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ABSTRACT |
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11.1. |
Introduction |
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The PhD thesis ‘Finite Element
Method Study of the Stresses and Strains in the Cylinder Blocks of the
Internal Combustion Engines’ is a multidisciplinary approach that offers an
accurate FEM model of a military vessel internal combustion engine (ICE)
together with a proposal regarding a research methodology of the
three-dimensional structures subjected to a complex state of loads.
In this section are presented the
main ideas of the thesis and the future directions to be followed in this
study. |
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11.2. |
Aspects Regarding the Suitability of the Research Theme |
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The
design of the complex three-dimensional frames of a new engine was done once
taking into account the huge experience of the designers, the similar models
already created, and the technological constraints. The computer aided design of this
class of structures is a remarkably effective method but it can offer
unrealistic solutions if a computing unit is not employed to predict the
state of strains and stresses. The creation of a finite element
model is the most rational method of investigation of the strains and
stresses. This solution must be validated by the use of some alternate
research methods: the development of several models to evaluate the accuracy
of the numerical solutions and the use of the accurate experimental methods
to validate the numerical models. There must be noticed that the
geometry of the structure is extremely complex and the loads are both mechanical
and thermal. The only flexible method that offers a complete solution inside
the method and effective tools to be employed in the study is the finite
element method. Moreover, the finite element model
once created and validated by the use of the alternate methods, may be used
for additional studies such as: weight optimization of the structure,
different materials to be used for the manufacturing of the frame, structure
assembled of a flexible foundation that may present deflections in running
conditions, shape modes, etc. Taking into account the
aforementioned ideas it results that the accurate estimation of the state of
strains and stresses inside the cylinder block of an ICE is an important
research issue that requires modern numerical and experimental techniques of
investigation. |
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11.3. |
Aspects Regarding the Numerical Basics of the Study |
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A
numerical investigation of the phenomena in an ICE employs a set of numerical
methods: general numerical methods, finite difference method, finite element
method, etc. General numerical methods are
employed to solve a large number of problems. The knowledge of these
computing methods is a necessary basic step in the engineering culture. In
this study are employed the following general numerical methods: ¨
interpolation methods (experimental data processing, SYSTHERM, etc.); ¨
solving of general equations (experimental data processing, SYSTHERM,
etc.); ¨
solving of linear systems (in original MDF and MEF programs); ¨
numerical calculus of the integrals (SYSTHERM); The finite difference method is the
scientific basis of a set of programs created to study the conductive heat
transfer (appendix 6). The finite element method is
employed for: ¨
the calculus of the field of temperatures in the cylinder block based
on the solution of the heat transfer equation; ¨
the calculus of the state of strains and stresses; ¨
the numerical optimization of the structure. The finite element method is also the theoretical
background of the original programs used for the calculus of the field of
temperatures (appendix 7) and for the calculus of the stresses [85]. |
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11.4. |
Aspects Regarding the Experimental Basics of the Study |
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The aim
of the experimental study is to offer the values of the strains and stresses
measured in a set of points located on the cylinder block. During the
measurement the engine is running, being subjected to a military reception
test. This is why the running parameters of the engine are kept under a
strict control and there was not allowed any destructive modification of the
frame (holes in the structure for the leadwires to some measurement point
located inside the engine) fact that imposed additional constraints. To measure the strains there was
designed an experimental program consisting of stages and procedures that
takes into account the phenomena in the ICE. In this way there were
forewarned the apparition of the parasitic effects. This is why the
experimental data have a high level of confidence. The experimental program was
conceived to be run by the use of an original measurement chain consisting
of: 10 VISHAY three-element rosettes, 3 VISHAY SB10 switch and balance units,
3 VISHAY Wheatstone bridges, data acquisition card, pressure transducer,
non-contact infrared thermometer. The output characteristics of these units
were separately studied to verify if they are compatible and then they were
put in accord and included in the measurement chain. There must be noticed
that the facilities of the units were used to the best (as number of
measurement point, frequency of the data acquisition card, number of engine
cycles to be used to check the uniformity of the parameters of the running engine,
the best settings of the pressure sensor). Several tests were done to check ¨
the possible parasitic influences that were eliminated,
selfcompensated, compensated or corrected; ¨
the repeatability of the experimental values supplied by the measurement
chain. There were also acquired all the
data supplied by the units that fit the laboratory where the experiment took
place. Another direction of the study was
the measurement of the field of temperatures in the cylinder block in a large
set of measurement points together with the measurement of the temperatures
and the strains during the natural cooling process of the engine. The experimental data were
processed with the ESAM program created by VISHAY and with a set of original
programs based on the data processing techniques presented in books, on the
