CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2014 119 671.6 filed Dec. 29, 2014, the entire contents of which are incorporated herein by reference. FIELD OF THE INVENTION The present invention pertains to a mixer for an exhaust gas duct system of an internal combustion engine. Such an exhaust gas duct system comprises, in general, a flow channel, through which exhaust gases can flow in a main direction of flow, in a flow channel element. Such a flow channel may lead from an internal combustion engine, for example, a diesel internal combustion engine, to a catalytic converter device and then farther to the surrounding area. BACKGROUND OF THE INVENTION To make it possible to meet increasingly stringent requirements imposed in regard to pollutant emissions from internal combustion engines, especially diesel internal combustion engines, it is known that additives, e.g., urea, can be introduced into the exhaust gas stream downstream of the internal combustion engine. To attain the greatest possible effect, it is necessary to efficiently mix such additives with the exhaust gases. However, the problem arises in this connection that the integration of mixer elements in the exhaust gas stream increases the exhaust gas back pressure because of the flow resistance generated by such mixer elements, which leads to a reduction of the output of an internal combustion engine. SUMMARY OF THE INVENTION An object of the present invention is to provide a mixer for an exhaust gas duct system of an internal combustion engine, which makes it possible to attain high engine outputs while basically ensuring an efficient mixing of exhaust gases and additives introduced into these. This object is accomplished according to the present invention by a mixer for an exhaust gas duct system of an internal combustion engine, comprising a flow channel, through which exhaust gases can flow in a main direction of flow, in a flow channel element, as well as at least one mixer element movable in the flow channel element to change the mixing effect. The mixer according to the present invention is configured such that the partial damming (blocking) of the flow channel, which is inevitable due to the mixer to attain the mixing effect, can also be varied because of the change in the mixing effect of the mixer. Especially when an internal combustion engine is operated such as to have the highest outputs, the mixer can be brought into a state in which a lower exhaust gas back pressure is generated, but a comparatively low mixing effect is also attained, so that a very high engine output can be reached, even through the mixing of the combustion waste gases and the additives is somewhat inferior. In states in which such a high output is not necessary or is not required, the mixer can be brought into a state with strong mixing effect, so that an efficient reduction of the percentage of pollutants is achieved in the exhaust gases released into the surrounding area. To make it possible to bring the mixer into states with different mixing effects, it is provided that the at least one mixer element of the mixer is pivotable about a pivot axis that is essentially at right angles to the main direction of flow. As an alternative or in addition, it may be provided that at least one mixer element be pivotable about a pivot axis extending essentially in the main direction of flow. Provisions may be made in another possibility of changing the mixing effect by moving the mixer element for at least one mixer element of the mixer to be able to be displaced in a direction extending essentially at right angles to the main direction of flow. In the mixer according to the present invention, at least one mixer element can be brought into a first operating state with maximum mixing effect of the mixer and into a second operating state with minimal mixing effect of the mixer. These different operating states equally correspond to states of different damming and different flow resistance. In the first operating state, i.e., in the state in which maximum mixing effect of the mixer is attained, very intense swirling of the flow is brought about in the flow channel, which is associated with a comparatively high flow resistance and a corresponding exhaust gas back pressure. In the second operating state, i.e., in the state with minimal mixing effect, the effect on the flow is correspondingly weak due to a comparatively low flow resistance, so that the mixer also brings about only a comparatively small increase in the exhaust gas back pressure and thus makes it possible to attain very high engine outputs. To make it possible to achieve a compact design of the mixer according to the present invention, it is provided that at least one mixer element be positioned essentially completely in the flow channel in the first operating state and in the second operating state. A minimal effect of the mixer on the flow in the flow channel in the second operating position can be achieved by at least one mixer element not being essentially positioned in the flow channel in the second operating state. Provisions may be made for this, for