Abstract:
The invention relates to a heat shield for the shielding of hot areas of components, e.g. of internal combustion engines. The heat shield has an insulating layer ( 3 ) and a metallic sheet layer ( 2 ) that are located next to each other. The insulation contains or consists of hollow fibers ( 10 ) as insulating material. The hollow fibers ( 10 ), at least in sections or over their entire length, have a tubular, hollow, unfilled cross section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a heat shield, in particular of the type used to shield hot areas of components, e.g. of an internal combustion engine, and to a system with a heat shield of this type. 
         [0002]    Heat shields of this type often have a housing that is composed of two metallic sheet layers. These metallic sheet layers are connected with each other at least in some areas on their edges, wherein as an insulating layer an additional layer, namely an intermediate later, is frequently located between the layers of sheet metal. This intermediate layer contains or consists of thermal and/or acoustical insulating material. 
         [0003]    The prior art also includes heat shields in which one or more metallic sheet layers are located on only one side of an insulating layer. For assembly, the thermal and/or acoustical insulating layer is then laid directly on the component to be shielded. For this purpose, however, it is necessary to use sufficiently stable materials for the materials of the insulating layer. 
         [0004]    Insulating materials used for the insulating layer are conventionally particles, for example, such as graphite, mica, expanded graphite and/or fibers such as glass fibers or silicate fibers. Fibers generally have a length that is more than three times their diameter. Both the particles as well as the fibers all have a filled cross section over their entire surface. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    One disadvantage of all these heat shields is that they are heavy and offer only limited insulating power. Relatively thick insulating layers are therefore conventionally used to achieve sufficient insulating power. However, that also increases the space requirement for the heat shield. 
         [0006]    The object of this invention is therefore to make available a heat shield that has improved insulation and in which a specified insulating power can be achieved with a reduced use of material. An additional object of the invention is to make available a system consisting of a component to be shielded and a heat shield of the type described above. 
         [0007]    This object is accomplished by the heat shield described in the claims. 
         [0008]    This invention now makes available a heat shield for the shielding and insulation of hot areas of components. Components of this type include, for example, an internal combustion engine, including its exhaust components such as the exhaust manifold, the exhaust gas treatment unit and similar components, and/or heat exchangers, e.g. to heat motor oil or transmission oil, for auxiliary interior heating and/or for battery conditioning. 
         [0009]    The heat shield according to the invention has an insulating layer that contains or consists of a thermal and/or acoustical insulating material. On one side of the insulating layer is a metallic sheet layer. Of course, a plurality of sheet metal layers or coatings can also be located on this side of the insulating layer. This sheet metal layer, in the assembled state, forms the outside of the heat shield. If one or more sheet metal layers are located only on one side of the insulating layer, the insulation can be located immediately next to the component to be shielded. In that case, an additional sheet metal layer on the second side of the insulating layer is not necessary. 
         [0010]    In this application, it should be noted that the term “layer” means a layer that extends over at least 50% of the respective surface of the heat shield, preferably over at least 70% and particularly preferably over at least 85% of the respective surface area of the heat shield. Flanges that merely grip the insulating layer from one side do not represent their own layer on the other side of the insulating layer. Their purpose is only to hold the insulating layer in position on the one-sided sheet metal layer. 
         [0011]    According to the invention, as the insulating material, hollow fibers are used at least partly, predominantly or even exclusively that have a tubular hollow, unfilled cross section at least in segments or over their entire length. The use of hollow fibers of this type has several decisive advantages compared to the prior art. As a result of the hollow space in the fibers, the insulating power, both in terms of thermal insulation as well as acoustic insulation, is better with an insulating layer of identical thickness than the insulating power of conventional materials. On the other hand, with similar insulating properties, this makes it possible to reduce the amount of material and the thickness of the insulating layer and thus to reduce the space required for the heat shield according to the invention. As a result of the hollow spaces in the fibers, the weight of the component is also reduced. Overall, when the hollow fibers according to the invention are used, the coefficient of thermal conduction of the insulating layer can be reduced from 0.4 W/mK for materials of the prior art to 0.2 W/mK, for example. 
