Journal of Spectroscopy
Volume 2017 (2017), Article ID 5392864, 8 pages
https://doi.org/10.1155/2017/5392864

Research Article

The Deterioration Mechanism of Diester Aero Lubricating Oil at High Temperature

Ting Yao,1,2 Hongwei Yang,³ Li Guo,³ Yiwei Fei,³ Huize Jiang,³ Sen Bian,³ and Tonghai Wu²

¹Analysis and Testing Center, Huangshan University, Huangshan, Anhui 245041, China
²Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China
³Department of Aero Oil and Material, Air Force Logistics College, Xuzhou, Jiangsu 221000, China

Correspondence should be addressed to Ting Yao; ttyaoting@163.com

Received 29 June 2017; Accepted 21 August 2017; Published 31 October 2017

Academic Editor: Xing Fan

Copyright © 2017 Ting Yao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The deterioration of aero lubricating oil at high temperatures was accelerated by using a specific device simulating the operating conditions of engines, where the deterioration mechanism was obtained. Structures of the deteriorated lubricating oils were analyzed by gas chromatograph/mass spectrometer. From the results, it can be concluded that deterioration of aero lubricating oil at high temperatures was composed of thermal pyrolysis, oxidation, and polymerization, with the generation of a variety of products, such as alcohols, aldehydes, acids, and esters, which caused the deterioration of physicochemical properties of the aero lubricating oil.

1. Introduction

The change of molecular structure of aero lubricating oil is the fundamental cause that affects the functions of oil at high temperature [1–5]. The deterioration process, which includes thermal pyrolysis, oxidation, and polymerization, is complicated. The identification of products and investigation of reaction process can effectively explain the cause of deterioration mechanism of aero lubricating oil at high temperature, which can provide the theoretical basis for developing new and high-end lubricating oils.

The tested aero lubricating oil is a kind of synthetic lubricating oil with diisooctyl sebacate (DIOS) as its based oil, which has been extensively used in a certain type of aircraft engine lubrication system. The lubricating properties of aero lubricating oil will change due to deterioration caused by high temperature, high pressure, and high shearing, along with metallic catalysis (i.e., copper and iron) under the conditions of being exposed to the air. The deteriorated oil will do harm to the functions of the engine afterwards [6, 7]. In recent years, the deterioration of diester lubricating oil was focused on the performance deteriorated at high temperatures [8–11]. Chen et al. explored the change about antioxidants during the decay of some aero lubricating oil. It was found that thermal oxidation of antioxidant was a pseudo-first-order reaction and the equation of reaction kinetics was obtained [8]. Fei et al. analyzed the pyroreaction products of diisooctyl adipate of aero lubricating oil using gas chromatograph/mass spectrometer (GC/MS) and indicated that the formation of substances (i.e., ketones, ethers, alcohols, and acids) was the primary cause of color deepening and increase of acid number [9]. Wu et al. simulated the oxidation of DIOS to investigate the lubricating properties of oil products under different degrees of oxidation by a four ball tester. The results showed that the formation of peroxide during the oxidation process of lubricating oil led to the deterioration of lubricating properties of oil products and occurrence of some polar products decreased abrasion loss [10]. Santos et al. analyzed the product of thermal deterioration by using FTIR and GC/MS, who found that the structure change was related with property change [11]. In these studies, the effect of factors and property changes of diester-like lubricating oil were focused on thermal oxidation. However, few studies concern the process and mechanism of thermal deterioration.

In the present investigation, some aero lubricating oil was subjected to simulating its oxidation under different high temperatures using the simulative device and subsequently analyzed by GC/MS. The deterioration mechanism was focused on investigation. This is of a great significance in improving the high-temperature performance of an aero lubricating oil, determining the appropriate oil change intervals and realizing the long-term quality monitoring on aero lubricating oil.

2. Experimental Section

2.1. Materials

Some aero lubricating oil was collected from some Fuel Research Institute. The reagents used in the experiment are analytical ones which are commercially available. All reagents (petroleum ether, n-hexane, acetone, and so on) used in the experiments were analytical reagents, and all the organic solvents had been distilled before use.

