Abstract:
A pressure equalization system reduces or eliminates a pressure differential across supercharger rotor shaft seals. Under high boost, rotor shaft seals often fail, allowing hot compressed air into an oil lubricated space containing rotor bearings and gears (and vented to ambient pressure), reducing oil lubricating effectiveness and resulting in increased wear and failure. Under low or non boost operation, the pressure differential is reversed causing the lubricating oil to leak into the supercharger interior and accelerated rotor seal wear. The pressure equalization system includes flow restrictive seals on both rotor shafts, separated from the rotor shaft seals by vented spaces, thereby isolating the rotor shaft seals from boost or vacuum in the supercharger interior and reducing or eliminating the pressure differential across the rotor shaft seals. Maintaining close to atmospheric pressure on both sides of the rotor shaft seals during boost and vacuum operation reduces wear and failures.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to supercharger seals and in particular to equalizing a pressure difference across a supercharger rotor shaft seal. 
     Power production of an internal combustion engine is ultimately limited by the amount of air pumped through each engine cylinder. Fuel systems can at best provide an optimal amount of fuel to burn with the air contained in the cylinder, and adding more fuel than required for a stoichiometric air-fuel ratio does not result in more energy being produced. The power production of non-supercharged engines is thus limited by the engine&#39;s ability to draw air into each cylinder, referred to the Volumetric Efficiency (VE) of the engine, where 100 percent VE is equivalent to complete filling of the cylinder at bottom dead center at one atmosphere of pressure. While some engines achieve greater than 100 percent VE using tuned intake manifolds providing a ram effect, the effects are generally limited to a small RPM range which the intake is tuned to. 
     Power production may also be realized by raising the RPM that an engine is operated at, thereby pumping more air through the engine. Unfortunately, high RPM operation requires cam lobe designs which are inefficient at low RPM, and is also stressful on engine parts. 
     An alternative method for increasing power production is to pump (or force) air into the engine. This approach is commonly called supercharging because more air is forced into each cylinder than 100 percent VE produces. For many years, supercharging was limited to special applications because of the power required to operate the supercharger (i.e., the parasitic draw of the supercharger) resulting in reduced fuel economy under all operating conditions. 
     One known supercharger is a screw compressor type supercharger employed to pump air into the engine at greater than atmospheric pressure to increasing horsepower. Screw compressor superchargers employ a pair of rotating screw elements (or rotors), within a confined cylindrical housing. The rotating screw elements draw air from a throttle body at a rear end of the housing and push the air progressing toward a forward end of the housing thereby compressing the air. The compressed air then flows into an intake manifold of the internal combustion engine. Providing the compressed air (commonly referred to as boost) and a corresponding amount of fuel, dramatically increases engine horsepower production and allows immediate and tremendous acceleration. 
     Twin screw type superchargers draw air into the rear of the supercharger and compress the air as it travels from the rear to the front of the supercharger between supercharger rotors, resulting in high pressures at the front of the supercharger. Because the rear of the supercharger must be open to provide a passage for air to enter the supercharger housing, the rotor (or timing) gears are generally at the front of the supercharger, along with rotor shaft bearings, and lubricating oil is present to lubricate the rotor gears and bearings. Front rotor shaft seals are necessary to prevent hot compressed air in the front of the housing from escaping from the housing and heating the lubrication oil, thereby reducing the effectiveness of the oil and causing gear and/or bearing failure, and to prevent the lubricating oil from leaking into the interior of the supercharger. 
