Patent Publication Number: US-7722828-B2

Title: Catalytic fibrous exhaust system and method for catalyzing an exhaust gas

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 10/833,298, filed Apr. 28, 2004, and entitled “Nonwoven Composites and Related Products and Processes”, which is a continuation-in-part of U.S. patent application Ser. No. 10/281,179, filed Oct. 28, 2002, and entitled “Ceramic Exhaust Filter”, now U.S. Pat. No. 6,946,013, issued Sep. 20, 2005, both of which are incorporated herein as if set forth in their entirety. 
     BACKGROUND 
     1. Field 
     The present invention relates generally to a catalytic device for cleaning and thermally managing a contaminated fluid, and more particularly to a catalytic device for use on a vehicle exhaust system. 
     2. Description of Related Art 
     Exhaust systems perform several functions for a modern engine. For example, the exhaust system is expected to manage heat, reduce pollutants, control noise, and sometimes filter particulate matter. Generally, these individual functions are performed by separate and distinct components. Take, for example, the exhaust system of a typical small gasoline engine. The small engine exhaust system may use a set of heat exchangers or external baffles to capture and dissipate heat and/or heat shields to protect the vehicle and/or the operator from excessive heat. A separate muffler may be coupled to the exhaust outlet to control noise, while a catalytic converter assembly may be placed in the exhaust path to reduce non-particulate pollutants. Although particulates may not generally be a concern in the small gasoline engine, some applications may benefit from the use of a separate particulate filter. Due to space limitations, costs, and engine performance issues, it is not always possible to include separate devices to perform all the desired functions, thereby resulting in an exhaust system that is undesirably hot, polluting, or noisy. 
     Known exhaust systems are often arranged with catalytic devices to support non-particulate emission control. Due to the physical size and reactivity requirements for these devices, their placement options are quite limited. Each device that must be placed adds additional design time, cost, and consumes valuable and limited space in the product. As emission requirements tighten, it is likely that more catalytic effect will be required, as well as further particulate control. In general, there has been a trend to place catalytic converters closer to the engine manifold in order to improve the transfer of heat to the catalysts and to decrease the time it takes for the catalysts to reach the operating or ‘light off’ temperature. However, it is not always possible to find a safe and effective placement for catalytic devices. Further, it is desirable and efficient for a for the amount of heat conveyed into the catalytic converter or a thermoelectric generator from the exhaust gas to be maximized and the waste heat lost to the surroundings to be minimized. Moreover, in the case of a typical catalytic converter, once they have begun, the catalytic reactions taking place are exothermic and can thus excessively heat the outside of the catalytic device housing assembly if not insulated properly. Such heating may pose human risk, such as burning the operator&#39;s hands or legs, as well as harm to the surrounding environment, if, for example, the heat causes dry grass to catch fire. These engines, such as small diesel or gasoline internal combustion engines (ICEs), are often found on motorcycles, lawn equipment, and recreational vehicles. Unfortunately, these small engines have generally not been able to benefit from catalytic technologies. In many applications, there is a need for a flexible, yet highly effective method to catalyze and remove the harmful emissions without producing excessive heat generation and transfer to the surrounding structure an/or environment. The ability to reduce noise pollution, as well as prevent injuries or harm due to excess heat is also desirable. 
     Known catalytic systems do not effectively operate until a threshold operational temperature is reached. During this “light-off” period, substantial particulate and non-particulate pollution is emitted into the atmosphere. Accordingly, it is often desirable to place a catalytic device close to the engine manifold, where exhaust gasses are hottest. In this way, the catalyst may more quickly reach its operational temperature. However, design or safety constraints may limit placement of the catalytic converter to a position spaced away from the manifold. In such a case, known exhaust systems have provided insulation on the inside of the pipe leading from the manifold to the catalytic converter. Again, similar constrains apply to the use of other devices that rely on engine heat for their operation, such as thermoelectric generation and electric power production. This insulation is used to direct heat from the manifold to the catalytic converter, where the converter may more quickly reach operational temperature. Additionally, if the insulated pipe is positioned where there is risk of human contact, the insulation may aid in keeping the exterior surface of the pipe cooler, thus reducing the risk of burn. 
