Patent Publication Number: US-2007113548-A1

Title: Reduced stall capacity torque converter

Description:
TECHNICAL FIELD  
      The present invention relates to an apparatus for enhancing operating characteristics of a torque converter.  
     BACKGROUND OF THE INVENTION  
      A torque converter is a hydrodynamic unit that transfers torque between an engine and an automatic transmission. The torque converter generally includes an impeller (driving member), a turbine (a driven member), and a stator that are disposed in a housing full of working fluid. The impeller is generally disposed at a rear portion of the housing (away from the engine), and it turns with a crankshaft of an engine. The turbine is generally disposed at a front portion of the housing (near the engine), and is connected to a transmission input shaft. The turbine is free to rotate independently from the impeller.  
      The working fluid flows from the impeller toward the turbine in a radial outer portion of the torque converter. The working fluid then flows from the turbine back toward the impeller by way of the stator in a radial inner portion of the torque converter.  
      As is well known in the art, in order to optimize efficiency it is necessary to select an appropriate torque converter for a particular engine configuration and application. For example, torque converters have traditionally been matched to a diesel engine by selecting a torque converter with a stall curve that crosses the engine torque curve approximately 100 rpm above peak engine torque, and a 0.8 speed ratio curve that crosses the engine torque curve at the engine governed speed.  
      For some engines such as, for example, highly turbocharged diesel engines, it may be necessary to match the stall curve of the converter with the engine&#39;s “lug up” curve. As will be described in more detail, a lug up curve is approximately the engines naturally aspirated torque curve up to the engine speed at which the turbocharger spins up. To properly match the stall curve of a torque converter with an engine&#39;s lug up curve, it may be necessary to provide a torque converter that is “looser” at stall than it is at 0.8 speed ratio. A “tighter” torque converter is one that can absorb more torque (has a higher capacity), and a “looser” torque converter is one that can absorb less torque (has a lower capacity).  
     SUMMARY OF THE INVENTION  
      The apparatus of the present invention provides a torque converter that is “looser” at stall than it is at 0.8 speed ratio. The torque converter includes a turbine, a stator and an impeller. The stator includes an inner hub portion, an outer shell, and a plurality of stator blades disposed therebetween. The hub portion is connected via a one way clutch to a stator that is fixed to the transmission. The stator blades are connected to both the hub and the shell, and are circumferentially disposed at equal intervals. Each stator blade is connected to an inner surface of the shell and an outer surface of the hub.  
      The stator blades each define a primary leading edge, a trailing edge, a concave surface, and a convex surface. The concave surface of each stator blade is commonly referred to as the pressure surface, and the convex surface is commonly referred to as the suction surface. The stator blades also preferably include a flange extending from the concave surface and terminating in a secondary leading edge. When the working fluid flows around the stator blade, pressure acting on the concave surface is greater than that acting on the convex surface. The stator rotates due to the pressure difference between the sides of the stator blades.  
      The stator blade flange extends away from the concave surface of each blade such that the secondary leading edge extends generally in the direction of the primary leading edge of a preceding stator blade. In this manner, the secondary leading edge is designed at stall to catch flow coming past the primary leading edge of the upstream blade and to redirect the flow back out of the front of the stator toward the turbine.  
      The flow redirected out of the front of the stator by the secondary leading edge forms a large separation region or bubble. A separation bubble is a region at which the approaching flow recirculates in a direction which is reversed with respect to the mean flow. The separation bubble creates a region of restricted flow which limits the amount of working fluid passing through the stator toward the impeller at stall. This reduction of flow through the stator reduces torque converter capacity at stall.  
      The secondary leading edge is designed to hide in the wake of the primary leading edge at higher speed ratios. The secondary leading edge therefore only minimally affects the flow rate of the working fluid at higher speed ratios, and correspondingly the torque converter capacity at such higher speed ratios is not significantly altered.  
