Patent Application
20100209256
1. Field of the Invention
The invention relates to an element for attachment of a compressor wheel to a turboshaft of an exhaust-gas turbocharger that produces a magnetic field, having a base body that holds a magnet, which rotates with the turboshaft.
2. Prior Art
The power produced by an internal combustion engine depends on the mass of air and the amount of fuel supplied to the internal combustion engine. To increase the power, it is generally necessary to supply more combustion air and fuel to the internal combustion engine. In the case of a naturally-aspirated engine, this power increase is achieved by enlarging the swept volume or by increasing the rotation speed. However, fundamentally increasing the swept volume leads to heavier internal combustion engines, with larger dimensions, which are therefore more expensive. Particularly in the case of large internal combustion engines, increasing the rotation speed leads to considerable problems and disadvantages.
One widely used technical solution for increasing the power of an internal combustion engine is boosting. This refers to compression of the combustion air in advance by an exhaust-gas turbocharger or else by means of a compressor which is mechanically driven by the engine. An exhaust-gas turbocharger essentially comprises a compressor and a turbine which are connected to a common shaft and rotate at the same rotation speed. The turbine converts the energy in the exhaust gas, which is normally blown out without being used, to rotation energy, and drives the compressor. The compressor sucks in fresh air and feeds the air which is being compressed in advance to the individual cylinders of the engine. The greater amount of air in the cylinders can have an increased amount of fuel added to it, as a result of which the internal combustion engine emits more power. The combustion process is furthermore advantageously influenced, as a result of which the internal combustion engine achieves a better overall efficiency. Furthermore, the torque profile of an internal combustion engine that is boosted by a turbocharger can be configured to be extremely advantageous.
As the amount of exhaust gas increases, the maximum permissible rotation speed of the combination comprising the turbine wheel, the compressor wheel and the turboshaft, which is also referred to as the exhaust-gas turbocharger train, can be exceeded. If the rotation speed of the train were to be impermissibly exceeded, it would be destroyed, leading to total damage to the turbocharger. In particular, small modem turbochargers, with considerably smaller turbine and compressor wheel diameters, which have a better rotation acceleration response because the mass moment of inertia is considerably smaller are particularly affected by the problem of the maximum permissible rotation speed being exceeded. Depending on the design of the turbocharger, the turbocharger may be completely destroyed just by the rotation speed limit being exceeded by about 5%.
The German patent application No. 10 2004 052 695.8 discloses an exhaust-gas turbocharger having a sensor at the compressor end of the turboshaft to measure the rotation speed of the turboshaft directly. In this case, the sensor is passed through the compressor housing and is directed at an element for variation of a magnetic field. The element for variation of the magnetic field is in the form of a permanent magnet which is arranged in an attachment element. The attachment element has a socket in which the magnet is mounted, with the magnet resting directly on the compressor wheel. If the attachment element is pressed against the compressor wheel with the desired tightening torque, then this results in forces which must be absorbed by the permanent magnet. This can damage the brittle material of the magnet. Furthermore, gases in the air inlet to the compressor can chemically attack and damage the permanent magnet.
An element which produces a magnetic field, for attachment of a compressor wheel to a turboshaft of an exhaust-gas turbocharger, is therefore subject to particularly stringent technical requirements. It should have a uniform mass distribution which is as perfect as possible with respect to the turboshaft, and which does not change, even during operation of the element which produces a magnetic field. It should also be able to withstand a high mechanical tightening torque and produce a high magnetic field strength. It should also be resistant to the gases and the high temperatures in the air inlet of the compressor. One object of the present invention is to provide an element that produces a magnetic field, the element being for attachment of a compressor wheel to a turboshaft of an exhaust-gas turbocharger. The element satisfies the abovementioned requirements and the magnet is fixed securely. There is substantially no change in the mass distribution in the element that produces the magnetic field, even if the magnet fractures. A further object of the present invention is to specify a method by which an element which produces a magnetic field and has the stated characteristics can be produced.
