A sealing device for prevention of exhaust gas in a scroll passage from leaking via a gap to a turbine impeller is arranged upstream, in a travel direction of the exhaust gas, of through-holes via which the vane shafts extend through a rear exhaust introduction wall, whereby pressure in a gap communicating with the through-holes of the rear wall is kept lower than pressure in an exhaust nozzle so as to displace nozzle vanes to the rear wall.

TECHNICAL FIELD

The present invention relates to a turbocharger which is simple in structure and which can reduce turbulence of exhaust gas in a turbine impeller outlet to improve efficiency of a turbine.

BACKGROUND ART

FIG. 1shows a conventional variable displacement turbocharger to which the invention is applied. In the turbocharger, turbine and compressor housings1and2are integrally assembled through a bearing housing3by fastening bolts3aand3b, a turbine impeller4in the turbine housing1being connected to a compressor impeller5in the compressor housing2by a turbine shaft7rotatably supported via a bearing6in the bearing housing3.

As shown inFIG. 2in enlarged scale, the bearing housing3is provided, on its turbine housing side, with an exhaust nozzle9by which the exhaust gas introduced into a scroll passage8in the turbine housing1is guided to the turbine impeller4.

The exhaust nozzle9comprises front and rear exhaust introduction walls10and11on sides of the bearing and turbine housings3and1, respectively, integrally assembled together with a required distance between them by, for example, three fixing members12arranged circumferentially. Upon assembling of the turbine and bearing housings1and3, an attachment member13fixed on a front surface of the front wall10(a side surface of the bearing housing3) is clamped by the housings1and3to fix the exhaust nozzle9. Upon the assembling, the exhaust nozzle9is positioned with respect to the bearing housing3by a positioning pin14.

Annularly arranged between the front and rear walls10and11are a plurality of nozzle vanes15. InFIGS. 1 and 2, each of the nozzle vanes15is dually supported such that the nozzle vane15has vane shafts16aand16bfixed to opposite sides of the vane15and extending through the front and rear walls10and11, respectively.

InFIG. 1, reference numerals17a,17b,17cand17ddesignate a linked transmission mechanism for control of opening angle of the vanes15through the vane shafts16a; and18, a scroll passage formed in the compressor housing2.

Provided between the turbine housing1and the rear wall11of the exhaust nozzle9is a gap19which is unwanted by nature and which is however provided for countermeasure to, for example, possible thermal deformation of the turbine housing1between during being hot and during being cold and possible variations in accuracy of parts to be assembled.

The gap19may disadvantageously cause the exhaust gas in the scroll passage8to vainly leak to a turbine impeller outlet20. Thus, in order to block the gap19, it has been proposed to arrange sealing piston rings21between an outer periphery on a downstream extension11′ of the rear wall11and an inner surface1′ of the turbine housing1confronting the extension11′ so as to prevent the gas leakage and absorb thermal deformation (see Patent Literature 1).

In Patent Literature 1, as shown inFIG. 2, formed on the outer periphery of the extension11′ of the rear wall11is an annular groove22into which generally two sealing piston rings21are inserted with their closed gaps or cutouts being not aligned or overlapped with each other, thereby providing a sealing device23. The piston rings21are pressed at their outer peripheries against the inner surface1′ of the turbine housing1by spring force of the piston rings themselves to prevent the gas leakage.

SUMMARY OF INVENTION

Technical Problems

In the conventional turbochargers, as shown inFIG. 2, some sealing device23has been devised to prevent gas leakage from the gap19; however, even with such devised sealing device23, it is difficult and limitative to substantially improve turbine efficiency.

Thus, the inventor has made various researches and experiments on factors other than the gas leakage affecting the turbine efficiency to find out that the more the exhaust gas in the turbine impeller outlet20is turbulent, the more the turbine efficiency is lowered and that the less the exhaust gas in the turbine impeller outlet20is turbulent, the more the turbine efficiency is improved.

