Abstract:
A transport refrigeration unit includes an integrally mounted unitary engine driven generator wherein the generator is cooled by the circulation of oil over the stator coils and the rotor to thereby provide a cooling system that is closed from the outside environment and capable of increased cooling efficiencies. In one embodiment, the circulation of the engine oil is integrated into the generator such that its serves the dual purpose of cooling the generator. In other embodiments, the oil is contained entirely within the generator and is circulated by various means such as by a pump and spray, a slinger or total immersion.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to transport refrigeration systems and, more particularly, to the cooling of an engine driven generator therefor. 
     Generally, transport refrigeration systems such as those used on truck trailers, have employed belt driven and/or mechanically linked, shaft driven compressor units and/or belt driven, or otherwise mechanically linked, fan powering systems. That is, the unit engine, such as a diesel engine, is mechanically connected to drive both the compressor unit(s) and the unit refrigeration systems fans and other components. 
     A significant improvement to such an arrangement was a so called “all electric” transport refrigeration system as described in U.S. Pat. No. 6,223,546, assigned to the assignee of the present invention. In such a unit, a generator is mechanically connected to the system engine and is used to generate the electrical power needed to drive the compressor and the various refrigeration system fans and other components. 
     Heretofore, such an engine driven generator has been cooled by a fan disposed on one end thereof and adapted to circulate air over the stator and armature components for the cooling thereof. A significant cooling effect is also achieved by moving air over the generator by the condenser fans. 
     The applicants have recognized that a significant volume of air at a substantially high pressure is required to provide the necessary cooling, particularly for obtaining sufficient circulation through the small openings in the rotor to obtain the required cooling. 
     Another problem that has been recognized in such air cooled systems is the occurrence of generator shorting that is caused by outside contamination. That is, the transportation environment is a harsh environment and, as the outside air is circulated through the generator, there is a tendency for contaminants to enter the unit and cause an electrical shorting of the windings. This results in damage to the components and to expenses due to down time. 
     What is needed is an improved method and apparatus for cooling the generator of an electrically powered transport refrigeration unit. Of particular interest is a reduction of size and weight. 
     SUMMARY OF THE INVENTION 
     Briefly, in accordance with one aspect of the invention, oil is caused to circulate through and around the critical areas of both the rotor and the stator of a generator so as to bring about the cooling thereof. 
     By another aspect of the invention, the generator is attached to and driven by the system engine in a cantilevered fashion such that the oil in the generator is used only for cooling and not for lubrication. 
     In accordance with another aspect of the invention, the oil system of the driving engine is integrated with the generator such that the engine lubrication oil is used to cool the generator. 
     By yet another aspect of the invention, the oil is circulated to the critical areas within the generator by way of a controlled oil spray 
     By still another aspect of the invention, the oil is isolated within the generator such that the rotor and/or stator components are partially or totally immersed in the oil. 
     These features provide substantially improved cooling efficiency, which, in turn, may allow for the reduction of weight by a downsizing of the components. Further, they overcome the problems of contamination that has occurred with air cooled products. 
     In the drawings as hereinafter described, a preferred embodiment and alternative embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a transport refrigeration unit as installed in a truck trailer in accordance with the prior art. 
         FIG. 2  is an exploded perspective view of an engine driven generator in accordance with the prior art. 
         FIG. 3  is a schematic illustration of a generator cooling system in accordance with one embodiment of the invention. 
         FIG. 4  is an alternative embodiment thereof. 
         FIG. 5  is another alternative embodiment thereof. 
         FIG. 6  is yet another alternative embodiment thereof. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1  there is shown a typical illustration of a trailer refrigeration unit  11  which is enclosed with an outer cover  12  and attached to a truck trailer  13  that is being towed by a truck  14 . All of the components of the trailer refrigeration unit  11  and the outer cover  12  are structurally supported by the structural framework  16 . As is common for transport refrigeration units, various panels of the outer cover are hinged and are removable to provide ready access to the refrigeration system to perform routine maintenance. 