2.2. Simulation Testing on High-Temperature Oxidation

128 g aero lubricating oil was added into a device (500 mL volume, 32 MPa, 450°C, 1100 r/min, and 2 kW, SongLing Co. Ltd, Yantai, China) simulating the operating conditions of the engine. The experimental lubricating oils were heated to the required temperatures (180, 200, 230, 250, 270, and 300°C) for 2 h, respectively, and the stirring rate was 800 r/min. For the convenience of description, the six experimental oils were marked with EO₁₈₀, EO₂₀₀, EO₂₃₀, EO₂₅₀, EO₂₇₀, and EO₃₀₀, respectively. After the reaction, the device was immediately placed in ice water until room temperature. The experimental oils were filtered and kept in dark conditions.

2.3. Acid Value Testing

The determination of acid values in testing samples is made in accordance with GB/T 264-1983(1991) Standard Testing Methods.

2.4. GC/MS Analyses

Oil samples were analyzed with a Hewlett-Packard 6890/5973 GC/MS, which was equipped with a capillary column coated with HP-5MS (cross-link 0.5% PH ME siloxane, 60 m length, 0.25 mm inner diameter, 0.25 μm film thickness) and a quadrupole analyzer with an m/z range from 33 to 500 and operated in electron impact (70 eV) mode. The capillary column was heated from 120 to 274°C at a rate of 13°C min⁻¹ and held for 2 min, then raised to 281°C at a rate of 0.5°C and held for 2 min, finally raised to 300°C at a rate of 12°C and held at 300°C for 3 min. Data were acquired and processed using software of Agilent MSD Productivity Chemstation. Compounds were identified by comparing mass spectra with NIST05a library data.

3. Results and Discussion

3.1. Color and Molecular Compositions of Oils

As a direct appearance index, color change of oils can be determined by various factors, including both internal ones (base oil, additive, oil pigment, and so forth) and external conditions (temperature, high pressure, high shearing, metal catalysis, oxidation, run time, and so forth) [12]. In this study, color of oils in different high-temperature reactions became more noticeable. EO₁₈₀ is almost pale yellow and transparent at 180°C. As the reaction temperature rose, the color gradually deepens. It is worth noting that color becomes most remarkable at 230°C, even looking reddish brown. EO₃₀₀ became not transparent but showed deep chocolate brown at 300°C. Most of aero lubricating oils were undergone significant color change under the high temperature, which provided a clue to explore the performance analysis for aero lubricating oils.

Figure 1 listed the total ion chromatogram of EO₁₈₀–EO₃₀₀ before 20.4 min. As shown in Figure 1, in total, 50 compounds were detected with GC/MS, which included alkanes, alkenes, alcohols, acids, amines, ketones, monoesters, and diesters, and were displayed in Table 1. As demonstrated in Table 1, ester compounds dominated in detected compositions, which were 25 compounds. In addition, some chromogenic compounds are detected in EO₁₈₀–EO₃₀₀, including amines (peak 23, 25), phosphates (peak 30, 33, 42, 44–47), ketones (peak 12, 31, 36), and acids (peak 11, 43). Though few in content, they played a crucial role in significantly deepening color of aero lubricating oils. Meanwhile, these compounds, stronger in polarity, contributed to the enhancement of the acid number, increasing from 0.173 mg KOH/g in EO₁₈₀ to 10.450 mg KOH/g in EO₃₀₀.

Figure 1: Total ion chromatograms of EO₁₈₀–EO₃₀₀ before 20.4 min.

Table 1: Distribution of compounds in EO₁₈₀–EO₃₀₀.