With the conventional sealing device23with the piston rings21between the outer periphery of the extension11′ and the inner surface1′ of the turbine housing1as shown inFIG. 2, pressure P2in the gap19is greater than pressure P1in the exhaust nozzle9; that is, P2>P1. Thus, as shown by an arrow A, the exhaust gas in the gap19passes through a gap S between the vane shaft16band a through-hole24into the exhaust nozzle9while there are preliminarily provided clearances between the nozzle vanes15and the front and rear walls10and11, respectively, so as to make the nozzle vanes15pivotable and rotatable; such clearances may be different in dimension depending on individual turbochargers. Thus, it was found out that, due to pressure difference P2>P1, the respective vane shafts16bof the nozzle vanes15are urged to the front wall10to thereby provide greater clearance C between the respective nozzle vanes15and the rear wall11.

The inventor found out that such greater clearance C produced between the respective nozzle vanes15and the rear wall11increases turbulence of the exhaust gas in the turbine impeller outlet20, resulting in lowering of the efficiency of the turbine.

The invention was made in view of the above and has its object to provide a turbocharger which is simple in structure and which can reduce turbulence of exhaust gas in a turbine impeller outlet to improve efficiency of a turbine.

Solution to Problems

The invention is directed to a turbocharger with a turbine housing having a scroll passage outwardly of an exhaust nozzle which in turn is arranged outwardly of a turbine impeller rotatably supported on a bearing housing and serves for guiding exhaust gas from the scroll passage to the turbine impeller, said exhaust nozzle having a plurality of nozzle vanes between front and rear exhaust introduction walls on sides of the bearing and turbine housings, respectively, vane shafts fixed to opposite sides of each of said nozzle vanes extending through the front and rear walls and being rotatably supported, the rear wall being arranged to have a gap between said rear wall and the turbine housing,

characterized in that a sealing device for preventing the exhaust gas from said scroll passage from leaking via said gap to the turbine impeller is arranged upstream, in a travel direction of the exhaust gas, of through-holes via which the vane shafts extend through the rear wall, whereby pressure in said gap communicating with said through-holes of the rear wall is kept lower than pressure in the exhaust nozzle so as to displace the nozzle vanes to the rear wall.

In the turbocharger, preferably, a portion of each of the vane shafts extending through the rear wall which is fixed to a corresponding nozzle vane is formed with a flange shrouding a corresponding through-hole.

Advantageous Effects of Invention

According to a turbocharger of the invention which comprises a sealing device for prevention of exhaust gas from a scroll passage from passing through a gap between a turbine housing and a rear exhaust introduction wall and leaking to the turbine impeller is arranged upstream, in a travel direction of the exhaust gas, of through-holes on the rear wall, pressure in a gap communicating with the through-holes of the rear wall is kept lower than pressure in an exhaust nozzle, whereby the nozzle vanes can be displaced to the rear wall by the simple structure to minimize a clearance between the nozzle vanes and the rear wall, resulting in advantageous effect that turbulence of the exhaust gas in the turbine impeller outlet can be reduced to substantially improve efficiency of the turbine.

REFERENCE SIGNS LIST

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described in conjunction with the attached drawings.

FIG. 3is an embodiment of the invention applied to the turbocharger shown inFIG. 1with nozzle vanes15being arranged between front and rear introduction walls10and11of an exhaust nozzle9and each dually supported at its opposite sides such that vane shafts16aand16boppositely fixed to the vane15extend through the walls10and11, respectively, wherein a sealing device25for prevention of the exhaust gas from the scroll passage8from leaking to the turbine impeller4via a gap19between the turbine housing1and the rear wall11is arranged upstream, in a travel direction of the exhaust gas, of (or nearer to the scroll passage8than) the through-holes24via which the vane shafts16bextend through the rear wall11.

