Thermal and Stress Analysis of Ceramic Coated Piston

Transcription

1 Thermal and Stress Analysis of Ceramic Coated Piston Vivek J Shah Bits Pilani Department of Mechanical Engineering Dubai Campus Dr. R Udayakumar Bits Pilani Department of Mechanical Engineering Dubai Campus. ABSTRACT Diesel engines are very popular for their fuel economy and efficiency, ceramic coating over the piston is provided to reduce the heat transfer and increase the efficiency and decrease the fuel consumption. The thermal and stress analysis was conducted by finite element method using Ansys. The results of piston coated with three different coatings namely MgZrO 3, Yttria stabilized Zirconia, Lanthanum hexaaluminate of different coating thickness 0.8mm, 1.0mm, 1.2mm were analyzed. The influence of coating thickness on the temperature distribution over the piston and the stresses acting on the piston were studied. Keywords Lanthanum Hexaaluminate (LHA), Mg-PSZ, Thermal Analysis, Stress Analysis, Y-PSZ INTRODUCTION The diesel engines are increasingly becoming important because of their fuel economy and efficiency but the main disadvantage of diesel engines is that lot of energy/heat is lost. The diesel engine rejects about two thirds of the heat energy of the fuel, one-third to the coolant, and one third to the exhaust, leaving only about onethird as useful power output so if the heat rejected can be lowered then the thermal efficiency and fuel economy of the vehicle would improve. Low heat rejection can be achieved by insulating the piston, combustion chamber walls, and valves with ceramic coating. Ceramic thermal based coatings are the best candidate materials for coating of piston top since they have low thermal conductivity, low coefficient of thermal expansion, they are more resistant to wear and corrosion than metals and have better thermal durability than metals.lower heat rejection from the combustion chamber through thermally insulated components causes an increase in available energy that would increase the in cylinder work and the amount of energy carried by the exhaust gases, which could be also utilized. In this study thermal and stress analysis was performed for various coating thickness and for three different materials Mg-PSZ, Y-PSZ, LHA. 2. THERMAL ANALYSIS AND STRESS ANALYSIS In this simulation catia V5 design software was used for modelling of the piston and it was imported in Ansys simulation software. Thermal analysis was done to understand the temperature on top of the piston with respect to various coating thickness of different coating materials Mg-PSZ, Y-PSZ and LHA. The top coat, substrate and bond coat material properties as shown in Table 1 were taken from literature. Bond coat is an intermetallic coat between the top coat and the substrate which provides oxidation resistance and internal stress reduction at high temperature. 2.1 Thermal Analysis Boundary Conditions Steady State Thermal Analysis was done on the Piston of various thickness and different top coat material.the Steady State thermal Analysis was done using Ansys Software and the Piston model was created in CATIA V5. The model is made up of 3 Layers namely the Top coat, Bond coat and the substrate respectively. All the materials are assumed to be isotropic and linearly elastic. It is assumed that all the heat transfer that takes places between piston and the combustion chamber is Convection Heat Transfer. 224 Vivek J Shah, Dr. R Udayakumar

2 Table 1: Material Properties of Metal Substrate, Bond Coat, Top Coat. Material Thermal conductivity (W/m-C) Thermal Expansion 10e-6(1/c) Density (Kg/m3) Specific Heat (J/kg-c) Poisson ratio Alsi Mg-PSZ Young s modulus (Gpa) Y-PSZ LHA NiCrAl Based on literature boundary conditions for the piston were taken as The average heat transfer coefficient and temperature for the porting above the rings are 500w/m 2 c and c see Figure 1 boundary condition 1. The heat transfer coefficient and temperature for the piston crown are 700w/m 2 c and c see Figure 2 boundary condition 2. The heat transfer coefficient and temperature for the inside portion of the piston are 1500w/m 2 c and c see Figure 3 boundary condition 3. The heat transfer coefficient and temperature for the lateral portion of the piston are 1500w/m 2 c and c see Figure 4 boundary condition 4. The heat transfer coefficient for 1 st, 2nd, 3rd piston rings is 400w/m 2 cand temperatures are c, c and c respectively see Figure 5, 6,7 boundary conditions 5,6,7. Figure 1: Boundary condition 1 Figure 2: Boundary condition 2 Figure 3: Boundary condition 3 Figure 4: Boundary condition Vivek J Shah, Dr. R Udayakumar

3 Figure 4: Boundary condition 4 Figure 5: Boundary condition 5 Figure 6: Boundary condition 6 Figure 7 Boundary condition Stress Analysis Strength of the material largely depends on the temperature, so therefore if the operating temperature of the engine is low then the strength of the piston increases and correspondingly the piston life also increases. As the temperature inside the engine is very high its normal for thermal stress to occur which my leads to cracks which damages the top coat so as result Stress analysis is very important and main disadvantage of ceramic material is its brittleness and cracking under high temperature Stress Analysis Boundary Conditions In this study Maximum normal stress were determined for various coating thickness and different coating material. Fixed support see Figure 8 Figure 8: Boundary condition Vivek J Shah, Dr. R Udayakumar