Peak
Compound
EO₁₈₀
EO₂₀₀
EO₂₃₀
EO₂₅₀
EO₂₇₀
EO₃₀₀

1
2-Pentene

×
×
×

2
4-Methyl-1-hexene

×
×

3
5-Methyl-3-heptene

×
×

4
3-Methylene

×
×
×
×
×

5
2-Ethylhexanal

×
×
×

6
2-Ethylhexane-1-alcohol
×
×
×
×
×
×

7
2-Ethylhexyl acetate

×

8
2-Ethylhexyl propionate

×

9
2-Ethylhexyl butyrate

×
×
×

10
2-Ethylhexyl valerate

×
×
×
×

11
12-Hydroxydodecanoic acid

×

12
4-Isopropylcyclohexane-1,3-diketone

×

13
2-Ethylhexyl-2-methoxyethyl acetate
×
×
×
×
×
×

14
2-Ethylhexylhexanoate
×
×
×
×
×
×

15
3-Ethylhexylhexanoate
×
×
×

×

16
Unknown
×
×
×
×
×
×

17
Unsaturated esters

×
×
×
×

18
2-Ethylhexyloctanoate

×

19
Unknown
×
×
×
×
×
×

20
2-Ethylhexyldecanoate

×

21
2-Ethylhexylacryloyloate
×
×
×
×
×
×

22
Unsaturated esters
×
×
×
×
×
×

23
(E)-N-(4-Butoxybenzylidene)-4-ethylaniline
×
×
×
×
×
×

24
Unknown
×
×
×
×
×
×

25
N-Phenylnaphthalene-2-amine
×
×
×
×
×
×

26
Unknown

×

27
(E)-1-(Pentyloxy)-3-cinnamenylbenzene
×
×
×
×
×
×

28
4-Methoxy-7-(2-methylbut-3-yn-2-oxy)furano[2,3-b]quinoline
×
×
×
×
×
×

29
Diisooctyl hexanedioate
×
×
×
×
×
×

30
Triphenyl phosphate
×
×
×
×
×
×

31
6-Heptyl-6-methyl-3-(pyridin-3-yl)-6,7-dihydro-1H-indazole-4(5H)-one
×
×
×
×
×
×

32
6-(Benzyloxymethyl)-3-butyl-2,4-dihydroxybenzene(methane)al
×
×
×
×
×
×

33
Diphenyl p-benzylphosphate
×
×
×
×
×
×

34
Isomer of 36

×
×
×
×

35
Bis(2-ethylhexyl)decanedioate
×
×
×
×
×
×

36
1-(3′-Benzoyl-2,6-dimethoxybiphenyl-3-yl)ethanone
×
×
×
×
×
×

37
Isomer of 36
×
×
×
×
×
×

38
Isomer of 36
×
×
×
×
×
×

39
Isomer of 36
×
×
×
×
×
×

40
Isomer of 36
×
×
×
×
×
×

41
Isomer of 36
×
×
×
×
×
×

42
Trio-benzylphosphate
×
×
×
×
×
×

43
3-(5-(3-Hydroxy-4-methylphenyl)-1-(3-hydroxyphenyl)-1H-pyrrol-2-yl)propanoic acid
×
×
×
×
×
×

44
Phenyldip-benzylphosphate
×
×
×
×
×
×

45
4-Ethylphenylphenyl p-benzylphosphate
×
×
×
×
×
×

46
Isomer of 42
×
×
×
×
×
×

47
Isomer of 42
×
×
×
×
×
×

48
Bis(2-ethylhexyl)decanedioate
×
×
×
×
×
×

49
Isomer of 48
×
×
×
×
×
×

50
1-(2-Ethyl-5-oxohexyl) 10-(2-ethylhexyl) decanedioate
×
×
×
×
×
×

3.2. The Deterioration Mechanism of Oils

Temperature is one of the most important factors for the deterioration of aero lubricating oil. As displayed in Figure 1 and Table 1, the product amounts were markedly increased with raising temperature and reached maxima at 300°C and properties of aero lubricating oil change with the oil’s compositions. The deterioration of oils is generally considered tending to form free radicals and follow the chain reactions under high temperature and oxygen conditions. But little work has been published to date about the deterioration mechanism of aero lubricating oil. We focus our attention on possible reaction mechanism at the molecular level in order to slower or prevent oil deterioration. As shown in Scheme 1, possible chemical reactions are thermal pyrolysis, thermal oxidation, and thermal polymerization according to detected compounds in Table 1.

Scheme 1: Possible reaction process of thermal pyrolysis of DIOS.

3.2.1. Thermal Pyrolysis

DIOS, as based oil of experimental oil, is a long-chain diester with favorable symmetric structure with presented hydrogen of β-carbon at alcohol side. Hydrogen of β-carbon at alcohol side and oxygen of ester carbonyl groups formed intramolecular hydrogen bond by molecular movement, forming six-membered ring with C, O, and H atoms [13]. At high temperature, hexatomic ring was cracked into free radical I and II (Scheme (a)), and then free radical I and II recurred disproportionation reaction and generated 10-[(2-ethylhexyl)oxy]-10-oxodecanoate and 2-ethylhexene (peak 4, Scheme (b)). Along with the increase of reaction temperature, the relative abundances of 2-ethylhexene (peak 4) in EO₁₈₀ to EO₃₀₀ were 0.018, 0.012, 0.068, 0.293, 0.500, and 5.213%, respectively, indicating the pyrolysis extent of oil were significantly enhanced with raising temperature. 2-ethylhexene further produced ethylene and hexylene at the higher temperature (Scheme (c)). As depicted in Scheme (d and e), parts of free radical II reacted with ethyl free radical and ethylene to generate 3-ethyloctane and 4-ethyloctene, respectively.

