Influence of Engine Age on Morphology and Chemistry of Diesel Soot

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Influence of Engine Age on Morphology and Chemistry of Diesel Soot Extracted from Crankcase Oil Vibhu Sharma, Sujay Bagi,† Mihir Patel,‡ Olusanmi Aderniran,§ and Pranesh B. Aswath* Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, Texas 76019, United States ABSTRACT: In this study, the role of engine age on the structure and chemistry of crankcase soot was studied using X-ray diffraction, high-resolution transmission electron microscopy, energy-dispersive spectroscopy, and Raman spectroscopy. Results indicate that the basic structure of the carbonaceous species remains the same in all cases and is composed of turbostratic carbon. However, there are some very subtle changes in the structure of the soot as the age of the engine increases. Older engines have a greater proportion of non-crystalline amorphous carbonaceous constituents in the soot relative to newer engines. In addition, the wear-induced debris increase with the age of the engine, with a larger proportion of phosphates of Ca and Zn as well as sulfates of Ca in the soot of older engines. The presence of these wear debris particles incorporated in the soot extracted from the crankcase indicates that they arise from three-body abrasive wear between the soot and the tribofilms formed within the engine. study31,32 to develop a comprehensive understanding on the subsequent changes in the morphology, structure, and chemistry of diesel soot extracted from crankcase oil acquired from engines with different accumulated mileages (86 000, 395 000, and 495 000 km) and maintaining a constant drain interval at approximately 80 000 km. All of the oils in this study conformed to SAE 15W-40 viscosity classification (API CJ-4) and were blended using mineral oil base stock presumably with a similar additive package. These results are compared to drain interval soot from an engine with an unknown accumulated mileage, the same as used in our previous study.31,32 Highresolution transmission electron microscopy (HR-TEM), highresolution X-ray diffraction (HR-XRD), and Raman spectroscopy were used to characterize the soot studied.

1. INTRODUCTION Recent regulatory mandates, such as API CJ4, to reduce nitrogen oxides (NOx) and other particulate matters (PMs) imposed by the United States Environmental Protection Agency (U.S. EPA) have resulted in the use of exhaust gas recirculation (EGR) in an internal combustion engine.1−3 Currently, EGR is the most effective technique available for reducing NOx emissions in a heavy-duty diesel engine. Use of EGR partially replaces the excess oxygen as well as lowers the combustion chamber temperature, thus reducing the formation of NOx; NOx primarily forms when a mixture of oxygen and nitrogen is subjected to high temperatures.4−6 At the same time, the application of EGR also results in higher soot loading in diesel engines because the deficiency of oxygen for the reaction with carbon in the time scale characteristic of the combustion process results in incomplete combustion of the fuel.7−9 Higher soot loading in engines causes a deleterious effect on engine component wear, functionality of lubricant oil, and particulate emissions.10−17 Many studies have been conducted to understand the characteristics of soot emitted under a diesel engine environment and its impact on the wear of engine components equipped with EGR.18−30 It has been established that crankcase soot degrades the lubricant oil properties physically and chemically, thereby inducing wear on engine parts. However, a study on the time-scale interaction of soot with lubricant chemistry and subsequent changes in the structure and chemistry of soot would provide better insight into the understanding on the role of soot in an engine. In our previous study, comprehensive analysis was conducted on the diesel soot extracted from crankcase oil from a truck, and we have shown that the chemical and structural makeup of diesel soot is very different from that of carbon black. In particular, a significant interaction between decomposed lubricant additives, tribofilm, and diesel soot was observed, responsible for modification in composition of diesel soot. In the previous study, various parameters, such as age of the engine, accumulated mileage, typical drain interval, etc., were unknown.10−12,31,32 This study is an extension of our previous © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Extraction of Diesel Soot. Three samples of diesel soot were extracted from engine oils (API CJ-4 Oil, Speedco Service Station, Indianapolis, IN) that were sampled from trucks that had a 15 L inline, wet sleeve Detroit Diesel engine with 86 000, 395 000 and 495 000 km, respectively, while the fourth diesel soot sample was extracted from drain interval oil from a Freightliner truck with an unknown age. The first oil was the very first drain from the engine, and the other engines had typical drain intervals of 80 000 km. The soot was extracted from the oil using the protocol described in the previous literature.31 Drained engine oil was diluted with hexane in a 50−50 wt % ratio and ultrasonicated for 10 min in a bath to ensure that the solution is thoroughly mixed. The oil−hexane mixture was centrifuged using a Sorvall SS 34 centrifuge at 12 000 rpm for a period of 2 h. The supernatant was discarded, and the solid soot particles at the bottom were dissolved in hexane and ultrasonicated, followed by a centrifuge process to remove contaminants in the soot. This process had three iterations that yielded a thick residual comprising of soot particles. The residual from the centrifuge process is further cleaned using a Soxhlet process for 48 h to eliminate the possibility of any trapped oil. In the Received: October 26, 2015 Revised: January 10, 2016

