Controllable Exhaust Gas Turbocharger

This invention relates to exhaust gas turbochargers. More specifically, the present invention relates to variable intake volute geometry for gas turbochargers and control means therefor.

Particularly in the case of small turbines, for example for motor vehicle exhaust gas turbochargers, a volute entry duct is often used before the rotor blading, instead of guide equipment, because a turbine casing of this sort is less expensive to manufacture without guide equipment.

The torque of a normal supercharger turbine with constant, non-adjustable entry duct flow section falls greatly at throughputs beneath the nominal throughput for which the turbine is designed. For a given throughput, the required tangential component of the flow velocity at the wheel inlet is determined by the narrowest flow section at the beginning of the volute part of the inlet duct and its radial position, relative to the inlet diameter of the rotor. In order to achieve acceptable turbine powers in the partial load range with varying exhaust flow, proposals have been made to alter these geometrical data. In general, these proposals consist of altering the volume of the inlet volute space by movable walls to correspond to the variable exhaust gas flow.

The ASME publication "Aerodynamic Design of Fixed and Variable Geometry Nozzleless Turbine Casings" describes the possibility of altering the inlet volute space by an axially movable wall corresponding to the exhaust gas throughput. This involves the acceptance of a clearance relative to the casing inner wall. This solution is shown only schematically in the paper without any information about the adjustment equipment and the design arrangement.

Another very complex design solution with large leakage flow losses is described in the U.S. Pat. No. 2,916,198. The volume of the effective inlet volute space can, in this case, be altered by two mutually opposed half moon shaped flaps which can be swivelled about axes lying approximately tangentially to the volute periphery. The operation of the flaps is carried out by a rod situated outside the casing. It is not possible by this structure to use the exhaust gas pressure in a simple manner to adjust the flaps.

The German Offenlegungsschrift No. 2,934,041 describes an appliance for solving the present task in which the intake in the volute space is subdivided into several flutes whose cross sections, inter alia, can be altered by walls movable in the axial direction from outside. Variants of this appliance have closing elements in these flutes and the degree of opening of these elements can be adapted to the exhaust gas flow. These solutions are not satisfactory in the case of partial opening of the closing elements.

The present invention, in which at least one wall element of an exhaust gas turbine is movable along a rotor shaft axis in a cylindrical part of an inlet volute, arose from the object of creating an embodiment of a controllable turbocharger which is usable in practice and avoids the disadvantages of known solutions.

The present invention is described in more detail below with reference to the illustrative embodiments shown schematically in the drawings. In the drawings:

FIG. 1 is a cross-sectional view of a radial turbine of an exhaust gas turbocharger according to a first preferred embodiment of the present invention,

FIG. 2 is a cross-sectional view of a first modified embodiment of the turbine of FIG. 1 having an integrated waste gate,

FIG. 3 is an enlarged cross-sectional view of a second preferred embodiment of the present invention with a bypass between a compressor and the tubine,

FIG. 4 is an end view of a control piston for the turbine of FIG. 3 along lines IV--IV of FIG. 3,

FIG. 5 is a plan view of a portion of FIG. 4, along the section line V--V shown in FIG. 4,

FIG. 6 is a reduced axonometric representation of the basic shape of the control piston of FIG. 4 and the boundaries of the inlet volute,

FIG. 7 is a cross-sectional view of a third preferred embodiment of the present invention illustrating a turbine similar to FIG. 1 with a control possibility using an external pressure source,

FIG. 8 is an enlarged cross-sectional view of a fourth preferred embodiment of the present invention, illustrating a turbine with a twin volute inlet, and

FIG. 9 is a cross-sectional view of an axial turbine embodiment of an exhaust gas turbocharger designed in accordance with the present invention .

