2. 1. ABSTRACT
For a vehicle engine, the piston is the essential part that bears heavy mechanical and
thermal loads; therefore it has very many influences on the reliability and durability of an
engine. So the piston greatly hinders the increase of power of an engine. Piston dynamics
and friction are two important characteristics determining the performance and the
efficiency of an internal combustion engine. In this project an approach to the behavioral
analysis of the piston skirt is presented that is based mainly on the Finite Element Method
representation. The Finite Element Method was used to predict the deformation of the
piston skirt and also the stress distribution across the piston.
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3. 2. INTRODUCTION
A piston is a cylindrical engine component that slides back and forth in the cylinder bore
by forces produced during the combustion process. The piston acts as a movable end of
the combustion chamber. The stationary end of the combustion chamber is the cylinder
head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight
thermal conductivity. Thermal conductivity is the ability of a material to conduct and
transfer heat. Aluminum expands when heated and proper clearance must be provided to
maintain free piston movement in the cylinder bore. Insufficient clearance can cause the
piston to seize in the cylinder. Excessive clearance can cause a loss of compression and
an increase in piston noise. A complete illustration of the piston and its parts are shown
below.
Figure 1 – Piston Features.
Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves,
ring lands, and piston rings. The piston head is the top surface (closest to the cylinder
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4. head) of the piston which is subjected to tremendous forces and heat during normal
engine operation.
A piston pin bore is a through hole in the side of the piston perpendicular to piston travel
that receives the piston pin. A piston pin is a hollow shaft that connects the small end of
the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to
the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts
have profiles cut into them to reduce piston mass and to provide clearance for the rotating
crankshaft counterweights.
A ring groove is a recessed area located around the perimeter of the piston that is used to
retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which
function as the sealing surface for the piston ring. A piston ring is an expandable split
ring used to provide a seal between the piston and the cylinder wall. Piston rings are
commonly made from cast iron. Cast iron retains the integrity of its original shape under
heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct
heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size
and configuration vary depending on engine design and cylinder material.
Piston rings commonly used on small engines include the compression ring, wiper ring,
and oil ring. A compression ring is the piston ring located in the ring groove closest to the
piston head. The compression ring seals the combustion chamber from any leakage
during the combustion process. When the air-fuel mixture is ignited, pressure from
combustion gases is applied to the piston head, forcing the piston toward the crankshaft.
The pressurized gases travel through the gap between the cylinder wall and the piston and
into the piston ring groove. Combustion gas pressure forces the piston ring against the
cylinder wall to form a seal. Pressure applied to the piston ring is approximately
proportional to the combustion gas pressure.
A wiper ring is the piston ring with a tapered face located in the ring groove between the
compression ring and the oil ring. The wiper ring is used to further seal the combustion
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5. chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by
the compression ring are stopped by the wiper ring.
An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil
ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil
is returned through ring openings to the oil reservoir in the engine block. Two-stroke
cycle engines do not require oil rings because lubrication is supplied by mixing oil in the
gasoline, and an oil reservoir is not required.
Pistons are usually made of an aluminum alloy. They are a sliding fit in the cylinders.
This serves several purposes as follows:
Transmits the force of combustion to the crankshaft through the connecting rod.
Acts as a guide for the upper end of the connecting rod.
Serves as a carrier for the piston rings that are used to seal the compression in the
cylinder.
The piston must withstand incredible punishment under temperature extremes. The
following are examples of conditions that a piston must withstand at normal highway
speed:
As the piston moves from the top of the cylinder to the bottom (or vice versa), it
accelerates from a stop to a speed approximately 50 mph at midpoint, and then
decelerates to a stop again. It does this approximately 80 times per second.
The piston is subjected to pressures on its head in excess of 1,000 psi.
The piston head is subjected to temperatures well above 600°F.
