Abstract:
To impede soot leakage around a shaft which extends through a bore connecting volumes of differing pressures, e.g., a turbocharger turbine housing and the ambient air, a soot seal is provided to capture particulate matter while allowing the passage of gas.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention concerns an improved seal for a shaft which passes through, e.g., the turbine housing of a turbocharger, and the turbocharger having such a seal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine&#39;s horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle. 
         [0003]    Turbochargers use the exhaust flow from the engine exhaust manifold to drive a turbine wheel ( 21 ), which is located in the turbine housing ( 2 ). Once the exhaust gas has passed through the turbine wheel, and the turbine wheel has extracted energy from the exhaust gas, the spent exhaust gas exits the turbine housing and is ducted to the vehicle downpipe and usually to after-treatment devices such as catalytic converters, particulate traps, and NO x  traps. 
         [0004]    In a wastegated turbocharger, the turbine volute is fluidly connected to the turbine exducer by a bypass duct. Flow through the bypass duct is controlled by a wastegate valve ( 61 ). Because the inlet of the bypass duct is on the inlet side of the volute, which is upstream of the turbine wheel, and the outlet of the bypass duct is on the exducer side of the volute, which is downstream of the turbine wheel, flow through the bypass duct, when in the bypass mode, bypasses the turbine wheel, thus not powering the turbine wheel. To operate the wastegate, an actuating or control force must be transmitted from outside the turbine housing, through the turbine housing, to the wastegate valve inside the turbine housing. For example, a wastegate pivot shaft may extend through the turbine housing. Outside the turbine housing, an actuator ( 73 ) is connected to a wastegate arm ( 62 ) via a linkage ( 74 ), and the wastegate arm ( 62 ) is connected to the wastegate pivot shaft ( 63 ). Inside the turbine housing, the pivot shaft ( 63 ) is connected to the wastegate valve ( 61 ). Actuating force from the actuator is translated into rotation of the pivot shaft ( 63 ), pivoting the wastegate valve ( 61 ) inside the turbine housing. The wastegate pivot shaft rotates in a cylindrical bushing ( 68 ), or directly contacts the turbine housing. Because an annular gap exists between the shaft and the bore of the bushing, in which the shaft is located, an escape of hot, toxic exhaust gas and soot from the pressurized turbine housing is possible through this clearance. 
         [0005]    Turbine housings experience great temperature flux. The outside of the turbine housing faces ambient air temperature while the volute surfaces contact exhaust gases ranging from 740° C. to 1050° C., depending on the fuel used in the engine. Typically the temperature around the wastegate pivot shaft is around 400° C. to 450° C. It is essential that the actuator, via the translated motions described above, be able to control the wastegate to thereby control flow to the turbine wheel in an accurate, repeatable, non-jamming manner. 
         [0006]    A VTG is used not only to control the flow of exhaust gas to the turbine wheel but also to control the turbine back pressure required to drive EGR exhaust gas, against a pressure gradient, into the compressor system to be re-admitted into the combustion chamber. The back-pressure within the turbine system can be in the region of up to 500 kPa. High pressure inside the turbine stage can result in the escape of exhaust gas to the atmosphere through any apertures or gaps. Passage of the exhaust gas through these apertures is usually accompanied by black soot residue on the exit side of the gas escape path. Deposits of this soot, generated by the engine combustion process, is unwanted from a cosmetic standpoint. This makes exhaust leaks a particularly sensitive concern in vehicles such as ambulances and buses. From an emissions standpoint, the soot which escapes from the turbine stage is not captured and treated by the engine/vehicle aftertreatment systems. The test for the escape of particulate matter is to simply wrap the turbine stage in aluminum foil, run the engine for a period of time, and visually inspect the foil for traces of soot which has escaped from the turbine stage of the turbocharger. 
         [0007]    The soot one sees coming from the exhaust stack of a Diesel engine is a mix of exhaust by-products in three basic phases: coarse phase, accumulation phase, and nuclei phase. Most of the particulate mass consists of carbonaceous agglomerates and associated adsorbed materials and is passed in the accumulation mode which has sizes in the 0.05 μm to 1.0 μm diameter range. The nuclei phase particles are typically volatile organics and sulfur compounds which have sizes in the 0.005 μm to 0.05 μm diameter range. While the particles in the nuclei phase are the greatest in number, they are only about 20% of the mass. Particles in the coarse phase range from 0.1 μm to 8 μm and contribute another 5-20% to the particulate mass. Coarse phase particles are typically accumulated on the walls of combustion and exhaust vessels and then re-entrained in the exhaust flow. 
         [0008]    Typically, some of the leakage of gas and soot through the annulus formed by a shaft rotating within a cylindrical bore was tolerated, as one or both of the end faces of the bushing are usually in contact with either the inboard flange of the valve arm or the outboard flange or surface of the driving arm of the wastegate control mechanism, thus blocking leakage some of the time. 
