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Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/268509070
DesignAnalysisofaCircularandSquareShapedPistonHeadConsideringMechanicalStressesInduced
ARTICLE·OCTOBER2013
DOI:10.13140/2.1.1000.9287
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SeanD'silva
Friedrich-Alexander-UniversityofErlangen-N…
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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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International Journal of Mechanical Engineering Research & Applications (IJMERA)
Vol. 1 Issue 5, October - 2013ISSN: 2347-1719
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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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International Journal of Mechanical Engineering Research & Applications (IJMERA)
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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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International Journal of Mechanical Engineering Research & Applications (IJMERA)
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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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International Journal of Mechanical Engineering Research & Applications (IJMERA)
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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