*Pavan P. Jorwekar¹, Yogesh V. Birari², Mayur M. Nadgouda³

¹Kirloskar Copeland Limited, Product Engineering Department,
Karad, Maharashtra, India
Phone +91 2164 241002 Fax +91 2164 241122 Email – [email protected]

²Kirloskar Copeland Limited, Product Engineering Department,
Karad, Maharashtra, India
Phone +91 2164 241002 Fax +91 2164 241122 Email – [email protected]

³Kirloskar Copeland Limited, Product Engineering Department,
Karad, Maharashtra, India
Phone +91 2164 241002 Fax +91 2164 241122 Email – [email protected]

*Indicates Corresponding Author

Abstract
As per the Montreal Protocol, the CFC refrigerants are being phased out and alternate refrigerants are being introduced for refrigeration system. In view of this the existing design schemes are required to be evaluated. The paper deals with evaluation of the contact pressure distribution of cylinder-head gasket to address the adverse pressure conditions.

Pump assembly consists of reciprocating mechanism in which a cylinder head is fastened to crank case deck face with gasket in between them. The cylinder head essentially consists of suction and discharge plenums separated by walls and are subjected to respective pressures. With alternate refrigerant, the pressure difference across the suction and discharge plenum is increased. In order to address this increased pressure differential, the cylinder head gasket must have enough contact pressure across the discharge plenum and separating walls.

This paper elaborates the use of FEA tool – ANSYS, to analyze the cylinder head and gasket for gasket-joint. This analysis helped in finding the contact pressures across the gasket due to pre-torques applied through bolts. Design modifications were done for removing the low contact pressure zone areas on gasket. The gasket contact pressure pattern obtained in ANSYS was validated using film.

1.Introduction
Gasket joints are essential components in hermetic compressor. A gasket is a compressible material, or a combination of materials, which when clamped between two stationary members prevents the passage of the media across those members. The gasket material selected must be capable of sealing mating surfaces, resistant to the medium being sealed, and able to withstand the operating temperatures and pressures. In hermetic compressors gasket joints are most critical parts as sealing must be leak proof at all the operating conditions. Any damage to gasket sealing leads the entire compressor unusable. The rapid change from CFC refrigerant to non CFC and natural refrigerant put a very high demand on compressor manufacturers to develop new design for new environment friendly alternate refrigerants. For these alternate refrigerants, suction and discharge pressures are higher than conventional CFC refrigerants. So while designing the gasket joints one must consider the increased pressure across the suction and discharge plenum of the cylinder head.

In order to ensure the maintenance of the seal throughout the life expectancy of the assembly, sufficient pressure must remain on the gasket surface to prevent the damage to the gasket. A traditional way to find the contact pressure is by means of films. This activity requires lots of prototyping and testing. It is costly and time consuming as there is chance of re-looping when the expected results are not obtained. Here in this paper, explained the use of FEA tool for virtual method to find out the gasket pressure.

2.Problem Description
Gasket pressure depends on bolt tension and the amount of this bolt tension is transferred to the gasket is dependent on the stiffness of the cylinder head. Stiffer the cylinder head, more uniform distribution of bolt tension on the gasket.

The objective of the analysis was to find out the distribution of cylinder head bolt tension on the gasket surface in terms of pressure for existing design.

3.Gasket Material
Gasket as sealing component between structural components are usually very thin and made of various materials, such as steel, rubber compounds and composites. From a mechanics point of view, gaskets act to transfer force between components. The primary deformation of a gasket is usually confined to one direction, i.e. through thickness. This behavior is most important sealing joint. In-plane stiffness (the stiffness contribution from membrane) and transverse shear is negligible when compared to the through thickness (ANSYS user guide). The stiffness contribution therefore is assumed to be negligible. Due to this, gasket behaves non-linearly in loading and unloading with permanent deformation. Typical gasket behavior is shown in Figure3 for synthetic material which is currently used in aforesaid Gasket. The loading and unloading curve was obtained from the following compression curve function of the gasket material.

Equation 1 gives the compression of gasket for the particular value of stress X. U parameter in equation1 is unloading offset which is equal to zero for initial loading of the gasket. Equation 2 and equation 3 gives the slop and tangent modulus of the compression curve. Equation 4 gives the unloading offset of the gasket due to unrecoverable strain. Using equation 1 and equation 4, gasket compression and unloading behavior was calculated. Figure3 shows the gasket compression and unloading curve that were input to the ANSYS.

