Diesel passenger car PM emissions: From Euro 1 to Euro 4 with particle filter

Atmospheric Environment 44 (2010) 909e916

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

Diesel passenger car PM emissions: From Euro 1 to Euro 4 with particle filter Theodoros Tzamkiozis, Leonidas Ntziachristos*, Zissis Samaras Laboratory of Applied Thermodynamics, Aristotle University of Thessaloniki, P.O. Box 458, GR 54124, Thessaloniki, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2009 Received in revised form 27 November 2009 Accepted 2 December 2009

This paper examines the impact of the emission control and fuel technology development on the emissions of gaseous and, in particular, PM pollutants from diesel passenger cars. Three cars in five configurations in total were measured, and covered the range from Euro 1 to Euro 4 standards. The emission control ranged from no aftertreatment in the Euro 1 case, an oxidation catalyst in Euro 2, two oxidation catalysts and exhaust gas recirculation in Euro 3 and Euro 4, while a catalyzed diesel particle filter (DPF) fitted in the Euro 4 car led to a Euro 4 þ DPF configuration. Both certification test and realworld driving cycles were employed. The results showed that CO and HC emissions were much lower than the emission standard over the hot-start real-world cycles. However, vehicle technologies from Euro 2 to Euro 4 exceeded the NOx and PM emission levels over at least one real-world cycle. The NOx emission level reached up to 3.6 times the certification level in case of the Euro 4 car. PM were up to 40% and 60% higher than certification level for the Euro 2 and Euro 3 cars, while the Euro 4 car emitted close or slightly below the certification level over the real-world driving cycles. PM mass reductions from Euro 1 to Euro 4 were associated with a relevant decrease in the total particle number, in particular over the certification test. This was not followed by a respective reduction in the solid particle number which remained rather constant between the four technologies at 0.86  1014 km1 (coefficient of variation 9%). As a result, the ratio of solid vs. total particle number ranged from w50% in Euro 1e100% in Euro 4. A significant reduction of more than three orders of magnitude in solid particle number is achieved with the introduction of the DPF. However, the potential for nucleation mode formation at high speed from the DPF car is an issue that needs to be considered in the over all assessment of its environmental benefit. Finally, comparison of the mobility and aerodynamic diameters of airborne particles led to fractal dimensions dropping from 2.60 (Euro 1) to 2.51 (Euro 4), denoting a more loose structure with improving technology. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Particle number Exhaust emissions DPF Emission standard Particle density

1. Introduction The diesel engine has historically been the powertrain of choice for trucks, trains, ships and other heavy-duty applications worldwide, due to its advantageous characteristics over other concepts, namely efficiency, reliability and durability. More recently, and especially in Europe, diesel engines have also become widespread for passenger cars and light-duty trucks. Diesel car registrations increased from 13% of all car registrations in 1990e53% in 2007 (ACEA, 2009). The increasing penetration of diesel cars in the market forced regulations to introduce more stringent emission standards. Euro 1 standards were introduced in 1992 and then, gradually, Euro 2 in 1996, Euro 3 in 2000 and Euro 4 in 2005. Euro 5 is expected in 2010 which will require the use of a diesel particle

* Corresponding author. Tel.: þ302310996003; fax: þ302310996019. E-mail address: [email protected] (L. Ntziachristos). 1352-2310/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.12.003

filter (DPF) to reach the PM emission limit. The significant emission reductions over the years were achieved with a combination of engine design optimization, availability of refined fuels, and the use of exhaust aftertreatment devices. Diesel combustion is characterised by lean air-to-fuel ratios, which lead to very low total hydrocarbon (THC) and carbon monoxide (CO) emissions. On the other hand, the high combustion temperature is responsible for increased nitrogen oxides (NOx) production, while the diffusion combustion of fuel is responsible for elevated emissions of particulate matter (PM), compared to gasoline cars. Particular focus is given to NOx and PM emissions, because of their significant impact on both health and the environment (e.g. Dockery and Pope, 1994; Chapman, 2007). Regarding PM, a number of epidemiologic studies reviewed by the California Air Resources Board (ARB, 1998) confirm an association between ambient PM and adverse health outcomes, including mortality rates, respiratory related hospital admissions, asthma attacks, and aggravation of chronic diseases. Additionally, animal studies

