Abstract:
A method and system for utilization of power converters in an aircraft engine start system includes measurement of power converter operation data that is utilized with a mathematical model of the power converter thermal characteristics to calculate operation limits for subsequent start duty cycles. A warning indicator is utilized in the event the start duty cycle limits are exceeded. This invention can be extended for any More Electric Vehicle applications, which utilizes an electric engine start system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to apparatus and methods of optimally utilizing power converters and, more specifically, to apparatus and methods of optimally utilizing power converters as part of an engine start system. 
     Aircraft engines require a start system to generate the mechanical torque required to bring the engine from a stopped state up to a target speed, at which point the engine is considered to have transitioned to a running state. Modern aircraft, such as More Electric Aircraft, have engine start systems that may include an AC power generator, a power converter, and an input power source. For these modern aircraft, during an engine start mode, the generator is operated as a torque-producing motor that uses power supplied at varying voltage and frequency by the power converter. The power converter is supplied with input power from an input power source. When the engine has transitioned from the stopped to the running state, the power converter is disconnected as an AC power supply. In the running state the engine produces mechanical torque which is transformed to AC power by the generator. 
     Utilization of the power converter as an AC power supply that receives input power can cause the power converter temperature to increase to a level unsafe for continued operation. This increase in temperature necessitates power converter thermal protections which may include operation time limits. Power converters have a rating that specifies limits for their continued operation followed by a minimum wait period. In an existing engine start application for an aircraft, the maximum duration of power converter operation is typically 135 seconds followed by a minimum wait period of 15 minutes. According to these limits, when the power converter has been utilized for the maximum rated duration of 135 seconds, the power converter should not be used for at least the next 15 minutes. The power converter rating serves as a guide to operating the power converter within the power converter operation limits. The power converter operation limits define the onset of thermal damage to the power converter. During engine start mode, the power converter is utilized continuously with input power during each start attempt. Typically current engine start systems do not track the actual operation time of the power converter. The start duty cycle normally expected consists of a single successful start attempt typically having a 40 second duration. The wait period is required so that the temperature of the power converter is reduced. The maximum start duty cycle is defined as the maximum number of consecutive start attempts estimated to occur within the power converter maximum utilization duration followed by the minimum wait period. In an existing engine start application for an aircraft, the maximum number of consecutive start attempts may be three. Thus after three start attempts, the engine start system is required to wait in an idle state for at least 15 minutes before another engine start attempt can be made. 
     Duty cycle abuse is defined as exceeding the maximum number of start attempts within the fixed maximum start duty cycle duration. When there are unsuccessful starts, it can be expected that the engine start system will encounter duty cycle abuse when the operator attempts multiple starts. In an existing engine start application for an aircraft, duty cycle abuse may be defined as four start attempts without pause, which are caused by consecutive unsuccessful starts. Duty cycle abuse avoidance is used in current engine start systems as a way to prevent exceeding the power converter rating. 
     Engine start systems that use a maximum start duty cycle based on a fixed number of start attempts are prone to unnecessarily long wait periods. This arises because the actual power converter utilization during the start duty cycle may be less than the estimated utilization. In addition, preventing duty cycle abuse does not necessarily prevent exceeding the power converter rating. This can arise if the actual power converter utilization during the start duty cycle is more than the estimated utilization. Current engine start system maximum start duty cycles can therefore introduce unnecessarily long wait periods while also allowing the power converter rating to be exceeded. 
     As can be seen, there is a need for a more accurate and efficient power converter utilization method in engine start systems. Additionally there is a need for a power converter utilization method that provides better protection against exceeding the power converter thermal rating. This invention can be extended for any More Electric Vehicle applications, which utilize an electric engine start system. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention a method of utilizing a power converter comprises the steps of measuring operation data of the power converter and using these data in developing a mathematical model of the power converter thermal characteristics. Further operation data of the power converter is used in conjunction with the mathematical model to calculate power converter operation parameters to be used as the maximum start duty cycle parameters. 
     In another aspect of the present invention an engine start system includes an indication of the maximum start cycle duty parameters and a warning indication in the event of duty cycle abuse. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an engine start system according to the present invention; 
         FIG. 2  is a graph of power converter thermal data showing the linear relationship of a next power converter utilization duration versus a wait period following a maximum power converter utilization duration; 
         FIG. 3  is a time event graph of power converter utilization indicating the relationships between a previous power converter utilization duration, a wait period, and a next power converter utilization duration; 
         FIG. 4  is a flow chart illustrating a method for determining the operation limits of a power converter for a subsequent utilization of the power converter according to the present invention; and 
         FIG. 5  is a time event graph showing the variation of a next available power converter operation duration with respect to the previous engine start attempt durations and wait periods. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     The present invention generally provides a method of utilization of a power converter by employing an empirically derived mathematical model of the thermal characteristics of the power converter in conjunction with measured power converter operation data. Other operation data such as ambient temperature, power dissipation, and cooling method may be used in developing the mathematical model. In contrast to the prior art that relies on estimates of the power converter operation data in conjunction with the power converter rating, the present invention produces power converter operation parameters that more closely represent the actual power converter operation limits. As a result, the present invention facilitates shorter wait periods during power converter utilization while more precisely and responsively indicating an occurrence of the power converter rating being exceeded. 
