Scholarly article on topic 'Unregulated emissions from light-duty hybrid electric vehicles'

Unregulated emissions from light-duty hybrid electric vehicles Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Atmospheric Environment
Keywords
{"Vehicle emissions" / "Plug-in hybrid" / WLTC / Ammonia / "Cold start emissions"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — R. Suarez-Bertoa, C. Astorga

Abstract The number of registrations of light duty hybrid electric vehicles has systematically increased over the last years and it is expected to keep growing. Hence, evaluation of their emissions becomes very important in order to be able to anticipate their impact and share in the total emissions from the transport sector. For that reason the emissions from a Euro 5 compliant hybrid electric vehicle (HV2) and a Euro 5 plug-in hybrid electric vehicle (PHV1) were investigated with special interest on exhaust emissions of ammonia, acetaldehyde and ethanol. Vehicles were tested over the World harmonized Light-duty Test Cycle (WLTC) at 23 and −7 °C using two different commercial fuels E5 and E10 (gasoline containing 5% and 10% vol/vol of ethanol, respectively). PHV1 resulted in lower emissions than HV2 due to the pure electric strategy used by the former. PHV1 and HV2 showed lower regulated emissions than conventional Euro 5 gasoline light duty vehicles. However, emissions of ammonia (2–8 and 6–15 mg km−1 at 22 and −7 °C, respectively), ethanol (0.3–0.8 and 2.6–7.2 mg km−1 at 22 and −7 °C, respectively) and acetaldehyde (∼0.2 and 0.8–2.7 mg km−1 at 22 and −7 °C, respectively) were in the same range of those recently reported for conventional gasoline light duty vehicles.

Academic research paper on topic "Unregulated emissions from light-duty hybrid electric vehicles"

Contents lists available at ScienceDirect

Atmospheric Environment

journal homepage: www.elsevier.com/locate/atmosenv

Unregulated emissions from light-duty hybrid electric vehicles

R. Suarez-Bertoa*, C. Astorga**

European Commission, Joint Research Centre (JRC), Institute for Energy and Transport (IET), Sustainable Transport Unit, 21027 Ispra, VA, Italy

CrossMark

HIGHLIGHTS

> NH3, ethanol and acetaldehyde emissions are in the same range of gasoline vehicles.

> Regulated emissions are in the same range as gasoline vehicles.

> Higher regulated and unregulated emissions were observed at -7 °C than at 23 °C.

> The battery state of charge strongly impacts the plug-in hybrid emissions.

ARTICLE INFO

ABSTRACT

Article history: Received 27 January 2016 Received in revised form 17 April 2016 Accepted 18 April 2016 Available online 20 April 2016

Keywords: Vehicle emissions Plug-in hybrid WLTC Ammonia

Cold start emissions

The number of registrations of light duty hybrid electric vehicles has systematically increased over the last years and it is expected to keep growing. Hence, evaluation of their emissions becomes very important in order to be able to anticipate their impact and share in the total emissions from the transport sector. For that reason the emissions from a Euro 5 compliant hybrid electric vehicle (HV2) and a Euro 5 plug-in hybrid electric vehicle (PHV1) were investigated with special interest on exhaust emissions of ammonia, acetaldehyde and ethanol. Vehicles were tested over the World harmonized Light-duty Test Cycle (WLTC) at 23 and -7 °C using two different commercial fuels E5 and E10 (gasoline containing 5% and 10% vol/vol of ethanol, respectively). PHV1 resulted in lower emissions than HV2 due to the pure electric strategy used by the former. PHV1 and HV2 showed lower regulated emissions than conventional Euro 5 gasoline light duty vehicles. However, emissions of ammonia (2—8 and 6 -15 mg km-1 at 22 and -7 °C, respectively), ethanol (0.3—0.8 and 2.6—7.2 mg km-1 at 22 and -7 °C, respectively) and acetaldehyde (-0.2 and 0.8—2.7 mg km-1 at 22 and -7 °C, respectively) were in the same range of those recently reported for conventional gasoline light duty vehicles.

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Hybrid electric vehicles (HEVs) have been presented as a promising approach to reduce the emissions of pollutants from vehicle exhaust. HEVs registrations has continuously increased over the last years, reaching share of 1.4% of all new car sales in the EU in 2014, which is more than twice the registrations of 2011 (ICCT, 2014). Hybrid electric vehicles comprise two alternative powertrains: an internal combustion engine (ICE) combined with an electric motor that includes an energy storage system recharged by the vehicle means (ICE, regenerative braking, etc.). There are three hybrid electric configurations: parallel, series and parallel/

* Corresponding author.

** Corresponding author.

E-mail addresses: ricardo.suarez-bertoa@jrc.ec.europa.eu (R. Suarez-Bertoa), covadonga.astorga-llorens@jrc.ec.europa.eu (C. Astorga).

series. Parallel design uses both ICE and electric motor to move the vehicle, using the ICE as the main power source and the electric one to assist according to the driving condition. In this configuration, the battery provides energy to the vehicle through an electric motor, which acts also as generator, recharging the battery (^agatay Bayindir et al., 2011). Series configuration uses the ICE as generator, supplying electricity to the electric motor, which provides the energy needed to move the vehicle, and recharging the battery (Hannan et al., 2014). In the case of parallel/series hybrid vehicles a power split device allows moving the vehicle using only the 1EC, or the electric motor or even both simultaneously.

Plug-in hybrid electric vehicles (PHVs), equipped with an electric powertrain and a battery pack which can be directly charged from the electric grid, have been recently introduced. The PHVs allow for pure electric driving under certain conditions. While running in pure electric mode there is no exhaust emission. While PHVs generally have higher power electric motor and higher

http://dx.doi.org/10.1016/j.atmosenv.2016.04.021

1352-2310/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

battery capacity than HEVs, some HEVs (non-plugin) also allow operation in pure electric mode. The main difference between HEVs and PHVs is the external charging capability. In Europe, PHVs registrations increased from a few hundred in 2011 to nearly nine thousand in 2012. The number of PHVs registrations tripled from 2012 to 2013. This growth cooled down from 2013 to 2014, showing an increase of 30% (Christian Thiel and Dilara, 2015).

