Shaft seal arrangement of turbocharger

Abstract: A shaft seal arrangement of a turbocharger (1) for an internal combustion engine having a throttle valve disposed upstream of a compressor of the turbocharger. First and second seal rings (12,13) are fitted on a cylindrical thrust spacer (11) fixed on a rotatable shaft (2) on which turbine and compressor wheels (4,3) are mounted. The seal rings (12,13) are spaced from each other and positioned between the compressor and bearings (6) for the shaft (2). An annular air chamber 14 between the seal rings (12,13) is supplied with atmospheric air through an air supply passage (15). The cross-sectional area of the air supply passage (15) is such that a pressure difference between the air chamber (14) and the bearing side is prevented from exceeding an experimental pressure difference value at which oil leak from the bearing side to the compressor side begins, thereby effectively avoiding oil leakage even in an engine operating condition in which the throttle valve is fully closed.

Description: This invention relates to improvements in a shaft seal arrangement in a turbocharger, and more particularly to a shaft seal arrangement for preventing oil leak from a bearing side to a compressor side of the turbocharger. A turbocharger installed in an internal combustion engine is usually provided with a shaft seal arrangement for preventing lubricating oil supplied to bearings from leaking out to the compressor side of the turbocharger. The bearings support a rotatable shaft on which a turbine wheel and a compressor wheel are fixedly mounted. Such a shaft seal arrangement is disclosed, for example, in GB-A-1 045 973 and in Japanese Utility Model Provisional Publication Nos. 60-116035 and 61-166134, in which the rotatable shaft of the turbocharger is provided with two seal rings for preventing oil leakage. An air supply passage is formed to supply air into between the two seal rings. In this case, oil leakage from the bearings is suppressed by cancelling a pressure difference between the seal rings and the bearing housing. When a throttle valve of an engine is disposed upstream of the compressor, as in the turbocharger arrangement disclosed in WO-88100283, for example, a greater vacuum is developed in the compressor in an engine operating condition in which the throttle valve is closed. The present inventors have found that the amount of air sucked out from between the seal rings to the compressor side may increase to such an extent that the air pressure between the seal rings falls, owing to a pressure drop of air flowing through the air supply passage. The inventors have also found that this fall in air pressure can cause lubricating oil to leak from the bearing housing to the compressor side of the turbocharger. It is an object of the present invention to improve a shaft seal arrangement in a turbocharger, in order to prevent lubricating oil from leaking out from the side of a bearing for a rotatable shaft of the turbocharger, thereby to reduce lubricating oil consumption. A method in accordance with the present invention is defined in claim 1. A shaft seal arrangement in accordance with the present invention is defined in claim 5. With the arrangement designed and provided by the method according to the invention, a sufficient amount of air can be supplied to the location between the first and seal seal rings even under an operating condition in which air in the above-mentioned location is sucked out to the compressor side through the clearance of the seal ring. This prevents an increase in pressure difference between the above-mentioned location and the bearing side (at atmospheric pressure) over the minimum seal performance, i.e. ΔPmin (or an experimental pressure difference value at which oil leak from the bearing side to the compressor side begins), thereby preventing oil leakage from the bearing side to the compressor side. This largely supresses oil consumption while preventing generation of white smoke in exhaust gas. The invention will be described further, by way of example, with reference to the accompanying drawings, like reference numerals designate corresponding elements and parts throughout all the figures, and in which: Fig. 1 is a longitudinal section view of a turbocharger including a first embodiment of a shaft seal arrangement according to the present invention; Fig. 2 is an enlarged fragmentary sectional view of an essential part of the shaft seal arrangement of Fig. 1; Fig. 3 is a fragmentary sectional view of a turbocharger including a second embodiment of the shaft seal arrangement according to the present invention; Fig. 4 is an enlarged sectional view of the shaft seal arrangement of Fig. 3; Fig. 5 is an enlarged sectional view similar to Fig. 4 but showing a modified example of the shaft seal arrangement of Figs. 3 and 4; and Fig. 6 is a fragmentary sectional view similar to Fig. 3 but showing another modified example of the shaft seal arrangement of Figs. 3 and 4. Referring now to Figs. 1 and 2, there is shown a first embodiment of a shaft seal arrangement in a turbocharger 1, in accordance with the present invention. The turbocharger 1 in this instance is in an automotive vehicle and includes a rotatable shaft 2 on which a compressor wheel 3 and a turbine wheel 4 are coaxially fixedly mounted spaced from each other. The compressor wheel 3 forms part of a compressor (no numeral) for intake air. The turbine wheel 4 forms part of a turbine and is rotated by exhaust gas. The rotatable shaft 2 is journalled through a floating metal 6 on a bearing housing 5. The floating metal 6 is maintained in a floating state relative to the inner supporting surface of the bearing housing 5 under the influence of engine lubricating oil supplied through an oil passage 7 leading from an engine oil lubricating system (not shown). A compressor housing 8 surrounding the compressor wheel 3 is connected through a