5th International Conference on Mechanics and Materials in Design

REF: A0319.0006

FEA OF TWO ENGINE PISTONS MADE OF ALUMINIUM CAST ALLOY A390 AND DUCTILE IRON 65-45-12 UNDER SERVICE CONDITIONS

P. Carvalheira1, and P. Gonçalves Departamento de Engenharia Mecânica, Faculdade de Ciências e Tecnologia da Universidade Coimbra Coimbra, Portugal Email: (1)[email protected]

SYNOPSIS As part of the design of a low fuel consumption internal combustion engine to be applied to our new vehicle XC20i (a prototype car for the European Eco-Shell Marathon), the aim of this work is to compare two different materials for the engine piston: Aluminium Alloy A390-T5 and Ductile Iron 65-45-12 using a Finite Element Analysis (FEA) and thus choose the best suited material. A number of FEA were carried out to predict the thermal and mechanical stresses in the piston and thus optimise its shape. The original design of the piston was fully analysed under wide open throttle (WOT) conditions at maximum design engine speed. Results showed three critical areas in the piston with high stresses and/or high temperatures. For each material, a parametric study of various design features/dimensions was conducted to make the design stronger and improve its safety. As a result of this work, we made a comparative study of some relevant parameters for each piston material. Despite low thermal expansion, thermal conductivity and high mechanical strength of Ductile Iron 65-45-12, its high density causes high inertial forces on the engine. Although the Aluminium Alloy A390- T5 has a larger thermal expansion, a larger thermal conductivity and a lower mechanical strength at high temperature than ductile iron, due to its lower density it was possible to design a piston meeting all the design requirements and weighting about 66 % less than Ductile Iron piston design.

1. INTRODUCTION The piston is the engine part that receives the energy from combustion and transmits it to the crankshaft. It must withstand heavy stresses under severe temperature conditions. The piston is considered the most critical component of an internal combustion engine and the prediction of the mechanical and thermal behaviour under service conditions requires great attention in the design phase. The material of the piston is chosen according to its mechanical strength, density, wear characteristics and thermal expansion properties. The design of the piston starts with the definition of the piston geometry using 3D CAD software. This 3D CAD geometric model is then imported to FEA software and analysed under the predicted service conditions before anything is made. That speeds up the design and testing process, reduces the lead time to create new pistons designs, and produces a better product. The idea behind finite analysis is to divide a model piston into a fixed finite number of elements. Computer software generates and predicts the overall stiffness of the entire piston. Analyzing the data it is possible predict

Chapter III: Product Engineering & Development in Design 1 Porto-Portugal, 24-26 July 2006 how the piston will behave in a real engine and allows the engineer to see where the stresses and temperatures will be the greatest and how the piston will behave [1] About 50 percent of unburned hydrocarbon (HC) emissions from conventional four stroke SI engines can be attributed to flame quenching within the top ring crevice [2]. Crevice volume can be reduced by moving the ring closer to the crown, although the possibility of the ring failure due to exposure to high temperature is high. Other good alternative design solution to reduce the crevice volume is using tighter clearances between the piston and cylinder. Cast iron has the advantages of low thermal expansion and high temperature strength over aluminium alloys allowing the reduction of the clearances between the piston and cylinder. Also, the lower thermal conductivity of the cast iron causes more of heat energy produced in combustion to be put into useful work, rather than wasted in the cooling system. The larger weight of the ductile iron piston increases the inertial forces created by the rapid change in piston direction. On the other side, the aluminium alloy has a high strength-to-weight ratio but also larger thermal conductivity and larger thermal expansion. Both materials in analysis have different characteristics with some positive and negative aspects. The FEA carried out for these two different materials predict the thermal and mechanical stress under operating conditions. Detailed comparative thermal and mechanical stress analyses are presented together as well as sensitivity reports of some piston geometrical parameters. With these results we can make a robust component and make a more rational selection tool for the material of the piston.

