Abstract:
In a locomotive having a dynamic braking capability, a method for maintaining an airflow path to a resistive grid disposed within a resistive grid enclosure having an enclosure inlet and an enclosure outlet is provided, wherein the method includes generating an inlet airflow into the enclosure inlet, creating a heated inlet airflow by associating the inlet airflow with the resistive grid such that the inlet airflow absorbs heat generated by the resistive grid, directing the heated inlet airflow toward the enclosure outlet to generate a heated outlet airflow, redirecting at least a portion of the heated outlet airflow to be thermally associated with a structural portion of the enclosure inlet such that the temperature of the structural portion of the enclosure inlet is increased above a predetermined temperature to reduce blockage of the enclosure inlet by snow and recirculating the redirected heated outlet airflow with ambient air being drawn into the resistive grid enclosure.

Description:
RELATED APPLICATIONS 
   This application claims priority of U.S. Provisional Patent Application Ser. No. 60/590,554 filed Jul. 23, 2004, the contents of which are incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to locomotive braking systems and more particularly to dynamic braking grids used in locomotives. 
   BACKGROUND OF THE INVENTION 
   Conventional rail locomotive designs typically employ a plurality of traction motors for propelling and retarding the forward and reverse motion of the locomotive. When being used to propel the locomotive the traction motors function as electric motors which convert electrical current into kinetic or mechanical energy. For example, current locomotives typically include a diesel engine which is used to drive an alternator which generates and supplies an electrical current to a plurality of traction motors which, in turn, converts this electrical energy into mechanical energy causing the locomotive to be propelled in the forward or reverse direction. However, the traction motors may also be configured to function as generators to produce a dynamic braking action which is used to slow the movement of the locomotive by converting the kinetic energy of the locomotive into electrical energy. Unfortunately however, this electrical energy cannot be used or stored conveniently on-board the locomotive. As such, this energy must be dissipated. To accomplish this the traction motors are connected to a bank of electrical resistors called a dynamic braking grid and the electrical energy generated during the dynamic braking action is converted into thermal energy, or heat, using resistive elements within the dynamic braking grid wherein the dynamic braking grid is typically force cooled by fan-driven airflow which transfers the heat energy into the ambient environment. 
   However, certain design and physical characteristics of current dynamic braking grid resistor packages, such as the package size and the upper temperature limit of the materials used to construct the dynamic braking grid tend to limit the amount of dynamic braking power that may be applied to the locomotive and still be efficiently transferred into the ambient environment. This acts to limit the amount of power that may be dissipated by a grid at a given ambient condition based on temperature and pressure. For example, a typical stack of braking grids occupying a volume of approximately 50 cubic feet may only be able to dissipate 1.8 MW of power. As such, because the efficient transfer of heat energy from the resistors to the ambient environment is a critical factor to the proper performance of a dynamic braking system, it is desirable to maximize this efficiency. Unfortunately however, because current dynamic braking grid resistor package designs are subject to cost, size, weight and noise limitations and the amount of space available on board the locomotive, it is not practical to simply increase the size of the grid enclosure or the size, quantity and/or the capacity of the cooling fans and resistors. 
   One way that has been developed to help solve this problem involves maximizing the energy dissipated across the entire grid by minimizing “hot spots” in the braking grid while avoiding localized material failure. Referring to  FIG. 1 ,  FIG. 2  and  FIG. 3 , typical cooling fans provide an uneven airflow velocity distribution at the outlet of the fan, wherein the outlet airflow velocity is highest proximate the center of the fan blade and lowest at the root and tips of the fan blades. In order to control the airflow more efficiently, an airflow diffuser plate is disposed between the fan outlet and the grid package inlet. For example, a typical cooling fan  100  used in a dynamic braking grid resistor package is shown and includes an impeller fan blade  102  and a flat plate  104 , wherein the flat plate  104  defines a plurality of diffuser holes  106 . Flat plate  104  includes an annular ring portion  108 , a central portion  110  and a plurality of corner portions  112 , wherein the annular ring portion  108  is aligned with the high velocity components of the fan air flow and wherein the central portion  110  and the corner portions  112  are aligned with the low velocity components of the fan air flow. The annular ring portion  108  defines a plurality of holes  114  having a relatively low quantity and/or size and the central portion  110  and the corner portions  112  define a plurality of holes  116  having a relatively high quantity and/or size. 