fixed support member or back plate 9 to the bearing housing 5. A cylindrical thrust spacer 11 is mounted or fitted on the rotatable shaft 2 and rotatably fitted in a bore (no numeral) of the back plate 9. Two seal rings 12, 13 are generally coaxially interposed between the thrust spacer 11 and the back plate 9 thereby to accomplish a fluid-tight seal between the bearing housing 5 and the compressor housing 8. More specifically, each seal ring 12, 13 is disposed in an annular groove (21 in Fig. 2) formed coaxially on the peripheral surface of the thrust spacer 11 as shown in Fig. 2, in which the outer periphery of each seal ring 12, 13 is contactable with the inner peripheral surface (defining the bore) of the back plate 9. An annular air chamber 14 surrounds the peripheral surface of a longitudinally intermediate part of the thrust spacer 11. The air chamber 14 communicates through an air supply passage 15 with an air intake passageway 17 upstream of a throttle valve 16. The air intake passageway 17 is fluidly connected through the compressor wheel 3 to engine cylinders (not shown) of the engine. The throttle valve 16 is rotatably disposed in the intake air passageway 17 upstream of the compressor wheel 3 to control the amount of air to be supplied to the engine cylinders. In the above arrangement, the volume flow rate Q of air sucked from the air chamber 14 to the side of the compressor wheel 3 is smaller than a so-called choke flow rate or a flow rate of air flowing when the throttle valve 16 is fully closed. Therefore, the volume flow rate Q for a maximum clearance area S for the seal ring 13 under a standard condition of atmospheric pressure (P₀) and a standard temperature, T, of 288K (15°C) is given by the following equation: where K is the ratio of specific heats; T is the temperature in the air chamber 14; g is gravitational acceleration; and R is the gas constant of air. The maximum clearance area S is determined by the equation S = min(x₁, x₂) x 2πr where x₁ and x₂ are respectively the opposite side clearances of the seal ring 13 relative to the wall surfaces of the groove 21; and r is the inner diameter of the seal ring 13 as shown in Fig. 2. A pressure difference ΔP between the bearing housing 5 and the air chamber 14 must be smaller than a minimum seal performance ΔPmin. Here, the flow velocity v of air flowing through the air supply passage 15 in the standard condition is given by the following equation: where γ is the density of air. In the air supply passage 15, air must flow at this flow velocity and in the above-mentioned flow rate Q, and therefore the cross-sectional area A of a through-hole 18 forming part of the air flow passage 15 should be in a relationship as represented by the following equation derived from the above-equations (1) and (2): where K is the ratio of specific heats; T is the temperature in the air chamber; R is the gas constant of air; and γ is the density of air. The minimum seal performance is determined by the following experiment: An experimental device is arranged in the same condition as in the turbocharger 1 shown in Fig. 1. The rotatable shaft (2) of the experimental device is rotated under a condition in which oil mist is filled on the side of the bearing housing (5), during which the pressure difference between the opposite positions of the ring seals is altered. A pressure value of the pressure difference at which oil mist begins to leak through the seal rings is determined as the minimum seal performance ΔPmin. The operation of the thus arranged intake system will be discussed hereinafter. The inside of the bearing housing 5 is maintained at about atmospheric pressure although there is a slight leak of exhaust gas from the side of the turbine wheel 4 into the bearing housing 5. The air chamber 14 is also maintained at about atmospheric pressure because of being supplied with atmospheric pressure prevailing in the air intake passageway 17 upstream of the throttle valve 16. In an engine operating condition in which the throttle valve 16 is closed, the back side of the compressor wheel 3 is under vacuum and therefore air is sucked out from the air chamber 14 to the side of the compressor wheel 3. The amount of the sucked out air is considerably small and therefore negligible. At this time, if the amount of air flowing through the air supply passage 15 becomes larger over a predetermined level, the air chamber 14 becomes under vacuum owing to a pressure drop in the air supply passage 15. This seems to cause oil leak through the seal ring 12 from the bearing housing 5. However, according to this embodiment, the cross-sectional area of the air supply passage 15 is determined so that the pressure difference between the air chamber 14 and the bearing housing 5 does not exceed the minimum seal performance ΔPmin. Accordingly, the air chamber 14 is prevented from becoming vacuumized, thereby securely avoiding oil leakage from the bearing housing 5. Figs. 3 and 4 illustrate a second embodiment of the shaft seal arrangement in accordance with the present invention, which is similar to the first embodiment shaft seal arrangement. In this embodiment, the two seal rings 12, 13 are disposed between the cylindrical thrust spacer 11 and the back plate 9. More specifically, the seal rings 12, 13 are respectively fitted in the two annular grooves 21A, 21 formed coaxially on the outer peripheral surface of the thrust spacer 11, and contactable at its outer peripheral surface with the surface 38 of the bore of the back plate 9. The annular air chamber 14 is defined between the seal rings 12, 13 and between the thrust spacer 11 and the back plate 9. The air chamber 14 is open to atmospheric air through the through-hole 18 and the air supply passage 15. The rotatable shaft 2 is journalled in the bearing housing 5 through a bearing metal 29 which is supplied with lubricating oil through a lubricating oil passage 27. The lubricating