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Effects of fresh lubricant oils on particle emissions emitted by a modern GDI passenger car Liisa Pirjola, Panu Karjalainen, Juha Heikkilä, Sampo Saari, Theodoros Tzamkiozis, Leonidas Ntziachristos, Kari Kulmala, Jorma Keskinen, and Topi Rönkkö Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es505109u • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Environmental Science & Technology

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Effects of fresh lubricant oils on particle emissions emitted by a

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modern GDI passenger car

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Liisa Pirjola1,2,*, Panu Karjalainen3, Juha Heikkilä3,6, Sampo Saari3, Theodoros

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Tzamkiozis4, Leonidas Ntziachristos4, Kari Kulmala5, Jorma Keskinen3, Topi Rönkkö3

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1

7

00180 Helsinki, Finland

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2

Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland

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3

Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O.

Department of Technology, Metropolia University of Applied Sciences, P.O. Box 4021, FI-

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Box 692, FI-33101 Tampere, Finland

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4

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Thessaloniki GR 54124, Greece

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5

Neste Oil Oyj, Keilaranta 21, P.O. Box 95, FI-00095 Neste Oil, Finland

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6

Wärtsilä Finland Oy, Järvikatu 2-4, FI-65200 Vaasa, Finland

Laboratory of Applied Thermodynamics, Aristotle University of Thessaloniki, P.O. Box 458,

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*Corresponding author contact information: phone +358 9 74246117;

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email: [email protected], [email protected]

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Supporting Information

20 21

Revision version submitted to Environmental Science & Technology 9.2.2015

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ACS Paragon Plus Environment


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TOC

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Abstract

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Particle emissions from a modern turbocharged GDI passenger car equipped with a three-way

28

catalyst and an exhaust gas recirculation system were studied while the vehicle was running on

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low-sulfur gasoline and, consecutively, with five different lubrication oils. Exhaust particle

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number concentration, size distribution and volatility were determined both at laboratory and on-

31

road conditions. The results indicated that the choice of lubricant affected particle emissions both

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during the cold start and warm driving cycles. However, the contribution of engine oil depended

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on driving conditions being higher during acceleration and steady state driving than during

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deceleration. The highest emission factors were found with two oils that had the highest metal

35

content. The results indicate that a 10 % decrease in the Zn content of engine oils is linked with

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an 11-13% decrease to the non-volatile particle number emissions in steady driving conditions

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and a 5 % decrease over the New European Driving Cycle (NEDC). The effect of lubricant on

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volatile particles was even higher, in the order of 20%.


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Introduction

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Despite their improved fuel economy and lower CO2 emissions, gasoline direct injection

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(GDI) engines have been reported to emit more particles in number than port fuel injection

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(PFI) engines. Most importantly, GDI engines emit more particles than modern diesel

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vehicles equipped with diesel particulate filters1,2,3. Elevated particle emissions affect local air

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quality and are hazardous to public health4,5,6,7,8. Exhaust particles also affect the climate by

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scattering or absorbing solar radiation and participating in cloud formation9,10.

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The particulate mass emitted from GDI vehicles has been reported to be composed primarily of

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carbonaceous material originating from unburned components of the fuel and the lubrication oil.11

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Sonntag et al.12 estimated that the contribution of lubrication oil to the fleet-averaged particulate

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mass emission rates can be even around 25%. Lubricants related emission primarily contributes to

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the organic fraction of particulate mass13,14,12 and is thus a source of secondary organic

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aerosol15,16, whereas fuel combustion products have been reported to dominate the elemental

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carbon fraction.14 Furthermore, some studies13, 54 showed that the emissions of aged lubrication

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oils can also be a significant source of PAH emissions.

