Static Stress Analysis of Piston

Transcription

1 Static Stress Analysis of Piston Kevin Agrawal B. E. Student, Mechanical Engineering, BITS Pilani K. K. Birla Goa Campus. AH7-352, BITS Pilani, K. K. Birla Goa Campus, NH 17B, Zuarinagar Parva Thole B. E. Student, Mechanical Engineering, BITS Pilani K. K. Birla Goa Campus. AH7-344, BITS Pilani, K. K. Birla Goa Campus, NH 17B, Zuarinagar ABSTRACT In this study, static stress analysis is investigated on a piston made of aluminum alloy A2618 and high grade steel 42CrMo4. The aim is to find the areas of stress concentration of piston at the real engine condition during combustion process using the engine specifications of the motorcycle Bajaj Platina 100ES. The study also aims to understand the advantages and disadvantages involved in using both the materials. The finite element analysis has been performed by using Computer Aided Design software ANSYS. The results predict the Von mises stress, deformation and the factor of safety following which the best material is selected. Keywords Piston, A2618, 42CrMo4, Bajaj Platina 100ES, PTC Creo, Stress Analysis 1. INTRODUCTION The piston inside an Internal Combustion Engine is responsible for transferring the force from the expanding gas in the cylinder to the crankshaft. The piston is connected to the crankshaft through a connecting rod. The connecting rod is responsible for converting the reciprocating motion of the piston into rotary motion of the crankshaft. The most common type of piston is a reciprocating piston. The piston comprises of the following general parts: Piston Head, Piston Skirt, Ring Grooves, Oil holes, Piston Pin Boss. The commonly used materials for piston are cast iron, cast steel, forged steel, cast aluminum alloy and forged aluminum alloys. Because of its operating conditions the piston is subjected to high thermal and mechanical stresses. The continuously varying temperature inside the internal combustion engine is responsible for thermal stresses inside piston. Further the extreme pressure due to the combustion of the gas is responsible for mechanical stress. Considering the thermal and the mechanical stresses the design requirements for piston are as follows: 1. Sufficient strength to withstand the force due to combustion and inertia force of the reciprocating parts. 2. Sufficient Rigidity to withstand the thermal and the mechanical distortions. 3. Minimum weight to reduce the inertia force due to reciprocating motion. 4. Adequate capacity for the heat dissipation from the piston to the cylinder walls through the piston rings and piston skirt. 1.1 Engine Specification The piston has been designed based upon the engine specifications of Bajaj Platina 100 ES. Table 1. Engine Specifications Parameters Values Engine Type Four Stroke (Petrol) Induction Air Cooled Number of Cylinders Single Cylinder Bore 48 mm Stroke 58.8 mm Displacement 102 cc Maximum Power 6.03 kw at 7500 rpm Maximum Torque 8.6 Nm at 5000 rpm No. of Revolutions/Cycle 2 45

2 1.2 Properties of Materials The materials chosen are A2618 and 42CrMo4. A2618 is an aluminum alloy while 42CrMo4 is a high grade steel alloy. Difference between the properties of aluminum and steel pistons are as follows: Table 2. Difference between the properties of Aluminum and Steel pistons Sr. No. Properties Aluminum Steel 1 Thermal Conductivity Higher Lower 2 Temperature Gradient inside piston Less More 3 Density Lower Higher 4 Weight Less More 5 Strength Less More 6 Wear Strength Less More 7 Coefficient of Thermal Expansion Higher Lower Table 3. The mechanical properties of A2618 and 42CrMo4 Sr. No. Parameters A CrMo4 1 Young s Modulus (GPa) Ultimate Tensile Strength (MPa) Poisson s Ratio Density (kg/m 3 ) METHODOLOGY Design of piston using engine specifications of Bajaj Platina 100ES petrol engine. Creation of 3D models of piston using the computer aided design software PTC Creo. Meshing of 3D models using ANSYS Workbench. Analysis of pistons under mechanical loads i.e. the pistons are subjected to a uniform gas pressure Static Stress Analysis of piston using aluminium alloy (A2618) and high grade steel (42CrMo4) as the piston materials. Comparative analysis of the two piston considering the Von Mises Stress, deformation and the factor of safety. Selecting the best suited piston materials based on the obtained results. Brake power (B.P) = (2*π*N*T) / 60 = (2*π*8.6*5000) / 60 = 4.502kW Mechanical efficiency of the engine (η) = 80 %. Now the mechanical efficiency is defined as η = Brake power (B.P) / Indicating power (I.P) Therefore, I.P = B.P/ η = / 0.8 = KW Also, the indicated power i.e. I.P is defined by I.P = P x A x L x N/2 Hence the I.P = P*( π*d*d/4)*l*(n/2) Substituting these values in the above equation we get:- P = MPa Now the Maximum Pressure pmax = 10 x P = 10 x = MPa 46