         [0012]    Of course it is not necessary, although it is advantageous, if the predominant portion or all of the hollow fibers have a tubular, hollow, unfilled cross-section over their entire length. However, it is also sufficient if they have a hollow cross-section over only ≧75% of their length, preferably over ≧90% of their length. Interruptions of the hollow form can in particular result from shrunken cavities that are formed during the manufacture of the glass fibers. To compensate for the different rates of thermal expansion of glass material and air in the hollow space, it is preferable if the exposed ends of the hollow fibers are open. 
         [0013]    Hollow fibers, the outside diameter of which is between 5 μm and 30 μm (each inclusive or exclusive of the limit values) have been found to be particularly advantageous. With regard to the inside diameter, it is advantageous if the hollow fibers have an inside diameter that is between 0.2 times the outside diameter and 0.9 times the outside diameter, in particular between 1 μm and 10 μm (each inclusive or exclusive of the limit values). 
         [0014]    The hollow fibers in the insulating layer need not be all identical, but it is sufficient if they are similar or identical in terms of one or more properties. The insulating layer, in the longitudinal plane and/or at a right angle to the longitudinal plane, can also have two or more segments in which the hollow fibers are different in terms of a property. This option makes it possible to have particularly effective insulation in hot spots, i.e. areas in which a particularly large amount of heat is generated, for example by an accumulation of hollow fibers with a particularly large hollow proportion, compared to other areas that are subjected to lower thermal loads. 
         [0015]    It is also possible to arrange fibers with different properties in layers, i.e., for example, to apply a layer of hollow fibers directly to the sheet metal layer, and to cover that layer with a layer of solid fibers. 
         [0016]    However, it is particularly advantageous if at least one property is the same, for example the material, the length and/or the outside diameter of the hollow fibers, the volumetric proportion of the hollow space in the hollow fibers to the volume of the overall hollow fibers and/or the length and/or the diameter of the hollow cross section. 
         [0017]    For this invention it is also harmless if the insulating layer contains sections that contain or consist of solid fibers without hollow spaces or if the insulating layer, over a portion of its surface or over its entire surface, contains a certain proportion of solid fibers without hollow spaces in addition to hollow fibers. 
         [0018]    This invention can be implemented in particular by the use of glass fibers and/or silicate fibers. For example, the fibers can be produced using or from alkali-free aluminum borosilicate glass. The types of glass cited here, e.g. alkali-free aluminum borosilicate glass, can have an outside diameter of 10 to 12 μm with an inside capillary tunnel diameter between 5 and 6 μm. Glass fibers of this type have a higher bending strength than solid fibers, in spite of their approximately 40% lower density. Consequently, the fibers are more elastic and have a higher compression strength as well as higher specific strength, improved thermal and/or acoustic insulating properties and a lower thermal conductivity. The density can be, for example, 1.4 g/cm 3  and the thermal conductivity 0.21 W/mK. If they are suitably connected to one another, the fiber material that is used for the insulating layer, which at least has a defined contour, is also self-supporting with many types of connections. For a particularly flexible configuration of the insulating layer, however, it is preferable if it has a filled material at least in sections, which is held in place, for example, by a layer of fibers connected with one another at the points in question. For this purpose, the self-supporting sections of the insulating layer, in particular sections of the insulating layer solidified with binder, can be in the form of shaped parts that have depressions or protuberances to hold the filled material. The shaping of the sheet metal layer with depressions, for example, can facilitate the realization of embodiments shaped in this manner. 
         [0019]    To improve the stability of the insulating layer, the fibers can be needled, sewn, crocheted, woven or in the form of fleece, for example. Alternatively, they can be at least partly, predominantly or completely connected with one another by means of a binder, to give the insulating layer inherent stabilization. 
         [0020]    The metallic sheet metal layer located on the one side of the insulating layer can have a sheet metal thickness, at least in areas over its entire area, between 0.075 mm and 0.8 mm (both including and excluding limit values). The sheet metal layer can therefore also be metal films, in particular structured, e.g. burled or micro-perforated metal films. 