     One known exhaust pipe insulator uses insulating materials, such as beads, between two layers of metallic tubes to reduce the exterior temperature of the exhaust pipe. The inner metal pipe is used to conduct heat away from its source. Another known insulator system uses a particulate based lining on the exhaust manifold to achieve some degree of thermal insulation and noise attenuation, with fiber mats filling the void spaces and providing cushioning. However, particulate-based systems are relatively non-porous, have limited less surface area, and are not very effective thermal insulators. Still another known insulation system places a particulate-based insulation liner on the exhaust manifold. Yet another known insulator system uses metal fibers in manifold-based noise abatement system for small engines. This system has higher backpressures and the metal fibers have relatively low melting point. Moreover, the metal fibers are incompatible with most catalyst materials and, since they are typically better thermal conductors, they do not provide as much thermal insulation as do the ceramic systems. Yet another insulation system incorporates a coated metallic mesh- or screen-type catalytic device; however, this device is characterized by a relatively low conversion efficiency; stacking multiple screens increases the effective conversion but likewise increases backpressure on the engine. In addition, the system offers little in the area of heat and/or noise insulation. Although these known insulated exhaust systems may offer some assistance in reducing light-off times and improving exhaust gas remediation, increasingly stringent emission standards demand further reductions in light-off time and the addition of known insulation systems alone is simply not enough to provide the requisite emissions reductions. Further, even when using these known insulators, a typical vehicle exhaust system sometimes still has to have both a pre-cat and an under-mount cat, the additions of which consume valuable space; moreover, these converters must be positioned to avoid heat hazards such as risk of burn injuries. In the case of small engines, space limitations are extremely constraining, and catalytic devices with high conversion efficiencies are much needed. Thus, there remains a need for a means of decreasing light off time, reducing noise, decreasing exhaust system surface temperature, and/or otherwise reducing pollutant emissions that does not add substantial size and weight to the exhaust system. The present invention addresses this need. 
     SUMMARY 
     Briefly, the present invention provides an engine system with a conduit portion for directing the flow of a contaminated or ‘dirty’ fluid from the engine. The conduit portion defines an inner surface and an outer surface. A substantially fibrous porous nonwoven refractory layer is connected to the inner surface of the conduit portion, wherein the substantially fibrous porous nonwoven refractory layer is characterized by a substantially low thermal conductivity and a substantially high surface area. 
     In a more specific example, an engine exhaust system conduit is provided, including a generally cylindrical outer portion and a generally cylindrical inner portion. The inner portion is disposed within the outer portion to define a generally cylindrical fluid-flow path. The generally cylindrical inner portion further includes a substantially fibrous porous nonwoven refractory monolith and a catalyst material at least partially coating the monolith. 
     Advantageously, the flow of exhaust gas may be directed from the engine through an exhaust gas pathway extending between the engine and the atmosphere. The passageway may include a manifold portion fluidically connected to an engine, a muffler and/or catalytic converter and/or thermoelectric generator portion fluidically connected to the atmosphere, a conduit portion fluidically connected between the manifold portion and the muffler and/or catalytic converter and/or thermoelectric generator portion, and/or a plurality of baffles operationally connected within the muffler. A substantially fibrous porous nonwoven refractory material at least partially coats the exhaust gas pathway, wherein exhaust gas from the engine flowing through the exhaust gas pathway to the atmosphere flows over the substantially fibrous porous nonwoven material. The substantially fibrous porous nonwoven material may further be at least partially coated with washcoat and/or catalyst for converting exhaust stream pollutants into non-pollutant gasses. In general, the substantially fibrous porous nonwoven material forms the inner coating of a fluid-flow pathway such that the fluid is able to interact with the substantially fibrous porous nonwoven material and also interact with any chemically active, reactive or catalytic material present on the surface of the fibers. While the specific examples recited herein relate primarily to internal combustion engines, it will be apparent to practitioners in the art that the described methods and apparati may likewise be applied to any system where a conduit is formed to transfer fluids from one location to the other, where reactions take place to convert certain species present in the flowing fluid, and/or where the management of heat, fluid-flow, fluid-dynamics and interaction between fluid and the substantially fibrous porous nonwoven material is advantageous for reaction and/or insulation. 
     These and other features of the present invention will become apparent from a reading of the following description, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The drawings constitute a part of this specification and include exemplary embodiments of the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. 
         FIG. 1  is a cross-sectional view of a manifold, pipe, and muffler in accordance with the present invention. 
         FIG. 2A  is a cross-sectional view of an exhaust system conduit component of  FIG. 1   
         FIG. 2B  is a side-sectional view of  FIG. 2B . 
         FIG. 2C  is a perspective view of  FIG. 2A . 
         FIG. 2D  is a perspective view of  FIG. 2C  with an adhesive layer between the conduit and fibrous insert layer. 
         FIG. 2E  is a schematic view of  FIG. 2C  showing the outer tube being wrapped around the ceramic inner core. 
         FIG. 3A  is a cross-sectional view of an exhaust system component in accordance with the present invention. 
         FIG. 3B  is an enlarged perspective view of a portion of  FIG. 2A  showing the fibers in greater detail. 
         FIG. 3C  is an illustration of a portion of  FIG. 2A  in greater detail. 
         FIG. 4  is a cross-sectional view of an exhaust system component in accordance with a second embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of an exhaust system component in accordance with a third embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of an exhaust system component in accordance with a third embodiment of the present invention. 
