Patent Publication Number: US-2017355518-A1

Title: Carbon dioxide injection in a transport unit

Description:
FIELD 
     This disclosure relates generally to a controlled atmosphere system (CAS) and a method for controlling the CAS. More specifically, the disclosure relates to a method for controlling an atmospheric gas in a transport refrigeration system (TRS). 
     BACKGROUND 
     A controlled atmosphere system (CAS) is generally used to control an atmospheric parameter such as, but not limited to, a nitrogen content, an oxygen content, and/or a carbon dioxide content within a storage space such as, but not limited to, a transport unit. Examples of a transport unit include, but are not limited to, a container on a flat car, an intermodal container, a truck, a boxcar, or other similar transport unit. A transport unit is commonly used to transport perishable cargo such as, but not limited to, produce, frozen foods, and/or meat products. By controlling one or more atmospheric parameters within the transport unit, the rate of, for example, ripening of perishable cargo stored in the transport unit can be reduced. 
     SUMMARY 
     This disclosure relates generally to a controlled atmosphere system (CAS) and a method for controlling the CAS. More specifically, the disclosure relates to a method for controlling an atmospheric gas in a transport refrigeration system (TRS). 
     A system and method for maintaining a desired carbon dioxide concentration within an interior space of a transport unit during transport are disclosed. In some embodiments, the system and method can be used while the transport unit is in transport. In some embodiments, the system and method can be used while the transport unit is stationary (e.g.,. at a storage facility, etc.). 
     In some embodiments, a carbon dioxide injection system can be used to control a carbon dioxide concentration within a transport unit. In some embodiments, the carbon dioxide injection system can be used to increase the carbon dioxide concentration within the transport unit. In some embodiments, the carbon dioxide injection system can include a conventional. CAS which can be used to decrease the carbon dioxide and/or oxygen concentration within the transport unit. In some embodiments, the carbon dioxide injection system can be used to identify a carbon dioxide leak rate of an interior space of a transport unit. In some embodiments, identifying a carbon dioxide leak rate of the interior space of the transport unit can provide an indication as to whether a CAS will function properly. In some embodiments, identifying a carbon dioxide leak rate of the interior space of the transport unit can be used to determine whether an injected amount of carbon dioxide will be unreasonably high (e.g., too high of a concentration of the carbon dioxide in the interior space of the transport unit). 
     A method for controlling a carbon dioxide concentration within an interior space of a transport unit is disclosed. The method includes determining the carbon dioxide concentration within the interior space; enabling a carbon dioxide injection system when the carbon dioxide concentration is not at a set point value; and disabling the carbon dioxide injection system when the carbon dioxide concentration is at the set point value. 
     A carbon dioxide injection system for controlling a carbon dioxide concentration within an interior space of a transport unit during transport is disclosed. The system includes a carbon dioxide source; a pressure control device; and a flow control device, the carbon dioxide source, the pressure control device, and the flow control device being fluidly connected and in fluid communication with the interior space of the transport unit. The system further includes a controller configured to selectively enable and/or disable flow from the carbon dioxide source to the interior space of the transport unit. 
     A transport unit is disclosed. The transport unit includes a transport refrigeration unit; and a carbon dioxide injection system for controlling a carbon dioxide concentration within an interior space of the transport unit during transport. The carbon dioxide injection system includes a carbon dioxide source; a pressure control device; and a flow control device. The carbon dioxide source, the pressure control device, and the flow control device are fluidly connected and in fluid communication with the interior space of the transport unit. The carbon dioxide injection system also includes a controller configured to selectively enable and/or disable flow from the carbon dioxide source to the interior space of the transport unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       References are made to the accompanying drawings that form a part of this disclosure, and which illustrate the embodiments in which the systems and methods described in this specification can be practiced. 
         FIG. 1  illustrates a transport unit with which the embodiments described in this specification can be practiced, according to some embodiments. 
         FIG. 2  illustrates a block diagram of a carbon dioxide injection system for use in a transport unit, according to some embodiments. 
         FIG. 3  illustrates a block diagram of a controlled atmosphere system (CAS) for use as a carbon dioxide source in a carbon dioxide injection system, according to some embodiments. 
         FIG. 4  illustrates a flowchart for a method to control a carbon dioxide concentration within a transport unit, according to some embodiments. 
     
    
    
     Like reference numbers represent like parts throughout. 
