Abstract:
A method for controlling speed  122  of an engine, the engine providing power to a compressor of a transport refrigeration unit, the method including generating a speed request  124,  the engine speed responsive to the speed request; and generating a speed request offset, the speed request offset being added to the speed request to adjust the speed of the engine.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to the operation of a transport refrigeration system and, more particularly, to adjusting engine speed in response to loads in a transport refrigeration system. 
         [0002]    Fruits, vegetables and other perishable items, including meat, poultry and fish, fresh or frozen, are commonly transported in the cargo box of a truck or trailer, or in an intermodal container, collectively referred to herein as a container. Accordingly, it is customary to provide a transport refrigeration system in operative association with the container for cooling the atmosphere within the container. The transport refrigeration system includes a refrigerant vapor compression system, also referred to as a transport refrigeration unit, and an on-board power unit. The refrigerant vapor compression system typically includes a compressor, a condenser, an expansion device and an evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The power unit includes an engine, typically diesel powered. 
         [0003]    In many truck/trailer transport refrigeration systems, the compressor of the transport refrigeration unit is driven by the engine shaft either through a belt drive or by mechanical shaft-to-shaft link. Other systems employ all electric transport refrigeration systems wherein the engine drives an on-board generator for generating sufficient electrical power to drive an electric motor operatively associated with the compressor of the transport refrigeration unit. For example, U.S. Pat. No. 6,223,546, assigned to Carrier Corporation, the same assignee to which this application is subject to assignment, the entire disclosure of which is incorporated herein by reference in its entirety, discloses an electrically powered transport refrigeration unit powered by an engine driven synchronous generator capable of producing sufficient power to operate the compressor drive motor and at least one fan motor. With respect to intermodal containers, clip-on power units, commonly referred to as generator sets or gensets, are available for mounting to the intermodal container, typically when the container is being transported by road or rail, to provide electrical power for operating the compressor drive motor of the transport refrigeration unit associated with the container. The genset includes a diesel engine and a generator driven by the diesel engine. 
         [0004]    During transport of such perishable items, the temperature within the container must be maintained within strict temperature limits associated with the particular items being transported, regardless of potentially severe operating conditions imposed by the local environment in which the system is operating. For example, when the transport refrigeration system is operated at high ambient temperatures and/or high altitude operation, the power demanded by the refrigeration unit at high cooling capacity demand may exceed the limited shaft power available from the engine, raising the potential for an engine stall or engine overload. In the event of an engine stall or engine overload, the loss of power from the generator will result in an undesired shutdown of the refrigeration unit. 
         [0005]    Existing transport refrigeration systems employ an independently controlled diesel engine that directly drives the compressor and fans, or indirectly drives the compressor and fans by providing them with electrical power that is generated with a generator. In either case, the system load (e.g., power required of the refrigeration or heating system) often changes to meet temperature requirements of the container. When the various components in the system are cycled, the system load can change very quickly. When this happens, the engine speed (i.e., RPM) can either droop or overdrive until the engine speed control algorithm has time to adjust to the new load. This causes instability within the system. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one embodiment a method for controlling speed of an engine, the engine providing power to a compressor of a transport refrigeration unit, the method including generating a speed request, the engine speed responsive to the speed request; and generating a speed request offset, the speed request offset being added to the speed request to adjust the speed of the engine. 
         [0007]    In another embodiment, a transport refrigeration system includes an engine; a compressor powered by the engine; an engine controller controlling a speed of the engine; and a refrigeration unit controller controlling a transport refrigeration unit, the refrigeration unit controller implementing a process including: generating a speed request, the engine speed responsive to the speed request; generating a speed request offset, the speed request offset being added to the speed request to adjust the speed of the engine. 