With respect to the sealing device25inFIG. 3, the turbine housing1has a portion26confronting a vertical surface of the rear wall11to provide a gap19therebetween. The portion26is cut out at its outer periphery to provide a step27, a disc spring seal28being fitted between the step27and a rear surface of the rear wall11. The step27comprises a vertical opposed surface27a(surface substantially in parallel with the rear surface of the rear wall11and substantially perpendicular to an axis of the turbine impeller4in the embodiment shown) confronting the rear wall11and an annular tapered surface27bwith its diameters being decreased in a direction away from the rear wall11. Though the bottom of the step27may be not the tapered surface27bbut, for example, a cylindrical surface with uniform radius with respect to the axis (even this, the disc spring seal28can be supported), the tapered surface27bmakes it possible to retain the disc spring seal28more stably and thus improve the sealing effect. Further, the tapered surface can prevent the disc spring seal28from moving and dropping from the step27upon, for example, assembling of the turbine housing.

The disc spring seal28is ring-shaped as shown inFIG. 4and has a vertical rectilinear portion30contiguous with an inner peripheral end29and pressed against the vertical surface27a, the inner peripheral end29being curved from the rectilinear portion30in a direction approaching to the rear wall11and then extending vertically and inwardly into substantially S-shape. Such S-shape makes it easy to push the inner peripheral end29onto the tapered surface27band the shape of the tapered surface27bmakes it difficult for the pushed-in inner peripheral end29to escape from the step27. An outer peripheral end31of the disc spring seal28(outwardly of the rectilinear portion30) has a slant portion32extending slantingly from the rectilinear portion30toward the rear wall11and a curved portion33peripherally of the slant portion32and curved in a direction away from the rear wall11and pressed against the rear wall11.

As shown inFIG. 5, the disc spring seal28is frustconical and has inner and outer peripheral edges29and31mutually offset with respect to an axis of the spring. An axial height H of the disc spring seal28with the frustconical shape is set such that a curved outer peripheral portion33of the seal28is pushed against the rear wall11when the inner peripheral end29is fitted onto the tapered surface27bto make the rectilinear portion30abut against the rectilinear surface27a.

As shown inFIGS. 3 and 6, portions of the vane shafts16aand16bfixed to each of the nozzle vanes15for extension through the front and rear walls10and11, respectively, are formed with flanges35which shroud corresponding through-holes24. Such flanges35can prevent foreign matters from invading the through-holes24and prevent the exhaust gas from passing through the through-holes24into the gap19. Moreover, as mentioned hereinafter, enough force can be obtained in utilization of pressure of the exhaust gas against the flanges35so as to make the nozzle vanes15move to the rear wall11.

FIGS. 7 and 8show a further embodiment of the sealing device25shown inFIGS. 3 and 4. With respect to the sealing device25inFIG. 7, a turbine housing1has a portion26confronting a vertical surface of a rear exhaust introduction wall11to provide a gap19therebetween. The portion26has a step36at its outer periphery, a disc spring seal37being fitted between the step36and a rear surface of the rear wall11. The step36comprises a deeply cut out surface36ain confrontation to the rear wall11and a cylindrical surface36bin parallel with an axis of the turbine shaft7.

The disc spring seal37is substantially ring-shaped and circumferentially cut out partly at38with a width of, for example, 0.2-0.8 mm as shown inFIG. 8. The disc spring seal37is shaped such that it has inner peripheral end29curved in a direction away from the rear wall11and movably and minutely fitted on the cylindrical surface36b, is frustoconical to have increased diameters from the fitted portion toward the rear wall11and has an outer peripheral end31formed with a curved portion33abutting on a rear surface of the rear wall11.

The disc spring seal37arranged in fitted onto the cylindrical surface36bis moved along the cylindrical surface36bby pressure of the exhaust gas in the scroll passage8(difference in pressure between the scroll passage8and the gap19) so that the curved portion33of the outer peripheral end31is automatically pushed on the rear surface of the rear wall11; the disc spring seal37is preliminarily shaped such that, in this case, the disc spring seal is reduced in diameter to null the cutout38shown inFIG. 8with its opposite ends just abutting to each other. Also in the embodiment inFIG. 7, portions of vane shafts16aand16bfixed to each of nozzle vanes15are formed with flanges35shrouding the corresponding through-holes24.