       FIG. 2  depicts a unitary engine driven generator which is cooled by the circulation of air in accordance with the prior art. The generator  17  includes an outer stator assembly  18  that is fixedly attached to the bell housing  19  of a suitable prime mover such as a diesel engine  21 . A rotor assembly  22  is affixed directly to the engine flywheel  23  to create a continuous drive connection between the engine drive shaft, the engine flywheel and the rotor assembly  22  of the generator  17 . A cover  24  and a generator cooling fan  26  have been removed to show the details of the rotor assembly  22 . 
     The outer stator assembly  18  includes a core section which may be fabricated from ferrous laminations or powdered metal. A main winding that provides primary power to the refrigeration system and an auxiliary winding that is electrically connected to the battery charging device are disposed in slots in the stator core in the conventional manner. Attachment of the stator assembly  18  to the bell housing  19  is accomplished by use of a series of elongated threaded fasteners  27  passing through mating openings  28  in the stator core. The fasteners  27  in turn pass through axially aligned openings  29  provided in an adaptor plate  31  and thence into axially aligned threaded openings  32  in the bell housing  19 . 
     The rotor assembly  22  includes a steel rotor hub  33  having a plurality of axial openings therethrough which are adapted to receive a plurality of elongated threaded fasteners  34  therethrough. The threaded fasteners  34  are adapted to be received in axially aligned threaded openings  36  in the engine flywheel  23  to provide the integral connection between the rotor assembly  22  and the engine flywheel  23  and drive shaft. 
     In operation, the diesel engine  21  drives the flywheel  23  and the rotor assembly  22  with its associated rotor magnets, thereby inducing in the stator windings, synchronous voltages in a manner well familiar to those skilled in the art of synchronous generator design. Other details of the prior art system are shown and described in U.S. Pat. No. 6,223,546, assigned to the assignee of the present invention and incorporated herein by reference. 
     It should be understood that while some cooling of the generator  17  occurs by radiation from the outer stator assembly  18 , the primary cooling function is accomplished by the fan  26  which is attached to and driven by the rotor assembly  22  to bring air in through the cover  24  and circulate it through both the stator windings and the rotor. While the stator windings develop a significant amount of heat and require the cooling air to prevent them from reaching excessive temperatures and being damaged thereby, it is also necessary to cool the rotor in order to prevent the permanent magnets from demagnetizing. In this regard, the applicants have found that it is difficult to circulate the air through the rotor because of the rather small openings that are provided for that purpose. For that reason, it is necessary to develop a substantial pressure with the fan  26  in order to penetrate those openings. 
     In  FIG. 3  there is shown an engine block  37  which is attached to a bell housing  38  and the supported stator assembly  39  in a manner similar to that described hereinabove. In this regard, it should be understood, that the stator may be attached to the bell housing in any of various ways so long as it is prevented from rotation and is sufficiently secured. 
     The drive shaft of the engine  37  is drivingly attached to the flywheel  41  and the rotor  42  in a similar manner, with the rotor  42  being supported in a cantilevered manner by way of the main bearings in the engine  37 . A starter  35  is mechanically connected to the flywheel housing such that the pinion gear  40  makes intermittent contact to crank the engine with a seal being provided between the pinion gear  40  and the bell housing  38 . However, rather than being cooled by the circulation of air, the generator  43  is cooled by the circulation of oil therethrough. One advantage to the use of oil is that the specific heat of oil is greater than that of air and greater cooling efficiencies can thus be obtained. 
     In the embodiment of  FIG. 3 , the cooling function of the generator  43  is integrated with that of lubricating the engine  37 , with the cooling medium being the engine oil. 
     As standard features of the engine, an oil sump  44  is provided at the bottom end of the engine and a pump  46  circulates the cooling oil to a plurality of galleys one of which is shown at  47 . After circulating through those portions of the engine which require lubricating, such as the bearings and the like, the oil again returns to the oil sump  44  to complete the process. 