example, for a mixer element mounting chamber adjoining the flow channel for receiving the mixer element to be provided in its second operating state. In order to make it possible to achieve efficient mixing of the exhaust gases with additives introduced into them with the mixer according to the present invention, it is provided that at least one mixer element have at least one flow deflection element, wherein said at least one flow deflection element has at least one flow deflection surface arranged at an angle in relation to a main direction of flow at least in the first operating state. Provisions may be made in this connection, for example, for at least one row of flow deflection elements following each other in the direction of a row to be provided. In case of such an arrangement in a row of the flow deflection elements, provisions may be made for moving at least one mixer element between the different operating states thereof for the pivot axis to extend essentially in the direction of the row or in a direction extending at right angles to the direction of the row. The mixing effect of the mixer can be increased by providing at least two rows of flow deflection elements arranged next to one another essentially at right angles to the main direction of flow, or/and by providing at least two rows of flow deflection elements following one another in the main direction of flow. To make it possible to move at least one mixer element to attain different mixing states in the mixer according to the present invention, it is provided that a moving drive be associated with at least one mixer element of the mixer. The present invention pertains, furthermore, to an exhaust gas duct system for an internal combustion engine, comprising at least one mixer according to the present invention. The present invention will be described in detail below with reference to the attached figures. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a partially schematic diagram of a mixer for an exhaust gas duct system of an internal combustion engine, viewed in a direction at right angles to a main direction of flow of exhaust gases; FIG. 2 is a view of the mixer according to FIG. 1, viewed in the main direction of flow of the exhaust gases; FIG. 3 is a perspective view of the mixer according to FIG. 1; FIG. 4 is a perspective view of a mixer element of the mixer according to FIG. 1; FIG. 5 is a view of the mixer, which view corresponds to FIG. 1, with a mixer element positioned in a state with minimal mixing effect; FIG. 6 is a view of the mixer, which view corresponds to FIG. 2, with the mixer element positioned in the state with minimal mixing effect; FIG. 7 is the mixer according to FIG. 5 in a perspective view; FIG. 8 is a view corresponding to FIG. 1 of an alternative embodiment of a mixer; FIG. 9 is the mixer according to FIG. 8 viewed in the main direction of flow of the exhaust gases; FIG. 10 is the mixer according to FIG. 8 in a perspective view; FIG. 11 is a mixer element of the mixer according to FIG. 8 in a perspective view; FIG. 12 is a view corresponding to FIG. 8 of the mixer with a mixer element positioned in a state with minimal mixing effect; FIG. 13 is a view corresponding to FIG. 9 with the mixer element positioned in the state with minimal mixing effect; FIG. 14 is the mixer according to FIG. 12 in a perspective view; FIG. 15 is a view of an alternative design embodiment of a mixer, viewed in a main direction of flow of the exhaust gases; FIG. 16 is a view corresponding to FIG. 15 of the mixer with a mixer element positioned in a state with minimal mixing effect; FIG. 17 is a schematic diagram of another alternative design embodiment of a mixer for an exhaust gas duct system; FIG. 18 is the mixer according to FIG. 17 in a state with minimal mixing effect; FIG. 19 is another schematic diagram of an alternative design embodiment of a mixer for an exhaust gas duct system; and FIG. 20 is the mixer according to FIG. 19 in a state of minimal mixing effect. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a mixer 10 for an exhaust gas duct system will be described below with reference to FIGS. 1 through 7. FIGS. 1 through 4 show a first operating state of a mixer element of the mixer 10, which mixer element is generally designated by 12, in which first operating state the mixer 10 generates a maximum mixing effect for exhaust gases flowing in a main direction of flow S in flow channel and for additive introduced into these. The flow channel 14 is formed in a flow channel element 16 having a tubular design. FIGS. 5 through 7 show the mixer 10 with the mixer element 12 positioned in a second operating state with minimal mixing effect of the mixer 10. In the embodiment of the mixer 10 shown in FIGS. 1 through 7, the tubular flow channel element 16 is configured such that it provides a nearly elliptical flow cross section for the exhaust gases and additives flowing, in general, in the main direction of flow S. It should be noted here that other flow cross section geometries, for example, a circular flow cross section, are also feasible while maintaining the principles of