Mode of operation of the embodiments shown inFIGS. 3 and 7will be described.

In the embodiment shown inFIG. 3, the turbine housing1ofFIG. 1is integrally assembled onto the bearing housing3, using the fastening bolts3a, with the inner peripheral end29of the disc spring seal28being fitted on the tapered surface27aof the step27. In this case, as shown inFIG. 5, the axial height H of the disc spring seal28with the frustoconical shape between the rectilinear and curved portions30and33is greater than a distance between the opposed surface27aand the rear surface of the rear wall so that, after the assembling, the rectilinear portion30of the disc spring seal28is pressed against the opposed surface27a, the curved portion33of the outer peripheral end31of the disc spring seal28being pressed against the rear surface of the rear wall11. Thus, the sealing device25in the form of the disc spring seal28can prevent the exhaust gas in the scroll passage8from leaking through the gap between the turbine housing1and the rear wall11.

In the embodiment shown inFIG. 7, with the disc spring seal37being arranged with its inner peripheral end29being fitted on the cylindrical surface36bof the step36of the turbine housing1, the outer peripheral end31of the disc spring seal37bbecomes automatically pressed against the rear surface of the rear wall11by the pressure of the exhaust gas in the scroll passage8and concurrently the disc spring seal37is reduced in diameter to null the cutout38inFIG. 8with its opposed ends abutting to each other. Thus, the above-mentioned sealing device25in the form of the disc spring seal37can prevent the exhaust gas in the scroll passage8from leaking through the gap19between the turbine housing1and the rear wall11.

In this case, the sealing device25is arranged upstream, in the travel direction of the exhaust gas, of (or nearer to the scroll passage8than) the through-holes24through which the vane shafts16bextend through the rear wall11, so that the gap19downstream of the disc spring seal28or37is low in pressure P2and has relationship of P1>P2with respect to the pressure P1in the exhaust nozzle9; as a result, the exhaust gas in the exhaust nozzle9is allowed to flow downstream of the disc spring seal28or37as shown by arrow B. Due to the pressure difference P1>P2as mentioned above, the nozzle vanes15are pushed and displaced to the rear wall11into abutment with the latter; thus, the clearance between the respective nozzle vanes15and the rear wall11is minimized. In this case, a portion of each of the vane shafts16bextending through the rear wall11which is fixed to the nozzle vane15is provided with a flange35shrouding the through-hole24, so that the pressure of the exhaust gas in the exhaust nozzle9acts on the flange35which is then pushed against the rear wall11to close the through-hole24. This reduces flow rate of exhaust gas leaking as shown by the arrow B and can prevent the exhaust gas passing through between the front and rear walls10and11from escaping into the through-holes24and gap19. Thus, the exhaust gas can be effectively fed to the turbine impeller4with no gas leakage and effectively rotate the turbine impeller4. Even if no flanges35are provided, by lowering the pressure P2in the gap19than ever, i.e., by reducing the force acting on the rear surface of the vane shaft16badjacent to the gap19, the nozzle vanes15may be displaced toward the rear wall11; however, the displacement of the nozzle vanes15is ensured by providing the flanges35.

FIG. 9shows a further embodiment of the invention in which a turbine housing1has an extension39extending into a position spaced apart from an outer periphery of a rear exhaust introduction wall11. Arranged between the extension39and the outer periphery of the rear wall11is a sealing device25which comprises sealing piston rings21fitted in a groove22, just likeFIGS. 1 and 2, which is formed on the extension39.

Also in the embodiment ofFIG. 9, the sealing device25is arranged upstream, in a travel direction of exhaust gas, of through-holes24via which vane shafts16bextend through the rear wall11, so that pressure P2of a gap19downstream of the sealing device25is lowered to have a relationship P1>P2with respect to pressure P1in the exhaust nozzle9. Thus, the exhaust gas in a exhaust nozzle9flows as shown by arrow B into the gap19downstream of the sealing device25. Due to the pressure difference of P1>P2, nozzle vanes15are pushed to the rear wall11, clearance between the respective nozzle vanes15and the rear wall11being minimized.