2. CHARACTERIZATION OF MATERIALS The materials chosen for our study are two metallic alloys usually used as substrate material for internal combustion engine pistons.

2.1 ALUMINIUM A390-T5 A unique combination of properties makes aluminium one of our most versatile engineering and construction materials. Besides its lower density, it has high resistance to corrosion under the majority of service conditions. A wide range of mechanical characteristics, or tempers, are available in aluminium alloys through various combinations of cold work and heat treatments. Aluminium and its alloys lose part of their strength at elevated temperatures, although some alloys retain good strength at temperatures from 200-260 ºC [5]. Aluminium Alloy A390 is a hypereutectic alloy for which silicon is the main alloying element. The low thermal expansion coefficient is exploited for pistons, the high hardness of silicon particles for wear resistance. [6] The hypereutectic alloys pistons features increased fatigue strength, lower thermal conductivity, increased scuff resistance, lower friction, and better wear properties than pistons made with other alloys. Increasing fatigue strength allows for pistons with thinner wall sections and lower weight. Lower thermal conductivity allows more of the heat energy produced in combustion to be converted to useful work, rather than wasted in the engine cooling system [7]. Table 1 shows the composition by weight percent of each component of Aluminium A390- T5.

2 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

Table 1. Composition of Aluminium A390-T5 [8]. Component Percent of weight Al 74.4-79.6 Cu 4-5 Fe Max 1.3 Mg 0.45-0.65 Mn Max 0.1 Si 16-18 Others --

Table 2 shows relevant mechanical and thermal properties of the Aluminium A390-T5 for the FEA.

Table 2. Relevant mechanical and thermal properties of Aluminium A390-T5 [8] for the FEA. Poisson Ratio 0.33 Modulus of Elasticity /GPa 81.2 Density at 20ºC / (kg/m3) 2730 Thermal Conductivity at 25ºC /(W/m K) 134 Thermal Expansion Coefficient (100-300ºC)/(1/ºC) 18.0E-06 - 22.5E-06 Specific Heat Capacity at 100ºC /(J/kg K) 963 Ultimate Tensile Strength at 25ºC /MPa 280 Yield Tensile Strength at 25ºC /MPa 240 Fatigue Tensile Strength (5e8 cycles R.R Moore Test) /MPa 110

The evolution with temperature of yield tensile strength is shown on Table 3.

Table 3. Evolution of yield tensile strength with temperature for Aluminium A390-T5 [8]. Temperature /ºC Yield Strength /MPa 25 240 95 225 150 195 205 160 260 85

Due to the lack of information of fatigue tensile strength with temperature, we used the same ratio of fatigue tensile strength to yield tensile strength at ambient temperature to obtain the fatigue tensile strength at each temperature. Fig. 1 shows the approximation made.

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300

250

200

150 Stress /MPa 100

50

0 20 60 100 140 180 220 260 Temperature /ºC

Yield Tensile Strength /MPa Fatigue Tensile Strength (Approximation)/MPa

Fig.1. Evolution of tensile yield strength and fatigue strength (approximation) with temperature for Aluminium A390-T5.

2.2 DUCTILE IRON 65-45-12 Ductile iron 65-45-12 is a high silicon ductile iron intended for use at high temperatures or when a part is subjected to thermal cycling. The ferritic structure remains stable so that no significant transformation takes place minimizing stresses that lead to cracks and distortion of the finished part. The composition of ductile iron 65-45-12 is presented on Table 4.

Table 4. Composition of ductile iron 65-45-12 in percentage of weight of each element.

Element Percentage

Carbon 3.45 – 3.75

Silicon 3.25 – 4.00

Manganese 0.15 – 0.35

Sulphur 0.025

Phosphorus 0.05

Relevant mechanical and thermal properties of ductile iron 65-45-12 for the FEA are shown on Table 5 [9].