Piston is one of the main components composing an engine and a lot of empirical
knowledge and advanced technologies are fully used for the piston design. It has to be
able to handle the abrupt changes in temperature and pressure just above its crown due to
combustion, hence during a cycle it has to handle efficiently the thermal and pressure
loads as well as maintaining the geometry with minimum deformation. The design of a
piston is crucial in determining the efficiency of an engine. Piston quality depends on
optimal geometrical design, body mass and material selection that ensures minimal
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6. energy loss due to inertia, heat transfer and bore interaction. Structure strength must be
retained against mechanical failure. However, finding the optimal set of the design
parameters is a very challenging job as at every crank angle the piston experiences
different loading. Consequently, the computational tools become very important in piston
design.
Over the years several piston models have been developed. Early piston models were
primarily developed to investigate the piston dynamics and its impact on the cylinder
bore that result in engine vibration and noise, in particular the piston slap phenomenon.
The effects from the lubrication and the piston skirt elasticity in the piston dynamics were
either not considered or assumed insignificant. Nevertheless, valuable information
insights and principles obtained from these studies regarding the piston dynamics have
contributed to the later efforts that attempt to couple lubrication and piston dynamics.
There are essentially two types of piston used in today’s automotive engine and we will
be analyzing about the Mono piston type.
2.1 Mono Piston
By far the most common, and the one used in all passenger car engines, is the mono
piston. As the name suggests, the piston comprises of a single component, which is
usually made from aluminum. The upper part of the piston that supports the combustion
force and holds the piston rings is called the crown. The lower part of the piston that
supports the lateral forces against the inner walls is called the skirt. The piston is linked
to the connecting rod via the wrist pin on which both components are hinged. An
example of a mono piston assembly can be seen in Figure 2 below.
Figure 2 - Schematic drawing of a mono piston assembly.
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7. 2.2 Articulated Piston
In some heavy duty diesel engines, due to extremely high combustion temperatures and
pressures, it is desirable to have a stainless steel crown section. However, an aluminum
skirt is still preferable due to its low weight and elasticity. Two components of different
materials cannot be rigid joined together in the piston, as their differing coefficients of
expansion would lead to failure. Instead, articulated pistons comprise a stainless steel
crown and an aluminum skirt which are separate components and are hinged separately
on the wrist pin. An example of an articulated piston assembly can be seen in Figure 3
below.
Figure 3 - Schematic drawing of an articulated piston assembly.
The piston motion equations are traditional Newton’s law of vertical and lateral motion
and rotation applied to the piston in standard manner. The forces and moments acting on
a piston are exemplified in Figure 4. These forces and moments come from interactions
of the piston with the liner, rings, wrist pin, connecting rod, cylinder pressure and inertia.
Figure 4 – Forces & Moments acting on the piston.
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8. 3. Finite Element Analysis
Finite Element Analysis (FEA) is a computer-based numerical technique for calculating
the strength and behavior of engineering structures. It can be used to calculate deflection,
stress, vibration, buckling behavior and many other phenomena. It can be used to analyze
either small or large-scale deflection under loading or applied displacement. It can
analyze elastic deformation, or “permanently bent out of shape” plastic deformation. The
computer is required because of the astronomical number of calculations needed to
analyze a large structure. The power and low cost of modern computers has made Finite
Element Analysis available to many disciplines and companies.
In the Finite Element Method, a structure is broken down into many small simple blocks
or elements. The behavior of an individual element can be described with a relatively
simple set of equations. Just as the set of elements would be joined together to build the
whole structure, the equations describing the behaviors of the individual elements are
joined into an extremely large set of equations that has the behavior of the whole
structure. The computer solves these large sets of simultaneous equations. From the
solution, the computer extracts the behavior of the individual elements. Fro this, it can get
the stress and deflection of all the parts of the structure.
The term “Finite Element” distinguishes the technique from the use of infinitesimal
“differential elements” used in calculus, differential equations and partial differential
equations. The method is also distinguished from finite difference equations, for which
although the steps into which space is divided are finite in size, there is little freedom in
shapes that the discreet steps can be taken. Finite element analysis is a way to deal with
structures that are complex than can be dealt with analytically using partial differential
equations. FEA deals with complex boundaries better than finite differential equations
will, and gives answers to “real world” structural problems.