     The present invention provides a system for utilization of power converters within an aircraft engine start system that may enable the determination of a maximum start duty cycle based on accurate power converter operation parameters. Unlike prior art aircraft engine start systems that define a maximum start duty cycle as a fixed number of start attempts followed by a fixed wait period, the present invention provides maximum start duty cycle limits based on power converter operation parameters that are calculated and updated from actual power converter operation data. As a result, the present invention may enable shorter start system wait periods, resulting in optimal utilization of power converters, and better detection of duty cycle abuse, defined as exceeding the maximum start duty cycle limits, than prior art start systems. 
     In more specifically describing the present invention, and as can be appreciated from  FIG. 1 , an embodiment of the present invention provides an input power source  100  electrically connected to a power converter  110 . The input power source  100  and power converter  110  may receive an operation signal  115  that directs the input power source  100  and power converter  110  to apply or remove the application of AC power  120  to the generator  130 . The power converter  110  may receive input power  105  from the input power source  100  and may convert input power  105  to AC power  120 . A generator  130  may receive AC power  120 . An operation duration of the power converter  110  is defined as the time duration of an application of AC power  120  to the generator  130  by the power converter  110 . A wait period of the power converter  110  is defined as the time elapsed since the removal of AC power  120  from the generator  130 . The operation data of the power converter  110  may include an operation duration and a wait period. 
     The generator  130  may operate as a standard electrical generator by producing electrical power from mechanical torque input and may also operate as a motor by producing mechanical torque from AC power  120 . 
     The generator  130  may be mechanically connected to an engine  140 . When the engine  140  is in a running state, the engine  140  may produce mechanical torque  135 . When the generator  130  is receiving AC power  120 , the generator  130  may produce mechanical torque  135  and the mechanical torque  135  may be applied to the engine  140 . The application of mechanical torque  135  to the engine  140  may result in an increase in the operating speed of the engine  140 . The engine  140  may provide a speed signal  150  that indicates the current value of the operating speed of the engine  140 . A system controller  160  may include an engine mode switch  162 , a start duty cycle indicator  164 , and a duty cycle abuse indicator  166 . The engine mode switch  162  may be in one of a plurality of states for controlling the state of the engine  140  including a) OFF for controlling the engine  140  to a stopped state; b) ON for controlling the engine  140  to remain in the running state or the stopped state; and c) START for controlling the engine  140  to transition from the stopped state to the running state. 
     When the engine mode switch  162  is switched to the START state while the engine  140  is in the stopped state, the system controller  160  may send the operation signal  115  to the input power source  100  and power converter  110  indicating to begin sending AC power  120  to the generator  130 . When the speed signal  150  reaches a minimum threshold value while the engine mode switch  162  is in the START state, the engine mode switch  162  can be switched to the ON state. 
     A start attempt duration is defined as the elapsed time from the engine mode switch  162  switching into the START state to switching out of the START state and is equivalent to an operation duration of the power converter  110 . A start attempt wait period is defined as the time elapsed since the engine mode switch  162  was most recently in the START state and is equivalent to a wait period of the power converter  110 . It should be noted that when the engine mode switch  162  is in the START state, by definition the start attempt wait period is zero. 
     The system controller  160  may provide start system operation data  170  to a start duty cycle limits processor  180 . The start system operation data  170  includes, but is not limited to, the most recent start attempt duration and the start attempt wait period. The start duty cycle limits processor  180  may include a mathematical model  185  of the thermal characteristics of the power converter  110 . The mathematical model  185  may be represented as digital data and stored on a machine-readable medium including a hard drive and an optical disk, as well as being processed on a computer. The start duty cycle limits processor  180  may utilize the mathematical model  185  in conjunction with the start system operation data  170  to calculate power converter  110  operation parameters that may be used to determine start duty cycle limits  190 . The start duty cycle limits processor  180  may receive the start system operation data  170  and may determine the start duty cycle limits  190 . The start duty cycle limits  190  may be received by the system controller  160  and may be indicated in the start duty cycle indicator  164 . The start duty cycle limits  190  may be represented as digital data and stored on a machine-readable medium including a hard drive and an optical disk, as well as being processed on a computer. 
     Certain electrical components of the present invention including, but not limited to, the speed signal  150 , the system controller  160 , the operation data  170 , the start duty cycle limits processor  180 , the mathematical model  185 , and the start duty cycle limits  190  may be implemented or represented fully or in various combinations of analog and digital electrical signals and circuitry. 
     The start duty cycle limits  190  define parameters for the operation of the engine mode switch  162  and may be based on the operation parameters of the power converter  110 . The start duty cycle limits  190  parameters may include, but are not limited to, a start attempt wait period until the engine mode switch  162  may be transitioned into the START state and a start attempt duration the engine mode switch  162  may remain in the START state. The duty cycle abuse indicator  166  may indicate operation of the engine mode switch  162  outside the start duty cycle limits  190 . 
     In more specifically describing the present invention, and as can be appreciated from  FIG. 2  and  FIG. 3 , another embodiment of the present invention provides a mathematical model of the thermal characteristics of a power converter  110 .  FIG. 2  shows the relationship between a power converter  110  wait period G and a next power converter  110  operation duration Y w  following a maximum power converter utilization duration. The wait period G is defined as the time elapsed since the termination of the most recent operation duration of the power converter  110 . In this exemplary embodiment of the present invention, an operation duration of the power converter  110  may be the application of AC power  120  to the generator  130 . As can be seen in  FIG. 2 , the next operation duration Y w  increases until the next operation duration Y w  is equal to power converter  110  rating maximum operation duration S. Y w  being equal to S coincides with the wait period G being equal to the power converter  110  rating minimum wait period W. This relationship may be expressed as
 