According to the structural formula, DIOS presented straight chain with eight carbons between ester groups. The straight chain could be cracked at high temperature, which is similar to straight-chain alkanes. The bonding energy of C-H bond of alkanes was higher than that of the C-C bond; therefore, homolysis of thermal pyrolysis started with C-C bond [14, 15], so did DIOS. All C-C bonds between ester groups can cause homolytic reaction at high-temperature conditions. The generated free radicals caused disproportionation reaction to produce saturation and unsaturation monoester. The reaction processes and products of the cracked C-C bond at different positions will be consequently discussed with the basis of GC/MS analysis.

As exhibited in Scheme , when the breakpoint is at the C₁₁-C₁₂ bond, DIOS could homolytically cracked two III free radicals (f). When two free radicals happened to have a collision, it will lead to the loss of H·and generated unsaturated 2-ethylhexyle-4-olefine acid monoester (C), whereas the other free radical generated saturated 2-ethylhexyl-olefine acid monoester (B). When the breakpoint is at the C₈-C₉ bond, DIOS generated IV and V free radicals and then two IV free radicals can generate low molecular weight di-2-ethylhexyl diester by coupling reaction (D). In this system, II free radical attacked the β-carbon of V free radical to generate VII free radical and alkyl radical with molecular weight of 141, whereas alkyl radical lost H atom and formed the alkene with molecular weight of 140 (Scheme (j and k)). When point of fracture is at the C₁₀-C₁₁ bond, DIOS generated two VII and VIII free radicals, and two VIII free radicals bumped together and formed caprylyl glycol adipate (E) by coupling reaction (Scheme (m)). VIII free radical was oxidized by oxygen at some temperature and formed the 6-(2-ethylhexyloxy)-6-diakyl hexylic acid (Scheme (n)). When the breakpoint is at the C₉-C₁₀ bond, DIOS could generate two free radicals and then formed diisooctyl adipate (F) by coupling reaction. Table 2 displayed the part ester compounds of EO₃₀₀.

Table 2: Several ester compounds of EO₃₀₀.

Peak
Compound
Structural formula

7
2-Ethylhexylacetate

8
2-Ethylhexylpropanoate

9
2-Ethylhexylbutanoate

10
2-Ethylhexylvalerate

14
2-Ethylhexylhexanoate

15
3-Ethylhexylhexanoate

18
2-Ethylhexyloctanoate

20
2-Ethylhexyldecanoate

29
Diisooctyl hexanedioate

35
Diisooctyl decanedioate

Spatial position and the near groups varied with the type of the C-C bond, resulting in different interactions involved. Therefore, the hemolytic cleavage of the different C-C bonds at high temperatures leads to the different abilities of free radicals. Generally, bond length of chemical bond is inversely proportional to the bond energy: the more low bond energies, the more crack of chemical bond. Zhou et al. [16] calculated the bonding energies of C-C bond of n-decane and xylene by computational chemistry method. The results indicated that the low bonding energy of n-decane can more easily cause thermal pyrolysis.

The structure of DIOS was optimized by using semiempirical method (PM3 method) with an aid of Hyperchem 7.0 software (Figure 2). Table 3 exhibited the bond length of various C-C bonds between ester groups. The fitted results of HyperChem indicated that the bonding length of the C₇-C₈ bond was significantly lower than that of other C-C bonds, revealing that C₇-C₈ bond was not easy to break. The difference in bond length of other four C-C bonds suggested that the C-C bond could be broken at high temperature, which was consistent with the deterioration mechanism of DIOS as based oil. Thermal pyrolysis included the cracking of six-membered ring and long carbon chain between ester groups. The cracking of long carbon chain between ester groups at different positions formed the different free radicals [17]. The collision of free radicals caused coupling and disproportionation reaction, resulting in formation of unsaturation and saturation monoesters, diesters with molecular weight less than DIOS, and part diesters with molecular weight more than DIOS.

