Keywords

1 Introduction

The main cause of head gasket failure is extreme engine temperature. High engine temperatures are often caused by a refrigerant leak. Different gaskets will fail at different stages and temperatures. Temperature changes are relevant when a metal has a high thermal expansion rate [1]. Analytical calculations are difficult and experimental evaluation are expensive. Nowadays, numerical analyses are a reasonable solution. Figure 1 shows the temperature distributions predicted by FEA [2]. It shows a highest temperature of 280 ºC, located by the flame cover of the cylinder head. The max. Temperature experienced by the gasket is approximately 200 ºC. These resulting temperatures, in the form of nodal temperatures, form the important heat load for structural analysis of the engine assembly. Therefore, in all 3D analysis, we must include a temperature map and simulate the various operating conditions of the engine [2]. Temperatures vary a lot in a very small space. The fuel mixture when ignited can reach between 700 ºC to 900 ºC. The hottest point of the engine is the exhaust manifold, just beyond the cylinder head cooling, where it can reach 500 ºC to 700 ºC.

Fig. 1.
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Temperature distribution on engine cover.

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This work investigates engine gasket sealing efficiency and cylinder head stress behavior under load conditions, using contact theory and thermal stress analysis. The thermal simulation analysis will provide some important thermal parameters, namely the heat transfer coefficients and the corresponding bulk temperatures. A finite element analysis (FEA) thermal simulation with those heat transfer coefficients and global temperatures will provide the temperature distribution of the engine assembly [2].

2 Methodology for Performing Thermal Analysis

Thermal analysis is an analytical technique most used in the field of materials science. Where changes in material properties are examined concerning the component at service temperature. The numerical thermal analysis procedure can be divided into four steps: containing defining elements, material properties, discretization, and results [3]. It requires to consider the structural evaluation of the component to conduct the thermal evaluation. The type of analysis is known as coupled. Discretization is relevant to simulate composite materials with coatings (Fig. 2).

Fig. 2.
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MLS Gasket Composite Material

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Regarding the definition of material data, it is necessary to evaluate them carefully, since one of the objectives of the analysis is to verify the existence of regions where there are losses of stiffness, caused by the phenomenon of plasticity. The effect of temperature can be significant, because the material can suffer a great loss of stiffness and consequent reduction in the ability to follow the movements of the flange. This is the result of stored energy being lost with temperature stress relaxation (Tables 1 and 2).

Table 1. Materials stress-strain data.
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Table 2. Basic elastic properties for the gasket components
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The engine head gaskets modeling is complex due to the nonlinear response of the materials. Material responses can lead to significant errors in the results [8]. This paper provides an overview of the construction and assembly process to create a head gasket model, describes the nonlinear nature of the materials used and presents the results.

Engine sealing components must be designed to provide adequate tightness to avoid leakage [9]. But they must not induce stresses on engine components that could impair engine performance and/or function. Engine sealing components must also be designed to operate for many millions of cycles without failure. The thermal problem can be classified as steady-state or transient, linear or nonlinear. Transient analysis is characterized by the evolution of the solution over time and, in addition to the exchange of energy with the environment, involves thermal energy storage. Steady-state analysis refers to the state-point solution to fixed-boundary condition problems [9]. Nonlinearities go into both stationary and transient solutions across several areas. The most common nonlinearity is associated with temperature-dependent material properties (Figs. 3 and 4).

Fig. 3.
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Temperature distribution.

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Fig. 4.
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Total heat flux.

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With the generation of the gasket model, a preliminary thermal analysis is carried out to estimate the heat flow and identify the critical zones. With these data, a nonlinear static analysis is executed to evaluate the distribution of loads. The boundary conditions applied to the motor head gasket sealing system include the displacement limit condition, the contact limit condition and the load limit condition [5]. For the contact boundary condition, it is assumed that the contact surfaces between the cylinder head, the bolts and the gasket are not completely flat. The cylinder head gasket suffers an elasto-plastic deformation process until the full contact force is supported. The size of the region of the plastically deformed gasket is directly proportional to the contact pressure and inversely proportional to the hardness of the material [5]. To avoid insufficient gasket sealing, bolts are pre-stressed into the range of 28–80 kN [10]. The initial effects that occur into the gasket depends directly on the tightening of the screws to close the motor, which is responsible to achieve the perfect seal of the system. A force of 80 kN was applied for the screws tightening in the monoblock assembly. The total force exercised in the engine and gasket system is 800 kN. The behavior of the combustion process depends on maximum pressure, temperature, gasket and monoblock materials (bolts, force, location, etc.) and sealing. The model presents difficulties for the analysis convergence process. The analysis is restricted to the number of significant digits, which directly interferes with the analysis result, once the cutoff errors accumulate, they create difficulties in reaching equilibrium and solving the problem. Complexity is due to non-linearities [11, 12] and care must be taken with discretization.