   As can be seen in  FIG. 3 , the uneven distribution of openings in the flat plate  104  has the effect of making the distribution of airflow volume and velocity downstream of the flat plate  104  much more uniform than that provided at the fan outlet. Additionally, the flat plate  104  also serves to reshape the air stream from the generally circular cross-sectional shape of the cooling fan  102  into the generally rectangular cross-sectional shape of the grid package, providing a more evenly distributed airflow within the grid package. This acts to cool the resistor grid in a more evenly distributed manner. Unfortunately however, the flat plate  104  also acts as a flow restriction and causes a significant pressure drop in the air stream which reduces the mass flow rate of cooling air available for flow through the resistor grid package. This is undesirable because the cooling of the resistive elements remains inefficient. 
   With conventional systems having a fan and six resistors, it is difficult to lower the maximum grid operating temperature. For a standard system, due to successive grid heating, the discharge resistor experiences the highest temperature which limits the power and environmental capability of the system. Thus, the cooling of a conventional system is difficult to improve upon for a number of reasons such as, airflow could be improved via increased flow via increased fan size, but space is limiting, airflow could be improved via increased fan speed, but physical fan/motor stresses are constraining, airflow could be increased but noise levels are constraining, airflow could be increased by reducing the pressure drop across the system, but inlet losses are limited by the size of the system, and grid and diffuser losses are limited by the size of the system as well. For instance, grid pressure drops could be reduced by increasing grid spacing, but the grid would have to be larger to still maintain sufficient heat transfer area. In fact, with conventional systems, blowing snow and freezing condensation can enter the fan, resulting in it freezing. 
   SUMMARY OF THE INVENTION 
   In a locomotive having a dynamic braking capability, a method for maintaining an airflow path to a resistive grid disposed within a resistive grid enclosure having an enclosure inlet and an enclosure outlet is provided, wherein the method includes generating an inlet airflow into the enclosure inlet, creating a heated inlet airflow by associating the inlet airflow with the resistive grid such that the inlet airflow absorbs heat generated by the resistive grid, directing the heated inlet airflow toward the enclosure outlet to generate a heated outlet airflow, redirecting at least a portion of the heated outlet airflow to be thermally associated with a structural portion of the enclosure inlet such that the temperature of the structural portion of the enclosure inlet is increased above a predetermined temperature to reduce blockage of the enclosure inlet by snow and recirculating the redirected heated outlet airflow with ambient air being drawn into the resistive grid enclosure. 
   In a locomotive with dynamic braking capability, wherein the locomotive includes a resistive grid enclosure having an enclosure inlet and an enclosure outlet, a method for reducing blockage of the enclosure inlet is provided and includes controllably operating an electric power resistance energy source to generate output energy and associating the output energy with at least one of the enclosure inlet and a structure adjacent the enclosure inlet, such that an airflow path to the enclosure inlet is kept free from obstruction. 