Previous works reported that hybrid electric vehicles present improved energy efficiency and lower pollutants emissions compared to conventional vehicles (Fontaras et al., 2008; Alvarez and Weilenmann, 2012). As a consequence of the fast introduction of the HEVs and PHVs in the market, evaluation of their regulated and unregulated pollutant emissions becomes very important in order to be able to anticipate their impact and share in the total emissions from the transport sector.

Low temperatures typically lead to higher vehicular emissions, especially for conventional gasoline vehicles (Dardiotis et al., 2013). Most of these emissions take place during the cold start, which is the period elapsing from the start of the ICE until hot engine operation and optimal temperature of the catalytic converter (hereinafter catalyst) are reached. Alvarez et al. suggested that the influence of low ambient temperatures on regulated emissions from hybrid electric vehicles equipped with a spark ignition engine is similar to that of conventional gasoline vehicles (Alvarez and Weilenmann, 2012). Combustion engine and after-treatment could then be affected by the presence of an electrical motor and an energy storage system that supply the needed energy during the low speed regime of the vehicle, delaying and/or modifying the vehicle cold start.

Ammonia (NH3), ethanol and acetaldehyde are pollutants that can be present in vehicle exhaust (Bishop and Stedman, 2015; Clairotte et al., 2012; Durbin et al., 2007; Graham et al., 2008; Karavalakis et al., 2014; Suarez-Bertoa et al., 2014, 2015a,b). While NH3 emissions are linked to the use of three-way catalyst in conventional gasoline vehicles, emissions of ethanol and acetal-dehyde are related to the use of ethanol containing blends. NH3 is classified under the European dangerous substances directive (67/ 548/EEC) as toxic, corrosive and dangerous for the environment. It is a precursor of atmospheric secondary aerosols (Behera and Sharma, 2010; Pope et al., 2002). Its reaction with nitric and sulfuric acid leads to the formation of atmospheric secondary aerosols, namely, ammonium nitrate and ammonium sulfate (Behera and Sharma, 2010; Pope et al., 2002). The deposition of the ammonium salts leads to hypertrophication of waters and acidification of soils with negative effects on nitrogen-containing ecosystems (Bouwman et al., 2002; Erisman et al., 2003; Sutton et al., 2000). Acetaldehyde is classified as probable carcinogenic by the US Department of Health and Human Services, and ethanol is an atmospheric precursor of acetaldehyde and peroxyacetyl nitrate (PAN) (Millet et al., 2010, 2012). Furthermore, acetaldehyde is classified as a hazardous air pollutant by the U.S. EPA (U.S. EPA), and its subsequent oxidation can lead to production of ozone (O3) and PAN. Hence, these two compounds are also associated with urban air pollution (Durbin et al., 2007; Graham et al., 2008; Andrade et al., 1998).

Hybrid electric and plug-in hybrid vehicles are expected to take over a large fraction of the vehicle market thanks to their low energy consumption and low CO2 emissions. Therefore, it is of major importance to understand how these new technologies and improvements impact on the emissions under different conditions (e.g., temperature, fuel, battery state of charge). In that frame, the present work presents the first comprehensive study of the exhaust emissions (regulated pollutants, CO2, NH3, acetaldehyde and ethanol) from a HEV (parallel, mild hybrid) and a PHV (seriesparallel, full hybrid), both spark ignition and Euro 5 compliant, over

the world harmonized light-duty test cycle (WLTC), which is considered to be representative of real world driving conditions. This test cycle will be used for type approval of light-duty vehicles (LDVs) in the European Union and potentially other countries who are signatories of the agreements to the United Nations Economic Commission for Europe (UNECE) (UNECE, 2015). The new step (Euro 6c) of the EU legislation will be implemented for new types on September 2017 and for new vehicles on September 2018. The vehicles were tested at 23 °C and also at -7 °C to study the effect of low ambient temperature. This temperature was chosen since it is used during the low temperature emissions test, also known as Type 6 test (European Commission, 2008). Finally, two different commercial fuels E5 and E10 (gasoline containing 5% and 10% vol/ vol of ethanol, respectively) were used to evaluate if different ethanol content in commercial fuel blends could influence the emissions of these type of vehicles.

2. Experimental

A plug-in hybrid electric vehicle (PHV1) and a hybrid electric vehicle (HV2), both Euro 5 compliant ((EC) No 692/2008) (European Commission, 2008), were tested (see technical details in Table 1) over the WLTC in the Vehicle Emission Laboratory (VELA) at the European Commission Joint Research Centre Ispra, Italy using a certified chassis dynamometer (see Fig. 1 for test cell configuration). The VELA facility comprises a climatic test cell with controlled temperature and relative humidity (RH) to simulate a variety of ambient conditions (temperature range: -10 to 35 °C; RH range: 50—80%). Tests were performed on a chassis dynamometer (inertia range: 454—4500 kg), designed for two and four-wheel drive light-duty vehicles (LDVs) (two 1.22 m roller benches — MAHA GmbH, Germany). The emissions were fed to a Constant Volume Sampler (CVS, Horiba, Japan) using a critical Venturi nozzle to regulate the flow (CVS flow range: 3—30 m3 min-1). A series of thermocouples monitored the temperature of the oil, cooling water, exhaust, and ambient conditions. A universal exhaust gas oxygen (UEGO) type sensor was connected to the tailpipe to follow the air to fuel ratio. Vehicles were kept inside the climatic cell under the described conditions for a 24 h soaking period.