A

DOI: 10.1021/acs.energyfuels.5b02512 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Soxhlet process, hexane was used as a solvent, which was heated to its boiling point and allowed to condense in the condenser tube and trickle down through a thimble that contains the residual soot particles. This results in the extracted soot being dry and further ground in a mortar and pestle to reduce agglomeration, and the powdered soot was available for further analysis. 2.2. HR-TEM. HR-TEM of the diesel soot was conducted using a Hitachi H-9500 microscope at an accelerating voltage of 300 kV (using a Lab6 filament) with a point-to-point resolution of 0.18 nm and a lattice resolution of 0.10 nm. Soot was deposited on a lacey C only 300-mesh Cu grid by submerging the grid in soot suspension, created by bath ultrasonication in ethanol. High-resolution lattice images of crystalline nanoparticles and the turbostratic structure of soot were acquired using a SC-1000 Orius TEM charge-coupled device (CCD) camera (4008 × 2672 pixels) attached with TEM. Energy-dispersive Xray spectroscopy (EDS) and electron diffraction were acquired from selected regions to determine the chemical makeup of different regions within the soot particles and their structure. 2.3. HR-XRD. HR-XRD spectra of the diesel soot samples and carbon black were acquired using a Bruker D8 Advanced X-ray diffractometer. X-ray scans were run in locked couple mode from 10° to 55° 2θ scan range with a step size of 0.015. Each scan was recorded with a 3 s acquisition time per scan step. Characteristic X-ray spectra of each soot and carbon black were analyzed using Jade software. 2.4. Raman Spectroscopy. Visible Raman spectra of three diesel soot samples were acquired using Thermo Scientific DXR Raman spectroscopy using a diode-pumped solid-state type laser as a source of illumination. The collection system was used in backscattered configuration. Raman spectra were recorded using a solid-state laser with a characteristic frequency of 532 nm at 1 mW laser power. Low laser power was used to avoid excessive sample heating. The laser spot diameter was 2 μm with 25 μm slit size for the fully focused laser beam. The spectral resolution was 5 cm−1 at 532 nm with a wavelength range from 50 to 3500 cm−1. About a 1 mm thick layer of soot sample was place on a glass slide and pressed using a spatula to create a macroscopically smooth surface. The sample is then placed under a microscope and focused using a 10× objective lens and white light. Scans were run at three different locations for each sample and averaged. Each spectrum had 16 repetitions with 99 s exposure time. Full width at half maximum (fwhm) values of the peaks, peak intensities, and peak positions were analyzed to determine the graphitic to non-graphitic content in soot samples using a curve-fitting program in OriginPro software.