     DETAILED DESCRIPTION 
     This disclosure relates generally to a controlled atmosphere system (CAS) and a method for controlling the CAS. More specifically, the disclosure relates to a method for controlling an atmospheric gas in a transport refrigeration system (TRS). 
     Perishable goods, such as fruits and vegetables, can consume oxygen and produce carbon dioxide (e.g., due to a ripening effect of the perishable goods) when being stored or during transportation. The ripening effect can reduce shelf life of the perishable goods. To help prolong the shelf life of perishable goods, atmosphere in an interior space of, for example, a transport unit can be controlled. During the transportation, the ripening effect of the perishable goods can continuously cause the concentrations of oxygen and/or carbon dioxide in the atmosphere of the interior space to change, which can cause undesirable effects on the shelf life of the perishable goods. It may be desired to control the atmosphere in the storage space during the transportation and/or storage of the perishable goods. 
     A “controlled atmosphere system” (CAS) includes, for example, a controlled atmosphere circuit for controlling one or more atmospheric parameters within an interior space of a transport unit. Examples of atmospheric parameters within the interior space include, but are not limited to, a content of nitrogen, a content of oxygen, and/or a content of carbon dioxide in the air contained within the interior space. 
     A “transport refrigeration system” (IRS) includes, for example, a refrigeration circuit for controlling the refrigeration of an interior space of a transport unit. The TRS may be a vapor-compression type refrigeration system, or any other suitable refrigeration system that can use refrigerant, cold plate technology, or the like. 
     A “transport unit” includes, for example, a container (e.g., a container on a flat car, an intermodal container, etc.), a truck, a boxcar, or other similar transport unit. 
     A “CAS controller” includes, for example, an electronic device (e.g., a processor, memory, etc.) that is configured to manage, command, direct, and regulate the behavior of one or more components of a CAS (e.g., an air compressor, one or more flow valves, one or more sensors, one or more switches, etc.). In some embodiments, the CAS controller can be part of a controller configured to manage, command, direct, and regulate the behavior of one or more components of a refrigeration circuit (e.g., an evaporator, a condenser, a compressor, an expansion valve (EXV), an electronic throttling valve (ETV), etc.), one or more components of an power unit powering, for example, the CAS and a refrigeration circuit, etc. 
       FIG. 1  illustrates a transport unit  100  with which the embodiments described in this specification can be practiced, according to some embodiments. The transport unit  100  includes a transport refrigeration unit (TRU)  10  and a carbon dioxide injection system  140 . It is to be appreciated that the transport unit  100  can include one or more additional components. It is further to be appreciated that the carbon dioxide injection system  140  can be incorporated within the TRU  10 , according to some embodiments. In other embodiments, the carbon dioxide injection system  140  can be incorporated within an interior space of the transport unit  100 . 
     The TRU  10  can generally be used to control one or more environmental conditions within the interior space of the transport unit  100 . Examples of the one or more environmental conditions include, but are not limited to, temperature, air quality, humidity, or the like. The TRU  10  generally operates according to principles known in the art. 
     It is to be appreciated that the transport unit  100  can be a variety of other types of transport units other than a container as illustrated. Examples of alternative transport units include, but are not limited to, a trailer, a boxcar, a truck with a cargo space, or other similar storage compartment designed for transporting cargo. The type of the transport unit  100  is not intended to be limiting, though it is to be appreciated that the systems and methods described in this specification may have varying levels of efficacy depending upon the type of the transport unit  100 . 
     The carbon dioxide injection system  140  can be configured to control one or more atmospheric parameters (e.g., an oxygen content, a carbon dioxide content, a nitrogen content, content of other gases (e.g., ethylene, ozone, etc.), or the like) within the interior space of the transport unit  100 . In particular, the carbon dioxide injection system  140  can be configured to add carbon dioxide to the atmosphere of the interior space in the transport unit  100 . In some embodiments, the carbon dioxide injection system  140  can also be configured to separate nitrogen from, for example, ambient air and supply nitrogen to the interior space of the transport unit  100 . In some embodiments, the carbon dioxide injection system  140  is configured for controlling the carbon dioxide concentration within the interior space of the transport unit  100  while the transport unit  100  is in transit. In some embodiments, the carbon dioxide injection system  140  is configured for controlling the carbon dioxide concentration within the interior space of the transport unit  100  while the transport unit  100  is not in transit (e.g., while at a storage facility, etc.). 
