) [2–4,7]. Among vehicle categories, the public bus fleet is one of the major targets for more stringent emission control in China’s megacities [11]. For example, the Euro IV emission standard was implemented for public buses in Beijing in July 2008, five years earlier than other heavy-duty diesel vehicles (HDDVs) (e.g., freight trucks and long-distance coaches) [2,4]. In 2008, the Beijing Environmental Protection Bureau launched a comprehensive vehicle emission measurement program, including hundreds of on-road tests for HDDVs using portable emission measurement systems (PEMS) [2,4]. Those on-road measurements together with other on-road studies identified considerable reductions of total hydrocarbons (THC), carbon monoxide (CO) and primary PM2.5 emission factors for diesel buses, as their emission standards became increasingly stringent from Euro II to Euro IV [4,12,13]. For example, the average PM2.5 emission factor for Euro IV buses during the tests was reduced by over 80% compared to Euro II [4]. However, for nitrogen oxides (NOX) which is a major precursor of regional O3 and secondary PM2.5, no improvement was observed; even Euro IV diesel buses with selective catalyst reduction (SCR) systems saw little to no improvement. NOX emission factors of diesel buses were all about 12 g km1 from Euro II to Euro IV under the typical driving cycle, the highest emission factor among all vehicle categories [2]. This has posed a considerable challenge to NOX emission control for the public bus fleet under low-speed driving conditions [4,13]. Although public buses composed only 0.5% of the total vehicle population in Beijing, they were estimated to be responsible for 19% of vehicle NOX emissions during 2010 [2]. Therefore, tremendous efforts have been made during the past five years to mitigate emissions from public buses, in particular to explore effective NOX emission control strategies. For example, the emission standard for diesel public buses in Beijing was tightened to Euro V in February 2013 [2,11], which is currently the most stringent emission standard for HDDVs in China. In addition to conventional fuel powered vehicles, diesel hybrid [6] and alternative fuel technologies, such as natural gas vehicles [14,15], biofuel vehicles [16,17] and battery electric vehicles [6,18,19], have been promoted in China. These diesel hybrid and alternative fuel vehicles diversify the vehicle technologies of the public bus fleet in Beijing and help to work toward the goals of cutting oil dependence and improving urban air quality [2,7,11]. Compressed natural gas (CNG) buses were introduced into the public bus fleet in Beijing in 1999. By 2010, over 3000 CNG buses were operating in Beijing, nearly 15% of the total public bus fleet [7,11]. In 2012, liquefied natural gas (LNG) buses were introduced into the bus fleet. Compared to CNG, LNG has higher energy density and lower investment in filling infrastructure, and the buses have longer service ranges [11,20,21]. Diesel hybrid buses and battery electric buses have been in service in Beijing since 2009 [7,11]. The stock of diesel hybrid public buses reached nearly 1000 vehicles in 2011. In Aug 2013, the Beijing municipal government has proposed an ambitious goal in the Clean Air Action Plan 2013– 2017 that the share of alternative fuel buses in the public bus fleet would increase to 65% by 2017 [2]. CNG/LNG buses, Euro V diesel buses and electric buses are soon going to constitute a major part of the public bus fleet in Beijing [2,22]. Therefore, considering the failure of on-road NOX emission control for Euro IV diesel buses, the real-world emissions and potential impacts of operating conditions should be carefully evaluated for more advanced diesel buses and alternative fuel technologies. In this study, we collected on-road measurement profiles for 15 urban public buses in Beijing, which covered almost of the recently