3. RESULTS AND DISCUSSION 3.1. HR-TEM Analysis of Diesel Soot. HR-TEM was used to study the structure and morphology of carbonaceous soot particles and changes in the chemical composition as a result of the interaction with lubricant oil chemistry during engine operation. Figure 1 shows selected area electron diffraction (SAED) images of four soot samples. SAED images obtained from soot samples show the ring pattern with spots, indicating the presence of embedded crystalline particles along with turbostratic carbonaceous soot particles. Patel et al.31 studied the structure of carbon black using HR-TEM and reported a similar ring pattern that arises from the (200) basal plane, (10) prism, and (11) pyramidal plane. A detailed analysis of the ring pattern suggesting turbostratic carbon as the calculated value of the ratio of radii (R1/R3 and R2/R3) is found to be in close agreement with the value reported in the earlier studies,31 shown in Table 1. Bright field (BF) images of the soot agglomerates of four diesel soot samples are shown in Figures 2−5. BF images of soot agglomerates show various nanocrystalline particles embedded in turbostratic carbon. A careful analysis of lattice fringes observed in BF images reveals the chemistry of each nanoparticle structure. SP A (Figure 2) shows more calcium-based compounds in the soot structure,

Figure 1. SAED images of SP A, SP B, SP C, and drain interval soot.

Table 1. Analysis of the Ring Pattern Observed in SAED Images of Soot (in Figure 1)

carbon black SP A SP B SP C drain interval soot

first ring d spacing (R1) (nm)

second ring d spacing (R2) (nm)

third ring d spacing (R3) (nm)

0.361

0.209

0.121

2.93

1.71

0.321 0.318 0.323 0.367

0.197 0.196 0.194 0.221

0.117 0.115 0.118 0.127

2.74 2.76 2.73 2.91

1.68 1.7 1.64 2.91

R1/R3 R2/R3

such as gypsum (CaSO4·2H2O) and calcium oxide (CaO), along with oxides of iron and zinc (Fe2O3 and ZnO), suggesting that most of the nanocrystalline particles generated in the low-mileage engine are calcium-based particles originated from calcium sulfonate detergent. While crystalline particles B


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Figure 2. BF HR-TEM image of SP A soot showing various nanocrystalline species embedded in the turbostratic soot.

Figure 3. BF HR-TEM image of SP B soot showing various nanocrystalline species embedded in the turbostratic soot.

has some contribution from the initial break in the period of the engine and, hence, has a higher content of Cu and Pb from bearings. The oldest engine SP C has a higher amount of Na and K likely from the coolant and other sources of contamination. Zn, P, and S are main constituents of antiwear additive components (zinc dialkyldithiophosphate) used in most commercial lubricant oils. The presence of these chemistries in crankcase oil could be a result of either adsorption of additives by soot particles or their physical embedment during the three-body wear mechanism induced by the presence of soot on the interacting counter surfaces. As indicated from earlier results, soot particles could initially interact with detergent chemistry of lubricant oils and form nanocomposite particles of turbostratic carbon with embedded calcium phosphates and calcium sulfates. It is possible that, because calcium phosphates are hard in nature, the incorporation of such chemical compounds results in the abrasive characteristics of the soot structure. These abrasive nanocomposite particles of soot when they reach the counter surface act as an abrasive third bodies under tribological

present on the soot samples extracted from the drained oil from older vehicles SP B (Figure 3) and SP C (Figure 4) are predominantly made up of iron oxide (Fe2O3/Fe3O4/FeO) and iron sulfate/sulfides (FeSO4/FeS), along with zinc sulfide (ZnS), zinc oxide (ZnO), and zinc phosphate [Zn3(PO4)2]. However, the possibility of calcium-based compounds cannot be completely ruled out because a small sample size is analyzed under HR-TEM and the fact that some of the calcium sulfate particles are much larger and do not deposit on the TEM grid. The presence of larger amounts of crystalline iron oxide particles embedded in the soot agglomerates is likely wear debris from severe wear conditions in high-mileage engines compared to an engine with fewer accumulated miles, as in the case of SP A. BF images of drain interval soot (Figure 5) primarily exhibit calcium species Ca5(PO4,CO3)3OH and Ca5(PO4)3)OH, along with ZnO and FeS. Calculation of the lattice spacing from the lattice imaging of the BF HR-TEM images is shown in Table 2. An elemental analysis of the used oil (shown in Table 3) provides further insight into the chemistry of the oil. The oil from the first drain interval SP A C


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Figure 4. BF HR-TEM image of SP C soot showing various nanocrystalline species embedded in the turbostratic soot.