     An unresolved weakness of twin screw superchargers has been the reliability of front rotor shaft seals at high boost levels. While the seals work well at between eight and twenty pounds of boost, increased wear has been observed above twenty pounds of boost. In the past, when boost was typically below twenty pounds the seal failure was not a significant problem. However, modern twin screw superchargers often produce greater than twenty pounds of boost and as a result, seal reliability has become a significant issue. Further, during part boost or no boost, the front rotor shaft seals are known to fail under vacuum and allow the lubricating oil to enter the supercharger interior. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing a pressure equalization system which reduces or eliminates a pressure differential across supercharger rotor shaft seals. Under high boost, rotor shaft seals often fail, allowing hot compressed air into an oil lubricated space containing rotor bearings and gears (and vented to ambient pressure), reducing oil lubricating effectiveness and resulting in increased wear and failure. Under low or non boost operation the pressure differential is reversed causing the lubricating oil to leak into the supercharger interior and accelerated rotor seal wear. The pressure equalization system includes flow restrictive seals on both rotor shafts, separated from the rotor shaft seals by vented spaces, thereby isolating the rotor shaft seals from boost or vacuum in the supercharger interior and reducing or eliminating the pressure differential across the rotor shaft seals. Maintaining close to atmospheric pressure on both sides of the rotor shaft seals during boost and vacuum operation reduces wear and failures. 
     In accordance with one aspect of the invention, there is provided a combination of the close clearance between rotor ends and an outlet end wall, flow restrictive seals, and the vented intermediate spaces between the flow restrictive seals and rotor shaft seals. The combination of elements reduces a pressure differential across the rotor shaft seals and thereby allows high boost without increased wear and supercharger failure. The reduction of the pressure differential further prevents damage and wear during negative boost (vacuum) conditions. 
     In accordance with another aspect of the invention, there are provided flow restrictive seals and vented spaces between the flow restrictive seals and rotor shaft seals. The combination of the flow restrictive seals and the vented spaces allows use of lower friction rotor shaft seals, and a reduced pressure differential across the rotor shaft seals further reduces friction, thereby reducing the creation of heat by the rotor shaft seals and the power consumption by the rotor shaft seals. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1A  is a side view of a supercharged engine according to the present invention. 
         FIG. 1B  is a top view of the supercharged engine according to the present invention. 
         FIG. 1C  is a front view of the supercharged engine according to the present invention. 
         FIG. 2A  is a side view of a supercharger and intake manifold according to the present invention. 
         FIG. 2B  is a top view of the supercharger and intake manifold according to the present invention. 
         FIG. 3  is a cross-sectional view of the supercharger and intake manifold according to the present invention taken along line  3 - 3  of  FIG. 2B . 
         FIG. 4  is a cross-sectional view of the supercharger outlet end wall taken along line  4 - 4  of  FIG. 2A . 
         FIG. 5  is a cross-sectional view of the supercharger outlet end wall taken along line  5 - 5  of  FIG. 4 . 
         FIG. 6  is a detailed view of the supercharger outlet end seals according to the present invention. 
         FIG. 7  shows a top view of the supercharger with the outlet end wall vented to the supercharger air intake. 
         FIG. 8  shows the outlet end wall vented to a filter. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims. 
     A side view of a supercharged engine  10  according to the present invention is shown in  FIG. 1A  and a top view of the supercharged engine  10  is shown in  FIG. 1B . The supercharged engine  10  includes a screw compressor type supercharger  12  attached to an intake manifold  20 . The screw compressor type supercharger  12  compresses air received through a throttle body  16  and provides the compressed air to the supercharged engine  10  through the intake manifold  20  and into the engine  10 . The screw compressor type supercharger  12  is driven by a belt  14  connecting a crankshaft pulley to a supercharger pulley. 
     A side view of the screw compressor type supercharger  12  according to the present invention is shown in  FIG. 2A  and a top view of the screw compressor type supercharger  12  is shown in  FIG. 2B . A supercharger pulley  18  is attached to the screw compressor type supercharger  12  at a front (outlet) end  12   a  of the supercharger, and the throttle body  16  is attached at a rearward end  12   b . While the supercharger is shown as having the outlet end to the front, belt drives may also be provided to position the inlet end of the supercharger to the front and the supercharger driven from the rear and both the belt and inlet can be at the same end, and such variations are intended to come within the scope of the present invention. 