All tests were performed following the WLTC which is a cold start driving cycle. The vehicle and its components (oil, coolant, catalyst, etc) must be at 23 or -7 °C, ± 1 °C, at the beginning of the test cycle. The driving cycle consists of four phases with different speed distributions (see Fig. 1) and it intends to be representative of the real world driving conditions being based on real world vehicle journeys from several countries. The length of the entire cycle is 1800 s and is comprised of the low speed (589 s), medium speed (433 s), high speed (455 s) and extra-high speed (323 s) phases. It reaches a maximum speed of 131.3 km h-1 and is about 23.3 km long. Three different driving cycles have been developed on the basis of the vehicle's power-to-mass ratio and its maximum speed,

Table 1

Fleet general features.

Denomination PHV1 HV2

Internal combustion engine Spark ignition Spark ignition

Type by drivetrain layout Series- Parallel Parallel

Type by level of hybridization Full hybrid Mild hybrid

EU emission standard Euro 5 Euro 5

After-treatment TWC TWC

Engine displacement (cm3) 1798 1497

Engine power (kW) 73 97

Odometer (km) 3546 9969

Three-Way Catalyst (TWC)

Calibration gat

Fig. 1. WLTC (top) and schematic diagram of the experimental setup (bottom).

to represent three different vehicle classes. The vehicle tested in the present study pertains to class 3 (power/mass >34 kW/ton), which is the highest power class. Fig. 1 illustrates the version WLTC 5.3 of the speed profile applicable for this class of vehicle (Marotta et al., 2015).

Battery pack level of charge was recorded by the on-board diagnosis (OBD) and expressed as the percent remaining level of charge for the battery, which is commonly known as the battery state of charge (SOC) (SAE, 2014). Tests were performed using the vehicles with the battery system at their maximum SOC. While the PHV1 was charged using its own plug connected to the electrical grid, the battery of HV2 was charged to its maximum using a car battery charger. Therefore, the performed tests could be considered as the best case scenario, and for the PHV1 it is an example of the emissions resulting from a PHV that is used after a typical overnight

charging period reaching it maximum SOC. Even though measured, the effect that the battery SOC has on vehicles emissions is not within the scope of the present study. The number and type of tests need for the type approval of these vehicles over the WLTC is still under discussion at international level.

2.1. Fuel blends

The EU has set a 10% renewable energy share in the transport sector to be complied with by 2020 (2009/28/EC) (EC, 2009). In 2010, the use of renewable energy by the transport sector was 4.70% and the 91% of this share was covered by biofuels (European Commission, 2013). The latest version of the principle European gasoline (EN228) standards allows blending up to E10 (gasoline containing up to 10% volume of ethanol). E5 (gasoline containing up

to 5% vol/vol of ethanol) and E10 are the most common ethanol blends in Europe as today and have been formalised in the CEN EN 228 standard for motor gasoline. Ethanol, even at low concentrations in motor gasoline, is known to impact both the fuel consumption and emissions from vehicles. Volumetric fuel consumption generally increases when running in ethanol/gasoline blends because ethanol has lower energy content per litre compared to conventional hydrocarbon gasoline.

2.2. Analytical instrumentation

Fig. 3a). During tests at -7 °C, PHV1 used the ICE during most of the phase 1, then run as pure electric during the phase 2, and after the second 1200 (phase 3) it used both IEC and pure electric mode, as it did at 23 °C. This shows a clear effect of the low temperature on PHV1 performance. Hence, in order to assure a good drivability, the plug-in vehicle, PHV1, used the ICE during the first part of the cycle when tested at -7 °C (see Fig. 3a). HV2, however, run using the ICE during most of the WLTC at the two temperatures (see Fig. 3b). For this reason, PHV1 presented significantly lower CO and NOx emissions than HV2.

The regulated gaseous emissions were measured using standard methodologies defined by the light-duty vehicle Global Technical Regulation (GTR). An integrated setup (MEXA-7400HTR-LE, HORIBA) analyzes diluted gas from a set of Tedlar bags connected to the CVS (Automatic Bag Sampler, CGM electronics) using non-dispersive infrared (for CO/CO2), a chemiluminescence (for NOx) and a heated (191 °C) flame ionization detector (FID; for THC). The GTR specifies globally harmonized performance-related equipment specifications and test procedures (UNECE, 2015). The tests were done following the World-harmonized Light-duty vehicle Test Procedure (WLTP) (UNECE, 2015). More than twenty gaseous compounds contained in the raw exhaust (e.g. NO, N2O, CH4, NH3, CH3CHO, CH3CH2OH) were monitored at 1 Hz acquisition frequency by a High Resolution Fourier Transform Infrared spectrometer (FTIR — MKS Multigas analyzer 2030-HS, Wilmington, MA, USA). The method and instrument are described in more detail in the literature (Clairotte et al., 2012; Suarez-Bertoa et al., 2015c), therefore, only a brief description is given here. The device consists of a silicon carbide source (at 1200 ° C), a multipath cell (optical length: 5.11 m), a Michelson interferometer (spectral resolution: 0.5 cm-1, spectral range: 600—3500 cm-1) and a liquid nitrogen cooled mercury cadmium telluride detector (MCT). A second set of analyzers, similar to the one used for gaseous regulated emission measurement was directly connected to the vehicle's exhaust pipe allowing a time-resolved (at 1 Hz) measurement of THC, NOx and CO/CO2 from the raw exhaust. CO, CO2 and NOx measurements from these analyzers were used to synchronize the FTIR time-resolved signal (for more information see Clairotte et al., 2012 (Clairotte et al., 2012)). The raw exhaust flow was determined by subtracting the flow of dilution air introduced into the tunnel, measured with a Venturi system, to the total flow of the dilution tunnel, measured by a sonic Venturi (Horiba). Mass flows were derived from the exhaust gas flow rates (m3 s-1) and from the measured concentration (ppmv). Emission factors (EFs, mg km-1) were calculated from the integrated mass flow and the total driving distance of the WLTC (23.3 km).

