Patent Publication Number: US-2005123394-A1

Title: Compressor diffuser

Description:
BACKGROUND AND DESCRIPTION  
      The present invention relates to a diffuser for a compressor for a vehicle engine turbocharger.  
      A turbocharger for an internal combustion engine comprises a turbine side receiving exhaust gas from the engine to drive a turbine wheel connected to a shaft on which is mounted a compressor impeller wheel. Exhaust gas from the engine turns the turbine wheel and thus the shaft and causes rotation of the compressor impeller wheel. Intake air is drawn into the impeller wheel and its pressure boosted before it is fed to the engine and mixed with fuel for the combustion process. The increased pressure of the engine intake air increases the performance of the engine.  
      A turbocharger compressor operates at relatively low temperatures but relatively high pressure compared to the turbine.  
      It is important to control the flow of gas in turbochargers to ensure a steady flow and avoid surges and stalls. A diffuser typically is positioned in the flow path from the compressor wheel to the air outlet to control the flow of air by means of vanes in the gas flow path which even out or diffuse the air flow.  
      These vanes may be fixed in position or may be arranged to be moveable to vary their angle so as to better suit the gas flow in the diffuser to the operating conditions of the engine.  
      According to one aspect of the present invention there is provided a compressor for a vehicle engine turbocharger. The compressor comprises a diffuser assembly and an impeller assembly. The diffuser assembly comprises: a diffuser housing having a gas flow path having a side wall connecting a gas inlet to a gas outlet; a plurality of pivotally mounted diffuser vanes arranged in the flow path to control gas flow, and a vane angle control device for adjusting the angle of each of the plurality of vanes in the flow path; the control device comprising a unison ring coupled to the plurality of vanes in such a way that rotation of the unison ring relative to the vanes pivots each of the vanes by interaction of a cam surface with a respective cam follower. The impeller assembly, located upstream of the diffuser, comprises an impeller wheel and a plurality of impeller blades, a venting chamber and a shroud wall extending around the impeller blades, separating the blades from said venting chamber, wherein the shroud wall comprises at least one vent pneumatically connecting the impeller to said venting chamber.  
      Venting in the shroud wall acts to pull extra air into the compressor when the compressor is in a choke condition and to recirculate the air flow back toward the intake when the compressor is in surge condition. This suction and recirculation action is driven by pressure differentials between the intake and diffuser section. The larger the pressure differential, the larger the flow of air through the venting hole(s).  
      Some of the impeller blades may extend only partially across the space between the impeller wheel and the shroud wall. This form of the blades is known as “splitter blades”. The “splitter blades” form of impeller is generally considered to have a better flow range, i.e. operates over a wider range of operating conditions, because the removal of part, typically about half, of the blade, opens up the throat area of the inducer and allows the flow to adjust itself in choking conditions.  
      While such splitter blades can be used with the present invention, the impeller blades in the present invention are preferably all full blades, extending from the wheel across the flow path to substantially adjacent the shroud wall.  
      This has been found to increase the frequency of noise to levels above human sensitivity, and to reduce the noise level of the compressor because blade loading is decreased. Also, surprisingly, to have a flow range comparable with that of an impeller having splitter blades. This may be at least partly because the full blades cause a larger pressure change in the inducer throat region when the inducer is choked and thus causes increased suction to compensate the choke flow. It has also been found that when vents are provided in the shroud wall the full bladed impeller is more effective in the surge region than splitter blades.  
      In one of the more advanced embodiments of the present invention, there are two or more vents in the shroud wall and the air flow through them is controlled, advantageously independently of each other, for example by a sliding or rotating cover.  
      The design can be further simplified by having the unison ring comprise a substantial part of the flow path side wall, for example between 40-80% of the distance between the trailing edge of the impeller blade and the diffuser exit.  
      According to a preferred embodiment of the present invention the unison ring is mounted for rotation in a recess in the diffuser housing such that the side of the ring exposed to the gas path is generally flush with the remainder of the diffuser housing making up the flow path side wall.  
      Preferably each diffuser vane comprises a leading end and a trailing end and is pivotally mounted about a pivot point close to the leading edge.  
