Abstract:
A method of operating a transport refrigeration unit that is operable to regulate a temperature of a cargo space. The method includes providing a controller, driving a refrigerant compressor of the transport refrigeration unit with an internal combustion engine to compress a refrigerant defining an engine operating state of the transport refrigeration unit, and driving the refrigerant compressor of the transport refrigeration unit with an electric motor to compress the refrigerant defining a motor operating state of the transport refrigeration unit. The method further includes sensing the temperature of the cargo space, receiving into the controller a signal indicative of the temperature of the cargo space, determining the temperature of the cargo space using the controller, and switching between the engine operating state and the motor operating state in response to a signal generated by the controller based on the temperature of the cargo space.

Description:
BACKGROUND 
       [0001]    The present invention relates to a transport refrigeration unit and a method of operating a transport refrigeration unit. 
         [0002]    Trucks and tractor-trailer combinations frequently transport cargo that must be maintained at a predetermined temperature (i.e., a set point temperature) or within a predetermined temperature range during transportation. Vehicles that transport temperature sensitive cargo typically have one or more cargo spaces that are maintained within the temperature range by a transport refrigeration unit having an electronic controller, a compressor, a condenser, a flow control valve, an expansion valve, and an evaporator coil. Operation of the transport refrigeration unit is generally controlled and monitored by the electronic controller. 
         [0003]    Typically, transport refrigeration units operate in cooling and heating modes, depending, at least in part, upon the temperature of the cargo space and the ambient temperature outside the air-conditioned cargo space. When the temperature of the cargo space is above the set point temperature, the transport refrigeration unit operates in the cooling mode to pull down the temperature in the cargo space. During operation in the cooling mode, refrigerant is directed along a refrigerant circuit, which extends between the compressor, the flow control valve, the condenser, the expansion valve, and the evaporator coil. The cargo space air is then exposed to the relatively cool evaporator coil. 
         [0004]    When the temperature of the cargo space is below the set point temperature, the transport refrigeration unit operates in the heating mode. During operation in the heating mode, relatively warm refrigerant is directed through a heating circuit, which extends from the compressor, the flow control valve, and the evaporator coil. The cargo space air is then exposed to the relatively warm evaporator coil. 
       SUMMARY 
       [0005]    In one embodiment, the invention provides a method of operating a transport refrigeration unit that is operable to regulate a temperature of a cargo space. The method includes providing a controller, driving a refrigerant compressor of the transport refrigeration unit with an internal combustion engine to compress a refrigerant defining an engine operating state of the transport refrigeration unit, and driving the refrigerant compressor of the transport refrigeration unit with an electric motor to compress the refrigerant defining a motor operating state of the transport refrigeration unit. The method further includes sensing the temperature of the cargo space, receiving into the controller a signal indicative of the temperature of the cargo space, determining the temperature of the cargo space using the controller, and switching between the engine operating state and the motor operating state in response to a signal generated by the controller based on the temperature of the cargo space. 
         [0006]    In another embodiment the invention provides a transport refrigeration unit that is operable to regulate a temperature of a cargo space. The transport refrigeration unit includes a sensor configured to sense the temperature of the cargo space and a controller configured to receive a signal from the sensor indicative of the temperature and configured to determine the temperature. The transport refrigeration unit further includes a refrigerant compressor operable to compress a refrigerant, an internal combustion engine configured to drive the refrigerant compressor to compress the refrigerant defining an engine operating state of the transport refrigeration unit, and an electric motor configured to drive the refrigerant compressor to compress the refrigerant defining a motor operating state of the transport refrigeration unit. A coupling is configured to selectively couple at least one of the internal combustion engine and the electric motor to the refrigerant compressor to drive the refrigerant compressor, and the controller is configured to switch between the engine operating state and the motor operating state in response to a signal generated by the controller based on the temperature of the cargo space. 
         [0007]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side view, partially in section, of a vehicle having a transport refrigeration unit according to one embodiment of the invention. 
           [0009]      FIG. 2  is a schematic representation of the transport refrigeration unit of  FIG. 1 . 
           [0010]      FIG. 3  is a flowchart illustrating a method of operating the transport refrigeration unit of  FIG. 1 . 
       
    
    
       [0011]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates a transport refrigeration unit (“TRU”)  10 . The TRU  10  is especially suitable for use in transport applications and can be mounted on a container, truck, trailer, and the like.  FIG. 1  shows the TRU  10  mounted on a trailer  14  having a cargo space  16 . The trailer  14  is pulled by a tractor  18 . In other constructions (not shown), the TRU  10  can be mounted on a storage container or another vehicle, such as, for example, a truck. Furthermore, although the unit  10  is referred to as a refrigeration unit, as will be discussed in more detail below, the TRU  10  is not limited to use in cooling modes and the TRU can also be used in heating modes. 