         [0009]    Seal means such as seal rings, sometimes also called piston rings, are commonly used within a turbocharger to create a seal between the static bearing housing and the dynamic rotating assembly (i.e., turbine wheel, compressor wheel, and shaft) to control the passage of oil and gas from the bearing housing to both compressor and turbine stages and vice versa. BorgWarner Inc. has had seal rings for this purpose in production since at least 1954 when the first turbochargers were mass produced. For a shaft with a seal ring boss of 19 mm diameter, rotating at 150,000 RPM, the relative rubbing speed between the seal ring cheek and the side wall of the seal ring groove is of the order of 149,225 mm/sec. 
         [0010]    Seal rings, of the variety which are used as described above, are sometimes used as a sealing device for relatively slowly rotating shafts (as compared to the 150,000 RPM turbocharger rapidly rotating assembly seals). These slowly rotating shafts move in rotational speeds of the order of 15 RPM which equates to a relative rubbing speed of 7 to 8 mm/sec. 
         [0011]    Seal rings, as used in turbochargers, create a seal by contacting part of the side wall of the seal ring against one side wall of a seal ring groove and contacting the outside diameter of the seal ring against the inside diameter of the bore in which the shaft resides. In order for the ring to be assembled to the shaft and then the shaft and ring be assembled into a bore, the depth of the seal ring groove must be such that the ring can collapse in outside diameter (and thus effective circumference and inside diameter) so that the outside diameter of the seal ring can assume approximately the inner diameter of the bore in which it operates.  FIG. 2A  depicts a seal ring ( 80 ) in the naturally expanded condition, albeit assembled to the shaft by forcibly expanding the ring over the diameter of the shaft ( 63 ) and then allowing the ring to relax into the groove. As the shaft, with the ring assembled on it, is pushed into the bore of the bushing ( 68 ), a chamfer ( 69 ) compresses the ring until the outside diameter of the ring can slide in the inside diameter ( 70 ) of the bushing. The now-compressed ring seals against the inside diameter of the bushing at any axial position of the shaft. 
         [0012]    In this condition, as depicted in  FIG. 3 , the seal ring ( 80 ) can axially reside at any axial position within the confines of the ring groove, the seal ring groove being defined as: the volume radially between the outside diameter of the shaft ( 63 ) and the diameter of the floor ( 82 ) of the seal ring groove and axially the distance between the inner ( 83 ) and outer ( 81 ) walls of the seal ring groove. With this definition of the seal ring groove, it can be seen that there always exists a volume under the ring (i.e., between the inside diameter ( 84 ) of the compressed piston ring and the diameter of the floor ( 82 ) of the seal ring groove). There also can exist a volume between the inner wall ( 83 ) of the seal ring groove and the adjacent wall of the seal ring. On the opposite side of the seal ring groove, there can also exist a volume between the outer wall ( 81 ) of the seal ring groove and the adjacent wall of the seal ring.  FIG. 3  depicts a condition in which the seal ring ( 80 ) is somewhat centered between the inner and outer walls ( 83  and  81 ) of the seal ring groove, thus allowing flow of gas and soot ( 86 ) around the seal ring. Since the axial position of the seal ring is controlled by the friction between the inner diameter of the bore in the bushing, and the ring is only moved by any contact with a side wall of a groove, a nearly complete sealing condition only exists when the seal ring sidewall is in direct contact with a seal ring groove side wall. In any other axial condition, the leakage path depicted in  FIG. 3  exists, and gas and particulate matter can escape the turbine stage through the shaft area. 
         [0013]    There are a number of patents which teach designs to reduce this leakage in the case of high speed rotating shafts by introducing multiple seal rings, and by modifying the pressure differential across the plurality of seal rings by introducing a pressure or vacuum between the rings; however, the potential for gas and soot leakage always exists unless the rings are in direct contact with the side wall(s) of the groove. In practice there is always some leakage, thus the aesthetics of the engine compartment can be compromised by the passage of soot or particulate matter associated with that leakage. 
         [0014]    Thus it can be seen that there is a need for a seal design which minimizes the passage of particulate matter independent of the exhaust gas leakage. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention solves the above problems by incorporating a semi-permeable sealing media within the elements which constrain and support the rotating or sliding shaft assemblies which pierce the wall of a turbocharger housing, thus minimizing the escape of potentially aesthetically compromising or potentially harmful soot or particulate matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention is illustrated by way of example and not limitation in the accompanying drawings in which like reference numbers indicate similar parts, and in which: 
           [0017]      FIG. 1  depicts a section for a typical wastegate turbocharger; 
           [0018]    FIGS.  2 A,B depict two sections showing seal ring compression; 
           [0019]      FIG. 3  depicts a section view showing gas leakage passage; 
           [0020]      FIG. 4  depicts a section view of the first embodiment of the invention; 
           [0021]      FIG. 5  depicts a section view of a variation to the first embodiment of the invention; and 
           [0022]      FIG. 6  depicts a view of the second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Gas and soot leakage from within a turbocharger to the ambient clean air surrounding a turbocharger is not permitted by engine manufacturers. Turbocharger manufacturers have been using solid piston rings, or seal rings, to seal gases, soot, and oil from communicating between the bearing housing cavity and either or both turbine and compressor stages ever since turbochargers were first in mass production in Diesel engines in the 1950s. Thus, it would be logical to engineer and apply such a seal to block any gas or material in less demanding locations on a turbocharger. 