      The unison ring is coupled to the plurality of vanes in such a way that rotation of the unison ring pivots each of the vanes by interaction of a cam surface with a respective cam follower, and the cam follower has a generally elongate oval shape in cross section to engage the cam surface over a contact surface. The cam follower may be formed as a tab on each vane and the respective cam surfaces are formed as an internal surface of an elongate slot in the unison ring. The slot preferably has an arcuate form. The elongated oval shape of the cam follower may comprise a central generally rectangular region and two curved end regions, and a region having a trapezium cross-section formed between the rectangular region and each curved end section, so as to present at least three generally planar sides on each side of the cam follower. The cam surface is preferably contoured to be complementary to the engaging surface of the cam follower so as to maximize the area of the contact surface between the cam and the cam follower. Each vane may have an elongate isosceles triangle shape with the apex of the triangle forming said one end, wherein the angle subtended at the apex of the triangle is between about 5 degrees and 15 degrees, preferably about 10 degrees. At least one side of each vane may be curved or straight. The vane angle control device preferably further comprises a rack and pinion driven crank shaft, and a spring biased variable current solenoid, wherein the crank shaft is coupled to the solenoid via a cam on the crank shaft to provide direct position feedback to the solenoid. Each vane may be pivotally mounted by means of a pivot pin on the vane which engages with a hole in the diffuser housing. The pivot pin may be formed by grinding and may be mounted on the same side of the vane as the cam follower with the pivot pin extending beyond the tab formed by injection molding.  
      According to another aspect of the invention there is provided a compressor for a turbocharger, comprising a diffuser assembly and an impeller assembly, the diffuser assembly comprising a diffuser housing having a gas flow path having a side wall connecting a gas inlet to a gas outlet and a plurality of diffuser vanes arranged in the flow path to control gas flow. The impeller assembly, located upstream of the diffuser assembly, has a plurality of impeller blades mounted on an impeller wheel, a venting chamber and a shroud wall extending around the impeller blades and separating them from said venting chamber, wherein the shroud wall comprises at least one vent pneumatically connecting the impeller to said venting chamber. Preferably the impeller blades are all full blades extending from substantially adjacent the base of the impeller wheel to substantially adjacent the shroud wall extending to an inlet portion of the shroud wall.  
      The invention can provide for a more robust and controllable compressor with better operating conditions and performance. It can be used for compressor wheels with or without splitter blades but it works most efficiently for compressor wheels without splitter blades. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the present invention and to show how the same may be carried into effect, reference is made to the accompanying drawings in which:  
       FIG. 1  is a cross-section of a vehicle engine turbocharger compressor incorporating a diffuser according to the present invention;  
       FIG. 2  is a plan view of a part of the compressor diffuser shown in  FIG. 1 ;  
       FIG. 3  is a plan view of a vane forming part of the compressor diffuser in  FIGS. 1 and 2  illustrating its path of movement;  
       FIG. 4  is a plan view of an alternative design shape for the vane;  
       FIG. 5  is a cross-sectional view of the vane of  FIG. 3 ;  
       FIGS. 6   a  and  6   b  are cross-sectional views of alternative arrangements of the vane of  FIG. 3 .  
       FIG. 7  is a graph showing pressure ratio plotted against corrected air flow for the embodiment of  FIG. 1 .  
       FIG. 8  is a perspective view, from the side, of a compressor wheel used in a preferred embodiment of the invention.  
       FIG. 9  is a front view of a compressor wheel which may be used in an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      In  FIG. 1 a  turbine housing  12  is adapted to receive exhaust gas from a vehicle engine and channel the gas to a turbine wheel  14  coupled to one end of a shaft  16 . The exhaust gas drives the turbine wheel  14  and thus rotates the shaft  16 . The other end of the shaft  16  is connected to a compressor wheel  18 , which is mounted in a compressor housing  19 . The compressor wheel  18  rotates with the shaft  16  and draws in air through the intake  20 . The pressure of this air is boosted by the compressor wheel  18  and channeled through a diffuser section  22  of the compressor to an air outlet  24  and ultimately to the vehicle engine for use in the combustion process.  