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showed that prolonged exposure of rats to high concentrations of diesel PM (2e10 mg m3) initiated a dose-dependent progression of cellular changes that eventually led towards the development of benign and malicious lung tumours (NIOSH, 1988). With respect to environmental effects, PM is characterised as an absorbing aerosol (Menon et al., 2002). Absorbing aerosols heat the air, alter regional atmospheric stability and vertical motions, and affect the largescale circulation and hydrologic cycle with significant regional climate effects. NOx, on the other hand, when combined with HC in the presence of sunlight, form a secondary class of pollutants, the photochemical oxidants, among them ozone and peroxyacetyl nitrate (PAN) (Jakobi and Fabian, 1997; Rappengluck et al., 2004). NO also reacts with oxygen to form nitrogen dioxide (NO2) which has longterm effects on the respiratory system and on lung function, especially in children (Pandey et al., 2005; Neuberger et al., 2007). One major deficit of the current emission control regulations is that type-approval is given by testing the vehicle under certification on a non-realistic driving cycle. The New European Driving Cycle (NEDC) is a cycle which involves long parts at constant speeds and mild accelerations. To put it in perspective, while a current mid-range passenger car can achieve accelerations in the order of 2.8 m s2, the NEDC acceleration is only 0.74 m s2. As it is wellknown that driving dynamics have a strong impact on pollutant emissions (Joumard et al., 2000; Ntziachristos and Samaras, 2000), the emission level over the NEDC is an under-representation of the real-world emission performance. Moreover, it is questionable whether emission benefits achieved over the NEDC by the various technology steps are also valid over more realistic driving conditions. In this direction, this paper examines the real-world efficiency of the emission control technologies employed over the years by conducting tests over real-world cycles. To this aim, three vehicles were tested in total four configurations to cover the whole range from Euro 1 to Euro 4 emission standards. In addition, retrofitting one of the vehicles with a diesel particle filter made possible to simulate the expected effectiveness of the upcoming Euro 5 PM regulation. Gaseous and particulate pollutants were measured, including the number of airborne particles. The information presented can provide useful input to inventories as well as studies on the actual impact of diesel passenger cars on urban air quality.

(NOx limit remained the same between the two classes). The second car was a 1.9 l Euro 3 common-rail diesel (Renault Laguna 1.9 dCi) equipped with its original exhaust line, consisting of a precatalyst and a main underfloor catalyst. The third vehicle was a 2.2 l Euro 4 diesel car (Honda Accord 2.2i-CTDi) equipped with highpressure common-rail fuel injection, an oxidation pre-catalyst (DOC) and a two-stage oxidation underfloor catalyst with de-NOx characteristics, the so-called “4-way catalyst” (Tomoya et al., 2004). This vehicle was measured in two different configurations; first in its original configuration, and also by using a diesel particle filter (DPF) which replaced the underfloor catalyst. The DPF was a Ptcoated 5.66  600 (DL) SiC filter, consisting of 16 segments and a cell density of 279 (cells in2). This technology (oxidation catalyst and DPF) is expected to be the technology of choice in meeting the Euro 5 emission limits. Therefore this vehicle can be considered to simulate the Euro 5 PM emission control with regard to its exhaust aftertreatment. We will call this configuration as Euro 4 þ DPF. Table 1 summarizes the most important information about the vehicles.

2.2. Fuels and lubricants Two different fuel grades were used in these tests. Both of them complied with the specifications of the European Directive 2003/ 17/EC but their sulphur content was selected to be representative of the fuel sulphur level at the time of introduction of each vehicle technology. Hence, the sulphur content was 8 ppm wt. in case of the Honda Accord 2.2 CTDi in both the Euro 4 and Euro 4 þ DPF configurations, and 50 ppm wt. for the other two vehicles. The lubricating oil used in each vehicle was of viscosity and quality grade recommended by the manufacturers.

2.3. Test protocol The tests were conducted on a chassis dynamometer. The first driving cycle over a measurement day was a cold-start New European Driving Cycle (NEDC), which was followed by the so-called Artemis driving cycles. The suite of Artemis cycles consists of three distinct cycles, each considered representative of real-world urban (Urban), rural (Road) and highway (Motorway) conditions in Europe (André, 2004). Compared to the certification test, the Artemis driving cycles exhibit more frequent speed variation and stronger accelerations. In addition, the vehicles were tested at a constant cruising speed of 50 and 120 km per hour (kph) in order to sample exhaust at steady-state conditions. At least two measurements were conducted per configuration. The day before each measurement, the vehicle and the CVS tunnel were conditioned by driving three consecutive extra-urban driving cycle repetitions. After this procedure the vehicle was left to soak for at least 8 h overnight in order to achieve a cold-start in the following day.

