Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a technique for cooling a power transfer unit by flowing coolant over surface where heat can be transferred to the coolant and away from the unit. 
         [0003]    2. Description of the Prior Art 
         [0004]    A power transfer unit (PTU) is a mechanical device that transmits power from the output of a transmission to a driveshaft leading to the real vehicle wheels. In a vehicle whose front wheels are continually driven by the transmission, the PTU changes the direction of rotating power flow from a lateral direction to a longitudinal direction. 
         [0005]    A recent trend in vehicle powertrain design provides all wheel drive in more vehicles and delivers more power to the wheels of a secondary axle. 
         [0006]    The PTU is subject to temperature increase in service. Friction developed in the PTU produces internal heat. Because automotive components that operate at high temperatures, such an engine, turbocharger, engine exhaust pipe and catalytic converter, are located near each PTU in the engine compartment, the air surrounding the PTU is at high temperature, which limits potential heat transfer from the PTU to ambient air. 
         [0007]    Conventionally no cooling system is provided to carry heat from a PTU. Therefore, heat is rejected from a PTU primarily by conduction to the transmission and convection to the surrounding air stream. But underbody shields limit heat transfer potential by reducing air movement over the PTU. 
         [0008]    Due to these factors and conditions, the temperature of the PTU and its case are reaching levels where the functional of the PTU can be compromised or adversely affected. 
         [0009]    As a result of transmitting more rotating power through the PTU, more heat rejection from the PTU is required to maintain its temperature at an acceptable magnitude. Consequently, a need exists in the industry for a system that cools a PTU, preferably by continually circulating coolant over surfaces of the PTU so that heat can be transferred to the coolant and away from the PTU. 
       SUMMARY OF THE INVENTION 
       [0010]    A system for cooling a power transfer unit includes a case enclosing said unit and including an outer surface, a chamber bounded by said outer surface, an inlet for carrying coolant from a source of pressurized coolant into the chamber, and an outlet for carrying coolant from the chamber. 
         [0011]    The coolant chamber occupies a large void between the transmission case and PTU. Coolant is supplied from a source of coolant at relatively low temperature. 
         [0012]    Stiffening ribs on the outer surface of the PTU case increase the surface area in contact with coolant and redirect coolant flow for enhanced heat transfer. Cutouts in the ribs ensure coolant flow through each cavity of the coolant chamber. 
         [0013]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a cross section taken at a horizontal plane showing a portion of a differential mechanism and a power transfer unit; 
           [0016]      FIG. 2  is a perspective bottom view showing components of a motor vehicle installed in an engine compartment; and 
           [0017]      FIG. 3  is a side view showing the end face of the PTU case of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Referring now to the drawings,  FIG. 1  shows a differential mechanism  10  located in a transmission case  12 , the differential being adapted to transmit rotating power to halfshafts  14 ,  16 , which extend laterally to driven wheels located at the outboard ends of the halfshafts. A ring gear  18  of the differential  10  is secured by bolts  20  to the housing  22 , which is supported on the transmission case  12  by bearings  24 ,  25 . A spindle,  26  secured to the differential housing  22 , supports bevel pinions, which rotate about axis  30  and revolve about axis  32 . Side bevel gears  34 ,  35  meshing with bevel gears  28 ,  29  are secured to halfshafts  14 ,  16 , respectively. 
         [0019]    The PTU  36 , enclosed in a PTU case  38 , is secured to transmission case  12  by bolts  40 . The PTU  36  includes a bevel gear  42 , supported by bearings  44 ,  45  on the PTU case  38 ; a bevel gear  46  meshing with bevel gear  42  and supported by bearing  48 ,  49  on PTU case  38 ; and a bolted connection  50  to a driveshaft  52 , which transmits power to a second set of wheels. 