the present invention. In the example being shown, the mixer element 12 comprises two mixer element parts 18, 20, which are provided, for example, as shaped sheet metal parts and are permanently carried on a common rocking shaft 22. The rocking shaft 22 is carried pivotably or rotatably about a pivot axis A, for example, at a flow channel element 16 and may extend out of the flow channel element 16, for example, on one side. The rocking shaft 22 can be driven, for example, by an electric motor as a moving drive 24 for pivoting or rotation about the pivot axis A, as a result of which pivoting of the mixer elements 12 and of the two mixer element parts 18, 20 carried on the rocking shaft 22 between the first operating state shown in FIG. 1 and the second operating state shown in FIG. 5 can be achieved. As can be seen in FIG. 1 on the basis of the mixer element part 18, a plurality of flow deflection elements 26 are provided on each of the mixer element parts 18, 20. The flow deflection elements 26 are arranged in two rows 28, 30 following each other in the main direction of flow S, row 28 of flow deflection elements 26 being positioned upstream in relation to the row 30 of flow deflection elements 26. The flow deflection elements 26 are arranged in each row 28, 30 such that they follow each other in a row direction R, the row direction R being essentially at right angles to the main direction of flow S and also at right angles to the pivot axis A in case of the mixer element 12 positioned in the first operating state. Each flow deflection element 26 provides at least one flow deflection surface 32 positioned at an angle in relation to the main direction of flow S. As is shown by a comparison of FIGS. 1 and 5, the flow deflection elements 26 of the downstream row 30 of flow deflection elements 26 provide two flow deflection surfaces 32 each positioned at an angle in relation to the main direction of flow S, namely a flow deflection surface 32, which is active in the first operating state (FIG. 1) and faces the row 28 of flow deflection elements 26, which is positioned upstream in the first operating state, and a flow deflection surface 32, which is active with minimal mixing effect in the second operating state shown in FIG. 5 and is arranged such that it is oriented away from the row 28 of flow deflection elements 26. It should be noted here once again that the mixer element part 20 may have a corresponding design. As is shown especially in FIG. 4, the flow deflection elements 26 of the mixer element part 20 may be arranged following one another in the row direction, which is present there as well, in such a way that they are positioned at mutually opposite angles in relation to the pitch of the flow deflection surfaces 32 of the flow deflection elements 26 of the mixer element part 18, as a result of which improved mixing effect can be achieved. A comparison of FIGS. 1 through 4, on the one hand, and FIGS. 5 through 7, on the other hand, shows clearly that in the first operating state according to FIGS. 1 through 4, the mixer element 12 brings about a substantially greater damming of the flow cross section of the flow channel 14, especially a substantially larger incoming flow area for the exhaust gases flowing in the flow channel 14 in the main direction of flow S in the first operating state according to FIGS. 1 through 4 because of its orientation essentially at right angles to the main direction of flow S. The mixer element 12 is oriented essentially such that it extends in the main direction of flow S in the second operating state according to FIG. 5. Even though mixing of the exhaust gases with the additive introduced into them is generated, as before, at each of the mixer element parts 18, 20 because the flow deflection surfaces 32 of the two rows 28, 30 are positioned at an angle, as before, in relation to the main direction of flow S, this mixing takes place with a markedly reduced mixing effect compared to the positioning according to FIGS. 1 through 4. Since the mixer element 12 is positioned in the second operating state according to FIGS. 5 through 7 such that it covers only a comparatively small percentage of the flow cross-sectional area of the channel 14, a substantially lower flow resistance is generated, and the exhaust gas back pressure in the flow channel 14 or in an exhaust gas duct system is markedly reduced compared to the first operating state according to FIGS. 1 through 4. It becomes possible with the design of a mixer 12 shown in FIGS. 1 through 7 either to position the mixer element 12, depending on the required output of an internal combustion engine, in case of a lower required output, such that the mixer 10 generates a maximum or comparatively strong mixing effect, so that the pollutant emission is minimized because of the highly efficient mixing. If a higher or maximum output is required from an internal combustion engine, the mixer element 12 can be brought by the moving drive 24 into a state in which the compromise of the free flow cross section in the channel 14 is minimized and the mixing effect is also likewise minimal The