Figure 2: Optimized structure of DIOS.

Table 3: Simulated results of bond length by HyperChem.

Number
1
2
3
4
5

Bond length
1.5134
1.5209
1.5213
1.5202
1.5207

3.2.2. Thermal Oxidization

Besides thermal pyrolysis, DIOS can cause thermal oxidization by the effect of oxygen under metal catalysis and high-temperature conditions. Alkene and reactive free radicals derived from thermal pyrolysis could be oxidized to form acid oxygen-containing compounds (peaks 11, 12, 31, 36, and 43), which lead to increasing acid number. Table 4 listed the acid number of experimental oils under different temperatures. As shown in Table 4, the acid number of EO₁₈₀ is 0.173 mg KOH/g, which was lower than critical value (0.5 mg KOH/g) with less corrosion for engine. However, the acid number was 1.011 mg KOH/g at 250°C, which could mildly lead to the corrosion of the engine. Furthermore, when the heated temperature increases to 300°C, the acid number dramatically reached to 10.450 mg KOH/g, which meant that the engine was corroded severely and the aero lubricating oil must be replaced.

Table 4: Acid number (mg KOH/g) of EO₁₈₀–EO₃₀₀.

Sample
Acid number
Sample
Acid number

EO₁₈₀
0.173
EO₂₅₀
1.011

EO₂₀₀
0.221
EO₂₇₀
2.092

EO₂₃₀
0.728
EO₃₀₀
10.450

3.2.3. Thermal Polymerization

The deterioration process not only is the cleavage of bonds but also includes polymerization, especially polycondensation at high temperature. It was reported that the oxidation products of lubricating oil could be condensed to polyoxides and then further formed the macromolecules [18, 19]. As expected from the GC/MS results, compounds of high molecular weight were detected in total ion chromatograms after 20.4 min. Due to the disadvantage of GC/MS, which only detects compounds with a molecular weight of less than 500, compounds of high molecular weight could not be detected and identified. Therefore, further research will focus on identifying the macromolecules from oils. These macromolecules, which combined with degraded products of additives and wear metallic particles, formed oil sludge. Oil sludge could be dissolved into oils and/or deposited to the surface of engine or blocked the strainer and gallery, which decreased lubrication effect due to lack of oil supply.

4. Conclusions

In summary, it is demonstrated that some aero lubricating oil significantly cause deterioration with air under high temperature. According to analysis with GC/MS, three reactions were presented, which mainly included thermal pyrolysis, thermal oxidation, and thermal polymerization, and thermal pyrolysis dominated at high temperature. Thermal pyrolysis included the pyrolysis of six-ring esters and pyrolysis of long carbon chain of ester groups, resulting in formation of acid, alkene, and various reactive intermediates. And thermal reactions between alkene/reactive intermediates and oxygen generated chromogenic compounds, such as alcohol, aldehyde, and acid. In addition, coupling and disproportionation reactions were observed by collision of free radicals, which generated unsaturated monoester, saturated monoester, diesters, and high-molecular-height polymers. The thermal pyrolysis of DIOS generated acid substances, and derivative of part additives resulted in the increase of acid number of oil products. Moreover, these products influenced the aero lubricating oil properties. The products of thermal oxidation were ester- and hydrocarbon-like compounds. The former and the latter were the chromophoric and auxochromous substance, respectively, which lead to the dark color of oil products after the oxidation process. The thermal polymerization generated the large molecular weight ester compounds. Then these ester compounds and degradation products generated oil sludge. The overabundance of oil sludge will block oil line and decreased the lubrication effect afterwards.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

Financial supports from the Project of Youth Fund of Jiangsu Province (no. BK20150166), Scientific Researching Youth Fund Projects of Air Force Logistics College (no. 2016xkjq006), Natural Science Foundation of the Anhui Higher Education Institutions of China (no. KJ2017A400), and National Natural Science Fund of China (no. 51575525) were acknowledged.

References

D. C. Kramer, J. N. Ziemer, and M. T. Cheng, “Influence of group II & III base oil composition on VI and oxidation stability,” Nlgi Spokesman, vol. 63, no. 10, pp. 20–39, 2000.

