Piston-Cylinder Liner - Science Direct

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ScienceDirect Procedia Engineering 150 (2016) 541 – 546

International Conference on Industrial Engineering, ICIE 2016

Effect of the Heat Insulating Coating of the Piston Crown on Characteristics of the "Piston-Cylinder Liner" Pair Y. Rozhdestvensky, E. Lazarev, A. Doikin* South Ural State University, 76, Lenin Avenue, Chelyabinsk, 454080, The Russian Federation

Abstract Based on experimental findings and thermophysical properties, the effect of a heat insulating coating on a piston in a diesel engine is estimated. Using a set of boundary conditions for heat transfer, a finite element model is applied to the piston – cylinder liner tribosystem to evaluate thermal and deformed conditions. The distributions of temperature and deformation for the coated and uncoated piston crown are compared. © Published byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors.Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICIE 2016. Peer-review under responsibility of the organizing committee of ICIE 2016 Keywords: piston-cylinder liner tribosystem; heat insulating coating

1. Introduction The piston is one of the most critical elements in a diesel engine crank mechanism that largely determines its resource. It is operating under very high thermal- and mechanical loads and has to exhibit minimal tribological losses. To reduce the thermal load on the system an insulating coating of different chemical composition to the bulk material is applied to the piston crown with a thickness of 60 to 120 microns or in special cases even more. The coating process can be described as a solid-anodization, where thermo-chemical conversion, under an electric potential, of the upper aluminum alloy layer into a solid ceramic (aluminum oxide Al2O3) is performed [1–5]. The heat insulating coating is designed to reduce the temperature of the piston head. However, it is unclear how it can affect the geometry of the piston skirt during operating conditions because a change in the heat flow originated from the combustion of fuel in the combustion chamber will likely lead to different mechanical strains between

* Corresponding author. Tel.: +7-908-579-3269; fax: +7-351-267-9214. E-mail address: [email protected]

1877-7058 © 2016 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 the organizing committee of ICIE 2016

doi:10.1016/j.proeng.2016.07.039

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Y. Rozhdestvensky et al. / Procedia Engineering 150 (2016) 541 – 546

piston and cylinder liner. The question arises about the need to correct the geometry of the profile of the piston skirt to obtain acceptable hydromechanical characteristics of the "piston-cylinder" tribopair. The available literature failes to find the answer to this question [6–16]. In this connection a computational and experimental study is conducted to describe the effect of a piston crown coating on hydromechanical characteristics in the "piston-cylinder liner" tribopair. The bulk material of the used piston is an aluminum alloy. The chemical composition and thickness of the coating were found by electron microscope investigations. 2. Experimental studies of the composition and thermophysical properties of the heat insulating coating of the piston crown Surface investigations were carried out by using a scanning Jeol JSM-7001F electron microscope. The surface was studied in secondary and backscattered electron mode. As a result, maps of the element distribution over the surface are collected. Fluorescence X-ray analysis was carried out using an energy dispersive spectrometer OxfordINCAX-max 80, installed on the microscope. In a small angle cut of the coating (Fig. 1) there can be seen different layers representing a contamination layer, and the initial coating.

Fig. 1. Maps of elements in the oblique cut of coating

The most interested distributions can be seen for Si and K (Fig. 2), as they pertain to the lower (main) coating on a silicon base. In a similar specimen, located on the opposite side of the piston, the pattern can be repeated. The outer layer is dirty (with calcium sulfate and calcium and zinc phosphate) whereas the inner layer is the coating with potassium and silicon. The grain structure of the layer shows many rounded shape pores of 0.2-10 microns in diameter.


Thus, the preliminary results of the composition and morphology analysis suggests that the coating of about 100 microns thickness has low thermal conductivity which implies that it protects the bulk aluminum from overheating and possibly has an anticorrosive function (protection from acid oxides).