exhaust gas back pressure, which is thus reduced as well, makes it possible to attain maximum output in an internal combustion engine. An alternative design embodiment of a mixer is shown in FIGS. 8 through 14. Components that correspond to above-described components in terms of design and function are designated by the same reference numbers with an “a” added. FIGS. 8 through 11 show the mixer in a state of maximum mixing effect, while the mixer is shown in FIGS. 12 through 14 in its state of minimal mixing effect. The mixer 10a comprises a plate-like mixer element 12a, on which two rows 28a, 30a of flow deflection elements 26a with respective flow deflection surfaces 32a are formed. It is seen, especially in FIG. 11, that the flow deflection surfaces 32a of the flow deflection elements 26a of both rows 28a, 30a are pitched or set in angular positions in opposite directions in relation to one another relative to the main direction of flow S. To make it possible to provide mutually partially overlapping flow deflection elements 26a positioned at an angle in the main direction of flow S and opposite the main direction of flow S, two plate-like parts made, for example, of sheet metal material may be placed on one another. These may be carried on a rocking shaft 22a, whose pivot axis or axis of rotation A extends in the row direction R in this exemplary embodiment. Furthermore, the pivot axis A extends in this exemplary embodiment in the direction of the larger of the two semiaxes of an ellipse describing essentially the cross-sectional geometry of the flow channel 14a, while the pivot axis is oriented in the previously described design embodiment essentially in the direction of the smaller of the two semiaxes, i.e., in the direction of the smaller dimension of the flow channel element. The mixer element 12a can be moved between the first operating state shown in FIGS. 8 through 11 and the second operating state shown in FIGS. 12 through 14 by the moving drive 24a coupled with the rocking shaft 22a. The portion of the flow cross section of the flow channel 14a that is essentially covered by the mixer element 12a has its maximum in the first operating state, so that a maximum mixing effect is generated with the flow deflection surfaces 32a. The mixer element 12a is oriented in the second operating state such that the flow deflection surfaces 32a of the flow deflection elements 26a are essentially parallel to the main direction of flow S, so that hardly any swirling and hence a minimal mixing effect is generated by the flow deflection elements 26a. The exhaust gas back pressure in the flow channel 14a is minimal in this state, so that it is possible to attain the maximum output in the internal combustion engine. Another alternative embodiment of a mixer is shown in FIGS. 15 and 16. Components that correspond to above-described components in terms of design and function are designated here by the same reference numbers with a “b” added. In the first operating state of the mixer 10b, which is shown in FIG. 15, the mixer 10b of this design embodiment comprises two mixer elements 12b, 12b′, which are, for example, plate-like elements and are oriented essentially at right angles to the main direction of flow, which is also at right angles to the drawing plane in this case. It should be noted here that even through the flow cross section is represented here with an ellipsoid contour, the design principle according to FIGS. 15 and 16 may also be combined with any other flow cross section. The two mixer elements 12b, 12b′ are arranged nested in one another in the embodiment shown in FIGS. 15 and 16, i.e., the mixer element 12b is or can be arranged such that it surrounds the mixer element 12b′. In the first operating state of the mixer 10b shown in FIG. 15, the two mixer elements 12b, 12b′ cover a maximum portion of the flow cross section of the flow channel 14b, so that a maximum mixing effect is generated by the flow deflection elements 26b of the two mixer elements 12b, 12b′, which said flow deflection elements 26b are indicated here in a schematic form only and may be arranged in a plurality of rows in this case as well. The outer of the two mixer elements, i.e., the mixer element 12b, is pivotable by a moving drive, not shown here, about a pivot axis A extending, for example, in the direction of the shorter extension of the flow channel element 16b. If the mixer element 12b is brought into its position shown in FIG. 16, in which it is essentially at right angles to the drawing plane, the flow cross section of the flow channel 14b covered by the two mixer elements 12b, 12b′ is markedly smaller, so that a lower exhaust gas back pressure is also generated along with a weaker mixing effect and higher engine outputs can be reached. The inner mixer element 12b′ may be configured in this arrangement, in principle, as a stationary mixer element. To make it possible to achieve an even more sensitive adjustment between a state of maximum mixing effect and a state of minimal mixing effect, the mixer element 12b′ could also be adjustable and be brought, for example, into a state that