119

promoted technologies, including Euro V diesel, diesel hybrid, CNG, and LNG engines. Instantaneous emission rates of NOX as well as THC, CO and CO2 were measured using PEMS and further related to on-road driving conditions using an operating mode binning methodology. We evaluated distance-based and fuel consumption-based emission factors under a typical driving cycle, with a particular focus on real-world NOX emissions. Furthermore, their relationships between real-world emission factors and operating conditions were explored, including average speed and air conditioning operation. Our objective is to provide a snapshot of on-road NOX emissions from diesel buses designed to achieve the most stringent limit in China as well as diesel hybrid and alternative fuel buses (e.g., CNG and LNG), and thus provide policy-makers with scientific support for further vehicle emission control and urban air quality improvement in the future. 2. Experimental section and data processing 2.1. On-road vehicle emission measurement The on-road emission tests were conducted during 2010–2012. The buses tested were from Beijing Public Transportation Holdings, Ltd. Vehicle information regarding the 15 measured buses is summarized in Table 1. All those tested buses had a vehicle length of 12 m and gross vehicle weight (GVW) of approximately 18 tons. Two conventional diesel buses of the same vehicle model were manufactured to Euro V emission standards (No. 1–2, see Table 1). Selective catalyst reduction (SCR) systems were installed in each Euro V diesel bus. Urea was used as the reducing agent in the SCR to control on-road NOX emissions. Two Euro IV parallel diesel hybrid buses manufactured in 2009 were also tested (No. 3–4). They were both equipped with a 165 kW diesel internal combustion engine and a 44 kW electric motor. Similar to the Euro IV and Euro V diesel buses in Beijing, urea-SCR systems were installed in the hybrid vehicle model. Nine CNG buses and two LNG buses were also measured in this study. Two models of CNG buses were recruited in this study with model years (MY) of 2007 and 2012 respectively. Those MY 2007 CNG buses (No. 5–12) were powered by a 147 kW stoichiometric spark-ignition engine. Three-way catalyst (TWC) after-treatment systems were installed for this vehicle type, which claimed to comply with the enhanced environmentally-friendly vehicle (EEV) emission standard. The MY 2012 CNG vehicle (No. 13) was also equipped with a stoichiometric sparkignition engine and a TWC system, which were manufactured to comply with Euro V emission standards. The LNG buses (No. 14– 15) were also powered by a stoichiometric ignition-spark engine. Unlike the CNG counterparts, they were equipped with urea-SCR systems that were primarily used to control NOX emissions. Two Sensor Inc. SEMTECH-DS PEMSs were used to measure instantaneous exhaust NOX emissions under real driving conditions with a non-dispersive ultraviolet analyzer [4,11,23]. Furthermore, the carbonaceous gaseous emissions were measured using the SEMTECH-DS PEMS (e.g., CO and CO2 measured by non-dispersive infrared analyser and THC measured by heated flame ionization detector) to calculate their real-time fuel consumption for further discussions [4,11]. The instantaneous volume rates of exhaust flow from the vehicle tailpipe were recorded by the SEMTECH exhaust flow meter (SEMTECH-EFM). Second-by-second vehicle speeds and location information for each bus were recorded by a GPS receiver [4,11]. The SEMTECH-EFM also employed a thermocouple temperature probe to measure the temperature of exhaust flow, approximately 3 m away from the tailpipe (i.e., tailpipe exhaust temperature). In particular, for the two diesel Euro V buses, a factory-produced special on-board diagnostics (OBD) decoder was applied to record real-world operating conditions, including both instantaneous engine conditions (e.g.,

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S. Zhang et al. / Applied Energy 132 (2014) 118–126

Table 1 Vehicle information of on-road tested bus samples. Vehicle sample number

Fuel type

Emission standard

Vehicle model

Engine manufacturer and model

After-treatment

Model year

GVW (tons)

Engine power rating (kW)

Bus operation route

1 2 3

Diesel Diesel Hybrid diesel

Euro V Euro V Euro IV

BJ6123C7NJB-1 BJ6123C7NJB-1 BJ6123C7C4D

Cummins ISB6.7E5250B

SCRb SCR SCR

2011 2011 2009

17.4 17.4 17.4

Bus 505 Bus 505 Bus 308

4

Hybrid diesel

Euro IV

BJ6123C7C4D

SCR

2009

17.4

CNG CNG CNG CNG CNG CNG CNG CNG CNG LNG LNG

EEVa EEV EEV EEV EEV EEV EEV EEV Euro V Euro V Euro V

JNP6120GC JNP6120GC JNP6120GC JNP6120GC JNP6120GC JNP6120GC JNP6120GC JNP6120GC HFF6121G15C BJ6123C7BTD BJ6123C7BTD

TWCc TWC TWC TWC TWC TWC TWC TWC TWC SCR SCR

2007 2007 2007 2007 2007 2007 2007 2007 2012 2012 2012

18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0

165 165 165, plus a 44 kW electric motor 165, plus a 44 kW electric motor 147 147 147 147 147 147 147 147 172 177 177

5 6 7 8 9 10 11 12 13 14 15 a b c

Cummins ISBE4 + 225B

IVECO F4BE0641A*G

Cummins BGe5 230 YC 6L240N-50

Bus 308 Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus Bus

33 33 33 68 32 32 32 32 32 32 32

Note: Enhanced environmentally-friendly vehicle. It is notable that the diesel engines were required to comply with Euro III emission standard for the model year of 2007. Selective catalyst reduction. Three-way catalyst.

