STATIC ANALYSIS OF PISTON HEAD
STATIC ANALYSIS OF PISTON
Introduction
Piston
is one of n the mechanical component, piston invented in a German scientist
Nicholas August Otto in year 1866.
2. To develop structural analysis of the piston.
Major Force Acting Over Piston
1. Due to explosion of fuel gases
2. Due to compression of fuel gases
3. Side wall friction and forces
4. Thermal load
5. Inertia force due to high
frequency of reciprocation of piston
6. Friction and forces at crank pin
hole
Functions of Piston
1. To reciprocate
in the cylinder as a gas tight plug causing suction,
Compression,
expansion, and exhaust strokes.
2. To receive the
thrust generated by the explosion of the gas in the cylinder
And transmit it to
the connecting rod.
3.To form a guide
and bearing to the small end of the connecting rod and to take the side thrust
due to obliquity of the rod.
Factors Considered for Proper Functioning of Piston
1. The piston
should have enormous strength and heat resistance properties to withstand gas
pressure and inertia forces. They should have minimum weight to minimize the
inertia forces.
2. The material of
the piston should have good and quick dissipation of heat from the crown to the
rings and bearing area to the cylinder walls. It should form an effective gas
and oil seal.
3. Material of the
piston must possess good wearing qualities, so that the piston is able to
maintain sufficient surface-hardness unto the operating temperatures.
4. Piston should
have rigid construction to withstand thermal, mechanical distortion and
sufficient area to prevent undue wear. It has even expansion under thermal
loads so should be free as possible from discontinuities.
Piston Assemble Model
DESIGN AND PROPERTIES
The automobile
piston must have enormous strength and heat resistance properties to resist gas
pressure and inertia forces. Also it should have minimum weight to minimize the
inertia forces also it should have rigid construction to endure thermal, mechanical
distortion and sufficient area to avoid undue wear. Automobile piston is an
extremely key part of combustion engine, fulfilling several functions as
transfer of gas force, kinematic guidance (sometimes with cross head), sealing
(in combination with piston rings) and Shaping the combustion chamber.
Nomenclature of Piston
Piston Head or Crown
The piston head or crown is designed
keeping in view the following two main considerations, i.e.
1. It should have
adequate strength to withstand the straining action due to pressure of
explosion inside the engine cylinder, and
2. It should dissipate the heat of
combustion to the cylinder walls as quickly as possible. On the basis of first
consideration of straining action, the thickness of the piston head is
determined by treating it as a flat circular plate of uniform thickness, fixed
at the outer edges and subjected to a uniformly distributed load due to the gas
pressure over the entire Cross-section.
Piston Rings
The
piston rings are used to impart the necessary radial pressure to maintain the
seal between the piston and the cylinder bore. These are usually made of grey
cast iron or alloy cast iron because of their good wearing properties and also
they retain spring characteristics even at high temperatures.
The
piston rings are of the following two types :
1.
Compression rings or pressure rings, and
2. Oil
control rings or oil scraper.
The
compression rings or pressure rings are inserted in the grooves at the top
portion of the piston and may be three to seven in number. These rings also
transfer heat from the piston to the cylinder liner and absorb some part of the
piston fluctuation due to the side thrust. The oil control rings or oil
scrapers are provided below the compression rings. These rings provide proper
lubrication to the liner by allowing sufficient oil to move up during upward
stroke and at the same time scrap the lubricating oil from the surface of the
liner in order to minimize the flow of the oil to the combustion chamber.
The
compression rings are usually made of rectangular cross-section and the
diameter of the ring is slightly larger than the cylinder bore. A part of the
ring is cut- off in order to permit it to go into the cylinder against the
liner wall. The gap between the ends should be sufficiently large when the ring
is put cold so that even at the highest temperature, the ends do not touch each
other when the ring expands, otherwise there might be buckling of the ring.
Piston Skirt
The
portion of the piston below the ring section is known as piston skirt. In acts
as a bearing for the side thrust of the connecting rod. The length of the
piston skirt should be such that the bearing pressure on the piston barrel due
to the side thrust does not exceed0.25 N/mm2 of the projected area for low
speed engines and 0.5 N/mm2 for high speed engines. It may be noted that the
maximum thrust will be during the expansion stroke. The side thrust (R) on the
cylinder liner is usually taken as 1/10 of the maximum gas load on the piston.
