Impact of Lubricating Oil on Morphology of Particles ... - Science Direct

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ScienceDirect Energy Procedia 75 (2015) 2388 – 2393

The 7th International Conference on Applied Energy – ICAE2015

Impact of Lubricating Oil on Morphology of Particles from a Diesel Engine Yuesen Wang, Xingyu Liang*, Gequn Shu, Lihui Dong State Key Laboratory of Engines, Tianjin University, Tianjin 300072, P.R.China

Abstract This study focuses on the impact of lubricating oil on morphology of diesel particles by experimental works. A four cylinder diesel engine was used for generating particles. Neat diesel and oil-dosed fuel were burned, and samples were collected by thermophretic sampling system. Samples were analyzed by transmission electron microscopy (TEM) technique, and size distributions of primary particles, fractal dimension information of aggregates, and aggregate size were studied. Results show that oil-related primary particles are larger than fuel related under low engine load, while the influence of oil is subtle under high load condition. Lubricating oil influences the fractal geometry differently depends on load conditions. Loose structure was observed under low load, however, more compact one appeared under high load. Statistical results indicate that aggregates from oil-dosed fuel have larger gyration diameter compare with neat diesel aggregates, which may be caused by the organic fractions in oil-related particles. Overall, the various combustion ratio under different conditions is responsible for the influence of oil on particle morphology. © The Authors. Published by Elsevier Ltd. This an open access article under the CC BY-NC-ND license ©2015 2015 The Authors. Published by isElsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute Keywords: Diesel particle; Lubricating oil; Morphology; TEM

1. Introduction Particle emission problem has been one of the most important interest topics in the research field of diesel engine and environmental protection. Also, more evidence shows that particles from diesel engine cause irreversible human health problem. Therefore, stricter emission regulations are needed and promoted. To meet the requirements, engine manufactures and research institute have done considerable research works. Factors, like fuel quality, alternative fuels, advanced combustion modes, and aftertreatment system were taken into considered. However, lubricating oil is the one factor that has been ignored somehow. With the research results, fuel-related particles can be reduced efficiently. Consequently, oil-related particles become proportionally larger in remaining particles.

* Corresponding author. Tel.: +86-22-27891285; Fax: +86-22-27891285. E-mail address: [email protected].

1876-6102 © 2015 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/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.182

Yuesen Wang et al. / Energy Procedia 75 (2015) 2388 – 2393

Few researchers had done research works about the contribution of lubricating oil to particle emission, and found that oil can increase the number of particles. Andrews et al. did research about the impact of oil age on emissions and concluded that a decrease followed by an increase trend exists for particle emission due to the evaporation of volatile fraction in fresh oil [1]. Taylor et al. studied the effect of evaporation property of oil on diesel emissions, and found that there was about 9-54% reduction of particle emission with the continuous removal of light fractions of the lubricating oil below 200 to 260°C [2]. Also the particle emission from part-synthetic and full synthetic oil were compared by Jefferd et al. [3], they drawn the conclusion that part-synthetic contributes over 20% less particulate matters than full synthetic oil. Physical property of oil like viscosity and volatility study show that increase in oil viscosity provided a larger reduction in particulate emissions than the decrease in volatility [4]. Miller et al. excluded the fuel related particles through doing experiments on a hydrogen engine, and found that the oil-related particles contained organic carbon, little or no elemental carbon, and a much larger percentage of metals than particles from diesel engines [5]. All these former studies focus on the effect of oil quality, oil age on particle emissions, but ignored the effect of oil on particle morphology, which determines the oxidability in aftertreatment system and in the atmosphere directly. The objective of this study is investigating the impact of lubricating oil on particle morphology via mixing a certain weight percent of oil into diesel fuel, as suggested by former studies [6-8]. Experimental works were performed in a diesel engine, and particles were collected with grid and analyzed by transmission electron microscopy (TEM) technique. Parameters like diameter, fractal dimension geometry, and aggregate size were evaluated and, comparison was made between results of fuel-related particles and that of oil-related particles. 2. Experimental method 2.1. Test engine and operating conditions All of the experiments in this study were conducted on a light duty diesel engine (YN4100QB) with a peak power of 80 kW at 3200 rpm and a maximum torque of 310 Nm at about 2000 rpm. This is a four cylinder, 4-stroke, turbocharged engine with a compression ratio of 17.5 to 1 and a displacement of 3.612 L. During the study, the engine was installed on the test bench based on a hydraulic dynamometer and is coupled with a control and data acquisition system. In the current study, break mean efficient pressure (BMEP) of 3.49 bar and 6.98 bar loads at medium engine speed of 2000rpm were chosen. All of the measurements were conducted after the engine was fully warmed up for about 15 min, and keep the engine run under the specific steady-state for about 10 min prior to each sampling. 2.2. Test fuel and oil Table 1. Specifications of lubricating oil Items

