Patent Publication Number: US-2019195133-A1

Title: Method and system for turbine engine temperature regulation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/532,829, filed Nov. 4, 2014, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This description relates to turbine engine controls, and, more particularly, to a method and system for turbine engine starting with inter-turbine temperature (ITT) gradient regulation. 
     At least some known turbine engine systems monitor temperature signals, such as, but, not limited to, an inter-turbine temperature (ITT) signal, which is supplied from an inter-turbine temperature sensor positioned between the high pressure turbine and the low pressure turbine in the turbine engine and a temperature signal from an exhaust gas temperature (EGT) sensor positioned at the outlet of the low pressure turbine. During a startup or during load changes on the turbine engine, the ITT gradient typically changes at a rate determined by many factors relating to the combustion process parameters and the physics of the particular configuration of the physical components in and adjacent to the gas path through the high pressure turbine and the low pressure turbine. Known turbine engines do not intervene or control ITT gradients during a starting sequence for the turbine engine, but may regulate to a predetermined maximum allowable temperature. Rapid changes in the ITT gradient can add stress to the turbine components due to repeated thermal shock. 
     Some small turbine/turboprop engines are known to use electronic intervention, which does monitor the rate of change of ITT and utilizes a comparator to determine an exceedance with respect to rate of change of ITT. Based on the exceedance, a binary activated fuel valve is commanded to deliver fixed, binary reductions in fuel flow when the threshold is exceeded. These fixed reductions, applied abruptly, tend to cause sharp changes in the ITT temperature and stall the acceleration of the engine to ground idle speed. These fuel flow interventions typically occur several times within the first few seconds of engine light-off until the ITT gradient reaches equilibrium below a predetermined exceedance threshold. Excessive magnitude, rapid changes and/or oscillations in ITT temperature can thermal shock the turbine components, eventually leading to limited life and potential damage. 
     BRIEF DESCRIPTION 
     In one aspect, the disclosure relates to a turbine engine temperature management system includes a turbine engine having a first turbine and a second turbine, a temperature sensor positioned between the first turbine and the second turbine and configured to sense a turbine engine temperature between the first turbine and the second turbine, a modulating fuel flow valve configured to control a fuel flow to the turbine engine, and a temperature controller. The temperature controls is configured to limit a rate of change of the fuel flow to the turbine engine to less than a predetermined maximum rate of change of the fuel flow that will reduce a rate of change of the temperature and maintain a positive rate of change of a rotational speed of the turbine engine through a turbine engine starting sequence, and limit a rate of the fuel flow to greater than a predetermined minimum rate of the fuel flow that maintains a positive rate of change of temperature and a positive rate of change of the rotational speed of the turbine engine through the turbine engine starting sequence. 
     In another aspect, the disclosure relate to a turbine engine temperature management system including a turbine engine having a high pressure turbine and a low pressure turbine, an inter-turbine temperature sensor positioned between the high pressure turbine and the low pressure turbine and configured to sense a turbine engine temperature between the high pressure turbine and the low pressure turbine, a modulating fuel flow valve configured to control a fuel flow to the turbine engine, and a temperature controller. The temperature controller is configured to limit a rate of change of the fuel flow to the turbine engine during a starting sequence to less than a predetermined maximum rate of change of the fuel flow that will reduce a rate of change of the temperature and maintain a positive rate of change of a rotational speed of the turbine engine until the engine starting sequence reaches an engine idle speed, and limit a rate of the fuel flow during the starting sequence to greater than a predetermined minimum rate of the fuel flow that maintains a positive rate of change of temperature and a positive rate of change of the rotational speed of the turbine engine until the engine starting sequence reaches an engine idle speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-4  show example aspects of the method and apparatus described herein. 
     
    
    
       FIG. 1  is a schematic block diagram of an inter-turbine temperature (ITT) gradient management system in accordance with an example aspects of the present disclosure. 
       FIG. 2  is a graph of ITT during a representative startup of the turbine engine shown in  FIG. 1  and a graph of fuel flow W f  during the startup without using the ITT gradient management system shown in  FIG. 1 . 
       FIG. 3  is a graph of ITT during a startup of the turbine engine shown in  FIG. 1  and a graph of fuel flow W f  during the startup in accordance with an example aspects of the present disclosure. 
       FIG. 4  is a flow chart of a method of managing inter-turbine temperature in the turbine engine shown in  FIG. 1 . 
     Although specific features of various aspects may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of aspects of the disclosure. These features are believed to be applicable in a wide variety of systems including one or more aspects of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the aspects disclosed herein. 
     DETAILED DESCRIPTION 
     The following detailed description illustrates aspects of the disclosure by way of example and not by way of limitation. It is contemplated that the disclosure has general application to analytical and methodical aspects of modulating fuel flow to a gas turbine engine to control a rate of change of inter-turbine temperature in industrial, commercial, and residential applications. 
