Abstract:
A gasket system is described and depicting having a gasket located between and sealing a cylinder head and a cylinder block. The gasket has a first metal layer and a second metal layer adjacent said first metal layer. The second metal layer has a bead region and a stopper region. At least a portion of the bead region has a predetermined tensile strength and a predetermined range of yield strength.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional application filed off of U.S. patent application Ser. No. 11/066,084 filed on Feb. 25, 2005, which is incorporated by reference in its entirety herein. This divisional application is being filed during the pendency of U.S. patent application Ser. No. 11/066,084. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to multi-layer steel (MLS) gaskets for cylinder head gaskets and in particular to MLS gasket materials that exhibit improved sealing characteristics. 
       BACKGROUND OF THE INVENTION 
       [0003]    The trends to reduce fuel consumption and emissions in internal combustion engine powered vehicles have placed increased demands on the performance of many components. Reducing fuel consumption by using lighter materials in engine cylinder blocks and head assemblies has proven successful, although the lighter alloys used typically experience greater deflection with equivalent cylinder compression ratios. This reduced stiffness may result in additional deflection within the head assembly and cylinder block, resulting in greater deflection between the head assembly and cylinder block, and thus, increased demand on a cylinder head gasket to accommodate relative deflection. 
         [0004]    Reducing emissions by increasing the engine compression ratio has also proven successful. However, this increase in cylinder pressure typically results in increased motion between the mating surfaces of the head assembly and cylinder block. These factors, and others, have resulted in the technology of MLS cylinder head gaskets becoming an area of constant innovation. 
         [0005]    The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. These gasket areas immediately adjacent the circumference of engine cylinder bore apertures also experience greater displacement between the mating surfaces than areas of the gasket radially remote from the apertures. Typically, MLS gaskets incorporate at least one beaded region to ensure an adequate seal. 
         [0006]    This displacement between the mating surfaces results in axial motion within the active layers and creates a micro-motion between the active layer and any adjoining surface. This motion requires a minimum durability in the beaded region to ensure that the gasket repeatably seals between the mating surfaces. 
         [0007]    Layers with beaded regions, also called “active” layers, exert pressure on the sealing surfaces to ensure an adequate seal. Generally, the higher the surface pressures, the better the sealing function, or sealing capability. MLS gaskets sealing between a cylinder head assembly and a cylinder block are typically exposed to temperatures that often exceed 1600 degrees F. 
         [0008]    Selection of materials for the active layers is typically limited due to the desired hardness, durability, spring rate, ductility, softening point, and other characteristics. Conventionally, materials have been selected by considering the available desirable properties of materials that have evolved into use. 
         [0009]    A typical material for active layers of MLS gaskets is 301 stainless steel (301 SS). Tensile strength for 301 SS is in a range of about 1350 to 1600 MPA, and yield strength is in a range of about 1050 to 1250 MPA. While 301 SS has the capacity to retain adequate properties during engine operation, improved materials are sought to increase gasket sealability, reliability, and/or capacity to withstand increased displacement between the mating surfaces. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates to a gasket system that includes in pertinent part a gasket located between and sealing a cylinder head and a cylinder block. The gasket has a plurality of cylinder apertures, bolt apertures and jacket apertures. The gasket has a first metal layer and a second metal layer adjacent the first metal layer. The second metal layer has a bead region and a stopper region. At least a portion of the bead region has a predetermined tensile strength in a range of about 1000 MPA to about 1150 MPA, and a predetermined range of yield strength. The yield strength is greater than about 90% of the tensile strength, wherein approximately 70% of the tensile strength of about 1000 MPA to about 1150 MPA and approximately 70% of the yield strength is throughout the operating conditions of an internal combustion engine for a vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a partial view of a multi-layer steel cylinder head gasket in accordance with an embodiment of the present invention; 
           [0013]      FIG. 2  is a sectional view of the gasket of  FIG. 1 , taken along line  2 - 2 , with the layers separated for clarity; and 
           [0014]      FIG. 3  is a sectional side view of the gasket of  FIG. 1  installed between a cylinder head assembly and a cylinder block assembly. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 1  shows an embodiment of a metal gasket  20  which is a cylinder head gasket. The gasket  20  is positioned between mating surfaces of a cylinder head assembly (not shown) and a cylinder block (not shown) of an internal combustion engine. The gasket  20  includes at least one metal layer  24 . As depicted, only the uppermost metal layer  24  is shown. Each metal layer  24  is defined by a plurality of cylinder apertures  26 , bolt apertures  28 , and jacket apertures  30 . Each jacket aperture  30  may transport a cooling fluid, or a lubricating fluid. The metal layers  24  are arranged such that the apertures  26 ,  28 ,  30  are generally aligned. 