3.1 How is Finite Element Analysis Useful
Finite Element Analysis makes it possible to evaluate a detailed and complex structure, in
a computer, during the planning of the structure. The demonstration in the computer of
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9. the adequate strength of the structure and the possibility of improving the design during
planning can justify the cost of this analysis work.
In the absence of Finite Element Analysis, development of structures must be based on
hand calculations only. For complex structures like an automobile, the simplifying
assumptions required to make any calculations possible can lead to a conservative and
heavy design. A considerable factor of ignorance can remain as to whether the structure
will be adequate for all design loads. Significant changes in designs involve risk. With
FEA, the weight of a design can be minimized, and there can be a reduction in the
number of prototypes built.
3.2 Types of Analysis on Structures
Structures can be analyzed for small deflection and elastic material properties (linear
analysis), small deflection and plastic material properties (material non-linearity), large
deflections and elastic material properties (geometric non-linearity) and for simultaneous
large deflection and plastic material properties.
Loads on structures can be represented by using the force of gravity on the mass of the
structure, by applying distributed pressure over surfaces of the structure, or by applying
forces directly to positions in the structure. Displacements of the structure can be
specified at positions in the structure. This can include boundary conditions that imply
symmetric structures where only one portion of the structure is modeled.
3.3 FEA Applications
In theory there is no limit to the number of applications that FEA can be used for. FEA
was born and nurtured in the automotive and aerospace industries but has since spread to
encompass all other sectors of industry, from medical instruments and F1 car design to
plastic molding and watch springs. If it can be designed, then it can be modeled using
FEA.
3.4 Finite Element Analysis Modeling Issues
FEA is approximate: The first issue to understand in finite element analysis is that it is
fundamentally an approximation. The underlying mathematical model may be an
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10. approximation of the real physical system (for example, the Bernoulli beam ignoring
shear deformation). The finite element itself approximates what happens in its interior
with interpolation formulas. The interior of a 2-D or 3-D finite element has been mapped
to the interior of an element with a perfect shape, so a severely distorted element cannot
deform in a manner that has an accurate match to the real physical response. Integration
over the body of the element is often approximated by Gaussian Quadrature. The
continuity of deformation between connected elements is interrupted at some level. Badly
shaped elements can give less accurate results. A linear analysis is an approximation of
the real behavior. The boundary conditions approximate how the structure is supported
by the outside world. The material properties assumed are approximate. Stress and strain
results are based on the derivatives of the displacement solution, amplifying the errors.
Meshing: Production of good quality mesh is a major issue. The mesh should be fine
enough for good detail where information is needed, but not too fine, or the analysis will
require considerable time and space in the computer. A mesh should have well-shaped
elements – only mild distortion and moderate aspect ratios. This can require considerable
user intervention, despite FEA software promotional claims of automatic good meshing.
Warning about Nodal Coupling: Nodal coupling has its uses: one is a quick and dirty
representation of a bolted or riveted connection with shell elements. More exotic
applications can be invented. When nodal coupling is used to represent a bolted
connection of 3D shells, the nodes that are coupled must occupy the same position in
space. Otherwise, body rotation at that part of the structure will result in artificial
mechanism acting on the structure. High local stresses, and an external couple would
result if the coupled nodes were not located at the same position.
Application of boundary conditions: Structural FEA displacement boundary conditions
are the limitations on movement of the structure at places such as anchor locations. The
boundary conditions in a finite element model must limit translation or rotation in a
manner appropriate to the case at hand. Boundary conditions can be used to imply
symmetric behavior in a structure that has symmetry, so that the model size can be
halved, quartered or similarly reduced, if the loading of the structure is also symmetrical.
Boundary conditions can also be used to imply anti-symmetry, for example, where a
warping displacement is applied to a symmetric structure.
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11. There are occasions when a displacement boundary condition needs to be applied to a
single node so that the structure can rotate around the support point. This single node
support, however, can result in a serious local stress spike.
Application of loading in a manner that’s of satisfactory accuracy, without becoming
overly complex: It is often sufficient to apply forces directly to a small set of nodes.
However, better representation of loading can be needed to avoid local stress spikes in
some analyses. Application of pressure over a region of elements, producing the desired
force, can help avoid a local stress spike.
