27-06-2013, 03:51 PM
THERMOMECHANICAL FE ANALYSIS OF THE ENGINE PISTON MADE OF COMPOSITE MATERIAL WITH LOW HISTERESIS
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Abstract
The main purpose of the preliminary analyses presented in the paper was to compare the behaviour of the
combustion engine piston made of different type of materials under thermal load. A thermomechanical FE analysis of
the engine piston made of composite material was shown. A selected engine is installed in one of the popular polish
tanks. The proposed new material is characterized by a low hysteresis – the differences of the coefficient of thermal
expansion for heating and cooling are not significant. The results obtained for the piston made of a new material were
compared with those for the current standard material. The piston was loaded by a temperature field inside it.
Appropriate averaged thermal boundary conditions such as temperatures and heat fluxes were set on different
surfaces of the FE model. FE analyses were carried out using MSC.Marc software. Development of the FE model was
also presented. Geometrical CAD model of the piston was developed based on the actual engine piston, which was
scanned using a 3D laser scanner. A cloud of points obtained from the scanner was processed and converted into a 3-
dimensional solid model. FE model of a quarter part of the piston was developed for the preliminary analysis
presented in the paper. 4-node tetrahedron finite elements were applied since there was no axial symmetry of the
considered object. The temperature field inside the piston was determined and presented in the form of contour bands.
Contours of displacement and stress in a radial direction were shown as well.
Introduction
A total weight of modern military vehicle has been still increased due to additional equipment,
armours, shields etc. Moreover, today’s military vehicles are supposed to be able to better
manoeuvre on the battlefield. Therefore, increasing the power of internal combustion engines used
to drive such vehicles is necessary.
The engine pistons are the most loaded elements of the internal combustion engine. They must
satisfy the requirements concerning durability and functionality. Therefore, a new type of material
with high strength properties at high temperatures is still searched. In addition, the new materials
should be characterized by a low hysteresis – the differences of the coefficient of thermal expansion
for heating and cooling are not supposed to be significant. It allows increasing the piston resistance
to fatigue damage and thermal shock.
A research object
A piston of the S12U diesel engine was selected as a representative for the study. Such engines
are installed in one of the popular polish tanks – PT-91 Twardy. The engine is a 12-cylinder
engine. It is worth to mention that the V-configuration of the S12U engine is not fully symmetric.
Its left and right row of cylinders is slightly different. The main and visible difference is related to
the length of the blade connecting rod.
Development of a geometrical and a FE model
A geometrical model of the piston was developed based on geometry of the actual object,
which was scanned using a three dimensional laser scanner. An obtained cloud of points was
processed and converted into triangular surfaces (Fig. 2a). The outer surfaces of the piston had to
be converted into the 3D solid geometrical model. A function of matching the cloud points to the
corresponding solids, surfaces and interfaces was applied (Fig. 2b). Reading the respective
properties of the geometrical objects allowed determining such parameters as the piston diameter,
the diameter of the piston pin hole, dimensions of the piston ring grooves, etc. An ovalization of
the piston skirt was not taken into consideration at this stage of analysis.
Material properties
The original pistons of the S12U engine are made of PA12 aluminum alloy. Its material
properties, necessary from a thermomechanical analysis viewpoint, were provided in Tab. 2.
Moreover, a new composite material with low hysteresis was also considered. Such material
allows reducing the differences of the coefficient of thermal expansion for heating and cooling,
and it improves a dimensional stability of the piston, consequently. Courses of changes of the
coefficient of thermal expansion for both materials, the PA12 aluminum alloy and the new
composite material, are depicted in Fig. 5.