         FIG. 7  is a cross-sectional view of an exhaust system component in accordance with a fourth embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of an exhaust system conduit component supporting a catalytic converter device within in accordance with the present invention. 
         FIG. 9  is a cross-sectional view of an exhaust system component in accordance with a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed descriptions of examples of the invention are provided herein. It is to be understood, however, that the present invention may be exemplified in various forms. Therefore, the specific details disclosed herein are not to be interpreted as limiting, but rather as a representative basis for teaching one skilled in the art how to employ the present invention in virtually any detailed system, structure, or manner. 
     The drawing figures herein illustrate and refer to an exhaust system pathway  10  that is specifically described as a component of an internal combustion engine  12  exhaust system. However, it should be appreciated that exhaust pathway  10  may be used on other types of fluid flow systems. For example, the fluid-flow system may be utilized for heat insulation or catalytic conversion for the petrochemical, biomedical, chemical processing, painting shops, laundromat, industrial exhaust, hot-gas filtration, power generation plant, or commercial kitchen applications. 
     Heat is conducted in a body via three different and distinct mechanisms, conduction, convection and radiation. Conduction in a solid, a liquid, or a gas is the movement of heat through a material by the transfer of kinetic energy between atoms or molecules. Convection in a gas or a liquid arises from the bulk movement of fluid caused by the tendency for hot areas to rise due to their lower density. Radiation is the dissemination of electromagnetic energy from a source and is the only mechanism not requiring any intervening medium; in fact, radiation occurs most efficiently through a vacuum. Generally, all three mechanisms work simultaneously, combining to produce the overall heat transfer effect. The thermal conductivity of a material is a physical property that describes its ability to transfer heat. In order to maximize insulation, the insulator is desired to be capable of reducing all modes of heat transfer. The system  5  described herein includes the ability to provide insulation, and hence more effective transfer of heat to the location where it can be utilized usefully, such as in catalytic conversion. 
     A catalytic device or converter here refers to a solid structure having catalytic activity. The solid structure may be enclosed in a housing, i.e. a metal can or a metal tube, or another attachment. In general, a catalytic device consists of a host or a structural substrate support, and a catalyst that coats the support. The device may include other components, such as washcoats, modifiers, surface enhancing agents, stabilizers, and the like. A catalytic device contains the appropriate type and mass of support and catalyst so that it can fulfill a precise catalytic function. For example, it may perform a conversion function. The conversion can be of gases into other gaseous products, liquids into other liquids, liquids into gaseous products, gasses into liquid products, solids into liquids, solids into gaseous products or any combination of these specific conversions. In all cases, the conversion reaction or reactions are deliberate and well-defined in the context of a particular application, e.g. the simultaneous conversion of NOx, HC, CO (such as occurs in 3-way converters), conversion of CO to CO2, conversion of reactive organic component in soot particulates to CO2, conversion of MTBE to CO2 and steam, soot to CO2 and steam, etc. 
       FIGS. 1-3  illustrate a first embodiment of the present invention, an exhaust system  5  with an exhaust gas apparatus or pathway  10  extending between an engine  12  and the atmosphere with a substantially fibrous porous nonwoven refractory material layer  14  at least partially coating the exhaust gas pathway  10 . As shown in  FIG. 1 , the pathway  10  is typically made up of exhaust system elements such as a manifold portion  20  fluidically connected to the engine  12 , a muffler portion  22  fluidically connected to the atmosphere, and a conduit portion  24  fluidically connected between the manifold portion  20  and the muffler portion  22 . The muffler portion  22  may further include one or a plurality of baffles  26  operationally connected therein. Such a pathway  10  might typically be found in an automobile exhaust system. 
     The respective portions  20 ,  22 ,  24 ,  26  of the exhaust gas pathway are typically made of metal, such as iron, stainless steel, aluminum, tin, alloy or the like and thus exhibit “metallic” thermal conductivity behavior. In other words, the metallic components  20 ,  22 ,  24 ,  26  are good conductors of heat. The substantially fibrous porous nonwoven refractory material layer  14 , in contrast, is typically made of a fibrous refractory material that is more typically mostly or completely composed of ceramic fibers. Thus, the substantially fibrous porous nonwoven refractory material layer  14  has a relatively low thermal conductivity (although it may have a relatively high heat capacity) and functions as an insulator to prevent heat from escaping through the respective portions  20 ,  22 ,  24 ,  26  of the exhaust gas pathway and instead be retained in the system  5  to more quickly raise the temperature of the catalyst located on the substantially fibrous porous nonwoven refractory material layer  14  or further downstream on another catalytic converter device. Alternately, the exhaust pathway components  20 ,  22 ,  24 ,  26  may be made of non-metallic structural materials, such as ceramics, ceramic composites, plastics or the like. These materials may have relatively high or low thermal conductivities. In either case, the substantially fibrous porous nonwoven refractory material layer portion  14  still functions as a thermal insulator to redirect heat away from the pathway  10  and to the catalyst. Further, the insulating effects of the substantially fibrous porous nonwoven refractory material layer  14  may make it possible to make the components  20 ,  22 ,  24 ,  26  out of materials having lower thermal conductivities and/or lower melting points than otherwise possible, thus broadening the field of materials possible for the construction of the exhaust pathway  10 . The substantially fibrous porous nonwoven refractory material layer  14  typically prevents a substantial amount of reactive exhaust gas condensates and components from reaching the surfaces of components  20 ,  22 ,  24 ,  26  defining the exhaust pathway  10 , hence reducing the likelihood of failure due to chemical stress on the shell materials. 
