Engine Cylinder Head Thermal and Structural Stress Analysis
Ing. Radek Tichánek, Ing. Miroslav Španiel, CSc.Czech Technical University in Prague Technická 4, Prague 6 CZ-166 07, Czech Republic Radek.Tichanek@fs.cvut.cz
spaniel@lin.fsid.cvut.cz Introduction.
The steady state heat transfer analysis of a diesel engine head assembly under operational conditions was presented at the 20th Danubia-Adria Symposium on Experimental Methods in Solid Mechanics. A structural analysis has been performed and the results of computation are introduced in this article. Special attention was paid to contact between both the intake and exhaust valves and their seats. The valve/seat interface is responsible for sealing of a combustion chamber and is subject to a high thermal, mechanical and chemical load. These loads may have the consequences on both the life of the valves and the operation of an engine. A method for obtaining information about the behavior of the valve/seat interface during an engine operation is the analysis of contact pressure between the valve and valve seat. The FE modeling was found to be a useful tool for this task. The FE model must involve both a thermal and mechanical load. This approach requires the complex FE model of the engine head assembly and the simulation of processes such as a heat transfer and the mechanical load caused by combustion pressure, valve springs, pre-pressed bolt connections and molded valve seats.
Developments and improvements to steady state heat transfer analysis.
Recently some input parameters have been refined.
The most important differences related to the conductance of the valves which are made of the special steel. At the same time the more realistic dependency of a film coefficient, describing the heat transfer between cooling water and cooled surfaces were determined in [3]. It depends on overheating instead of absolute temperature and its values are several times higher. In Figure 1 is presented the comparison between measured and computed temperature at 12 measured points.
Figure 1
Structural Analysis.
A structural model is constructed as an assembly of several parts. The head itself is fastened to the cylinder by 6 pre-stressed bolts through the gasket (steel ring).
Both the bolts and gasket are constrained on the side of the cylinder, which is not present in the model. The pressing of the seats and valve guides into the head is modeled using a contact with friction. The sliding of the valve guides is disabled by special constraints using Lagrange multipliers1. Vertical motion of the valves is determined by the pre-stressed springs acting between the valve top and the head body and limited by the contact interactions with the seats. In the structural model the same FE topology as in the previously presented steady state heat transfer was used [1]. The thermal elements were substituted with structural ones.
The solid parts contain C3D4 tetrahedrons, C3D6 pentahedrons and C3D8 hexahedrons; shells are meshed with S3 and S4 elements; B3D2 are beams, all from the ABAQUS element library. The solid elements in the model are of standard isoparametric formulation with linear/bilinear displacement interpolation; general shells and Bernoulli beams are used to model some details. The FE model assemblage was statically loaded to approximately five basic operational states:
1 Such constraints which define the complex interaction between two nodes and enable various kinds of constraining relationships between single degrees of freedom are called “connector elements” in ABAQUS.
1. The engine head assembly was not thermally and mechanically loaded. The only assembly self-balancing internal forces are present.
2. The engine head assembly was loaded by in- cylinder pressure. The pressure
96 .
= 1
p
MPa was applied to the bottom of the head and the valves, while the head temperature was equal to the ambient temperature.3. The engine head assembly was loaded by in- cylinder pressure and a temperature field. The pressure
p = 1 . 96
MPa was applied to the bottom of the head and valves, while the head temperature was distributed according to the previously computed steady-state temperature field.4. The engine head assembly was loaded by maximum in-cylinder pressure. The pressure
= 15
p
MPa was applied to the bottom of the head and valves, while the head temperature was equal to the ambient temperature.5. The engine head assembly was loaded by maximum in-cylinder pressure and a temperature field. The pressure
p = 15
MPa was applied to the bottom of the head and valves, while the head temperature is distributed according to the previously computed steady-state temperature field.As we expected, it was most difficult to obtain a convergence of the assemblage process. Some enhancements were implemented in the FE model to improve convergence of assembling. Now, the structural model has some new properties:
The reference nodes are added and constrained with the valve seats and the guides to help to fix them in space during assembling. The head itself was fixed in three nodes at its top. The gasket ring was constrained by the bottom surface. This substitutes a contact with the cylinder. The six bolts modeled using beam and shell elements were fully constrained at the bottom (in cylinder), while their heads are in a contact with the cylinder head. Pre-stressing the bolts causes head connection with the cylinder through the gasket ring.
The injector is connected with the head by two bolts (modeled as beams) constrained by the head top. Pre- stressing the bolts presses the injector slightly against the bottom wall of the head. The valves are drawn up by springs acting against the head top. Their motion is limited by the contact with the seats.
Results.
Complete stress/deformation tensors, displacements and contact pressures are available as the results of structural analysis. All these results give us new knowledge about the loads of parts under operational conditions. Their interpretation is general and very complex, mainly due to the uncertain influence of model simplifications and approximations. The results may be used for various particular purposes. For instance, at present the experimental testing of
vermicular cast iron (which the head is made of) including the effects of fatigue under high temperature is in progress. Evaluation of head low-cycle fatigue due to heating and cooling based on the presented structural analysis is going to be performed in the near future.
As mentioned previously, our attention was focused on the valve/seat contact interactions. Our investigation was based on the detailed analysis of valve/seat contact pressure. Undeformed contact surfaces are conical and meshed regularly so that their nodes lie on 4 circles having the centers on the common valve/seat axis. This fact was used to describe the contact pressure distribution along four paths in relation to the angle from 0 to 360 degrees on all seats.
Conclusion.
The thermal model of the cylinder head was further developed using some improved input data and the FE model enhancements. The heat transfer coefficient between cooling water and the cooled wall is now based on experimental data obtained from [3]. The material parameters of cast irons and steels were updated according to the producer. The structural analysis results were obtained from the partially modified FE model. The evaluation of the seat/valve interaction shows the different influence of valve and seat deformation. Valve head deformation by the temperature field moves the contact into the outer edge, while applying pure pressure leads to contact along the inner one. Seat deformation due to the head stiffness and heating causes the non-uniformity of contact pressure distribution along the contact area.
Acknowledgements.
This research was accomplished in the Josef Božek Research Center of Engine and Automotive Engineering, supported by the Ministry of Education of the Czech Republic, project No. LN00B073.
References.
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Széchenyi István University of Applied Sciences, 2003, s. 74-75.
ISBN 963-9058-20-3.
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