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The exhaust particle number size distributions of GDI vehicles have been observed to possess

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two modes.17,18,19,20,21 The larger in size, i.e. the so-called soot mode, has a mean diameter

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between 45-60 nm21 and is composed primarily of carbonaceous agglomerates and adsorbed

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material. A second, smaller in size mode, often appears at and below 20 nm. These very small

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particles have previously been observed to be non-volatile and to mainly consist of

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amorphous carbon.19,17,22 The study of Sgro et al.19 indicated that these nanoparticles were

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formed in the combustion chamber, and they were composed of 5 nm non-volatile core onto

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which hydrocarbons were condensed. Some studies1,24,22 indicate that when driving with high

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engine load (high exhaust temperature) the GDI vehicle exhaust can also contain semi-volatile

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nucleation particles. The semi-volatile particle formation in GDI vehicle exhaust seems to



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depend on the fuel and/or lubrication oil originating sulfur,22 and the formation is qualitatively

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similar to the formation of semi-volatile nucleation particles in diesel engine exhaust.24,25,26

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Metallic compounds from lubrication oil or fuel have been found to increase the diesel

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exhaust particle number concentration in the size range below 20 nm and may affect the soot

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mode characteristics at high metal concentrations.27,28 Lubrication oil related metals (Ca, Zn,

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Mg) have been found in the sub-20 nm particles28, associated with emissions during engine

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braking,24,22 whereas metals (e.g. Fe, Mn) from the fuel have been found in larger particles in

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the accumulation mode.28,29 For example, Abdul-Khalek et al.52 proposed that solid

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nanoparticles of diesel exhaust originate from the lubrication oil metals, that is primarily Ca.

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Lähde et al.53 reported that, in the case of heavy duty off-road diesel engine without exhaust

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after-treatment, the change of engine oil affected the concentration of nanoparticles (mean

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diameter smaller than 10 nm) at idle conditions. This effect was suggested to be caused by the

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differences in the Ca concentration of lubrication oils. No effects of oil on soot particles was

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identified in that study. In addition, the content of oils in P and S has been observed to affect

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nanoparticle emissions of diesel engines.30,31,32

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The harmfulness of the exposure to particles has been shown to increase together with the

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content of metals in nanoparticles.33,34,35 PM-induced toxic activity of vehicular exhaust is

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strongly associated with tracers of lubrication oil emissions, such as Zn, P, Ca, and hopanes,

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suggesting that the incomplete combustion of lubrication oils is associated with increased

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health risks.36 Engine oil has been reported to also affect the gaseous exhaust compounds. De

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Albuquerque et al.37 showed that synthetic oils produce less hydrocarbon emissions than

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semi-synthetic and mineral oils.

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To our knowledge, no earlier studies have investigated the effect of lubrication oil

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properties on particle number emissions from gasoline vehicles. However, it is reasonable to

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assume that also in this case, the metallic compounds associated with negative health effects

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primarily originate from engine oil. Recent studies22,24 indicate that the lubrication oil may be ACS Paragon Plus Environment

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dominant in emissions during deceleration conditions and may cause very high instantaneous

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particle number concentrations in the exhaust of gasoline vehicles.

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In this study, dedicated experiments were conducted to provide more insights on the

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impacts of lubrication oil to particle emissions of GDI vehicles. In total, five different oils

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were tested in laboratory experiments and two oils over on road drive emissions. The study

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aimed at addressing several research questions including the impact of lubrication oil

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composition on total particle number emissions, cold start particle overemissions, volatility of

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nanoparticles and differences between steady state and transient conditions.

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Experimental Vehicle laboratory measurements. The vehicle tested on the chassis dynamometer was a

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Euro 4 GDI car (displacement 1.8 dm3, model year 2011, 88 kW rated power). The engine is

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powered by a turbocharged engine which operates on stoichiometric stratified combustion

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below about 3000 rpm. The vehicle was equipped with a conventional three-way catalyst and

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exhaust gas recirculation (EGR). The engine ran on low sulfur (< 10 mg/kg) 95-octane

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gasoline-ethanol blend fuel, with an ethanol content of approximately 10% vol.