corresponds to the state of the mixer element 12b in FIG. 4, so that a minimal coverage of the flow cross section of the flow channel 14b is achieved because the two mixer elements 12b, 12b′ are positioned now such that they are oriented essentially parallel to the main direction of flow S. A separate moving drive could be associated with each of the two mixer elements 12b, 12b′ in this embodiment. Another alternative embodiment of a mixer is shown in FIGS. 17 and 18. Components that correspond to above-described components in terms of design and function are designated by the same reference numbers with a “c” added. While the mixer elements present in the embodiments described above with reference to FIGS. 1 through 16 are positioned completely in the flow channel and only their effective cross section is varied by the different positioning in both the first operating state and the second operating state, the embodiment shown in FIGS. 17 through 18 provides for a mixer element 12c that is essentially not positioned in the flow channel 14c any longer in the second operating state shown in FIG. 18, so that the exhaust gases flowing in the main direction of flow S can flow essentially unhindered through the flow channel element 16c, even though said mixer element 12c is positioned in the flow channel 14c in the first operating state shown in FIG. 17 and can thus ensure swirling and hence mixing with its schematically shown flow deflection elements 26c. To make it possible to achieve this, a mixer element mounting chamber 34c may be provided, which laterally adjoins the flow channel 14c and in which the mixer element 12c can be positioned to attain the second operating state, i.e., the operating state with minimal mixing effect. Driven, for example, by a moving drive, not shown, about a pivot axis A positioned essentially parallel to the main direction of flow S, the mixer element 12c can be pivoted out of the flow channel 14c and into the mixer element mounting chamber 34c. If the mixer element 12c is received in the mixer element mounting chamber 34c, the flow channel 14c is released for the flow essentially without being compromised by the mixer element 12c. The motion of the mixer element 12c from the state shown in FIG. 17 into the state shown in FIG. 18 may be achieved by a displacement, for example, a linear displacement, as an alternative to a pivoting about a pivot axis A directed essentially parallel to the main direction of flow S. An embodiment of a mixer, in which such a displacing motion of a mixer element is used to make it possible to reach states of different mixing effects is shown in FIGS. 19 and 20. Components that correspond to above-described components in terms of design and function are designated by the same reference numbers with a “d” added. One of the mixer elements, namely, the mixer element 12d, is arranged movably in the embodiment of a mixer 10d shown in FIGS. 19 and 20, while the other mixer element, namely, the mixer element 12d′, may be positioned in a fixed manner in the flow channel 14d. Each of the mixer elements 12d, 12d′ may be designed with a plurality of flow deflection elements 26d, positioned, for example, in rows 28d, 30d following each other in the main direction of flow S. The two mixer elements 12d, 12d′ are positioned in FIG. 19 such that they have a minimal or no overlap at right angles to the main direction of flow S and thus cover the flow cross section in the flow channel 14d to the maximum extent. Maximum mixing effect of the exhaust gases flowing in the flow channel 14d with the additive being introduced into these is thus guaranteed. If it is necessary to change over from the first operating state of the mixer 10d shown in FIG. 19 to the second operating state of the mixer 10d shown in FIG. 20, the mixer element 14d is displaced essentially at right angles to the main direction of flow S in a displacement direction V, for example, linearly. A greater overlap of the two mixer elements 12d, 12d′ is brought about in the process, as a result of which the cross section covered in the flow channel 14d decreases. The coverage of the flow cross section is minimal and the mixing effect is correspondingly minimal as well in case of maximum overlap of the two mixer elements 12d, 12d′. The exhaust gas back pressure in the exhaust gas duct system is also minimal in this state, so that maximum outputs of an internal combustion engine can be reached. The motion of the mixer element 12d in the flow channel 14d can be generated by a linear displacing drive, configured, for example, as an electric motor, not shown in FIGS. 19 and 20. The arrangement could, in principle, also be such that, similarly to the embodiment described above with reference to FIGS. 17 and 18, a mixer element mounting chamber, into which the mixer element 12d can be moved, so that a mixing effect is achieved in the second operating state only by the mixer element 12d′ positioned in the flow channel 14d, is provided for the movable mixer element 12d, laterally adjoining the flow channel 14d. While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. |