4 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

Table 5. Relevant mechanical and thermal properties of ductile iron 65-45-12 for the FEA. [9] Poisson Ratio 0.33 Modulus of Elasticity /GPa 168 Density /(kg/m3) 7100 Thermal Conductivity /(W/m K) 36 Thermal Expansion Coefficient /(1/ºC) 12.8 E-6 Specific Heat Capacity at 100ºC/(J/kg K) 494 Ultimate Tensile Strength at 25ºC /MPa 448 Yield Tensile Strength at 25ºC /MPa 310 Elongation at rupture /% 12 Fatigue Tensile Strength /MPa (50% of the ultimate tensile strength) /MPa 224

Due to the lack of data for ductile iron 65-45-12 to predict the evolution of the fatigue strength limit (R = -1) with temperature of ductile iron 65-45-12 we considered at each temperature the same ratio between the fatigue strength limit (R = -1) and the ultimate tensile strength. The ratio considered was 50 %. Fig. 2 shows the results of the assumptions made to find the evolution of fatigue strength limit with temperature.

500

450

400

350

300

250

200 Stress /MPa 150

100

50

0 0 50 100 150 200 250 300 350 400 450 500 550 Temperature /ºC

Ultimate Tensile Strength /MPa Yield Tensile Strength /MPa Fatigue Tensile Strength /MPa

Fig 2. Evolution of ultimate tensile strength, yield tensile strength and the approximation made to fatigue strength with temperature for ductile iron 65-45-12.

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3. BOUNDARY CONDITIONS The boundary conditions for the FEA of the service conditions of the piston were predicted using the computer code “4SSI” developed by Carvalheira. The program provides a detailed analysis of in-cylinder processes, including combustion, heat transfer, pressure evolution, flame propagation and others. The thermal performance of the engine was simulated by a model of single-cylinder two- valve engine using the engine specification data presented on Table 6. Some of this data was optimized by D. Guilherme, J. Ramos (2002) [3] and it is presented on Table 6.

Table 6. Engine specification data. Bore /mm 33.0 Stroke /mm 37.0 Con Rod Length/ mm 69.0 Displacement/ cm3 31.65 Compression Ratio 15 Combustion Chamber Hemispheric No. of Inlet Valves 1 No. of Exhaust Valves 1 Inlet Valve Open Timing /deg (BTDC) 10 Inlet Valve Close Timing /deg (ABDC) 75 Exhaust Valve Open Timing /deg (BBDC) 44 Exhaust Valve Close Timing /deg (ATDC) 0 Fueling System Indirect Injection Coolant Temperature 90ºC Oil Temperature 100ºC

The engine performance simulations using the “4SSI” computer code were carried out with the engine operating at wide open throttle (WOT) conditions at 5000 rpm. Experimental data obtained with a similar engine, Honda GX22, show a good agreement with the results calculated with the “4SSI” computer code.

The boundary conditions required for thermal and mechanical FEA are:

Maximum Cylinder Pressure 9.0 MPa Cycle Average Heat Flux 405 000 W/m2

These results were obtained for a spark ignition timing of 10 deg BTDC and an equivalence ratio of 0.74 at 5000 rpm:

6 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

The other boundary conditions, as the heat removed by the coolant and oil were calculate taking into account basic expressions of heat transfer and some of the data used by the engine simulation computer program:

Thermal and geometrical conditions:

Average temperature of the coolant: Tc = 90 ºC

Average temperature of cylinder walls: Tcw = 100 ºC

Average temperature of piston walls: Tpw = 150 ºC Clearance between cylinder and piston: l = 60 E-6 m Thermal conductivity of film oil: k = 134E-3 W/m K Skirt area in contact with cylinder walls: 621 E-6 mm2

The main heat transfer, Q through film oil contacting with cylinder walls is given by the heat conduction equation, Eq. (1) [4].

(T − T ) Q = k.A. pw cw [W] (1) l

Considering Tcw and Tpw constant and dividing Q by (Tpw-Tcw).A, we can write Eq. (1) in the form of Eq. (2).