3. Results and discussion

3.1. Regulated gases emissions

Fig. 2 illustrates the EFs of the regulated gases obtained for two fully charged Euro 5 hybrids, PHV1 and HV2, fueled with E5 and E10 blends and tested over the WLTC at 23 and -7 °C. Regulated pollutants EFs over the WLTC are summarized in Table 2, EFs from each of the four phases (low, medium, high and extra-high speed) are shown in Tables S1 and S2 in the supplementary material. PHV1 and HV2, regulated emissions were well below Euro 5 limits (Regulations 715/2007/EC and 692/2008/EC) at all studied conditions.

PHV1 run on pure electric mode during the first 1200s of the WLTC (WLTC 1800s) when tested fully charged at 23 °C. These 1200s correspond to a distance of 10 km, which was the maximum autonomy of the vehicle battery at pure electric operation (see

Fig. 2. Regulated compounds emission factors (mg km-1) over the WLTC. Hatched bar plots refer to experiments at -7 °C. Error bars represent maximum semi-dispersion from two tests.

PHVl's THC and NMHC emissions were similar than those of HV2 at 23 ° C, and higher at -7 °C. THC and NMHC are typically emitted during the cold start. As soon as the ICE kicked in, THCs started being emitted, a similar behavior to that presented by conventional gasoline vehicles (Dardiotis et al., 2013). Overall, very similar emissions were obtained from tests performed using E5 and E10 blends for each vehicle. Higher emissions were always observed at -7 °C.

In Europe, vehicles manufacturers will have to guarantee that their entire LDV fleet does not emit on average more than 95 g km-1 of CO2 by 2020 (EU, 2014). PHV1 emitted on average 52 ± 4 and 52.0 ± 0.8 g km-1 and of CO2 when tested at 23 °C with E5 and E10 respectively. CO2 tailpipe emissions from PHV1 were 1.5 times higher during the tests at -7 °C. HV2 emitted on average 133.4 ± 0.8 and 133.7 ± 0.3 g km-1 at 23 °C with E5 and E10 respectively. HV2's CO2 tailpipe emissions were 1.2 times higher at -7 °C.

THC and CO EFs of PHV1 and HV2 are similar to those reported for a Euro 4 compliant HEV tested over the NEDC at 21 °C and 27 °C by Fontaras et al. (Fontaras et al., 2008). NOx emissions, however, were respectively 2 times and more than 30 times lower for HV2 and PHV1, compared to the Euro 4 hybrid electric vehicle reported in that study.

EFs of all regulated compounds were substantially lower for the PHV1 and the HV2 than those reported by Marotta et al. (Marotta et al., 2015) for a fleet of Euro 5 and Euro 6 gasoline LDVs tested over the WLTC at 23 °C. The largest differences were observed in the case of the PHV1 whose NOx emissions were more than 80 times lower than the average of the gasoline fleet. THC and CO emissions from HV2 and PHV1 were 2—7 times lower than those reported for the gasoline LDVs. While CO2 emissions from HV2 at 23 °C were similar to the average of the gasoline fleet reported in that study, those from PHV1 were 3 times lower. Regulated emissions from PHV1 and HV2 were also lower than what reported for a Euro 5 flex-fuel vehicle (FFV) running in E5 over the WLTC at 23 °C by Suarez-Bertoa et al. (Suarez-Bertoa et al., 2015a). However, THC, CO and CO2 emissions from PHV1 and HV2 were similar to those for the FFV when the vehicles were tested at -7 °C (NOx emissions were respectively 15 and 5 times lower for PHV1 and HV2 compared to the FFV).

3.2. Unregulated emissions

NH3, acetaldehyde and ethanol exhaust emissions from PHV1 and HV2 fueled with E5 and E10 blends and tested over the WLTC at 23 and -7 °C were measured on-line with a high resolution FTIR at the vehicles' tailpipe. EFs of these three pollutants are summarized in Tables 2 and 3. As already explained, hybrid electric vehicles

comprised an electric powertrain combined with an ICE, which in the case of PHV1 and HV2 is a spark ignition engine. Like any recent spark ignition vehicle, both vehicles were also equipped with a TWC as after-treatment system. In TWC NH3 is produced, through a mechanism that involves NO and molecular hydrogen (H2), right after catalyst light-off (Bradow and Stump, 1977). H2 is formed on the catalyst from a water-gas shift reaction between CO and water or via steam reforming from hydrocarbons (Whittington et al., 1995). Not surprisingly, the two studied hybrid electric vehicles presented NH3 emissions while running in the ICE at all studied conditions (see Table 2, Figs. 3 and 4).

PHV1 run on pure electric mode for 1200 s before using the ICE at 23 ° C. For that reason emissions of NH3 were not observed until the very end on the WLTC. HV2 uses the ICE from the very beginning, and during most part of the WLTC. Furthermore, HV2's catalyst lights off after a few seconds. Longer request of the ICE and sooner catalyst light-off led to higher NH3 emissions from HV2 than from PHV1 at all studied conditions (see Table 2). In general, NH3 formation over the catalyst is enhanced at low lambda (air/fuel ratio), also known as rich combustion, where conditions are reductive and higher concentrations of CO and H2 are present (Whittington et al., 1995; Czerwinski et al., 2010). The studied vehicles tended to run on rich conditions (air/fuel ratio < 1) for a longer period at -7 °C than at 23 °C (see Figures S1-S4 of the supplementary material). As a consequence, the NH3 emissions of PHV1 and HV2 were approximately 2 times higher at -7 °C than at 23 °C for both E5 and E10 fuel blends (see Table 2). The same trends were already reported in previous studies for conventional gasoline LDVs (Suarez-Bertoa et al., 2014, 2015c; Heeb et al., 2006; Huai et al., 2003). Since NH3 emissions are not regulated for LDVs, lambda control aims at reducing NOx and CO emissions.