         [0021]    Optionally, the insulating layer can also contain phase change materials. For this purpose, for example, phase change materials with phase change temperatures between 70 and 300° C., preferably between 80° C. and 200° C., can be used. Phase change materials of this type include, for example, paraffin, salt hydrates or eutectic salt hydrate mixtures, salts or eutectic salt mixtures or similar materials. As a result of the use of phase change materials, the areas of a component next to the heat shield, in particular the areas of a component surrounded by the heat shield or a component completely surrounded by the heat shield, for example, can be heated when an engine is started even more quickly than is possible with the use of the hollow fibers according to the invention. As the result of a rapid heating, for example of an internal combustion engine as the component, harmful emissions in the startup phase of the internal combustion engine are reduced. 
         [0022]    Several examples of heat shields according to the invention and systems according to the invention are described below. In all the accompanying figures, identical or similar reference numbers are used for identical or similar elements, so that in some cases an explanation is not repeated. 
         [0023]    The following examples illustrate heat shields and systems in which a plurality of optional features of this invention are represented together with mandatory features of this invention in a single exemplary embodiment. The optional features, however, can also be implemented individually, together with the mandatory features of this invention, regardless of how they are portrayed in combination with other optional features in the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In the figures: 
           [0025]      FIG. 1  illustrates a heat shield according to the invention; 
           [0026]      FIG. 2  is an exploded view of a system claimed by the invention; 
           [0027]      FIG. 3  illustrates a heat shield according to the invention; 
           [0028]      FIG. 4  is a detail in cross-section of a heat shield according to the invention; 
           [0029]      FIG. 5  is a plan view of an insulating layer claimed by the invention; 
           [0030]      FIGS. 6 and 7  are details on a larger scale in the form of schematic drawings of an insulating material claimed by the invention; 
           [0031]      FIG. 8  is a detail in cross-section of a heat shield according to the invention comprising hollow fibers; 
           [0032]      FIG. 9A  is a cross-section of a heat shield according to the invention; 
           [0033]      FIG. 9B  is another cross-section of a heat shield according to the invention; 
           [0034]      FIGS. 10 and 11  are views of insulating materials according to the invention; and 
           [0035]      FIGS. 12A and 12B  are views of heat shields according to the invention. 
           [0036]      FIG. 13  is a view of a heat shield according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]      FIG. 1  is a plan view of the inside of a heat shield  1  with a sheet metal layer  2  and an insulating layer  3 . The view in  FIG. 1  is a plan view of the inside of the curved heat shield  1  on the exposed surface of the insulating layer  3 . Openings  7  extend through the sheet metal layer  2  that covers the insulating layer  3  on the side away from the viewer and through the insulating layer  3 , which are in the form of screw holes, for example. 
         [0038]      FIG. 2  illustrates a system claimed by the invention with a heat shield claimed by the invention as illustrated in  FIG. 1 , in a schematic view in a combination view with an exhaust gas treatment system as the component  9 , the hot areas of which are to be shielded. In this exploded view, the component  9  is surrounded and covered by two half-shells  3   a  and  3   b  of the insulating material as an insulating layer. Located above this insulating material are two half shells  2   a  and  2   b  of a sheet metal layer, that extend on their sides beyond the edges  6   a  and  6   b  or the facing edges of the insulating layers  3   a  and  3   b . Thus the component  9  is surrounded by the insulating layer  3   a / 3   b  and is simultaneously completely enclosed by the sheet metal layers  2   a  and  2   b.    
