Static Structural and Thermal Analysis of Aluminum Alloy Piston For Design Optimization Using FEA Kashyap Vyas 1 Milan Pandya 2

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1 IJSRD - International Journal for Scientific Research & Development Vol. 2, Issue 03, 2014 ISSN (online): Static Structural and Thermal Analysis of Aluminum Alloy Piston For Design Optimization Using FEA Kashyap Vyas 1 Milan Pandya 2 1 PG Student, M.E. Thermal 2 Associate Professor, Department of Mechanical Engineering, L.J.I.E.T, Ahmedabad-54, Gujarat, India Abstract--- This paper describes the structural and thermal analysis study conducted on Aluminum alloy piston using Finite Element Method in ANSYS. The design of the piston is developed through analytical approach, considering the operating gas pressure, temperature and material properties. The results obtained through structural and thermal analysis is used to predict the stress and temperature distribution across the piston geometry respectively. Based on the stress and temperature values obtained, the piston dimensions are optimized sequentially by first reducing the thickness of the piston barrel at open end and later by providing equal slots inside the open end piston barrel. The optimized piston geometry is further analyzed considering the structural and thermal aspects to ensure that the design is well within the permissible stress and temperature ranges. I. INTRODUCTION Piston being the heart of IC engines, it is one of the first components exposed to high temperatures and pressures due to combustion phenomenon. As such, piston is designed to withstand the stresses developed due to high pressure and temperature. Modern day pistons are made up of Aluminum alloys that have the advantages of light weight and high heat transfer rates. However, through the use of computer aided tools, it is possible to further optimize the geometry of the piston while maintain the structural and thermal integrity. So far, numerous researches have been done to evaluate different Aluminum alloys applicable for piston manufacturing [1]. There is also enough literature available on thermal and structural analysis of piston to measure deformation due to Von Mises stresses [2, 3, 4]. Yet, only little work has been done to optimize lightweight Aluminum alloy pistons for further mass reduction using computer aided tools like Finite Element Analysis in ANSYS. The purpose of this paper is to conduct one such experiment on Aluminum alloy piston to predict the possibility of mass reduction without affecting stress and deformation values. II. PISTON DETAILS A. Piston Selection Table 1 Thermal and Mechanical Properties Ofalghs1300 Aluminum Alloy Properties Values Elastic Modulus Ultimate Tensile Strength (MPa) % Yield Strength (MPa) 1220 Poisson s Ratio 0.3 Thermal Conductivity (W/m 0 C) 120 Co-efficient of thermal expansion (l/k) 18 x 10-6 Density (Kg/m 3 ) 2780 Fig. 1. Piston Model In order to carry out thermal and structural analysis, the piston used is from four stroke single cylinder engine of a motorcycle. The material of the piston considered is ALGHS1300 Aluminum alloy and its properties are mentioned below: B. Engine Specifications The engine performance data related to the selected piston is shown below in the table. Table 2 Performance Parameters of Single Cylinder Petrol (Gasoline) Engine Induction Parameters Number of cylinders Bore size Stroke length Length of connecting rod Values Air cooled type Single mm mm mm Displacement volume cm 3 Compression ratio 8.4 Maximum power Maximum torque Number of revolutions/cycle KW at 7500 RPM 8.05 Nm at 5500 RPM III. METHODOLOGY 1. Step-1 Perform analytical design using given engine specifications. 2. Step-2 Develop CAD model based on calculated dimensions 3. Step-3 Mesh generation using ANSYS All rights reserved by 671