To execute the thermal analysis of the gasket, the maximum temperature is be 200 °C in the area of cylinders (Fig. 5a). In the part of the water and oil passages, a temperature of 90 °C is taken (Fig. 5b). Figure 6 and Fig. 7 show the result of the distribution of temperatures acting on the gasket. The highest point of heat is the cylinder area, as the water and oil passages influence the temperature, they should maintain the gasket at a controlled temperature, so it does not suffer deterioration due to temperature changes.

Fig. 5.
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a. Introduction of temperature. b. Introduction of temperature for water and oil passages

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3 Solution and Results of Static Analysis with Thermal Loads

The objective of the global model is to observe the general performance of the component identified by the distribution of loads and stresses, temperatures, and contact pressures. Therefore, it is possible to identify the critical areas from the point of view of sealing. For this case, the temperature characteristics are imported into the static analysis (Fig. 8 and 9). In other words, the ones with the smallest applied load to provide the seal, as well as the areas subjected to the most rigorous conditions where the phenomenon of plasticity can be witnessed (deeply undesirable). Subsequently, the creation of a detailed model can be carried out based on this information.

Inherent in the uncertain contact problem, which determines the existence of a variable charge vector. Another detail concerns the geometry approach. The use of contact elements determines the choice of linear elements, to avoid errors related to the application of load in the intermediate nodes of the element edges. Since the elements have an isoparametric formulation, the geometry is defined by linear interpolation functions. As an unavoidable consequence, there are sharp corners joining elements, in the definition of curvilinear surfaces.

Fig. 6.
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Upper gasket temperatures

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Fig. 7.
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Lower gasket temperatures

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Fig. 8.
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Maximum principal stress in cylinder zone, refrigerant and lubrication passages in lower and upper seal

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Fig. 9.
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von Mises stress at lower and upper gasket

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4 Conclusions

The results consider the effect of temperature, acting load and contact. This kind of numerical evaluation shows the importance on the engine head gasket sealing reliability and proves its importance and efficiency on the finding results. Also, has the advantage in time saving and cost reduction. Concluding that heat dissipation flows from the highest to the lowest temperature zones. The results show the maximum main stresses to which the engine gasket is subjected, already with the action of temperature. They also show an increase in the area of water and oil passages. This is due to the actions of the thermal loads in the actuation of the system, improving the sealing of the joint. Results illustrate the importance of the local distribution of the ribbed means. Also, a continuous sealing area is formed where all stresses are similar around the coolant holes and the lubrication holes that according to the material specifications meets the initial requirements of the customer. While von Mises stresses shows that a ductile material begins to yield at a location when the stress is equal to the elastic limit, it would indicate if the material could fail in this zone. When a head gasket is installed between the cylinder head and the engine block, tightening the head bolts slightly compresses the gasket, allowing the soft material in the gasket to conform to minor surface irregularities of the platform and the block. This allows the gasket to seal cold, so refrigerant doesn’t leak out until the engine is started. There is a decrease in height in ribbed areas and near the combustion chamber. As well as, in areas of the passage of water and oil. This indicates that due to the pressure of the screws when mounting them to the engine, they generate a reaction force that creates the seal through the ribs. The gasket assembled with the two layers shows that the displacements suffered are minimal and do not affect the function of the engine gasket. According to the results obtained this kind of gasket would present no problem for operation, since the stress limits of the materials used are not exceeded. Meanwhile, the contact forces are the most basic and important index on the sealing reliability of the engine head gasket. The general distribution of stresses in the contacts in the system were estimated. The full stresses of the ribs in the cylinder bores are greater than half the stresses of the beads in the cooling bores and oil bores. It is also observed that in the cylinder area there is a stress of approximately 50 MPa, enough to meet the factory requirement on sealing pressure. The pressures in the cylinder area must be more than 20 MPa according to the customer’s requirements. By producing a proper sealing, the engine efficiency and performance could be increase.