 Y   w   =G *( S/W ),  (1)
 
where S is in seconds, W is in minutes, G is in minutes, and Y w  is in seconds. Equation (1) is an exemplary embodiment of the present invention describing a linear relationship between Y w  and G. The mathematical relationship of equation (1) may also be expressed in other linear and non-linear equation forms and in associations such as lookup tables.
 
       FIG. 3  shows the relationships between an operation duration X of the power converter  110 , a wait period G, the power converter  110  rating maximum operation duration S, and a remaining operation duration Y r  of the power converter  110 . According to the definition of the power converter  110  rating, at any point in time during an operation duration the remaining operation duration is the difference between the power converter  110  rated maximum operation duration S and the value of the operation duration X. This can be expressed as
 
 Y   r =( S−X ),  (2)
 
where S, X, and Y r  are all in the same time units, typically seconds.
 
     At any point in time during the wait period G the total next operation duration may be expressed as the sum of Y w  and Y r , which, according to equations (1) and (2), may be expressed as
 
 Y   T   =Y   r   +Y   w =[( S−X )+ G *( S/W )],  (3)
 
where Y T  is the next available power converter operation duration in the same time units as Y r  and Y w . Equation (3) is applicable at any point in time during a power converter  110  utilization as shown in  FIG. 3  with the limitations 0≦X≦S and 0≦Y T ≦S. X and G may be included in the power converter  110  operation data. Y T  may be included in the operation parameters of the power converter  110 . Note: If G≧G MIN , then Y T =S, otherwise Y T  is given by Equation (3). G MIN  is the is the minimum wait period required in order for Y T  to reach its maximum value of S.
 
 G   MIN =( S−Y   T )*( W/S )  (4)
 
     Equations (3) and (4) have been used to develop  FIG. 5  which shows the variations of Y T  and G MIN  with respect to multiple engine start attempt durations X and wait periods G. It should be noted that the value of Y T  is calculated based on the previous start attempt durations and wait periods in accordance with equation (3) and G MIN  is calculated in accordance with equation (4). The value of Y T  increases as the wait period increases until the wait period equals the minimum wait period G MIN , at which point Y T  reaches its maximum value of S. 
     In more specifically describing the present invention, and as can be appreciated from  FIG. 4 , another embodiment of the present invention provides a method  400  for optimal utilization of a power converter  110 . A step  410  of acquiring thermal data of a power converter  110  may comprise acquiring empirically obtained data as well as published data. A step  420  of defining a mathematical model  185  of the thermal characteristics of the power converter  110  may include rigorous statistical and other mathematical analysis of the data of step  410 . The mathematical model  185  may be a function of any number of input variables including power converter  110  operation data and power converter  110  ratings. An example of a mathematical model  185  of the thermal characteristics of a power converter  110  is equation (3). The mathematical model  185  may calculate any number of output variables including a next operation duration, a wait period G, or some combination of operation durations and wait periods G. A step  430  may include obtaining sufficient operation data of the power converter  110  for the mathematical model  185  of step  420 . A step  440  may include utilization of the mathematical model  185  of step  420  and the operation data of step  430  in order to calculate operation parameters for subsequent utilization of the power converter  110 . A step  450  may include the utilization of the operation parameters of step  440  in the subsequent utilization of the power converter  110 . 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.