oil circulates through a bearing chamber Y. A thrust collar 28 is disposed between the bearing metal 29 and the thrust spacer 11. In this embodiment, the surface 38 of the bore of the back plate 9 is cylindrical so that the seal rings 12, 13 are axially slidably supported by the bore surface 38. The back plate 9 is integrally formed with an annular stop section 39 which extends radially inwardly from the bore surface 38. The annular stop section 39 is formed on the side of the compressor wheel 3. The seal ring 13 at the compressor wheel side is contactable at its side surface with the annular stop section 39, thereby restricting movement of the seal ring 13 toward the compressor wheel 3. Each seal ring 12, 13 is fitted in the annular groove 21A, 21 with a predetermined side clearance (a total clearance between the side surface of the seal ring and the side surface of the annular groove). The side clearance for the annular groove 21A and seal ring 12 at the bearing metal side is set at about 50 µm taking only machining error into consideration. The side clearance for the annular groove 21 and for the seal ring 13 on the compressor wheel side is set at a range of from 90 to 150 µm which is not less than 1.8 times the above side clearance of about 50 µm, taking into account machining error and the axial displacement of the rotatable shaft 2. In operation, the seal ring 12 seems to displace toward the side of the compressor wheel 3 under the action of a pressure difference (about 600 mmHg in maximum) between the back side X of the compressor wheel 3 and the bearing chamber Y. However, the seal ring 13 is brought into contact with the annular stop 39 and therefore is prevented from displacement toward the compressor wheel 3. As a result, the seal ring 13 can be prevented from being pressed against the side surface of the groove 21. This prevents wear of seal ring 13 while suppressing the friction loss of the rotatable shaft 2 to a lower level, thus maintaining a high acceleration response of the turbocharger. The annular air chamber 14 is supplied with atmospheric air and therefore always maintained at about atmospheric pressure regardless of engine operating conditions. Consequently, a pressure difference is hardly developed between the annular chamber 14 and the bearing chamber Y so that no pressure difference is applied to the seal ring 12 on the bearing metal side. As a result, a force for pressing the seal ring 12 against the side wall of the groove 21A is very small, and accordingly wear of the seal ring 12 can be prevented though there is no annular stop section for the seal ring 12. This suppresses the friction loss of the rotatable shaft 2 at a lower value, thereby maintaining a high acceleration response of the turbocharger. Since no step section is formed at the bore surface 38 of the back plate 9, the seal ring 12 is axially slidably supported and therefore the side clearance for the seal ring 12 and the annular groove 21A is not required to be set larger to take account of the axial displacement amount (about 40 to 100 µm) of the rotatable shaft 2. Consequently, the side clearance for the seal ring 12 is set, for example, at 50 µm, taking into account only machining accuracy of annular groove 21A. As a result, the amount of lubricating oil leaking through the clearance between the seal ring 12 and the side surfaces of the annular groove 21A is minimized thereby providing a sufficient sealing ability. The back plate 9 is formed with only one annular stop section (a step section) which requires a relatively high machining accuracy, thereby lowering production cost of the turbocharger. Besides, since only an annular tapered section 40 at an end section of the back plate bore surface 38 is required, the axial length of the rotatable shaft required for shaft seal is shortened and therefore the flexural rigidity of the rotatable shaft 2 is increased, thereby improving the vibrational characteristics and durability of the rotatable shaft 2. Fig. 5 shows a modified example of the second example shaft seal arrangement. In this example, a further annular groove 46 is formed at the outer peripheral surface of the thrust spacer 11 and located between the annular grooves 21A, 21. The annular groove 46 forms part of the annular air chamber 14, thereby increasing the volume of the air chamber 14 formed between the thrust spacer 11 and the back plate 9. By virtue of this air chamber 14 having an increased volume, a pressure variation within the annular air chamber 14 is suppressed thereby providing a stable sealing ability. Besides, the annular groove 46 decreases the rotating mass of the turbocharger and does not obstruct the installation operation of the seal ring 12 as compared with a case in which the corresponding annular groove (46) is formed in the back plate (9). Fig. 6 shows another example of the second embodiment shaft seal arrangement in accordance with the present invention. In this example, an annular stopper plate 52 is installed to the back plate 9 at the end face 57 opposite to the compressor wheel 3 by means of small screws 53. The outer peripheral section of the annular stopper plate 52 extends radially inwardly from the bore surface 38 of the back plate 9 and therefore serves as the annular stop section (39) for restricting the movement of the seal ring 13. In this example, the back plate 9 is formed at its end surface opposite to the compressor wheel 3 with an annular groove 55 in which an annular projection 56 of the stopper plate 52 is fitted, so that installation accuracy of the stopper plate 52 is improved. It will be understood that installation of the annular projection 56 of the stopper plate 52 may be made by means of press-fitting or with small screws 53. With this arrangement, precise locating of the seal ring 13 is controlled by the side end face 57 of the back plate 9 thereby to lower the production cost of the turbocharger.