engine speed, engine load) and the inlet temperature just before the catalyst (i.e., SCR temperature). For hybrid diesel buses, the state of charge (SOC) of their batteries at the end of tests were also recorded, which were both almost at the same level as the beginning of the tests. The bus drivers tried to keep the test cycles of those buses consistent with their real-world operating conditions, including a one-minute halt at each bus stop for simulating discharging and receiving passengers. Their regular operation routes were used as test routes (see Table 1), including urban highways (e.g., the West Fourth-Ring Road), arterial roads and residential roads. Measurement for each bus was repeated twice under similar traffic conditions on the same operation route (e.g., inbound and outbound). Buses stopped for about 1 h between two parallel measurement tests for checking the status of instruments. In addition, each test lasted more than 30 min so that enough on-road vehicle driving data and emission profiles could be obtained. The load mass of each individual test was generally 1.0–1.5 tons, which was equivalent to approximately 15–25 passengers (25–40% of rating passenger capacity). It represents a typical passenger load for public buses during non-rush hours in Beijing [11]. Low sulfur diesel was used for diesel buses during our study period, with a sulfur content of 50 ppm or less. In addition, two diesel hybrid buses (No. 3–4) and two CNG buses (No. 9–10) were selected to explore the impacts of air conditioning usage on NOX emissions. In the first step, air conditioning was not in operation. Later, the vehicles were tested on the same routes with air conditioning operating at a maximum load. 2.2. Data processing An operating mode binning methodology was used to relate instantaneous emission rates of gaseous pollutants to real-time vehicle driving conditions. Each operating mode was classified by vehicle speed and vehicle specific power (VSP). VSP is a proxy parameter representing on-road driving conditions, which has been widely used in developing new modal emission factor models [2,4,24–26]. In this study, we referred to the MOVES model and estimated VSP for buses with Eq. (1) [4,24,27].

VSP ¼ 0:0643v þ 0:000279v 3 þ av þ 9:81v sin h

ð1Þ

where VSP is estimated vehicle specific power, kW ton1; v is vehicle speed, m s1; a is vehicle acceleration, m s2; h is road grade, radians. Twenty-two operating modes were established, including

a deceleration mode, an idling mode and 20 modes representing cruise or acceleration driving modes, illustrated in Table 2. Those 20 modes of cruise and acceleration driving conditions were further categorized into three speed zones – low-speed (1:6 6 v < 40), medium-speed (40 6 v < 80) and high-speed (v P 80) zones. No measurement profiles with vehicle speed exceeding 80 km h1 were obtained from urban buses, so high-speed operating modes (bins 35–38) were ignored in this study. As shown in Eq. (2), we first averaged the instantaneous emission rates of gaseous pollutants by vehicle and operating mode bin. Then emission rates were averaged by bus technology group (e.g., Euro V diesel, Euro IV diesel hybrid, CNG and LNG). The mean emission rates of gaseous pollutants for each bus category by fuel system and operating mode bin were estimated with Eq. (2) [4,24]. N

ERi;j;k

T

i k 1X 1X ¼ ERj Ni 1 T k 1

! ð2Þ

where ERi;j;k is the average emission rate of pollutant j for vehicle category i and operating mode bin k, g s1, Ni is the total number of vehicles in vehicle category i; Tk is the number of second-by-second data for each vehicle in operating mode bin k; ERj is the instantaneous emission rate of pollutant j, g s1; [4]. Standard deviations for all emission rates were also calculated in the suggested way by the MOVES2009 model, by combining the within-vehicle and between-vehicle variations [24]. The distance-based emission factors of THC, CO and NOX in g km1 were developed based on the average emission rates and time allocation of operating modes within a given driving cycle (see Eq. (3)) [4].

EF dis

i;j

¼

P 3600 k ðERi;j;k  Pk  TÞ PT 1

v

ð3Þ

where EF dis i;j is the emission factor for bus category i of pollutant j, g km1; P k is the time allocation of operating mode bin k to the total driving cycle; T is the total time of the driving cycle, s; and v is the instantaneous speed of the driving cycle, km h1. In this study, emission factors were all estimated for a typical driving cycle for urban buses in Beijing (BJBC) to eliminate distinction of driving conditions between each individual measurement [4]. The average speed of BJBC was 18 km h1 and idling time as high as 33%. To facilitate comparison among various fuel systems, fuel consumption-based emission factors of THC, CO and NOX in g MJ1 were estimated using a carbon mass balance approach (see Eq. (4)) [4,11].

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S. Zhang et al. / Applied Energy 132 (2014) 118–126 Table 2 Definition of instantaneous operating mode bins by using vehicle specific power (VSP) and vehicle speed (v). VSP (kW ton1)

VSP < 4 4 6 VSP < 2 2 6 VSP < 0 0 6 VSP < 2 2 6 VSP < 4 4 6 VSP < 6 6 6 VSP < 8 VSP P 8

EF fuel j ¼

Vehicle speed (km h1)