      The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic illustration of a torque converter connected to an engine and a transmission in accordance with an aspect of the invention;  
       FIG. 2  is a graph showing the torque curve for the engine of  FIG. 1 , and the stall and 0.8 speed ratio curves for the torque converter of  FIG. 1 ;  
       FIG. 3  is a detailed view of a stator of the torque converter of  FIG. 1 ;  
       FIG. 4  is a detailed view of the stator blades of the stator of  FIG. 3 ;  
       FIG. 5   a  is flow diagram showing the flow of working fluid across a stator blade of  FIG. 4  while the torque converter of  FIG. 1  is operating at stall; and  
       FIG. 5   b  is flow diagram showing the flow of working fluid across a stator blade of  FIG. 4  while the torque converter of  FIG. 1  is operating at 0.8 speed ratio.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As shown in  FIG. 1 , a torque converter  10  includes a pump impeller  12  (a driving member), a turbine runner  14  (a driven member), and a stator  16 . An engine  18  and a transmission  20  are respectively disposed at opposing sides of the torque converter  10 . The torque converter  10  further includes a housing member (not shown) attached to the impeller  12  such that a chamber full of working fluid (not shown) is formed therebetween.  
      The impeller  12  is connected to a crankshaft of the engine  18 , and engine torque is transmitted from the impeller  12  to the turbine  14  through an operation of the working fluid. The turbine  14  is connected to an input shaft of the transmission  12  to transmit the engine torque to the transmission  12 .  
      When the engine  18  is running, the rotating impeller  12  causes fluid to be directed outward toward the turbine vanes (not shown). When this occurs with sufficient force to overcome the resistance to rotation, the turbine  14  begins to turn, turning the transmission input shaft (not shown). The fluid flow exiting the turbine  14  is directed back into the impeller  12  by way of the stator  16 . The stator  16  redirects the fluid flow from the turbine  14  to the impeller  12  in the same direction as impeller rotation, thereby reducing impeller torque and causing torque multiplication.  
      As is well known in the art, in order to optimize efficiency it is necessary to select an appropriate torque converter for a particular application and engine configuration. For example, torque converters have traditionally been matched to a diesel engine by selecting a torque converter with a stall curve that crosses the engine torque curve approximately 100 rpm above peak engine torque, and a 0.8 speed ratio curve that crosses the engine torque curve at the engine&#39;s governed speed. The engine&#39;s governed speed is the maximum speed at which the engine is designed to operate.  
      Some turbocharged diesel engines have a condition occurring at low speed at which they do not produce sufficient exhaust to spin up the turbochargers, and which therefore cannot be matched with a torque converter in the previously described traditional manner. The engine torque curve of such an engine is reduced to approximately its naturally aspirated torque curve until the engine speed increases enough to spin up the turbochargers. This reduced torque portion of the engine torque curve is commonly called a “lug up” curve.  
      To better illustrate the present invention, the engine  18  will hereinafter be described as a turbocharged diesel engine used in a fracturing rig, however, it should be appreciated that the apparatus of the present invention may be implemented with other engines as well. As is known by one skilled in the art, a “fracturing rig” is a stationary device used to improve the production rate and increase recoverable reserves of an oil well.  
      Referring to  FIG. 2 , a graph of torque (measured in ft-lbs) versus speed (measured in rpm) is shown. The solid line represents the torque curve of the engine  18  (shown in  FIG. 1 ). As shown, the engine  18  produces 2,000 ft-lbs of torque from idle (600 rpm) up to 1,500 rpm. At 1,500 rpm, the turbochargers of the engine  18  come up to speed and the torque output increases quickly. The engine generates 6,100 ft-lbs of torque from 1,650 rpm up to the engine&#39;s governed speed (1,850 rpm). The portion of the engine torque curve between 600 rpm and 1,500 rpm is the lug up curve.  
      The previously described traditional method for matching an engine and a torque converter does not apply to the engine  18  (shown in  FIG. 1 ). More precisely, matching a torque converter&#39;s stall curve with the engine  18  in the traditional manner could produce a lower stall point if the torque converter stall curve crosses the lug up curve of the engine  18 . As is known in the art, a “stall point” is an operating point at which the engine at full throttle is being held by the torque converter at stall. To avoid producing a lower stall point that improperly matches a torque converter to the engine, the torque converter stall curve cannot cross the engine&#39;s lug up curve.  
      The dashed line of  FIG. 2  labeled “stall” represents the stall curve for the torque converter  10  (shown in  FIG. 1 ). The stall curve shows the torque capacity of a torque converter operating at stall and within an engine speed range. As shown in  FIG. 2 , the stall curve for the torque converter  10  meets the lug up curve of the engine  18  at 1,500 rpm but does not cross the lug up curve. As the stall curve of the torque converter  10  does not cross the lug up curve of the engine  18 , the engine  18  can operate up to its governed speed without encountering a premature stall point.  