Claims: A method of designing and providing a shaft seal arrangement in a turbocharger (1) of an internal combustion engine having a throttle valve disposed upstream of a compressor of the turbocharger, the shaft seal arrangement comprising: first and second seal rings (12, 13) disposed around and sealingly connected to a rotatable shaft (2) of the compressor and located between the compressor and a bearing (6) for the rotatable shaft (2), the first and second seal rings (12, 13) being located at the bearing side and the compressor side, respectively, to prevent oil leakage from the bearing side to the compressor side; and an air supply passage (18) between the compressor and the bearing (6), through which passage (18) a location between the seal rings (12, 13) is supplied with substantially atmospheric pressure; the method including the steps of: (a) determining the minimum seal performance, ΔPmin, of the first seal ring (12); (b) determining the maximum clearance, S, of the second seal ring; and (c) setting the cross-sectional area, A, of the air supply passage (18) in accordance with the following relationship at standard atmospheric pressure: where K is the ratio of specific heats, R is the gas constant of air, γ is the density of air, and T is the absolute temperature which prevails in the said location between the seal rings (12, 13) and which is taken to be the standard temperature of 288K (15°C). A method as claimed in claim 1, in which the shaft seal arrangement includes a cylindrical thrust spacer (11) fitted on the rotatable shaft (2), the thrust spacer (11) being formed with first and second coaxial annular grooves in which the first and second seal rings (12, 13) are disposed respectively, the said maximum clearance S being defined between the second seal ring (13) and a wall surface of the thrust spacer defining the first annular groove, the seal rings (12, 13) being slidingly contactable with a fixed support member (9) for rotably supporting the thrust spacer (11). A method as claimed in claim 1 or 2, in which the shaft seal arrangement includes means for maintaining the bearing (6) in a floating state under the influence of oil, and means for supplying the said oil around the bearing. A method as claimed in claim 1 or 2, in which the shaft seal arrangement includes means defining a generally cylindrical supporting surface of constant diameter on which the first and second seal rings (12, 13) are slidably supported at their outer periphery, the said cylindrical supporting surface being formed around the rotatable shaft (2), and a stop section in fixed relation to the said cylindrical supporting surface defining means and located at the compressor side relative to the second seal ring (13), the stop section extending radially inwardly relative to the cylindrical supporting surface, said second seal ring being contactable with the stop section, and means defining first and second annular grooves provided on the side of the rotatable shaft (2), the first and second seal rings (12,13) being fitted in the respective first and second annular grooves with predetermined side clearances. A shaft seal arrangement in a turbocharger (1) for an internal combustion engine having a throttle valve disposed upstream of a compressor of the turbocharger, the shaft seal arrangement comprising: first and second seal rings (12, 13) disposed around and sealingly connected to a rotatable shaft (2) of the compressor and located between the compressor and a bearing (6) for the rotatable shaft (2), the first and second seal rings (12, 13) being located at the bearing side and the compressor side, respectively, to prevent oil leakage from the bearing side to the compressor side; and an air supply passage (18) through which a location between the seal rings (12, 13) is communicated with a region at substantially atmospheric pressure; characterised by: means defining a generally cylindrical supporting surface of constant diameter on which the seal rings (12, 13) are slidably supported at their outer periphery, the cylindrical supporting surface being formed around the rotatable shaft (2); a stop section in fixed relation to the cylindrical supporting surface defining means and located at the compressor side relative to the second seal ring (13), the stop section extending radially inwardly relative to the cylindrical supporting surface, the second seal ring (13) being contactable with the stop section; and means defining first and second annular grooves provided at the side of the rotatable shaft (2), the first and second seal rings (12, 13) being fitted in the respective first and second annular grooves with predetermined side clearances; the cross-sectional area, A, of the air supply passage (18) being in accordance with the relationship specified in claim 1.

Inventors: AIDA MASAHIRO, YAGI YOICHI, AIDA, MASAHIRO, YAGI, YOICHI
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: F01D25
IPC codes: F16J15, F01D25