research contracts completed by the author and on the VISHAY tech-notes. The
information supplied by these sources was verified on the bases of some
complex criteria in order to test the convergence of the results computed by
the use of the recommended analytical and graphical methods. There must be reminded that in the
processing and the interpretation of the experimental data there were
employed two points of view: ¨
the first one belongs to the specialist with large experience in
complex general experimental mechanics projects who knows the limits of the
units involved in the experiment and of the experimental data computing
methods; ¨
the second point of view is in accord with the ICE expert who takes
into account the phenomena inside the engine. Obviously the two points of view are not contradictory,
moreover they complete each other. The experimental values acquired
(millions of values) were subjected to a complex processing and they were
graphically represented to establish the range of values, the extreme values,
the location of these maximum values during an engine cycle, general trends,
general shape of the curves, etc. The final values of the strains and
stresses are used to verify the results of the studies which use finite
element models. |
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11.5. |
Future Research Directions To Be Followed |
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Taking into account the previously
mentioned ideas there may be identified a series of future research themes
which may be classified in main future research directions. Directions for a follow-up study of this completed project The finite element model may be
optimized taking into account the following directions: ¨
study of the stiffness of the interior walls which support the main
bearings in a complex structural optimization process of the cylinder block; ¨
evaluation of the rigidity/elasticity of the supports and their
modeling with more improved accuracy; ¨
study of the state of strains and stresses in the supports and in the
frame assembled in a oblique position as it would be on the running military
vessel; ¨
study of the shape modes of the engine. Directions for the generalization of the study The generalization of the study may
be done by the creation of a chain of software applications which solve the
following problems: ¨
in-cylinder gas exchange phenomena; ¨
thermal in-cylinder phenomena; ¨
exhaust gas - cylinder wall heat transfer phenomena; ¨
conductive heat transfer in the cylinder block; ¨
calculus of the mechanical and thermal strains and stresses. Analytical methods employed for the calculus of the
stresses The basic hypotheses of the Theory
of Elasticity and from the Strength of Materials discipline may be employed
to study the strains and stresses in the frame. In figure 11.1 it is
presented a qualitative comparison between the classic methods (Strength of
Materials) and the finite element method. These methods complete each other
and may be used in complex studies that use an analytical approach. There already have been developed
methods, algorithms and computer codes which compute: ¨
the normal stresses created by tensile forces and bending moments; ¨
the tangential stresses created by the share forces. From this point of view the direction identified is ¨
computer aided calculus of the tangential stresses for a torque
applied onto a multiconnected domain with holes. The main ideas and concepts are presented in [87], [104],
[102], [110]. |
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Analytical method for the calculus of the thermal stresses The thermal stresses may be
computed using analytical approaches following the stages:
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heat transfer phenomenon using analytical methods by the use of the
decomposition of the structure in simple shaped parts: bars, plates, ribs,
etc. [13], [23], [26], [27], [34], [37];
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using the temperatures computed in the previous stage there can be
calculated the thermal stresses [13]. |
Figure 11.1 - Comparison between the
computer aided methods for the calculus of the stresses, solutions that use
classic and FEM computing methods |
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11.6. |
Original Elements of the Thesis |
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The author of the thesis is responsible
for the contents of this essay including the original elements, being the
effective accomplisher of all the parts of the paper (analysis, research,
programming, experimental and numerical studies, editing, etc.). Regarding the numerical study
there can be noticed the following ideas: ¨
the concern was to use some common
sense scientific hypotheses, not some ‘brave’ and generous ones which could
lead to unrealistic results; ¨
there have been done several
separate studies regarding: |
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1. |
the discretization of the structure (two studies which
employ different finite elements); |
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2. |
the creation of the models of the supports (evaluation of
6 different types of supports); |
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3. |
the calculus of the mechanical loads (there was written a
computer code which offers the set of forces applied onto the structures for
each crankshaft angle); |
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the conductive heat transfer was studied to establish the temperatures
in the nodes of the model, temperatures which are used, further on, for the
calculus of the thermal strains and stresses; ¨
the field of strains and stresses was studied for each simple type of
loads: |
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1. |
thermal loads; |
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2. |
mechanical loads (forces applied on the structure for 4
different positions of the crankshaft) |
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3. |
weight of the engine |
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¨
the complex state of loads in the structure was studied adding the
simple types of loads; ¨
the accuracy of the results was verified by the use of the
experimental values in order to validate the finite element model; ¨
once validated, the numerical model was employed in three additional
studies: |
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1. |
