Dry insulated parts and method of manufacture

Abstract

An engine component such as a cast iron engine exhaust manifold having an outer irregular-shaped surface is covered with an inorganic heat insulating layer preferably having a thermal conductivity in the range of from about 0.01 BTU/hr/ft/.degree.F to 0.2 BTU/hr/ft/.degree.F. An encapsulating, cast metal layer is formed over the insulating layer so that when hot fluids within the manifold reach temperatures of up to 2,000.degree. F and higher, the temperature at the outer case metal layer does not exceed 450.degree. F.

Parent Case Text

RELATED APPLICATION

This application is a continuation-in-part of applicant's copending patent application Ser. No. 264,199 filed June 19, 1972 now abandoned.

Description

BACKGROUND OF THE INVENTION

Particularly in large engines such as marine diesel engines, the high temperature of the exhaust manifold, located between the engine exhaust gas passage and a supercharger as well as the exhaust piping from the engine supercharger to atmosphere and for non-supercharged engines from the engine to the atmosphere, is known to cause problems. When flammable liquids or gases are present, high temperatures at the external surface of the manifolds can cause a fire or explosion hazard. Moreover, such high temperatures can be a substantial safety hazard since engine operators can be easily burned on accidental contact with the manifolds.

The prior art has sought to alleviate this problem in most cases by providing for various means for cooling the outside of the manifold. Such means often include the use of a fluid jacket to pass a cooling fluid around the manifold and thus reduce surface temperatures. However, the use of cooling fluids adds to the space and weight requirements of power packages due to the accessory pumps, radiators and fluid supply means which also add to cost. When manifolds and conduits are liquid cooled as between the engine exhaust gas passageway such as the cylinder head and the supercharger, hot exhaust gases passed to the supercharger are reduced in temperature thus causing some drop in the energy available to the supercharger thus reducing the power available to supply the engine with air for combustion resulting in lower engine power output per unit of engine displacement. When engine exhaust gas is passed through a reactor located in the engine exhaust manifold or piping as part of an emission reduction system, the gas temperature entering the reactor influences the performance of the reactor. Higher temperatures are desirable and the absence of a cooling medium in the manifold provides such higher temperatures. Engine power reduction can be substantial, for example as much as 2% or more when temperatures in the manifold are reduced to 850.degree. F from a normal temperature of about 1,100.degree. F. The power loss can be reduced by merely leaving the manifold uninsulated and uncooled but, here again, heat dissipation through the exposed iron or steel surfaces of the manifold causes a temperature loss and in addition, the safety problem is substantially multiplied.

It is not a problem to merely insulate the manifolds as by the use of organic or inorganic insulating layers. However, such layers often become degraded by moisture or contaminated with hydrocarbons normally present around an operating engine unless they are protected. Such hydrocarbon gases and liquids tend to degrade the insulation reducing its resistance to the flow of heat and can cause fire hazards. Other means of encapsulating such as fabricating covers by formed sheet metal have proven costly and unsatisfactory in protecting the thermal insulation from mechanical failures in handling of the sheet metal and manifold casting in service, and in preventing permeation and degrading by hydrocarbons as well as other liquids and gases.

SUMMARY OF THE INVENTION

It is an important object of this invention to provide a dry, heat insulated engine part comprising a conduit for hot fluid flow, which engine part is efficiently heat insulated with minimized cost and complexity.

Another object of this invention is to provide an engine part having a complex form in accordance with the preceding object which can be manufactured without complicated procedures and in a highly efficient manner with good reliability and with outstanding safety features.

Still another object of this invention is to provide a method of forming a dry, heat insulated engine part in accordance with the preceding objects.

Additional objects of the invention are to provide external metal encapsulation totally enclosing the thermal insulation to prevent deterioration due to permeation of the thermal insulation by engine room air borne contaminants and by contaminants accidently spilled on the insulation.