2. Methodology 2.1. Test vehicles The sample of vehicles comprised of three diesel passenger cars. The first was a 1.9 l Euro 2 diesel (VW 1.9 TDI Golf), originally equipped with an oxidation catalyst. This vehicle was tested in an alternative configuration with its catalyst removed to simulate Euro 1 levels, as the main difference of Euro 2 over Euro 1 was the addition of an oxidation catalyst in the former to reduce PM levels

Table 1 Vehicle and fuel characteristics used in the experiments. Vehicle type

Engine size (l)

Emission standard

VW Golf TDI VW Golf TDI Renault Laguna dCi

1.9 1.9 1.9

Euro 1 (simul.) Euro 2 Euro 3

Honda Accord CTDi Honda Accord CTDi

2.2 2.2

Euro 4 Euro 4 þ DPF

Fuel S content (ppm wt.) 50 50 50 <10 <10

Emission control

Mileage (  103 km)

None Underfloor oxidation catalyst Closed-coupled (pre-catalyst) & underfloor oxidation catalysts, EGR Oxidation pre-catalyst, 4-way catalyst, EGR Pre-catalyst, ceramic DPF, EGR

150 165 85 16 25

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2.4. Emissions sampling Fuel consumption and regulated pollutant emissions (CO, HC, NOx, PM) were measured following the regulations (70/220/EEC with amendments), by first conditioning the exhaust in a constant volume sampling (CVS) dilution tunnel. A non-dispersive infrared (NDIR e Horiba VIA-510) analyzer was used for CO determination, a flame ionization detector (FID e Signal 3000) downstream of a heated line was used for HC monitoring, and a chemiluminscence detector (CLD e Signal 4000) was used for NOx. Airborne particle number concentration and size distributions were determined using a dedicated sampling system, shown in Fig. 1. Samples were drawn from the CVS dilution tunnel with a Dekati Fine Particle Sampler (FPS- 4000) operating at a nominal dilution ratio of 12:1. The dilution ratio of the FPS was determined daily by measuring the upstream (CVS) and downstream (FPS) CO2 concentration. Depending on the emission levels of the vehicle tested, one or two additional ejector-type calibrated diluters (Giechaskel et al., 2004) were employed, each providing a dilution ratio of about 10:1 (the exact value was verified in each measurement day), in order to bring the particle emissions levels within the measurement range of the instruments. In some cases (Euro 1, 2 vehicles), where further dilution of the sample was necessary, a third dilution stage was used. This extra dilution was achieved by installing an in-house dilution stage, consisting of a mixing chamber and a HEPA capsule filter between the two ejector dilutors. The dilution stage provided an additional dilution ratio of about 10:1. Hence, the total DR downstream of the CVS ranged from w120:1 (DPF vehicle) to w12 000:1 (Euro 1 vehicle). The exact dilution ratio of the secondary system was determined with gas concentration measurements before measurements with each vehicle configuration commenced. A Condensation Particle Counter (TSI 3010 CPC) was employed to monitor the total particle number concentration. A Dekati Electrical Low Pressure Impactor (ELPI) provided the aerodynamic size distribution in real time. The ELPI operated with wet

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(oil-soaked) sintered plates and a filter stage that extended the lower cutpoint to w7 nm (Marjamäki et al., 2002). ELPI was sampling through a thermodenuder (Dekati Ltd), operating at 250  C, which removed volatile and semi-volatile exhaust components. A scanning mobility particle sizer (TSI 3936 SMPS) was used instead of the CPC over steady-speed tests (50e120 kph) to record the mobility size distribution. The sampling system, consisting of an FPS and a thermodenuder to isolate non-volatile particles, has been shown to be consistent with the PMP sampling system within 1% (Giechaskiel et al., 2008b). The ELPI data reduction requires knowledge of the effective particle density to provide an accurate measurement of the particle number concentration. The effective density was calculated according to the fitting procedure described by (Virtanen et al., 2004) over the steady-state tests for each vehicle configuration, and then it was also used for the transient cycles. The procedure and the effective density values per vehicle are presented in the Results section. Moreover the ELPI results have been corrected for diffusion and space charge losses (Virtanen et al., 2001) as well as for thermophoretic losses (Giechaskel et al., 2009) inside the thermodenuder. It should be noted that our results of solid particle number concentration include particles down to 7 nm, compared to 23 nm defined by the PMP protocol. However, our aim was not to replicate the PMP protocol but to provide information on the volatile and non-volatile particle emissions of the selected vehicle technologies. 3. Results and discussion 3.1. Regulated pollutants All vehicles were first measured over the certification test and all regulated pollutants were found to be within the conformity of production emission levels. Emission over the Artemis real-world cycles are shown in Fig. 2, in comparison with the emission limits per Euro standard. Additionally, Table 2 shows the ratio of emissions rate (g km1) in the Artemis cycles compared to the NEDC.