         [0008]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of embodiments of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a view of an exemplary refrigerated trailer equipped with a transport refrigeration system; 
           [0011]      FIG. 2  is a schematic diagram of an exemplary embodiment of a transport refrigeration system wherein the compressor is directly driven by a fuel-fired engine; 
           [0012]      FIG. 3  is a schematic diagram of an exemplary embodiment of a transport refrigeration system wherein the compressor is driven by a motor powered by an electric generator driven by a fuel-fired engine; 
           [0013]      FIG. 4  depicts an engine control method in an exemplary embodiment; and 
           [0014]      FIG. 5  is a plot of engine speed, system load and speed request in an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  depicts a transport refrigeration system  10  associated with a trailer  12  pulled by a tractor  14  as depicted in  FIG. 1 . The exemplary trailer  12  includes a cargo container  16  defining an interior space  18  wherein perishable product is stowed for transport. The transport refrigeration system  10  is operative to climate control the atmosphere within the interior space  18  of the cargo container  16  of the trailer  12 . It is to be understood that the method disclosed herein may be applied not only to refrigeration systems associated with trailers, but also to refrigeration systems applied to refrigerated trucks, to intermodal containers equipped with gensets, and to other refrigeration systems including a refrigerant unit having an engine driven compressor. 
         [0016]    Referring to  FIGS. 2 and 3  of the drawings, there are depicted exemplary embodiments of transport refrigeration systems for cooling the atmosphere of a truck, trailer, intermodal container or similar cargo transport unit, collectively referred to herein as a container. The transport refrigeration system  10  includes a transport refrigeration unit  20  including a compressor  22 , a refrigerant heat rejection heat exchanger  24  (shown as a condenser in the depicted embodiments) with its associated fan(s)  25 , an expansion device  26 , a refrigerant evaporator heat exchanger  28  with its associated fan(s)  29 , and a suction modulation valve  30  connected in a closed loop refrigerant circuit and arranged in a conventional refrigeration cycle. The transport refrigeration system  10  further includes an engine  32  (e.g., diesel), an electronic refrigeration unit controller  34  and an electronic engine controller  36 . The transport refrigeration system  10  is mounted as in conventional practice to an exterior wall of the container with the compressor  22  and the condenser heat exchanger  24  with its associated condenser fan(s)  25 , and diesel engine  32  disposed externally of the refrigerated container  16 . 
         [0017]    As in conventional practice, when the transport refrigerant unit  20  is operating in a cooling mode, low temperature, low pressure refrigerant vapor is compressed by the compressor  22  to a high pressure, high temperature refrigerant vapor and passed from the discharge outlet of the compressor  14  to circulate through the refrigerant circuit to return to the suction inlet of the compressor  22 . The high temperature, high pressure refrigerant vapor passes into and through the heat exchange tube coil or tube bank of the condenser heat exchanger  24 , wherein the refrigerant vapor condenses to a liquid, thence through the receiver  38 , which provides storage for excess liquid refrigerant, and thence through the subcooler coil of the condenser heat exchanger  24 . The subcooled liquid refrigerant then passes through a first refrigerant pass of the refrigerant-to-refrigerant heat exchanger  40 , and thence traverses the expansion device  26  before passing through the evaporator heat exchanger  28 . In traversing the expansion device  26 , which may be an electronic expansion valve (“EXV”) as depicted in  FIGS. 2 and 3 , or a mechanical thermostatic expansion valve (“TXV”), the liquid refrigerant is expanded to a lower temperature and lower pressure prior to passing to the evaporator heat exchanger  28 . 
         [0018]    In flowing through the heat exchange tube coil or tube bank of the evaporator heat exchanger  28 , the refrigerant evaporates, and is typically superheated, as it passes in heat exchange relationship return air drawn from the cargo space  18  passing through the airside pass of the evaporator heat exchanger  28 . The refrigerant vapor thence traverses a second refrigerant pass of the refrigerant-to-refrigerant heat exchanger  40  in heat exchange relationship with the liquid refrigerant passing through the first refrigerant pass thereof. Before entering the suction inlet of the compressor  22 , the refrigerant vapor passes through the suction modulation valve  30  disposed downstream with respect to refrigerant flow of the refrigerant-to-refrigerant heat exchanger  40  and upstream with respect to refrigerant flow of the suction inlet of the compressor  22 . The refrigeration unit controller  34  controls operation of the suction modulation valve  30  and selectively modulates the open flow area through the suction modulation valve  30  so as to regulate the flow of refrigerant passing through the suction modulation valve to the suction inlet of the compressor  22 . By selectively reducing the open flow area through the suction modulation valve  30 , the refrigeration unit controller  30  can selectively restrict the flow of refrigerant vapor supplied to the compressor  22 , thereby reducing the capacity output of the transport refrigeration unit  20  and in turn reducing the power demand imposed on the engine  32 . 