Figure 5. BF HR-TEM image of drain interval soot showing various nanocrystalline species embedded in the turbostratic soot.

contacts and abrade antiwear tribofilms of polyphosphates. These observations correlate with the previous studies,

suggesting the abrasive nature of soot particles in a diesel engine environment.1,15,33 Gautam et al.34 investigated the D


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Energy & Fuels Table 2. Analysis of Lattice Fringes from Nanocrystalline Particles in Figures 2−5 soot sample SP A (86 000 km)

SP B (395 000 km)

SP C (495 000 km)

drain interval soot

suspected compound

indices of crystallographic plane (hkl)

hematite

Fe2O3 (104)

calcium oxide calcium sulfate gypsum zinc oxide zinc phosphate hematite magnetite wustite iron sulfate wurtzite 2H

CaO CaSO4 CaSO4·2H2O ZnO

zinc oxide

ZnO

hematite zinc oxide iron sulfate

Fe2O3 (104) ZnO FeSO4

iron sulfide zinc sulfate wurtzite 2H

FeS ZnSO4 [(Zn,Fe)S]

hametite

Fe2O3 (104)

carbonate hydroxylapatite hydroxylapatite

Ca5(PO4,CO3)3OH (300) Ca5(PO4)3)OH (211) Ca5(PO4)3OH (112) ZnO FeS

interplanar crystal spacing d (Å)

nanocrystalline particles

1.69 2.51 2.7 2.41 2.82 2.87 2.43/2.47 3 1.69 2.53 2.22 2.62 1.91 2.93 3.12 2.43 2.47 2.7 2.43 2.75 3.6 2.97 4.15 3.12 3.31 2.69 2.51 2.7 2.81 2.77 2.47 2.66

C S F, G, O, Q, Z A, B, H, I, J, K, L, M, N, T, Y P, R, X V D, E, W, U A, B, C, F, I D G K, L, W J E N, T V H, M, P, Q, R, S O E, F H A K D C B, G I, J A, C, H E, P B, G, O D, F, J, K, M, N, Q K, M, N I, J L

Fe2O3 (104) Fe3O4 FeO FeSO4 [(Zn,Fe)S]

zinc oxide iron sulfide

Table 3. Chemical Composition of the Oils Used in the Engine Age Study elemental composition metals (ppm)

contaminants (ppm)

contributions from equipment wear

from engine coolant

additives (ppm) from detergents

from ZDDP antiwear

friction modifier

anti foam additive

sample

engine age (km)

Fe

Cr

Pb

Cu

Sn

Al

Ni

K

Na

Mg

B

Ca

P

Zn

Mo

Si

SP A SP B SP C

86000 395000 495000

61 38 42

5 3 2

4 2 2

432 16 12

8 4 4

68 18 17

2 1 1

221 32 507

13 18 676

791 1149 1355

83 13 15

2393 2644 2395

1041 1034 1042

2104 2017 2059

112 55 133

17 15 15

characteristics using HR-TEM, energy electron loss spectroscopy (EELS), electron spin resonance (ESR), and X-ray photoelectron spectroscopy (XPS) techniques. It was concluded that a higher wear rate as a result of soot is influenced by the soot particulate concentration, morphology, surface chemistry, and reactivity of soot particulate species. Patel et al.10 studied the structure and chemistry of soot extracted from crankcase, cylinder wall, and tribofilms on piston rings of a Mack T-12 engine test using HR-TEM and X-ray absorption near edge structure (XANES). Nanoparticles of calcium phosphate compounds, ZnO, and Fe2O3 embedded at the surface of turbostratic soot were reported. It was concluded that the abrasive wear observed on the cylinder liners and piston ring-pack components resulted from three-body wear of