Fig. 1. Schematic of the airborne particle measurement sampling system.

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a

1.2 Euro 2 1.0

Euro 2 Euro 3

CO [g/km]

0.8

Euro 3

Euro 4 0.6 Euro4

0.4 0.2 0.0 0.00

0.02

0.04

0.06

0.08

HC [g/km]

b

1,4

Euro 2

NOx or HC+NOx [g/km]

1,2

Artemis Urban

Euro 3 Euro 4

1,0

Euro 2

0,8

Euro4+DPF 0,6 0,4

Euro 3 Euro 4

0,2

Euro 5

0,0 0,00

0,02

0,04

0,06

0,08

0,10

0,12

PM [g/km] Fig. 2. Emission levels over the real-world driving cycles and comparison with the emission standards. (a) CO and HC emissions, (b) NOx and PM emissions (vertical axis is the sum NOx þ HC for the Euro 2 car as this is the definition of the emission standard. Emissions are practically dominated by NOx).

All vehicles tested emit below the CO and HC emission standards and also lower than the NEDC emission levels. This result is explained by the fact that after the “cold-start” NEDC, the catalyst has reached its light-off temperature and its oxidation efficiency has been increased. The only exception is the HC emission of the Euro 4 vehicle over the Artemis Urban cycles. Over this cycle the HC emissions are 21% higher compared to the emission levels over the NEDC and this can be attributed to the HC trapping effect of the zeolite catalyst (Leyrer, 1996). This catalyst tends to trap HC, when

Table 2 Relative emissions over the artemis cycles expressed in percentage difference over the certification (NEDC) emission level. Pollutant

CO NOx HC PM

Euro 2

Euro 3

Euro 4

Urban

Road

Mtw

Urban

Road

Mtw

Urban

Road

Mtw

59 þ121 36 þ30

90 þ6 73 þ25

93 þ48 83 þ40

60 þ121 43 þ59

91 þ11 63 þ18

94 þ39 84 þ40

82 þ261 þ21 þ28

91 þ113 41 30

91 þ165 77 28

the temperature is low and release them into a temperature range where the true catalytic oxidation of HC starts. The HC stored during the NEDC are emitted over the Urban cycle. In any case, CO and HC have not been considered priority pollutants from diesel vehicles. The picture looks different in case of the PM and, in particular, NOx emissions. These two pollutants increased substantially over the three Artemis cycles compared to the NEDC levels. As a result, all vehicles exceeded their certification level over the real-world Artemis cycles. The relative magnitude of this exceedance is shown in Table 2. Over the Urban cycle, NOx emissions were 2.2 times higher for Euro 2 and Euro 3 vehicles, while a minor increase was observed over the Road cycle, in the range of 10%. Under motorway driving, both vehicles emitted approximately 45% higher. The departure from the NEDC level is more significant for the Euro 4 car with the Urban cycle leading to w 3.6 times higher emissions, compared to the NEDC. These results confirm the significant effect of driving conditions on the emissions, which has been demonstrated in several works (Watson, 1995; USEPA, 1997; Joumard et al., 2000; Ericsson, 2001; Frey et al., 2006). (André and Rapone, 2009) showed that the urban congested driving (with a lot of stops and accelerations) produces high NOx emissions. The more dynamic speed pattern of the Artemis cycles compared to the NEDC is therefore responsible of the increase in the emission level. The important relative departure of the Euro 4 car emission rate, compared to the earlier technologies is the result of the exhaust gas recirculation (EGR) and de-NOx catalyst optimization inside the NEDC operation window. Operation of the vehicle at higher engine speed and/or load may lead to reduced efficiency (de-NOx catalyst) or complete switch-off of the emission control (EGR). Certification level exceedances also appear for PM, but they are generally less than NOx. In addition, the Euro 4 car emits below the emission standard over the Road and Motorway cycles. The moderate increase, or even decrease, should primarily be associated with the higher catalyst efficiency over the hot exhaust Artemis cycles compared to the cold-start NEDC. This could counterbalance, to a certain extent, the effect of the cycle dynamics on the emission performance. In particular, the Euro 4 decrease over Road and Motorway could be a result of engine tuning towards a higher NOx-lower PM mode, outside the NEDC window, in order to reduce fuel consumption. This is a common practice, especially in heavy-duty engines (Hausberger and Rexeis, 2004) and it is justified by the high NOx emissions of the particular vehicle over the same driving cycles. 3.2. Particle number emissions Emission regulations in Europe move to the direction of controlling particle number. The PMP programme developed the technical requirements for conducting non-volatile particle measurements (Giechaskiel et al., 2008a) and Euro 5 regulations introduce an emission standard for DPF-equipped cars (the current standard being at 6  1011 km1). The next sections discuss the evolution of particle number of the vehicle technologies examined, in the light of the upcoming regulations. 3.2.1. Effective particle density The particle effective density (reff) may be used to provide useful input on the effect of vehicle technology on particle characteristics. Diesel exhaust particles are agglomerates with a complex morphology (Burtscher, 2005). Their aerodynamic diameter can be linked to the mobility one, by means of their “effective density”, i.e. the density of a sphere which has the same aerodynamic and mobility diameters as the agglomerated particle (Kelly and McMurry, 1992). Due to the ELPI operation characteristics,