     Aspects of the present disclosure describe a method of regulating fuel flow in response to excessive inter-turbine temperature gradient conditions with limited changes in the commanded fuel metering valve flow rate during the engine starting sequence. The control sequence operates such that negative (speed or temperature) transitions are avoided during the scheduled speed increases during the start, utilizing an interaction sequence balancing the time of the fuel intervention and the amount of the fuel inhibited through the fuel metering valve. The implementation is facilitated with circuitry pre-configured for engine fuel metering valve fuel flow and engine starting speed algorithms. Engine mechanical life benefits from reduced thermal stress during engine starts is expected from the elimination of aggressive temperature transitions experienced during typical starting. 
     The method measures the rate change in the engine inter-turbine temperature (ITT) and reduces fuel flow if the rate change exceeds a maximum threshold. The amount of the fuel reduction, the slope at which the fuel flow drops and recovers, and the ultimate duration of the fuel reduction are all constrained by the control system implementation to gently reduce the temperature rate of change, or gradient, without causing a reversal of the temperature (negative gradient) or stalling of the engine (negative change in core engine speed). 
     The method actively manages hot starts of the turbine engine and can ultimately limit the maximum temperatures achieved, preventing exceedances, which could cause immediate or latent damage to the turbo-machinery. The method extends the life of the turbine engine, allowing for longer times between scheduled inspection and repairs, which results in lower overall ownership and maintenance costs. This method&#39;s implementation of ITT gradient management serves to prevent or reduce prolonged operation in the turbine engine&#39;s flat spot, which creates a discontinuity in the engine response and won&#39;t allow it to accelerate as required. The manipulation of the engine fuel flow is contained within reasonable limits such that the acceleration of engine speed is not stifled in an attempt to control the ITT temperature. 
     The following description refers to the accompanying drawings, in which, in the absence of a contrary representation, the same numbers in different drawings represent similar elements. 
       FIG. 1  is a schematic block diagram of an inter-turbine temperature (ITT) gradient management system  100 . In the example aspects, an inter-turbine temperature (ITT) signal  102 , which is supplied from an inter-turbine temperature sensor  104  positioned between a high pressure turbine  106  and a low pressure turbine  108  of, for example, but not limited to, a free turbine engine  110 . Signal  102  is processed using a differentiator or derivative circuit  112  to provide a signal  114  representative of the rate of change of ITT, (ITT-Dot). ITT-Dot signal  114  is input to a comparator  116  where it is compared to a predetermined ITT-Dot threshold value  118 , which sets the maximum rate change of ITT that is acceptable. Comparator  116  generates an ITT-Dot error signal  120  that is multiplied by a gain, k in an amplifier  122 . When ITT-Dot signal  114  exceeds ITT-Dot threshold value  118 , an output  124  of amplifier  122  is negative, which tends to drive fuel flow rate (W f )  126  down. The rate at which fuel flow (W f )  126  is reduced is proportional to the measured exceedances represented by ITT-Dot error signal  120 . A first W f -Dot saturation limiter  128  sets a maximum rate of change for fuel flow, W f    126 , such that the rate of fuel reduction is less than a predetermined maximum, which could result in an undesired response in ITT. The saturation limited W f -Dot signal  130  is then processed by an integrator  132  to produce a desired fuel flow rate signal  134  proportional to the desired flow rate, W f    126 . A magnitude of desired flow rate, W f    126  is bound by a second Delta-W f  saturation limiter  136  (which may be implemented using minimum-maximum selectors against scheduled limits that vary based on the turbine engine core operating speed). A core engine speed detector  137  is used to provide the turbine engine core operating speed. An analog schedule or memory table  139  includes the selections that are made based on the turbine engine core operating speed by a max-min selector  141 . Second Delta-W f  saturation limiter  136  is configured to prevent desired fuel flow rate signal  134  from exceeding predetermined boundaries, which are set appropriate for the engine operating mode. 
     During a start sequence, when ITT gradient management system  100  is designed to operate, a lower threshold  138  provides a maximum reduction in fuel flow, Delta-W f , from a nominal start flow rate. A final output  140  is converted from an electronic signal to an actual fuel flow rate by an engine fuel system  142 . 
       FIG. 2  is a graph  200  of ITT during a representative startup of turbine engine  110  (shown in  FIG. 1 ) and a graph  202  of fuel flow W f  during the startup without using ITT gradient management system  100 . Graph  200  includes an x-axis  204  graduated in units of time (seconds) and a y-axis  206  graduated in units of temperature. A trace  208  illustrates the ITT during the startup. 
     Graph  202  includes an x-axis  210  graduated in units of time (seconds) and a y-axis  212  graduated in units of flow. A trace  214  illustrates the fuel flow W f  to turbine engine  110  during the startup. 
     When initial light-off (t 0 ) occurs, an ITT gradient (ITT-Dot 1 )  216  (i.e., a slope of trace  208 ) at t 0  typically exceeds a desired threshold (i.e. the slope of trace  208  exceeds the threshold). Based on a comparison of ITT-Dot 1   216  to the threshold, a binary activated fuel valve in fuel system  142  is commanded to deliver a fixed, binary reduction  218  in fuel flow when the threshold is exceeded. This fixed reduction  218 , applied abruptly, tends to cause sharp changes in ITT trace  208  and stall the acceleration of the engine to ground idle speed. Similarly, when ITT recovers after fuel flow is restored, an ITT gradient (ITT-Dot 2 )  220  again exceeds the threshold causing the binary activated fuel valve in fuel system  142  to close again in a fixed reduction  222  of fuel flow. ITT again is reduced before the comparator can open the binary activated fuel valve, restoring fuel flow and increasing ITT. These fuel flow reductions  218 ,  222 , and  224  typically occur several times within the first few seconds of engine light-off until the ITT gradient reaches equilibrium below the predetermined exceedance threshold. 
