Abstract:
An improved heat shield provides thermal insulation and reduced noise transmission of vehicular engine components, including exhaust manifolds. The structure has three layers; an outer structural metal layer, a center insulation layer to isolate heat and dampen noise, and an inner metal layer directly adjacent the shielded component for reflecting heat back to the shielded component. The heat shield is attached by bolts to the shielded component. In the described embodiment, the volume of the insulation layer is expanded by approximately 15 to 20 percent over conventional shields to produce a softer, thicker material having a lower density but unchanged mass. The invention provides a technique to achieve desired thickness and density in insulation layers via modal finite element analysis. The relatively thicker heat shield more effectively absorbs vibration and attenuates noise without increase in mass. In the described embodiment, the layer contains cellulose, diatomaceous earth, talc, and fiberglass.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to protective structures for vehicular engine parts, such as for example engine exhaust manifolds that generate substantial heat and vibration during engine operation. More specifically, the invention relates to the fabrication of protective heat shields applied to such engine parts, and particularly for enhancements of insulation layers employed in such shields for reducing transmission of noise and vibration. 
     2. Description of the Prior Art 
     The exhaust manifolds of internal combustion engines in today&#39;s modern vehicles can reach under-the-hood temperatures in the neighborhood of 1600 degrees Fahrenheit. Such high temperatures create significant risks of damage to electronic components sharing under-the-hood space with the manifolds. Thus, protection has been provided for such components via use of heat shields designed to at least partially cover up and insulate exhaust manifolds and other heat generating components. In some cases, the shields have been effective to reduce measured temperature levels to within a range of 300 degrees Fahrenheit. 
     One recurrent shortcoming with respect to current shield designs, however, has been in their inability to reduce or attenuate noise down to satisfactory levels. Generally, the insulation layer is normally the center layer interposed between two metal layers, is relatively thin, and has a relatively high density that is makes it rather stiff. The insulation layer, while often quite adequate to desirably thwart heat transfer at desired values, has been stubbornly insufficient to dampen noise. Unfortunately, the relatively stiff and thin structures for producing heat shields tend to be prone to producing echoes rather than to absorb vibrations and/or noise. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved insulated heat shield for a variety of heat generating components, such as engine exhaust manifolds of internal combustion engines, engine mounts, and catalytic converters of exhaust systems. In one described embodiment, a heat shield is formed as a unitary structure adapted for securement via bolted connection to an engine manifold, and includes three layers; an outer metal layer to provide overall structural integrity, a center layer formed of a relatively thick insulation material of relatively low density to isolate heat and to dampen noise, and an inner metal layer adjacent the shielded component for reflecting heat back to the shielded component. 
     In the described embodiment, the insulated heat shield includes at least one bolt aperture for attachment of the shield to an under-the-hood shielded component, such as an exhaust manifold; the heat shield is attached by bolts to the shielded component. As disclosed, the volume of the insulation layer is expanded by approximately 15 to 20 percent over conventional insulation materials to produce a softer material having lower density but conventional values of mass. The invention provides that desired values of thickness and density of such layers are determined via modal finite element analysis. The relatively thicker insulation layer of the heat shield is more effective to absorb vibration and to attenuate noise. With no increase in mass, the improved insulation layer is generally no more expensive. In the described embodiment, the insulation layer contains cellulose, diatomaceous earth, talc, and fiberglass. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of one described embodiment of a heat shield of the present invention installed over an exhaust manifold (shown in phantom) of an internal combustion engine (shown fragmentarily). 
     FIG. 2 is a cross-sectional view of the heat shield of FIG. 1 as shown installed over an exhaust manifold in accordance with the present invention, as viewed along lines  2 — 2  of FIG.  1 . 
     FIG. 3 is an elevation view of the heat shield of FIG. 2, shown detached, and constructed in accordance with the present invention. 
     FIG. 4 is a cross-sectional view of a portion of the heat shield of FIG. 3, as viewed along lines  4 — 4  of FIG.  3 . 
     FIG. 5 is a similar cross-sectional view of a portion of a relatively thinner prior art heat shield, displayed for comparative discussion purposes, only. 