2 4. Step-4 Performing static structural analysis 5. Step-5 Performing thermal analysis by considering thermal and mechanical loads. 6. Step-6 Optimizing piston design by reducing thickness of the piston barrel at open end. 7. Step-7 Re-analyzing the piston design using structural and thermal analysis. 8. Step-8 Repeating the design optimization process by applying equal slots on two sides of the piston barrel at open end. 9. Step-9 Performing structural and thermal analysis for the new optimized piston design. 10. Step-10 Selecting the best piston design having the capability to withstand stress and deformation with reduction in weight. A. Analytical Design Standard Parameters and Nomenclature used in calculation [1, 5]: IP: Indicated power produced inside the cylinder (W) η : mechanical efficiency: 0.8 n : number of working stroke per minute o : N/2 (for four stroke engine) N : Engine speed (rpm) L : Length of stroke (mm) A : Cross-section area of cylinder (mm 2 ) r : Crank radius (mm) lc : Length of connecting rod (mm) a : Acceleration of the reciprocating part (m/s 2 ) m p : Mass of the piston (Kg) V : Volume of the piston (mm 3 ) th : Thickness of piston head (mm) D : Cylinder bore (mm) P max : Maximum gas pressure or explosion pressure (MPa) σ t : Allowable tensile strength (MPa) σ ut : Ultimate tensile strength (MPa) F.O.S : Factor of Safety : 6 K: Thermal conductivity (W/m 0 K) Tc : Temperature at the centre of the piston head (K) Te : Temperature at the edge of the piston head (K) HCV : Higher Calorific Value of fuel (KJ/Kg) : KJ/Kg BP : Brake power of the engine per cylinder (KW) m : Mass of fuel used per brake power per second (Kg/KW s) C : Ratio of heat absorbed by the piston to the total heat developed in the cylinder: 5% or 0.05 b : Radial width of ring (mm) Pw: Allowable radial pressure on cylinder wall (N/mm 2 ) : MPa σ p : Permissible tensile strength for ring material (N/mm 2 ): 1110 N/mm 2 h: Axial thickness of piston ring (mm) h1: Width of top lands (mm) h2 : Width of ring lands (mm) t1 : Thickness of piston barrel at the top end (mm) t2 : Thickness of piston barrel at the open end (mm) ls : Length of skirt (mm) µ : Co-efficient of friction (0.01) l1: Length of piston pin in the bush of the small end of the connecting rod (mm) do : outer diameter of piston pin (mm) Mechanical efficiency (η) = 80 %. But, η = Brake power (B.P) / Indicating power (I.P) Therefore, I.P = B.P / η = 6.2 / 0.80 = 7.75 KW Also, I.P = P x A x L x N / 2 I.P = P x (π/4) D2 x L x N/2 We have, 7.75 x 1000 = P x ( π / 4 ) x (0.051)2 x (0.0488) x (5000) / ( 2 x 60 ) P = x 105 N/m 2 P = MPa Maximum gas pressure acted upon the Piston : P max = 10 x P = 10 x = MPa B. Analytical design for ALGHS1300 alloy piston By considering the Factor of Safety as 6, the allowable stress for the piston design is calculated as under: σ t = Ultimate Tensile Strength / FOS = 635 / 6 = MPa Thickness of the Piston Head According to Grashoff s formula the thickness of the piston head is given by th = D (3pmax/16σt) Therefore, th = 51 x (3 x 18.66)/(16 x ) = 3.97 mm 1) Empirical formula: th = D = x = 3.2 mm On the basis of the heat dissipation, the thickness of the piston head is given by: th = [ C x HCV x m x BP] x 106 / x K (Tc Te) = [0.05 x x x 10-3 x 6.2] x 106 / x 147 x 20 x 3600 = x 106 / = 3.775mm The maximum thickness from the above formula is th is 3.92 mm. 2) Piston Rings The radial width of the ring is given by: b = D (3 pw/σp) = 51 (3 x 0.025/110) = 1.33 mm Axial thickness of the piston ring is given by: h = (0.7b to b) = 0.7 x 1.33 = mm 1 mm All rights reserved by 672

