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DesignAnalysisofaCircularandSquareShapedPistonHeadConsideringMechanicalStressesInduced

ARTICLE·OCTOBER2013

DOI:10.13140/2.1.1000.9287

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Design Analysis of a Circular and Square Shaped Piston Head Considering

Mechanical Stresses Induced

Sean D’Silva

Department of mechanical

engineering, Rajiv Gandhi

Institute of Technology,

Mumbai,India.

Sumit Jain

Department of mechanical

engineering, Rajiv Gandhi

Institute of Technology,

Mumbai,India.

Mayur Ingale

Department of mechanical

engineering, Rajiv Gandhi

Institute of Technology,

Mumbai,India.

Abstract

The piston is a very important part in various

machines like IC engines, hydraulic pumps, air

compressors, etc. So the piston design is very

important in these machines. Design of a piston has a

very important role in the performance of IC engines.

In this paper we are considering pistons used in IC

engines. Normally pistons with a circular cross

section are used, but here a circular piston is

compared with a square shaped piston (piston with

square-shaped crown). In this comparison we are

focusing mainly on mechanical parameters like stress

induced, strain induced and deformation of the

piston. The paper explains why square pistons cannot

be used once the two are compared. The pistons have

been designed using Autodesk INVENTOR and the

analysis has been done using ANSYS static structural

neglecting the frictional losses.

1. Introduction

In recent years, digital simulation technology has

been developing rapidly. Virtual piston is established

by Autodesk INVENTOR Professional easily with

the necessary material properties. As is well-known

that virtual piston can simulate the product all kinds

of character in the real environment. Piston is one of

the key components in a motor and it closely relates

to the machine performance, carbon emissions and

the economy. With the engine, the higher speed and

strength developing, its higher pressure ratio and

higher power improve constantly. Pistons work

condition is more and more bad, so its reliability has

become the key factors to improve engine reliability.

Structure and working environment of pistons are

very complex. In the working environment, the

pistons will produce stress and deformation because

of the periodic load effect, which are from high gas

pressure, high speed reciprocating motion from the

inertia force, lateral pressure, friction and so on.

Burning of the high pressure gas products high

temperature, which makes piston expands in order

that its interior produces thermal stress and thermal

deformation. The thermal deformation and

mechanical deformation will cause piston cracks,

tortuosity, etc. Therefore, it is essential to reduce the

stress field, temperature field, heat transfer,thermal

load and mechanical load coupling of piston in order

to lower the heat load and improve the thermal stress

distribution and improve its working reliability

during the piston designed. Analysis method of the

finite element provides a powerful calculation tool,

which is better than test method and theory analysis

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method and has become an important means for

internal combustion engine performance study.

The efficiency and economy of the engine

primarily depends on the working of piston. It must

operate in the cylinder with minimum friction and

should be able to with stand high explosive force

developed in the cylinder and also the very high

temperature ranging from 750K to 3100K(500ºC to

2,800ºC) during operation. The piston should be as

strong as possible. However its weight should be

minimized as far as possible in order to reduce the

inertia due to its reciprocating mass. Among engine

components exposed to thermal effects, the piston is

considered to be one of the most severely stressed,

where a high amount of the heat transferred to a

coolant fluid goes through it, this amount depends on

the thermal conductivity of the materials employed,

average speed and geometry of the piston. Conventional pistons used everywhere are circular in

shape. These pistons are easier to manufacture and

have good stress distribution but if we use square

piston instead, we will get good result as compared to

circular because stress induced will be less. This is

shown in this paper. To find the various dimensions

of the piston some empirical relations are used.

2. Material Properties of Piston:-

Material of Piston: - Aluminum 6061

Young’s Modulus [E] – 69 Gpa

Poisson’s ratio [μ]– 0.33

Ultimate Tensile strength –310 Mpa.

Tensile Yield strength – 276 Mpa.

Shear strength –207 Mpa.

Elongation – 12 %.

3. Geometry:

The below image shows the geometry of piston

imported into the simulation software for Analysis.

Before going to import a geometrical model of piston

which can be prepared by modeling software’s like

Autodesk Inventor, the geometrical modeling can

also done in the analysis software’s like ANSYS.

Figure 1 and 2 shows the piston created by CAD

software for further analysis.