PHV1 presented lower NH3 emissions using E10 (7 ± 4 and 11.1 ± 0.9 mg km-1 at 23 and -7 °C, respectively) than using E5 blend (1.9 ± 0.6 and 6 ± 1 mg km-1 at 23 and -7 °C, respectively). This could arise as a consequence of PHV1 's longer rich combustion present during the tests performed using E5 compared to those using E10. HV2 presented similar air/fuel ratio with both blends, which lead to similar NH3 emissions at -7 °C (13 ± 2 and 15.4 mg km-1, for E5 and E10, respectively) and slightly higher NH3 emissions for E10 at 23 °C (6.1 ± 0.3 and 8.0 mg km-1 for E5 and E10, respectively). Higher emissions of CO and lower emissions of NO (NH3 precursors) were present at conditions where higher emissions of NH3 were observed. Therefore, as suggested by Kean at al. (Kean et al., 2009) and Livingston et al. (Livingston et al., 2009), CO emissions from gasoline vehicles are indicative of NH3 formation over the catalyst at expenses of NO (Kean et al., 2009; Livingston et al., 2009). Even though PHV1 and HV2 showed lower regulated emissions than gasoline LDVs, their NH3 EFs were

Table 2

Regulated and unregulated emission factors (mg km-1, *CO2 emission factor (g km-1)) over the WLTC at 23 and -7 °C. In parentheses, maximum semi-dispersion. PHV1 HV2

23 °C -7 °C 23 °C -7 °C

E5 E10 E5 E10 E5 E10 E5 E10

THC 9 (±2) 6.8(± 0.1) 108 (±5) 98 (±5) 12.9 (±0.8) 13 (±1) 79 (±6) 76 (±3)

CH4 1(±1) 0.7 (±0.1) 3.5 (±0.3) 3.3 (±0.3) 1.0 (±0.1) 1.2 (±0.1) 3.8 (±0.3) 3.6 (±0.1)

NMHC 7.8 (±0.6) 6.1 (±0.0) 104 (±5) 95 (±5) 11.9 (±0.7) 11 (±1) 75 (±6) 72 (±3)

CO 80 (±22) 62 (±4) 323 (±52) 363 (±56) 206 (±23) 238 (±25) 568 (±36) 623 (±5)

NOx 0.4 (±0.7) 0.5 (±0.0) 5 (±2) 3.7 (±0.5) 7 (±2) 7.8 (±0.3) 13.5 (±0.3) 12.0 (±0.2)

CO2 g/km 52 (±4) 52.0 (±0.8) 77.3 (±0.1) 79 (±3) 133.2 (±0.8) 133.7 (±0.7) 164.5 (±0.2) 157.9 (±0.1)

NH3 7 (±4) 1.9 (±0.6) 11.1 (±0.9) 6(±1) 6.1 (±0.3) 8.0 (±0.0) 13.0 (±2) 15.4 (±0.0)

Ethanol 0.3 (±0.3) 0.8 (±0.1) 2.6 (±0.1) 7.2 (±2.7) 0.6 (±0.1) 0.7 (±0.0) 3.1 (±0.1) 7.0 (±0.9)

CH3CHOa 0.1 (±0.0) 0.2 (±0.1) 1.8 (±0.0) 2.7 (±0.0) 0.2 (±0.1) 0.2 (±0.1) 0.8 (±0.0) 1.4 (±0.1)

a Acetaldehyde; Euro 5 spark ignition emission limits (mg km-1) at 20—30 °C over the New European Driving Cycle (NEDC): THC = 100; NMHC = 68; CO = 1000; NOx = 60.

Fig. 3. Exhaust flow (Flow TP; m3 min-1) and NO, CO and NH3 emission rates (g s-1) of(a) PHV1 and (b) HV2 over the WLTC (grey) at 23 °C (left plots) and -7 °C (right plots) at full SOC.

within the range reported in previous studies for Euro 5 spark ignition vehicles at 23 and -7 °C (Suarez-Bertoa et al., 2014; Kean et al., 2009; Livingston et al., 2009).

Ethanol and acetaldehyde exhaust emissions of conventional vehicles have been shown to be associated to uncombusted ethanol fraction from the fuel blends. Ethanol and acetaldehyde EFs are

Fig. 3. (continued).

summarized in Table 2. Emissions of ethanol and acetaldehyde all conditions. Once the catalyst reached the optimal operating were present during the cold start and until the catalyst light-off at temperature, the emissions of these unregulated compounds were

Table 3

Total cold start emissions (g) of ethanol and acetaldehyde (CH3CHO) using E5 and E10 blends at 23 and -7 °C. In parentheses, maximum semi-dispersion.

PHV1 HV2

23 °C -7 °C 23 °C -7 °C

E5 E10 E5 E10 E5 E10 E5 E10

Ethanol CH3CHO 5.6 (±5.3) 1.9 (±0.0) 15 (±3) 3.8 (±0.0) 59 (±1) 41.6 (±0.7) 159 (±9) 63.2 (±0.0) 12 (±3) 5.0 (±0.3) 13 (±4) 5 (±2) 67 (±7) 19.5 (±0.4) 130 (±4) 32 (±1)

Fig. 4. Ammonia emission factors (mg km 1) over the WLTC. Hatched bar plots refer to experiments at -7 °C. Error bars represent maximum semi-dispersion from two tests.