         [0013]    As used herein and in the claims, the term “refrigerant” includes any conventional refrigeration fluid, such as, for example, chlorofluorocarbons (CFCs), hydrocarbons, cryogens (e.g., CO 2 , and N 2 ), etc. In addition, as used herein and in the claims, the term “refrigerant” refers to fluids also commonly used for heating and defrosting purposes. 
         [0014]    The TRU  10  controls or regulates the temperature of the cargo space  16  to a specified temperature range adjacent to a predetermined set point temperature (“SP”). More particularly, the TRU  10  maintains the temperature of the cargo space  16  within a range surrounding the set point temperature SP (e.g., SP ±5° F.). As shown in  FIG. 2 , the TRU  10  has a closed refrigerant circuit or flow path  20 , which includes a refrigerant compressor  22  driven by a prime mover arrangement  24 . The prime mover arrangement  24  includes an internal-combustion engine  26  and an electric motor  28 . In one embodiment, the internal-combustion engine  26  is a diesel engine. The TRU  10  further includes a clutch or coupling  30 . The coupling  30  is configured to selectively drive the compressor  22  with either the engine  26  or the motor  28 . Accordingly, as will be discussed in more detail below, the refrigerant compressor  22  can be driven by engine  26  when the coupling  30  is in a first configuration and the compressor  22  can be driven by the electric motor  28  and disengaged from the engine  26  when the coupling  30  is in a second configuration. 
         [0015]    With continued reference to  FIG. 2 , the TRU  10  further includes a discharge valve  34  and a discharge line  36  that connects the compressor  22  to a three-way valve  38 . A discharge pressure transducer  40  is located along the discharge line  36 , upstream from the three-way valve  38  to measure the discharge pressure of the compressed refrigerant. The three-way valve  38  includes a first outlet port  42  and a second outlet port  44 . When the TRU  10  is operated in a COOLING mode, the three-way valve  38  is adjusted to direct refrigerant from the compressor  22  through the first outlet port  42  and along a first circuit or flow path (represented by arrows  48 ). When the TRU  10  is operated in HEATING and DEFROST modes, the three-way valve  28  is adjusted to direct refrigerant through the second outlet port  44  and along a second circuit or flow path (represented by arrows  50 ). 
         [0016]    The first flow path  48  extends from the compressor  22  through the first outlet port  42  of the three-way valve  38 , a condenser coil  52 , a one-way condenser check valve  54 , a receiver  56 , a liquid line  58 , a refrigerant drier  60 , a heat exchanger  62 , an expansion valve  64 , a refrigerant distributor  66 , an evaporator coil  68 , an electronic throttling valve  70 , a suction pressure transducer  72 , a second path  74  through the heat exchanger  62 , an accumulator  76 , a suction line  78 , and back to the compressor  22  through a suction port  80 . The expansion valve  64  is controlled by a thermal bulb  82  and an equalizer line  84 . 
         [0017]    The second flow path  50  bypasses a section of the refrigeration circuit  86 , including the condenser coil  52  and the expansion valve  64 , and connects the hot gas output of compressor  22  to the refrigerant distributor  66  via a hot gas line  88  and a defrost pan heater  90 . The second flow path  50  continues from the refrigerant distributor  66  through the evaporator coil  68 , the throttling valve  70 , the suction pressure transducer  72 , the second path  74  through the heat exchanger  62 , and the accumulator  76  and back to the compressor  22  via the suction line  78  and the suction port  80 . 
         [0018]    A hot gas bypass solenoid valve  92  is disposed to inject hot gas into the hot gas line  88  during operation in the COOLING mode. A bypass or pressurizing line  96  connects the hot gas line  88  to the receiver  56  via check valves  98  to force refrigerant from the receiver  56  into the second flow path  50  during operation in the HEATING and DEFROST modes. 
         [0019]    Line  100  connects the three-way valve  38  to the low-pressure side of the compressor  22  via a normally closed pilot solenoid valve  102 . When the solenoid valve  102  is closed, the three-way valve  38  is biased (e.g., spring biased) to select the first outlet port  42  of the three-way valve  38 . When the evaporator coil  52  requires defrosting and when heating is required, valve  92  is energized and the low pressure side of the compressor  22  operates the three-way valve  38  to select the second outlet port  44  to begin operation in the HEATING mode or DEFROST modes. 
