Abstract:
A method for reducing fuel permeation through gasketed joints in an internal combustion engine ( 10 ) includes surface treating the gaskets in a halogenated medium before placing them into service. A low-cost elastomer such as acrylic, EPDM or HNBR, can be used for the gasket body. The halogenated medium may comprise either a liquid bath or a gaseous bath, into which the gasket is immersed. The halogenated medium bath is excited by heating to accelerate the halogenation process. A gasket treated according to this invention possesses a chemically altered exterior surface which substantially enhances its fuel permeability resistance at relatively low cost.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     NONE. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates generally to a gasket treated to reduce fuel permeation, and more specifically to a polymer gasket that is surface treated with a halogenated solution to substantially enhance the gasket&#39;s resistance to fuel permeability. 
     2. Related Art 
     In internal combustion engines, highly volatile fuel is used for the combustion process. Ever increasing emission regulations motivate engine and automobile manufacturers to minimize the escape of unburnt fuel to the environment. Typically, unburnt fuel escapes by permeating through static gaskets of the type used throughout an engine to seal various bolt-on components. Gaskets in the fuel delivery path are particularly vulnerable to fuel permissivity, but gaskets on the oil side of engines are also at risk. Because fuel mixes with engine oil up to 20% between oil changes, even an oil pan gasket is vulnerable to fuel permeation. 
     Gaskets made from plentiful, low-cost acrylic elastomers are typically used in non-fuel path applications. In order to reduce fuel vapor permeation through fuel path gaskets, the prior art has taught to manufacture those gaskets from fluorinated elastomers. Fluorinated elastomer materials, while effective as a fuel permeation barrier, are typically four times more expensive than the same gasket made from an acrylic elastomer. In other words, because of the high cost differential, gaskets made from a homogenous fluorinated elastomer material have been used mainly in intake manifolds and other fuel path applications. Whereas, the more cost effective acrylic, EPDM or HNBR elastomer gaskets tend to be used for valve covers, oil pan gaskets and the like where small amounts of fuel permeation have been considered tolerable. 
     Because of the heightened concern about the escape of fuel from engines for environmental reasons, there is a need to more completely outfit an internal combustion engine with gaskets resistive to fuel permeation. However, this desire must be offset by the high cost of prior art solutions which manufacture the entire gasket body from fluorinated elastomer. 
     SUMMARY OF THE INVENTION 
     The subject invention overcomes the shortcomings and disadvantages found in prior art gasket systems by providing a method for reducing fuel permeation through gasketed joints in an internal combustion engine. The subject method comprises the steps of providing a first engine component, providing a second engine component adapted for direct connection to the first engine component, and providing a gasket of the type to be compressed between opposing surfaces for the purpose of establishing a fluid-tight interface therebetween. The gasket includes an exterior surface containing hydrogen atoms. The method further includes compressing the gasket between the first engine component and the second engine component to create a fluid-tight sealed interface therebetween. The advantages of this invention are achieved by the step of surface treating the gasket in a halogenated medium prior to the compressing step, whereby halogen atoms in the halogenated medium replace hydrogen atoms in the exterior surface of the gasket thereby altering the chemical composition of the exterior surface and substantially enhancing the fuel permeability resistance of the gasket. 
     Whereas prior art solutions required manufacturing the gasket from a homogenous fluorinated elastomer material, the subject invention treats only the surface of the gasket in a halogenation process. The exterior surface of the gasket is thereby chemically modified, substituting halogen atoms for the original hydrogen atoms and creating a fuel resistant barrier on the exposed surface of the gasket. Accordingly, a substantial cost savings can be realized through the use of standard, lower cost elastomers as the constituent or base material when treated according to this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
         FIG. 1  is a simplified, exploded view of an internal combustion engine depicting an engine block with several components exploded therefrom, each with an associated gasket exposed to unburnt fuel; 
         FIG. 2  is a schematic representation of the subject halogenation process, wherein a gasket is immersed in a liquid bath to create a halogenated surface treatment; 
         FIG. 3  is a schematic view as in  FIG. 2 , but representing an alternative technique of immersing the gasket in a halogenated gaseous bath; and 
         FIG. 4  is an enlarged, illustrative view depicting a gasket treated according to the subject process and placed into service whereby fuel vapor permeation is retarded from left to right. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, an internal engine is generally shown at  10  in an exploded illustration. The engine  10  includes a block  12  to which a number of engine components are affixed, usually via threaded fasteners. For example, an oil pan  14  is affixed to the underside of the block  12 , whereas an intake manifold  16  is indirectly affixed to the block  12  via a cylinder head  18 . A valve cover  20  is also indirectly connected to the engine block  12  by way of the cylinder head  18 . 