         [0019]    Air drawn from within the container  16  by the evaporator fan(s)  29  associated with the evaporator heat exchanger  28 , is passed over the external heat transfer surface of the heat exchange tube coil or tube bank of the evaporator heat exchanger  28  and circulated back into the interior space  18  of the container  16 . The air drawn from the cargo box is referred to as “return air” and the air circulated back to the cargo box is referred to as “supply air.” It is to be understood that the term “air’ as used herein includes mixtures of air and other gases, such as for example, but not limited to nitrogen or carbon dioxide, sometimes introduced into a refrigerated container for transport of perishable product such as produce. 
         [0020]    In the embodiment of the transport refrigeration system depicted in  FIG. 2 , the compressor  22  comprises a reciprocating compressor having a compressing mechanism (not shown) mounted on a shaft that is directly coupled to and driven by the fuel-fired engine  32 . In this embodiment, the fan(s)  25  and the fan(s)  29  may also be driven by the fuel-fired engine  32  through a belt or chain drive. Additionally, the engine  32  may also power an alternator, again through a belt or chain drive, to generate electric current for powering the refrigerant unit controller and other on-board electrical or electronic components of the transport refrigeration system  10 . 
         [0021]    In the embodiment of the transport refrigeration system depicted in  FIG. 3 , the compressor  22  comprises a semi-hermetic scroll compressor having an internal electric drive motor and a compression mechanism having an orbital scroll mounted on a drive shaft driven by the internal electric drive motor that are all sealed within a common housing of the compressor  22 . The fueled-fired engine  32  drives an electric generator  42  that generates electrical power for driving the compressor motor which in turn drives the compression mechanism of the compressor  22 . The drive shaft of the fueled-fired engine drives the shaft of the generator  42 . In this embodiment, the fan(s)  25  and the fan(s)  29  may be driven by electric motors that are supplied with electric current produced by the generator  42 . In an electrically powered embodiment of the transport refrigeration system  10 , the generator  42  comprises a single on-board engine driven synchronous generator configured to selectively produce at least one AC voltage at one or more frequencies. 
         [0022]    In an embodiment, the fueled-fired engine  32  comprises a diesel fueled piston engine, such as for example a diesel engine of the type manufactured by Kubota Corporation. However, it is to be understood that virtually any engine may be used that meets the space requirements and is capable of powering the compressor  22  or the generator  42 . By way of example, the engine  32  may comprise a diesel fueled piston engine, a gasoline fueled piston engine, a natural gas or propane fuel piston engine, as well as other piston or non-piston engines that are fuel-fired. 
         [0023]    As noted previously, the transport refrigeration system  10  also includes an electronic refrigeration unit controller  34  that is configured to operate the transport refrigeration unit  20  to maintain a predetermined thermal environment within the interior space  18  defined within the cargo box  16  wherein the product is stored during transport. The controller  30  maintains the predetermined thermal environment by selectively activating and deactivating the various components of the refrigerant vapor compression system, including the compressor  22 , the fan(s)  25  associated with the condenser heat exchanger  24 , the fan(s)  29  associated with the evaporator heat exchanger  28 , and various valves in the refrigerant circuit, including but not limited to the suction modulation valve  30  to selectively varying the refrigeration load capacity of the transport refrigeration unit  20 . 
         [0024]    In one embodiment, the refrigeration unit controller  34  includes a microprocessor and an associated memory. The memory of the controller  34  may be programmed to contain preselected operator or owner desired values for various operating parameters within the system. The programming of the controller is within the ordinary skill in the art. The controller  34  may include a microprocessor board that includes the microprocessor, an associated memory, and an input/output board that contains an analog-to-digital converter which receives temperature inputs and pressure inputs from a plurality of sensors located at various points throughout the refrigerant circuit and the refrigerated container, current inputs, voltage inputs, and humidity levels. The input/output board may also include drive circuits or field effect transistors and relays which receive signals or current from the controller  34  and in turn control various external or peripheral devices associated with the transport refrigeration system. The particular type and design of the controller  34  is within the discretion of one of ordinary skill in the art to select and is not limiting of the invention. 