According to the invention a dry, heat insulated metallic engine part has a conduit therethrough allowing hot fluid flow as for example in an engine exhaust manifold. The part defines an outer irregularly-shaped surface with an inorganic insulating layer overlying the surface and having a thermal conductivity preferably in the range of from about 0.01 BTU/hr/ft/.degree.F to 0.2 BTU/hr/ft/.degree.F. A cast metallic, encapsulating layer conforms to the shape of the insulating layer and covers the insulating layer preferably encapsulating it. The engine part is designed to have a surface temperature at the surface of the encapsulating layer of below about 450.degree. F at fluid temperatures within the conduit of from 500.degree. F to at least 2,000.degree. F when the engine part is operated in normal atmospheric air environments with air temperatures from 60.degree. to 150.degree. F.

In the preferred embodiment, the engine part is a cast iron or steel engine exhaust manifold or conduit having a conventionally irregular shape to suit the internal fluid flow and external geometric constraints imposed by the over-all engine configuration. The insulation conforms to the outer irregular configuration of the manifold as does an overlying cast metallic encapsulating layer preferably having a thickness of one-sixteenth to one-half inch which provides mechanical protection and preferably hermetically encapsulates the insulation through sealing of the outer metallic layer to the engine part.

According to the method of this invention, an irregularly-shaped member subject to operating temperatures in the range of from 500.degree. F to at least 2,000.degree. F, is insulated by forming an inorganic self-supporting insulating layer over a surface of the member with the insulating layer defining an outer irregular configuration. A thin mechanically strong impervious metal casing is then formed over the insulating layer and sealed to the member to encapsulate the insulation by direct seals or through the use of heat flow restricting joining means. It is important that the method of this invention provide a cast outer metallic layer over an irregularly shaped member which outer layer can be maintained at a temperature below 450.degree. F regardless of the temperature of the underlying engine part. In the preferred method, a transient coating such as wax is placed over the insulation layer, a mold conforming to the so insulated engine part is formed using the engine part as a form, the transient layer is removed and the outer metallic layer cast directly in the mold replacing the transient layer over the insulation layer and engine part preferably at temperatures high enough to form required hermetic seals with the engine part thus encapsulating the insulation.

In some cases the outer metallic layer can be cast first and sealed to the metallic engine part with an insulation layer later formed to fill the space between the metallic part and the cast layer.

It is a feature of this invention that engine efficiency can be substantially increased by maintaining the heat within a manifold rather than dissipating any substantial portion of the heat to an outside environment.

Another feature of the invention is its anti-pollution aspects when used to maintain heat in a flowing gas which gas is later processed in a reactor more efficiently since the heat is maintained to the reactor inlet.

Still another feature of the invention relates to the safety increase permitted by encapsulation of the insulating layer, cooling of the outer surface and prevention of moisture or hydrocarbon contamination of an insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, objects and advantages of the present invention will be better understood from the following specification when read in conjunction with the accompanying drawings in which:

FIG. 1 is a top perspective view of an insulated exhaust manifold in accordance with a preferred embodiment of this invention;

FIG. 2 is a bottom view thereof;

FIG. 3 is a cross sectional view thereof taken through line 3--3 of FIG. 2;

FIG. 4 is a top perspective view of an element thereof;

FIG. 5 is a diagrammatic showing of the manifold connected between a supercharger and an engine;

FIG. 6 is a cross sectional view through line 6--6 of FIG. 2 showing a modified seal construction; and

FIG. 7 is a top view of the manifold before insulation is applied.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference now to the drawings and more particularly FIGS. 1 and 5, a preferred embodiment of an engine part in the form of a heat insulated engine exhaust manifold 10 is shown for mounting between an engine block 11 and an engine supercharger 12 as conventionally found in a marine diesel engine.