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Oil properties. Five different commercial synthetic lubricant oils were tested. All oils

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were of SAE 5W-30 viscosity specifications. Oils 1 and 2 were full SAPS (sulphated ash,

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phosphorus and sulfur) gasoline and diesel engine oils meeting ACEA A3/B4 baseline

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approvals.38 Oils 3, 4 and 5 were of limited catalyst chemical compatibility, i.e. so-called mid

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SAPS engine oils, developed for vehicles with after- treatment devices. Oils 3, 4 and 5 met

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and exceeded ACEA C3 baseline approval. Oils 1, 2 and 3 had the OEM approval so that the

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maximum oil drain interval (ODI) was 15 000 km or one year, while Oil 4 was so called long

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life oil for maximum 30 000 km ODI or two years usage. Oil 5 can be used as a temporary

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replacement if no oil with specific OEM approval is available. Since the viscosity properties

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were rather similar for all oils, including their high temperature - high shear values, it is ACS Paragon Plus Environment


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expected that viscosity does not play any major role in these studies. However, the amounts of

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the additives varied from oil to oil (Table 1). The oil change was always done in the previous

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evening of the measurement day. Rinsing was done carefully by running the car

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approximately 10 minutes with the new oil, followed by the actual oil change after which the

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engine was again warmed up (~10 minutes ) to normal operating temperature.

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Table 1. Lubrication oil specifications. All oils were tested in the laboratory; Oil 1 and Oil 4

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also on-road. Oil properties

Oil 1

Oil 2

Oil 3

Oil 4

Oil 5

SAE viscosity grade

5W-30

5W-30

5W-30

5W-30

5W-30

12.1

12.1

12.2

12.5

12.1

Kinematic viscosity at 100ºC, mm2/s Kinematic viscosity at 40ºC, mm2/s VISCO-IND

71.1

71.5

71.8

72.4

71.0

168

168

169

173

169

VISCO-CCS-30C, mPas

5580

5540

5120

5860

5970

MRV at -35C, cP

23800

25300

29400

25900

17100

HTHS, mPas Phosphorus, mg/kg

3.5 900

3.5 940

3.5 790

3.5 770

3.5 763

Sulfur, mg/kg

2780

2240

1650

2230

2390

Calcium, mg/kg

3200

2420

1680

1660

4650

Magnesium, mg/kg

7.2

12.5

8.6

4.2

4.8

Zinc, mg/kg

920

980

820

700

922

124 125

Driving modes. The measurements were conducted during the standardized NEDC - New

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European Driving Cycle (1180 s). This is composed of four urban-like sub-cycles constituting

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the Urban Driving Cycle (UDC) with a total duration of 780 s, followed by a 400 s long Extra

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Urban Driving Cycle (EUDC). The UDC simulates city driving and is characterized by low

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engine load, low exhaust gas temperature, and a maximum speed of 50 km/h, whereas the

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EUDC simulates highway driving and accounts for more aggressive, high speed driving

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modes, up to a maximum speed of 120 km/h (Fig. 1). The vehicle was soaked overnight at 30 ACS Paragon Plus Environment

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o

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hereafter called ”cold start cycle”. After the cold start cycle, the NEDC cycle was repeated

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four times, and all of these are considered as “warm start cycles”. After the NEDC tests

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selected steady state points were driven each for 10-15 minutes at a speed of 80 km/h, fifth

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gear, and consecutive absorbed power of 5 kW, 10 kW and 20 kW. The steady-state tests

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were repeated three times.

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C prior to each measurement with a fresh oil. The first driven NEDC cycle of each day is

Exhaust sampling and instrumentation. The measurement setup is the same as described

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in Karjalainen et al.22 and Happonen et al.39 Exhaust dilution was conducted using a partial

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exhaust flow dilution system40 consisting of a porous tube diluter with the dilution ratio of

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about 12, a short ageing chamber with a residence time of 2.6 s, and a secondary diluter (a

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modified Dekati diluter) with a dilution ratio of about 4.5 (Fig. S1 Supporting Information).