     In accordance with one aspect of the present invention, an oil supply line  48  is connected to receive oil from the oil galley  47 , with the line  48  then passing in a sealed manner through the bell housing  38  and into the generator  43  to be dispersed to those components requiring cooling. One method of dispersion may be by way of a spraying device  49  which sprays oil in a controlled manner on the windings  51  of the stator. The oil then flows downwardly over the rotor  42  and over the stator windings at the lower end of the generator  43 . The oil then collects in the bottom of the generator  43  or other collection point if appropriate, and is caused to flow by gravity and pressure along the oil return line  52  to the engine oil sump  44 . 
     It should be recognized that the oil supply line  48  may be brought into the generator  43  at other locations other than the bell housing as shown. Further, the spraying device  49  may be located at other locations or on the outer side of the generator rather than on the inside thereof as shown. With the design as shown, however, it is necessary to have a sealed relationship between the oil supply line  48  and the bell housing. 
     A described, the oil that is normally in the engine  37  is pressurized and filtered by the normal engine apparatus and functions. The heat that is transferred to the oil from the generator  43  will be cooled somewhat by radiation from the generator casing. However, the primary oil cooling function is accomplished by way of radiation from the oil sump  44  and by the radiator coolant that is being circulated through the engine  37 . 
     Such a system allows the generator  43  to be cooled by the existing engine oil system with its existing engine pump and filtration system. Further, the existing oil from the engine is sufficient for both purposes, thereby allowing a reduction in weight of the total system. Further, because of higher cooling efficiencies, the size of the rotor and stator components may be reduced to further reduce the weight and fabrication costs of the system. 
     The embodiments shown in  FIGS. 4 ,  5  and  6  are similar to the embodiment shown in  FIG. 3  in that they use oil as the coolant to be circulated within the generator for cooling purposes. However, rather than being integrated with the oil system of the engine, these embodiments have their own separate oil supply and circulation system which is self-contained within the generator. 
     As shown in  FIG. 4 , the cooling function is accomplished by the spraying of oil on the components to be cooled by way of a spraying device  49 . However, rather than using the engine oil, a dedicated oil system within the generator is provided. It is estimated that about a quart of oil would be required in the generator  53 . An oil pickup tube and a dedicated oil pump  56  are provided to pump the oil upwardly to the spraying device  49 . The oil should not have to be changed since it is self contained from contamination and it serves no purpose other than cooling. That is, the rotor  42  is cantilevered out from the engine and the support bearings are lubricated by the engine oil. Thus, the oil in the generator does not serve a lubrication function and should not need to be filtered or changed since it is self contained and not exposed to contamination in the way the engine is. 
     In this system, the oil is cooled by way of radiation from the stator casing  57 . A plurality of fins  58  may be added to the front cover to assist in this process. 
     In the embodiment shown in  FIG. 5 , the generator  59  has a dedicated supply of oil in the bottom thereof. Again, it is estimated that a quart of oil will be sufficient for this purpose. However, rather than providing a spraying device, the distribution of the oil is accomplished by way of an oil slinger  61  that is attached to and driven by the rotor  42  such that its periphery extends downwardly into the oil sump such that it tends to sling the oil upwardly to reach the critical areas of the stator and rotor for the cooling thereof. The slinger may have knurled or similar surfaces to assist in the oil pick-up process. It should also be recognized that the flywheel  41 , with its associated attached starter ring gear, which extends downwardly below the level of the oil, may also be used for slinging oil upwardly. For that purpose, it may be desired to place fins or the like on the periphery of the flywheel  41 . However, it should be kept in mind that the ring gear by itself may be sufficient to accomplish this function. 
     In the  FIG. 6  embodiment, the entire inner space of the generator  62  is substantially filled with oil such that the rotor  42  and the stator windings  51  are immersed in oil. For that purpose, it is estimated that about a gallon of oil should be sufficient for this purpose. Such a system is advantageous in that the device otherwise needed for circulating the oil, such as the spraying device or the oil slinger, is not required. Further, because the components are always entirely immersed in the oil, more heat will be transferred to that oil. Cooling of the oil is accomplished by way of radiation from the stator casing and from the fins  58 .