Bin 0 Deceleration or braking

v < 1.6

1.6 6 v < 40

40 6 v < 80

v > 80

Bin Bin Bin Bin Bin Bin Bin Bin

Bin 35

Bin 1 Idle

Bin Bin Bin Bin Bin Bin Bin Bin

1000  W c  EF dis j ð0:273  EF dis CO2 þ 0:429  EF dis CO þ 0:866  EF dis THC Þ  HL ð4Þ

where EF fuel j is the fuel consumption based emission factor for the tested bus of pollutant j, g MJ1; W c is the ratio of carbon mass to total fuel mass, 0.866 for diesel and 0.736 for natural gas; and HL is the low heating value of the fuel, 42.7 MJ kg1 for diesel and 46.2 MJ kg1 for natural gas. For emission limits of HDDVs, the brake-specific emission factor in g kW h1 is used instead. Therefore, for those two tested Euro V diesel buses, we collected the second-bysecond engine performance data with the OBD detector and further estimated their real-world NOX brake-specific emission factors. To better map real-world traffic patterns, each trip was divided into several micro-trips. Each micro-trip lasted approximately 250–300 s, representing a link-level driving cycle [11]. Previous studies identified average speed as the leading traffic indicator in real-world fuel consumption and CO2 emission factors for urban buses [11]. Relative emission factors of THC, CO and NOX were developed with Eq. (5). As non-dimensional and speed-dependent variables, they could eliminate other effects, such as vehicle weight, engine displacement and accumulated distance travelled, that would occur with the original emission factors, therefore, evaluate the impacts from traffic patterns [11].

REF m;j ¼

EF dis m;j EF dis 0;j

ð5Þ

where REF m;j is relative emission factor of pollutant j for micro-trip cycle m of a tested bus; EF dis m;j is the distance based emission factor of pollutant j for the micro-trip m of that tested bus, g km1; and EF dis 0;j is the emission factor of pollutant j emission factor under the BJBC baseline driving cycle for the same bus, which is estimated in Eq. (3). 3. Results and discussion 3.1. Instantaneous emission rates by operating mode Fig. 1 presented the average emission rates of NOX for each vehicle technology group by operating mode (see Figs. S1 and S2 in supplementary for emission rates of CO and THC). Average tailpipe exhaust temperatures by operating mode for Euro V diesel, Euro IV hybrid and LNG buses, and average SCR temperatures for Euro V diesel buses were summarized in Fig. 2. The emission rates of all gaseous pollutants generally increased with VSP for both lowspeed and medium-speed zones (e.g., bins 11–18 and bins 21–28), which is consistent with previous studies [4,27]. For example, average NOX emission rates for Euro V diesel buses increased from 0.009 ± 0.001 g s1 (mean ± standard deviation) of bin 11 to 0.153 ± 0.006 g s1 of bin 18. In addition, although real-world SCR performance is a complicated issue associated with operating conditions (e.g., vehicle speed and engine load), characteristics of the SCR systems (e.g., reaction temperature range), and urea manage-

11 12 13 14 15 16 17 18

21 22 23 24 25 26 27 28

Bin 36 Bin 37 Bin 38

ment and injection strategies [28–30], we clearly observed that SCR temperature was a key factor in real-world NOX emissions. As an example, the OBD decoder detected that SCR temperatures associated with medium-speed operating modes were significantly higher (by almost 30 °C) compared to low-speed modes (see Fig. 2). Therefore, average NOX emission rates for medium-speed modes (bins 21–28) were 40% lower relative to low-speed modes (bins 11–18) (see Fig. 1). On the other hand, when compared to Euro IV diesel buses [4], average NOX emission rates for Euro V diesel buses were lower by 30% for bins 11–18 and 50% for bins 21– 28 on average. Considering the limited testing of Euro V diesel buses in this study [4], more tests for Euro V diesel buses are needed to confirm their benefits of mitigating NOX emissions relative to Euro IV diesel buses. Recently, various potential strategies to improve the low-temperature performance of SCR technologies have been reported by Johnson [29]. These strategies include improved catalyst, new ammonia sources (e.g., gaseous ammonia injected instead of urea), and thermal management. Therefore, to precisely evaluate the SCR performance of new diesel buses (e.g., Euro V), for example, to test and evaluate the catalyst performance needs to be carefully addressed in the future. The average NOX emission rates of Euro IV diesel hybrid buses were substantially lower for most bins than Euro V diesel buses (see Fig. 1). This is especially true for low-speed modes. For example, average NOX emission rates of bins 16–18 were 0.07 g s1 for diesel hybrid buses, 50% lower than Euro V diesel buses. Low-speed operating modes with high VSP (bins 16–18) occurred frequently when buses accelerated from stop or idling conditions. Therefore, hybrid buses could effectively reduce NOX emissions under low-speed stop-and-go driving conditions. For CNG buses, the average NOX emission rates were also significantly lower than Euro V diesel buses for most bins. For example, average emission NOX rates increased from 0.006 ± 0.002 g s1 for bin 11 to 0.049 ± 0.017 g s1 for bin 18, reduced by 60% on average compared to Euro V diesel buses. For LNG buses, although we did not obtain their SCR temperature data during the tests, we could estimate this value based on the difference between SCR temperature and tailpipe exhaust temperature for Euro V diesel buses (70–120 °C, see Fig. 2). The average tailpipe exhaust temperature of LNG buses was 30 °C higher than diesel buses for most bins (see Fig. 2), which indicated that the average SCR temperature could be over 250 °C for low-speed modes (e.g., bins 11–18). As a result, LNG buses should have lower NOX emission rates for all driving conditions, less than 0.004 g s1. We compared average instantaneous mass ratios of NOX to CO2 (i.e., ERNOX =ERCO2 ) by operating mode among various vehicle technology groups (see Fig. 3). Emission ratios (g NOX per kg CO2) are a widely used indicator of real-world NOX emissions [31–33]. Euro V diesel buses had significant decreasing average ERNOX =ERCO2 with VSP for both low-speed and medium-speed modes. These trends were similar to the on-road results for SCR-Euro V HDDVs previously reported in Europe [31–34]. For the other three newenergy bus categories, the variations in average ERNOX =ERCO2 were smaller across all operating modes relative to Euro V buses. For