     The TRU  10  and the carbon dioxide injection system  140  can be configured to work together in order to provide a desired atmospheric condition for the interior space that is suitable, for example, for transporting perishable goods such as, but not limited to, fruits and vegetables. 
       FIG. 2  illustrates a block diagram of the carbon dioxide injection system  140  ( FIG. 1 ), according to some embodiments. The carbon dioxide injection system  140  generally includes a carbon dioxide source  150 , a pressure control device  155 , a flow control device  160 , a controller  165 , and a sensor  170  (e.g. a carbon dioxide concentration sensor or the like that is disposed within an interior space  105  of the transport unit  100 ). 
     The carbon dioxide source  150  can be any means for supplying carbon dioxide gas to the interior space of the transport unit  100 . In some embodiments, the carbon dioxide source  150  can be a pressurized cylinder including carbon dioxide. In some embodiments, the pressurized cylinder can have a volume between about 10 and about 100 liters. In some embodiments, the pressurized cylinder including carbon dioxide can be stored at about 50 bar. In some embodiments the carbon dioxide  150  can be placed inside the interior space of the transport unit  100 . In other embodiments, the carbon dioxide source  150  can be placed outside the interior space of the transport unit  100 . In some embodiments, the carbon dioxide source  150  can be a high pressure storage container having a volume between about 30 and about 200 liters. In some embodiments, the high-pressure storage container can store the carbon dioxide at about 50 bar and can be placed either in the interior space or outside the interior space of the transport unit  100 . In some embodiments, the carbon dioxide source  150  includes a controlled atmosphere system (CAS) such as CAS  200  shown and described in accordance with  FIG. 3  below. It is to be appreciated that the carbon dioxide source  150  is not intended to be limited to the above-described embodiments and that other carbon dioxide sources may be implemented within the scope of this disclosure. It is to be appreciated that this volume range is intended to be exemplary and that the volume of the carbon dioxide source  150  can vary depending upon the embodiment. Further, the pressures are intended to be exemplary and can vary according to the principles described herein. 
     The pressure control device  155  is generally configured to reduce a pressure of the carbon dioxide coining from the carbon dioxide source  155 . As described above, the carbon dioxide source  150  can include carbon dioxide stored under pressure. The pressure control device  155  can reduce the pressure of the carbon dioxide. In some embodiments, the carbon dioxide can be reduced from a pressure of about 50 bar to a pressure of about 2 bar. In some embodiments, the pressure can be reduced to about atmospheric pressure. The pressure reduction can, for example, be based on a safety requirement. In some embodiments, the pressure reduction can be selected such that the pressure control device  155  does not freeze due to the reduction in pressure. The pressure control device  155  generally functions according to principles known in the art. 
     The flow control device  160  can generally be configured to allow flow of the carbon dioxide to the interior space of the transport unit  100  or the prevent flow of the carbon dioxide to the interior space of the transport unit  100 . In some embodiments, the flow control device  160  can be a solenoid valve having a flow enabled position and a flow disabled position. In some embodiments, the flow control device  160  can include a valve having a flow-enabled position, a flow disabled position, and one or more intermediate positions in which a partial flow of carbon dioxide is enabled. The flow control device  160  generally operates according to principles known in the art. The controller  165  can control the state (e.g., flow enabled, flow disabled, etc.) of the flow control device  160 . A method for controlling the state of the flow control device  160  is discussed in additional detail in accordance with  FIG. 4  below. 
     The controller  165  can be, for example, an electronic device that is configured to manage, command, direct, and regulate the behavior of one or more components (e.g., the flow control device  160 , etc.) of the carbon dioxide injection system  140 . The controller  165  controls the carbon dioxide injection system  140  to obtain an environmental condition (e.g., a concentration of carbon dioxide) in the interior space of the transport unit  100 . The controller  165  can be in communication with the flow control device  160  and the sensor  170 . The controller  165  can be powered by, for example, a battery (not shown). 
     The sensor  170  is disposed within the interior space of the transport unit  100 . In some embodiments, the sensor  170  can be a carbon dioxide sensor configured to determine a concentration of carbon dioxide within the atmosphere of the interior space of the transport unit  100 . The sensor  170  is configured to determine the concentration of carbon dioxide in the interior space of the transport unit  100  and provide the determined carbon dioxide concentration to the controller  165 . The controller  165  can use the determined carbon dioxide concentration to control the flow control device  160  (e.g., flow enabled, flow disabled, etc., as discussed in additional detail in accordance with  FIG. 4  below). 