         [0024]    Seal rings, of the variety which are used as described above, are sometimes used as a sealing device for relatively slowly rotating shafts (as compared to the 150,000 RPM turbocharger rotating assembly seals). These slowly rotating shafts move in rotational speeds of the order of 15 RPM which equates to a relative rubbing speed of 7 to 8 mm/sec. Even with the seal means described above, on both fast and relatively slowly moving shafts, there can be small escape of gases, soot, and other particulate matter which can negatively impact the aesthetics of an engine compartment. 
         [0025]    The inventor studied many complicated methods for reduction of the escape of gases, soot, and other particulate matter and came to the conclusion that, even with greatly reduced leakage rates, soot and other particulate matter still escaped and accumulated on the foil which was wrapped around the turbine stage during the test. Thus a method for minimizing leakage of soot and other particulate matter, independent of any measured leakage flow rate, was developed. 
         [0026]    To do this, the inventor would provide a seal which would be permeable to gas but non-permeable to the soot. For the purposes of this invention the definition of such a seal will be referred to as a gas-permeable-non-soot-permeable (GPNSP) seal. 
         [0027]    Several tests were performed with a simple packing of glass fiber, which is capable of withstanding temperatures up to 650° C., around the rotatable shaft, and these tests produced desirable results. For production purposes, in order to control the variability of the media, to quantify the amount of soot retention and to make assembly simple, several different materials presented in several forms were tested. Materials such as glass fiber, steel wool, ceramic mesh, and PTFE, were evaluated in structural forms such as resin impregnated fibers, woven fibers, and fibers with different stiffeners. 
         [0028]    In a first embodiment to the invention, as depicted in  FIG. 4 , an annular volume of the bushing ( 68 ), at the (outer) valve arm ( 62 ) end of the bushing, was provided by either widening the bushing internal diameter or, as shown in  FIG. 4 , narrowing the shaft diameter to allow space for the GPNSP media ( 34 ). The annular volume taken up by the GPNSP has an inside diameter close to that of the shaft against which it seals, an outside diameter contacting the bore in the bushing in which the GPNSP media is radially constrained, and a length to fill the “provided for” space. Gas is free to travel through the GPNSP, but particulate matter will travel up the aforementioned annular volume and be trapped in the GPNSP media. 
         [0029]    In a variation to the first embodiment of the invention, as depicted in  FIG. 5 , the GPNSP media is located as close as possible to the (inner) wastegate valve ( 61 ) end of the bushing ( 68 ). 
         [0030]    In a second embodiment to the invention, as depicted in  FIG. 6 , the full length of the bore ( 70 ) bushing ( 68 ) is used to control the position of the pivot shaft ( 63 ), and a counterbore or cylindrical extension ( 66 ) is provided in the valve arm ( 62 ) to retain a donut-like piece of GPNSP media ( 34 ). In this embodiment of the invention, the seal to the turbine housing is provided by the contact of the GPNSP media ( 37 ) with the lower surface ( 67 ) of the valve arm ( 62 ) and the upper surface ( 65 ) of the end of the bushing ( 68 ). Almost any material that can be used in a Diesel soot filter can be used as the seal material of the present invention. Catalysts used in regeneration of Diesel filters are not required in the present invention, but may provide some benefit in maintaining or prolonging the gas permeability of the seal. Preferred materials are materials that can withstand mechanical vibration. Thus, metal based materials such as steel wool and glass based materials such as fiberglass are preferred over ceramic based materials. 
         [0031]    The device within the turbine housing actuated by an actuating mechanism located outside the turbine housing is preferably a wastegate, wherein the shaft is a wastegate pivot shaft, wherein the actuator is connected to a wastegate arm and the wastegate arm is connected to the wastegate pivot shaft, and wherein the wastegate pivot shaft extends through the turbine housing and is connected to the wastegate valve. The wastegate arm ( 62 ) is preferably provided with a counterbore or cylindrical extension ( 66 ) to retain a donut-shaped piece of sealing material against the outside the turbine housing or the end of the bushing. 
         [0032]    Alternatively, the device within the turbine housing actuated by an actuating mechanism located outside the turbine housing is a variable turbine geometry (VTG) device comprising a unison ring for actuating vanes forming nozzle passages, wherein a VTG actuator is connected to an arm on the actuator shaft, and wherein the actuator shaft extends through the turbine housing or the bearing housing and is connected to a link arm connected to the unison ring. 
         [0033]    The sealing material is preferably fiberglass woven into a fabric and compressed into an annular shape. The sealing material is preferably in the form of glass fiber, steel wool, or ceramic mesh.