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Both the primary dilution ratio and the total dilution ratio (~50-80) were calculated based

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on the CO2 concentration in raw and diluted exhaust samples. The sample reached a

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temperature of about 25○C after secondary dilution. This dilution system is considered to

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fairly well mimic the cooling and dilution processes encountered during real-world driving,

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especially in what concerns exhaust nucleation particle formation.41,42

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During the transient tests, the number size distribution of the exhaust particles in the size

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range of 7 nm - 10 µm were measured with two Electrical Low-Pressure Impactors (ELPI1

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and ELPI2, Dekati Oy). The ELPIs were equipped with a filter stage43 and an additional

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impactor stage installed for nanoparticles.44 In this work, ELPI1 was used after a

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thermodenuder TD,45 where the diluted sample was heated to 265°C and was then led into the

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denuding part, where cooled active charcoal on the denuder’s wall collected evaporated

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compounds. The TD was used for half of the measurement cycles. The size distributions

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measured after the TD were corrected for particle losses.46 In addition to the ELPIs, a

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scanning mobility particle sizer SMPS47 was used during the steady state tests. The

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measurement range of the SMPS equipped with DMA 3071 (TSI Inc.) and CPC 3375 (TSI ACS Paragon Plus Environment


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Inc.) was 10-225 nm in 102 size bins and a total scan time of 150 s. The ELPI with 1 s time

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resolution is more suitable for transient driving conditions whereas the SMPS provides better

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size resolution during steady driving conditions.

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Regulated emissions. Regulated emissions were measured during the cold start NEDC as

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well as during the first warm NEDC with all oils. Due to a malfunction of the fuel flow meter

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(AVL PLU 116H), fuel consumption could not be measured in all tests. In these cases, fuel

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consumption was calculated using the formula given in Supporting Information (Table S1).

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As seen from Table S1, the CO2-emissions were higher for the cold start cycles compared to

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the warm cycles, while the various oils led to 6% maximum difference.

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On-road measurements. Exhaust particle measurements were also performed on road to

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study how well the laboratory results for Oil 1 and Oil 4 compare to real-world dilution

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conditions. For these tests, a mobile laboratory van called “Sniffer”48,49,41chased a similar

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passenger car as the one tested on the dynamometer. This second car (displacement 1.8 dm3,

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model year 2012, 132 kW rated power) was Euro 5 compatible with some engine differences

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compared to the laboratory tested car. Its engine operated on a combination of GDI and PFI

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fuel injection practices to suppress soot emissions. PFI was implemented under low load

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conditions, while fuel injection takes place during both the intake and compression strokes at

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stratified operation. Overall, this engine was also stoichiometric in both stratified and

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premixed modes. Constant speed tests (15, 30, 50, 70 km/h with gears 2, 3, 4, 5, respectively)

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and transient acceleration tests as well as deceleration by engine braking (30 - 90 - 30 km/h,

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gear 3) were carried out.

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The sampling probe on the Sniffer was located above the front bumper, at 0.7 m height

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from ground, and the distance with the preceding vehicle was kept at 12 m. The particle

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number concentration and size distribution were measured utilizing similar instrumentation

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with the one used in the laboratory studies: two UCPCs 3025 (TSI Inc.) and two ELPIs,


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before and after the TD, and an EEPS (Engine exhaust particle sizer, model 3090, TSI Inc.)

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for particle size range of 5.6-560 nm.

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Results

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Particle number emissions during NEDC cycles. Figure 1a-b presents the time series the

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total and non-volatile particle numbers over the NEDC with Oil 1 and Oil 4 (see Supporting

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Information Fig. S2 for other oils), and Fig. 1c-d depicts the time series of the particle size

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distributions. The instantaneous concentrations measured by the ELPI were multiplied by the

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corresponding exhaust flow volume, and then averaged over the measured cycles; in total

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over four cycles for all particle emissions and two cycles for the non-volatile particle

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emissions.