Piston Pin
The piston pin (also called gudgeon pin or wrist pin)
is used to connect the piston and the connecting rod. It is usually made hollow
and tapered on the inside, the smallest inside diameter being at the center of
the pin. The piston pin passes through the bosses provided on the inside of the
piston skirt and the bush of the small end of the connecting rod. The Centre of
piston pin should be 0.02 D to 0.04 D above the center of the skirt, in order
to off-set the turning effect of the friction and to obtain uniform
distribution of pressure between the piston and the cylinder liner. The
material used for the piston pin is usually case hardened steel alloy
containing nickel, chromium, molybdenum or vanadium having tensile strength
from 710 MPa to 910 MPa.
Design
Considerations of a Piston
In designing a piston for I.C. engine, the following
points should be taken into consideration
1. It should have enormous strength to withstand the
high gas pressure and inertia forces.
2. It should have minimum mass to minimize the inertia
forces.
3. It should form an effective gas and oil sealing of
the cylinder.
4. It should provide sufficient bearing area to
prevent undue wear.
5. It should disperse the heat of combustion quickly
to the cylinder walls.
6. It should have high speed reciprocation without
noise.
7. It should be of sufficient rigid construction to
withstand thermal and mechanical distortion.
8. It should have sufficient support for the piston
pin.
Material for Pistons
The most commonly
used materials for pistons of I.C. engines are cast iron, cast aluminum, forged
aluminum, cast steel and forged steel. The cast iron pistons are used for
moderately rated engines with piston speeds below 6 m / s and aluminum alloy
pistons are used for highly rated engines running at higher piston speeds. It
may be noted
1. Since the
coefficient of thermal expansion for aluminum is about 2.5 times that of cast
iron, therefore, a greater clearance must be provided between the piston and
the cylinder wall in order to prevent seizing of the piston when engine runs continuously
under heavy loads. But if excessive clearance is allowed, then the piston will
develop ‘piston slap’ while it is cold and this tendency increases with wear.
The less clearance between the piston and the cylinder wall will lead to
seizing of piston.
2. Since the
aluminum alloys used for pistons have high **heat conductivity (nearly four
times that of cast iron), therefore, these pistons ensure high rate of heat
transfer and thus keeps down the maximum temperature difference between the
center and edges of the piston head or crown.
3. Since the
aluminum alloys are about three times lighter than cast iron, therefore, its
mechanical strength is good at low temperatures, but they lose their strength
(about 50%) at temperatures above 325°C. Sometimes, the pistons of aluminum
alloys are coated with aluminum oxide by an electrical method.
|
Parameters |
AlSiC(40-60)% |
|
Young’s
Modulus |
230GPa |
|
Poison
Ratio |
0.24 |
|
Density |
2937kg/m3 |
|
Thermal
Conductivity |
197w/m-.C |
|
Specific
Gravity |
894J/kg-.C |
Damage From Running Unmixed Fuel
Fig.2.2: Damage From Unmixed Fuel
The piston above has severe scouring
on the exhaust skirt with the heaviest damage on the clutch side of the piston.
All of this damage was caused from running straight fuel. The lack of
lubrication on the piston has caused it to seize to the cylinder wall. The
damage you see was caused in the moments before the piston "stuck,"
which seized the engine.
This kind of piston damage can also
be found on a saw that was run with the carburetor set too lean or one that was
run with an air leak. If you didn't know this saw had been run with no oil in
the fuel, how would you know it wasn't a heat seizure To fully understand the
cause of this failure, it is important to look at the rest of the piston. The
photo below is of the same piston. It shows additional damage that's usually
only found on a saw engine that had been run with unmixed.
Damage From Over-Speeding The
Engine
Fig.2.3: Damage From Over Speeding Engine
The piston above has been damaged by over-speeding. Look at the piston material between the ring-lands. You can see a big chunk of it is missing and some has been "squished" thinner, creating a super-wide ring-land. Look at the top ring (bottom of photo). You can see the edge is rounded-over, a sure sign the rings were catching in the exhaust port. When this occurs, this sets off a high frequency vibration, eventually breaking the ring-land.
Damage From Detonation
Fig.2.4: Damage from Detonation
The piston above
has been damaged by detonation. Notice the damage on the top and the edges of
the piston. The heat caused by detonation made the piston so hot, the rings
stuck and the piston seized in the cylinder. You can see the seizure marks on
the side of the piston. This damage usually ruins both the cylinder and piston.
Detonation can be caused by a number of things. In this case, changing to higher octane supreme grade fuel was the answer.
Wrong Out Failure
Damage from Debris Getting
Through the Air Filter
Fig.2.6: Damage from Getting Through The Air Filter
The damage on this piston skirt is
caused by debris getting through the air filtering system. Notice the
horizontal machine marks have been scrubbed off all across the bottom
indicating extreme wear on the lower part of the skirt. Not shown, but the other
side of the piston looked perfect. This damage was only found only on the
intake side of the piston. This is typical for damage caused by intake debris.