     Referring to  FIGS. 2A-2D , an exhaust system conduit portion  24  is shown with a substantially fibrous porous nonwoven refractory material layer portion  14  connected therein. Typically, both the exhaust system conduit portion  24  and the substantially fibrous porous nonwoven refractory material layer portion  14  are generally cylindrical. The substantially fibrous porous nonwoven refractory material layer portion  14  may be deposited onto the interior of the conduit  24  by such familiar processing techniques as dipping, spraying, casting, or extrusion thereinto. Alternately, the substantially fibrous porous nonwoven refractory material layer portion  14  may be separately formed and inserted into the conduit portion  24 . In this case, the outer diameter of the (relaxed) substantially fibrous porous nonwoven refractory material layer portion  14  is substantially equal to or slightly greater than the inner diameter of the exhaust system conduit portion  24 . The substantially fibrous porous nonwoven refractory material layer portion  14  may be held in place in the conduit portion  24  by frictional forces (such a substantially fibrous porous nonwoven refractory material cylinder  14  is illustrated in  FIG. 2C ) such as via an interference fit. Alternately, the substantially fibrous porous nonwoven refractory material layer portion  14  may be held in place in the conduit portion  24  by an adhesive or cementitious layer  30  disposed therebetween (see  FIG. 2D ). Still alternately, the substantially fibrous porous nonwoven refractory material layer portion  14  may be wrapped in a piece of sheet metal that is then welded  25  or otherwise fastened in place to define a conduit portion  24  (see  FIG. 2E ). 
     Regardless of the forming and application techniques selected the substantially fibrous porous nonwoven refractory material layer  14  is typically made of a matrix of tangled (non-woven) refractory fibers  32 . The fibers are typically chopped to a relatively short length and more typically have diameter to length aspect ratios of between about 1:3 to about 1:500. Typical fiber diameters range from about 1.5 to about 15 microns and greater. Typical fiber lengths range from several microns to about 1-2 centimeters. More typically, a bimodal or multimodal distribution of fiber aspect rations is used to enhance the strength of the substantially fibrous porous nonwoven refractory material layer portion  14 . For example, the aspect ratios may peak at about 1:10 and about 1:100. In other words, the layer portion  14  may be made of fibers having a bimodal aspect ratio, with a first mean at a first predetermined aspect ratio, and a second mean at a second predetermined aspect ratio. 
     As shown in  FIG. 3B , the fibers  32  are typically refractory, are more typically metal, metal oxide, metal carbide and/or metal nitride, and are still more typically made of one or more of the following materials: alumina, silica, mullite, alumina-silica, aluminoborosilicate, mixtures of alumina and silica, alumina enhanced thermal barrier (“AETB”) material (made from aluminoborosilicate fibers, silica fibers, and alumina fibers), zirconia, aluminum titanate, titania, yttrium aluminum garnet (YAG), aluminoborosilicate, alumina-zirconia, alumina-silica-zirconia, magnesium silicate, magnesium aluminosilicate, sodium zirconia phosphate, silicon carbide, silicon nitride, iron-chromium alloys, iron-nickel alloys, stainless steel, mixtures of the same, and the like. For example, fibers  32  made from components of AETB are attractive since AETB composite has a high melting point, low heat conductance, low coefficient of thermal expansion, the ability to withstand substantial thermal and vibrational shock, low density, and very high porosity and permeability. Alternately, the substantially fibrous porous nonwoven refractory material  14  comprises ceramic fibers  32  having amorphous, vitreous, vitreous-crystalline, crystalline, metallic, toughened unipiece fibrous insulation (TUFI) and/or reaction cured glass (RCG) coatings. Still alternately, the substantially fibrous porous nonwoven refractory material  14  comprises Fibrous Refractory Ceramic Insulation (FRCI) material. The refractory fibers  32  may be amorphous, vitreous, partially crystalline, crystalline or poly crystalline. The substantially fibrous porous nonwoven refractory material  14  may also include non-fibrous materials (in addition to catalysts) added as binders or other compositional modifiers. These include non-fibrous materials added as clays, whiskers, ceramic powders, colloidal and gel materials, vitreous materials, ceramic precursors, and the like. During the forming (typically firing) process, some of the non-fibrous additives bond to the fibers  32  and effectively become fibrous; others remain non-fibrous. Some of the coatings may be placed on the substantially fibrous porous refractory material post-firing in the form of vapor depositions, solutions or slurries. 