Q k = [W/m2.K] (2) (Tpw − Tcw ).A l

Substituting in Eq. (2) the variables by the numeric values we obtain Eq. (3).

−3 Q 134 ×10 2 = −6 = 2233 W/m K (3) (Tpw − Tcw ).A 60 ×10

Thus the boundary conditions for the piston walls in contact with the cylinder can be approximated by a convection coefficient of 2233 W/m2K, with a wall temperature of 100 ºC. We used a convection heat transfer mode instead of a conduction heat transfer mode because the program didn’t allow introducing the conduction heat transfer mode in a way that the reality is properly modelled. To calculate the heat removed by the carter oil in the carter we consider the situation of maximum rotation of the engine (5000 rpm) and we made an estimative of the oil projected (9 % approx.) to the piston. On this way we can calculate the average mass flux of the oil that contact with the piston and thus find the convection coefficient. The convection coefficient calculated for the piston walls in contact with the oil in the carter was h = 750 W/m2K.

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3. FINITE ELEMENT ANALYSIS The FEA of the temperature, stress and deformations in the piston were carried out using ANSYS (Workbench Simulation module). The ANSYS Workbench solution is a new- generation tool that offers an efficient and intuitive user interface, superior CAD integration, automatic meshing, automatic contact detection and creation and access to model parameters. ANSYS Workbench Simulation is build upon the core ANSYS solver technology that the industry has recognized for the benefits it offers for advance analysis.

3.1 THE MODEL To create the model we applied some geometric features optimized using the “4SSI” computer program, while others were treated as free parameters and optimized later using the ANSYS Workbench Simulation. Geometrical parameters such as the skirt area, pin boss bearings diameter, pin diameter or filets radius were some of the parameters that we optimized using the ANSYS Workbench Simulation. The variables used to evaluate the design were the equivalent stress and temperature on critical points and radial deformations relative to cylinder axis. The aim of this FEA is to design a robust component, without risk of rupture under maximum thermal and mechanical cyclic loads for a specified service life and with a minimum mass, to reduce the inertial forces. To create our model we used Autodesk Inventor, 3D CAD software. The model is then imported to ANSYS Workbench Simulation, where after the definition of boundary conditions and types of contacts, stress, temperature and deformations distributions in the piston are calculated. The body mesh generation is fully automatic and it is integrated in the solver time. The element size is calculated based on the size of the body box (the smallest box that the body will fit in), the proximity of others topologies, body curvature, and complexity of the feature. Contact conditions are formed where the bodies meet. Contact conditions can transfer structural loads and heat flows across the contact boundaries and “connect” the various bodies. Depending on the type of contact, the analysis can be linear or nonlinear.

Fig. 3. Piston model

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3.2 ENVIRONMENT CONDITIONS, SUPPORTS AND CONTACT DEFINITIONS

Environment conditions Maximum cylinder pressure: 9.0 MPa Heat flux on top of the piston: 405 000 W/m2 (hot piston) Heat flux on top of the piston: 0 W/m2 (cold piston) Convection heat transfer coefficient (between the piston walls and the cylinder): 2333 W/m2 K Convection heat transfer coefficient (walls inside the piston, in contact with oil): 750 W/m2 K

Supports Type: Cylindrical Support (pin-connecting rod interface) Definition: Radial Free Axial Free Tangential Fixed

Contacts definitions Contact pin-piston Definition: Type: Frictional Friction coefficient: 0.01

4. RESULTS AND DISCUSSION

4.1 COLD PISTON AND HOT PISTON CONDITIONS

The first two types of conditions considered when analysing the piston were: 1. Mechanical load only (cold piston), i.e., the piston is subjected to an uniform temperature distribution; 2. Thermal and mechanical loads (hot piston), i.e., the piston is subjected to an uniform gas pressure and a non-uniform temperature distribution.