On the conditioning that a conventional turbocharger (conventional one) and a turbocharger of the invention (claimed one) shown inFIG. 3are made to have substantially same pressure ratios between upstream and downstream sides of turbine impeller4as shown inFIG. 10, the inventor determined velocity distribution of the exhaust gas at radial positions in turbine impeller outlet20through numerical analysis (at three points). The results are shown inFIG. 11.

As is clear fromFIG. 11, in comparison with the conventional one, the claimed one has radially flattened flow velocity distribution with less deviation. This means that the claimed one has less turbulence of exhaust gas in turbine impeller outlet20in comparison with the conventional one.

Moreover, turbine efficiency was compared between the claimed and conventional ones through numerical analysis. As a result, it was found out as shown inFIG. 12that the claimed one has turbine efficiency improved by about 10% relative to the conventional one.

On the conditioning that the conventional turbocharger (conventional one) and the turbocharger of the invention (claimed one) as shown inFIG. 3are made to have substantially same pressure ratios as shown inFIG. 10, the inventor actually measured turbine efficiency (at four points) with respect to three different rotational velocities a, b and c. The results are shown inFIG. 14. Also in this actual measurement, just like the above-mentioned results by the numerical analysis, the claimed one has turbine efficiency improved by about 10% relative to the conventional one.

The exhaust gas in the scroll passage8passes through the nozzle vanes15of the exhaust nozzle9to the turbine impeller4. Because of such exhaust gas flow being a complex three-dimensional stream, it is much difficult to find out factors in turbulence of the exhaust gas in the turbine impeller outlet20.

However, as mentioned in the above, the sealing device25for sealing of the gap19between the turbine housing1and the rear wall11is arranged upstream, in the travel direction of the exhaust gas, of the through-holes via which the vane shafts16bextend through the rear wall11, so that the difference in pressure between the gap19downstream of the sealing device25and the exhaust nozzle9can urge the respective nozzle vanes15to the rear wall11to minimize the clearance between the respective nozzle vanes15and the rear wall11, whereby the velocity distribution of the exhaust gas at radial positions in the turbine impeller outlet20become flattened to reduce turbulence of the exhaust gas in the turbine impeller outlet20; this can be construed to be considerable improvement in turbine efficiency. Thus, it was found out that the clearance between the respective nozzle vanes15and the rear wall11is one of factors affecting turbulence of the exhaust gas in the turbine impeller outlet20and thus affecting turbine efficiency. According to the embodiments of the invention, clearance between the respective nozzle vanes15and the front exhaust introduction wall10is increased by the preliminarily existing clearance for rotation and pivot of the nozzle vanes15(that is, clearance between the nozzle vanes15and the front wall10is widened by a degree corresponding to the decreased clearance between the nozzle vanes15and the rear wall11); it was found out even in such a case that the clearance between the nozzle vanes15and the front wall10hardly affects the turbulence of the exhaust gas in the turbine impeller outlet20and thus the turbine efficiency.

Thus, according to the invention, by the simple structure that the sealing device25for sealing of the gap19is arranged upstream, in the travel direction of the exhaust gas, of the through-holes24via which the vane shafts16bextend through the rear wall11, the nozzle vanes15are displaced to the rear wall11to minimize the clearance between the respective nozzle vanes15and the rear wall11, whereby turbine efficiency can be substantially improved.

In the embodiments mentioned in the above, the disc spring seal28has substantially S-shaped section (seeFIG. 3); however, the invention is not limited thereto. For example, the disc spring seal28inFIG. 3may be without the curved portion33or the rectilinear portion30; alternatively, it may comprises only the slant portion32(that is, it may be frustoconical with its diameter gradually and constantly increased toward the rear wall11. In such cases, the disc spring seal28may be simplified in structure, advantageously resulting in, for example, reduction in production cost.

It is to be understood that the present invention is not limited to the above embodiments and that various changes and modifications may be made without leaving the scope of the invention. For example, the sealing device may be of any structure among various structures.