3.1 Calculations:

Diameter of the piston = 150 mm

∴ Area = 𝜋𝑟2= 0.0176 𝑚2

But,

Area of the circular piston surface= Area of the

square shaped piston surface

∴ 0.0176 = (𝑠𝑖𝑑𝑒)2

∴ side = 132.66 mm

Now consider,

Torque produced by the crankshaft= Force*radius of

the crank

Torque = 220 N-m (for 4 cylinders)

Stroke = 96 mm (radius is 48 mm)

∴ 220/4 = 55 N-m per cylinder

1000mm/48mm =20.833 (conversion factor)

20.833×55 =1145.83 N per cylinder

Pressure= 𝐹𝑜𝑟𝑐𝑒

𝐴𝑟𝑒𝑎

∴ Pressure on the cylinder crown = 1145.83

0.0176

Pressure = 64973.74 N/𝑚2

This pressure value is used for analysis

∴∴∴∴

Figure 1: Circular piston

Figure 2 : Square shaped piston

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4. Finite Element Model:

The element selected for meshing the piston model

is solid-187 tetrahedron type of element which is

higher order tetrahedral element. The mesh count for

the circular piston model contains 233174 number of

nodes and 146755 number of elements. While the

square shaped piston contains 119347 nodes and

72975 elements .Figures 3 and 4 show the meshed

model of the pistons.

Figure 3: Mesh for circular piston

Figure 4: Mesh for square shaped piston head

5. Piston Structure Design

5.1 Design of the Engine Piston

According to the design parameters, we use

Autodesk Inventor software to establish the piston

model. In this paper, we consider the symmetry of the

piston geometry structure and adopt 1:1 model to

analyze the piston.

5.2 Model Parameter Settings

Select the "Mechanical" in the "application" of

menu and set the parameters. First, set model

material for aluminum alloy AL6061. Then, set the

tensile strength definition for 240 Mpa, tensile

stress limit definition for 290 Mpa, surface finish for

the model is glazing.

5.3 Defining Constraints and defining Load

Piston pin hole is constrained by displacement and

symmetry in order to make the pin hole produce the

correct constraint condition. By analysis of the piston

working process, we find that stress and deformation

of the piston is the most serious under the steady

speed conditions when the gas-fired pressure is the

maximum. At the same time, the strength of piston is

especially outstanding. Therefore, it is essential to

choose the piston under the rated power and we only

analyze distribution force in the axis of the force,

including the maximum explosion pressure and

reciprocating inertia force. Pressure load of piston is

that gas pressure effects piston top surface by high

pressure in the cylinder. For simplified analysis, we

can use the steady state process, but cannot ignore the

effect that combustion power stroke products

impact load for piston. Using cylinder fluid dynamics

simulation results, we can calculate that average

pressure of piston top is 64973 Pa in a working cycle,

which will be surface pressure load.

6. Mechanical Analysis of the Piston

After the simulation model is established, we can

get strain distribution and fatigue analysis under

the effect of mechanical load by operating the

mechanical program. The simulation results of the

piston are shown in the figures below.

Piston is affected by gas explosion pressure and the

reciprocating inertia force and their common feature

is that they affect along the axis direction of the

piston, so the axis direction of piston bears the bigger

load.The results of these simulations analysis

provides a strong theory basis for failure problems of

the piston.

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6.1 Loading & Boundary Conditions:

Figure 5 show the loading and boundary conditions

considered for the analysis. The uniform pressure of

64973 Pa is applied on crown of piston which is

indicated by red color and the model is constrained

on the piston pin hole.

Figure 5: Load on the circular piston

Figure 6: Load on the square piston

6.2 Total deformation

Maximum deformation can be seen at the centre of

the piston as expected. The maximum deformation in

case of a circular piston head is 0.00269 mm while

for the square shaped piston head it is 0.00245 mm.

Figure 7: Total deformation on the circular piston

Figure 8: Total deformation on the square shaped piston

6.3 Equivalent (Von-Mises) stress

In materials science and engineering the Von-Mises

yield criterion can be also formulated in terms of

the Von-Mises stress or equivalent tensile stress, ,

a scalar stress value that can be computed from

the Cauchy stress tensor. In this case, a material is

said to start yielding when its Von-Mises stress

reaches a critical value known as the yield

strength, . The Von-Mises stress is used to predict

yielding of materials under any loading condition

from results of simple uniaxial tensile tests. The Von-

Mises stress satisfies the property that two stress

states with equal distortion energy have equal Von-

Mises stress. In the case of the circular piston head

the equivalent stress is 3.458× 106 Pa ,whereas for

the square shaped piston head the value is 2.899×

106 Pa.

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Figure 9: Equivalent stress for the circular piston

Figure 10: Equivalent stress on the square piston

6.4 Maximum principle stress

According to the maximum principle stress theory,

the material will fail when one of the principal

stresses exceeds the yield strength in tension. In this

case the maximum principle stress for a circular

piston head is 3.0505× 106 Pa and for the square

shaped piston head the value is 2.029× 106 Pa.

Figure 11: Maximum principle stress for the circular piston

Figure 12: Maximum principle stress for the square piston

7. Disadvantages of using a square

shaped piston head

1) Circular shaped pistons are much easier to bore

and machine compared to a square shaped piston

head.