In FIG. 1, the turbine casing of an exhaust gas turbocharger is indicated by the numeral 1. A rotor 2 is partially surrounded by an outlet diffuser 3 having a circular cylindrical outer surface 4. Concentric with this, the turbine casing 1 has a circular cylindrical bore 5 and the circular cylindrical space between the surfaces of this bore 5 and the outer surface 4 is closed at the outlet end of the diffuser 3 by a cylinder end 6. Furthermore, this circular cylindrical space contains a control piston 7 whose precise shape and function are described below. It is sealed against the surfaces 4 and 5 by an inner piston ring 8, seated in a groove of the diffuser 3, and an outer piston ring 9, situated in a groove of the piston 7. The piston 7 separates in this way the inlet volute space 10 from a rear cylinder space 11. The full line position of the piston is the one for which the inlet volute space is a maximum whereas, in the dot-and-dash line position, it makes available the smallest volume for the exhaust gas.

The piston 7 is loaded by a series of helical compression springs 12 distributed around the periphery and these press the piston into the position of minimum volute space volume, in which the piston touches the plane end 13 of the cylindrical space. The contact pins 14 concentric with the springs 12 limit the displacement of the piston 7 in the flow direction of the turbine. The rear cylinder space 11 is connected with the diffuser space via at least one balance bore 15.

In this embodiment and also that of FIG. 2, whose parts carry the same numbers wherever they are identical to the parts of the embodiment shown in FIG. 1, the piston 7 is pressed by the exhaust pressure in a self-regulating fashion into the position at which a sufficient pressure drop is produced for the conversion of the exhaust gas energy into mechanical energy of the rotor. The opening characteristics of the piston 7 necessary for this is obtained by appropriate dimensioning of the springs 12, which are shown as simple helical compression springs in FIGS. 1 and 2. In order to obtain desired spring characteristics, various spring types are available in practice, such as conical springs, springs with varying slope or wire thickness, weak plate springs, leaf springs and the like as well as combinations of the same.

Control pistons with piston rings, such as are preferably used, have the advantage of approximately constant friction with good sealing. However, annular diaphragms with appropriate lift fastened to the casing and the diffuser are also suitable even if their lift characteristic is more difficult to calculate in advance.

In the embodiment according to FIG. 1, the piston displacement is effected by the difference between the exhaust gas pressure in the volute space and the pressure in the diffuser 3. For the same geometrical data, this also applies to the type of construction in FIG. 2, in the diffuser 3 of which waste gate ducts 16 are provided. The piston displacement effected by a pressure difference in FIG. 2 applies to the extent that the piston 7 covers these waste gate ducts 16, i.e. while permissible exhaust pressures are still present in the inlet volute space. If the pressure is too high, the openings of the waste gate ducts 16 become free and the surplus exhaust gases flow into the diffuser. Due to the ejector effect, the pressure in the diffuser then decreases and with it the pressure in the rear cylinder space. However, since the pressure in the volute space also decreases because of the waste gate flow, the pressure difference across the piston 7 remains substantially the same.

FIG. 3 shows an embodiment with a bypass pipe 17 which, at small exhaust gas flows, connects the compressor air pipe of the compressor 18 with the inlet volute space 19 via a plurality of bypass ducts 20 in a control piston 21, in order to avoid surging of the compressor. The ducts 20 communicate with an annular groove 22, with which the bypass pipe 17 is connected at low exhaust gas flows. A non-return valve 23 can be provided in the bypass pipe 17.

FIG. 4 is a cross-sectional view of the control piston 21 and the casing normal to the sections shown in FIG. 3, there being no scale association with FIG. 3. The full lines of FIG. 4 refer to the section IV--IV,, while the dot-and-dash lines show the cross section of the inner boundary of the inlet volute space 19 along the section line IV'--IV'. The section through the bypass pipe 17 is omitted as is the representation of the piston rings 9, 8 in the control piston and the diffuser 3. The annular groove 22 of the bypass and the bypass duct 20 are shown in dotted outline.

It may be seen from FIGS. 3 and 4 that the control piston 21 is made up of a circular cylindrical body 24 and a volute body 25. The circular cylindrical body 24 contain the outer piston rings 9 which, in common with the piston ring 8 in the diffuser 3 seals the inlet volute space 19 against the rear cylinder space 26. In addition, the circular cylindrical body 24 contains the annular groove 22 and the ducts 20 of the bypass, of which only two are shown in FIG. 4 to represent all the ducts distributed evenly around the periphery.