     FIG. 6 is an elevation view of another embodiment of a shield, showing various degrees of shading in various areas of the body of the shield to reflect data generated during a modal finite element analysis of the shield. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIGS. 1 and 2, a multi-layered heat shield  10  is adapted to encase or closely surround at least portions of an under-the-hood engine component  30 . In the described embodiment, the component  30  (shown in phantom in FIG. 1) is a heavy-duty cast-iron exhaust manifold ( 30 ). The manifold  30  is bolted via bolts (not shown) to a plurality of engine exhaust ports  52  on the flank or side  54 , of an internal combustion engine  50  (shown fragmentarily). The manifold  30  includes cooperating ports  56  having associated mounting bosses  58  for securement of the manifold  30  to the plurality of engine exhaust ports  52 . 
     The engine exhaust ports  52  operate to collectively receive exhaust gases from individual combustion chambers (not shown) of the engine  50 , and to funnel those exhaust gases into a common exhaust pipe portion  60  (FIG. 1) of the manifold  30 . An exhaust pipe flange (not shown) is integrally provided at an end of the exhaust pipe portion  60  for securement to a separate exhaust pipe (not shown) to facilitate passage of exhaust gases from the engine  50  to the atmosphere. 
     A particular aspect of this invention involves control of vibration and noise attenuation properties of the shield  10 , particularly as related to the means by which the shield  10  is attached to an engine component, such as the manifold  30 . Referring now also to FIG. 3, an enlarged view of the manifold  30  is shown in greater detail. The heat shield  10  is secured to the manifold  30  by bolts  40  that extend through apertures  22  of the shield  10 . 
     For this purpose, the exterior surface  34  of the manifold  30  includes at least two bolt attachment bosses  32  (FIG. 2) that are positioned on and protrude from the exterior surface  34  of the manifold  30 . It will further be noted that the heat shield  10  is displaced away from the surface  34  by the bosses  32  to provide an air space S. Those skilled in the art will appreciate that the air space S is effective to impart an insulating effect in addition to that imparted by the actual construction of the heat shield  10 . 
     Those skilled in the art will also appreciate that vibration (and associated noise) are transmitted from the engine  50  into the manifold  30 . The vibration then travels from the manifold  30  through the bosses  32  (FIG.  2 ), and into the heat shield  10 . The transmittal of vibration is particularly facilitated by the bolts  40  which have a shank portion (not shown) attached to a bolt head portion  44  (FIG.  2 ), secured in a manner such as to rigidly retain the shield  10  between the head  44  and the boss  32  as shown. 
     If not arrested or at least attenuated, those skilled in the art will further appreciate that the vibration will travel through the bosses  32  and bolts  40  and thus into the structure of the shield  10 . 
     Referring now also to FIG. 4, the heat shield  10  has a body consisting of three layers; an external or outer metal layer  12  to provide structural integrity and overall rigidity, a center layer  14  of thermal insulation material to isolate temperature and to dampen vibration and noise, and an inner metal layer  16  adjacent the shielded component for reflecting heat back to the shielded component. The respective layers are sandwiched together to form a unitary body as particularly shown in FIG.  3 . 
     The outer metal layer may be preferably formed of cold rolled steel, aluminized steel, aluminum, and even stainless steel for more exotic vehicles where cost is less of a factor. If cold rolled steel is utilized, the exterior of the shield may be coated with a corrosion-resistant material to enhance longevity of the shield. 
     The inner metal layer  16  is the portion of the shield  10  in closest contact with the exhaust manifold. To the extent that the temperatures of the manifold can reach the 1600 degrees Fahrenheit range, the material of the inner metal layer should be able to withstand significant heat. In some applications the inner layer may be relatively shiny formed of high-temperature alloys, and adapted to reflect heat back to the shielded component. In others, the inner layer  16  can be formed of cheaper materials, including aluminum-clad steel. Those skilled in the art will appreciate choice of materials may be critical for avoiding degradation associated with elevated temperatures and for handling considerable vibrations in particular applications. 
     Although described with three layers, the shield  10  could be effectively manufactured without the outer layer  12  for some lower budget shields. The inner layer  16  would provide the requisite stiffness and support in such cases, and may need to be relatively thicker in some applications. The material choices for the thermally insulating and vibration and noise dampening center layer  14  are fairly broad. Such choices may include non-metallic fibers such as aramid fibers, or ceramic fiber paper. Depending on anticipated temperature ranges, even non-fiber compositions may be employed, such as densified vermiculite powders, for example. 