The manifold 10 comprises a conventional single or alloyed iron or steel fabricated or cast part 13 (FIG. 7) in a substantially conventional form having engine block rectangular mounting flanges 14, 15 and 16 preferably provided with bolt holes 17 and bolt receiving notches 18 to allow the manifold to be attached to an engine block. The manifold 10 receives hot fluid in the form of combustion gases passing through inlet openings 20, 21, and 22 converging to a center passageway extending substantially from one closed end to the other closed end of the manifold and from thence through an outlet integral passageway opening at 23. Opening 23 carries an outwardly extending flange 24 with suitable bolt holes 24' for attachment to an engine supercharger. Hot gases from the engine cylinders pass in the direction of arrows 25 out of the opening 23 as known in the art.

The cast iron component 13 of the manifold 10 is of a conventional irregular configuration. Overlying the outer configuration is an insulating layer 31 formed of an inorganic material preferably having a thermal conductivity in the range of from 0.01 BTU/hr/ft/.degree.F to 0.2 BTU/hr/ft/.degree.F. The thickness of the insulation layer can be whatever is required to balance the heat from the cast iron component of the manifold portions which reaches substantially the temperature of the hot inside fluids or gases to an outside manifold temperature, at the surface of a skin casing 40, to maintain the surface at a level below 450.degree. F and preferably lower for safety reasons. Insulation thicknesses of from 0.1 inch to 1.5 inches are preferred. Inorganic insulating materials such as Fiberfrax, a trademarked product of Carborundum Corporation of Niagara Falls, N.Y., and comprising an aluminum silicate, Min-K, a trademarked product of Union Carbide Corporation of New York, N.Y., an aluminum oxide powder, foamed ceramic, sand, refractory oxides, mixtures thereof and the like can be used for the insulating material. It is necessary that the insulating material not be degraded at the temperatures normally encountered in engine operation or encountered in subsequent casting processes of the outer metallic layer as will be described.

The insulating layer is preferably formed with a substantially uniform thickness and preferably conforms to the outer configuration of the cast iron component 13 of the manifold. However, in some cases, it may be desirable to form the insulation layer with nonuniform thicknesses and/or with an outer configuration different than that of the outer configuration of the cast iron component. For example, thicker sections of insulation can be used at local points of high heat flux as at the turn in the passageway illustrated in FIG. 3.

Preferably the insulating layer is directly adjacent the outer surface of the cast iron component 13 with no voids present. Over the insulating layer 31 is a thin metallic, outer skin casing layer 40 which is cast and which acts to provide mechanical protection and encapsulation to the insulating layer. The cast layer 40 is preferably formed of a good heat dissipating material such as aluminum, aluminum alloys which melt at low enough temperatures for casting and are non-destructive to the insulating layer. It is preferred to use aluminum or magnesium or alloys thereof having a thermal conductivity of at least about 50 BTU/hr/ft/.degree. F. These materials serve to minimize local hot spots because of the high thermal conductivity of the layer 40. Materials such as carbon or alloy steel, cast iron and the like can also be used for the casing 40. Thin coatings are preferred, preferably in the range of from one-sixteenth to one-half inch. The layer 40 preferably has a substantially uniform thickness and entirely surrounds and conforms to the outer configuration of the insulating layer being bound thereto at least because of the encircling nature of the layer to the irregular configuration of the manifold.