   In a locomotive having a dynamic braking capability, a resistive grid enclosure is provided and includes a first enclosure portion, wherein the first enclosure portion defines a first enclosure portion cavity. Additionally, a second enclosure portion is provided, wherein the second enclosure portion defines a second enclosure portion cavity having at least one second enclosure portion inlet, wherein the second enclosure portion is disposed adjacent to and separated from the first enclosure portion via a first enclosure portion wall, wherein the first enclosure portion wall includes at least one convection opening which communicates the first enclosure portion cavity with the second enclosure portion cavity. A third enclosure portion is also provided, wherein the third enclosure portion defines a grid cavity having at least one grid cavity outlet, wherein the third enclosure portion is disposed adjacent to and separated from the second enclosure portion via a third enclosure portion wall, wherein the third enclosure portion wall includes at least one airflow opening which communicates the second enclosure portion cavity with the third enclosure portion cavity and wherein the third enclosure portion further includes at least one airflow directional device associated with the at least one grid cavity outlet to direct air flowing out of the at least one grid cavity outlet away from the at least one second enclosure portion inlet. Furthermore, at least one transfer duct portion is provided, wherein the at least one transfer duct portion defines a duct cavity having at least one duct inlet and at least one duct outlet, wherein the at least one duct inlet is disposed to be associated with the grid cavity and wherein the at least one duct outlet is disposed within the first enclosure cavity to communicate the grid cavity with the first enclosure cavity. 
   A resistive grid assembly for dissipating power generated in a locomotive having a dynamic braking capability is provide, wherein the resistive grid assembly includes a resistive grid enclosure, wherein the resistive grid enclosure defines at least one enclosure inlet, at least one enclosure outlet and a resistive grid cavity, wherein the at least one enclosure inlet is communicated with the at least one enclosure outlet via the resistive grid cavity. A plurality of resistive grid elements are also provided, wherein the plurality of resistive grid elements are disposed within the resistive grid cavity in a plurality of separate element groups. Furthermore, at least one cooling device is provided, wherein the at least one cooling device is disposed to separate at least one of the plurality of separate element groups from the others of the plurality of separate element groups and wherein the at least one cooling device is configured to generate an airflow incident upon at least one of the plurality of separate element groups. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments, taken in conjunction with the accompanying drawings in which like elements are numbered alike in the several Figures: 
       FIG. 1  is a top down view of a dynamic resistive grid package, in accordance with the prior art; 
       FIG. 2  is a front view of a flat plate of the prior art dynamic resistive grid package of  FIG. 1 ; 
       FIG. 3  is a side illustrative view of the airflow incident on a flat plate generated by a cooling fan of the prior art dynamic resistive grid package of  FIG. 1 ; 
       FIG. 4  is a top down view of a dynamic resistive grid package, in accordance with a first embodiment; 
       FIG. 5  is a sectional view of the dynamic resistive grid package of  FIG. 4 , along the lines A-A in  FIG. 4 ; 
       FIG. 6  is a partial side perspective view of a dynamic resistive grid package, in accordance with a second embodiment; 
       FIG. 7  is a partial top down perspective view of the dynamic resistive grid package in  FIG. 6 ; 
       FIG. 8  is a front view of the dynamic resistive grid package in  FIG. 6 ; 
       FIG. 9  is a sectional view of the dynamic resistive grid package in  FIG. 6  with a resistive grid disposed within; 
       FIG. 10  is a block diagram illustrating a method for maintaining an airflow path to the resistive grid package of  FIG. 1  and  FIG. 6 ; 
       FIG. 11  is a sectional side view of the dynamic resistive grid package in  FIG. 6 , showing the airflow during operation; and 
       FIG. 12  is a sectional front view of the dynamic resistive grid package in  FIG. 6 , showing the airflow during operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 4  and  FIG. 5 , a first embodiment of a braking grid resistor package  200  is shown and includes an enclosure structure  202  defining an enclosure cavity  204  for containing a plurality of resistive elements  206  and a plurality of cooling fans  208 . Enclosure structure  202  includes an outer structure  210  and an inner structure  212 , wherein outer structure  210  defines a plurality of structure inlets  214  and a plurality of structure outlets  216  and wherein inner structure  212  may be used to support plurality of cooling fans  208  and plurality of resistive elements  206 . 