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knowledge of the effective density leads to an improved number measurement by the ELPI (Mamakos et al., 2006). In order to derive the effective density function with size, the method proposed by (Virtanen et al., 2004) has been used, by composing the ELPI and SMPS size distributions at 50 kph (Fig. 3). For these measurements, both the SMPS and the ELPI were sampling downstream of the same dilution stage, in order to eliminate any uncertainties related to the sampling position and the dilution ratio determination. This fitting procedure led to the effective density profile presented in Table 3 for each vehicle. The particle density is considered constant for particles smaller than 40 nm, at a value equal to the reff,ref presented in Table 3. For larger particles the effective density scales with the fractal particle dimension (DF) according to Eq. (1).

14

12

Euro 1-4 (×10 )

Euro 4+DPF (×10 )

4.0

4.0 Euro 1

3.5

3.5 Euro 2 3.0 Euro 3

-1

dN/dlog dp (km )

3.0 2.5

2.5 Euro 4

2.0

Euro 4+DPF

1.5

2.0 1.5

1.0

1.0

0.5

0.5

0.0 1

10

100

1000

0.0 10000

Aerodynamic Diameter (nm)

b

14

12

Euro 1-4 (×10 )

Euro 4+DPF (×10 )

2.0

2.0 Euro 1

1.8

1.6

Euro 2

1.6

1.4

Euro 3

1.4

1.2

Euro 4

1.2

1.0

Euro 4+DPF

1.0

-1

dN/dlogdp (km )

1.8

0.8

0.6

0.4

0.4

0.2

0.2 1

10

100

1000

Vehicle

Emission standard

DF

reff,ref @ 40 nm (g cm3)

VW Golf TDI VW Golf TDI Renault Laguna dCi Honda Accord CTDi Honda Accord CTDi

Euro Euro Euro Euro Euro

2.60 2.62 2.50 2.51 2.48

0.91 0.85 0.86 0.87 0.83

reff ðdb Þ ¼ reff;ref$

0.0 10000

Mobility Diameter (nm) Fig. 3. Solid (a) and total (b) particle size distributions at 50 kph. ELPI distribution calculated assuming unit particle density. These distributions have been used to derive the effective density profile.

db

1 (simul.) 2 3 4 4 þ DPF

!DF3 (1)