     Gaskets are provided for establishing fluid-tight interfaces between attached components. In the case of the oil pan  14 , an oil pan gasket  22  is provided. Here, the oil pan  14  includes a periphery  24  which is adapted for direct connection to the bottom of the engine block  12 . In service, the gasket  22  is compressed between the bottom of the engine block  12  and the periphery  24  of the oil pan  14 , and thereby establishes a fluid-tight sealed interface between these two engine components. Similarly, an intake manifold gasket  26  is compressed between a periphery  28  of the intake manifold  16  and the mating surface on the cylinder head  18 . And again, a valve cover gasket  30  is compressed between a periphery  32  of the valve cover  20  and the opposing surface of the cylinder head  18 . Indeed, other engine components are likewise connected, either directly or indirectly to the engine block  12 , with gaskets inserted therebetween and compressed to perfect tight sealing interfaces. The above-mentioned gaskets are merely representative of all the static gaskets in an engine which may be susceptible to fuel permeation. 
     Because many of these engine gaskets are exposed to volatile fuel, either during the aspiration process or as a contaminant in the engine oil, it is environmentally beneficial to enhance the fuel permeability resistance of many of the engine gaskets, including but not limited to the above-noted gaskets  22 ,  26  and  30 . The subject invention includes the step of surface treating the relevant gaskets in a halogenated medium prior to being placed into service. During the halogenation process, halogen atoms in a halogenated gas or solution replace hydrogen atoms in the exterior surface of the gasket, thereby altering the chemical composition of the gasket&#39;s exterior surface. This halogenation treatment substantially enhances the fuel permeability resistance of the treated gasket. 
     Preferably, the gasket is fabricated from a low-cost elastomer material like acrylic of the type including hydrocarbon compounds. However, other low-cost elastomer compounds such as EPDM or HNBR can also be used instead of acrylic. Indeed, any suitable elastomer that contains the element hydrogen (H) may be used in the context of this invention. If the halogen in the halogenated medium is selected from the element fluorine, for example, three potential reaction mechanisms are expressed by the following equation: 
                                
These reaction mechanisms depict at least three ways in which a halogen atom, fluorine (F) in this example, replaces a hydrogen atom in the material composition of the gasket surface layer. Similar reaction mechanisms for the halogens of bromine (Br) and chlorine (Cl) can be readily predicted by those of skill in the art.
 
       FIG. 2  illustrates a surface treating technique according to this invention wherein a representative gasket  30  is immersed in a tank  36  containing a liquid form of halogenated medium  38 . The liquid solution  38  containing a halogen element may be excited by the application of heat, schematically represented by a burner  40 . While the gasket  34  is immersed in the solution  38 , halogen atoms in the medium are chemically substituted for the hydrogen atoms in the material composition of the gasket  34  according to at least one of the reaction mechanisms described above. The treated gasket  34 ′ is then removed from the tank  36  and dried as suggested illustratively by fan  42 . Of course, other drying techniques may be employed. 
     The manufacturing method described above in connection with  FIG. 2 , wherein the halogenated medium is maintained in liquid form, is perhaps best used when the halogen in the halogenated medium is bromine (Br) or chlorine (Cl). When it is desired to use fluorine (F) as the halogen in the medium, it may be preferred to maintain the medium in gaseous form instead of liquid form.  FIG. 3  illustrates an exemplary manufacturing process wherein a gasket  44  is placed into a chamber  46  into which a gaseous medium containing fluorine  48  is introduced through a supply pipe  50 . Thus, the surface treatment is carried out in a fuming process wherein the fluorine atoms within the gaseous medium are chemically substituted for the hydrogen atoms present in the material composition of the elastomer gasket  44  according to one (or more) of the reaction mechanisms described above. Removed from the chamber  46 , the surface-treated gasket  44 ′ is possessed of an exterior surface which functions as a retarder to fuel permeation. As with the preceding example, the use of a heat source  52  to elevate the temperature of the gaseous halogenated medium  48  inside the chamber  46  is effective to provide an excited atmosphere more conducive to the halogenation process. Of course, other techniques and methods for carrying out the halogenation process according to the subject invention may be possible and applied in appropriate circumstances with equal or ever greater effectiveness than that described herein. 
       FIG. 4  illustrates, conceptually, the subject invention when used in a gasket application where fuel vapors are present. Fuel molecules  60  (i.e., hydrocarbons) are represented in a gaseous or vapor state. Directional arrows  62  indicate the permeation direction, wherein the left side of gasket  56  would be an internal engine zone and the right hand side of gasket  56  would represent the environment. Prior art gaskets made from acrylic elastomers would not create any appreciable barrier to fuel vapor  60  permeation. As shown in  FIG. 4  however, the subject gasket  56  does retard permeation, and in fact blocks a substantial quantity of fuel vapor molecules  60  from passing through to the environment. As shown in this highly illustrative cross section, the treated layer of the gasket  56  extends only to a defined depth below the surface  54 . This is distinguished from prior art techniques where the entire material composition of the gasket would be manufactured from a fluorinated elastomer. Thus, according to the subject invention, the core  58  of the gasket  56  remains generally unaffected by the halogenation process. 
     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly the scope of legal protection afforded this invention can only be determined by studying the following claims.