   Plurality of resistive elements  206  are shown as including twelve resistive elements  206  disposed within enclosure cavity  204  to create a first resistive element group  218 , a second resistive element group  220 , a third resistive element group  222  and a fourth resistive element group  224 . Additionally, plurality of cooling fans  208  are shown as including a first cooling fan  226 , a second cooling fan  228 , a third cooling fan  230  and a fourth cooling fan  232 , wherein first cooling fan  226  is associated with first resistive element group  218 , second cooling fan  228  is associated with second resistive element group  220 , third cooling fan  230  is associated with third resistive element group  222  and fourth cooling fan  232  is associated with fourth resistive element group  224 . 
   It should be appreciated that although first cooling fan  226  and second cooling fan  228  are shown as being powered by one electric motor and third cooling fan  230  and fourth cooling fan  232  are shown as being powered by an additional electric motor, plurality of cooling fans  208  may be powered individually via a single motor or in groups via a plurality of motors. In fact, the number of plurality of resistive elements  206  per cooling fan  208  may be chosen to optimize the performance of the system. 
   In the configuration of  FIG. 4 , each of the plurality of cooling fans  208  cools three of the plurality of resistive elements  206  to dissipate a portion of the electrical braking power of the locomotive. It should be appreciated that a fewer number of resistive element groups reduces the airflow backpressure as seen by each of the plurality of cooling fans  208 , thus allowing for an increased mass flow rate of air through the resistive element groups. Furthermore, it should be appreciated that limiting the total number of resistive element groups also reduces the successive group to group heating and thus allows the maximum group operating temperature to be greatly reduced. 
   Referring to  FIG. 5 , the inlet airflow paths  234  of braking grid resistor package  200  are shown and include a top inlet airflow path  236 , a side inlet airflow path  238  and a bottom inlet airflow path  240 . The top inlet airflow path  236  includes four top inlet airflow elements occurring through the four sides of the top inlet section braking grid resistor package  200 . The side inlet airflow path  238  also includes four airflow elements occurring through the four regions on both sides of the braking grid resistor package  200 , fore and aft of the plurality of structure outlets  216 . Additionally, the bottom inlet airflow paths  240  are disposed on both sides of braking grid resistor package  200  below the plurality of structure outlets  216 . Typically, airflow is not permitted to enter the system from below or from the fore or aft end of the system directly into the sides of the blower motor due to interfaces with adjoining cab structures, however, bottom inlet flow path  240  allows for an improved airflow distribution and overall inlet pressure drop versus top inlet flow path  236  alone. It should be appreciated that the bottom inlet flow path  240  allows for an adequate inlet area without extending the length of the system as well as valuable inlet area which may be used in the event that the top inlet flow path  236  is blocked or covered with snow, ice, etc. . . . which is prevalent in extreme operating environments. It should also be appreciated that bottom inlet flow path  240  may be implemented as a separate duct, a plurality of separate ducts or air may be drawn in from directly and/or indirectly under the resistive elements. 
   However, operation of the locomotive in extreme altitudes and temperatures may even result in bottom inlet flow path  240  becoming plugged with snow, ice, etc. . . . as well. As such, venting may be used to control the inlet air temperature and provide warm air circulation when the locomotive is not in dynamic braking mode, as well as when it is in dynamic brake mode. This venting may or may not be operated as a function of the ambient temperature and will allow for a mixture of warm air from within the locomotive to be mixed with cold ambient inlet air to keep the intake region(s) free from snow and ice buildup with the sloped floor allowing for adequate drainage. 
   Referring again to  FIG. 5 , plurality of structure outlets  216  are shown and are disposed to minimize uncontrolled recirculation into the inlet paths, which may be detrimental to the cooling of the system. It should be appreciated that each of the discharge paths may share a common duct surface with the air inlet to provide heating to a portion of an inlet surface to reduce inlet freeze-up and associated blockage. Moreover, the surface may be designed to allow for a small amount of hot discharge air to bleed into the inlet via small ducts, vents, etc. . . . to further reduce the possibility of freeze-up and associated blockage. It should be appreciated that although the discharge ducting is shown as being packaged together, the discharge ducts may be separate for each grid path or may be Y-ed together to reduce discharge pressure loss. 