These two loading conditions represent two extreme engine working conditions for the piston. The cold piston represents the running condition when the engine load suddenly changes from idle condition to maximum engine rotation speed (5000 rpm) at WOT. The hot piston represents the steady-state condition when the engine is running at maximum engine rotation speed (5000 rpm) at WOT. Fig. 4 shows the evolution of maximum equivalent stresses and piston mass for Aluminium A390-T5 piston and for Ductile Iron 65-45-12 piston under two types of running conditions.

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250 80 Maximum Equivalent Stress (Hot Piston) Maximum Equivalent Stress (Cold Piston) 70 Piston Mass /g 200 60

50 150

40 Mass /g Mass

Stress /MPa 100 30

20 50

10

0 0 Ductile Iron 65-45-12 Aluminium A390-T5 Fig. 4. Comparative analysis of maximum equivalent stress (von Mises) and piston mass for Ductile Iron 65-45- 12 and Aluminium A390-T5 pistons under two extreme loading conditions.

In both situations the maximum stress is located on the piston boss surface (inner side). Fig. 5a and Fig. 5b show the stress distributions in ductile iron 65-45-12 piston for these two different conditions. The highest value of equivalent stress is verified for the piston made of ductile iron 65-45-12 and under application of mechanical loads only or cold piston condition. In both materials we verified a reduction of the maximum stress when we also apply thermal loads (hot piston condition). These results show the importance of the heating the engine before working at high loads to reduce the stress levels in the piston and improve the tribological behaviour of the piston bearings because the maximum stresses are located in a sliding region.

Fig. 5a. Ductile Iron 65-45-12 Piston (Hot Piston). Fig. 5b. Ductile Iron 65-45-12 Piston (Cold Piston)

10 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

4.2 OPTIMIZATION OF PARAMETERS To study the influence of some parameters on piston stress levels, several FEA were made, changing only one parameter each time. Through these results it was possible to choose the best value for each parameter taking into account the stresses levels on the piston and the mass of the assembly (piston + pin). The aim is to obtain an assembly as light as possible and with some safety margin. As the methodology followed was the same for the pistons made with two different materials, only the results of Aluminium A390-T5 piston were reported.

4.2.1 PIN DIAMETER The first parameter analysed was the pin diameter because the region where we have the highest stress is the region of contact pin-piston. We wanted to know the influence of this parameter on the maximum equivalent stress in the piston. These results are shown on Fig. 6.

300 50 Maximum Equivalent Stress (Piston) Pin Mass 250 40

200 30

150 Mass /gMass 20 Stress /MPa 100

10 50

0 0 7 8 9 10 11 12 13 Pin Diameter /mm Fig. 6. Evolution of the maximum equivalent stress (von Mises), located on the piston boss surface, and pin mass with the pin diameter (Aluminium A390-T5 piston).

The reduction of the pin diameter from 12 mm to 8 mm increases the maximum equivalent stress two times. The fatigue strength of Aluminium A390-T5 at boss temperature of 150 ºC is about 154 MPa. Only diameter of 12 mm for the pin allows us to have some safety margin.

4.2.2 PIN WALL THICKNESS Established the pin diameter of 12 mm, the next step is to evaluate the influence of the pin wall thickness on the maximum equivalent stress verified on the piston. These results are shown on Fig. 7.

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180 50 Maximum Equivalent Stress 45 160 Pin Mass 40 140

35 120 30 100 25 80 Mass /gMass 20 Stress /MPa 60 15

40 10

20 5

0 0 1,5 2 2,5 3 3,5 4 4,5 Pin Wall Thickness /mm Fig 7. Evolution of maximum equivalent stress verified on the piston and pin mass with pin wall thickness.

We verified a reduction of about 40 % on equivalent stress when we increase the pin wall thickness from 2 to 3 mm. This modification increased the pin mass about 30 %. The increase of the pin wall thickness from 3 to 4 mm leads to an increase of pin mass of about 16 % and only 11 % of reduction of the maximum equivalent stress. With these results, we concluded that the pin wall thickness of 3 mm is the value for this parameter that corresponds to a minimum pin mass with some safety margin. All the following FEA are made with these two parameters values: pin diameter (12.0 mm); pin wall thickness (3.0 mm).