effect on the engine wear as a result of soot-contaminated oil using a three-body wear test and reported higher average wear from soot-contaminated oil than wear observed from oil without soot contamination. They hypothesized that diesel soot reduces the antiwear properties of the oil possibly by an abrasive wear mechanism. Recently, Patel et al.12 examined the impact of soot on wear and friction using treated and untreated carbon black as a soot surrogate as well as extracted diesel soot under extreme pressure conditions. They concluded that the mechanism of wear with treated carbon black and with diesel soot appears to be by the third-body abrasive wear mechanism when used in a four-ball test configuration. Antusch et al.35 studied the tribological behavior of soot using a pin on disk tribometer and proposed a correlation between their tribological properties to their structural and chemical E


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Figure 6. HR-XRD spectra of SP A, SP B, SP C, and drain interval soot.

modified soot particles as a result of the interaction of soot with lubricant additives. 3.2. HR-XRD. The HR-XRD technique was employed to identify the various crystalline species present in diesel soot extracted from crankcase oil at different mileage intervals of the vehicle. XRD spectra of diesel soot SP A, SP B, SP C, and drain interval soot are plotted and compared to carbon black in Figure 6. Previous studied have shown that carbon black has a typical turbostratic carbon structure,36−39 which is clearly evident in Figure 6. Spectra of carbon black shows two peaks at 2θ positions 24.3 (D) (002) and 43.2 (J) (100) that are arising from turbostratic carbon40 in the 10−60° 2θ scan range. Upon comparison to carbon black, XRD spectra of all four diesel soot samples exhibit the presence of peaks D and J, confirming the

Figure 7. Raman spectra of SP A, SP B, SP C, and drain interval soot.

turbostratic structure, which is also seen in the previous HRTEM results. Moreover, the spectra of diesel soot show various other peaks superimposed on the background pattern from the turbostratic structure, which are labeled in an alphabetical manner. In-depth analysis of these peaks reveals the presence of various crystalline species in soot samples, which are summarized in Table 4. XRD spectra of SP A, SP B, and SP C primarily show Ca-based crystalline species that are CaSO4, CaSO4·2H2O, CaCO3, and β-Zn3(PO4)2. Drain interval soot from the Freightliner truck also exhibits similar crystalline particles in the structure; characteristic peaks associated with

Table 4. Analysis of HR-XRD sample type

peak identification

peak list

peak position (2θ deg)

possible compound

A

11.58

B C D E F G H

14.62 20.72 24.36 28.07 28.98 29.64 31.44

I J K L M

33.39 43.23 48.91 53.55 55.95

calcium sulfate hydrate β-zinc phosphate calcium sulfate turbostratic carbon calcium sulfate calcium carbonate β-zinc phosphate calcium sulfate hydrate calcium sulfate turbostratic carbon β-zinc phosphate calcium sulfate calcium carbonate

chemical formula

Miller indices

SP A

SP B

SP C

drain interval soot

carbon black

CaSO4·2H2O

(0 2 0)

absent

absent

absent

present

absent

Zn3(PO4)2 CaSO4 C CaSO4 CaCO3 Zn3(PO4)2 CaSO4·2H2O

(1 (0 (0 (1 (1 (2 (0

1 2 0 1 1 2 0

0) 1) 2) 1) 2) 0) 2)

present absent present present present absent present



present present present present present present present

absent absent present absent absent absent absent

CaSO4 C Zn3(PO4)2 CaSO4 CaCO3

(0 (1 (2 (1 (2

2 0 3 5 0

2) 0) 2) 2) 2)

absent present present absent absent

absent present present absent absent

absent present present present present

present present present absent absent

absent present absent absent absent

F


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Energy & Fuels Table 5. Raman Peak Analysis of the Soot Sample and Carbon Black sample

peak

peak position (cm−1)

intensity

G/D1

SP A (86 000 km)