      Compressor wheel  18  comprises a hub to which impeller blades  40  are attached. The blades  40  may be what are known in the industry as full blades or splitter blades. Full blades are shown in  FIG. 8  and a mixture of full and splitter blades is shown in  FIG. 9 . The full impeller blades  40  occupy the gap between the hub and an inner shroud wall  41  and have an outer edge substantially matching the profile of the inner surface of the shroud wall  41  to ensure a close tolerance. Splitter blades  50  do not extend as far axially forward as full blades. They are typically located between the full blades  40  as shown in  FIG. 9 . Compressors with splitter blades tend to be noisier than the compressors with full blades because the frequency of the noise is in the audible range and the noise level higher due to the extra loading on the blades. Returning to  FIG. 1 , there is an outer shroud wall  42  surrounding the inner shroud wall  41  and forming an annular venting chamber  43 .  FIG. 1  illustrates a single vent  44  that connects the venting chamber  43  to the impeller chamber within the inner shroud wall  41 . Such a venting arrangement provides a bleed path and improve the surge characteristics of the compressor by providing a vent path for back to return a small amount of pressurized gas to the intake. This arrangement is known as a ported shroud. More than one vent may be provided and each vent may take many forms such as individual bores or circumferential slots (with bridging support struts). Such vents may be arranged so that the airflow through them can be varied, for example with moveable covers so as to optimize the compressor performance depending upon operating conditions. These covers may be slidable or rotatable depending upon the form of the vents.  
      The compressor wheel may comprise splitter blades or full blades but full blades may be preferred in applications where the noises generated by the vents in the shroud wall are of concern. It has also been found that full bladed compressor wheel also makes the vented shroud more effective in improving the flow range of the compressor.  
      An arrangement of variable position vanes  26  is disposed in the diffuser section  22  and these cooperate with a unison ring  28  which controls their orientation relative to the air flow path. The unison ring  28  is rotatably disposed within the compressor housing  19  and is arranged to engage and rotate all of the compressor vanes in unison by cooperation of slots  32  in the unison ring  28  with tabs  34  on the vanes  26  acting as cam members.  
      The unison ring  28  is set into a recess in the wall of the diffuser section  22  and forms a part of the wall thereof. Since the diffuser effectively has two faces we are referring here to one half of the diffuser wall. This provides for a more robust arrangement and is more cost effective since less parts are required. Also the unison ring  28  has a pressure gradient across it which tends to move it axially toward the vanes  26  thus effectively eliminating any clearance gap between the vane side and the diffuser housing. Such a gap is a source of efficiency loss in known arrangements. The unison ring  28  may effectively be located radially inside of the vanes. It does not open to the gas path, that is to say that its outer peripheral edge is totally located within the recess and the side adjacent the gas path is arranged flush with the rest of the diffuser wall.  
      The unison ring  28  is a robust and hard wearing item which has a thickness of about 5% of the compressor wheel tip diameter. A thicker ring tends to reduce the effects of wear through contact but a thinner one reduces wear through vibration.  
      On the opposite wall of the diffuser section  22  an insert ring  30  is located, again set in an indentation in the compressor housing  19 .  
      The arrangement of the vanes  26  and the unison ring  28  is shown more clearly in  FIG. 2 . The vanes  26  are wedge shaped i.e. are relatively narrow tapering triangular members, each pivoted at pivot point  36  close to the apex of the triangle. Each has a tab  34  acting as a cam member to cooperate with the slot  32  on the unison ring  28 . Each cam member tab  34  has a relatively large surface area configured to provide a maximum area contact with the slots  32  on the unison ring  28 . In particular the tabs  34  are generally larger than pins and has a generally elongate oval shape. The slots  32  are shaped to match the shape of the tabs  34 . Such a tab and slot arrangement does not wear out as quickly as a pin and slot arrangement and provides better and more accurate connection and thus more accurate movement of the vanes. The major axis of each tab  34  is set at an inclined angle with respect to the longitudinal axis of each of the vanes  26  and the angle of each slot  32  in the unison ring  28  is adapted accordingly.  