     Example substantially fibrous porous nonwoven refractory material  14  compositions include: (1) 70% silica-28% alumina-2% boria; (2) 80% mullite; 20% bentonite; (3) 90% mullite, 10% kaolinite; (4) 100% aluminoborosilicate; (5) AETB composition; (6) 90% aluminosilicate, 10% silica; (7) 80% mullite fiber, 20% mullite whisker precursors (i.e., alumina and silica). All compositions are expressed in weight percents. The compositions may be present as combinations of individual fibers (i.e., composition (2) may include four alumina fibers  32  for every silica fiber  32 ) or as homogeneous fibers  32  (i.e., composition  1  may be homogenous fibers  32  of an aluminoborosilicate composition) or as a mixture of fibers and non-fibrous materials such as clays, whiskers, ceramic powders, colloidal ceramics, very high surface area materials (aerogels, fumed silica, microtherm insulation, etc), glass, opacifiers, rigidifiers, pore-modifiers, and the like. 
     The fibers  32  form a porous matrix and are typically sintered or otherwise bonded together at their intersections. The substantially fibrous porous nonwoven refractory material layer  14  is typically at least about 60% porous, is more typically at least about 80% porous, and is still more typically at least about 90% porous. Alternately, the substantially fibrous porous nonwoven refractory material layer  14  may be formed with a porosity gradient, such that the substantially fibrous porous nonwoven refractory material layer  14  is more porous (or less porous) adjacent the respective pathway component(s)  20 ,  22 ,  24 ,  26  and less porous (or more porous) away from the respective pathway component(s)  20 ,  22 ,  24 ,  26  (i.e., adjacent the flowing exhaust gas stream). (See  FIG. 3A ). Likewise, the substantially fibrous porous nonwoven refractory material layer  14  may have a uniform and typically low density or, alternately, may have a density gradient such that it is denser adjacent the respective pathway component(s)  20 ,  22 ,  24 ,  26  and less dense away from the respective pathway component(s)  20 ,  22 ,  24 ,  26 . This may be accomplished by varying the density and porosity of a single fibrous porous nonwoven refractory material layer  14  composition, or, alternately, by forming a fibrous porous nonwoven refractory material layer  14  from a plurality of sublayers  34 , wherein each sublayer  34  is characterized by fibers of different size, aspect ratio and/or density (see  FIG. 3C ) or by applying a densifying coating such as aluminosilicate glass (typically with alkaline or alkaline earth fluxes), borosilicate glass, yttria-alumina-silicate glass, aluminaborosilicate glass, clay suspensions, ceramic suspensions, ceramic powders and precursors with foaming agents (such as azodicarbamides), whiskers, or the like. 
     Typically, the substantially fibrous porous nonwoven refractory material  14  is selected such that its coefficient of thermal expansion (CTE) is similar to that of the pathway component  20 ,  22 ,  24 ,  26  material to which it is to be connected. This CTE matching is desirable but not critical, since the substantially fibrous porous nonwoven refractory material  14  is fibrous and highly porous, such that there is some ‘give’ built into the material  14 . In other words, compressive forces will first cause the material  14  to deform and not crack or fail. 
     In one embodiment, the system  5  minimizes conductive heat transfer from the typically relatively hot inner surface  33  to the typically cooler outer surface  35  of the substantially fibrous porous nonwoven refractory material layer  14  through the establishment of a porosity and thermal mass gradient in the layer  14 . In this embodiment, porosity is defined by substantially closed cell structures. The porosity increases from the inner surface  33  to the outer surface  35  while the thermal mass likewise decreases, yielding an increase in the concentration of closed cells approaching the outer surface  35 . The resulting reduction in the number of paths for heat conduction (generally via molecular vibrational energy transfer) thus reduces heat transfer to the outside surface  35  and the conduit portion  24 . Alternately, the porosity may be defined by substantially open cell structures and may be made to decrease from the inner surface  33  to the outer surface  35 , yielding an decrease in the concentration of open cells and, thus, convection paths as the outer surface  35  is approached. The resulting reduction in gas flow to the outer surface  35 , and thus convective/convection-like heat transfer opportunities, thus reduces heat transfer to the outside surface  35  and the conduit portion  24 . 