db;ref

In Eq. (1), reff,ref is the effective density value at an arbitrary reference mobility diameter (db,ref) and reff (db) is the effective density value at a different diameter (db). Both reff,ref and DF are determined by the method proposed by (Virtanen et al., 2004), using the algorithm developed by (Mamakos et al., 2006). The effective density profiles depend on vehicle technology. A rather monotonic decrease of the reference value from 0.91 to 0.83 g cm3 and DF values from 2.60 to 2.48 is observed as the technology improves. The absolute level of the values reported is within the range observed in other studies of passenger cars. (Maricq and Xu, 2004) measured a light-duty vehicle at 64 kph and found a DF of 2.3. (Virtanen et al., 2004) came up with a DF value of 2.8 from a light-duty Euro 2 vehicle at low load. Additionally, (Olfert et al., 2007) reported values from 2.22 to 2.48 at low loads from a Euro 3 passenger car. Our values, in particular from the Euro 1 car, appear within the range of the literature values. The high DF and reff,ref values correspond to more dense particles with a smaller void volume and a more spherical shape. This may be the effect of the higher in-cylinder soot concentration in older vehicle technologies. 3.2.2. Certification cycle Use of the effective density profile to correct the ELPI signal makes possible the one-to-one comparison of ELPI (solid particles) and CPC (total particles) in Fig. 4. With respect to solid particles, all vehicles except Euro 4 þ DPF emit at an average 0.86  1014 km1 with a coefficient of variation (CoV) of 9% around this value. When taking into account the individual variability of the measurements for each car, it becomes even more obvious that the solid particle emissions have remained rather unaffected by the various emission control technologies employed over the years, at least over the NEDC. Therefore the reduction in PM mass imposed by the emission standards (Fig. 2), should originate either from a decrease in mean particle size or control of the non-solid PM fraction.

Euro 4+DPF

Euro 1-4

0.8

0.6

0.0

Table 3 Estimated fractal dimension (DF) and effective density (reff,ref) at 40 nm.

2.0E+14

Parti cle n u mb er [# / k m]

a

913

2.0E+12

Total Solid

1.5E+14

1.5E+12

1.0E+14

1.0E+12

5.0E+13

5.0E+11

0.0E+00

0.0E+00

Euro 1

Euro 2

Euro 3

Euro 4

Euro 4+DPF

Fig. 4. Solid and total particle number emitted by the five vehicles over the NEDC. The error bars depict the standard deviation of the measurements and correspond to at least 2 repetitions.

T. Tzamkiozis et al. / Atmospheric Environment 44 (2010) 909e916

3.2.3. Real-world representative cycles Similar to regulated pollutants, driving conditions may have an effect on particle number emissions. Therefore, it is worth examining the total and solid particle number over the real-world driving cycles as well. These are shown in a correlation plot in Fig. 5a (no results available for Euro 1). The fact that most points lie close to the y ¼ x line points towards the conclusion that the emitted particles are mainly solid. However, Euro 2, Euro 4 þ DPF, and in particular Euro 3 vehicles depart from the y ¼ x line over the Motorway driving cycle, denoting increased emission of non-solid particles under highspeed driving conditions. Fig. 5b provides an explanation for this departure, showing the total particle size distribution at 120 kph, which is a common motorway cruising speed. The figure depicts a distinct nucleation mode being formed by the Euro 3 vehicle which peaks at 21 nm. The Euro 2 vehicle also shows a somewhat increased concentration of particles in the sub-30 nm size range compared to the Euro 4 car.

a

1E+14

3E+14

Euro 2 Euro 3

1E+12

Euro 4

Solid N [#/km]

In principle, the total particle number emissions over the NEDC follow a trend similar to the emission standards improvement. The total particle number dropped from w1.5  1014 km1 on Euro 1 to about 0.75  1014 km1 on Euro 4, corresponding to a w50% reduction. Only the use of the oxidation catalyst reduced the total particle number by about 20%, when comparing the Euro 2 (with catalyst) and Euro 1 (without catalyst) technologies. Fig. 4 also shows the relative difference of total and solid particle number per technology level. This may help explain the reasons between the different trends of total vs. solid particle number emissions. At Euro 1 level the total particle number appears twice as high as the solid one. This difference drops to 30% for Euro 2 and diminishes for Euro 3 and 4 technologies. The fact that Euro 4 sold particle number appears marginally higher than the total one at Euro 4 level should probably be explained by the measurement variability (the error bars of both concentration strongly overlap) and uncertainties in the determination of the effective density related to the method of (Virtanen et al., 2004). However, the important message from Fig. 4 is that regulations mainly led to a control of the non-solid particle emissions. The use of oxidation aftertreatment and a decrease in sulphur level of the fuel between Euro 1 and Euro 4 have resulted in a decrease of the number of volatile and non-volatile particles. The results of (Vogt et al., 2003) and (Giechaskiel et al., 2005) confirm the positive effect of low sulphur fuel and an oxidation catalyst on the reduction of total particle number, even under real-world ambient dilution of the exhaust plume. In addition to fuel sulphur, the control of the PM organic fraction by means of the oxidation catalysts has been important. (Horiucho et al., 1990) reported that up to 50% of exhaust PM could be organic material from engine technologies similar to Euro 1. This can be effectively controlled by the oxidation catalyst. The upcoming emission control at Euro 5 level with the use of DPFs is expected to have a dramatic effect of up to three orders of magnitude of both solid and total particle number over the NEDC. The Euro 4 þ DPF vehicle measured in this study emitted a solid particle number of w3.0  1011 km1, i.e. 50% below the currently agreed 6  1011 km1 standard from PMP. Even the total particle number remains below the limit with use of fuel complying with the 2009 specifications in sulphur content. Therefore, there is a clear benefit in terms of particle number reduction with the use of DPF over the certification test. This is consistent with the findings of (Mohr et al., 2006), when they considered vehicles fitted with a catalyzed DPF as in our case, and with the real-world results of (Bergmann et al., 2009), when testing a Euro 4 car equipped with a similar DPF system.