      The dashed line of  FIG. 2  labeled “0.8” represents the 0.8 speed ratio curve for the torque converter  10  (shown in  FIG. 1 ). The 0.8 speed ratio curve shows the torque capacity of a torque converter operating at a speed ratio of 0.8 and within a specified engine speed range. The 0.8 speed ratio curve of the torque converter  10  is optimally matched with the engine  18  (shown in  FIG. 1 ) in the previously described traditional manner in which the 0.8 speed ratio curve is configured to cross the engine torque curve at the engine&#39;s governed speed (1,850 rpm).  
      It should be appreciated that the engine  18  (shown in  FIG. 1 ) requires a torque converter that is “looser” at stall than it is at 0.8 speed ratio. In other words, while a typical torque converter gets “looser” with increasing speed ratio, a torque converter properly matched to engine  18  must get “tighter” with increasing speed ratio. A “tighter” torque converter is one that can absorb more torque (has a higher capacity), and a “looser” torque converter is one that can absorb less torque (has a lower capacity).  
      Referring again to  FIG. 1 , the stator  16  of the present invention is designed to reduce the stall capacity of the torque converter  10  without significantly impacting torque converter capacity at high speed ratios. The torque converter  10  of the present invention is therefore looser at stall than it is at 0.8 speed ratio, and can be matched with an engine such as the engine  18 .  
      Referring to  FIG. 3 , the stator  16  is shown in more detail. The stator  16  includes an inner hub portion  30 , an outer shell  32 , and a plurality of stator blades  34  disposed therebetween. The hub portion  30  is connected to a stator shaft (not shown) that is fixed to the transmission  20 . The stator blades  34  are connected to both the hub  30  and the shell  32 , and are circumferentially disposed at constant intervals. Each stator blade  34  is connected to an inner surface of the shell  32  and an outer surface of the hub  30 .  
      Referring to  FIG. 4 , the stator blades  34  each define a primary leading edge  36 , a trailing edge  40 , a concave surface  42 , and a convex surface  44 . The stator blades  34  also preferably include a flange  37  extending from the concave surface  42  and terminating in a secondary leading edge  38 . When the working fluid flows around the stator blade  34 , pressure acting on the concave surface  42  is greater than that acting on the convex surface  44 . For this reason, the concave surface  42  is generally referred to as a high-pressure surface or a positive pressure side, and the convex surface  44  is generally referred to as a low-pressure surface or a negative pressure side. The stator  16  (shown in  FIG. 3 ) reacts due to the pressure difference between the sides of the stator blades  34 . Referring to  FIGS. 1 and 4 , the stator  16  is disposed between the impeller  12  and the turbine  14  such that the convex surface  44  substantially faces the turbine  14 , and the concave surface  42  substantially faces the impeller  12 .  
      As shown in  FIG. 4 , the flange  37  extends away from the positive pressure side of each blade  34  such that the secondary leading edge  38  points generally to, or extends in the direction of, the primary leading edge of a preceding stator blade. In this manner, the secondary leading edge  38  is designed at stall to catch flow coming past the primary leading edge of the upstream blade and to redirect the flow back out of the front of the stator  16  toward the turbine  14  (shown in  FIG. 1 ).  
       FIG. 5   a  shows the flow of working fluid across the stator blade  34  (shown in  FIG. 3 ) when the torque converter  10  (shown in  FIG. 1 ) is operating at stall. The arrows indicate the flow direction of the working fluid. As shown, the flow redirected out of the front of the stator  16  (shown in  FIG. 1 ) by the secondary leading edge  38  forms a large separation region or bubble  50 . A separation bubble is a region at which the approaching flow recirculates in a direction which is reversed with respect to the mean flow. The separation bubble  50  creates a region  52  of restricted flow which limits the amount of working fluid passing through the stator  16  toward the impeller  12  (shown in  FIG. 1 ) at stall. This reduction of flow through the stator  16  reduces torque converter capacity at stall.  
      The secondary leading edge  38  is designed to hide in the wake of the primary leading edge  36  at higher speed ratios. The secondary leading edge  38  therefore only minimally affects the flow rate of the working fluid at higher speed ratios, and correspondingly the torque converter capacity at such higher speed ratios is not significantly altered.  FIG. 5   b  shows the flow of working fluid across the stator blade  34  when the torque converter  10  is operating at 0.8 speed ratio. It can be seen that the secondary leading edge  38  only slightly blocks the flow at region  54 , and the resultant separation bubble  56  is therefore also relatively small.  
      While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.