4.2.3 PISTON CROWN THICKNESS One of the most important parameters of piston design is the piston crown thickness. This important parameter influences a great number of variables: maximum equivalent stress, directional deformations, stress levels where the boss blends into the piston under crown, maximum temperatures and other variables. Fig. 8 shows the evolution of maximum equivalent stress, maximum temperature and equivalent stress in the region where the boss blends into the piston under crown with the piston crown thickness, on three critical points. The increase of the crown thickness leads to a little increase of maximum equivalent stress on boss surface region due to an increase of the stiffness of the piston. The stress levels for the maximum temperature point and for the point where the boss blends into the piston under crown decrease substantially with the increase of the crown thickness. Because the high uncertainty of aluminium properties at high temperature, it is convenient to have the minimum stresses levels on points where the temperature is high. So despite increase of the piston mass with the increase of piston crown, we choosed the value of 3 mm for the piston crown thickness to reduce the risk of failure at points at high temperatures, where the properties are more uncertain.

12 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

140 30

120 25

100 20 80 15 60 Mass /g

Stress /MPa Stress 10 40

20 5

0 0 1,5 2 3 Crown Thickness /mm Maximum Equivalent Stress Point (Piston Boss Surface) Maximum Temperature Point (Centre of piston crown) Point where the boss blends into piston under crown Piston Mass

Fig. 8. Evolution of maximum equivalent stress with the piston crown thickness in three critical points.

4.2.4 EVOLUTION OF MAXIMUM EQUIVALENT STRESS WITH THE FILLET RADIUS OF THE PISTON BOSS

122

121

120

119

118 Stress /MPa

117

116

115 1.522.533.544.5 Fillets Radius of Piston Boss /mm

Fig. 9. Evolution of maximum equivalent stress with the fillet radius of the piston boss.

As it is shown in Fig. 9, the increase of the fillets radius of the pin boss leads to a decrease of the maximum equivalent stress of the piston. Consequently to reduce the maximum equivalent stress it is convenient to use fillet radius of the pin boss as large as possible.

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4.3 CONVERGENCE ANALYSIS After performing the analysis to find the optimal values for the parameters that have more influence on the behaviour of piston, we made some mesh refinement to guarantee the convergence and thus the accuracy of the results. The results for the Aluminium A390-T5 piston are shown on Fig. 10.

160 35

Equivalent Stress (von Mises) /MPa Variation 140 30

25 120

20 100 15 80 10

Stress /MPa 60 5

40 0

20 -5

0 -10 25000 30000 35000 40000 45000 50000 55000 60000 65000 70000 75000 Number of Nodes

Fig. 10. Convergence history - Evolution of the equivalent stress with the number of nodes for Aluminium A390-T5 piston.

Fig 10 shows the influence of increasing the number of nodes (mesh refinement) on the maximum equivalent stress of the optimized piston. The results tend to converge to a constant value of 120 MPa, which prove their convergence and indicates their accuracy.

4.4 MAXIMUM DIRECTIONAL DEFORMATIONS After the optimization of both pistons (Aluminium A390-T5 piston and Ductile Iron 65-45-12 piston) we are interested to show the differences in terms of directional deformations of both materials. Fig. 11 shows the evolution of the directional deformation on YY axis and XX axis directions with the distance from top piston. The results show that using Ductile Iron 65-45-12 as piston material we reduce the directional deformations (YY axis) about 10 E-6 m. The directional deformations on both directions are similar which allows using turning as the machining manufacturing process for the external shape of the piston.