G D1 D3 D4 G D1 D3 D4 G D1 D3 D4 G D1 D3 D4 G D1 D3 D4

1596 1349 1537

741 1704 168

0.43

1596 1348 1531

838 2344 254

0.36

1598 1348 1539

1110 3784 436

0.29

1600 1349 1531

1402 4684 484

0.3

1590 1347 1509 1200

829 2172 534 169

SP B (395 000 km)

SP C (495 000 km)

drain interval soot

carbon black

G/D3

G/D4

G/(D1 + D2 + D3 + D4) 0.40

4.4 0.32 3.3 0.26 2.5 0.27 2.9 0.29 0.38 1.6 4.9

degree of graphitization.31,40,48 Moreover, an integrated intensity ratio of disordered graphite (“D” band) to graphite (“G” band) is inversely proportional to microcrystalline planar size La that corresponds to the in-plane dimension of the single microcrystalline domain in graphite.49−51 Analysis of spectral features, such as peak position, intensity, line shape, and bandwidth between 800 and 2000 cm−1, offer valuable information on the nature of carbonaceous soot samples. Figure 7 illustrates the Raman spectra of four diesel soot samples and carbon black. The Raman spectra of drain interval soot and carbon black exhibit two broad overlapping peaks at wavenumbers ∼1590 and ∼1350 cm−1. The Raman shift at wavenumber ∼1590 cm−1 (known as the G band) arises from the vibration of the crystalline graphitic structure.48 Raman spectra of graphite discussed in this work have been adapted from a previous study.31 Another broad peak observed at wavenumber ∼1350 cm−1 illustrates a higher degree of disorder in the graphite structure corresponding to a disordered graphite phase (labeled as peak D1). Curve fitting of the each spectra further revealed two peaks at wavenumbers ∼1500 and ∼1210 cm−1. Curve fitting was performed using a Lorentzian function to achieve the best peak fit for diesel soot and carbon black spectra and is in good agreement with recent studies by Sadezky et al.48 The peak observed at Raman shift ∼1500 cm−1 is attributed to the high signal intensity between G and D1 bands and is identified as the D3 band originating from the amorphous carbon fraction of soot (organic molecules, fragments, or functional groups) and/or sp2-bonded forms of carbon. Table 5 summarizes curve-fitting data obtained for four soot samples and carbon black. SP A soot extracted from the engine with the least mileage shows the highest graphitic content in the soot, whereas soot extracted from higher mileage engines exhibit a similar graphitic content as in carbon black and other drain interval soot. When the individual contribution from each phase is analyzed, a further understanding into the carbonaceous soot structure can be obtained. The ratio of G/ D1 represents the amount of crystalline graphitic content with respect to disordered graphitic content. The ratio of G/D1 is highest in the case of SP A that subsequently decreases to the

individual crystalline phases could be used to confirm their presence in the samples. Crystalline peaks with a higher intensity are observed for soot samples extracted from higher mileage engines (SP B and SP C), suggesting higher levels of lubricant interaction with the turbostratic carbonaceous soot. There are fewer crystalline peaks with a relatively lower peak intensity in the soot sample generated in a low-mileage engine (SP A), which is an indication of lower levels of lubricant impurities in turbostratic carbonaceous soot. XRD results indicate that the chemistry of crystalline phases present in diesel soot is dominated by the presence of basanite, gypsum, and other calcium-based compounds. Patel et al.31 using a synchrotron XRD also reported the presence of gypsum and basanite as well as the presence of amorphous zinc-based compounds in the soot extracted from crankcase oils. Tribofilms are typically the glassy phase of phosphate, sulfate, and sulfides of zinc and/or iron.41,42 In the current study, the existence of zinc-based compounds in soot is possibly as a result of the interaction between tribofilms and soot. Moreover, the influence of overbased detergents on the composition of tribofilms has been well-documented in previous studies43−45 that might explain the presence of the crystalline phase of calcium sulfates in the soot agglomerates. The hardness of Zn polyphosphates has been reported to be as high as 20 GPa,46,47 whereas the hardness of the steel substrate is typically around 7−8 GPa. In contrast to that, Ca sulfates have a hardness of 2 on the Mohs scale that is as soft as talc; it could be postulated that the presence hard particles of zinc polyphosphates as well as calcium phosphates (shown earlier in HR-TEM results) may play a critical role in third-body abrasive wear in a diesel engine, while calcium sulfates are not likely the source of any abrasive wear. Thus, it can be hypothesized that the higher service age of an engine results in more severe wear conditions and the threebody wear mechanism dominates as a result of the presence of a larger number of hard particles in the soot agglomerates. 3.3. Raman Spectroscopy. Raman spectroscopy has been used very effectively to investigate short-range highly disordered graphitic structures. Studies have shown that different soot types can be distinguished on the basis of the G