   Referring to  FIG. 6 ,  FIG. 7  and  FIG. 8 , a second embodiment of a braking grid resistor package  300  is shown and includes a first enclosure portion  302  defining a first enclosure portion cavity  304 , a second enclosure portion  306  defining a second enclosure portion cavity  308  and a third enclosure portion  310  defining a third enclosure portion cavity  312 , wherein the first enclosure portion cavity  304  and the third enclosure portion cavity  312  are separated from the second enclosure portion cavity  308  via a first enclosure portion wall  314  and a third enclosure portion wall  316 , respectively. The second enclosure portion  306  includes at least one second enclosure portion inlet opening  318  and at least one convection opening  320 , wherein the at least one second enclosure portion inlet opening  318  is communicated with the external environment of the braking grid resistor package  300  and wherein the at least one convection opening  320  communicates the first enclosure portion cavity  306  with the second enclosure portion cavity  308 . 
   The third enclosure portion  310  includes at least one third enclosure portion cavity outlet  322 , at least one third enclosure portion cavity inlet opening  323  and at least one airflow opening  324 , wherein the at least one third enclosure portion cavity outlet  322  communicates the third enclosure portion cavity  312  with the external environment of the braking grid resistor package  300  and wherein the at least one airflow opening  324  communicates the second enclosure portion cavity  308  with the third enclosure portion cavity  312 . Additionally, the third enclosure portion  310  includes at least one airflow directional device  326  disposed to be associated with the at least one third enclosure portion cavity outlet  322  and configurable to controllably direct air flowing out of the at least one third enclosure portion cavity outlet  322  away from the at least one second enclosure portion inlet opening  318 . Moreover, at least one transfer duct portion  328  is included, wherein the at least one transfer duct portion  328  defines a duct cavity  330  having at least one duct cavity inlet  332  and at least one duct cavity outlet  334 , wherein the at least one duct cavity inlet  332  is communicated with the at least one duct cavity outlet  334  via the duct cavity  330 . Furthermore, at least one transfer duct is disposed such that the at least one duct cavity inlet  332  is associated with the at least one third enclosure portion cavity outlet  322  and such that the at least one duct cavity outlet  334  is associated with the first enclosure portion cavity  304 . The at least one duct cavity outlet  334  may or may not include controllably movable louvers to increase and/or decrease the amount of discharge air being directed into the at least one transfer duct portion  328 . It should be appreciated that the at least one airflow directional device  326  may include a plurality of louvers  327  having a knifelike edge  329 . Louvers  327  may be movable to direct heated air flowing from the duct cavity outlet  322  either away from or partially into the duct cavity inlet  332 . As such, when the heated air flowing into the duct cavity inlet  332  encounters the first enclosure portion wall  314  and a third enclosure portion wall  316 , heat is transferred from the heated inlet air to the first enclosure portion wall  314  and to the third enclosure portion wall  316 . 
   Referring to  FIG. 9 , a plurality of resistive elements  340  are shown as being disposed within the braking grid resistor package  300  to be located within the third enclosure portion cavity  312 . Additionally, a plurality of cooling fans  350  are included and are disposed to be associated with at least one of the plurality of resistive elements  340 . Also as above, each of the plurality of cooling fans  350  may cool three (3) of the plurality of resistive elements  340  to dissipate a portion of the electrical braking power of the locomotive. It should be appreciated that a fewer number of resistive element groups reduces the airflow backpressure as seen by each of the plurality of cooling fans  350 , thus allowing for an increased mass flow rate of air through the resistive element groups. Furthermore, limiting the total number of resistive element groups may also reduce the successive group to group heating and thus may also allow the maximum group operating temperature to be greatly reduced. The embodiments disclosed herein provide for a compact design while allowing for an advance in the overall neatness of the packaging, while maintaining multiple (in this case at least four (4)) separate airflow paths. 