         [0020]      FIGS. 1 and 3  show that a portion of the outer surface  54  of the PTU case  38 , adjacent and facing the transmission case  12 , defines a cavity bounded by the outer surface  54 . A cover  56  closes an opening where the cavity faces the transmission case  12  and seals the opening against flow of coolant from a coolant chamber  58  bounded by the outer surface  54  and the cover  56 . A series of bolts  60  secures cover  56  to the PTU case  38 . 
         [0021]      FIG. 2  shows that the components of a motor vehicle located in the engine compartment include an engine  70 , transmission  72 , PTU  36 , engine exhaust pipe  74  and a catalytic converter  76 . Each bolt  60  that connects the transmission case  12  to the PTU case  36  is fitted in one of the bolt holes  80 ,  81 ,  82 ,  83 ,  84  that extending through a mounting surface  86 , formed at the lateral face of the PTU case  36  adjacent the transmission case  12 .  FIG. 3  show an external spline  88 , which connects bevel pinion  42  and the differential housing  22 . 
         [0022]    Mounting surface  86  is formed with a recess  90  that extends along the inner periphery of the mounting surface and is about 2 mm. deep. Cover  56 , which is fitted into recess  90 , seals coolant against the PTU case  38 , thereby reducing risk of cross contamination between coolant and automatic transmission fluid or coolant and PTU fluid. 
         [0023]    An inlet port  92  and outlet port  94  allow coolant to enter and exit coolant chamber  58 . Preferably inlet port  92  is located at a lower elevation than that of outlet port  94  and laterally spaced from the outlet port. Preferably outlet port  94  is located at the higher elevation to allow trapped air to rise and leave the coolant chamber  58 . Air trapped in chamber presents a risk of oxidation of both coolant and the aluminum alloy of which the PTU case  38  is formed. 
         [0024]      FIG. 3  shows that the outer surface  54  of the PTU case  38  is formed with stiffening ribs located in chamber  58  and dividing the chamber into cavities, each cavity bounded by at least one rib and the lower surface of the chamber. The ribs are used to produce turbulent flow of coolant in chamber  58 , to direct coolant flow from the inlet  92  to the outlet  94 , and to increase the area through which heat is transferred from the outer surface  54  of the PTU case  38  to coolant flowing in chamber  58 . 
         [0025]    A narrow passage  96  having a relatively small cross sectional area formed in rib  98 , allows air to flow toward the outlet  94  and to flow across rib  98 . Similar narrow passages  100 ,  102 , each having a relatively small cross sectional area are formed in other ribs of the PTU case  38  to allow air to flow toward the outlet  94 . Preferably the cross sectional area of passages  96 ,  100 ,  102  is small enough to limit coolant flow through the passages. 
         [0026]    Coolant flows through chamber  58  from cavity-to-cavity through relatively large slots  104 ,  105 ,  106 ,  107  formed in the ribs. Preferably the cross sectional area of each slot  104 - 107  is larger than that of each passage  96 ,  100 ,  102  and is large enough to allow coolant to flow through the slots but without weakening the rib or substantially reducing stiffness of the ribs. 
         [0027]    Cover  56  is formed with flow deflectors  108 ,  109 ,  110 ,  111 ,  112  which extend from cover  56  into chamber  58 , are located in a coolant flow path between inlet  92  and slots  104 - 107  such that the deflectors  108 - 111  cause coolant entering inlet  92  and exiting each slot to flow around the adjacent deflector rather than flowing directly into another of the slots  104 - 107 . Deflectors  108 - 111  further produce turbulent coolant flow along the surface of the ribs in coolant chamber  58 . Preferably cover  56  fit tight against the top of the webs of PTU case  38  to ensure that coolant flows as previously described and not just leak between cavities of chamber  58  that are separated by the ribs. 
         [0028]    Machining slots  104 - 107  in the ribs is less desirable than drilling holes through the ribs. Preferably the slots  104 - 107  are formed while casting the PTU case  38  of aluminum alloy. 
         [0029]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.

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