         [0039]      FIG. 3  shows the sheet metal layer half-shell  2   b  and the half-shell  3   b  of the insulating material of a heat shield  1  according to the invention as illustrated in  FIG. 2 . In addition, the peripheral areas  18  that protrude outward are visible only in the sheet metal layer half-shell  2   b , by means of which the half shells illustrated in  FIG. 2  are fastened to each other, e.g. by means of flanges or welding. The half-shell  3   b  has various segments  3   b ′,  3   b ″,  3   b ′″,  3   b ″″ etc. that have different orientations relative to one another. Between the segments  3   b ′,  3   b ″,  3   b ′″,  3   b ″″ etc. there are kinks or bends  16 . With loose or relatively loosely solidified hollow fiber material, the insulating layer  3  can be applied relatively easily to these kinks or bends. Likewise, it is possible to manufacture the half-shell  3  in the form of a shaped part, in particular if it is a hollow fiber material solidified by binder. Alternatively, however, it is also possible to cut out excess hollow fiber material or to use the hollow fiber mat material in sections. If the hollow fiber material is composed in sections of shaped parts, it is easily possible to combine different hollow fiber materials with one another. For example, the terminal segments  3   b ′″ and  3   b ″″ can consist of hollow fiber/solid fiber mixtures, while all the segments between them have no solid fibers. It is likewise possible for the segments  3   b ′ and  3   b ″, for example, to have hollow fibers of different sizes. 
         [0040]      FIG. 4  shows a detail of a heat shield according to the invention in the peripheral area  6  of the insulating layer  3  in cross-section. The insulating layer  3  has fibers  10  that are stabilized by means of a binder  14 . Only on one side of the insulating layer  3  is a sheet metal layer  2  provided that extends around the edge  6  of the insulating layer  3  with a flange. 
         [0041]      FIG. 5  shows a detail of a plan view of an insulating layer  3  and its edge  6 . The fibers  10  that are fastened to one another in the manner of a fleece in the central area of the insulating layer  3  are clearly visible and on the edge  6 , individual fibers project beyond the edge  6 . The dark areas thereby represent the areas of the fleece that are needled with each other. 
         [0042]      FIG. 6  is an enlarged detail of a fleece layer according to the invention. This layer has hollow fibers  10  that have a tubular hollow cross-section with hollow spaces  11 . In this example the fibers  10  are woven with one another into a fleece and are tubular and hollow over their entire length, with a ratio of 0.85 between the diameter of the hollow space and the diameter of the total fiber. 
         [0043]      FIG. 7  shows an enlarged detail of another fleece material. In this fleece material, in contrast to the one illustrated in  FIG. 4 , the fibers are different. The ratio between the diameter of the hollow space and the outside diameter of the fiber in the three illustrated fibers  10   a ,  10   b  and  10   c  with the hollow spaces  11 ,  11   b  and  11   c  is different with different diameters and is 0.94 fiber  10   a,  0.55 for fiber  10   b  and 0.33 for fiber  10   c.    
         [0044]      FIG. 8  shows a cross-section through a hollow fiber of the insulating material according to the invention. The hollow fiber  10  has a hollow interior  11 . The inside diameter of the hollow space is marked ID, while the outside diameter of the hollow fiber  10  is marked AD. In this example, the ratio between the inside diameter ID and the outside diameter AD is 0.6. 
         [0045]      FIG. 9  shows, in the sub-figures A and B, two additional heat shields  1  claimed by the invention, each with an outer sheet metal layer that consists of two sub-layers  2   a  and  2   b . These sub-layers are in contact with an insulating material that consists of two sub-layers  3   a  and  3   b . The partial area consisting of sheet metal layer  2   a  and insulating material  3   a  has end surfaces  18   a  and  18   a ′, while the partial area consisting of the sheet metal layer  2   b  and the insulating layer  3   b  has end surfaces  18   b  and  18   b′.    
         [0046]    The sheet metal layers  2   a  and  2   b  are rotationally connected to each other by means of a hinge so that they can be opened and closed relative to each other. In the closed position, the end surfaces  18   a ′ and  18   b ′ come to lie on each other and the end surfaces  18   a  and  18   b  on each other. 
         [0047]    The heat shields  1  illustrated in the sub-figures A and B differ in that both insulating material half-shells  3   a ,  3   b  in  FIG. 9A  are fabricated from identical insulating material, while in  FIG. 9B  the half-shell  3   b  is made of a different insulating material than half-shell  3   a . The hollow fibers from which the half-shell  3   b  is fabricated have a smaller inside diameter ID than those from which the half-shell  3   b  is fabricated. The outside diameter AD and the glass material, on the other hand, are essentially identical. Furthermore, the heat shields of the two sub-figures A and B differ in that the hinge  8  in  FIG. 9A  is filled with an insulating material, while in the heat shield illustrated in  FIG. 9B  the hinge  8  is unfilled. 