H. Nakanishi, K. Onodera, K. Inoue, Y. Yamada, and M. Hirata, “Oxidation stability of synthetic lubricants,” Lubrication Engineers, vol. 53, no. 5, pp. 29–37, 1997.

L. N. Edmund, “Neopentyl polyol ester lubricants-bulk property optimization,” Industrial & Engineering Chemistry Product Research and Development, vol. 15, no. 1, pp. 54–58, 1976.

W. Liu, J. Xu, and D. Feng, “Research status and development tendency of synthetic lubricating oil,” Tribology, vol. 33, no. 1, pp. 91–104, 2013, (In Chinese).

X. Fan, J. Jiang, L. Chen et al., “Identification of organic fluorides and distribution of organic species in an anthracite with high content of fluorine,” Fuel Processing Technology, vol. 142, pp. 54–58, 2016.

K. C. Jayaprakash, S. P. Srivastava, K. S. Anand, and P. K. Goel, “Oxidation stability of steam turbine oils and laboratory methods of evaluation,” Lubrication Engineers, vol. 40, no. 2, pp. 89–95, 1984.

A. C. M. Wilson, “Problems encountered with turbine lubricants and associated systems,” Lubrication Engineers, vol. 32, no. 2, pp. 59–65, 1976.

L. B. Chen, S. H. Guo, S. Y. Li, L. Q. Song, and Z. G. Zhang, “Thermal oxidization kinetics of antioxidant in aero lubricating oil,” Journal of Fuel Chemistry and Technology, vol. 30, no. 6, pp. 523–528, 2002, (In Chinese).

Y. W. Fei, X. L. Peng, T. Yao, F. Guo, and Y. H. Wang, “Component analysis of ester-like aero lubricating oil at high temperature,” Petrochemical Technology, vol. 43, no. 12, pp. 1444–1449, 2014, (In Chinese).

Y. Wu, W. Li, and X. Wang, “The influence of oxidation on the tribological performance of diester lubricant,” Lubrication Science, vol. 26, no. 1, pp. 55–65, 2013.

J. C. O. Santos, I. M. G. Santos, and A. Souza, “Thermal degradation process of synthetic lubricating oils: part I, spectroscopic study,” Petroleum Science and Technology, vol. 33, no. 11, pp. 1238–1245, 2015.

Y. W. Fei, Z. S. Cheng, H. W. Yang, and T. Yao, “Characteristic analysis of ester-like synthetic aero lubricating carrier oil,” Contemportary Chemical Industry, vol. 42, no. 9, pp. 1297–1300, 2014, (In Chinese).

N. Ponnekanti and K. Savita, “Study of synthetic complex esters as automotive gear lubricants,” Journal of Synthetic Lubrication, vol. 25, no. 25, pp. 131–136, 2008.

F. C. Li, J. S. Zhang, and Q. M. Yuan, “Reaction mechanism of thermal and catalytic pyrolysis of octane and formation of methane,” Journal of Fuel Chemistry and Technology, vol. 42, no. 6, pp. 697–703, 2014, (In Chinese).

B. Y. Wang, N. X. Tan, Q. Yao, and Z. R. Li, “Accurate calculation of the reaction barriers and rate constants of the pyrolysis of alkyl radicals in the β-position using the isodesmic reaction method,” Acta Physico-Chimica Sinica, vol. 28, no. 12, pp. 2824–2830, 2012, (In Chinese).

H. Zhou, J. Mao, B. Y. Wang, Q. Zhu, J. L. Wang, and X. Y. Li, “Thermal pyrolysis of decane and xylene under supercritical pressure conditions,” Acta Physico-Chimica Sinica, vol. 29, no. 4, pp. 689–694, 2013, (In Chinese).

M. Wang, X. Fan, X. Y. Wei et al., “Characterization of the oxidation products of Shengli lignite using mass spectrometers with ‘hard’, ‘soft’ and ambient ion sources,” Fuel, vol. 183, pp. 115–122, 2016.

J. F. Lahijani, F. E. Lockwood, and E. E. Klaus, “The influence of metals on sludge formation,” A S L E Transactions, vol. 25, no. 1, pp. 25–32, 1982.

J. A. Supp and R. E. Kornbrekke, “Deposit formation in gasoline engines: part 1. Base oil in sequence VE deposits,” Lubrication Engineers, vol. 50, no. 12, pp. 964–969, 1994.