Fig. 2. Maps of phases in the oblique cut of the coating

To determine the thermal insulating properties of the aluminum silicate protective layer experiments were carried out on two equally sized specimens in shape similar to a parallelepiped cuted from the piston. The specimens were placed in the center of a flat surface of a laboratory electric stove. For temperature measurements a chromel-alumel thermocouple with a diameter of 0.3 mm (flexible heat and electrical isolation made of fiberglass) with a hot junction of 1.3 mm diameter was used. A series of measurements revealed that at a stove surface temperature of 220°C the surface temperature of the coated sample was 204°C whereas the uncoated sample showed 218°C. Thus, at 220°C the temperature drop in the coating was 14°C. The measurement error was about 3°C. Considering the heat transfer coefficient in the air, the heat flow through the coated specimen, was 3800 W/m2, and through the uncoated specimen - 4300 W/m2. 3. Thermal and deformed state of the piston with a heat insulating coating on the crown Experimental results were used to evaluate the thermal and deformed state of the piston under the action of heat flow occurring during the combustion of fuel in a diesel engine combustion chamber. The determination of the thermal fields are produced by using the boundary conditions of heat exchange and a calculation of the operating cycle of the diesel engine according to the method described in [17, 18]. The calculated results for the thermal state of the piston cross-section are shown in Fig. 3 and demonstrate that the coated piston crown temperature has decreased by approximately 12-14 ° C, and the piston skirt temperature decreased by 5–6°C. 4. Effect of coating on the hydro-mechanical characteristics of the tribopair The dynamics of motion of the piston on the lubricating layer in the cylinder of a diesel engine depends to a large extent on the used profile of the piston skirt, because it has a significant influence on the formation of a hydrodynamic wedge in the "piston-cylinder liner" pair. Analysis of the thermal state of the piston demonstrates a temperature decrease of the coated piston crown, which reduces the thermal deformation of the piston (Fig. 4), primary in its upper part. Consequently, the application of the heat insulating coatings can change the profile of the piston skirt under the action of a given temperature gradient.

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Fig. 3. Calculation results of thermal stresses for an uncoated and coated piston crown.

Fig. 4. Calculation results of deformation for an uncoated and coated piston crown.

To evaluate the effect of geometrical changes due to thermal deformation pistons with and without a coating layer on their crowns were compared in axial profiles (Fig. 5). As can be seen, the coating has an insignificant effect on the deformation of the piston profile. Furthermore, the method described in [19-24] was used on axial piston profile was added to the temperature conditions (Fig. 6), to calculate the trajectory of the piston on the lubricating layer in the cylinder. Hydromechanical characteristics of pairing with into account changes of profile and the viscosity of the lubricant under the action of changes in the piston temperature view in Table.1. Based on the calculated results of the piston crown with a heat insulating coating we can conclude minor changes in the hydromechanical parameters of the tribopair is case of friction induced power loss, lubricant flow in the direction of the combustion chamber, and average lubricant film thickness. Therefore, when applying the thermomechanical protection layer similar physical and geometric parameters can be estimated resulting in an insignificant decrease in the temperature and tribological contact conditions.

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Fig. 5. Comparison of deformation

Fig. 6. The axial profile of the piston skirt Table1. Dependence of hydro mechanical characteristics on the presence of the heat insulating coating Piston crown

Power loss due to friction [W]

Lubricant flow in the direction of the combustion chamber [cm3/s]

uncoated

215.6

49.7

59.8

coated

228.3

48.5

61.2

Average lubricant film thickness [μm]

5. Conclusion The investigated heat insulating coating on the piston crown reduces the heat flow from the working gas in the combustion chamber of the diesel engine in the surface of the piston. As a result a reduction of the maximum temperature of piston crown of 12–14°C, and of the piston skirt of 5–6°C is estimated. However, this does not result in significant changes in mechanical deformation of the piston skirt. The tribological profile of the piston skirt stays

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practically unchanged. Hydromechanical characteristics of the "piston-cylinder liner" pair despite the change of lubricant viscosity do not change significantly. Thus, when physical and geometrical parameters of heat insulating coating of the piston crown are similar to those studied, tribological piston profile may be left unchanged. Acknowledgements This work has been carried out within financial support of Russian Foundation for Basic Research (project ʋ 1608-00990\16, project ʋ 16-08-01020\16). Experimental studies carried out in the South Ural State University by Assoc., PhD in Chemistry D. Zherebtsov. References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surface and Coatings Technology. 2௅3 (1999) 73௅93. [2] N.Yu. Dudareva, I.A. Butusov, R.V. Kalschikov, R.R. Grin, I.V. Alexandrov, F.F. 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