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Emission peaks could be observed during accelerations; the highest value ~8x1012 #/s was

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found with Oil 1, while the peak values with the other oils were 60-80% of that value. During

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idle, the emissions dropped to ~7x1011 #/s with Oil 1 and Oil 5, and to ~1 x1011 #/s with the

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others. Particle emissions were also observed during decelerations at engine braking. These

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particles seem to originate from the lubricant oil rather than from the fuel combustion because

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their emissions do not follow the exhaust CO2 concentration and they seem to include

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lubricant oil originated metals.22,24

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a)

b)

c)

d)

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Figure 1. Time series of particle number emission (# /s) and size distribution over the

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averaged NEDC for Oil 1 (a, c) and Oil 4 (b, d) measured with ELPI. Also shown is the

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speed, and the non-volatile number emission (a,b). The color bar in (c) and (d) refers to the 10

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base logarithm of dN/t.

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Table 2. Average particle number emission factors (#/km) for the GDI vehicle with different

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engine oils for the NEDC, UDC and EUDC cycles with and without the thermodenuder

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treatment. Also given are the emission factors for the cold start NEDC and steady state tests

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with 5, 10 and 20 kW wheel powers. The data was measured by the ELPI (particles >7 nm).

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The average particle number emission factors EFN over the NEDC are given in Table 2.

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The emission factors with Oil 3 and Oil 4 were similar, whereas the EFN with Oil 1 was

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almost five-fold compared to the lowest EFN with Oil 3. In fact, a 78% reduction in particle

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number emissions was observed solely by changing lubrication oil properties, i.e. from Oil 1

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to Oil 3, and a 20% reduction by changing Oil 5 to Oil 3 (Table 2). The reduction is even


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higher during cold start cycles reaching a 99% or 97% reduction if Oil 5 is changed to Oil 4 or

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Oil2 to Oil 4, respectively (Table 2). The trends are different when separately comparing the

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UDC and EUDC segments (Table 2). Over the UDC the lowest emissions were observed with

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Oil 2 and the highest with Oil 5. Emission rates per km over the EUDC (7.0 km) were lower

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than over the UDC (4.1 km) with the exception of Oil 1. The lowest emission was reached

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with Oil 4. In general the particle emission rate per second is generally higher over the EUDC

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(Fig. 1) than the UDC, as one would expect due the stronger accelerations of the former.

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However, particle number emissions over the EUDC are actually lower when expressed per

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unit of distance travelled.

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Volatile vs. non-volatile particles. Table 2 compares particle number emissions with and

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without the thermodenuder treatment. The majority of particles with oils 2-5 were of non-

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volatile nature, as shown by Fig. 1a,b and Fig. S2. In contrast, 70% of particles were volatile

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when using Oil 1. These were mostly formed during the high speed driving and the following

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deceleration from 120 km/h to 0 km/h (the EUDC segment) and were smaller than ~20 nm

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(Fig. 1c). Oil 1 had the highest sulfur content indicating that these particles could be of

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sulfuric acid origin. Although this peak emission with Oil 1 was observed during all four

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NEDC cycles, its values was halved from cycle to cycle. Thus, the results are in line with

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previous studies22,26,32,31 which indicated that the operation history may even have the

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dominant role in the formation of volatile nanoparticles. The phenomenon, yet weaker, was

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also observed during the first two NEDC cycles with Oil 2 but not with the other oils, as seen

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from Fig. 1d for Oil 4.