         [0020]    A condenser fan or blower  104  directs ambient air (represented by arrows  106 ) across the condenser coil  52 . Return air (represented by arrows  108 ) heated by contact with the condenser fan  104  is discharged to the atmosphere. An evaporator fan  110  draws cargo space air (represented by arrows  112 ) through an inlet  114  in a bulkhead or wall  116  and upwardly through conduit  118 . A return air temperature sensor  120  measures the temperature (T 1 ) of air entering the inlet  114 . In the illustrated embodiment, the fans  104 ,  110  are directly driven by the same power source that drives the compressor  22 . 
         [0021]    Discharge air (represented by arrow  122 ) is returned to the cargo space  16  via outlet  124 . Discharge air temperature sensor  126  is positioned adjacent to the outlet  124  and measures the discharge air temperature. During the DEFROST mode or during operation in a RECOVERY cycle, a damper  128  is moved from an opened position (shown in  FIG. 2 ) toward a closed position (not shown) to close the discharge air path to the cargo space  16 . 
         [0022]    The TRU  10  also includes a controller  130 . The controller  130  includes a microprocessor  132 , a database  134 , and a user interface  136 . The user interface  136  allows the user to enter load parameters, including the set point temperature (“SP”) and an acceptable range surrounding the set point temperature (e.g., SP ±5° F.). These values are then saved to the database  134 . Also, the database can store preprogrammed set point temperatures and the acceptable range surrounding the set point temperature for various types of cargo. Then, the user can enter the type of cargo (e.g., apples, bananas, flowers, etc.) into the controller  130  via the user interface  136  and the controller  130  automatically recalls the corresponding load parameters, including the set point temperature and acceptable range surrounding the set point temperature from the database  134 . 
         [0023]    The controller  130  receives data from sensors, including the return air temperature sensor  120  and the discharge air temperature sensor  126 . Additionally, given temperature data and programmed parameters, the controller  130  determines whether cooling, heating, or defrosting is required by comparing the data collected by the sensors with the set point temperature SP. Also, the TRU  10  includes a sensor  138 , which can be a voltage sensor, a current sensor, or the like. The sensor  138  senses whether an external alternating current electrical power source  140  is available to power the TRU  10 . The sensor  138  is in communication with the controller  130  so that the controller  130  can receive a signal from the sensor  130  to indicate whether the electrical source  140  is available to power the TRU  10 . The electrical source  140  can include any suitable external alternating current electrical power source. For example, the trailer  14  may parked at a loading dock and the user can plug an electrical cord of the TRU  10  into an electrical outlet near the loading dock to supply external power independent of the TRU  10  to the TRU  10 . 
         [0024]    Referring to  FIGS. 2 and 3 , in operation, the controller  130  prompts the operator to enter load parameters, represented by act  142  in  FIG. 3 . In one embodiment, the controller  130  prompts the operator to enter the set point temperature SP (e.g., 32° F.), a first high temperature limit X 1 (e.g., ° 5 F), a first low temperature limit X 2  (e.g., ° 5 F), a second high temperature limit Y 1 , and a second low temperature limit Y 2 . In some methods of operation and embodiments, the first and the second high temperature limits X 1  and Y 1  are equal and the first and the second low temperature limits X 2  and Y 2  are equal. The purpose of these temperature limits will be discussed in more detail below. The user enters these values into the controller using the interface  136 . In other constructions, the controller  130  prompts the operator to enter via the interface  136  the type of cargo (e.g., lettuce, bananas, flowers, ice cream, milk, etc.) and the anticipated travel time (e.g., one hour, two hours, etc.). In these constructions, the controller  130  recalls previously programmed load parameters, including set point temperature SP, first high temperature limit X 1 , first low temperature limit X 2 , second high temperature limit Y 1 , and second low temperature limit Y 2  values for the selected cargo type from the database  134  of the controller  130  and the load parameters are automatically entered. 
         [0025]    With continued reference to  FIGS. 2 and 3 , during operation of the TRU  10 , the controller  130  determines the return air temperature T 1  using the sensor  120  located in the return air conduit  118 , which is represented by act  144  in  FIG. 3 . If the return air temperature T 1  is greater than or equal to the sum of the set point temperature SP and the first high temperature limit X 1 (“YES” at act  146 ) the controller  130  operates the TRU  10  in the COOLING mode to provide relatively cool air to the cargo space  16 . During operation in the COOLING mode, the compressor  22  is driven to compress the refrigerant and the refrigerant is directed along the first flow path  48 . Additionally, the damper  128  is moved toward the opened position and the evaporator fan  110  is activated to draw cargo space air across the evaporator coil  68 . Relatively cold refrigerant flows through the evaporator coil  68  during operation in the COOLING mode and the cargo space air is cooled by contact with the relatively cold evaporator coil  68  before being returned to the cargo space  16  via the outlet  124 . 