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(7) Lawrence, L. Mach. Lubr. 2007. (8) Maiboom, A.; Tauzia, X.; Hétet, J. Energy 2008, 33, 22−34. (9) Haynes, B. S.; Wagner, G. H. Prog. Energy Combust. Sci. 1981, 7, 229−273. (10) Patel, M.; Aswath, P. B. Tribol. Int. 2014, 77, 111−121. (11) Patel, M.; Aswath, P. Tribol.-Mater., Surf. Interfaces 2015, 9, 19− 32. (12) Patel, M.; Aswath, P. Tribol.-Mater., Surf. Interfaces 2015, 9, 1− 18. (13) Esangbedo, C.; Boehman, A. L.; Perez, J. M. Tribol. Int. 2012, 47, 194−203. (14) George, S.; Balla, S.; Gautam, V.; Gautam, M. Tribol. Int. 2007, 40, 809−818. (15) George, S.; Balla, S.; Gautam, M. Wear 2007, 262, 1113−1122. (16) Bardasz, E. A.; Carrick, V. A.; Ebeling, V. L.; George, H. F.; Graf, M. M.; Kornbrekke, R. E.; Pocinki, S. B. Understanding soot mediated oil thickening through designed experimentationPart 2: GM 6.5L. Proceedings of the 1996 SAE International Fall Fuels and Lubricants Meeting and Exhibition; San Antonio, TX, Oct 14−17, 1996; DOI: 10.4271/961915. (17) Cadman, W. R.; Johnson, H. J. A study of the effect of exhaust gas recirculation on engine wear and lubricating oil degradation in a heavy-duty diesel engine using analytical ferrography and other oil analysis techniques. Proceedings of the SAE International Congress and Exposition; Detroit, MI, Feb 24−28, 1986; SAE Technical Paper 860378, DOI: 10.4271/860378. (18) Kimijima, T.; Haneishi, T.; Okabe, H. J. Jpn. Soc. Tribol. 1994, 39, 337−337. (19) Kaneta, M.; Irie, T.; Nishikawa, H.; Matsuda, K. Proc. Inst. Mech. Eng., Part J 2006, 220, 307−317. (20) Kagaya, M. Sekiyu Gakkaishi 1997, 40, 482−487. (21) Kagaya, M. Sekiyu Gakkaishi 1997, 40, 488−493. (22) Kagaya, M.; Kagawa, T.; Takahashi, Y. Sekiyu Gakkaishi 1997, 40, 494−499. (23) Hirose, Y.; Kunoki, T. J. Jpn. Soc. Lubr. Eng. 1987, 32, 159−164. (24) Kawamura, M.; Ishiguro, T.; Fujita, K.; Morimoto, H. Wear 1988, 123, 269−280. (25) Mainwaring, R. Soot and wear in heavy duty diesel engines. Proceedings of the International Spring Fuels & Lubricants Meeting & Exposition; Dearborn, MI, May 5−8, 1997; pp 47−63, DOI: 10.4271/ 971631. (26) Rounds, F. G. Proceedings of the SAE International Congress and Exposition; Detroit, MI, Feb 23−27, 1981; Paper Number 810499, DOI: 10.4271/810499. (27) Rounds, F. G. Proceedings of the Passenger Car Meeting & Exposition; Detroit, MI, Sept 26−30, 1977; Paper Number 770829, DOI: 10.4271/770829. (28) Akiyama, K.; Masunaga, K.; Kado, K.; Yoshioka, T. Proceedings of the 1987 SAE International Fall Fuels and Lubricants Meeting and Exhibition; Toronto, Ontario, Canada, Nov 2−5, 1987, DOI: 10.4271/ 872158. (29) Berbezier, I.; Martin, J. M.; Kapsa, P. Tribol. Int. 1986, 19, 115− 122. (30) Sasaki, M.; Kishi, Y.; Hyuga, T.; Okazaki, K.; Tanaka, M.; Kurihara, I. The effect of EGR on diesel engine oil, and its countermeasures. Proceedings of the International Spring Fuels & Lubricants Meeting & Exposition; Dearborn, MI, May 5−8, 1997; pp 39−44, DOI: 10.4271/971695. (31) Patel, M.; Ricardo, C. L. A.; Scardi, P.; Aswath, P. B. Tribol. Int. 2012, 52, 29−39. (32) Patel, M.; Aswath, P. B. Tribol. Int. 2012, 52, 17−28. (33) Yamaguchi, E. S.; Untermann, M.; Roby, S. H.; Ryason, P. R.; Yeh, S. W. Proc. Inst. Mech. Eng., Part J 2006, 220, 463−469. (34) Gautam, M.; Chitoor, K.; Durbha, M.; Summers, J. C. Tribol. Int. 1999, 32, 687−699. (35) Antusch, S.; Dienwiebel, M.; Nold, E.; Albers, P.; Spicher, U.; Scherge, M. Wear 2010, 269, 1−12. (36) Clague, A. D. H.; Donnet, J. B.; Wang, T. K.; Peng, J. C. M. Carbon 1999, 37, 1553−1565.