In addition to the above considerations for selection of the metal material of the outer skin layer 40, the material is preferably designed to have a suitable coefficient of thermal expansion to the coefficient of thermal expansion of the component 13 over the expected operating temperature range of the component 13 and skin 40 so that the actual physical differential in expansion between points on component 13 and between corresponding overlying points on skin 40 is as close to equal as possible at operating temperatures. Thus ideally for example, if the manifold 10 is to be operated at a temperature of substantially 1,000.degree. F at the component 13 and a resulting temperature of 400.degree. F at the skin 40 the thermal expansion of part 13 at 1,000.degree. F is preferably equal to the expansion of the skin 40 at 400.degree. F. Although exact matching of differential expansion at the two different operating temperatures is difficult to obtain, careful selection of materials can result in only small acceptable differences. For example, when the component 13 is formed of a ferritic material such as cast iron, ordinary carbon steel or a series 400 alloy steel having a thermal coefficient of expansion of from 6 .times. 10.sup..sup.-6 in/in/.degree.F to 8 .times. 10.sup..sup.-6 in/in/.degree.F the skin material is preferably selected to have a thermal coefficient of expansion of from 10 .times. 10.sup..sup.-6 in/in/.degree.F to 12 .times. 10.sup..sup.-6 in/in/.degree.F and thus could be a cast aluminum or an aluminum alloy. In this case, the physical expansion of the skin is closely parallel to the physical expansion of the inner ferritic part. Assuming both the skin 40 and component 13 are both at 80.degree. F and are then heated to their operating temperatures as described above, the differential expansion from point to point for the inner component 13 and the outer skin 40 would be determined by:

.DELTA.L.sub.1.sub.-2 = L[(.DELTA.T.sub.1) (E.sub.1) - (.DELTA.T.sub.2) (E.sub.2) ]

where

.DELTA.L.sub.1.sub.-2 = differential in thermal expansion

L = linear distance between points over which thermal expansion takes place

E = coefficient of thermal expansion for material

.DELTA.T = difference between starting temperature and operating temperature

subscript 1 refers to the inner component 13 and 2 refers to the encapsulating skin material.

In the case described above with 1,100.degree. F exhaust gas from an engine the inner part 13 metal temperature would be approximately 1,000.degree. F and the typical length for an exhaust manifold would be 30 inches long, the inner part is iron and the outer encapsulating skin material is aluminum so that:

.DELTA.L.sub.1.sub.-2 = 30 [(1,000-80) (6 .times. 10.sup..sup.-6) - (400-80) (11 .times. 10.sup..sup.-6)]

differential expansion = .DELTA.L.sub.1.sub.-2 = 0.06 in

This difference in thermal expansion would be absorbed mechanically by the system. It is preferred that .DELTA.L.sub.1.sub.-2 that is the differential expansion be maintained in the range of from 0 to 0.10 inch. However, the value can vary depending on the specific sizes of parts used. If the differences in expansion characteristics are not matched, particularly with long length or large diameter manifolds, they can cause destruction of the seals used or physical rupture of the skin or other parts of the device. In smaller manifolds, this feature is of lesser significance since over-all physical expansion of parts is small.

Heat dissipating fins 30 are preferably integrally formed in the layer 40. The fins improve natural convection to a surrounding fluid environment such as air, to dissipate the small amount of heat passing to the layer 40 through the insulation. The fins 30 have uniform fin heights of three-fourth inch with widths of one-fourth inch and a fin to fin spacing of 1 inch although the dimensions can vary as desired and in some cases the fins can be eliminated.

In the case of an exhaust manifold, the joint between the outer metallic layer 40 and the inner manifold 13 occurs at the flanges which connect the manifold to the engine block and at the flange 24 which connects the manifold to the turbo supercharger. Since the engine block is liquid cooled, the manifold connecting flanges are only slightly hotter than the engine block to which they are connected. Thus, at this location, there is not sufficient heat flow from the manifold to the outer metallic layer to cause the external temperature to exceed the objective of keeping it below 450.degree. F. At the other end, this is not the case. The supercharger is not liquid cooled and the connecting end of the manifold part 13 is at a high temperature. If it were directly connected to the outer metallic layer, a large heat leak would occur causing the outer metallic layer to exceed the desired temperature. In cases such as this, a flexible thin walled metallic seal is provided to restrict the flow of heat from the manifold to the outer metallic layer.