Specifications

Kinematic viscosity

15.41 mm2/s at 100 °C

Flash point

241 °C

Pour point

-27 °C

Kinematic viscosity

6810 mpa·s at -20 °C

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The engine was lubricated with SAE 15W-40 grade oil produced by Shell Inc., whose detail information was given in Table 1. Neat diesel fuel and 0.5% weight oil-dosed fuel were used. The oil that added into the diesel fuel was the same with that in engine oil pan. 2.3. Thermophoretic sampling and TEM image analysis A thermophoretic system was assembled including a mechanical actuator and a digital time controller. The mechanical actuator is composed of a probe with a width of 3.05mm, and allows the probe access into the exhaust pipe about 5-6 mm and the residence time is about 0.3 s (controlled by digital timer). A kind of Copper 200 mesh grids coated with carbon or silicon monoxide film were used as filters for the TEM sample. A JEOL 2010 FEF TEM (point resolution: 0.19 nm) was used to obtain the TEM images. More than 20 TEM images were taken for each condition and analyzed with the commercial image processing software Image-Pro Plus 6.0 (Media Cybernetics, Inc.). 3. Results and discussions 3.1. TEM observations Figure 1 presents the typical TEM images of particles from diesel engine when both pure diesel and oil-dosed fuel were burned under two load conditions. No matter which fuel was burned, the aggregates demonstrated grapelike or chainlike structure. For all of the aggregates, a number of spherical primary particles were accumulated together. With the comparison of TEM images of neat diesel particles and oilrelated particles, lubricating oil affects particle formation differently under different load conditions.

Fig. 1. Typical TEM images of diesel particles (a: 3.49 bar, b: 6.98 bar) and oil-related particles (c: 3.49 bar, d: 6.98 bar)

Under low engine load, it can be seen that large size primary particles appear in oil-related particle aggregates (Fig. 1, c). Unlike the large overlap phenomenon in diesel particle aggregates (Fig. 1, a), majority of oil-related primary particles connect with each other nearly with point-point style. Under high load condition, the diesel primary particle show with a relative average diameter (Fig. 1, b), and not as many small size particles as those under low load. However, more small size primary particles were found in oil-related aggregates, and they accumulated and stacked together with a large overlap area (Fig. 1, d). 3.2. Primary particle size distributions As shown in the Fig. 2, size of all primary particles from neat diesel and oil-dosed fuel show normal distribution. Similar conclusions can be found in previous studies [9, 10], which mainly focus on the influence of operation mode on particle morphology. Under the low load condition, about 80.7% primary diesel particles have a diameter smaller than 30 nm. However, the percent is only 44.2% for oil-related

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particles. Under high load condition, more oil-related primary particles (63.2%) have a diameter larger than 30 nm compare with diesel particles (39.2%).

Fig. 2. Size distributions of diesel and oil-related particles: a, BMEP=3.49 bar; b, BMEP= 6.98 bar

To quantify the differences, the average diameters of primary particles from two fuels under both load conditions were calculated and given in Fig. 2. Results show that lubricating oil increases the mean diameter under low engine load. However, there is only subtle difference under high load condition. The combustion involvement of lubricating oil contributes to the discrepancy. Under low load, firstly, oil could reduce the number of small particles via adsorbing the unburned hydrocarbons; secondly, combustion products of lubricating oil increase the number of large size particles. This is coordinate with the TEM observation results. Under high load, oil could be burned along with fuel relatively more efficient, and resulting in more large size particles. On the contrary, metallic additive and sulphuric composition in oil may contribute to more nucleation particles. That is why there is subtle difference of mean diameter under high load, and why more nucleation particles were found in TEM images (Fig. 1, d). 3.3. Aggregate fractal dimensions Fractal dimension (Df) provides a comprehensive overview of the agglomeration mechanism, namely, particle-cluster or cluster-cluster style. According to past studies, the following equation can be used to obtain the aggregate fractal dimension information [11, 12]:

N

k fL ( L / d p )

Df

(1)

where L is the length of aggregate, dp is the average diameter of primary particles, KfL is a correlation pre-factor and determines the magnitude of the least square linear fit to the data in the N versus L/dp plot. Fractal dimension results from statistical works for diesel particles and oil-related particles are shown in Figure 3. Fig. 3 (a) demonstrates the fractal information under low load condition, lubricating oil decreased the fractal dimension of aggregates from 1.848 to 1.424. Under high engine load condition, lubricating oil contributes different impact on fractal dimension (Fig. 3, b). The fractal dimension increased from 1.308 (diesel particle aggregates) to 1.913 (oil-related particle aggregates). A large fractal dimension indicates aggregates formed dominate with particle-cluster style, and corresponding to compact spatial structure. Therefore, results show that lubricating oil leads to looser structure under low load while more compact one under high load. Adsorbing mechanism under low load prevents the nucleation mode particles filling the structure gap, and unburned organic fraction which covered on the surface make the cluster-cluster style easier (Fig. 1, c). Consequently, loose aggregate structure was found for oil-related particles under low load. On the other hand, oil can be burned relative completely under high temperature of high load condition. The combustion ratio of oil is relative higher than that of low load. Thus, more nucleation mode particle

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(mainly sulphuric acid) formed, and consequently particle-cluster style dominates during the formation of aggregates.