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70.0E-06 Aluminium A390-T5 (YY Axis) 60.0E-06 Ductile Iron 65-45-12 (YY Axis) Aluminium A390-T5 (XX Axis) 50.0E-06 Ductile Iron 65-45-12 (XX Axis)

40.0E-06

30.0E-06

20.0E-06 Directional Deformation /m Deformation Directional

10.0E-06

00.0E+00 0.00 5.00 10.00 15.00 20.00 25.00 Distance from Top Piston /mm

Fig. 11. Evolution of the directional deformations with the distance from the top piston of Aluminium A390-T5 piston and Ductile Iron 65-45-12 piston.

4.5 COMPARATIVE ANALYSIS OF EQUIVALENT STRESS, TEMPERATURE AND FATIGUE STRENGTH OF ALUMINIUM A390-T5 PISTON AND DUCTILE IRON 65-45-12 PISTON IN THREE DIFFERENT POINTS.

250 350

300 200 250

150 200

150 100 Stress /MPaStress

100 /ºC Temperature 50 50

0 0 Maximum Equivalent Stress Point Maximum Temperature Point Point where the boss blends into piston under crown Aluminium A390-T5 Equivalent Stress /MPa Ductile Iron 65-45-12 Equivalent Stress /MPa Fatigue Strenght (Aluminium A390-T5) /MPa Fatigue Strenght (Ductile Iron 65-45-12) /MPa Ductile Iron 65-45-12 Temperature Aluminium A390-T5 Temperature Fig. 12. Equivalent stress, temperatures and fatigue strength limits of Aluminium A390-T5 piston and ductile iron 65-45-12 piston.

Fig. 12 compares the maximum equivalent stress, temperature and fatigue strength limits on three critical points (Point of maximum equivalent stress (moderated temperature), point of maximum temperature (moderated stress levels) and point where the equivalent stress and temperature are both high. The results show that for the same point the equivalent stress is always higher on Ductile Iron 65-45-12 piston than on Aluminium A390-T5 piston. For both materials, at elevated

Chapter III: Product Engineering & Development in Design 15 Porto-Portugal, 24-26 July 2006 temperature points, we looked for to maintain the lowest possible stress levels because of the uncertainty of material properties at high temperatures and in this way to reduce the risk of rupture.

4.6 FINAL RESULTS Table 7 shows the FEA results for each piston made of different materials and the differences that occur between them. Table 7. Results of FEA of the Aluminium A390-T5 piston and Ductile Iron 65-45-12 piston and the relative difference between them in %. Relative Aluminium Ductile Iron difference for A390-T5 65-45-12 the Aluminium A390-T5 /%

Piston Mass /kg 28.4E-3 71.7E-3 152.46

Pin Mass / kg 18.4E-3 13.2E-3 -28.26

Assembly Mass (Piston+Pin) /kg 46.8E-3 84.9E-3 81.41

Inertial Forces Piston+Pin @7000 rpm /N 5785 10494 81.41

Number of Nodes 65270 59508 -8.83

Number of Elements 40213 35585 -11.51

Max. Equivalent Stress (von Mises) /MPa 123 169.6 37.89

Temperature @Point of Max. Equivalent Stress /ºC 150 145 -3.33 Fatigue Strength @ Point Max. Equivalent Stress /MPa 154 222 44.16

Safety Factor @Point of Max. Equivalent Stress 1.25 1.31 4.55 Equivalent Stress (von Mises) @Point of Max. Temperature /MPa 20 38 90.00

Max Temperature /ºC 198 320 61.62

Fatigue Strength @ Point Max. Temperature /MPa 125 197 57.60

Safety Factor @Point of Max. Temperature 6.25 5.18 -17.05 Equivalent Stress (von Mises) where the boss blends into piston under crown /MPa 29 86 196.55 Temperature where the boss blends into piston under crown /ºC 187 267 42.78 Fatigue Strength @ Point where the boss blends into piston under crown /MPa 133 210 57.89 Safety Factor where the boss blends into piston under crown 4.59 2.40 -47.67