   Referring to  FIG. 10 , in a locomotive having a dynamic braking capability, a method  500  for maintaining an airflow path to a resistive grid disposed within a resistive grid enclosure is illustrated and discussed, wherein the method  500  includes generating an inlet airflow  402  into the at least one second enclosure portion inlet opening  318  and/or the at least one third enclosure portion cavity inlet opening  323 , as shown in operational block  502 . Referring to  FIG. 11  and  FIG. 12 , this may be accomplished by at least one of propelling the locomotive to receive ambient airflow or by operating the plurality of cooling fans  350  to generate an inlet airflow  402 . The inlet airflow  402  flows into the second enclosure portion cavity  308  and into the third enclosure portion cavity  312  via the at least one airflow opening  324 . As the inlet airflow  402  flows through the at least one airflow opening  324 , the inlet airflow  402  encounters and interacts with the plurality of resistive elements  340  to create a heated inlet airflow  404  by absorbing the heat radiated by the plurality of resistive elements  340 , as shown in operational block  504 . 
   As the heated inlet airflow  404  circulates within the third enclosure portion cavity  312  the flow of the circulation directs the heated inlet airflow  404  toward the at least one third enclosure portion cavity outlet  322  to generate a heated outlet airflow  406 , as shown in operational block  506 . As the heated outlet airflow  406  flows out of the at least one third enclosure portion cavity outlet  322 , a portion (which may or may not be predetermined) of the heated outlet airflow  406  is redirected into the at least one duct cavity inlet  332  and/or through the at least one transfer duct portion  328  and out of the at least one duct cavity outlet  334  into the first enclosure portion cavity  304 , as shown in operational block  508 . This redirection of a portion of the heated outlet airflow raises the temperature of the first enclosure portion wall  314  to prevent/reduce the buildup of blockage material, such as snow and/or ice. The remaining portion of the heat outlet airflow  406  flows out of the at least one third enclosure portion cavity outlet  322  and is controllably directed away from the at least one second enclosure portion inlet opening  318  to prevent heated air from being re-ingested into the braking grid resistor package  300 . The redirected portion of heated outlet airflow  406  is then re-circulated with the ambient air being drawn into the at least one second enclosure inlet opening  318  via at least one convection opening  320 . Additionally, as the heated outlet airflow  406  flows out of the at least one third enclosure portion cavity outlet  322 , a predetermined portion of the heated outlet airflow  406  may also be redirected into the at least one second enclosure portion inlet opening  318  to increase the temperature of the third enclosure portion wall  316 . 
   It should be appreciated that inlet and outlet airflow paths may be disposed adjacent each other by means of airflow directional control via louvers, etc. . . . in order to minimize uncontrolled mixing of the airflow paths and recirculation. It should also be appreciated that as disclosed herein, these issues are addressed by redistributing the grid arrangement such that three (3) grids are cooled per airflow path. Additionally, by utilizing top, bottom, and side inlets, appreciable inlet area is achieved, holding inlet pressure losses to a minimum, without increasing the length of the system. Warm air circulating techniques are being proposed to reduce inlet freezing concerns. Moreover, side or bottom inlet ducts may be implemented. This approach would add additional inlet area to lower inlet losses and opens up the possibility to bring a small amount of warm discharge air into the inlet area to reduce freeze-up risks. A top inlet may also be utilized to reduce inlet losses. It should be further appreciated that sensors, such as temperature, airflow and/or oxygen sensors, may be located within the braking grid resistor package  300  to monitor the temperature, airflow and/or oxygen within the braking grid resistor package  300 . Additionally, these sensors may be associated with the plurality of louvers  327  to allow the plurality of louvers  327  to be automatically adjusted to increase and/or decrease the amount of warm discharge air being recirculated. 
   While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes, omissions and/or additions may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, unless specifically stated any use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.