     In another embodiment, convective heat transfer through the system  5 ′ from the relatively hot inner surface  33 ′ to the relatively cold outer surface  35 ′ of the substantially fibrous porous nonwoven refractory material layer  14 ′ is minimized by the application of a semi-permeable layer  37 ′ on the inside surface  33 ′. (See  FIG. 4 ). The semi-permeable layer  37 ′ is typically vitreous, such as a glass matrix layer. The semi-permeable layer  37 ′ typically forms a fiber reinforced glass ceramic matrix composite that retards the penetration of gases into the insulation layer  14 ′, and hence reduces heat transfer to the outside surface  35 ′ and thus prevents excessive heating of the conduit portion  24 ′. 
     In still another embodiment, a suspension or slurry of crushed borosilicate glass is sprayed onto the inner surface  33 ″. (See  FIG. 5 ). Typically, the crushed glass contains about 6 percent boron content and the particles are on the order of about 1 micron across. Typically, the suspension or slurry may contain about 70% borosilicate glass frit (such as 7930 thirst glass frit available from Corning glassworks), about 30% MoSi 2 , and 2 or 3% SiB 6  in a liquid medium, such as ethanol, with the MoSi 2  and SiB 6  additives present to enhance emissivity. The slurry is sprayed onto the inside surface  33 ″ to form a coating about 2500 microns thick. The liquid medium is evaporated to yield a layer of powdered materials embedded into the fibrous matrix  14 ″. The fibrous matrix  14 ″ is then heated sufficiently to yield a semi-permeable fiber-reinforced glass ceramic matrix composite layer  37 ″ thereupon. Typically, heating to 2250 degrees Fahrenheit for about 2 hours is sufficient to form the layer  37 ″. The permeability of the coating  37 ″ may be controlled by adjusting the concentration of the slurry constituents, the thickness of the coating, and the firing time/temperature. Alternately, a suspension or slurry of other high temperature glass frits, crushed to finely grained powder, or ceramic precursors clays may be sprayed onto the inner surface  33 ″ to reduce porosity, increase strength and rigidity, enhance durability and to form closed pores. 
     In yet another embodiment, radiative heat transfer from the hot inner surface  33 ″′ to the cold outer surface  35 ″′ is minimized by the addition of thermally stable opacifiers  39 ″′ into the substantially fibrous porous nonwoven refractory material layer  14 ″′. (See  FIG. 6 ). The particle size distribution of the opacificers  39 ″′ is typically controlled to optimize the distribution thereof throughout the layer  14 ″′ and/or surface coating  37 ″′. Typically, the opacifiers  39 ″′ are metal oxides, carbides or the like. The particle diameter is typically sized to be about the same as the wavelength of the incident radiation. The opacifier particles  39 ″′ operate to scatter infrared radiation and thus retard transmission. Addition of opacifiers  39 ″′ such as SiC, SiB4, SiB6 and the like into the substantially fibrous porous nonwoven refractory material layer  14 ″′ increase the emissivity of the substantially fibrous porous nonwoven refractory material and of any surface coating  37 ″′ that may be present. Addition of about 2% SiC in the substantially fibrous porous nonwoven refractory material  14 ″′ increases its emissivity to about 0.7. 
     In the above embodiments, some of the pores, such as the pores on the top surface of the substantially fibrous porous nonwoven material, may be closed or filled by the impregnation or inclusion of non-porous material introduced by means of slurries composed including powders, glass, glass-ceramic, ceramics, ceramic precursors, ceramic foams, colloidals, clays, nano-clays or the like suspended therein. Upon heat treatment, such materials enable the formation of partially or fully closed pores in the surface layers, similar to the closed cell porosity commonly observed in dense ceramics or ceramic foams. The closed pore structure prevents hot fluid from flowing therethrough and thus reduces the amount of heat transferred via convection. The entrapped air also serves as a relatively efficient thermal insulator. The closing of the pores can also be achieved by such alternative methods as, casting, impregnation, infiltration, chemical vapor deposition, chemical vapor infiltration, physical vapor deposition, physical adsorption, chemical adsorption and the like. 