below the detection limit. Emissions of acetaldehyde and ethanol were affected by ambient temperature, increasing at -7 °C (see Table 2). Similar EFs of ethanol and acetaldehyde were observed during the tests performed using E5 and E10 at 23 °C for two studied HEVs (see Table 2). Emissions were higher for E10 than for E5 at -7 °C. Together, emissions of ethanol and acetaldehyde accounted, on average, for about 5 and 10% of THC emitted when using E5 and E10, respectively. Of those, 81—99% were emitted during the cold start. The cold start period of the vehicle was defined based on the legislation for heavy duty vehicles (EC No 582/ 2011) (EC, 2009), which considers the period elapsing from the start of the test until the coolant temperature reaches 70 °C for the first time as the cold start. This period lasted about 1300 s (18.8 km) at 23 °C and 1600 s (20.4 km) at -7 °C for PHV1 and 360 s (1.9 km) at 23 °C and 380 s (2.3 km) at -7 °C for HV2. Cold start of PHV depends on the pure electric mode strategy implemented. Since EFs are expressed as mass distance-1, cold emissions that take place later in the WLTC (e.g. emissions from PHV1), are divided by a higher number (kilometres covered). Therefore, EFs would result to be lower even at equal emitted mass. For that reason, cold start emissions are also reported as total emissions (see Table 3).

Similar acetaldehyde EFs were reported for a fleet of gasoline LDVs tested over the Federal Test Procedure (FTP) driving cycle (fleet average 0.4 ± 0.1 mg km-1) by Durbin et al. (Durbin et al., 2007) and for a flex-fuel vehicle (FFV), also tested over the FTP using E10 and a gasoline blend containing 5.7% ethanol, (0.6 ± 0.4 mg km-1) by Karavalakis et al. (Karavalakis et al., 2012). Suarez-Bertoa et al. (Suarez-Bertoa et al., 2015a) reported similar ethanol (1 mg km-1) and higher acetaldehyde (3 mg km-1) EFs for a FFV tested over the WLTC using hydrous and anhydrous E10 blends.

PHV1 and HV2 fueled with E5 were also tested using an 88% battery SOC to be able to probe for changes on their emissions arising from a different SOC. While no changes were observed for

HV2 at 88% SOC compared to full charge SOC, a drastic increase of NOx, CO, CO2 and NH3 emissions was obtained from PHV1 (see Fig. 5). At 88% SOC, NH3, CO and CO2 emissions from PHV1 were more than 2 times higher, and NOx were 6 times higher than those obtained at full charge SOC. The reason for this increase was a change on the powertrains use strategy. As explained, PHV1 run on pure electric mode for 10 km when tested at battery full charge SOC. However, at 88% battery SOC, the ICE is used for most part of the WLTC, leading to much higher emissions. Since THC are emitted, during cold start, i.e., from the ignition of the ICE until the catalyst light-off, and this took place within the time-frame of the WLTC at the two studied SOC, THC emissions did not vary at the different battery SOC. These results suggest that PHVs performance and emissions may depend on their battery SOC.

4. Conclusions

Regulated and unregulated emissions from one light duty hybrid electric vehicle (HV2) and one plug-in hybrid electric vehicle (PHV1) have been studied over the WLTC using E5 and E10 fuel blends.

Results suggest that, as for conventional gasoline vehicles, low ambient temperatures lead to higher emissions of regulated and unregulated pollutants. Furthermore, no significant differences were observed for the regulated emissions when vehicles were tested using either E5 or E10. PHV1 resulted in lower emissions than HV2 due to pure electric strategy of the former, which relied on the continuous use of battery for up to 10 km of the WLTC. However, since THC were emitted at cold start, both vehicles showed similar THC emissions.

While no changes were observed for HV2 when tested at 88% battery SOC instead of full charge SOC, PHV1 resulted in much higher emissions of NOx, CO, CO2 and NH3 at 88% SOC than at full charge SOC. Suggesting that PHVs may performed differently according to their battery SOC. THC emissions were not affected by the battery SOC. These important differences in the observed emissions and behavior shed light on the fundamental unlikeness between the two technologies, plug-in electric hybrid and electric hybrid, which are often seen (and studied) as the same category.

NH3 emissions presented variations depending on fuel blend used. Thus, NH3 emissions from PHV1 were higher with E5 than with E10 at both 23 °C (7 ± 4 and 1.9 ± 0.6 mg km-1, for E5 and E10, respectively) and -7 °C (11.1 ± 0.9 and 6 ± 1 mg km-1, for E5 and E10, respectively).

Ethanol and acetaldehyde emissions were similar during the tests performed using E5 and E10 at 23 ° C, and higher for E10 than for E5 at -7 °C. These two compounds accounted for about 5% (using E5) and 10% (using E10) of THC emitted. Acetaldehyde average EFs (0.2 ± 0.1 and 1.7 ± 0.8 mg km-1, at 23 and -7 °C, respectively) were similar to those reported in the literature for gasoline and flex-fuel vehicles.

PHV1 and HV2 showed lower regulated emissions than conventional Euro 5 gasoline LDVs. However, their NH3, ethanol and acetaldehyde EFs were in the same range reported for these vehicles at 23 and -7 °C using E5 and E10 blends. Hence, although

Fig. 5. PHV1 exhaust flow (Flow TP; m3 min 1) and emission rates (g s 1) of NO, CO, NH3 over the WLTC (grey) at 23 °C at 88% SOC.

hybrid electric vehicles are presented as an alternative to reduce exhaust emissions, this is only true for the regulated pollutants.

Disclaimer

The opinions expressed in this manuscript are those of the authors and should in no way be considered to represent an official opinion of the European Commission.

Acknowledgements

The VELA staff is acknowledged for the skilful technical assistance, in particular M. Cadario, R. Colombo, G. Lanappe, P. Le Lijour and M. Sculati.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2016.04.021

References

Alvarez, R., Weilenmann, M., 2012. Effect of low ambient temperature on fuel consumption and pollutant and CO2 emissions of hybrid electric vehicles in real-world conditions. Fuel 97, 119-124. http://dx.doi.org/10.1016/ j.fuel.2012.01.022.