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S. Zhang et al. / Applied Energy 132 (2014) 118–126

Fig. 1. Average emission rates of NOX for four bus categories by operating mode.

Fig. 2. Average tailpipe exhaust temperatures for three bus categories with SCR systems by operating mode, together with average SCR temperatures for Euro V diesel buses.

diesel hybrid buses, the average instantaneous emission ratios of low-speed modes (bins 11–18) were reduced by 30% compared to Euro V diesel buses. For CNG buses, average ERNOX =ERCO2 were approximately 0.01 on average. The mass ratios for LNG buses with SCR systems were even lower, around 0.005. Compared to Euro IV and Euro V diesel buses, CNG and LNG buses effectively controlled NOX emissions, even under suboptimal operating conditions (e.g., low speed, low engine load). 3.2. Emission factors of gaseous pollutants Table 3 illustrates estimated distance-based and fuel consumption-based emission factors of NOX, THC and CO for each sampled bus. Compared to on-road emission factors of Euro IV diesel buses in Beijing (11.8 ± 4.8 g km1 based on 24 samples) [4], the average distance-based NOX emission factor of Euro V diesel buses was 37% lower. This presents a moderate improvement in on-road NOX emission control for urban diesel buses that largely depend on

Fig. 3. Average mass ratios of NOX emission rates to CO2 emission rates for four bus categories by operating mode.

SCR systems. However, combined with real-time engine performance data collected by the OBD detector, the average derived brake-specific NOX emission factor under actual driving conditions was 5.6 ± 0.2 g kW h1. This was 41% lower than that for Euro IV buses (9.6 g kW h1) tested in Beijing [4] but still 180% higher than Euro V emission standards for HDDVs (2.0 g kW h1) over the European Steady Cycle (ESC) or European Transient Cycle (ETC). This is of concern due to the significant contributions of NOX from diesel vehicles under city traffic conditions that could cause urban NO2 concentrations to exceed European ambient air quality standards [31–33]. For example, NOX emission factors for Euro V urban diesel buses was revised in the COPERT4 model in 2010 (i.e., COPERT4 V8.0). For SCR-Euro V urban diesel buses, the NOX emission factor was increased from 4.4 g km1 to 10.4 g km1 under a typical European urban driving cycle with an average speed of 18 km h1 [35,36]. Therefore, the gap between real-world emissions and the regulatory limit may necessitate further NOX emission controls for buses under low-speed driving conditions. Compared to conventional Euro V diesel buses, diesel hybrid buses not only reduced fuel use by 18% [11], but also reduced

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S. Zhang et al. / Applied Energy 132 (2014) 118–126 Table 3 Distance-based and fuel consumption-based emission factors of NOX, THC and CO for all measured buses under the BJBC.

a

Vehicle Number

Technology group

Distance-based emission factor (g km1)

Fuel consumption-based emission factor (g MJ1)

THC

CO

NOX

THC

CO

NOX

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Diesel-SCR Diesel-SCR Hybrid diesel-SCR Hybrid diesel-SCR CNG-TWC CNG-TWC CNG-TWC CNG-TWC CNG-TWC CNG-TWC CNG-TWC CNG-TWC CNG-TWC LNG-SCR LNG-SCR