         [0026]    If the return air temperature T 1  is less then the sum of the set point temperature SP and the first high temperature limit X 1 (“NO” at act  146 ) and if the return air temperature T 1  is less than or equal to the set point temperature SP minus the first low temperature limit X 2  (“YES” at act  148 ) (i.e., if the return air temperature T 1  is below the predetermined acceptable temperature for the load), the controller  130  initiates the HEATING mode to provide relatively warm air to the cargo space  16 . During operation in the HEATING mode, the compressor  22  compresses the refrigerant and the refrigerant is directed along the second flow path  50 , bypassing portions of the refrigeration circuit  20 , including the condenser coil  52 , the check valve  54 , and the receiver  56 . 
         [0027]    In act  148 , if the return air temperature T 1  is greater than the set point temperature SP minus the first low temperature limit X 2  (“NO” at act  148 , which is also less than then set point temperature SP plus the first high temperature limit X 1  because of act  146 ), the controller  130  operates the TRU  10  in a NULL mode. In the NULL mode, the controller  130  shuts down the compressor  22  or operates the compressor  22  at reduced speed and reduced capacity. Additionally, the controller  130  shuts down or reduces the operating speed of the condenser fans  104  and the evaporator fans  110 . 
         [0028]    Referring to  FIGS. 2 and 3 , the compressor  22  can be driven to compress refrigerant for use in the HEATING, the COOLING, and the NULL modes using either the engine  26  or the electrical motor  28 . As discussed above, the coupling  30  can be configured by the controller  130  to transfer power from either the engine  26  or the motor  28  to the compressor  22  to drive the compressor  22 . As will be discussed below, the controller  130  can automatically switch between using the engine  26  to drive the compressor  22  and the motor  28  to drive the compressor  22 . 
         [0029]    When the user enters the cargo load parameters, the user can also enable a feature that allows the compressor  22  to automatically switch between being driven by the electrical motor  28  and the engine  26 . In act  162  of the flowchart illustrated in  FIG. 3 , the controller  130  determines whether this feature has been enabled by the user. If the feature has not been enabled (“NO” at act  162 ), the controller  130  continues to operate the TRU  10  using either the motor  28  or the engine  26  depending on whether the motor  28  or the engine  26  was manually selected by the user to drive the compressor  22  in the HEATING, the COOLING, and the NULL modes described above. If the feature has been enabled by the user (“YES” at act  162 ), the controller  130  proceeds to act  168  and determines whether the external electrical power source  140  is available. 
         [0030]    In act  168 , the sensor  138  senses a current, a voltage, or the like and the controller  130  receives a signal from the sensor  138  and determines whether the electrical power source  140  is available. If the controller  130  determines that the electrical power source  140  is not available (“NO” at act  168 ), the controller operates the TRU  10  using the engine  26  to drive the compressor  22  in the HEATING, the COOLING, and the NULL modes, which is generally indicated by act  172 . In order to drive the compressor  22  using the engine  26 , the controller  130  automatically starts the engine if the engine  26  is not already operating or running. Also, the controller  130  sends a signal to the coupling  30  to configure the coupling  30 , if not already so configured, so that the coupling  30  transfers power from the engine  26  to the compressor  22  in order to drive the compressor  22  to compress the refrigerant. 
         [0031]    If the controller  130  determines that the electrical power source  140  is available (“YES” at act  168 ), the controller  130  determines the temperature within the cargo space  16 , which is represented by act  176 . In act  176 , the return air temperature sensor  120  records the temperature T 1  of air entering the TRU  10  through inlet  114  of the return air conduit  118  and transmits the return air temperature data T 1  to the controller  130 . In general, the return air temperature T 1  is substantially equal to the average temperature of the load space air. 
         [0032]    After recording the return air temperature T 1 , the controller  130  determines whether the return air temperature T 1  is less than or equal to an upper limit temperature T 2  and greater than or equal to a lower limit temperature T 3  (act  178 ). The upper limit temperature T 2  is a first predetermined temperature equal to the set point temperature SP plus the second high temperature limit Y 1  and the lower limit temperature T 3  is a second predetermined temperature equal to the set point temperature SP minus the second low temperature limit Y 2 . As referenced earlier, the set point temperature SP minus the first low temperature limit X 2  defines a third predetermined temperature, and the set point temperature SP plus the first high temperature limit X 1  defines a fourth predetermined temperature. If the return air temperature T 1  is greater than the upper limit temperature T 2  or less than the lower limit temperature T 3  (“NO” at act  178 ) the controller  130  automatically operates the TRU  10  so that the engine  26  drives the refrigerant compressor  22  (represented by act  172 ) and the motor  28  is turned off. If the return air temperature T 1  is less than or equal to the upper limit temperature T 2  and greater than or equal to the lower limit temperature T 3  (“YES” at act  178 ), then the controller  130  automatically operates the TRU so that the electric motor  28  drives the refrigerant compressor  22  (represented by act  180 ) and the engine  26  is turned off. 