     A cross-sectional view of the screw compressor type supercharger  12  taken along line  3 - 3  of  FIG. 2B  is shown in  FIG. 3 . A first rotor  24  and a second rotor  26  are rotatably housed in an interior of a housing  13  of the screw compressor type supercharger  12 . The rotors  24  and  26  are turned by the pulley  18  and gears  47   a  and  47   b  (see  FIG. 4 ) and draw ambient air  28  through the throttle body  16  and through the rear (inlet) end  12   b  and into the screw compressor type supercharger  12 . The ambient air is compressed as it passes through the screw compressor type supercharger  12  by the rotors  24  and  26 . The compressed air  29  is pumped through compressed air passage  30  and the intake manifold  20  into the engine  10 . 
     Known superchargers include rotor shaft seals  45  between the rotors  24  and  26 , and the rotor shaft bearings  48 , and a outer shaft seal  49  between a gear space  46  at the outlet end  12   a  of the supercharger  12  containing the rotor gears  47   a  and  47   b , and the pulley  18 . The rotor shaft seals  45  may be single or double lip seals. The rotor gears  47   b  and  47   b  reside in the space  46  between the seals  45  and seal  49  and the rotor shaft bearings  48  are exposed to the space  46 . The space  46  contains lubricating oil for lubricating the gears  47   a  and  47   b  and the bearings  48 . The rotor shaft seals  45  are intended to prevent compressed air inside the interior  13   a  of the supercharger  12  from escaping into the space  46  and prevent the lubricating oil in the space  46  from entering the interior  13   a  of the supercharger  12 . 
     Under part or no load, the supercharger  12  internal pressure is reduced and is often below atmospheric pressure (i.e., positive vacuum). Under part load the absolute pressure inside the supercharger may be as low as 0.5 bars, and coasting, as low as 0.05 bars, resulting in a pressure difference across the rotor shaft seals  45  tending to urge the lubricating oil into the interior  13   a  of the supercharger  12 . As the pressure difference grows, the friction between the seal lips and the seal ring increases which is a primary cause of seal wear. 
     Further, the power produced by a supercharging internal combustion engine  10  is increased by increasing the supercharger  12  boost pressure. Increasing the boost pressure results in increased pressure and temperature at the outlet end  12   a  of the supercharger  12 . If the boost pressure is very high, for example, greater than twenty pounds, the increased pressure has resulted in the hot compressed air in the interior  13   a  of the supercharger  12  escaping past the seals  45  (see  FIG. 5 ) past the bearings  48  and into the space  46  at the outlet end  12   a  of the supercharger  12  which contains supercharger rotor (or timing) gears  47   a  and  47   b , thereby causing increased wear and eventually failure of the seals  45 , the bearings  48 , and the gears  47   a  and  47   b . One solution to the potential leakage of the hot compressed air into the space  46  is to add flow restrictive seals between the rotors  24  and  26  and the bearing  45 . Unfortunately, merely adding such seals does not sufficiently limit the flow of the hot compressed air into the space  46  to prevent wear and failure. 
     Single lip seals might be used, with the lips opening outward under boost, away from the rotor shafts and against the seal seat, and seal wear is not a problem, but the compressed air flowing from the supercharger interior into the space  46  carries lubricating oil out of the space  46  through the vent  53 . 
     Another potential measure is to controllably vent the space  46  to ambient air through a vent  53  to allow the hot compressed air to escape the space  46  in a controlled manner, for example, not blowing the lubricating oil onto the supercharger pulley  18  and belt  14 . However, such vent  53  still allows the escape of the lubricating oil under high boost when the hot compressed air pushes past the sealing lips of the shaft seals  45  and into the space  46  and create a mist of the hot compressed air and the lubricating oil from space  46  through the vent  53  to the ambient air. Such vent  53  also does not address the flow of lubricating oil from the space  46  into the supercharger interior  13   a  under vacuum. 
     A top cross-sectional view of the supercharger outlet end  12   a  according to the present invention, taken along line  4 - 4  of  FIG. 2A , is shown in  FIG. 4 , a front cross-sectional view of the supercharger outlet end wall  44  taken along line  5 - 5  of  FIG. 4  is shown on  FIG. 5 , and a detailed cross-sectional view of the seals  45  and  51  is shown in  FIG. 6 . The supercharger  12  according to the present invention addresses the above problems by providing a close clearance  41  between ends of the rotors  24  and  26  and outlet end wall  44 , flow restrictive seals  51 , and vented intermediate spaces  43  between the seals  51  and the shaft seals  45 . The clearance  41  is preferably approximately 0.2 mm. Both sides of the rotor shaft seals  45  are thus vented to ambient air pressure. 