Bucking analysis & failure: It can be pursued in two ways: Linear Eigen-value buckling
and geometrically non-linear (large displacement) buckling analysis. Eigen-value
buckling (also known as Euler buckling or classical buckling) will be sufficient for some
structures, but much greater details about stress amplification and margin of safety can be
found with geometrically non-linear analysis. Note that margin of safety is not a simple
concept in a non-linear analysis. The margin of safety will be based on the difference
between the intended design load and either the load that reaches failure conditions or the
load that exceeds allowable set by design codes.
3.5 Failure Modes
1. Static loads lead to stresses exceeding yield over a significant region.
2. Loads on bolts, rivets, spot-welds, plug-welds, stitch-welds, fillet-welds,
bevel-welds, full penetration-welds, adhesives, nails, tie-rods, links or other
connection devices are too high.
3. Strains reach fracture levels in brittle materials.
4. Surface strains cause damage to protective coatings.
5. Buckling of components leads to local damage or progressive collapse.
6. Combined bending and compression leads to excessive stress and failure.
7. Fatigue or sudden fracture is reached.
8. Vibration frequencies are located where applied loading causes damage
through large amplitude response.
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12. 3.6 Steps in Finite Element Analysis
There are a number of steps in the solution procedure using finite element methods.
1. Specifying Geometry: First the geometry of the structure to be analysed is
defined. This can be done either by entering the geometric information in the
finite element package through the keyboard or mouse, or by importing the model
from a solid modeller like Pro/ENGINEER.
2. Specify Element Type & Material Properties: Next, the material properties are
defined. In an elastic analysis of an isotropic solid these consist of the Young's
modulus and the Poisson's ratio of the material.
3. Mesh the Object: Then, the structure is broken (or meshed) into small elements.
This involves defining the types of elements into which the structure will be
broken, as well as specifying how the structure will be subdivided into elements
(how it will be meshed). This subdivision into elements can either be input by the
user or, with some finite element programs (or add-ons) can be chosen
automatically by the computer based on the geometry of the structure (this is
called auto meshing).
4. Apply Boundary Conditions & External Loads: Next, the boundary conditions
(e.g. location of supports) and the external loads are specified.
5. Generate Solution: Then the solution is generated based on the previously input
parameters.
6. Post – Processing: Based on the initial conditions and applied loads, data is
returned after a solution is processed. This data can be viewed in a variety of
graphs and displays.
7. Refine the Mesh: Finite element methods are approximate methods and, in
general, the accuracy of the approximation increases with the number of elements
used. The number of elements needed for an accurate model depends on the
problem and the specific results to be extracted from it. Thus, in order to judge the
accuracy of results from a single finite element run, you need to increase the
number of elements in the object and see if or how the results change.
8. Interpreting Result: This step is perhaps the most critical step in the entire
analysis because it requires that the modeller use his or her fundamental
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13. knowledge of mechanics to interpret and understand the output of the model. This
is critical for applying correct results to solve real engineering problems and in
identifying when modelling mistakes have been made (which can easily occur).
The eight steps mentioned above have to be carried out before any meaningful
information can be obtained regardless of the size and complexity of the problem to be
solved.
3.7 FEM Convergence Testing
The convergence testing is started with mesh - discretization, observe and record the
solution. The problem is repeated with a finer mesh (i.e. more elements) and then the
results are compared with the previous test. If the results are nearly similar, then the first
mesh is probably good enough for the particular geometry, loading and constraints. If the
results differ by a large amount however, it will be necessary to try a finer mesh yet.
Finer meshes come with a cost however: more calculation time and large memory
requirements (both disk and RAM)! It is desired to find the minimum number of elements
that gives a converged solution.
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14. 4. Modeling & Analysis
4.1 Modeling of Piston Assembly
Computer Aided Three Dimensional Application Software (CATIA) was found by
Dassault Systems of France. It was first used by BOEING Engineers and now it is used
widely in aerospace & automobile applications. It has become the global language for all
automobile manufacturers. It is widely used in surface modelling of automobile body. It
is highly user friendly. Its latest version is V5 and the release is R14 and the capabilities
of CATIA are immense.