NAa 0.25 0.11 0.07 0.47 1.4 2.2 1.0 1.5 1.3 3.6 2.2 1.3 1.5 NA

5.3 3.0 3.1 2.9 4.0 30.3 12.6 10.7 7.7 2.1 53.9 3.2 1.5 1.2 0.7

7.5 7.4 3.6 5.2 3.2 3.1 10.7 5.5 6.1 4.3 4.7 8.0 5.5 3.16 3.3

NA 0.023 0.012 0.008 0.025 0.060 0.141 0.052 0.093 0.112 0.238 0.171 0.098 0.111 NA

0.50 0.28 0.35 0.33 0.21 1.27 0.82 0.55 0.49 0.18 3.52 0.25 0.11 0.09 0.05

0.71 0.68 0.41 0.58 0.17 0.13 0.70 0.28 0.39 0.37 0.31 0.62 0.41 0.23 0.22

Note: Not available due to malfunction of the HFID sub-module for the SEMTECH-DS PEMS.

distance-based emission factors by 65% for THC, 28% for CO and 42% for NOX. This indicates a potential win–win strategy that mitigates both air pollutants and greenhouse emissions (e.g., CO2) using hybrid technologies. We also noticed significant uncertainties in hybrid diesel buses NOX emission factors from previous studies. Some previous on-road tests indicated higher NOX emission factors for hybrid diesel buses than conventional diesel buses, which probably could be attributed to the variety of hybrid diesel fuel systems and limited testing samples [37–39]. For example, Hallmark et al. [38] used a PEMS to test a parallel diesel hybrid urban bus and observed significantly higher NOX emission rates by approximately 2–9 times compared to a conventional diesel bus. Since the hybrid diesel bus is an emerging technology with a variety of technology types, the variation in hybrid technology can lead to substantial differences in on-road emissions. Some advanced simulation tools (e.g., the ADVISOR model) might be necessary to improve the understanding of on-road emissions and fuel consumption for hybrid vehicles [40–43]. On the other hand, manufacturers should take real-world operating conditions into consideration when designing advanced hybrid systems, energy management systems and emission control strategies for public buses to maximize co-benefits of improving fuel economy and reducing air pollutants. The average NOX emission factor of CNG buses was significantly lower than diesel buses, by 50% and 21% compared to the Euro IV and Euro V diesel counterparts, indicating CNG engines could achieve lower NOX emissions than diesel engines with SCR under the low-speed driving cycle. NOX emission factors from this study were significantly lower than an on-road remote sensing field study in Europe, i.e. 17.3 ± 8.7 g (kg fuel)1 in this study vs. 41 ± 26 g (kg fuel)1 from the Europe field study [44]. It should be noted that in this study, CNG buses were only tested using remote sensing technologies for a short period during acceleration, and vehicle maintenance might also bring about variability of emissions [44]. Furthermore, we noticed that a variety of natural gas engine technologies (e.g., stoichiometric engine with a TWC, lean-burn engine, lean-burn engine with an oxidation catalyst, and dual fuel engine) were applied to natural gas powered heavy-duty vehicles in the market [45–48]. Some recent studies in the U.S. and Europe indicated that the dedicated stoichiometric engine with TWC might provide superior NOX emission control performance compared to lean-burn engines [45,46]. This is because the best compromise between emissions and fuel economy for the lean-burn engine is not always easily achieved considering that driving conditions and fuel property could vary significantly in real-world operation [47]. By contrast, stoichiometric engines could enable TWC to

control NOX emissions at the cost of energy economy. Therefore, dedicated deNOX systems like SCR might be necessary for lean-burn and dual fuel engines to meet more stringent emission standards [46,47]. When compared to Euro V diesel buses, CNG buses had significantly higher THC emissions due to the leakage of methane emissions, which were estimated at approximately 1.0–1.3 g km1 with the COPERT4 model [35,36]. CO emission factors varied widely. For example, two CNG buses (i.e., No. 6 and No. 11) manufactured in 2007 with high accumulated distance travelled had high on-road CO emission factors. This was likely due to the deterioration of the after-treatment systems (i.e., TWC). However, the relative range of NOX emission factors for CNG buses is lower than THC and CO, indicating that CNG buses could be an effective NOX emission control option for the urban public bus fleet, even though they have been in service for several years (e.g., odometers of 200,000–250,000 km). The average NOX emission factor of LNG buses was further reduced by 45% compared to CNG buses. When compared to Euro IV and Euro V diesel buses, the reduction benefits in distance based NOX emission factor were as high as 73% and 57%, respectively. Higher exhaust temperatures were the leading factor in the efficiency of SCR systems for LNG buses. As Fig. 2 indicates, for most operating modes, the average tailpipe exhaust temperature for LNG buses was significantly higher compared to diesel buses (see Section 3.1). In August 2013, the Beijing Municipal Government initiated a comprehensive emission mitigation plan (i.e., ‘‘Clean Air Action Plan 2013–2017’’) to improve local air quality. The plan includes an effort to increase the number of alternative fuel technologies in the public fleets (e.g., public buses, taxis, sanitation and postal vehicles) [2]. Specifically, the natural gas fueled buses are expected to constitute over 50% of total public buses by 2017 under the plan [2]. Considering that the LNG bus model has substantial advantages in energy density, service distance, NOX emission control and infrastructure cost compared to the CNG counterpart, it will be one of the major technologies for public buses in Beijing [2,11]. There is significant uncertainty about onroad NOX emission factors for Euro VI diesel buses, which are expected to penetrate the Beijing fleet around 2017. Therefore, in the interim the combination of dedicated LNG engine and SCR technology can provide a promising and competitive solution to reduce traffic-related NOX emissions in urban areas. 3.3. Impacts of average speed and air conditioning usage Average speed is the leading factor affecting NOX emission rates, more so than other traffic parameters (e.g., road type) [11]. The