     Referring back to  FIG. 3B , the fibrous porous nonwoven refractory material layer  14  typically includes a catalyst material  36  at least partially coated thereon, typically coating at least portions of the individual fibers  32 . The catalyst material  36  is typically chosen from the noble metals, such as platinum, palladium, and rhodium (either alone or as alloys or combinations), and oxides thereof, but may also be selected from chromium, nickel, rhenium, ruthenium, cerium, titanium, silver, osmium, iridium, vanadium, gold, binary oxides of palladium and rate earth metals, transition metals and/or oxides thereof, rare-earth metal oxides (including, for example, Sm 4 PdO 7 , Nd 4 PdO 7 , Pr 4 PdO 7 , La4 PdO 7  and the like), and the like. The catalyst is typically a material that lowers the potential barrier for a chemical reaction, such as the conversion of a pollutant species to a to nonpollutant species (i.e., helping the reaction to occur faster and/or at lower temperatures). In general, a catalyst may be used to more readily convert one species to another species at a lower temperature or at a faster rate. Since different catalysts  36  require different threshold temperatures to begin to function, the fibrous porous nonwoven refractory material layer  14  may include more than one catalyst material  36  coated thereupon (either in discrete regions or intermixed with one and other). For example, the fibrous porous nonwoven refractory material layer  14  may include a first catalyst material  36  that begins to function at a first, relatively low temperature and a second catalyst material  36  that activates at a second, higher temperature. The second material  36  may be added because it is cheaper, more chemically and/or thermally stable, has a higher top end temperature for catalyst function, and/or is a more efficient catalyst  36 . Additionally multiple catalysts may also be utilized to assist in catalytic reactions of different species. Typically, a washcoat layer  38 , such as alumina, ceria, zirconia, titania or the like, is provided between the fibers  32  and the catalyst material  36  to promote adhesion and to increase the overall surface area available for chemical reactions. Both the layer  14  thickness and degree of catalyst  36  coating on the fibers  32  may be increased and/or decreased to tailor the temperature (i.e., the degree of thermal insulation provided) and catalytic activity (catalyst  36  is expensive, and thus it is desirable to not use more than is necessary for a given exhaust gas environment) of the exhaust system. The system  5  allows catalytic benefits coincident with temperature management to increase vehicle/equipment safety (by lowering exhaust system outer temperature), shorten light-off time, utilize otherwise wasted heat, and the like while simultaneously decreasing pollution emissions. The system  5  may be used in tandem with conventional and pre-existing pollution control methodology, or may be used alone to address pollution emissions from heretofore uncontrolled sources, such as lawn mowers. As there are fewer components in the exhaust pathway  10 , the complexity of the typical vehicular exhaust system may be reduced while the weight thereof is decreased; backpressure and cost may both be simultaneously reduced as well. 
     In operation, exhaust gas from the engine  12  typically flows through the exhaust gas pathway  10  to the atmosphere and also flows through the substantially fibrous porous nonwoven refractory material layer  14  positioned therein. Baffles  26  operate to make the gas flow more turbulent, as a tortuous flow path, along with high catalyst surface area, serves to increase catalytic efficiency of the system  5 . Since the fibrous nonwoven refractory material layer  14  is typically substantially porous, the diffusion forces urge the exhaust gas into the pores  40  of the substantially fibrous porous nonwoven refractory material layer  14 . The fibrous nonwoven refractory material layer  14  is typically thick enough to provide substantial thermal insulation to the pathway  10 , but not so thick so as to significantly impeded the flow of exhaust fluids from the engine  12  to the atmosphere and thus contribute to an unacceptable build-up of back pressure. Typically, the fibrous nonwoven refractory material layer  14  is between about 1 and about 3 centimeters thick, although the thickness may vary with exhaust system size, positioning in the pathway  10 , and the like. For instance, it may be desirable for the fibrous nonwoven refractory material layer  14  to be thicker adjacent portions of the pathway  10  more prone to operator contact (such as near the foot plate on a motorcycle exhaust system  5 ) to prevent burn injuries. Alternately, the fibrous nonwoven refractory material layer  14  may be made thinner near the engine  12 , such as in the manifold portion  20 , such that the catalyst material  36  thereon reaches light-off temperature sooner, thus beginning to convert pollutants to non-pollutants sooner. 
     Typically, the exhaust gas does not penetrate completely into the substantially fibrous porous nonwoven refractory material layer  14 , since the diffusion forces are relatively weak as compared to the pressure differential between the engine and the atmosphere that urges the exhaust gas along and out of the pathway  10  and into the atmosphere. The substantially fibrous porous nonwoven refractory material layer  14  also tends to become denser and less porous moving from its inner surface (adjacent the exhaust gas) to its outer surface (adjacent the manifold  20 , muffler  22 , conduit  24 , etc . . . portions of the exhaust gas pathway  10 ), further retarding the penetration of gas therethrough. 
     The exhaust gas transfers heat into the substantially fibrous porous nonwoven refractory material layer  14 , which tends to quickly raise the temperature of (at least the inner surface of) the layer  14  until it is in equilibrium with the exhaust gas temperature, since the substantially fibrous porous nonwoven refractory material layer  14  typically has a low thermal conductivity value and, more typically, a low thermal mass. If a catalyst  36  material is present thereon, its temperature is likewise quickly increased into its operating range, whereupon the catalyst material  36  begins to convert pollutants in the exhaust gas into relatively harmless nonpollutant gasses. 