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Furthermore, the UCPC data showed that with Oil 1 more than 50% of the emitted particles were even smaller than 7 nm (Fig. S3, Supporting Information). Contribution of cold starts. Compared to the warm start cycles, the emission factors were

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much higher during the cold-start NEDC (Table 2) before the engine and the catalyst reached

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their normal operating temperatures. At the beginning of the cold cycle the coolant ACS Paragon Plus Environment


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temperature was ~29 oC and was stabilized to around 100 oC after 540 s from engine start

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(Fig. 2a). In contrast, the coolant temperature was almost constantly at ~100 oC during the

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warm start cycle (Fig. 2b). During the last high speed acceleration the temperature rose by 7-8

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degrees in both cases. As seen from Fig. 2 the cumulative particle number steadily increased

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over the cold-start cycle, by as much as two orders of magnitude, depending on the oil. The

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total particle number ratios over the whole cold and warm cycles were 66, 54, 2.0, 1.5 and 2.5

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for oils 1 thru 5, respectively. Peckham51 found that around 50% of the cumulative particle

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number was emitted during the first 200 s of the cold start cycle whereas we found that 11%,

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46%, 57%, 68% and 72% of the total NEDC emissions with oils 1 thru 5 occurred during the

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UDC segment (780 s). The corresponding ratios for the warm cycles were 16%, 51%, 64%,

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67%, and 60%, respectively.

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Figure 2. Cumulative particle emissions over the cold and warm NEDC tests with all oils.

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Also shown are the coolant temperature and vehicle speed.

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Acceleration vs. deceleration. The differences in the particle size distributions with

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different oils are shown by Fig. S4 and Table S2 in Supporting Information. The average size

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distributions with Oil 1 thru 3 over the NEDC were bimodal in shape (Fig. S4a). One mode

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peaked at 10-20 nm and the larger one at 45-80 nm (Table S2). The size distribution was

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nearly unimodal with oils 4 and 5, with a peak at around 40 nm. Oil 3 led the largest particles

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(Table S2) and Oil 5 to the lowest particle number emission.


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When looking at the differences during the acceleration phases (altogether 366 s and 3.97

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km) (Fig. S4b) and during engine braking (the last deceleration phase) (36 s and 0.62 km)

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(Fig. S4c) two features can be emphasized. First, with all oils the average emissions in the

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soot mode were larger at the acceleration than at the deceleration phase. Second, with oils 1

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and 2 the smaller mode emissions were in the same order of magnitude at both acceleration

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and deceleration phases. The other oils led to much lower emissions during deceleration. With

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all oils the emissions of larger particles (>300 nm in diameter) were also observed. The

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existence of these particles is in line with the morphology analysis by Karjalainen et al.22

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Particles formed in accelerations comprised 45-68% of total NEDC emissions. Particles

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during decelerations comprised 23-41% of total NEDC emissions when oils 1 or 2 were used,

278

but less than 1 % when oils 3 to 5 were used. The particles formed during engine braking with

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oils 1 and 2 were predominantly in the sub 20 nm size range (Fig. S4c). The majority of these

280

particles were volatile based on the measurements with and without the TD (Fig. S4c).

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Steady state driving modes. The steady state tests revealed differences in the particle size

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distribution with different oils. With all oils tested, the number emission factors (#/km) of

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particles increased as the wheel power increased (Table 2), contrary to the particle

284

concentrations (#/cm3) in the exhaust that decreased respectively (not shown). The highest

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EFs were observed with Oil 1, and the lowest with Oil 4 (Fig. S5). The distinctions were clear

286

between the different oils, and the order remained the same during all power settings.

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The particle number size distributions measured with all oils are presented in Fig. 3. The

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size distributions were bimodal; the geometric mean diameter of the smaller mode was ~15

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nm while for the larger mode it was ~70 nm. For 5 kW and 10 kW wheel powers the highest

290

concentrations were observed with oils 1 and 2, both in the smaller and in the larger modes,

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whereas no difference between oils 3 to 5 could be observed. At 20 kW absorbed power, the

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concentration of the soot mode decreased and the smaller size mode increased with all oils,



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but predominantly with oils 1 and 2. Similar trade-offs between the small and large particles

294

concentrations has been also observed in diesel exhaust.50

295

The TD treatment (265 °C) did not result to any significant change in the size or number

296

of the exhaust particles except during the 20 kW power setting when a large fraction of the

297

particles in the smaller mode were removed, especially with Oil 1.

