Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC RADIAL TURBINE WHEEL
Technical Field
The invention relates to a ceramic turbine wheel, expecially a
ceramic radial turbine wheel for an exhaust gas driven turbine of a
turbocharger for motor vehicles, with a body which is formed in one
piece with radial blades and a hub which is connectable to a usually
metallic turbine shaft.
Background Art
With turbochargers for combustion engines in a power range suit-
able for motor vehic1es, the turbine driving the compressor is fed bythe exhaust gas of the combustion engine and to date has usually been
constructed out of metallic alloys having high strength at high
temperatures.
With reference to the increasing use of turbochargers in auto-
mobiles3 an increased high temperature strength and an improvedbehavior o-f accelerat;on is desired. These requirements could be
fulfilled by the use of ceramic materials in the turbocharger
turbine. Ceram;c materials, such as silicon nitride or silicon car-
bide, have a nearly constant high strength in the range of temper-
ature under consideration, and have a density which is only one-third
of that of a metallic material.
Due to the high temperature strength of such materials, the
operating temperatures can be raised without danger, while the mass
moment of inertia, due to the relative low density o-f the ceramic
turbocharger rotor, can be reduced to about 40% oF the moment of
inertia of a metallic type rotor, and therefore, the time of response
of the turbocharger correspondingly improves.
The experiments to develop such turbine wheels out of ceramic
materials have up to now not yet led to the desired success, as there
3~ have resulted diFFIculties in the production process of the wheel, as
well as other problems. Inadmissable defects to the components were
found in the preFerred, low-cost process, in which the radial turbine
wheels, produced out oF the ceramic material, are first oF all
fabricated as so-called green parts by injection molding or slip-
casting techniques and afterwards subjected to a burn-out procedure for
the binder followed by sintering or nitriding procedures. These
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de-fects are mainly attributed to a prevented escape of the gases
produced by the burn-out o~ the binder as well as by a non-uniform
shrinkage of the wheel with the solid body of the hub. A central,
hollow bore, which would be favorable for the production proc-
ess, is however not real;zable for strength reasonsa as by that shapethe tensions resulting in operation would be increased to double the
value of a hub without a bore.
The object of this invention is therefore construct;on of a
ceramic turbine wheel, which, although it has a sufficient high
temperature strength, does not show the above described dif~;culties
in production.
Disc10sure of InYention
This invention, accordingly, provides a ceramic turbine wheel,
comprising:
~a) a body portion, symmetrical about an axis;
(b) a plurality of blades, integral with and extending out-
wardly from the body portion; and
(c) a hub port;on, integral and coaxial with the body
portion, symmetrical about the axis, and connectable with a turbine
shaft;
characterized in that the body port;on is provided with a hollow
central core, on the side of the body portion opposite the hub
portion.
By providing the body portion with a hollow central core, gases
can emerge from the burn-out of the binder and escape out of the body,
so that the forma~ion of f1aws is largely prevented.
In addition, according to this invention, the portion of the
turbine from which the blades extend outwardly, will beco~e more
elastic, so that deformations of the blades can be taken up sooner
without damage.
A radial turbine wheel, d~sic~ned according to this 1nvention, is
producible without large difficu1ties by inexpenslve production proc-
esses 1ike injection molding or slip casting out of ceramic materials,
e.g. silicQn nitride and silicon carbide, wherein particularly the
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befPre mentioned difficulties in the necessary procedures of burn-
out of the binders as well as in sintering and nitriding are
prevented.
Especially with the material alpha silicon carbide, which shows
in the mentioned production processes under high temperatures a
large shrinkage, this design enables production with substantially
reduced difficulties. See U.S. Patents 4,124,667 and 43144,207,
regarding sintering and injection molding o-f alpha silicon carbide.
In addition9 the design of the hub, accorcling to the inven~ion,
results in a further reduction of the mass moment of inertia, already
lowered by the use of ceramic material.
Brief Description of Drawing
In the drawing an example of design o-f the invention is shown,
which will be explained in the following. The drawing shows sche-
matically a presentat;on of a cross section, according to ~heinvention, of a preferred embodiment of a ceramic radial turbine
wheel for the turbocharger o~ a combustion engine for a motor vehicle,
in which the hub portion is cylindr;cal.