3 2.1 Analytical design for A2618 alloy piston Based on the material properties of A2618 we have theoretically calculated the dimensions for the following components of a piston: 1. Thickness of the piston head 2. Axial and radial thickness of piston rings 3. Width of top land and ring land 4. Thickness of piston barrel 5. Length of the skirt 6. Length of the piston pin in connecting rod bushing 7. Piston pin diameter Thickness of Piston Head (th): The piston thickness of piston head calculated using the following Grashoff s formula, th = D (3pmax/16σt) th=48 ( (3*12.69) / ( 16*213.3) = 5.069mm Where P= maximum pressure in N/mm² D= cylinder bore/outside diameter of the piston in mm. σt = permissible tensile stress for the material of the piston. and σt = σut /2.25 = 480 / 2.25 = MPa Empirical formula: th = D = 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) th = [0.05 x x x 10-3 x 6.2] x 10 6 / x 147 x 20 x 3600 = mm Considering the Grashoffs formula, empirical formula and on the basis of the heat dissipation the maximum thickness from the above formula is th is mm. Piston Rings: The radial width of the ring is given by: b = D (3 pw/σp) = 48 (3 x 0.025/110) = 1.25 mm Axial thickness of the piston ring is given by: h = (0.7b to b) = 0.7 x 1.25 to 1.25 = mm to 1.25 mm We have considered b = 1.25mm and h = 1mm Width of Top Land and Ring Lands Width of top land: h1 = (th to 1.2 th) = mm Width of ring land: h2 = (0.75h to h) = 0.75 mm Piston Barrel Thickness of piston barrel at the top end: t1 = 0.03 D + b = 0.03 x = 7.59 mm Thickness of piston barrel at the open end: t2 = (0.25 t1 to 0.35 t1) = 0.25 x 7.59 = mm 2 mm Length of the skirt ls = (0.6 D to 0.8 D) = 0.6 x 48 = 28.8 mm Length of piston pin in the connecting rod bushing l1 = 45% of the piston diameter = 0.45 x 48 = mm Piston pin diameter do = (0.28 D to 0.38 D) = 0.28 x 48 to 0.38 x 48 We have considered the piston pin diameter do to be 12mm 47

4 Figure 1 : Model of the Piston Note that the centre of the piston pin should be 0.02 D to 0.04D above the centre of the skirt. As similar steps have been followed for the calculations for aluminium alloy, analytical design of 42CrMo4 is carried out and the results are summarized as follows: Sr. No Design - Dimensions Table 4. Analytical Design of 42CrMo4 and A2618 piston Size in mm (A2618) Size in mm (42CrMo4) 1 Length of the Piston(L) (mm) Cylinder bore/outside diameter of the piston(d) (mm) Radial thickness of the ring (t1) (mm) Axial thickness of the ring (t2) (mm) Width of the top land (h1) (mm) Width of other ring lands (h2) (mm) CAD AND FEA OF PISTON The piston design is first created using a 3D CAD software. The 3D CAD software is then imported to the FEA software ANSYS where a static structural analysis is to be performed on the piston. Under the finite element analysis the model of the piston is divided into a fixed number of elements and later ANSYS will predict the overall stiffness of the piston. From the analysis we will be able to find the areas of stress concentration inside the piston. 3.1 Meshing of the piston The piston is meshed taking solid 187. In the case of piston modelled using the high grade steel the total number of elements were 46490, nodes were and the volume being m 3 found in the meshed 48