2) For a given cross-sectional area, a cylinder has the

least amount of circumference when compared to any

other shape. In pistons, leakage at the circumference

is the biggest enemy, so to minimize this, a circular

cross section should be used.

3) It is much easier to seal a circular shape with O-

rings.

4) In the case of a combustion engine, the gases just

prior to combustion-explosion will be much more

evenly mixed in a circular cylinder than a square one,

hence resulting in a more efficient explosion and

better thrust. A square cylinder would have more

turbulence, areas of poor mixing in the corners, etc.

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5) Heat flow out of the piston head itself can be

considered. The heat from combustion has to go

somewhere, most of it into the cylinder head and

piston crown. The piston crown then transfers heat to

the rings which then imparts it to the cylinder walls.

The circular cross-section has the greatest amount of

edge area as compared to surface area. Thus the most

effective on heat transfer is obtained in case of a

circular piston head

6) Friction is a huge source of power loss. The

square having once again the larger edge area will

increase the amount of friction as well. All of these

add to power losses and heat added into the system.

As we know most gas engines are only 10% efficient

to start with and diesels at 20 to 30%. The rest is lost

in heat through the cooling system and heat out the

exhaust system.

7) Thermal expansion is another great reason. A

circle does what you would expect, expands under

heat in all directions at an even rate. A square does

not. The corners will become super heated and

expand at a greater rate than the sides. This will cause

more of a 4 pointed star shape to occur. This is

neither good for sealing of the combustion chamber

nor for the transfer of heat. A fillet around the edges

would help only to some extent. This would in turn

make for poor sealing during cold starts. This is the

time when the fuel to air ratio is the highest.

Excessive amounts of "blow by" would enter the

crankcase and contaminate the oil.

8. Results and conclusion:

It is observed that although fatigue is not

responsible for the biggest slice of damaged pistons,

but the stresses induced are the major factor for

piston failure. From the analysis it can be seen that

for some instances a square piston is better. This is

seen in case of the total deformation, equivalent

stress and maximum principle stress. All these results

have been formulated without considering friction

and other losses. Although it might seem better to use

a square shaped piston head instead of a circular

shaped one after looking at these results, but it will

require extra cooling arrangements and more

maintenance. Square shaped pistons may have

applications in some machines, but cannot be used in

the modern day practical automobiles, as the losses

incurred cannot be afforded. This paper thus explains

the shortcomings of square shaped piston heads

versus the circular shaped ones and it can be inferred

that the circular type of piston heads should be

preferred in most of the cases.

9. Acknowledgment

The authors would like to thank Prof. Rajkumar V.

Patil (Associate Professor of Mechanical

Engineering- Rajiv Gandhi Institute of Technology)

for his constant support and guidance during the

course of the project.

10. References

[1] Shuoguo Zhao,” Design the Piston of Internal

Combustion Engine by Pro\ENGEER”, Journal , 2nd

International Conference on Electronic & Mechanical

Engineering and Information Technology (EMEIT-2012).

[2] Ghodake A. P.*, Patil K.N.,”

Piston Design and Analysis by CAE Tools”, Journal,

IOSR Journal of Engineering (IOSRJEN) ISSN: 2250-3021

ISBN: 2878-8719 PP 33-36 National Symposium on

engineering and Research.

[3] B.R. Ramesh and Kishan Naik,” Thermal Stress

Analysis of Diesel Engine Piston”, Journal, International

Conference on Challenges and Opportunities in Mechanical

Engineering, Industrial Engineering and Management

Studies (ICCOMIM - 2012), 11-13 July, 2012

[4] Tetsuhiro Hosokawa, Hiroshi Tsukada, Yorishige

Maeda,” Development of computer aided engineering for

piston design”, SAE Paper. 890775:916 ~ 922.

[5] Robinson D, Palaninathan R, “Thermal analysis of

piston casting using 3- D finite element method”, Journal,

Finite Elements in Analysis and Design 2001, 37: 85~ 95

[6] C.H. Li., “Piston thermal deformation and friction

considerations”, SAE Paper 820086, 1982.

[7] Handbook of Internal Combustion Engines, Book Title

SAE International.

[8] F.S. Silva ,“Fatigue on engine pistons–A compendium

of case studies” ,Book Title, 31 March 2005

[9] Antoine Rios, Bruce Davis and Paul Gramann

,“Computer Aided Engineering in Compression Molding”,

Book Title, The Madison Group: Polymer Processing

Research Corporation 505 S. Rosa Rd. Madison, WI

53719)

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International Journal of Mechanical Engineering Research & Applications (IJMERA)

Vol. 1 Issue 5, October - 2013ISSN: 2347-1719

IJMERA

IJMERA

www.ijmera.orgIJMERAV1IS050003