         [0016]      FIG. 2  illustrates gasket  20  to further include a second metal layer  32 . Second metal layer  32  includes a first surface  36 , a second surface  38 , and a cylinder region surface  40 . Second metal layer  32  is also illustrated to include a bead region  46  and a stopper region  48 . As illustrated, second metal layer  32  extends in a radial direction R with bead region  46  and stopper region  48  having portions that extend in an axial direction A. Second metal layer  32  acts as an active layer, as described in greater detail below. 
         [0017]    During installation of the gasket of  FIGS. 1 and 2 , bead region  46  is partially compressed in the axial direction A, thereby causing bead region  46  to foreshorten in the axial direction A and portions of bead region  46  to experience some movement in the radial direction R. During engine operation, relative movement between the mating surfaces in the axial direction A requires portions of bead region  46  to elastically move in the axial direction A in order to properly seal the mating surfaces. This elastic movement in the axial direction A of bead region  46  causes micro-motion of portions of bead region  46  in the radial direction R relative to surfaces in contact with first surface  36  and second surface  38 . When bead region  46  does not have sufficient strength throughout bead region  46 , cracking and fretting may occur. 
         [0018]    In order to improve the desired physical properties of materials used in active layers, such as second metal layer  32 , a theoretical simulation has been performed to identify desirable properties for bead region  46 . Since many physical properties of metals vary as temperatures exceed several hundred degrees F., and some physical properties vary in relation to others, material selection is complicated by a variety of competing factors. 
         [0019]    One of these competing factors is internal stress. In order to improve durability, internal stress should be reduced. However, in order to improve sealing function, internal stress should be increased to correspondingly increase the surface pressure that bead region  46  applies in axial direction A. 
         [0020]    Computer simulations performed by the inventor for optimizing the physical properties of bead region  46  have indicated that a tensile strength (at about 70 degrees F.) in a range of about 1000 MPA (145 ksi) to about 1150 MPA (166 ksi) and a yield strength of at least about 90% of the tensile strength will result in a gasket that is optimized for both durability and sealing function. Preferably, the material will retain about 70% of these respective strengths at gasket operating temperatures. These properties were determined by keeping the internal stress at a minimum value to ensure adequate sealing function. 
         [0021]    Testing was performed on materials with a yield strength that is within 90% of the tensile strength, although these materials have a tensile strength below 1000 MPA. The durability of gaskets produced with these materials was adequate, but the sealing capabilities did not meet the desired test requirements. A material with an increase in both tensile strength and yield strength over the tested materials is expected to produce a gasket with desired durability and sealing capacity. 
         [0022]    Preferably, materials for second metal layer  32  are stainless steels, including those that are austenitic, martensitic, and ferritic, although other materials, such as an Inconel® that maintains these desired properties at operating temperatures, may also be used. 
         [0023]      FIG. 3  depicts the gasket described above located between a cylinder head assembly  52  and a cylinder block assembly  50  of an internal combustion engine for a vehicle. 
         [0024]    While the invention has been described with respect to specific examples including preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.