Euro 4+DPF 2E+14

1E+10 1E+10

1E+12

1E+14

1E+14

1E+10 1E+10

1E+14

2E+14

3E+14

Total N [#/km]

b

1E+15 Euro 2 Euro 3 Euro 4

1E+14

N [#/km/dlogDp].

914

1E+13

1E+12

1E+11 1

10

100

1000

Mobility Diameter [nm] Fig. 5. a) Solid and total particle number emitted by the four vehicles (Euro 2, 3, 4, and Euro 4 þ DPF) over the Artemis cycles, (b) Mobility size distribution at 120 kph.

Although both fuel and lube oil used in the Euro 2 and Euro 3 vehicles have the same sulphur content (50 ppm wt.), the Euro 3 vehicle tends to form bimodal distributions characterised by high levels of nanoparticle emissions. This is partially explained by the fact that the Euro 3 vehicle is equipped with more efficient oxidation aftertreatment compared to Euro 2, since it is also equipped with a pre-catalyst before the main catalyst. The increased oxidation potential of the Euro 3 vehicle exhaust system leads to higher conversion rates of SO2 to SO3, which is the precursor of hydrated H2SO4 nuclei formation (Baumgard and Johnson, 1996; Vogt et al., 2003). The sulphuric acid nuclei act as condensation sites which promote the formation of a discrete particle nucleation mode. This mechanism has been theoretically examined by (Vouitsis et al., 2005) and is also supported by the measurements of (Maricq et al., 2002). The calculation of the particle volume, assuming the particles emitted at 120 kph as spheres, further explains the tendency of the Euro 3 vehicle to form bimodal distributions. The particle volume of

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the Euro 3 vehicle is 20% lower compared to Euro 2 one. The lower surface thus available for condensation is another reason which enhances self-nucleation in the Euro 3, compared to the Euro 2 case. No nucleation mode forms in the case of the Euro 4 vehicle, despite the further reduction in the accumulation mode volume, as the fuel sulphur content is very low in this case. Fig. 5a also shows that the Euro 4 þ DPF vehicle seems to strongly depart from the y ¼ x line under motorway driving conditions, despite the near-zero sulphur content of the fuel. Over Motorway, it emits two orders of magnitude more particles than over the NEDC. Although no size distributions are available over this driving cycle, the fact that PM mass remains at 3 mg km1 and the number concentration exceeds 0.8  1014 km1 is a clear indication that these particles are in their majority within the nucleation mode size range (<40 nm). This is the effect of the almost complete removal of solid particles by the DPF. These particles could have acted as condensation sites for semi-volatile and volatile components (sulphates and organics). In this case, selfnucleation is practically the only option for super-saturated components, and this leads to the formation of a strong nucleation mode. In addition, the high exhaust temperature reached over this driving cycle (in excess of 300  C) may initiate a mild regeneration of the filter. According to (Bergmann et al., 2009), who measured particle size distributions during a forced regeneration, this may lead to the formation of nucleation mode, similar to our case. Nucleation leads to total particle concentrations at the same level with Euro 4 car (inset in Fig. 5a). Although the PMP regulations make no reference to the measurement and control of non-solid particle number, current evidence suggests that the (semi-)volatile PM fraction is at least as oxidative in nature as the non-volatile fraction (Biswas et al., 2009; Cheung et al., 2009). Therefore, the possibility of formation of a large number of such particles at high speed when a particle filter is installed needs to be thoroughly assessed. With regard to solid particle emissions over the non-certification driving modes, the Euro 2 vehicle seems to emit practically the same number of solid particles, with and average level of 1.3  1014 km1 and a CoV of 6%, over all Artemis cycles (points over Urban and Road cycles overlapping in Fig. 5a). This number is w50% higher than over the NEDC, which means that the more aggressive conditions introduced by the Artemis cycles lead to a rather significant increase in particle number. Euro 3 emits 1.1  1014 km1 on average over all Artemis cycles (with a CoV of 16%), which is translated into 15% higher emissions than over the NEDC. Compared to Euro 2, the Euro 3 level is lower by 12%. This is a rather marginal reduction, which shows that the technology improvement from Euro 2 to Euro 3 has not been as effective in reducing particle number as the PM emission standards would have targeted to. The Euro 4 car emits somewhat lower, by w40% and w30% than the Euro 2 and Euro 3 respectively. Moreover, its solid particle number emissions do not significantly differ compared to the NEDC, with the Urban cycle being 12% higher and the Road and Motorway actually being lower than the NEDC. This shows that the development in diesel fuel combustion (high injection pressure, electronic control of injection timing and duration, optimized EGR) led to some control of solid particle number, practically over all typical driving conditions. However, the reductions still remain much below the more than 3-fold reduction in the PM standard from Euro 2 to Euro 4. Solid particles will be significantly reduced with the introduction of Euro 5 cars in 2010. The solid particle number from the Euro 4 þ DPF car was found three orders of magnitude lower than the Euro 4 (at 1.1  1011 km1 on average with a CoV of 20%) over all Artemis cycles, denoting that DPFs may be effective over practically all engine operation modes. The effectiveness of the