3 3) Width of Top Land and Ring Lands Width of top land: h1 = (th to 1.2 th) = 3.97 mm Width of ring land: h2 = (0.75h to h) = 0.80 mm 4) Piston Barrel Thickness of piston barrel at the top end: t1 = 0.03 D + b = 0.03 x = 7.76 mm Thickness of piston barrel at the open end: t2 = (0.25 t1 to 0.35 t1) = 0.25 x 7.76 = 1.94 mm 2 mm Length of the skirt ls = (0.6 D to 0.8 D) = 0.6 x 51 = 30.6 mm Length of piston pin in the connecting rod bushing l1 = 45% of the piston diameter = 0.45 x 51 = mm Piston pin diameter do = (0.28 D to 0.38 D) = 0.28 x 51 =14.28 mm The centre of the piston pin should be 0.02 D to 0.04D above the centre of the skirt. C. Development of CAD Model of the Piston The geometry of the piston is modeled using CAD tools. The 3D model developed using Creo is show below: deformations of nearly incompressible elastoplastic materials, and fully incompressible hyperelastic materials. Fig. 3: SOLID186 Element Fig. 4: Applying mesh on piston model Table 3:Mesh Details No. of nodes No. of elements E. Static Structural Analysis of the Piston The piston model after proper meshing is used to conduct static structural analysis to record the values of Von Mises stress and deformation values under effect of gas pressure of MPa. The piston pin area is assumed as a frictionless support with all degrees of freedom fixed. Fig. 2: Piston model developed using Creo D. Mesh Generation using ANSYS The CAD model developed in Creo is imported in ANSYS Mechanical and the model is divided into number of small finite elements by mesh generation. In order to ensure accuracy at each node, the element selected is SOLID186 consisting of 20 nodes with each node having three degrees of freedom. This element supports plasticity, hyperelasticity, creep, stress stiffening, large deflection and large strains. It also has mixed formulation capability for simulating Fig. 5: Physics application on Piston Model All rights reserved by 673

4 Fig. 6: Total deformation in the piston geometry Fig. 9: Temperature distribution across the piston geometry F. Fig. 7: Equivalent Von Mises stress distribution on the piston geometry Steady State Thermal Analysis of the Piston The meshed piston is once again used to conduct steady state thermal analysis to calculate temperature distribution and total heat flux across the geometry. Values of temperature and heat transfer co-efficient are applied as shown below: Table 4 Temperature And Heat Transfer Co-Efficient Values At Different Regions Of The Piston Piston Region Temp (K) Piston Head Width of top land Piston ring area Piston skirt land Heat Transfer Coefficient (W/mm 2 K) Fig. 10: Total heat flux across the piston geometry G. Design Optimization The results obtained from the structural and thermal analysis are well within the permissible limits, allowing further mass reduction from the existing piston geometry. The optimization is done with two trials and the new design obtained in again analyzed for structural and thermal integrity. Fig. 11: Reducing the thickness of the piston barrel at open end by 2.61 mm and 0.25 mm deep slot Fig. 8: Application of temperature and heat transfer coefficient on piston geometry All rights reserved by 674

5 Fig. 12: Total heat flux on optimized piston Fig. 13: Total deformation Fig. 14: Equivalent Von Mises stress IV. RESULTS The selected piston was first analyzed both for structural as well thermal parameters. The weight of the selected piston was found to be 100 grams. Initial stress and deformation values obtained through the analysis were Mpa and mm respectively. Based on the number of design optimization trials performed, following results were obtained: Table 5: Stress, Deformation And Weight Reduction Obtained Through Optimization Trials Trials Stress (MPa) Deformation (mm) Weight (gm) 2.61 mm thickness mm thickness mm thickness & 0.25 mm slot 2.61 mm thickness & 0.50 mm deep slot V. CONCLUSION The results obtained through several optimization trials and subsequent structural and thermal analysis suggest that pistons made up of lightweight Aluminum alloy such as ALGHS1300 can also be further optimized for mass reduction without excessive stress generation and deformation. The methodology mentioned in this paper can be used to optimize piston or similar geometrical components made up of different materials. Using finite element analysis, it becomes easier to predict the stress concentration regions that can promote deformation. The research work carried out is also useful to conduct modal analysis of piston with optimized geometry. REFERENCES [1] Ajay Raj Singh and Dr. Pushpendra Kumar Sharma, Design, Analysis and Optimization of Three Aluminium Piston Alloys Using FEA, International Journal of Engineering Research and Applications, Vol. 4, Issue 1, (Version 3, Jan 2014) [2] F. S. Silva, Fatigue on engine pistons A Compendium of case studies, Department of Mechanical Engineering, University of Minho, Portugal, Engineering Failure Analysis 13 (2006) [3] A.R. Bhagat and Y. M. Jibhakte, Thermal Analysis and Optimization of IC Engine Piston, International Journal of Modern Engineering Research Vol. 2, Issue. 4, July-Aug [4] Ekrem Buyukkaya and Muhammet Cerit, Thermal analysis of ceramic coating diesel engine piston using 3D finite element method, Elsevier- Surface & Coating Technology 202 (2007) [5] V.B. Bhandari, Design of Machine Elements (Tata McGraw Hill, 2007). All rights reserved by 675