The metallic layer 40 is preferably hermetically sealed to the cast iron or stainless steel component of the manifold during the casting operation as by joints shown at 50 in FIG. 6 where the mounting flanges are overturned as shown, or by joints as shown in FIG. 3 directly to part 13 adjacent the flanges. Such joints are formed by contact of the molten outer layer with the part 13 during the casting process as will be described. Such joints are preferably made at the lower temperature or inlet end of the manifold where there is no great heat dissipation in operation, from the cast iron section of the manifold to the outer skin due to the small area of contact and the cooler operating temperature of this end of the manifold. At the outlet end 23 of the manifold, where temperature of the manifold is highly elevated, it is preferred to seal the outer metallic layer 40 to the inner cast iron or stainless steel component 13 through a heat restricting seal ring such as 51. The seal ring 51 is a convoluted circumferentially extending enclosing collar with its inner cylindrical surface 52 bonded to the cast iron component 13 of the manifold as by welding or brazing and its outer cylindrical surface 53 sealed directly to the skin or outer metallic layer 40 during casting procedure. The ring 51 is preferably formed of a low thermal conductivity metal in a thin convoluted encircling shape. For example, the ring can be formed of titanium or sheet steel having a thickness of from 0.01 to 0.03 inch to restrict heat flow. The ring is preferably resilient and can flex to allow for a small differential in expansion between the encapsulating layer 40 and the component 13 without destruction of the seal.

Because of the metallic mechanical properties of the outer skin 40, it provides mechanical protection to the insulating layer. In addition, it can be sealed as described above to prevent moisture, hydrocarbons or other fluids from contaminating the insulating layer as could otherwise be the case during normal operation of the engine. By proper selection of materials and design of layer thicknesses, it is possible to obtain any reasonable desired temperature at the outer layer 40. For example, when the gas flow in the manifold reaches temperatures of 900.degree. F, by the use of the construction noted, the outer skin temperature can easily be maintained below 350.degree. F.

In the preferred method of this invention, the manifold cast iron component 13 is 24 inches from end to end having an outside diameter of 5 inches with a roughly uniform wall thickness of one-fourth inch. Component 13 is coated with an inorganic insulating layer such as Fiberfrax as by applying it in conventional castable form containing a binder. The cast insulation is coated with a ceramic paint or other sealer known in the art to seal the porous insulation preventing permeation by the later applied molten metal.

The insulated part is then coated with a layer of a transient material such as a hydrocarbon wax having a thickness of one-fourth inch corresponding to the thickness desired for the outer metallic layer to be cast thereover. Where fins are to be formed, they can be carved in the wax. The so coated insulated part is then used as a form and a conventional sand casting mold is formed thereover conforming to the outer surface of the wax layer.

The temperature is raised to melt the wax and it runs out of the casting mold. Preferably the temperature of the part 13 and the mold is heated approximately to the melting temperature of aluminum and molten aluminum then poured into the mold to form the cast layer 40 bonded to the ring collar 51 and seals 50 during the casting operation. The mold is then cooled at ambient temperatures and the completed manifold 10 removed.

The above preferred method is basically the "lost wax" procedure for forming castings as well-known in the art. Application of this procedure to the specific materials and components noted has significant advantages as pointed out above. Costs are lowered and mechanically strong, advantageous results are obtained. However, the cast coating 40 can be formed by several other methods. For example, the insulation can be formed around the part 13 and a metal outer coating provided by dipping in molten metal to build up the desired thickness of metallic outer layer 40. In still another procedure, a wooden pattern representing the external shape of the outer layer 40 is formed and a split mold made of it. The part 13 is then coated with insulation and placed in the mold so formed. The outer layer 40 is then cast around the insulation. This method avoids the chance of contamination by wax. In all of these methods, the use of the temperature of the molten metallic layer aids in degassing the insulation and assuring removal of contaminants from the insulation.