298

a)

b)

c)

299 300

Figure 3. Average number size distribution of particles during the steady state cycles

301

measured by SMPS for 5 kW (a), 10 kW (b), and 20 kW (c) wheel power.

302 303

Contribution of lubricant oil additives. The impact of the concentration of metal and

304

nonmetal compounds in the lubrication oils on particle number emissions was studied. The

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most significant positive correlation was found for Zn (Fig. 4 a) under constant speed driving

306

conditions. Since the emission factors with oils 2 - 5 were so close to each other, no

307

correlation of particle number and lube oil species content could be established over the

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NEDC. However, such correlations could be established for non-volatile particles (Fig. 4b).

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Similar results were obtained for P (Fig. 4c,d) and Mg (not shown). Non-volatile particles

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positively correlated also with S at the 20 kW and NEDC tests (Fig. 4f). Total particles were

311

positively correlated also at smaller power settings. These results indicate that a 10 percent

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decrease in the amount of magnesium, zinc, sulfur, and phosphorus in the lubrication oil


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decreases non-volatile particle number emission factors by around 2-4%, 5-11%, 6-10%, and

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9-18%, respectively.

315

316

317 318

Figure 4. Correlation scatter plots of particle number emission factors and metal

319

concentrations in lubricant oils. Left-side panels refer to the steady state driving tests, and

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right-side panels to the steady state along with the NEDC with the thermodenuder treatment.

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Linear fitting by the method of least squares were used; equations and squares of correlation

322

coefficients (R2) are displayed.

323 324

On road tests. During the on-road constant speed tests, the particle number concentration

325

in the exhaust plume was around 1900-2200 cm-3, that is at background levels. During

326

transient tests and in particular, over accelerations, increased particle numbers in the exhaust ACS Paragon Plus Environment


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plume could be detected with both oils. The CPC and, the ELPI to a lesser extent, also

328

detected some higher particle numbers during decelerations. The EEPS recordings were at the

329

detection limit of the instrument.

330

As an example, Fig. S6 (Supporting Information) presents the particle concentrations when

331

the car rapidly accelerated from 30 km/h to 90 km/h with gear 2, and then after 23 s

332

decelerated back to 30 km/h by engine braking. The highest particle concentrations were

333

observed during the acceleration part, with an average value over all repetitions of (8.5±3.0)

334

x104 cm-3 with Oil 1 and much lower (1.5±0.5) x104 cm-3 with Oil 4 (Fig. 5a). These values

335

were significantly above background. The peak concentrations during engine braking were

336

rather close to each other, at (5.8±1.5)x103 cm-3 and (5.4±2.4)x103 cm-3 with oils 1 and 4,

337

respectively.

338

a)

b)

339 340

Figure 5. a) Average particle number concentration in the exhaust plume, measured by the

341

UCPC, during the acceleration from 30 km/h to 90 km/h, and during the deceleration by

342

engine braking from 90 km/h to 30 km/h. The error bars refer to the standard deviations. The

343

average background concentration was 2096 ± 445 cm-3. b) Average number size distributions

344

of the maximum concentrations by ELPI during the acceleration tests from 30 km/h to 90

345

km/h with Oil 1 and Oil 4. Effects of different dilution conditions have been taken into

346

account by calculating the ratio of the excess particle number concentration and the excess

347

CO2 concentration over background levels. Error bars refer to standard deviations of the

348

maximum values. ACS Paragon Plus Environment

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17 349 350

The average number size distribution of the maximum concentrations during acceleration

351

is shown in Fig. 5b. Again the bimodal shape of the distribution is evident. The smaller mode

352

peaks at around 20 nm and the larger one at around 70 nm with both oils.