       FIG. 3  illustrates a controlled atmosphere system (CAS)  200  for a carbon dioxide source (e.g., carbon dioxide source  150  of the carbon dioxide injection system  140  in  FIG. 2 ) for a transport unit  202 , such as the transport unit  100  shown in  FIG. 1 . 
     The basic components of the CAS  200  include an air compressor  205 , a particulate filter  210 , a heat exchanger  215 , a nitrogen separation membrane  220 , a system of metering valves  225 , a plurality of gas sensors  230 , and a CAS controller  235 . 
     The CAS  200  is configured to control the amount of oxygen and carbon dioxide inside the transport unit  202  to change the rate of ripening of cargo (not shown) stored in the transport unit  202 . The CAS  200  can control the amount of oxygen (O 2 ) and carbon dioxide (CO 2 ) by introducing nitrogen (N 2 ) generated from the nitrogen separation membrane  220 . 
     When the CAS  200  is running, ambient air  201  from outside the transport unit  202  enters the air compressor  205  through a dust filter  240 . In some embodiments, air from inside the transport unit  202  can also be directed to the air compressor  205  through the dust filter  240  via an intake line  275 . The atmospheric air is then compressed to a high pressure by the air compressor  205 . High pressure air from the particulate filter  210  passes to the heat exchanger  215  where it can be temperature conditioned (e.g., heated or cooled) to an optimum operating temperature. The CAS controller  235  receives temperature data from a heat exchanger temperature sensor  217  and can control operation of a heat exchanger switch  219  to maintain the temperature of compressed air leaving the heat exchanger  215 . The high-pressure, temperature conditioned air is then filtered by the particulate filter  210  to remove moisture, dirt, and/or other air contaminants (e.g., oil, ozone, hydrocarbons, etc.) before passing to the membrane  220 . In some embodiments, the particulate filter  210  can include a plurality of particulate filters  210 . A normally opened drain valve  245  is provided on the particulate filter  210 . It will be appreciated that the drain valve  245  can alternatively be a normally closed drain valve. A normally opened drain valve  245  can, for example, allow fluid to drain out in case of a power loss, which can prevent freezing of the particulate filter  210  and/or the drain valve  245 . In some embodiments, the drain valve  245  can be an automated drain valve in which the drain valve  245  is adapted to be opened and/or closed when instructed by the CAS controller  235 . The CAS controller  235  can be programmed to periodically open the drain valve  245 , for a short time, to remove residue which may build up in the particulate filter  210 . In some embodiments, the drain valve  245  may not be included if the particulate filter  210  includes an automatic drain. 
     The temperature conditioned, high pressure air passing from the heat exchanger  215  enters the nitrogen separation membrane  220 , where it can be separated into high purity nitrogen, which passes from a nitrogen outlet  212 , and oxygen/and other gases which are passed to an oxygen outlet  214 . The rate of separation occurring in the nitrogen separation membrane  220  depends on the flow of air through the nitrogen separation membrane  220 . This flow rate is controlled by the pressure in the nitrogen outlet  212 . The higher the pressure in the nitrogen outlet  212 , the higher the nitrogen purity generated, and the lower the flow rate of nitrogen. The nitrogen separation membrane  220  can be capable of generating nitrogen purity levels greater than, for example, about 99 percent. As the pressure in the nitrogen outlet  212  falls, the purity level of the nitrogen falls, and the flow rate increases. 
     The nitrogen enriched gas passing from the nitrogen separation membrane  220  through the nitrogen outlet  212  passes to the flow control valves  225 . The oxygen/other gasses from the oxygen outlet  214  are exhausted to the outside air. 
     The pressure on the nitrogen outlet  212  of the nitrogen separation membrane  220  is regulated by the aforementioned flow control valves  225 . To control the percentage of nitrogen present in the transport unit  202 , the CAS controller  235  can be programmed to cycle the flow control valves  225  to increase or decrease the amount/purity of nitrogen in the transport unit  202  as required. The CAS controller  235  may also add carbon dioxide from an external carbon dioxide source  250  if desired. 
     In some embodiments (e.g., during a ventilation mode), the temperature conditioned, high pressure air passing from the heat exchanger  215  can bypass the nitrogen separation membrane  220  and pass directly to the transport unit  202  via a bypass line  270 . Accordingly, the amount of oxygen in the transport unit  202  can be increased and the amount of carbon dioxide in the transport unit  202  can be decreased. 
