Thermal insulation lining of ceramic material for a hot gas duct enveloped in metal

A thermal insulation lining (1) in the exhaust duct (4) of a cylinder head (5) consists of a sintered compact (2) of ceramic material having an envelope containing or consisting of inorganic fibers.

The invention relates to a thermal insulation lining of ceramic material 
for a hot gas duct encased in metal, especially in the cylinder head of an 
internal combustion engine, and a method of producing same. 
Thermal insulation linings in the form of hollow sintered compacts of 
ceramic materials and their use in hot gas ducts, such as for example 
exhaust ducts in the cylinder head and exhaust elbows, are known. These 
hollow sintered compacts are encased in metal by the casting of a cylinder 
head, for example, during which the interior of the hollow sintered 
compact can be filled with casting sand so as to prevent it from being 
destroyed by the pressures to which it is subject in casting. 
Prior-art methods of making hollow sintered compacts from ceramics for 
encasement in metal are described in German Pat. Nos. 2,163,717 and 
2,354,254. These proposals call for mixtures of silicon oxide and aluminum 
oxide in the one case, and in the other case, for fireclay, alumina, 
silica, sillimanite, mullite, zirconium, chromite, magnesia clinker, 
silicon carbide, electrocorundum, opaque fused silica, kyanite, magnesia, 
fused spinel, silicon nitride, chrome magnesia, chrome magnesite, 
vermiculite, vermiculite asbestos, heavy spar, burnt diatomaceous earth, 
and pumice, bound by argillaceous earth or aluminum phosphate, for the 
production of the sintered compacts. 
Disadvantages of these proposals are the excessively low insulating power, 
as in the case of silicon carbide and aluminum oxide, for example, while 
sintered compacts made of other materials, such as opaque fused silica, 
for example, do not have adequate strength. 
German Auslegeschrift 2,750,290 calls for an aluminum titanate containing 
50 to 60 wt.% of aluminum oxide, 40 to 45 wt.% of titanium oxide, 2 to 5 
wt.% of kaolin, and 0.1 to 1 wt.% of magnesium silicate, plus material 
characteristics tailored to the application involved. Although aluminum 
titanate has a very low thermal conductivity, this patent has not been 
widely applied. One disadvantage is especially the still excessively low 
strength of aluminum titanate, and the fact that only slight wall 
thicknesses of about 2 to 3 mm can be achieved in the sintered compact, so 
that the insulating effect that can actually be achieved still remains 
unsatisfactory. Particularly in the case of hot gas ducts encased in 
aluminum, great energy losses are involved on account of the poor thermal 
stability of aluminum and the resultant need for intensive cooling. 
It is therefore the object of the present invention to eliminate these 
known disadvantages and to improve the thermal insulating action and the 
resistance to thermal and mechanical stresses of the sintered compacts 
which have been proposed for thermal insulation linings. In particular, 
the invention purports to offer a thermal insulation lining which will 
also be suitable for encasement in aluminum, while at the same time having 
an excellent insulating action and an improved resistance to thermal and 
mechanical stresses. 
Another object of the invention is to use even those ceramic materials 
which of themselves have not found widespread use heretofore on account of 
their insufficient insulating action and/or their low resistance to 
mechanical and thermal stresses in the known structural configurations. 
THE INVENTION 
For the achievement of these objects, the present invention provides for a 
thermal insulation lining of ceramic material for a metal-encased hot gas 
duct, especially in the cylinder head of an internal combustion engine, 
which is characterized by the fact that the thermal insulation lining 
consists of a sintered compact of ceramic material encased in a jacket 
containing inorganic fibers or consisting thereof. 
The present invention makes it possible to produce thermal insulation 
linings of considerably greater wall thickness for hot gas ducts which are 
to be encased in metal, and thereby to achieve improved insulating action. 