Best Mode for Carrying Out the Invention
In.the drawing there is designated by l, the radial turbine
wheel as a whole; by 2, the blades which extend outwardly, usually
radially, from the body portion, and by 3 the body portion, which is
formed in one piece with blades 2 and the hub portion 4. The hub
portion 4 ls prov;ded for connection with a usually metallic, some
times ceramic turbine shaft, which is not shown here.
The blades 2 are described as "radial", i.e. not "axial". In
the turbocharger art, the two main types of blade arrangements are
"radial" and "axial". In the "radial" type, portions of the
blades 2, near the hub portion 4, are located along radTl of the
body portTon 3; and portions o~ the blades 2, further from the hub
port10n 4, are curved gently in the same direction~
AccordTng to the invention, the body portion 3 has on the side
opposite to the hub psrtTon 4 a hollow central core 5, whose outer
diameter, designated with D5, should preferably not exceed 60% of the
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smallest outer diameter, designated with D3mjn~ of the body portion
3; and whose length, designated with L5, should preferably not exceed
60% of the axial blade length, designed by L. It should be noted
tha~ L5 extends only from the base of the hollow central core 5 to
the extremity of the blades 2, and not to the extre~ity of the
hollow central core 5. In other words, at least ~0% of the blade
length L should extend from a non-hollow part of body portion 3.
With such proportions, no noticible increases in tension appear ;n
the body portion 3. In the case of less stressed turbine wheels, the
hollow central core 5, rounded at its bottom, could be placed even
deeper, ;n consideration of expediency.
It is preferred that the minimum outer diameter, D3mjn, of the
body portion 3, be at the side of the body portion 3 opposite to the
hub portion 4, and that the maximum ou-ter diameter9 D3maX, of the
body portion 3, be at the side of the body portion 3 adjacent to the
hub portion 4, of the turbine wheel 1.
For the relative proportions of the outer diameters D3mjn and
D3maX it is preferred that these should amount to about 25% and 50%
respectively, of the outer diameter of the wheel, D. In following
these rules, in view of strength and production requirements, an
optimi~ed turbine wheel can be obtained.
The hub portion 4 preferably has a diameter D4 of about 15 to
25%, more preferably 15 to 20%, of the outer diameter D of the wheel.
According to one preference, the hub portion 4 has a length L4
of, at most, twice its diameter D4. According to another preferenceg
the hub portion 4 is integral with the shaft, not shown. The shaft
can be either a solid or hollow cylinder, made as an extension of
hub portion 4, ranging from about 6 up to about 12 times the
diameter D4 of the hub portion 4.
On the hub port;on 4, as shown in the drawing, there can option-
ally also be provided a hollow central core 6, rounded at its bottom,
whose dlameter D6 amounts to at most up to about 60% oF the diameter
of the hub portion 4 and whose length L6 iS at most 60% of the length
L4 of the hub portion 4.
Finally, at the transition 7 from the hub portion 4 to the body
portion 3, there should preferably be provided a radius of at least
20% oF the diameter of the hub portion 4.
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By adherence to these rules of dimension for the production out
of ceramic materials an optimally designed radial turbine wheel
results, which offers favora~le p~oper~i es regarding to its strength
as well as regarding to its production process.
The embodiment shown in the drawing has a cylindrical hub portion
4. In the case of an alternate con;cal design o~ the hub portion 4,
not shown, the basic diameter measured at about 60% of its length,
measured from its end, corresponds to the diameter D4 of the cylindri-
cal design. The cone angle of this design can amount to between 20
and 30 inclusive, preferably about 25.
Also, this conical hub portion, in consideration of expediency,
can possess a hollow central core, whose diameter is at most 60C~ of
the minimum diameter of the conical hub portion and whose axial
length is at most 60% of ~he length of the hub ~ortion. In acldition,
it is advantageous that the hollow central core, according to this
embodiment of the invention, is rounded at its bottom.
Finally, it is suggested that, as with the cylindrical embodiment,
the transition from the conical hub portion to the body is rounded
with a radius of at least 20% of the cliameter of the hub portion.
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