Andrade, J.B.d., Andrade, M.V., Pinheiro, H.L.C., 1998. Atmospheric levels of formaldehyde and acetaldehyde and their relationship with the vehicular fleet composition in Salvador, Bahia, Brazil. J. Braz. Chem. Soc. 9, 219-223. http:// dx.doi.org/10.1590/S0103-50531998000300004.

Behera, S.N., Sharma, M., 2010. Investigating the potential role of ammonia in ion chemistry of fine particulate matter formation for an urban environment. Sci. Total Environ. 408 (17), 3569-3575. http://dx.doi.org/10.1016/ j.scitotenv.2010.04.01 .

Bishop, G.A., Stedman, D.H., 2015. Reactive nitrogen species emission trends in three light-/medium-duty United States fleets. Environ. Sci. Technol. 49 (18), 11234-11240. http://dx.doi.org/10.1021/acs.est.5b02392.

Bouwman, A.F., Van Vuuren, D.P., Derwent, R.G., Posch, M., 2002. A global analysis of acidification and eutrophication of terrestrial ecosystems. Water Air Soil Poll. 141 (1-4), 349-382. http://dx.doi.org/10.1023/A:1021398008726.

Bradow, R.L., Stump, F.D., 1977. Unregulated Emissions from Three-way Catalyst Cars. In: SAE Technical Paper No. 770369. http://dx.doi.org/10.4271/770369.

Çagatay Bayindir, K., Gozùkùçùk, M.A., Teke, A., 2011. A comprehensive overview of hybrid electric vehicle: powertrain configurations, powertrain control techniques and electronic control units. Energ Convers. Manage. 52 (2), 1305-1313. http://dx.doi.org/10.1155/2011/571683.

Christian Thiel, J.K., Dilara, Panagiota, 2015. Electric vehicles in the EU from 2010 to 2014 is full scale commercialisation near? Jt. Res. Centre Eur. Comm.

Clairotte, M., Adam, T.W., Chirico, R., Giechaskiel, B., Manfredi, U., Elsasser, M., Sklorz, M., DeCarlo, P.F., Heringa, M.F., Zimmermann, R., Martini, G., Krasenbrink, A., Vicet, A., Tournié, E., Prévôt, A.S.H., Astorga, C., 2012. Online characterization of regulated and unregulated gaseous and particulate exhaust emissions from two-stroke mopeds: a chemometric approach. Anal. Chim. Acta 717 (0), 28-38. http://dx.doi.org/10.1016/j.aca.2011.12.029.

Czerwinski, J., Heeb, N., Zimmerli, Y., Forss, A., Hilfiker, T., Bach, C., 2010. Unregulated Emissions with TWC, Gasoline & CNG. In: SAE Technical Paper Series 2010-01-1286. http://dx.doi.org/10.4271/2010-01-1286.

Dardiotis, C., Martini, G., Marotta, A., Manfredi, U., 2013. Low-temperature cold-start gaseous emissions of late technology passenger cars. Appl. Energy 111 (0), 468-478. http://dx.doi.org/10.1016Zj.apenergy.2013.04.093.

Durbin, T.D., Miller, J.W., Younglove, T., Huai, T., Cocker, K., 2007. Effects of fuel ethanol content and volatility on regulated and unregulated exhaust emissions for the latest technology gasoline vehicles. Environ. Sci. Technol. 41 (11), 4059-4064. http://dx.doi.org/10.1021/es061776o.

EC. Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. Off. J. Eur. Union. L 140, 2009.

Erisman, J.W., Grennfelt, P., Sutton, M., 2003. The European perspective on nitrogen emission and deposition. Environ. Int. 29 (2-3), 311-325. http://dx.doi.org/ 10.1016/S0160-4120(02)00162-9.

EU, 2014. Regulation (EU) No 333/2014 of the European Parliament and of the Council of 11 March 2014 amending Regulation (EC) No 443/2009 to define the modalities for reaching the 2020 target to reduce CO2 emissions from new passenger cars. Off. J. Eur. Union. L 103/15.

European Commission, 2008. Commission regulation (EC) No 692/2008 of 18 July 2008 implementing and amending regulation (EC) No 715/2007 of the European parliament and of the council on type-approval of motor vehicles with respect to emissions from light passenger and commercial vehicles (Euro 5 and Euro 6) and on access to vehicle repair and maintenance information. Off. J. Eur. Commun. 1 -136.

European Commission, 2013. Renewable Energy Progress and Biofuels Sustain-ability. Report from the Commission to the European Parliament, The Council, The European Economic and Social Committee and the Committee of the Regions.

Fontaras, G., Pistikopoulos, P., Samaras, Z., 2008. Experimental evaluation of hybrid vehicle fuel economy and pollutant emissions over real-world simulation driving cycles. Atmos. Environ. 42 (18), 4023-4035. http://dx.doi.org/10.1016/ j.atmosenv.2008.01.053.

Graham, L.A., Belisle, S.L., Baas, C.-L., 2008. Emissions from light duty gasoline

vehicles operating on low blend ethanol gasoline and E85. Atmos. Environ. 42 (19), 4498—4516. http://dx.doi.org/10.1021/es403096v.

Hannan, M.A., Azidin, F.A., Mohamed, A., 2014. Hybrid electric vehicles and their challenges: a review. Renew. Sust. Energ Rev. 29, 135—150. http://dx.doi.org/ 10.1016/j.rser.2013.08.097.

Heeb, N.V., Forss, A.-M., Bruhlmann, S., Luscher, R., Saxer, C.J., Hug, P., 2006. Three-way catalyst-induced formation of ammonia—velocity- and acceleration-dependent emission factors. Atmos. Environ. 40 (31), 5986—5997. http:// dx.doi.org/10.1016/j.atmosenv.2005.12.035.