     The gas sensors  230  can include, for example, art oxygen concentration sensor, a carbon dioxide concentration sensor, an ethylene concentration sensor, etc. Periodic calibration of the gas sensors  230  to correct drifts with time and temperature can require sampling outside air via a line  260 . The gas sensors  230  can be provided at various locations within the transport unit  202 . 
     The CAS controller  235  is configured to monitor the amount of oxygen and carbon dioxide in the transport unit  202 , using the gas sensors  230  via a sample line  255 . The oxygen and carbon dioxide concentrations monitored by the CAS controller  235  can be stored in a data recorder  280 . 
       FIG. 4  illustrates a flowchart for a method  400  to control a carbon dioxide concentration within a transport unit (e.g., the transport unit  100  of  FIG. 1 ). according to some embodiments. 
     The method  400  generally is directed to determining a concentration of carbon dioxide in an atmosphere of an interior space of the transport unit  100  and adding carbon dioxide (e.g., increasing a concentration of carbon dioxide) or ventilating the interior space (e.g., reducing a concentration of carbon dioxide) for the transport unit  100 . 
     The method  400  begins at  405  when a controller (e.g., the controller  165  of  FIG. 2 ) determines a concentration of carbon dioxide in the interior space of the transport unit  100 . The controller  165  can determine the concentration of carbon dioxide in the interior space of the transport unit  100  through a sensor (e.g., the sensor  170  of  FIG. 2 ). 
     At  410  the controller determines whether the carbon dioxide concentration is about the same as a carbon dioxide concentration set point value. The carbon dioxide set point value can vary depending upon a variety of factors. For example, the carbon dioxide set point can vary based on a cargo being transported within the interior space of the transport unit  100 , a duration of a trip, a user preference, or the like. 
     If the carbon dioxide concentration is not at about the set point value in  410 , a carbon dioxide injection system (e.g., the carbon dioxide injection system  140  of  FIGS. 1-2 ) is enabled at  415 . Enabling the carbon dioxide injection system  140  can include enabling flow from a carbon dioxide source (e.g., the carbon dioxide source  150  of  FIG. 2 ) by modifying a flow control device (e.g., the flow control device  160  of  FIG. 2 ) such that flow of the carbon dioxide is enabled from the carbon dioxide source  150  to the interior space of the transport unit  100 . Once the carbon dioxide injection system  140  is enabled at  415 , the method  400  returns to  405  and the controller determines the concentration of carbon dioxide in the interior space of the transport unit  100 . 
     If the carbon dioxide concentration is about the same as the set point value, the carbon dioxide injection system  140  is disabled at  420 . In some embodiments, disabling the carbon dioxide injection system  140  can, for example, include modifying the flow control device  160  such that flow of carbon dioxide from the carbon dioxide source  150  to the interior space of the transport unit  100  is prevented. 
     At  425  the carbon dioxide concentration in the interior space of the transport unit  100  is determined again by the controller  165 . At  430 , the controller determines whether the carbon dioxide concentration is within a threshold range of the set point value. For example, the threshold range can be an acceptable deviation from the set point value (both above the set point value and below the set point value). In some embodiments, the controller  165  determines the carbon dioxide concentration from the sensor  170  disposed within the interior space of the transport unit  100 . 
     If the carbon dioxide concentration is within the threshold range, the method  400  continues to  425  and monitors the concentration of the carbon dioxide within the interior space of the transport unit  100  until the carbon dioxide concentration is outside the threshold range. If the carbon dioxide concentration is not within the threshold range, the controller determines whether the carbon dioxide concentration is above the threshold range at  435 . If the carbon dioxide concentration is above the threshold range at  435 , the controller  165  will ventilate the interior space of the transport unit  100  at  440 . In some embodiments, ventilating the interior space of the transport unit  100  can be accomplished by enabling a CAS (e.g. the CAS  200  of  FIG. 3 ). The method  400  then continues to  430  to monitor whether the carbon dioxide concentration returns to within the threshold range of the set point value. 
     If the carbon dioxide concentration is not within the threshold range and the carbon dioxide concentration is not above the threshold range at  435  (e.g., the carbon dioxide concentration in the interior space of the transport unit  100  is below the lower bound of the threshold range of the set point value), the method continues to  415  and enables the carbon dioxide injection system  140 . 