The wall thickness of the thermal insulation lining of the invention can 
be as much as three times that of the sintered compacts heretofore 
proposed for this application. The invention thus makes it possible to 
make cylinder heads and their attached exhaust ducts of aluminum instead 
of gray iron, without thereby requiring provisions for greater cooling. 
The thermal insulation lining of the invention, however, is also 
outstandingly suited for encasement in gray iron, especially when the wall 
thickness of the gray iron casing is very great and therefore results in 
great compressive forces as the iron solidifies. The resilient fiber 
envelope serves as a buffer against the compressive forces produced by the 
solidification of the molten iron, and prevents deformation or destruction 
of the thermal insulation lining by compensating for peak tensions 
produced by accumulations of mass, especially in the casting of gray iron, 
but also in the casting of aluminum. 
The thermal insulation lining in accordance with the invention is produced 
by first making a compact from ceramic material in a known manner, e.g., 
by slip casting, and sintering it in the usual manner. Then the compact is 
covered with a jacket containing or consisting of inorganic fibers. For 
this purpose the inorganic fibers are mixed with an organic or inorganic 
binding agent and applied to the sintered compact. Preferably the 
inorganic binding agent is the same ceramic material of which the compact 
to be covered by the fibers is made. To achieve an optimum resistance of 
the thermal insulation lining while molten metal is being cast around it, 
especially molten gray iron, it has been found advantageous to prepare the 
jacket only from those inorganic fibers which have a short-term heat 
resistance up to 1500.degree. C. Mineral fibers have proven outstandingly 
suitable for this purpose, especially fibers of aluminum oxide, zirconium 
oxide, mullite or kaolinite. 
The thickness of the coat is substantially what determines its handling 
characteristics when it is embedded in molten metal, and its thermal 
insulating action. Thicknesses of 1 to 7 mm have proven to be especially 
suitable. 
The following have proven particularly suitable ceramic materials for the 
production of the sintered compact to be covered with inorganic fibers: 
mullite, zirconium oxide, magnesium aluminum silicate (MAS), especially 
cordierite, magnesium aluminum titanate (MAT), aluminum titanate (AT) or 
lithium aluminum silicate (LAS), of which aluminum titanate is 
outstandingly suitable. Mixtures of these ceramic substances are also 
suitable for the preparation of the sintered compact. 
For the binding of the inorganic fibers to the compact, the above-named 
ceramic materials are used, in an especially desirable embodiment, in the 
form of slips. In an especially preferred embodiment of the invention, 
slips of those ceramic materials are used from which the sintered compact 
being enveloped is prepared. In this case a very especially preferred 
embodiment has proven to be a compact of aluminum titante which is 
enveloped in a jacket of inorganic fibers which are fastened to the 
compact with aluminum titanate as the binding agent. 
However, organic binding agents can also be used in the coating process. 
Even though these binding agents are burned out when the coat of inorganic 
fibers has been applied, for the purpose of driving off water or solvent, 
the adhesion of the inorganic fibers to the compact is sufficient for many 
kinds of applications. In this case the coating consists only of inorganic 
fibers. 
An especially preferred method for the production of the thermal insulating 
lining of the invention is to spray inorganic fibers contained in a slip 
consisting of ceramic material, onto the sintered compacts made from 
ceramic material.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In the drawing there is represented an exhaust duct 4 in a fragmentarily 
represented cylinder head 5 of an internal combustion engine. The thermal 
insulation lining 1 of the exhaust duct 4 consists of a sintered compact 2 
of aluminum titanate having a wall thickness of 2 mm, and of an envelope 3 
of kaolinite fibers which are joined to the sintered compact 2 with 
aluminum titanate as binder. The thickness of the envelope 3 amounts to 5 
mm. The density of the envelope amounts to 1.05 grams per cubic 
centimeter. 
It will be understood that the specification and examples are illustrative 
but not limitative of the present invention and that other embodiments 
within the spirit and scope of the invention will suggest themselves to 
those skilled in the art.