         [0033]    As indicated by loops  182 , the controller  130  continues to monitor whether the electrical power source  140  is available (act  168 ), whether the TRU should operate in the HEATING, the COOLING, or the NULL mode, and whether the return air temperature T 1  is less than or equal to the upper limit temperature T 2  and greater than or equal to the lower limit temperature T 3  (act  178 ). As long as the conditions of acts  168  and  178  are met, the controller  130  continues to operate the TRU  10  using the motor  28  to drive the compressor  22 . However, if the electric power source  140  is no longer available or if the return air temperature T 1  is greater than the upper limit temperature T 2  or less than the lower limit temperature T 3 , the controller  130  automatically switches from driving the compressor  22  with the motor  28  to driving the compressor  22  with the engine  22 . The controller  130  automatically switches between these driving arrangements by sending a signal to the coupling in order to configure the coupling so that power is transferred from either the motor  28  or the engine  26  to the compressor  22 . Also, if the controller  130  switches from driving the compressor  22  with the motor  28  to driving the compressor  22  with the engine  26 , the controller  130  can automatically restart the engine  26  if the engine  26  was shutdown or stopped by the controller  130  or user when the compressor  22  was being driven by the motor  28 . 
         [0034]    Accordingly, the controller  130  can automatically switch between driving the compressor  22  with the motor  28  and the engine  26 . In some applications of the TRU  10  and trailer  14 , the user will park the trailer  14  at a loading dock. Then, the user can plug the TRU  10  into the electrical power source  140  (e.g., electrical socket). The controller  130  determines whether the user has plugged the TRU  10  into the power source  140  and the controller  130  also determines the temperature within cargo space  16  of the trailer  14 . If the temperature within the cargo space  16  (e.g., T 1 ) is excessively high (e.g., above the upper limit temperature T 2 ), the controller  130  automatically uses the engine  26  to drive the compressor  22  in the COOLING mode. Typically, the engine  26  provides more power than the motor  28 , and therefore, the TRU  10  can reduce the temperature within the cargo space  16  (i.e., pull down) quicker by using the engine  26  to drive the compressor  22  than by using the motor  28  to drive the compressor  22 . Similarly, if the temperature within the cargo space  16  is excessively low (e.g., below the lower temperature limit T 3 ), the controller  130  automatically uses the engine  26  to drive the compressor  22  in the HEATING mode. Typically, the TRU  10  can increase the temperature within the cargo space  16  quicker by using the engine  26  to drive the compressor  22  than by using the motor  28  to drive the compressor  22  because the engine  26  often provides more power than the motor  28 . 
         [0035]    Once or if the temperature within the cargo space  16  (e.g., T 1 ) is within a predetermined temperature range (e.g., less than or equal to T 2  and greater than or equal to T 3 ), the controller  130  automatically uses the motor  28  to drive the compressor  22  in one or more of the HEATING, the COOLING, and the NULL modes. Therefore, the controller  130  automatically uses the motor  28  when generally less power is required. Using the motor  28  instead of the engine  26  to drive the compressor  22  saves fuel stored on-board the trailer  14  for the engine  26  and can also reduce the amount of noise generated by the TRU  10 . 
         [0036]    In the illustrated embodiment, the controller  130  automatically switches between driving the compressor  22  with the motor  28  and the engine  26  based on the temperature within the cargo space  16 . In other embodiments, the controller  130  can also include a timer that determines the time that has elapsed since start-up of the TRU  10 . In such an embodiment, the controller  130  can automatically switch from driving the compressor  22  with the engine  26  to driving the compressor  22  with the motor  28  after a predetermined elapsed time from start-up of the TRU  10 . Therefore, the engine  26 , which typically provides more power than the motor  28 , is used to drive the compressor  22  immediately after start-up of the TRU  10  to quickly pull-down the temperature T 1  within the cargo space  16 . Then, after the predetermined time has elapsed, the controller  130  automatically switches to driving the compressor  22  with the motor  28  when generally less power is required because the temperature T 1  within the cargo space  16  has been pulled down to an acceptable temperature using the engine  26 . 
         [0037]    Various features and advantages of the invention are set forth in the following claims.