Fig. 3. Statistical determination of the fractal dimension for particle aggregates: (a) BMEP=3.49 bar; (b) BMEP=6.98 bar.

3.4. Aggregate size distributions According to former research study [13], the gyration diameter can be estimated from following equation:

d g / L ( D f / ( D f 2))1/2

(2)

where dg is the gyration diameter of the aggregate. For any aggregate with a fractal dimension in the range of 1-3, this equation is valid.

Fig. 4. Aggregate size distributions for diesel and oil-related particles: (a) BMEP=3.49 bar; (b) BMEP=6.98 bar.

Figure 4 provides the detail information about aggregate size distributions. The statistical results show that all of the aggregates have a gyration diameter in the range of 50-350 nm. Results show that most of the aggregates from diesel engine have a size at range of 100-200 nm. Compare with aggregates from neat diesel, those from oil-dosed fuel shift to larger size. We deduced that organic and metallic fraction in particles is responsible for the large aggregate size. Metallic ash fraction contributes to large size particles and organic fraction leads to a stronger bond between them, resulting in larger aggregate size. 4. Conclusions z Lubricating oil contributes to both small and large size particles. However, conclusion that large particles mainly from the unburned organic and metallic additive fraction should be further verified in future works. z The combustion ratio of oil determines its influence. Low combustion ratio contributes to loose aggregate structure, while high ratio contributes to compact one. z Oil-related particles have larger aggregate size than neat diesel particles, which may be caused by the unburned organic and metallic ash fraction in particles. 5. Copyright


Authors keep full copyright over papers published in Energy Procedia Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51376136 and 51406132) and Natural Science Foundation of Tianjin (No. 14JCYBJC21300). References [1] Andrews GE, Abdelhalim SM. The influence of lubricating oil age on oil quality and emissions from idi passenger car diesels. SAE paper 1999-01-1135. [2] Taylor GW. The effect of lubricating oil volatility on diesel emissions.SAE paper 2001-01-1261. [3] Jefferd K, Rogerson J, Copp D, Brundle R. The impact of lubricants on heavy duty diesel engine fuel economy and exhaust emissions. SAE paper 2000-01-1983. [4] Froelund K, Owens EC, Frame E, Buckingham J, Garbak J, Tseregounis SI. Impact of lubricant oil on regulated emissions of a light-duty Mercedes-Benz Om 611 cidi-engine. SAE paper 2001-01-1901. [5] Miller AL, Stipe CB, Habjan MC, Ahlstrand GG. Role of lubrication oil in particulate emissions from a hydrogen-powered internal combustion engine. Environ Sci Technol 2007;41:6828-35. [6] Jung H, Kittelson DB, Zachariah MR. The influence of engine lubricating oil on diesel nanoparticle emissions and kinetics of oxidation. SAE paper 2003-01-3179. [7] Takeuchi Y, Hirano S, Kanauchi M, Ohkubo H, Nakazato M, Sutherland M, et al. The impact of diesel engine lubricants on deposit formation in diesel particulate filters. SAE paper 2003-01-1870. [8] Sappok AG, Beauboeuf D, Wong VW. A novel accelerated aging system to study lubricant additive effects on diesel aftertreatment system degradation. SAE paper 2008-01-1549. [9] Wang Y, Liang X, Shu G, Wang X, Sun X, Liu C. Effect of lubricant oil additive on size distribution, morphology, and nanostructure of diesel particulate matter. Appl Energy 2014;130:33-40. [10] Lee KO, Cole R, Sekar R, Choi MY, Kang JS, Bae CS, et al. Morphological investigation of the microstructure, dimensions, and fractal geometry of diesel particulates. Proc Combust Inst 2002;29:647-53. [11] Zhu J, Lee KO, Yozgatligil A, Choi MY. Effects of engine operating conditions on morphology, microstructure, and fractal geometry of light-duty diesel engine particulates. Proc Combust Inst 2005;30:2781-9. [12] Brasil A, Farias T, Carvalho M. A recipe for image characterization of fractal-like aggregates. J Aerosol Sci 1999;30:137989. [13] Neer A, Koylu U. Effect of operating conditions on the size, morphology, and concentration of submicrometer particulates emitted from a diesel engine. Combust Flame 2006;146:142-54.

Biography Xingyu Liang is a professor in State Key Laboratory of Engines, Tianjin University, China. His interest is the contribution of lubricating oil to particle emission, also wall-wet and oil dilution by fuel is his research topic.

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