Since the magnet is connected to the base body with a force fit, the mass cannot move at all in the base body even if the brittle material of the magnet fractures. A force which acts on the magnet and originates from the base body holds the magnet material in its position in all situations. Fracture of the magnet therefore does not cause the turboshaft to be unbalanced.
In one refinement, a thread is formed in the base body for screwing the element which produces a magnetic field to a thread on the turboshaft. The base body composed of high-strength material can accommodate a particularly fine thread, thus making it possible to screw the compressor wheel against the turboshaft with a high force. In this case, it is advantageous for the base body to be composed of non-magnetic, high-strength and weldable steel. This steel carries the magnetic field very well and can be welded well. The strength of this steel is extremely high.
In one embodiment of the invention, the magnet contains rare-earth metals. By way of example, rare-earth magnets such as NdFeB or SmCo magnets produce a relatively high magnetic field which can still be detected well, even by a sensor which is a relatively long distance away.
In embodiment of the invention, the base body has a higher coefficient of thermal expansion than the magnet. It is thus possible to insert the magnet into the heated base body and to produce the force fit between the base body and the magnet as the base body cools down. Even if the element which produces a magnetic field is heated to about 170° C. during operation in the air inlet of the turbocharger, the force fit is maintained between the base body and the magnet if the base body has been heated to about 330° C. for insertion of the magnet. In this case, the force fit between the base body and the magnet is created in the shrinking process after a heat treatment, which results in enormously high forces between the base body and the magnet, with the force fit being produced particularly intensively.
In one embodiment of the invention, the magnet is annular. An annular magnet allows a uniform mass distribution with respect to the rotation axis of the turboshaft to be achieved particularly easily.
In an embodiment of the invention, the element that produces a magnetic field additionally has a threaded body, wherein the threaded body is connected to the base body such that a tightening torque, which is transmitted to the base body, is also transmitted to the threaded body, and the magnet which is positioned between the base body and the threaded body is thus pressed by the threaded body against the base body, producing the force-fitting connection, or reinforcing a force-fitting connection which already exists between the base body and the magnet. This results in forces being applied to the magnet from all sides, as a result of which it cannot change its position in the socket even if the magnet material fractures. This prevents masses in the element which produces a magnetic field from moving, in all circumstances.
In this case, it is advantageous for the threaded body to be composed of 17-4PH steel also referred to as 1.4542 or 1.4548 steel. The strength of this steel is extremely high, and it can be welded. In this case, its soft-magnetic characteristics have no disruptive effect since the magnetic field can propagate well to the outside via the base body.
If the threaded body is connected to the base body by a weld, a torque acting on the base body can easily be passed to the threaded body. In combination with the weld, or as an alternative to this, the threaded body can be connected to the base body in an interlocking manner and/or by a crimp.
In one embodiment of the invention, the magnet is connected to the base body with a force fit, in that a sleeve body, which is connected to the base body using the magnetic pulse method, presses the magnet into the socket in the base body. When using the magnetic pulse method, the sleeve body produces an excellent force-fitting connection between the magnet and the base body. Elements which produce a magnetic field and have been manufactured using this method can be produced very cost-effectively and with high quality.
With respect to the method, a magnet is first of all inserted into a socket in a base body, and a sleeve body is then pushed over the base body with the magnet, after which the sleeve body is connected to the base body by a magnetic pulse, with a force being created which pushes the magnet into the socket, with a force-fitting connection being produced between the base body and the magnet.
Embodiments of the invention are illustrated by way of example in the figures, in which:
During this process, care should be taken to ensure that the permanent magnet 13 is as far as possible not heated above its Curie temperature. A Ferromagnet loses its spontaneous magnetization if it is heated above its Curie temperature. The Ferromagnet recovers its ferromagnetic characteristics somewhat below this temperature, that is to say it exhibits spontaneous magnetization even without any applied external field. Above the Curie temperature, the material then exhibits only a paramagnetic behavior, that is to say the material is magnetized by an external field but loses its magnetization again when the magnetic field is switched off. When the Curie temperature Tc is undershot, the magnet passes through a phase transition from the paramagnetic phase to the ferromagnetic phase. Despite the spontaneous resumption of the magnetic characteristics when the Curie temperature is undershot, there is actually no point in destroying the magnetic characteristics of annular magnets 13 because a magnet with characteristics that are not the same as the original characteristics may subsequently be formed. The Curie temperature of some typical magnetic materials is for: Cobalt 1394K (1121° C.), iron 1041 K (768° C.) and nickel 633 K (360° C.).