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DPF over all driving modes also means that its introduction at a Euro 5 level will lead to avoiding regulation loopholes of the past, at least for solid particles, where an emission target was met over the certification cycle and was exceeded most other driving conditions.

4. Conclusions The emissions of three different vehicles were characterised in this study over real-world driving cycles, in comparison to the certification one. The vehicles were measured in various configurations with the aim to cover the whole Euro-based range of emission standards with respect to PM. Although vehicle-specific emission effects cannot be excluded, all vehicles were equipped with engine technology and emission control systems which are typical in the European passenger cars of corresponding emission standards. The emission control ranged from no aftertreatment (Euro 1) to a car equipped with an oxidation pre-catalyst, exhaust gas recirculation and a diesel particle filter (Euro 4 þ DPF). The fuel used was representative of the grade used at time of introduction of each technology. Therefore, results of this study show the combined effect of vehicle and fuel technology improvement on emissions. The results showed that all technologies (from Euro 2 to Euro 4) exceeded the NOx emission standards over non-certification driving conditions. The exceedance reached 3.6 times the NEDC standard in the Euro 4 case. PM exceedances were also observed in the Euro 2 and Euro 3 cases, albeit at a lesser magnitude than NOx. Number emissions of both total and solid particles were also characterised, with the latter calculated from the ELPI signal after correcting for the effective particle density. Over the certification cycle, the technology improvement from Euro 1 to Euro 4 and the reduction of sulphur in the fuel led to a significant reduction in the non-volatile particle number, with solids representing 50% of the total number in Euro 1 and practically 100% in Euro 4. The solid particle number has not significantly decreased from Euro 1 to Euro 4 over the certification driving cycle. However, Euro 4 appears a lower emitter of solid particle number over real-world driving cycles, than both the Euro 2 and the Euro 3. The Euro 5 regulation will be introducing significant effects in PM emissions, with the particle filter leading to almost complete removal of solid particles, with levels more than three orders of magnitude less than current Euro 4 cars. However, results on a Euro 4 car retrofitted with DPF showed that despite the use of low sulphur fuel (<10 ppm), there is still the possibility to form nucleation mode particles at high-speed conditions. This needs to be looked at with regard to its environmental and toxicological effect. These observations lead to the conclusion that the technologies employed so far to reduce PM according to the Euro standards mainly aimed at reducing the non-solid particle fraction. This strategy led to new vehicles (first circulated in 2005), that despite the advanced technology and the sophisticated design, emit the same number of solid particles per kilometre as models first put in circulation in 1992. This is expected to change only with the introduction of diesel particle filters at Euro 5 level.

Acknowledgements The authors would also like to thank, Dr. Panayotis Pistikopoulos, Athanasios Papazacharias and Argiris Tzilvelis and for their support in the experimental part of this work.

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