       FIG. 3  is a graph  300  of ITT during a startup of turbine engine  110  (shown in  FIG. 1 ) and a graph  302  of fuel flow W f  during the startup in accordance with an example aspects of the present disclosure. Graph  300  includes an x-axis  304  graduated in units of time (seconds) and a y-axis  306  graduated in units of temperature. A trace  308  illustrates the ITT during the startup. 
     Graph  302  includes an x-axis  310  graduated in units of time (seconds) and a y-axis  312  graduated in units of flow. A trace  314  illustrates the fuel flow W f  to turbine engine  110  during the startup. 
     ITT gradient management system  100 , is one controller element of many that includes an Electronic Engine Control (EEC) (not shown) for turbine engine  110 , which may include a core speed governor, scheduled fuel flow limiters for start flow and overspeed prevention, and other limiting regulators (for torque, ITT magnitude or propeller speed, for example). The EEC determines which of these plurality of regulators drives a output via a min-max selection process. The results of this approach are illustrated in  FIG. 3 . 
     When initial light-off (t 0 ) of turbine engine  110  occurs, an ITT gradient (ITT-Dot 1 )  316  at t 0  typically exceeds a desired threshold (i.e. a slope of trace  308  exceeds the threshold). The EEC, employing this method, makes a calculated reduction in fuel flow rate, W f  bound by first W f -Dot saturation limiter  128  and second Delta-W f  limiter  136 , until the closed-loop feedback confirms an ITT gradient (ITT-Dot 2 )  318  has shifted below the exceedance threshold. 
     Rather than a series of abrupt binary reductions in fuel flow to reduce the ITT gradient during startup, the present method uses a calculated reduction to turn the rate of change of ITT in a controlled manner to provide a smooth transition of ITT from cold iron temperatures to ground idle speed of turbine engine  110 . ITT gradient management system  100  measures the rate change in the engine inter-turbine temperature (ITT) and reduces fuel flow if the rate change exceeds a maximum threshold. The amount of the fuel reduction, the slope at which the fuel flow drops and recovers, and the ultimate duration of the fuel reduction are all constrained by the control system implementation to gently reduce the temperature rate of change, or gradient, without causing a reversal of the temperature (negative gradient) or stalling of the engine (negative change in core engine speed). 
       FIG. 4  is a flow chart of a method  400  of managing inter-turbine temperature in turbine engine  110  (shown in  FIG. 1 ). In the example aspects, method  400  includes receiving  402  a signal representative of an inter-turbine temperature (ITT) of a turbine engine, determining  404  a rate of change of the ITT, comparing  406  the determined rate of change of the ITT to a predetermined rate of change of the ITT threshold, and limiting  408  the resultant of the comparison to less than a predetermined maximum rate of change of a fuel flow to the turbine engine that will reduce the rate of change of the ITT and maintain a positive rate of change of a rotational speed of the turbine engine. Method  400  also includes determining  410  a rate of fuel flow to the turbine engine corresponding to the limited rate of change of the fuel flow, limiting  412  the determined rate of the fuel flow to greater than a predetermined minimum rate of the fuel flow that maintains a positive rate of change of ITT and a positive rate of change of the rotational speed of the turbine engine. Method  400  further includes controlling  414  a fuel flow to the turbine engine based on the limited determined rate of the fuel flow. 
     The above-described aspects of a method and system of regulating fuel flow in response to excessive inter-turbine temperature gradient conditions during the turbine engine starting sequence provides a cost-effective and reliable means for avoiding negative (speed or temperature) transitions during the scheduled speed increases during the turbine engine starting sequence. More specifically, the methods and systems described herein facilitate balancing the time of the fuel intervention and the amount of the fuel inhibited through the fuel metering valve. In addition, the above-described methods and systems facilitate measuring the rate change in the engine inter-turbine temperature (ITT) and reducing fuel flow if the rate change exceeds a maximum threshold. The amount of the fuel reduction, the slope at which the fuel flow drops and recovers, and the ultimate duration of the fuel reduction are all constrained by the control system implementation to gently reduce the temperature rate of change, or gradient, without causing a reversal of the temperature (negative gradient) or stalling of the engine (negative change in core engine speed). As a result, the methods and systems described herein facilitate actively managing starts of the turbine engine and limiting the maximum temperatures achieved, preventing exceedances, which could cause immediate or latent damage to the turbine engine in a cost-effective and reliable manner. 
     Example methods and apparatus for managing a turbine engine ITT gradient are described above in detail. The apparatus illustrated is not limited to the specific aspects described herein, but rather, components of each may be utilized independently and separately from other components described herein. Each system component can also be used in combination with other system components. 
     This written description uses examples to describe the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.