The other side of the piston is not exposed to an intake port, so it isn't
affected at early stages.
What damages the intake skirt is
debris from a leaking filter wedging between the piston and cylinder wall
causing scuffing on the piston skirt. Since the piston is made of softer
material, the damage is more pronounced on the skirt than on the cylinder bore's
hard surface. This wear on the piston increases the clearance, which allows the
piston to "rock" in the cylinder's bore. As the skirt becomes thinner
and weaker, rocking increases. Eventually the piston will break. When it does,
the engine seizes. On a pro saw, the piston skirt performs another important
function. Not only does it guide the piston, the skirt serves as the engine's
intake valve. As the piston travels up and down the cylinder, its base opens
and closes the intake port as it passes. For the engine to run its best, it is
important for this valve to function well.
CATIA
CATIA stands for Computer Aided Three-Dimensional
Interactive Application. It’s much more than a CAD (Computer Aided Design)
software package. It’s a full software suite which incorporates CAD, CAE
(Computer-Aided Engineering) and CAM (Computer-Aided Manufacture).
So, let’s
take a look at each of these areas and the tools that CATIA offers
professionals to enhance innovation in product development.
What
is CAD (Computer Aided Design)?
Enabling
2D and 3D design
Computer-Aided Design is most employed by Engineers to
help create, modify and/or analyze graphical representations of product
designs. Its applications stretch across a multitude of industries due to its
many popular benefits.
CAD software innovation continues to improve the quality
of design through greater accuracy and the reduction of design errors. CAD
software is also capable of improving communication due the centralization of
design data and documentation, creating a single source of truth for engineers
and manufacturers.
Fig.3.1: Enabling 2D and 3D design
What is CAE (Computer Aided Engineering)?
Enabling verification through analysis of 3D
models
Computer-aided engineering most used by Engineers for the
simulation and analysis of product designs. These analyses commonly include
Finite Element Analysis (FEA), Computational Fluid Dynamics (CFD), and
Multibody Dynamics (MBD). Similar to prototyping, these processes help to
prepare a product design for real world stresses. This testing enables
Engineers to make better product design decisions and revisions – this
ultimately leads to better product performance and customer experiences.
Fig.3.2: Enabling verification
through analysis of 3D models
What is CAM
(Computer Aided Manufacture)?
Enabling manufacturing
processes to be designed for 3D model manufacture
Computer-aided manufacturing is commonly
used by Manufacturers – rather than Design Engineers – to plan, manage, control
and automate manufacturing operations. CAM software uses CAD designs to infer
machining instructions while optimizing part production efficiency and material
usage.
Fig.3.3: Enabling manufacturing processes to be designed for
3D model manufacture
Versions
CATIA
currently stands at version level 6, better known as CATIAV6.
The first release of CATIA was back in
1977 by Dassault Systems, who still maintain and develop the software. It
was initially developed for use in designing the Dassault Mirage fighter jet.
Nowadays, the most widely used version is CATIA V5,
with CATIA V4 still being used in some industries, mostly in conjunction with
V5. Between versions, CATIA has varied significantly in terms of usage and
appearance. Each Version brings significant additional functionality. Between
V4 and V5, the fundamentals to the design process were developed and between V5
and V6 the handling of data changed. Within each version, Dassault Systems also
offer updates in the form of releases. New releases are typically released
annually and also bring additional functionality within the Version as well as
bug fixes.
Process Methodology
DETAIL DRAWINGS
Fig.4.1: Detail Drawings
Model Analysis
The following FEA results
are got from CATIA software, when the piston head experience 20N of force over
the Piston Head.
Fig.4.2: Meshing
Fig.4.3: Stress
Fig.4.4: Displacement
Fig.4.5: Stress based on Von-Misses Theory
PISTON ANALYSIS ANIMATION
RESULTS AND CONCLUSIONS
The modeling is created in CATIA and then it is imported
in CATIA analysis Workbench and the piston head is made to fix at hole (where
the Gudgeon pin is inserted) and a force
of 20N is applied over the piston head. And the following results have been
Observed.
RESULTS
SL NO. | Material name | Young’s modulus (Gpa) | Poisson's ratio | Density (kg/m3) | Von Mises Stress (Mpa) | Max-principal stress (Mpa) | Deformation(mm ) |
1. | Al-SiC | 230e3 | 0.24 | 2937 | 216 | 58.3 | 6.46 |
CONCLUSION
Compared results based on Aluminum-(Silicon
Carbide)and cast iron .Al alloy gives best results. Because the stresses
induced in the piston is reduced. Hence the prepared piston with Al-Alloy
specimen show better results than the normal Cast Iron.
Comments
Post a Comment