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relationships between relative NOX emission factors for micro-trips and average speed values are presented in Fig. 4 by vehicle technology group (see Fig. S3 for THC and CO). Similar to the effect of traffic on fuel consumption and CO2 emission factors [11], average speed showed a strong nonlinear impact on NOX emission factors for all vehicle technology groups except diesel hybrid buses. In order to determine the best fit for average speed corrections, we applied power functions or polynomial functions (quadratic polynomial functions). In general, the correlation coefficients between relative NOX emission factors and average speed of micro-trips were reasonably good for Euro V diesel buses, CNG buses and LNG buses, with R2 at 0.56–0.77. The relative NOX emission factors showed clear inverse trends with average speed of micro-trips when average speeds were below 25 km h1. Using fitting functions, we performed sensitivity analysis to quantitatively evaluate the effects from variations in average speed. When average speed decreased from 25 km h1 to 15 km h1, representing typical traffic conditions during non-rush hours and rush hours respectively, NOX emission factors were estimated to increase by 67% for Euro V diesel buses, 55% for CNG buses, and 58% for LNG buses. If average speed declined to below 10 km h1, representing extremely congested driving conditions, Euro V diesel buses with SCR systems were the most sensitive to average speed, while CNG buses with TWC were less sensitive than SCR-equipped LNG buses (see Fig. 4). This indicates real-world performance of SCR is more sensitive than TWC to changes in average speed. In addition, the variation in NOX emissions for Euro V diesel buses was significantly greater than variations in fuel consumption corresponding to the same traffic scenario changes (i.e., +67% for NOX emissions vs. +33% fuel consumption) [11]. Higher sensitivity of NOX emissions to average speed changes could be attributed to unsatisfactory NOX emission control performance for SCR-Euro V diesel buses under low-speed driving conditions. By contrast, the correlation coefficients between NOX emission factors and average speed were fairly low for diesel hybrid vehicles compared to other bus groups. This is primarily because hybrid buses rely on electric motors under low-speed

driving conditions. Therefore, diesel hybrid bus can more effectively reduce NOX emissions under congested driving conditions than conventional diesel buses. It should be noted that all measurements were carried out on real-world bus operation routes, so few micro-trips with average speeds over 30 km h1 could be obtained for this study. Thus, the potential impacts from driving conditions with average speeds over 30 km h1 should be evaluated with more test profiles under high-speed driving conditions (e.g., buses operated on urban freeways or inter-city buses). Traffic management policies designed to reduce fuel consumption would also reduce emissions of gaseous pollutants [4,11,49]. For example, four bus rapid transit (BRT) routes have been put into operation in Beijing by the end of 2012. Those BRT routes provide special lanes for public buses even in traffic-dense areas. Other passenger and freight vehicles are prohibited from these special lanes. Lai et al. [50] collected real-world driving condition data on nine regular bus routes and two BRT bus routes in the urban areas of Beijing. The results showed that the average speed of BRT routes was 20.7 km h1, 36% higher than regular bus routes (15.2 km h1). If Euro V diesel buses, CNG buses and LNG buses operated on BRT routes, on-road NOX emission factors are estimated to decrease by 36%, 29%, and 32%, respectively, due to the improvement in driving conditions. For Euro V diesel buses, the estimated benefit from better driving conditions (i.e., 36% higher average speed) is comparable to the technology enhancements that Euro V diesel buses demonstrate versus Euro IV buses (e.g., average NOX emission factor was lowered by 37% compared to the Euro IV diesel buses under the BJBC). Therefore, policymakers should consider increasing and optimizing BRT routes to mitigate bus emissions at a lower cost of lower driving speed for private cars. For diesel hybrid buses, air conditioning usage was identified as a real-world operating condition that has strong impacts on fuel consumption and CO2 emissions (e.g., an average increase of 50% when air conditioning is operating, see Fig. 5) [11]. Air conditioning usage increased emission factors by 52% for THC and 14% for CO on average, indicating a similar trend for CO2 emission factors (49%)

Fig. 4. Correlations between relative emission factors of NOX and average speed of all the micro-trips for four bus categories.