         [0048]      FIG. 10  is a plan view of a fiber material that is used for an insulating layer  3  according to the invention. In the figure, for example, the hollow fibers illustrated in the detail are designated  10   a ,  10   b ,  10   c  and  10   d , which differ in terms of their outside diameter, i.e. in terms of one property. However, they are all made of the same glass material.  FIG. 10  also indicates the dimension of the glass hollow fibers with reference to the 100 μm scale. 
         [0049]      FIG. 11  shows a cross section through a detail of the insulating layer  3  which is illustrated in the plan view in  FIG. 10 . Clearly visible are cross sections through hollow fibers  10   a ,  10   b ,  10   c  and  10   d , which have different outside diameters and different inside diameters of their hollow spaces. 
         [0050]      FIG. 12 , in sub-figure A, shows a highly schematic overhead view of the sheet metal layer  2  of a heat shield  1 , which has two pot-shaped depressions  4 ,  4 ′ pointing toward the viewer. The sectional drawing in sub-figure B clearly shows that these pot-shaped depressions  4 ,  4 ′ are used to hold packed beds  12 ,  12 ′ of loose hollow fibers  10 , which are identified as  3 ′ and form a part of the insulating layer  3 . By means of these packed beds, a particularly thick insulating layer is achieved at the places in question, which is appropriate for isolating and shielding what are called hot spots, i.e. areas of especially high heat exposure. The hollow fibers  10  of the packed beds  12 ,  12 ′ can be identical or different in terms of their dimensioning and/or chemical composition and/or density of the packed bed. Likewise, they can be identical or different with reference to their dimensioning and/or chemical composition to the hollow fibers  10  of the needled section  3 ″ of the insulating layer  3  that covers the packed beds  12 ,  12 ′ and thereby stays securely in the depressions  4 ,  4 ′ even before installation. While it is preferable for the packed beds  12 ,  12 ′ to use exclusively hollow fibers—that can optionally have pipes as a result of production conditions—the needled section  3 ″ of the insulating layer  3  can also be a mixture of hollow fibers and solid fibers. 
         [0051]    Finally,  FIG. 13  is a schematic sectional drawing that shows an additional potential configuration for the layer structure of a heat shield  1  with a sheet metal or metal film layer  2  and an insulating layer  3  which in this case consists in sections—in section  3 ″—of hollow fibers  10  that have a fixed contour and in this case are even self-supporting, namely solidified with binder, and has sections  3 ′ that are formed by loose packed beds  12  of hollow fibers  10 . In this case, the sections lie on top of one another essentially perpendicular to the boundary surface  5  between the metal film layer  2  and the insulating layer  3 . The solidified section  3 ″ of the insulating layer  3 , together with the metal film layer  2 , therefore forms a sort of housing for the packed bed  12 . As in the previous example, the solidified hollow fibers  10  of the section  3 ″ can be identical or different to the hollow fibers of section  3 ′ in terms of their dimensioning and/or chemical composition. In turn, it is preferable if the fibers of section  3 ′ consist exclusively of hollow fibers, while for the solidified section  3 ″ both all hollow fibers as well as mixtures of hollow fibers and solid fibers can be used. By way of example, the two edges  6 ,  6 ″ illustrate possible configurations for the outer edge of the insulating layer  3 . While on the left edge  6 , the solidified section  3 ″ is configured so that it forms a lateral flange  15 , by means of which the edge of the packed bed  12  is protected against a detachment of hollow fibers  10 , on the right edge  6 ′ both the packed fiber material of section  3 ′ as well as the solidified section  3 ″ of the insulation project on the right edge  6 ′ as far as the illustrated edge of the metal film layer  2 . The latter is related to reduced manufacturing cost and effort and can be easily used if the relative edge of the heat shield  1 , as shown in  FIGS. 2 and 9 , is connected with another heat shield (section).