Huai, T., Durbin, T.D., Miller, J.W., Pisano, J.T., Sauer, C.G., Rhee, S.H., Norbeck, J.M., 2003. Investigation of NH3 emissions from new technology vehicles as a function of vehicle operating conditions. Environ. Sci. Technol. 37 (21), 4841—4847. http://dx.doi.org/10.1021/es030403.

ICCT, European, 2014. Vehicle Market Statistics.

Karavalakis, G., Durbin, T.D., Shrivastava, M., Zheng, Z., Villela, M., Jung, H., 2012. Impacts of ethanol fuel level on emissions of regulated and unregulated pollutants from a fleet of gasoline light-duty vehicles. Fuel 93 (0), 549—558. http:// dx.doi.org/10.1016/j.fuel.2011.09.021.

Karavalakis, G., Short, D., Vu, D., Villela, M., Asa-Awuku, A., Durbin, T.D., 2014. Evaluating the regulated emissions, air toxics, ultrafine particles, and black carbon from SI-PFI and SI-DI vehicles operating on different ethanol and iso-butanol blends. Fuel 128 (0), 410—421. http://dx.doi.org/10.1016/ j.fuel.2014.03.016.

Kean, A.J., Littlejohn, D., Ban-Weiss, G.A., Harley, R.A., Kirchstetter, T.W., Lunden, M.M., 2009. Trends in on-road vehicle emissions of ammonia. Atmos. Environ. 43 (8), 1565—1570. http://dx.doi.org/10.1016/j.atmosenv.2008.09.085.

Livingston, C., Rieger, P., Winer, A., 2009. Ammonia emissions from a representative in-use fleet of light and medium-duty vehicles in the California South Coast Air Basin. Atmos. Environ. 43 (21), 3326—3333. http://dx.doi.org/10.1016/ j.atmosenv.2009.04.009.

Marotta, A., Pavlovic, J., Ciuffo, B., Serra, S., Fontaras, G., 2015. Gaseous emissions from light-duty vehicles: moving from NEDC to the new WLTP test procedure. Environ. Sci. Technol. 49 (14), 8315—8322. http://dx.doi.org/10.1021/ acs.est.5b01364.

Millet, D.B., Guenther, A., Siegel, D.A., Nelson, N.B., Singh, H.B., de Gouw, J.A., Warneke, C., Williams, J., Eerdekens, G., Sinha, V., Karl, T., Flocke, F., Apel, E., Riemer, D.D., Palmer, P.I., Barkley, M., 2010. Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations. Atmos. Chem. Phys. 10 (7), 3405—3425. http://dx.doi.org/10.5194/ acp-10-3405-2010.

Millet, D.B., Apel, E., Henze, D.K., Hill, J., Marshall, J.D., Singh, H.B., Tessum, C.W., 2012. Response to comment on "natural and anthropogenic ethanol sources in North America and potential atmospheric impacts of ethanol fuel use". Environ. Sci. Technol. 47 (4) http://dx.doi.org/10.1021/es305112s, 2141—2141.

Pope, I.C., Burnett, R.T., Thun, M.J., et al., 2002. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. JAMA 287 (9), 1132—1141. http://dx.doi.org/10.1001/jama.287.9.1132.

SAE, 2014. J1979-DA, digital annex of E/E diagnostic test mode. SAE Int.

Suarez-Bertoa, R., Zardini, A.A., Astorga, C., 2014. Ammonia exhaust emissions from spark ignition vehicles over the New European driving cycle. Atmos. Environ. 97 (0), 43—53. http://dx.doi.org/10.1016/j.atmosenv.2014.07.050.

Suarez-Bertoa, R., Zardini, A.A., Keuken, H., Astorga, C., 2015. Impact of ethanol containing gasoline blends on emissions from a flex-fuel vehicle tested over the Worldwide Harmonized Light duty Test Cycle (WLTC). Fuel 143, 173—182. http://dx.doi.org/10.1016/j.fuel.2014.10.076.

Suarez-Bertoa, R., Zardini, A.A., Platt, S.M., Hellebust, S., Pieber, S.M., El Haddad, I., Temime-Roussel, B., Baltensperger, U., Marchand, N., Prévôt, A.S.H., Astorga, C., 2015. Primary emissions and secondary organic aerosol formation from the exhaust of a flex-fuel (ethanol) vehicle. Atmos. Environ. 117, 200—211. http:// dx.doi.org/10.1016/j.atmosenv.2015.07.006.

Suarez-Bertoa, R., Zardini, A., Lilova, V., Meyer, D., Nakatani, S., Hibel, F., Ewers, J., Clairotte, M., Hill, L., Astorga, C., 2015. Intercomparison of real-time tailpipe ammonia measurements from vehicles tested over the new world-harmonized light-duty vehicle test cycle (WLTC). Environ. Sci. Pollut. R. 22 (10), 7450—7460. http://dx.doi.org/10.1007/s11356-015-4267-3.

Sutton, M.A., Dragosits, U., Tang, Y.S., Fowler, D., 2000. Ammonia emissions from non-agricultural sources in the UK. Atmos. Environ. 34 (6), 855—869. http:// dx.doi.org/10.1016/S1352-2310(99)00362-3.

UNECE, 2015. Global Technical Regulation No. 15. In: Worldwide Harmonized Light Vehicles Test Procedure. UNECE, Geneva, Switzerland.

US Department of Health and Human Services, 2011. Report on carcinogens. In: P.H.S. US Department of Health and Human Services, National Toxicology Program.

U.S. EPA, Chemical Summary for Acetaldehyde, U.S.Environmental Protection Agency Washington, DC.

Whittington, B.I., Jiang, C.J., Trimm, D.L., 1995. Vehicle exhaust catalysis: I. The relative importance of catalytic oxidation, steam reforming and water-gas shift reactions. Catal. Today 26 (1), 41—45. http://dx.doi.org/10.1016/0920-5861(95) 00093-U.