      This is shown more clearly in  FIG. 3  which illustrates a series of positions which the tab  34  occupies in the slot  32  as it slides along the slot in response to the unison ring being rotated. This pivots the vane  26  about pivot point  36 , close to its leading edge.  
      An alternative shape and configuration of the tabs  34  is shown in  FIG. 4 . In this embodiment the vanes  26  are curved or cambered and take the shape of a fin with a wide end at the trailing edge where the tab  34  is located, tapering to a narrow end at the leading edge where the pivot  36  is located. The tab  34 , or cam follower, may be molded with the vane  26 .  
      The pivot point  36  of each vane  26  is set close to the apex of the triangle to ensure higher efficiency. It is generally desired to locate the pivot point of each vane within 10% of the apex and preferably within 10% of the trailing edges of the compressor wheel. This ensures that the leading edge of the vanes  26  is always at approximately the same distance from the compressor wheel  18  regardless of the angle of orientation of the vane and improves performance.  
      The pivot point  36  of each vane  34  is made as close to the apex of the triangular wedge as is practically possible to assist the aerodynamic loading of the vanes  34 , reducing stress on the vanes  34  under high compressor pressures.  
      The arrangement of the present invention provides a relatively simple and robust operating mechanism with relatively few parts, making it more hard wearing and cost effective to produce and assemble. Control of the vanes is particularly accurate and sensitive since a wider angle of rotation of the unison ring is required for a given rotation of the vanes.  
      The unison ring  28  is rotated by a crank mechanism  38  to alter the angle of the vanes  34 . One possible version of this crank mechanism  38  is described in U.S. 2003/0167767, which is incorporated herein by reference. The crank mechanism  38  is located at the top of the diffuser section  22 .  
       FIG. 5  is a cross-sectional representation of a vane  26  showing the tab  34  close to the trailing edge, engaged in a slot  32  in the unison ring  28 . The pivot  36  is close to the leading edge of the vane and is on the opposite side of the vane to the tab  34 . However, the pivot pin could be mounted on the same side of the vane as the tab  34  as shown in  FIG. 6   a , in which the pivot pin  36  is formed integrally with the vane  26 , and  FIG. 6   b , in which the pivot pin  36  is fixed to the vane  26  and less space is available for the unison ring  28 .  
      Adjusting the angle of the vanes  26  in the diffuser by rotating the unison ring  28 , causes the diffuser inlet and outlet areas to be adjusted and thus the diffuser flow area can be set at different values to suit different air mass flow rates. This helps to stabilize the diffuser flow and delay a compressor surge and thus extends the operating range of the compressor.  
      A combination of at least one vent  44  in the shroud wall  41  and the variable vanes  26  in the diffuser improves the operating range of the compressor and improves stability at higher compressor pressure ratios. Improved choke flows are also achievable with such an arrangement.  
      Referring to  FIG. 7 , it will be seen that the combination of ported shroud and variable vanes produces a more advantageous performance map than would otherwise be expected. The lines  50 - 56  joining the solid point markers represent the performance of a compressor using a variable diffuser and a vented shroud for compressor corrected speeds between 95,000 and 210,000 rpm. The lines  60 - 66  joining the shaded point markers, represent the performance of a compressor without a vented shroud for the same values of corrected compressor speeds. The corrected speed is the compressor physical speed corrected to a standard reference inlet condition. The numbers 62% to 75% on the Figure show compressor total to total efficiency.  
      It will clearly be seen that the results for the combination of the vented shroud and the variable values shown by the compressor map comprising lines  50 - 56  provides higher pressure ratios for given airflows and given corrected compressor speeds and thus results in superior performance, particularly at high compressor speeds.  
      Normally at higher pressure ratios, it is very difficult to achieve a wide flow range, but the vent  44  reduces the surge flow and increases the choke flow and thus improves the flow range whilst increasing the attainable compressor pressure ratios, and its efficiency. The combination also addresses the known problem of vaned diffusers having a tendency toward instability in that the vent  44  tends to make the compressor more stable.