value of 0.36 and 0.29 in the cases of SP B and SP C, respectively, suggesting that, with a longer service age of an engine, less graphitic content is present in the carbonaceous soot. The ratio of G/D3, which represents the ratio of crystalline graphitic content of soot to amorphous carbon content, also decreases from 4.4 (SP A) to 2.5 (SP C), indicating a similar decline in the relative proportion of the crystalline graphitic content in soot with respect to the amorphous and disordered parts of soot as the engine ages.

4. CONCLUSION In this study, the role of engine age on the structure and chemistry of crankcase soot was studied using XRD, HR-TEM, EDS, and Raman spectroscopy. Results indicate that the basic structure of the carbonaceous species remains the same in all cases and is composed of turbostratic carbon. However, there are some subtle changes in the structure of the soot as the age of the engine increases. Older engines have a greater proportion of non-crystalline amorphous carbonaceous constituents in the soot relative to newer engines. In addition, the wear-induced debris increase with the age of the engine, with a larger proportion of phosphates of Ca and Zn as well as sulfates of Ca in the soot of older engines. The presence of these wear debris particles incorporated in the soot extracted from the crankcase indicates that they arise from three-body abrasive wear between the soot and tribofilms formed within the engine.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: 817-272-0308. E-mail: [email protected]. Present Addresses †

Sujay Bagi: Paccar Technical Center, 12479 Farm to Market Road, Mt. Vernon, Washington 98273, United States. ‡ Mihir Patel: Vanderbilt Chemicals, LLC, 30 Winfield Street, Norwalk, Connecticut 06955, United States. § Olusanmi Aderniran: Schaeffler Group USA, Inc., 308 Springhill Farm Road, Fort Mill, South Carolina 29715, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Experimental facilities provided by the Center for Characterization for Materials and Biology are gratefully acknowledged. REFERENCES

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