weight optimization of the cylinder block; |
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2. |
frame assembled on a deflected structure (deflected vessel
caused by waves); |
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3. |
cylinder block manufactured from a different material
(cast iron instead of aluminium). |
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The
experimental study was employed to establish the strains, stresses and
temperatures in the measurement points. There were accomplished the following
stages: ¨
creation of an experimental measurement program applied onto the
running engine, program which must take into account all the existing
constraints and which must eliminate, selfcompensate, compensate or correct
the parasitic effects; ¨
the evaluation of the best locations of the measurement points was
done by the use of the ‘most probable shape of deflection’ concept; ¨
design of a measurement chain starting from a set of separate units:
Wheatstone bridges, switch and balance units, data acquisition card, pressure
transducer, non contact infrared thermometer; ¨
creation of some alternate measurement chains and experimental
programs in some ‘crisis scenarios’ which might appear during the experiment
(interrupted leadwires, failure of some of the units, etc.); ¨
repeated acquisition of the data in order to verify the constancy, the
accuracy of the experimental measurements; ¨
creation of a computer code for the experimental data processing; ¨
validation of the experimental values taking into account the
phenomena inside the engine (firing sequence, maximum pressure which occurs
during the combustion, location of the measurement points, etc.); ¨
measurement of the temperatures in several points onto the engine
block and the use of the temperatures supplied by the units of the
laboratory; ¨
study of the natural cooling phenomenon and measurement of the strains
and temperatures during this on-going process. There may be identified some other
original elements: ¨
the phenomena inside the cylinder block are placed in an upper level
of understanding that allows the creation of an computer aided advanced
approach which uses analytical, numerical and experimental methods (future
research directions); there must be noticed that the calculus of the strains
and stresses by the use of the classic analytical methods do not assume the
simply creation of a complex computer code, but the algorithmic
generalization of the method employed in the Theory of Elasticity, fact that
offers a high degree of versatility and a new youth of these classic methods;
in this way the concepts are extended being reached a metalevel of
understanding which is, further on, reflected onto the original classic
disciplines; ¨
there have been created several computer codes consisting of more than
40000 lines, some programs being used to accomplish this study and other
being included in the ‘future research directions’ module; the aim of the
presentation of the programs was not only to offer a software reference
manual but to briefly emphasize their main features; side remarks like the
aim of each procedure, user graphic interface, eventually the source listing
are presented in the appendices of the thesis. |
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11.7. |
Conclusion |
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The
scientific and applicative importance of the thesis may be noticed taking
into account the following remarks: ¨
the paper offers a methodology for the evaluation of the stresses
inside the cylinder block of an ICE even from the concept stage, this
methodology being used as an optimization tool of its frame; ¨
the structural analysts have the opportunity to use the information
regarding the particular aspects of the numerical models created in the
thesis and the numerical methods original software presented, consisting of
more than 40000 computer code lines are an obvious inspiring environment; ¨
the paper proposes an evaluation software method of the stresses in a
complex three dimensional frame by the use of a chain of programs, most of
them being already created and tested; ¨
the experimental research program and the software instrument created
may be readily used in other particular experimental mechanics studies [106],
[107], [112], [113], [114], [117], [119], [120]; ¨
the original software developed in the study uses the matrix-file
processing technique, that is an original and extremely effective method; it
represents a software general solution applied in all the matrix-based
approaches which can be easily extended or adapted for other programming
languages or other operating systems [83], [84], [85], [86], [92], [98]; ¨
from the numerical point of view, in the thesis are applied general
numerical methods (experimental data processing, SYSTHERM software, etc.),
the finite difference method [99], [100], [101], [105], [109], [111], [115],
[116] and the finite element method, [85], [90], [91] [93], [103] in this way
being reached an overview level regarding these numerical method that
complete each other; ¨
finally, an advanced theoretical instrument is offered, ‘tool’ which
can be used to accomplish several practical applications in a set of problems
different one to the other from the scientific point of view. |
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11.8. |
Final Remarks |
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This
paper was created in six years of research, but the scientific evolution
period is much longer being respected the classic stages: thesis, antithesis,
synthesis. Thus, the paper was supposed to be a gathering area between the
practical features of the assembly in question, an internal combustion
engine, and the general approach which is modern from both, experimental and
numerical points of view. It is neither a theory seeking for practical
applications nor a study oriented on a narrow topic. Some of the results of
the research work were presented, so far, in 17 scientific papers and 2
research projects and the future research directions are some of the most
important accomplishments. The overview approach was to use the general,
synthetizing and in the same time practical features of the programming work
which is applied in all the research methods: analytical, numerical and
experimental. Last but not least, I do express my
deep gratitude to Prof. Ph. D. Eng. Constantin Aramă, Member of the
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