Modeling in CATIA is comparatively easier than in any other modeling software. CATIA
has various supportive features such as easy profile drawing, extrude, shell, pocket,
revolve, rib. The modeling of the piston assembly is subdivided into following parts:
a) Sketching the profile of the piston and body.
b) Performing operation on the sketch to generate 3-D feature.
c) Additional features.
The piston modeled in Catia is sown below.
Figure 5 – Piston Model.
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15. 4.2 Steps Involved in FEA of the Piston Assembly
The analysis is done as per standard procedure as follows:
a) Modeling.
b) Material application (Aluminum) as per the specifications.
c) Boundary conditions and loads are loaded as per the real time working
environment.
d) Meshing is done using 10 noded tetrahedron element.
e) Solution phase.
4.3 Material property
The material properties for the piston and piston – pin are as mentioned below:
S. No Property Piston
1. Young’s modulus 68 x 109 Nmm-2
2. Poison ratio 0.35
3. Yield strength 400 x 106 Nmm-2
4. Density 2.7 g/cm3
5. Thermal expansion 24 X 10-6/°C
Table 1 – Material Property.
4.4 Boundary condition
In a numerical simulation, it is impossible and unnecessary to simulate the whole
universe. Generally we choose a region of interest in which we conduct a simulation. The
interesting region has a certain boundary with the surrounding environment. Numerical
simulations also have to consider the physical processes in the boundary region. Different
boundary conditions may cause quiet different simulation results. Improper sets of
boundary conditions may introduce non-physical influences on the simulation system,
while a proper set of boundary conditions can avoid that. The boundary conditions
applied here in the simulation are:
a) Fixed restraints.
b) Pressure force applied on the piston crown.
c) Connecting rod force applied on the piston-boss.
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16. The pressure force is applied on the top face of the piston crown due to gas force acting
during the combustion. The connecting rod force is calculated as follows:
Piston acceleration, a p = rω 2 (cos θ + λ cos 2θ )
2πn 2 × π × 410
Here θ = 360°, r = 0.05 m, ω = = = 42.94 rad
60 60 s
r 0.05
λ= = = 0.307
l 0.163
a p = 0.05 × (42.94) 2 × (cos 360 + (0.307) cos 720 ) = 120.5 m
s2
Connecting rod force, Fitr = −(m pa + mc −tr )a p
Here mpa = 2.043 kg, mc-tr = 0.735 kg
Fitr = −(2.043 + 0.735) × 120.5 = −335 N
where,
ap = Piston acceleration in m .
s2
θ = Crank angle in Degree.
r = Crank radius in m.
ω = Angular velocity in rad .
s
n = Engine speed in rpm.
l = Length of the connecting rod in m.
λ = Utilization factor, r .
l
Fitr = Connecting rod force in N.
mpa = Mass of piston assembly in kg.
mc-tr = Mass of connecting rod in translational side in kg.
In fixed restraints condition the piston boss outer face and the piston boss inner area are
constrained in 3 translational and 3 rotational directions (ux, uy, uz =0, θx, θy, θz = 0). And
the connecting rod force is applied on the piston boss area. The described boundary
condition is exemplified in the following figures.
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18. 4.5 Element selection
Once the component is modeled and boundary conditions are applied, the next in analysis
is to discretize the component by using pre-defined elements in the analysis package. The
piston assembly analysis is a three dimensional analysis. Hence a 3D element is used.
Various 3D elements could be used for discretization. The best element should be chosen
to get accurate results and convergence. Solid 95 element type is preferred over the Solid
45 element. Because solid 95 is a higher order version of Solid 45(8 noded element), it
can tolerate irregular shapes without as much loss of accuracy. Solid 95 have compatible
displacement shapes and are well suited to model curved boundaries.
4.6 Comparison of Solid 95 Elements
Criteria Tetrahedral Option Pyramid Option Prism Option
Diagram
No of Nodes 10 13 15
Solving Time Comparatively Less Large Large
Matrix Size RAM
Comparatively Less Large Large
required/
Table 2 –Comparison of Solid 95 Elements.