In order to fit the magnet 13, the base body 11 has a considerably higher coefficient of thermal expansion (CTE) than the permanent magnet 13. The base body 11 should have a coefficient of thermal expansion (CTE) of about 15 to 20 ppm/K while the magnet 13 should have a coefficient of thermal expansion (CTE) from about 5 to 10 ppm/K. This ensures that an element 17 which produces a magnetic field and is heated to a temperature of 330° C., which is below the Curie temperature of nickel, decreases its volume sufficiently when cooling down to build a stress between the base body 11 and the magnet 13 which leads to an adequate force-fitting connection between the base body 11 and the magnet 13. The process of reducing the volume of a material when it is cooling down is also referred to as shrinking. For the shrinking process, the magnet 13 is inserted into the socket 10 in the heated base body 13, and the element 17 which produces a magnetic field that has been assembled in this way is then cooled down. Even if the element 17 which produces a magnetic field is heated to about 170° C. during operation in the air inlet 16 of the turbocharger 1, the force-fitting connection between the base body 11 and the magnet 13 will continue to exist in an adequate manner since the operating temperature of 170° C. is well below the production temperature of 330° C. which was chosen as the initial temperature for the shrinking process.
To protect the magnet 13 against mechanical loads and chemically reactive gases, the socket 10 is closed with a protective cap 14. The socket 10 can be closed with the protective cap 14 by the application of weld beads 12 which, on the one hand, ensure high mechanical robustness and, on the other hand, result in a gas-tight seal between the socket and the external environment. In this example, the protective cap 14 is supported on a circumferential step 20 as a result of which the protective cap 14 and the upper area of the base body 11 form a flat surface. Furthermore, the base body 11 of the element 17 which produces a magnetic field has a thread 19, by which the element 17 which produces a magnetic field can be screwed onto an external thread on a turboshaft 5. By way of example, a hexagon 18 may be formed on the element 17 which produces a magnetic field, for a wrench to be fitted to. When the element 17, which produces a magnetic field, has been screwed onto the turboshaft 5, it presses the compressor wheel 9 firmly against a conical seat 25 on the turboshaft 5. Enormously high tightening torques are transmitted to the element 17 which produces a magnetic field, in order to achieve a secure and long-life connection between the compressor wheel 9 and the turboshaft 5. The material of the base body 11 and that of the threaded body 22 which will be introduced later are therefore subject to very stringent strength requirements. The material of the permanent magnet is brittle and fragile. In order not to change the center of gravity of the element 17 which produces a magnetic field and rotates at high speed, even if the permanent magnet 13 fractures, the mass of the element 17 which produces a magnetic field must not move at all in the base body 11. At this point, it should be noted that the train of a turbocharger can rotate at more than 270 000 revolutions per minute. Even a very minor non-uniform mass distribution with respect to the rotation axis of the train would also lead to enormous forces, which can attack the bearings of the turboshaft and can destroy them. The element 17 which produces a magnetic field is an extremely highly loaded component, since the element 17 which produces a magnetic field is subject to chemically highly reactive gases because of the exhaust gas being fed back into the air inlet 16 of the exhaust-gas turbocharger 1. All of these influences result in a requirement for particularly careful design of the element 17 which produces a magnetic field, well beyond the development of conventional attachment elements.
Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 041 901.7 | Sep 2007 | DE | national |
This is a U.S. national stage of application No. PCT/EP2008/061447, filed on Sep. 1, 2008, which claims priority to the German Application No.: 10 2007 041 901.7, filed: Sep. 4, 2007; the contents of both which are incorporated here by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/061447 | 9/1/2008 | WO | 00 | 3/4/2010 |