The section 27 of the inlet volute space 19, which is shown dash-dotted in FIG. 4 as already mentioned, runs with a small clearance parallel to the end view of the control piston as far as a sharp edged depression 28 at the transition of the volute body 25 into the circular cylindrical body 24. This depression may also of course have a larger radius. Its position may be seen from the section V--V of FIG. 4 shown in FIG. 5. The continuation of the section 27 beyond the depression 28 is the section of the inlet pipe 29. The dividing wall 30, see FIG. 5, is the result of the penetration of the inlet pipe and the inlet volute into the cylindrical casing which contains the control piston. The edge 31 of this separating wall 30 is also shown dash-dotted in FIG. 4, which is there considered cut away.

The upper run-out of the volute body 25 is formed by a rounded nose 32 so as to ensure an inlet flow with out separation of the exhaust gases from the inlet pipe 29 into the inlet volute space 19. The double arrow 33 indicates the displacement directions and the double arrow 34 the possible stroke of the control piston.

FIG. 6 shows in axonometric representation the basic shape of the control piston 21 in full lines and the basic shape of the inlet volute space in dot-and-dash lines. The shape elements have the same reference numbers as in FIGS. 4 and 5. The circle 35 is the peripheral line of the bore 36 of the casing for the control piston 21 at the transition plane into the inlet volute space 19 and has the same reference number as in FIGS. 3 and 4.

The control piston could be manufactured in one piece as a precision casting with machining required at most on the outer and inner piston running surfaces and the piston ring grooves. If the control piston is manufactured in separate parts 24 and 25, rolled and cast rod material are possibilities for the disk shaped sections of the volute body 25 and, in general, these only require machining on the plane surfaces.

FIG. 7 shows an illustrative embodiment in which only the exhaust gas pressure corresponding to the difference between the exhaust gas, side and rear, spiral ring surfaces 38 and 39 of the volute body 37 is available for altering the cross section of the inlet volute space 40. This surface difference is the surface contained within the annular ring of width b. In this case, the main force for adjusting the control piston 41 is provided by a pressure dependent directly on the turbine power or controlled by it. In the present case, this is the boost pressure p.sub.L. For this purpose, there is a connection between a cylindrical space 42 between the circular cylindrical body 43 of the control piston 41 and an end surface 45 of a bearing housing 44 and the boost air pipe. This connection is achieved via a boost air tapping pipe 46 and a radial bore 47 in the circular cylindrical body 43.

Mention should also be made of a balance bore 48 in the volute body 37, which is responsible for the reduction mentioned above of the force operating on the volute body because of the exhaust gas pressure and also of a guide pin 49 with a balance duct 50. This guide pin 49 slides in a guide bore 51 in the control piston 41 and ensures an even clearance between the volute body 37 and the casing of the inlet volute. Without such a guide, which can of course also be provided by other means, the volute body, despite the fact that it is guided in the casing of the same shape, could tip over because of the unavoidable clearance or of clearance deliberately provided for thermal expansion, and make displacement more difficult.

In the turbine of an exhaust gas turbocharger shown in FIG. 8, the exhaust gas is introduced through a twin volute 52 with two mirror image inlet volute spaces. The control pistons 53, which correspond to those of FIGS. 1 and 2, are placed symmetrically about the twin volute. The balance bores 54 and 55 in the rear cylinder spaces open into the exhaust gas diffuser in the righthand portion and into the open air in the lefthand portion.

The invention can also be applied to axial turbines, as shown in FIG. 9. The control pistons do not then differ in any way with respect to their basic shape from those for radial turbines.

Apart from the faultless sealing possibility using piston rings or a diaphragm, another particular feature of the invention is the possibility of inexpensive manufacture.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not as limiting to the scope and spirit of the invention as set forth in the appended claims.