5 model. Similarly in the piston modelled using aluminum alloy the total number of elements were 46784, nodes were and the volume being m 3 in the meshed model. 3.2 Applying the gas pressure on the piston The boundary condition for the mechanical simulation were defined as, firstly a pressure of 12.69MPa acting on the entire piston head surface. Here 12.69MPa being the maximum pressure in the internal combustion engine. The second boundary condition being that the base of the entire piston being fixed so that a static stress analysis can be performed on the piston under the pressure of 12.69MPa. 3.3 Results Figure 2 : Von Mises Stress in piston modelled from A2618(left) and 42CrMo4(right) Figure 3 : Deformation in piston modelled from A2618(left) and 42CrMo4(right) 49 Figure 4 : Safety factor in piston modelled from A2618(left) and 42CrMo4(right)

6 Table 5. Summary of Results Sr. No. Parameters A CrMo4 1 Von-Mises Stress (MPa) Total Deformation (mm) Mass (kg) Safety Factor CONCLUSION Based on the summary of results we can conclude that the Von-Mises Stress developed in 42CrMo4 piston is more than that developed in A2618 by 76%. Following which we can conclude that the total deformation in A2618 piston is more than that in 42CrMo4 piston by 39.58%. This can also be understood considering that the Ultimate Tensile Strength and the Young's Modulus is greater for high grade steel as compared to aluminum alloy. From the summary of results we also conclude that the weight of 42CrMo4 piston will be more than that of A2618 piston. Finally the Safety Factor for 42CrMo4 piston is more than that of A2618 piston by %. Hence the following properties of aluminum alloy and high grade steel pistons have been verified: Aluminum piston is lighter in weight than Steel Piston. The safety factor of Aluminum piston is less than Steel piston. The deformation in Aluminum piston is more than deformation in Steel Piston. 5. FUTURE SCOPE OF WORK Presently majority of the pistons use aluminum alloy as the piston material giving the advantage of a lightweight piston which reduce the stress and the pressure from the inertia forces. But since the pistons using aluminum alloys have a high coefficient of thermal expansion and decrease in strength at higher temperatures, pistons using high grade steel offer some advantages over the aluminum alloy pistons. Steel pistons are stronger than aluminum ones and can be operated at higher temperatures. Moreover the thermal conductivity of steel is lower than that of the aluminum, meaning a higher temperature can be reached inside the combustion chamber and hence a better ignition quality can be achieved. Hence following the comparative analysis of both the piston based on the static stress analysis a thermal analysis can also be done considering the piston to be modelled using aluminum alloy and high grade steel. 6. ACKNOWLEDGEMENT We would like to thank Dr. D. M. Kulkarni for giving us this opportunity of working on the project under his guidance. We would like to thank him for his constant guidance and support throughout the length of the project. We would also like to thank Dr. G. Karthikeyan and Dr. Ranjit Patil for providing their useful insights regarding the methodology to be followed during the analysis. 7. REFERENCES [1] V. B. Bhandari, Design of Machine Elements, 3rd Edition, McGraw Hill. [2] Shigley, Mechanical Engineering Design, 9th edition, McGraw-Hill. [3] Shuoguo Zhao Design the Piston of Internal Combustion Engine by Pro\ENGEER 2nd International Conference on Electronic & Mechanical Engineering and Information Technology (EMEIT-2012), Published by Atlantis Press, Paris, France. pp [4] Dr. P.C. Sharma, Aggarwal, R.D.K,, A Text Book of Machine Design, S. K. Kataraia and sons, New Delhi [5] Manjunatha.T.R, Dr. Byre Gowda.H.V, Prabhunandan.G.S, " Design and Static Structural Analysis of Cylinder and Piston of Two Stage Reciprocating Compressors Using ANSYS", International Journal of Innovative Research in Science, Engineering and Technology, Vol. 2, Issue 12, December 2013 [6] S. Srikanth Reddy, Dr. B. Sudheer Prem Kumar, "Thermal Analysis and Optimization of I.C. Engine Piston Using Finite Element Method", International Journal of Innovative Research in Science, Engineering and Technology, Vol. 2, Issue 12, December