     Aspects 
     It is to be appreciated that aspects 1-6 can be combined with any one of aspects 7-11 or 12-15. Further, any one of aspects 7-11 can be combined with any one of aspects 12-15. 
     Aspect 1. A method for controlling a carbon dioxide concentration within an interior space of a transport unit, the method comprising: 
     determining the carbon dioxide concentration within the interior space; 
     enabling a carbon dioxide injection system when the carbon dioxide concentration is not at a set point value; and 
     disabling the carbon dioxide injection system when the carbon dioxide concentration is at the set point value. 
     Aspect 2. The method according to aspect 1, wherein determining the carbon dioxide concentration includes determining a sensor reading from a sensor disposed within the interior space of the transport unit. 
     Aspect 3. The method according to any one of aspects 1-2, wherein enabling the carbon dioxide injection system includes positioning a flow control device such that flow from a carbon dioxide source to the interior space of the transport unit is enabled. 
     Aspect 4. The method according to aspect 3, wherein disabling the carbon dioxide injection system includes positioning the flow control device such that flow from the carbon dioxide source to the interior space of the transport unit is disabled. 
     Aspect 5. The method according to any one of aspects 1-4, further comprising: enabling the carbon dioxide injection system when the carbon dioxide concentration is lower than a threshold range based on the set point value. 
     Aspect 6. The method according to any one of aspects 1-5, further comprising: ventilating the interior space to decrease the carbon dioxide concentration when the carbon dioxide concentration is greater than a threshold range based on the set point value. 
     Aspect 7. A carbon dioxide injection system for controlling a carbon dioxide concentration within an interior space of a transport unit during transport, the system comprising: 
     a carbon dioxide source; 
     a pressure control device; 
     a flow control device, wherein the carbon dioxide source, the pressure control device, and the flow control device are fluidly connected and in fluid communication with the interior space of the transport unit; and 
     a controller configured to selectively enable and/or disable flow from the carbon dioxide source to the interior space of the transport unit. 
     Aspect 8. The system according to aspect 7, further comprising a carbon dioxide sensor, wherein the carbon dioxide sensor is configured to be in electronic communication with the controller. 
     Aspect 9. The system according to any one of aspects 7-8, wherein the controller is configured to selectively enable the flow from the carbon dioxide source to the interior space of the transport unit in response to a concentration of carbon dioxide within the interior space falling below a threshold. 
     Aspect 10. The system according to any one of aspects 7-9, wherein the controller is configured to selectively disable the flow from the carbon dioxide source to the interior space of the transport unit in response to a concentration of carbon dioxide within the interior space being above a threshold. 
     Aspect 11. The system according to any one of aspects 7-10, wherein the carbon dioxide source is one of a pressurized cylinder, a high pressure storage container, or a controlled atmosphere system. 
     Aspect 12. A transport unit, comprising: 
     a transport refrigeration unit; and 
     a carbon dioxide injection system, the carbon dioxide injection system for controlling a carbon dioxide concentration within an interior space of the transport unit during transport, the system including:
         a carbon dioxide source;   a pressure control device;   a flow control device, wherein the carbon dioxide source, the pressure control device, and the flow control device are fluidly connected and in fluid communication with the interior space of the transport unit; and   a controller configured to selectively enable and/or disable flow from the carbon dioxide source to the interior space of the transport unit.       

     Aspect 13. The transport unit according to aspect 12, further comprising a controlled atmosphere system. 
     Aspect 14. The transport unit according to any one of aspects 12-13, wherein the interior space of the transport unit includes a sensor configured to determine a carbon dioxide concentration within the interior space. 
     Aspect 15. The transport unit according to any one of aspects 12-14, wherein the transport unit is one of a container on a flat car, an intermodal container, a truck, or a boxcar. 
     The terminology used in this specification is intended to describe particular embodiments and is not intended to be limiting. The terms “a,” “an,” and “the” include the plural forms as well, unless clearly indicated otherwise. The terms “comprises” and/or “comprising,” when used in this specification, specify the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and/or components. 
     With regard to the preceding description, it is to be understood that changes may be made in detail, especially in matters of the construction materials employed and the shape, size, and arrangement of parts without departing from the scope of the present disclosure. This specification and the embodiments described are exemplary only, with the true scope and spirit of the disclosure being indicated by the claims that follow.