Maximum Directional Deformation (XX Axis) /m 62.1E-6 53.4E-6 -13.95

Maximum Directional Deformation (YY Axis) /m 65.4E-6 54.8E-6 -16.21

16 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

5. CONCLUSIONS The ductile iron used for the manufacture of the piston has the advantage of low thermal expansion and high strength at high temperature but it has the disadvantage of high density. On the other hand, the aluminium alloy has a high strength to weight ratio, but relatively low mechanical properties at high temperature and under cyclic loads. The FEA show that the gudgeon pin bends due the mechanical load of the piston exerted by cylinder gas pressure. As a result, the contact stresses peak in the pin hole area moves towards the inner side of the pin hole. Thermal deformation of the piston, on other hand, has an opposite effect and leads to a reduction of the stresses on the inner side of the pin hole. Reducing the pin diameter or reducing the pin wall thickness, the FEA indicate the stress levels in the pin hole increase. Another important parameter is the piston crown thickness in the boss blend area: the stress levels decreasing in this area with the increasing of the crown thickness. The results show that despite of the aluminium alloy piston having larger directional deformations than ductile iron piston (about 14-16 %), the ductile iron piston has about 152 % more mass than aluminium piston, increasing the inertial forces of the engine about 80 %. Also the highest maximum temperature of the ductile iron piston reduces the quantity of air that enters to the combustion chamber reducing the volumetric efficiency. So with theses results the best choice for the material for our application is the Aluminium A390-T5 alloy.

ACKNOWLEDGEMENTS The authors would like to express their thanks to Fundação para a Ciência e a Tecnologia for financing this work. This work was done under Project POCI/TRA/61209/2004.

REFERENCES [1] Carley, Larry, Piston Design, An Evolution Tale, Babcox. [2] Wentworth, J.T. “The Piston Crevice Volume Effect on Exhaust Hydrocarbon Emission”, Combustion Science Technology, Vol. 4, p 97-100, 1971. [3] David C. Guilherme, João F.C. Ramos, “Projecto de um motor de 4 tempos de ignição por faísca de baixo consumo especifico”, Departamento de Engenharia Mecânica, Faculdade de Ciencias e Tecnologia da Universidade de Coimbra, Coimbra, 2002. [4] Frank P. Incropera, David P. DeWitt, “Introduction to Heat Transfer”, Wiley, 3rd Ed, 1996. [5] Properties of Aluminium and Aluminium Alloys, Knowledge Article from www.key-to- metals.com, INI International & STEP-COMMERCE AG. [6] Aluminium-Silicon Alloys, Knowledge Article from www.key-to-metals.com, INI International & STEP-COMMERCE AG. [7] Lawrence S. Gould, “Reciprocating Engines Never Had it so Good”, Automotive Design™ and Production”, in http://www.autofieldguide.com. [8] ASM Handbook, 10th Edition 1990, Volume 2 p. 171-177. [9] ASM Handbook, 10th Edition 1990, Volume 1 p. 42-54.

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FEA RESULTS (FIGURE CAPTIONS)

Fig. FC1. Mesh used for the final FEA.

Fig. FC2. Pin deformation of Aluminium A390-T5 piston.

18 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

Fig. FC3. Stress distribution for Aluminium A390-T5 piston (hot piston condition).

Fig. FC4. Temperature distribution for Ductile Iron 65-45-12 (hot piston condition).

Chapter III: Product Engineering & Development in Design 19 Porto-Portugal, 24-26 July 2006

Fig. FC5. Temperature distribution for Aluminium A390-T5 piston (hot piston condition)

Fig. FC6. Directional Deformation for Ductile Iron 65-45-12 piston (Y Axis).

20 Editors: J.F. Silva Gomes and Shaker A. Meguid 5th International Conference on Mechanics and Materials in Design

Fig. FC7. Safety factors distributions for Ductile Iron 65-45-12 piston (Most critical condition: if the piston is all at maximum uniform temperature of 320 ºC).

Fig. FC8. Safety factors distributions for Aluminium A390-T5 piston (Most critical condition: if piston is all at maximum uniform temperature of 198 ºC).

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