         [0025]    The refrigeration unit controller  34  is also in communication with an electronic engine controller  36 . For example, the refrigeration unit controller  34  may be in closed loop communication with the electronic engine controller  36  by way of a controller area network (CAN) system. In operation, controller  34  executes a refrigeration control algorithm to control the refrigeration system components (as noted above). Controller  34  also executes an engine speed control algorithm to control a speed setting of engine  32 . Controller  34  detects when a change in load on the refrigeration unit occurs or is imminent, and feeds forward a speed request offset to engine controller  36  to request a temporary speed change so that the engine speed better matches the anticipated load of the refrigeration unit. The load change may be due to one or more components of the refrigeration unit, such as the compressor and/or one or more fans. Engine controller  36  can increase or decrease speed of the engine  32  by adjusting the engine throttle. This speed request offset can be ramped up and down over time to create a more stable engine speed, and reduce stalls or overdrives of engine  32 . 
         [0026]      FIG. 4  depicts an exemplary engine speed control algorithm. At  100 , controller  34  monitors a parameter of the transport refrigeration unit  20 . The monitored parameter may be a temperature (e.g., temperature within container  16 ) or a pressure (e.g., discharge pressure of compressor  22 ) or a combination of a plurality of temperatures and or pressures. At  102 , the parameter of the transport refrigeration unit  20  is compared to a threshold, such as an acceptable range for that parameter. For example, the temperature within container  16  may be compared to a setpoint range to determine if the temperature is within the desired range. 
         [0027]    If at  102 , the parameter of the transport refrigeration unit  20  does not exceed the range, flow proceeds to  104  where controller  34  predicts if the sensed parameter is expected to exceed the threshold (e.g., setpoint range) within a predetermined time period (e.g., within next 30 seconds). This prediction may be based on a rate of change of the parameter over time that indicates the parameter will exceed the threshold within the predetermined time period. Alternatively, parameter values may be applied to a model to predict upcoming parameter values. Other factors, such as ambient temperature measurements, may be used to predict if the parameter is expected to exceed the threshold within a predetermined time period. 
         [0028]    If the parameter does not exceed the threshold at  102 , and is not predicted to exceed the threshold at  104 , then at  106 , controller  34  provides a speed request signal to engine controller  36  that maintains the engine speed at a steady RPM. This is the steady state operational mode for the engine  32 . 
         [0029]    If the parameter exceeds the threshold at  102 , or is predicted to exceed the threshold at  104 , then flow proceeds to  108  where controller  34  determines an amplitude and duration of a speed request offset. The speed request offset is a signal provided to the engine controller  36  to adjust the speed of the engine (e.g., increase or decrease) to accommodate a change in the load on the refrigeration unit  20 . The speed request offset may have a fixed amplitude and duration stored in controller  34 . Alternatively, controller  34  may compute the amplitude and duration of the speed offset in response to sensed parameters of the system and/or load on the system. At  110 , controller  34  provides the speed request offset to engine controller  36 . 
         [0030]      FIG. 5  is a plot of engine speed  122 , system load  120  and speed request  124  in an exemplary embodiment. As shown in  FIG. 5 , the engine speed  122  and the speed request  124  are substantially equivalent when the load is at a steady state. At a time 2, an increase in load is anticipated, and the speed request  124  is increased by a speed request offset. This causes the engine speed  122  to increase briefly, until the load  120  begins increasing at time 3. As the load increases and stabilizes at a new level, the speed of the engine varies, but not dramatically due to the speed request offset. The speed request offset drops to zero after predetermined period of time, after which the speed request  124  and engine speed  122  are substantially equivalent. 
         [0031]    The use of a feed forward speed request offset reduces variation in engine speed, creates a stable control scheme and prevents the engine speed from over speeding, drooping, or even engine stalling. Although  FIG. 5  depicts an increase in engine speed in response to load change, it is understood that the speed request offset may reduce engine speed to prevent overdriving of engine  32 . 
         [0032]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.