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buses (i.e., No. 9 and No. 10 samples), average NOX emission factors increased by 15% when air conditioning systems were operating, accompanied by a 9% increase in on-road fuel consumption (see Fig. 5). THC emission factors were almost stable, while CO emission factors declined by 13% on average with air conditioning.

4. Conclusions

Fig. 5. Comparison of average NOX emission factors and fuel consumption of two diesel hybrid buses and two CNG buses with and without air conditioning use respectively.

and fuel consumption (48%) (see Table S1).For NOX, however, the effect from air conditioning was the opposite of the impact on fuel consumption. Average emission factors of NOX for No. 3 and No. 4 bus samples were 2.5 g km1 and 2.8 g km1 under the BJBC (see Table S1), 30% and 46% lower than emission factors with the air conditioning system switched off. This was because average tailpipe exhaust temperatures were substantially higher, by 25– 40 °C, for all operating modes. These higher temperatures could enhance the efficiency of SCR systems. Average NOX emission rates for all operating modes were below 0.03 g s3 when air conditioning systems were functioning (see Fig. 6). With air conditioning systems in operation, NOX emission factors were 50–65% lower for bins 11–18 and 50–95% lower for bins 21–28. Considering the penetration of public buses with air conditioning systems will reach 70% of the Beijing fleet by the end of 2015 [51], diesel hybrid buses can substantially mitigate NOX emissions at a cost of higher fuel consumption when their air conditioning systems are functioning, a trade-off policy-makers could try to balance. For CNG

Fig. 6. Comparison of average NOX emission rates and tailpipe temperatures by operating mode for two diesel hybrid buses with and without air conditioning use respectively.

On-road gaseous emissions for two Euro V diesel, two Euro IV hybrid diesel, nine CNG and two LNG public buses were tested using PEMS in Beijing. We employed an operating mode methodology to relate instantaneous emission rates of THC, CO and NOX to driving conditions and then estimate on-road emission factors under the BJBC, with a special focus on NOX emissions from those buses. Furthermore, we explored the impacts of operating conditions on real-world NOX emissions, including traffic conditions indicated by average speed and air conditioning usage. Our findings provide state-of-the-art real-world NOX emissions for diesel buses compliant with China’s emission standard of HDDVs and alternative fuel technologies. For Euro V diesel buses, the average NOX emission factor was 7.5 ± 0.1 g km1, under the BJBC. The NOX emission factor was 37% lower than the average NOX emission factor for Euro IV diesel buses in Beijing. However, the average on-road brake-specific NOX emission factor exceeded the Euro V emission standard by 180%, indicating that SCR technology could not provide satisfactory emission control for urban diesel buses under the BJBC. Therefore, special emission control strategies are needed to reduce NOX emissions from urban diesel buses during unfavorable low-speed driving conditions. Compared to conventional diesel counterparts, diesel hybrid and alternative fuel technologies were better options to mitigate NOX emissions from the public bus fleet. For example, the average NOX emission factor of Euro IV diesel hybrid buses was 4.4 ± 1.1 g km1, 63% and 42% lower than conventional Euro IV and Euro V diesel buses. Therefore, diesel hybrid buses could be a win–win technology option to reduce fuel consumption and improve urban air quality. For CNG buses with TWC, the average NOX emission factor was 5.7 ± 2.4 g km1 under the BJBC, significantly lower than conventional diesel buses. The average NOX emission factor of LNG buses equipped with SCR systems was 3.2 ± 0.7 g km1, 45% lower than CNG buses. The exhaust temperatures of LNG buses were much higher than diesel buses, which would enhance performance of SCR systems under most operating conditions. Therefore, diesel hybrid and alternative fuel technologies could play an essential role in mitigating vehicle NOX emissions for urban buses in China. We identified strong correlations between NOX emission factors and average speed for Euro V diesel, CNG and LNG buses. For example, NOX emission factors increased by 67% for Euro V diesel buses, 55% for CNG buses, and 58% for LNG buses when average speeds decreased from 25 km h1 to 15 km h1. This implies better traffic management policies (e.g., the BRT routes) could be as important as advanced emission control technologies for mitigating urban vehicle NOX emissions. Furthermore, diesel hybrid buses were not significantly sensitive to changes of average speed, indicating that they could be effective at reducing NOX emissions under low-speed driving conditions. In addition, average NOX emission factors for diesel hybrid buses were reduced by 40% when onboard air conditioning systems operated because of better SCR performance due to higher exhaust temperature. However, air conditioning usage increased fuel consumption. For CNG buses, we observed air conditioning usage increased NOX emission factors by 15%. Manufacturers should take special consideration of the impacts of emissions from real-world operating conditions when designing advanced vehicles in the future.

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