     The combination of the close clearance  41  and the flow restrictive seals  51  limits the escape of the hot compressed air to the bearings  48 , gears  47   a  and  47   b , and lubrication oil. The vents  50  and  53  keep the pressure in both the spaces  43  and  46  straddling the rotor shaft seals  45  near ambient air pressure and thus at about the same pressure, thereby limiting any flow past the rotor shaft seals  45 . The novel synergistic combination of the close clearance  41 , the flow restrictive seals  51 , and the vented intermediate spaces  43  between the seals  51  and the seals  45 , allow high boost without increased wear and supercharger failure by reducing the pressure difference across the rotor shaft seals  45  during both high boost and negative boost (vacuum) conditions. 
     The flow restrictive seals  51  rotate with the rotor shafts  25  and have a flange  51   a  residing in recesses in the rotor side of the outlet end wall  44 , and a cylindrical portion  51   b  reaching further into the outlet end wall  44 . The outer diameter of the flange  51   a  includes a sealing surface  42  for sealing against a cooperating surface of the outlet end wall  44 . The radial clearance between the sealing surface  42  and the recess in the outlet end wall  44  is extremely small and is preferably approximately 0.05 mm. The sealing surface  42  preferably includes several “sharp” edges  42   a  which allow the sealing surface  42  to contact the recesses in outlet end wall  44  without seizing or creating friction. The restrictive seals  51  are preferably made from hardened steel or the equivalent and the sealing surfaces  42  are preferably a labyrinth type seal to provide low friction while restricting the flow of air past the seal by providing a restrictive path for escaping air. A combination of a tight clearance  41  between ends of the rotors  24  and  26  and a rear face  44   b  of the outlet end wall  44 , and the labyrinth sealing surfaces  42 , allows only a small flow of the hot compressed air inside the supercharger interior  13   a  at the outlet end  12   a  to escape into an annular space  43  between the rotors  24  and  26  and the rotor shaft seals  45  in the outlet end wall  44 . 
     A passage  50  intersects both of the spaces  43  and vents the spaces  43  to ambient air pressure or to near ambient air pressure. Under high boost, an airflow  60  flows from the spaces  43  and under low or no boost (or vacuum), and the air flow  60  flows into the spaces  43 . The space  46  on the opposite side of the shaft seals  45  is also vented to ambient air by a passage  53 . Because spaces on both sides of the shaft seal  45  are vented to ambient air pressure or to near ambient air pressure, the present invention addresses both the pressure difference across the rotor shaft seals  45  at high load (boost in the supercharger interior  13   a ) as well as part or no load (vacuum in the supercharger interior  13   a ). The labyrinth sealing surfaces  42  allow a very small clearance to reduce the air flows into the spaces  43  and through the passage  50 , thereby not reducing performance under boost and providing safe operation. The labyrinth seal preferably has a radial clearance of approximately 0.05 mm. The passage  50  is drilled in the outlet end wall  44 , and communicating with the two spaces  43  for draining of those to the ambient pressure at high boost and pressurizing spaces  43  from the ambient at low or no boost. 
     A top view of the supercharger  12  according to the present invention showing a hose  62  connecting the passage  50  to the superchargers inlet manifold  64  downstream the air mass flow meter  66  and upstream the throttle body  16  is shown in  FIG. 7 . 
     An alternative embodiment with the hose  62  connected to a filter  68  is shown in  FIG. 8 . Because the air flow through the passage  50  is small it is often acceptable to let ambient air to enter the passage  50  directly via the small filter  68 . The filter  68  prevents particles from entering into the supercharger  12 , while providing ambient pressure at the spaces  43 . At high boost, space  43  will be drained down to the ambient pressure through an outflow through the filter  68 , thereby having a cleaning effect on the filter  68 . 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.