Based on the comparison details, the best element to achieve accurate meshing, solution
with least solving time and least memory required would be the 10 Noded Tetrahedron.
Also Solid 95 elements have special features to support Plasticity, Creep, Stress
stiffening, Swelling, Deflection, Large Strain, Birth and Death, Adaptive Descent.
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19. 5. Results & Discussion
I. FE Analysis on piston deformation at pressure = 3.8 Mpa & speed = 410 rpm.
Figure 8 –Displacement Contour.
Figure 9 – Displacement Contour.
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20. Figure 10 – Von Mises Stress Contour.
Figure 11 – Von Mises Stress Contour.
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21. II. FE Analysis on piston deformation at pressure = 5.5 Mpa & speed = 1060 rpm.
Figure 12 - Displacement Contour.
Figure 13 - Displacement Contour.
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22. Figure 14 – Von Mises Stress Contour.
Figure 15 - Von Mises Stress Contour.
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23. III. FE Analysis on piston deformation at pressure = 6 Mpa & speed = 1580 rpm.
Figure 16 - Displacement Contour.
Figure 17 - Displacement Contour.
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24. Figure 18 – Von Mises Stress Contour.
Figure 19 – Von Mises Stress Contour.
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25. Maximum
Pressure Speed, Reaction Force, Maximum Von
S. No Displacement,
Mpa RPM N Mises Stress, Nm-2
mm
1 3.8 410 -335 0.0857 4.93 x 107
2 5.5 1060 -2227 0.124 7.13 x 107
3 6 1580 -4943 0.135 7.8 x 107
Table 3 –Displacement & Von Mises Stress at different cases.
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26. 6. Conclusion
The piston has been designed to study the skirt deformation under several operating
conditions using finite element method. Empirical equations are used to calculate the
connecting rod forces and the boundary conditions are developed to simulate the exact
constraints with some simplifications. In the final observation the skirt deformation was
maximum on the thrust side of the piston and the reasons for this are the eccentricity in
the piston pin and the firing chamber.
The piston skirt profile relates to the piston slap phenomena that affect the skirt liner
impact forces, skirt lubrication, friction and liner cavitations. By predicting the secondary
motion of the piston, the power cylinder system can be modified in early design phase.
However, the piston design changes made to improve piston guidance in cylinder and
thereby reduce piston slap noise often have a negative impact on piston friction.
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27. 7. References
Andreas Panayi, Harold Schock, Boon-Keat Chui & Mikhail Ejakov,
“Parameterization and FEA Approach for the Assessment of Piston
Characterstics”, SAE Paper No. 2006-01-0429.
S.H.Mansouri & V.W.Wong, “Effects of Piston Design Parameters on Piston
Secondary Motion and Skirt-Liner Friction”, SAE Paper No. 2004-01-2911.
You Zhang, Yong Zhang, Bin Gao & Baozhong Zhang, “Structure Design & FEA
of LHR Piston for Vehicle Engines”, SAE Paper No. 981488.
Shivakanth N.Kurbet & R. Krishna Kumar, “Mechanics and Stress Analysis of
Piston Ring in Multibody Single Cylinder Internal Combustion Engine – FE
Analysis”, SAE Paper No. 2001-01-3371.
Tetsuya Kimura, Kazuki Takahashi & Shigeru Sugiyama, “Development of a
Piston Secondary Motion Analysis Program with Elastically Deformable Piston
Skirt”, SAE Paper No. 1999-01-3303.
Mikhail A. Ejakov, Analytical Powertrain Department, Ford Motor Company –
Piston/Piston Ring Dynamics CAE.
B.Lawton & D.E.G.Crutcher, “Mechanical Stresses in Pistons, Gudgeon Pins &
Connecting rods”, IMechE 2002.
B.L.Ruddy & F.H.Kinsella, “Computer Aided Engineering for Pistons, Rings &
Pins”, IMech 1990.
Conor P.McNally, “Development of a Numerical Model of Piston Secondary
Motion for Internal Combustion Engines”.
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