Figure 1: Cylinder Head and Gasket assembly.

4.1 Model Simplification and assumption

CAD models of cylinder head, gasket, valve plate and crank case was created in I-DEAS NX2. These models were geometrically simplified for ease of meshing. Only the crank case deck has considered for the analysis as there is negligible effect of bolt torque on remaining part of the crank case. This has reduced the model size and solution time.

Following are the assumption

o Bolt torque sequencing is ignored and torque applied simultaneously

o Gasket and cylinder head material assumed to be homogenous.

o Valve plate and crank-case assumed to be single part.

o Effect of operating temperature on gasket behavior is not considered in the analysis.

.

4.2 Pre-processing

ANSYS offers a series of elements to model gaskets. These elements belong to the ANSYS family of interface elements. For these elements, the membrane and transverse shear are neglected for gasket simulations. A gasket material option has been designed especially to account for the gasket through thickness behavior. This option should be used in a microscopic investigation of how gasket joints behave and does not predict the response of the gasket, but rather allows the user to input the response of the material. In this analysis, gasket material behavior was obtained form compression curve equation. One may focus on the behavior of the joint rather than detailed study of the gasket. If the detailed gasket results are desired, more sophisticated material models will be required

For this problem INTER194 is selected to model the gasket. INTER194 is a 3-D 16-node quadratic element. It is defined by 16 nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions. Cylinder head and crank-case is meshed using Solid 92 elements. Bolt toque is converted into axial force and applied on the cylinder as shown in Figure2. Base of crank-case part is constrained in all degrees of freedoms. Material properties for cylinder head and valve plate such as modulus of elasticity, poisons ratio were defined.

The gasket material compression and unloading data were input to the ANSYS. The GASKET option in ANSYS allows directly input data for complex pressure closure curves. The GASKET option also offers two sub-options to define gasket unloading behavior including linear and nonlinear unloading. The linear unloading option simplifies

the input by defining the starting closure at the compression curves and the slope. The nonlinear option allows to directly input unloading curves to more accurately model the gasket unloading behavior. In this simulation nonlinear option was used for both compression and unloading condition to simulate accurate behavior of the gasket.

Figure 2: Boundary condition

Fig. 5. Schematic diagram of the experimental apparatus.

Fig. 6 Photographs of (a) the wheel and rail specimens and (b) the loading fram
showing the location of the ultrasonic transducer.

Fig. 7. Schematic diagram showing the transducer lateral (i to v) and longitudinal (a to e)
locations with respect to the wheel-rail contact regions.

Fig. 8. Images from pressure sensitive film experiments for normal load P=80 kN
under different applied lateral forces, Q. (a) Q=0 kN, (b) Q=2 kN, (c) Q=4 kN, (d)
Q=6 kN, (e) Q=9 kN.

Fig. 9. Reflection coefficient profiles in the transverse direction across the contact
(recorded at position (iii) in Fig. 8) at the total load is increased from 5 to 80 kN.

Fig. 10. Reflection coefficient profiles in the longitudinal direction across the
contact (recorded at position (b) in Fig. 8) at the total load is increased from 20 to 80 kN.

Fig. 11. A map of reflection coefficient obtained by assembling together five
reflection coefficient profiles in the lateral direction under normal loads of (top to bottom)
P=20 kN, P=40 kN, P=60 kN, and P=80 kN.

Fig. 12. A map of reflection coefficient obtained by assembling together five
reflection coefficient profiles in the longitudinal direction under normal loads of
(top) P=20 kN, and P=40 kN, (bottom) P=60 kN and P=80 kN.

Fig. 13. Experimental and theoretical approaches for contact area measurement for
three normal loads 20 kN, 40 kN, and 80 kN plotted at the same scale. (a) Pressure
sensitive film, (b) scans of the contact using a single transducer, (c) scans of the
contact using an array transducer.

Table 1. Comparison of the semi-minor, a and semi-major radii, b of the contact
regions 1 and 2 measured by the experimental techniques.

Fig. 14. Comparison of the total contact area determined by the three experimental methods.

Fig. 15. The relationship between train speed, pulse repetition rate, and the number of
lateral measurements that could be recorded.