     One method of manufacturing of the heat shield  10  can be described as follows. Each of the inner and outer metal layers  16 ,  12  are stamped from sheet metal, and formed in a progressive die to the shapes depicted. The insulation layer  14  is then applied against the outer metal layer  12 , and the inner metal layer  16  is placed atop the insulation layer. 
     Ideally, the outer layer  12  will be relatively and slightly oversized compared to inner layer  16 , so that edges (not shown) of the layer  12  may be folded over respective mated edges of the inner metal layer, effectively encapsulating the insulation layer  14  between the metal layers  12  and  16 . 
     For comparative purposes, a heat shield embodiment  10 ′ of the prior art is depicted in FIG.  5 . The heat shield embodiment  10 ′ incorporates an external or outer metal layer  12 ′ to provide structural integrity and overall rigidity, a center layer  14 ′ of thermal insulation material to isolate temperature and to dampen vibration and noise, and an inner metal layer  16 ′ adjacent the shielded component for reflecting heat back to the shielded component, all similar to the heat shield  10 . However, it will be appreciated that the insulation layer  14 ′ is noticeably thinner, although having the same mass as the insulation layer  14 , because the insulation layer  10 ′ has not been expanded in accordance with the apparatus and method of the present invention. 
     Desired values of thickness and density of the insulation layer  10 ′ are determined via modal finite element analysis, a technique described herein that permits a simple trial and error approach to manufacturing what will generally be a relatively thicker insulation layer of the heat shield, and one more effective to absorb vibration and to attenuate noise. The resulting shield with the improved insulation layer will be without any increase in mass, and thus will produce no weight penalty. As such, the shield will generally be no more expensive than prior art heat shields. 
     EXAMPLE 
     One method of manufacturing a heat shield for an under-the-hood vehicular engine component produces a shield of three layers, including an inner metal layer, an outer metal layer, and a non-metallic insulation layer sandwiched therebetween. The inner metal layer adapted to be positioned directly adjacent or proximal the engine component, and the insulation layer is positioned radially outwardly of the inner metal layer. The layers collectively provide thermal insulation of, and reduced noise transmission from, the engine component. The specific method included the following steps: 
     a) establishing relative thickness and density values of an insulation layer by using non-linear modal finite element analysis. (For this step, the heat shield is attached to a test component via fastening bolts. The shield is vibrationally excited to measure and map relative amplitudes of vibration over the entire body of the shield.) 
     b) determining optimal values of the insulation layer thickness and density at a primary critical frequency of the shield as required to optimize level of reduced noise transmission when parameters of size and shape of the shield, and fastening bolt locations of the shield, are fixed. 
     Given that the parameters of shape of the shield  10 , thickness of the shield metal layers  12  and  16 , and location of bolts holes are fixed, the thickness of the insulation becomes the primary undetermined variable. 
     Establishing an operating requirements frequency is an initial objective. This involves identifying and isolating the critical frequency, i.e. the frequency that produces the greatest amounts of vibration over the body of the shield. Several tools are available to aid in this function. For example, Abaqus software manufactured by Hibbett, Karlsson, and Sorensen, of Pawtucket, R.I., was employed to map various levels of vibration produced by excitation of the body of the shield. 
     Referring now to FIG. 6, another embodiment of the heat shield  60  is shown under such vibratory conditions. The darkest regions  62  represent areas of the shield  60  undergoing most significant or greatest vibration generated at the particular excitation frequency. The intermediately grayed regions  64  represent areas of lesser vibration and lighter regions  66  even less vibration, etc. 
     The areas of vibration are less critical the lighter shaded they are. Thus, ideally the particular thickness of the insulation layer  14  should be increased to the point where there are virtually no areas  62 , if possible. In the example presented, after five iterations, and after starting with an insulation layer  14  having a test thickness of 0.9 mm, a heat shield  60  had no areas  62  after the test insulation thickness was increased by 20 percent. Thus, the final thickness of the insulation layer  14  in the example presented was 1.08 mm. The test thickness was based upon a given range of engine operating frequencies, a specific shape of the shield, and the specific location of bolts holes  22 ′. 
     It is to be understood that the above description is intended to be illustrative and not limiting. Many embodiments will be apparent to those of skill in the art upon reading the above description. Therefore, the scope of the invention should be determined, not with reference to the above description, but instead with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.