In still another method, a pattern can be made for the exterior shape of the outer layer 40. A core is then made representing the shape of the part 13 with its insulated coating. The aluminum or other metallic layer 40 is then cast using the core in the mold formed from the pattern. The mold is preferably a two-piece mold allowing the layer 40 to be formed as a two-piece jacket which can be opened and resealed around the part 13. The part 13 can then be left with an air gap in place of the insulating layer or alternatively, filled with a powdered insulation after sealing of the layer 40 to the part 13. Preferably the space is evacuated of moisture and other contaminants prior to filling through a fill opening and then the opening is sealed to form the final product.

While a specific embodiment of this invention has been shown and described, it should be understood that many variations are possible. For example, the engine part which is insulated by a dry insulating material need not be a manifold but could be other parts such as cylinder heads, liners, piping, casings and the like used in marine engines, diesel engines, gas turbine engines, steam generators and the like wherever the presence of unburned hydrocarbons, moisture and the spillage of fuel and lubricating oil could tend to contaminate thermal insulation not protected by a sealing capsule.

The seals formed are preferably formed as at the collar or ring 51 and seals 50. Such seals when formed during the casting operation unite parts 13 and 40 forming tight seals which restrict the flow therethrough to a value small enough so that there is no problem with whatever normally would pass through the seal. For example, if normal capillary action forces at one atmosphere pressure differential keep out contaminants, moisture and fluids, such a seal is sufficient even though the seal may not be a hermetic seal.

While the manifold part 13 is preferably cast iron or steel as conventionally used, other metallic parts can be insulated in accordance with this invention. The configurations of these parts are normally irregular in that geometrical, cylindrical, square or the like shapes are not present but irregular shapes are present in accordance with required engine specifications and the like. Thus, to form outer layers 40 by machining, sheet metal forming and conventional fabricating techniques, would involve high expense. However, the casting procedure of this invention enables low cost formation of the required parts with the required encapsulated insulation.

Claims

I claim:

1. A cast metallic engine part

said part defining an outer irregularly shaped surface and being formed of an iron-containing metal defining a fluid carrying conduit therein uninterrupted by fluid seals,

an inorganic dry insulation layer overlying said outer surface and having a thermal conductivity in the range of from about 0.01 BTU/hr/ft/.degree.F to 0.2 BTU/hr/ft/.degree.F, and a cast metallic, protective encapsulating layer conforming to the shape of and covering said insulating layer,

said cast encapsulating layer being formed of a metal having a high thermal conductivity and selected from the group consisting essentially of aluminum, magnesium and alloys of these metals,

said encapsulating layer having a surface temperature below about 450.degree. F at fluid temperatures within said part of from 500.degree. F to at least 2,000.degree. F,

said cast metallic encapsulating layer having a thermal coefficient of expansion matched to the thermal coefficient of expansion of said part so that over-all expansion of said part and metallic encapsulating layer are closely parallel to each other over the operating temperature range of said part.

2. An engine part in accordance with claim 1 wherein said part is in the form of an engine exhaust manifold interconnected with an engine exhaust gas passage and a supercharger,

said manifold having an inlet and an outlet,

said cast metallic encapsulating layer being sealed to said conduit at said inlet and outlet.

3. An engine part in accordance with claim 1 wherein said insulating layer thickness is from about 0.1 to 1.5 inch and said metallic encapsulating layer has a thickness of from about one-sixth to one-half inch.

4. An engine part in accordance with claim 1 wherein said cast metallic encapsulating layer has a thermal coefficient of expansion of from about 10 .times. 10.sup..sup.-6 to about 12 .times. 10.sup..sup.-6 in/in/.degree.F and said part has a thermal coefficient of expansion of from about 6 .times. 10.sup..sup.-6 to about 8 .times. 10.sup..sup.-6 in/in/.degree.F.

5. An engine part in accordance with claim 3 wherein said cast metallic layer carries outwardly extending heat dissipating fins.

6. An engine part in accordance with claim 5 wherein said cast layer comprises material selected from the group consisting essentially of aluminum and magnesium.

7. An engine part in accordance with claim 3 wherein said one seal is formed by a convoluted metallic collar providing a long thin heat restricting path.