353 354

Discussion

355

Modern lubricant oils protect against corrosion and sludge formation, and contain detergent

356

additives to keep the engine clean. For example, typical detergents are magnesium sulfonates

357

whereas zinc dithiophosphates are used as antioxidant and antiwear additives. Recently many

358

improvements have been made in the quality and durability of lubricant oils. Particularly the

359

content in metal compounds has been decreased in order to protect catalysts.

360

In this study, it was observed that the lubricant oil contributes to the formation of gasoline

361

vehicle exhaust particles in several different ways. Firstly, our laboratory tests showed that the

362

concentration of the additives (Zn, Mg, P and S) in the lubricant oils was positively correlated

363

with particle number emissions. The highest emission factors were observed with oils 1, 2 and

364

5, which had the highest metal (Zn, Ca, Mg) and S contents (Table 2). Our results indicate

365

that approximately a 10% decrease in P or Zn content may decrease non-volatile particle

366

emission factors by 9-11%.

367

Secondly, particle emissions during transient operation strongly depended on lubrication

368

oil; a 78% reduction in particle number emissions was observed solely by shifting from Oil 1

369

to Oil 3, and a 20% reduction by shifting from Oil 5 to Oil 3 (Table 2) in the warm start

370

NEDC. The change was even higher during cold start cycles, amounting to 99% and 97%

371

reduction if Oil 5 was changed to Oil 4 or Oil 2 to Oil 4, respectively (Table 2).

372

Thirdly, the lubrication oil contributed to both volatile and non-volatile particle emissions

373

during accelerations and steady state driving modes. With Oil 1, 70% of particles were

374

volatile, formed during the high speed driving and the following deceleration from 120 km/h ACS Paragon Plus Environment


Page 18 of 26

18 375

to 0 km/h, whereas with the other oils the majority of the particles over the full cycle were

376

non-volatile (99%).

377

However, the non-volatile particle emissions during engine braking were not significantly

378

affected by the lubricants. While the emissions of volatile particles may have depended on the

379

lubricant oil sulfur content, the non-volatile particle emissions more clearly depended on the

380

metal contents of lubricant oil, especially during steady state driving. It should be noted that

381

the majority of emitted particles were non-volatile, whereas, volatile particles were observed

382

at high speed driving only and their existence was affected by the driving history.

383

Therefore, it is important to recognize that the characteristics of gasoline engine lubrication

384

oils should be taken into account when considering the ways to improve air quality in urban

385

areas and to protect human health. Our results indicate that the gasoline vehicle particle

386

emissions can be significantly reduced by regulating lubricant oil additives. The choice of

387

lubrication oil becomes even more emphasized in winter conditions. For example, Pirjola et

388

al.49 reported that particle number emissions of traffic in Finland are 2-3 fold compared to

389

those in summer conditions. Part of that may be caused by differences in the functionality of

390

engine oil under cold conditions.

391

It should be noted that fresh lubrication oils were used in these experiments. Christianson

392

et al.55 reported that oil conditioning and ageing can have a significant impact on particle

393

mass emissions so that emissions decrease in the first 2000 miles of lubricant break-in. More

394

work in needed to assess the impacts of lubricant ageing on particle emissions.

395 396 397

Acknowledgements

398

This work was a part of the TREAM-project and supported by the Finnish Funding Agency

399

for Technology and Innovation (TEKES), AGCO Power, Neste Oil, Dinex Ecocat and Ab


Page 19 of 26


19 400

Nanol Technologies Oy. Aleksi Malinen and Kaapo Lindholm from the Metropolia

401

University of Applied Sciences are acknowledged for their contribution to the experiments.

402 403

Supporting Information Available

404

The measurement setup of the laboratory experiments (Figure S1), regulated emissions (Table

405

S1), average particle number emissions over the NEDC tests (Figures S2, S3; Table S2),

406

average particle size distributions in transient conditions (Figure S4), average particle number

407

emissions in steady driving tests (Figure S5), and the number concentrations in the transient

408

driving tests on-road (Figure S6).

409

This information is available free of charge via the Internet at http://pubs.acs.org/ .

410 411 412 413

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