     The system  5  may be used with any source of pollutant fluids, such as gasoline and diesel engines, including those in automobiles, motorcycles, lawn mowers, recreational equipment, power tools, chemical plants, power-generators, power-generation plants, and the like, to further reduce pollution emissions therefrom. Further, the system  5  provides an additional function of trapping particulate emissions in fibrous nonwoven refractory material layer  14  for later burnout or removal. The system may be present in the form of a ceramic insert  14  into an existing exhaust system  24  component (see  FIG. 2C ), an add-on internally coated  14  pipe  24  having couplings or connectors  42  operationally connected at one or both ends (see  FIG. 7 ), as a replacement segment or portion (i.e., conduit  24 , muffler  22 , etc . . . ) to an existing exhaust system having an inner insulator layer  14  for treating exhaust gasses, or as an exhaust system  5  as originally installed. 
     Referring more particularly to  FIG. 7 , a replacement conduit portion  24 A is provided with regards to aftermarket modification of pre-existing exhaust systems. The replacement conduit portion  24 A includes an inner fibrous nonwoven refractory material layer  14  attached thereto or formed therein and terminates at either end in a connector fitting  42 . In use, the replacement conduit portion  24 A is connected to an existing exhaust system by cutting into the exhaust system and removing a portion thereof of about the same length as the replacement conduit portion  24 A. The two thus-formed newly-cut exposed ends of the exhaust system are connected to the respective connector fittings  42 , such as by welding, to replace the cut out and removed original portion of the exhaust system with the replacement portion  24 A. Exhaust gasses flowing through the replacement portion  24 A will, at least in part, flow through the fibrous nonwoven refractory material layer  14  and thus at least some of the particulate matter therein will be filtered out. Further, if the fibrous nonwoven refractory material layer  14  supports catalyst material  36  on the fibers, certain exhaust gas species may be catalytically converted into other, more desirable species. 
     The system  5  is typically used in conjunction with other pollution reduction systems (such as in automobiles) to further reduce pollutant emissions, but may also be used alone where space is at a premium (such as in lawn mowers, hand-held motor-powered equipment, or the like). 
     The insulation layer  14  thus accomplishes two functions that, on the surface, may appear different and somewhat opposing, namely quickly heating the catalyst material  36  in (both in the insulation layer  14 , if present and in a separate catalytic converter device  46  that may be positioned in the system) and keeping the outer surface of the exhaust pathway  10  cool. (See  FIG. 8 ). First, the inside surfaces of the insulation layer  14  (i.e., the surface that interfaces with exhaust gas) capture heat to raise the temperature of the catalyst material  36  residing on the fibers  32  to quickly reach an operational temperature. These inside regions are therefore relatively less porous, with smaller pore-sizes and a high surface area contributed by exposed fibers  32 . The regions approaching and adjacent the outside wall  10  prevent or retard the flow of heat therethrough, and thus are typically relatively more porous with larger pore sizes to trap dead air. The large amount of trapped, noncirculating air near the wall  10  thus provides good thermal insulation. In some cases, the use of large sized pore-formers (such as organic particulates with sizes greater than 50 micron and, more typically, between about 100-200 microns) will result in a pore structure that roughly resembles a foam. In such cases, a substantially fibrous refractory foam-like body is formed having air is entrapped to provide a higher degree of thermal insulation. Heat is prevented from leaving the exhaust system  5  through the pathway  10  is thus present to raise the temperature of the catalyst  36  and eventually is eliminated from the system  5  via heated exhaust gasses escaping into the atmosphere. 
     The insulation layer  14  may be formed through a variety of means. For example, the substantially fibrous porous nonwoven refractory material layer  14  may be disposed upon a exhaust gas pathway surface  10  through such ceramic processing techniques as extrusion, molding, coating, spraying, tape casting, sol-gel application, vacuum forming, or the like. Alternately, the substantially fibrous porous nonwoven refractory material  14  may be applied on flat metal and then roll into a pipe  24 . Still alternately, the inner fibrous layer  14  may be cast and then the external housing  10  formed therearound. Yet alternately, the inner fibrous layer  14  may be formed as a tube for insertion into an existing external exhaust pathway  10  portion, such as a pipe  24 . 
     Likewise, the layer  14  may be formed to varying degrees of thickness. For example, the layer  14  may be formed as a thick, porous membrane. Alternately, the layer  14  may be made sufficiently thick so as to have more significant sound and thermal insulative properties. (See  FIG. 9 ). In this illustration, the exhaust system  5  is connected to a motorcycle. A thicker insulating layer  14 A is positioned within the conduit portion  24  of the exhaust system  5  proximate a foot rest, such that the foot rest  61  (and, presumably, a rider&#39;s foot) will benefit from the lower conduit temperatures provided by the increased thermal insulation. A thinner layer  14 B is provided elsewhere within the system  5 . Additionally, the layer  14  may be formed relatively thickly on baffles  26  to improve catalytic efficiency and noise attenuation (see  FIG. 1 ). 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the invention are desired to be protected.