4 The Pressure acting on top of the piston is 9.2 MPA; this value was obtained from literature for a naturally aspirated engine see Figure 9 Figure 9: Boundary condition RESULTS AND DISCUSSIONS Highest temperature is obtained on top of the piston. For Mg-PSZ and 0.8mm top coat thickness see figure 10.The maximum temperature on top of the piston crown is c, under similar condition simulation was carried out for 1.2mm and 1.0mm thickness and max temperature on the piston top coating were cand c respectively Figure 10 For Y-PSZ and 0.8mm top coat thickness see figure 11.The maximum temperature on top of the coating surface is c, under similar conditions simulation was carried out for 1.2mm and 1.0mm thickness and max temperature on the piston top coating were c and c respectively 227 Vivek J Shah, Dr. R Udayakumar Figure 11

5 For LHA and 0.8mm top coat thickness see figure 12. The maximum temperature on top of the coating surface is c, under similar conditions simulation was carried out for 1.2mm and 1.0mm thickness and max temperature on the piston top coating were c and c respectively Figure 12 From the figure 13 it can be seen that as the top coat thickness increases the maximum temperature on the coating surface also increase for all the three coating materials Max 400 Temperature deg c Mg-PSZ Y-PSZ LHA mm 1mm 1.2mm Thickness Figure: 13.Max Temperature on piston vs Coating Thickness. Due to lower thermal conductivity of Mg-PSZ it has the maximum temperature on the top coat and since LHA has higher thermal conductivity compared to other two materials the temperature on top of the coat is low. The temperatures are higher because coated surface does not allow for heat transfer into the piston surface. The Results for Stress analysis were as following For Mg-PSZ and coating thickness of 0.8mm the maximum normal stress on top of the coat is MPA, under similar conditions maximum normal stress for 1.2mm and 1.0mm thickness were found and the values were and MPA respectively. For Y-PSZ and coating thickness of 0.8mm the maximum normal stress on top of the coat is MPA, under similar conditions maximum normal stress for 1.2mm and 1.0mm thickness were found and the values were and MPA respectively. 228 Vivek J Shah, Dr. R Udayakumar

6 For LHA and coating thickness of 0.8mm the maximum normal stress on top of the coat is MPA, under similar conditions maximum normal stress for 1.2mm and 1.0mm thickness were found and the values were and MPA respectively From Figure 14 it can be seen that as the coating thickness increases the stress acting on the top coat also increases for all the three top coat materials. LHA has the least max stress acting on it when compared to other materials at the same thickness and is less prone to crack formation than other materials in this study. 850 Max Equivalent stress MPA mm 1.0mm 1.2mm Mg-PSZ Y-PSZ LHA Thickness Figure: 14 Max Equivalent Stress vs Coating Thickness. 3. CONCLUSION The study shows that having a thermal barrier coating on the piston tends to improve the engine performance and efficiency of the vehicle. From the study it was noted that as the coating thickness increases the temperature acting on the piston as well as the thermal stress increases. From thermal analysis point of view Mg-PSZ is the best candidate for thermal barrier coating but when viewed from stress analysis LHA has the least effect of stress when compared to other materials in the study. Therefore more amount of research must be done on LHA to increase its thermal conductivity because stress analysis proves that it has the least amount of stress acting on it and it is stable upto c. It is desirable to use higher coating thickness but from the study it can be seen that as coating thickness increases thermal stress also increases so as a result nominal coating thickness must be used. 4. REFERENCES [1] S. Krishnamani and T. Mohanraj.Thermal Analysis of Ceramic Coated Aluminum Alloy Piston using Finite Element Method. Indian Journal of Science and Technology [2] Thermal Properties of Lanthanum Hexaaluminate. Zahra Neghadari, Monika Willert, Florian. Journal of European ceramic society. [3] Sumana Ghosh. Thermal Barrier Ceramic Coatings A Review. [4] Matsumoto M, Takayama H, Yokoe D, Mukai K, Matsubara H, Kagiya Y, Sugita Y. Thermal cycle behavior of plasma sprayed La2O3,Y2O3 stabilized ZrO2 coatings. ScriptaMaterialia. 2006; 54: [5] Murat Ciniviz, Mustafa Sahir Salman, EyübCanlı,Hüseyin Köse1 and ÖzgürSolmaz.Ceramic Coating Applications and Research Fields for Internal Combustion Engines., Prof. Feng Shi (Ed.), ISBN: , InTech. [6] G Sivakumar1, Dr. V. Shankar2, Dr. G. Hemath Kumar3, N. G. Renganathan4, V. U. Garud5. Is Thermal Barrier Coating for Low Heat Rejection in SI Engines or Diesel Engine. International Journal of Emerging Technology and Advanced Engineering(ISSN , ISO 9001:2008 Certified Journal, Volume 2, Issue 12, December Vivek J Shah, Dr. R Udayakumar