Patent Description:
Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a "bowed rotor" condition. If a gas turbine engine is in such a "bowed rotor" condition it is undesirable to restart or start the engine.

Accordingly, it is desirable to provide a method and/or apparatus for mitigating a "bowed rotor" condition. Known methods for starting a turbine engine are disclosed in <CIT> and <CIT>.

According to a first aspect of the invention, there is provided a bowed rotor start mitigation system comprising: a gas turbine engine comprising an air turbine starter; a variable position starter valve fluidly connected to the air turbine starter; and a controller configured to dynamically adjust the variable position starter valve to deliver a starter air supply to the air turbine starter to drive rotation of a starting spool of the gas turbine engine according to a dry motoring profile that continuously increases a rotor speed of the starting spool to approach a critical rotor speed gradually while bowed rotor start mitigation is active, wherein the critical rotor speed refers to a major resonance speed of the starting spool, wherein the controller monitors the rotor speed and dynamically adjusts a valve angle of the variable position starter valve to maintain the rotor speed along a target rotor speed profile defined in the dry motoring profile that drives the rotor speed to a set value and accelerates the rotor speed through the critical rotor speed; and wherein the target rotor speed profile in the dry motoring profile maintains a positive slope while bowed rotor start mitigation is active.

Optionally, the target rotor speed profile extends above the critical rotor speed and the controller dynamically adjusts the valve angle of the variable position starter valve to maintain the rotor speed along the target rotor speed profile across the critical rotor speed up to an engine idle speed.

Optionally, a slope of the target rotor speed profile in the dry motoring profile is adjusted and maintains a positive slope while bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine is less than a targeted maximum range.

Optionally, the variable position starter valve has a response rate of <NUM>% to <NUM>% open in less than <NUM> seconds.

Optionally, the dry motoring profile is determined based on reading data from one or more temperature sensors of the gas turbine engine.

Optionally, the gas turbine engine is a turbofan and the starter air supply is provided from an auxiliary power unit.

Optionally, based on determining that bowed rotor start mitigation is complete, the controller is operable to monitor a vibration level of the gas turbine engine while sweeping through a range of rotor speeds including the critical rotor speed and determine whether the bowed rotor start mitigation was successful prior to starting the gas turbine engine.

According to a second aspect of the present invention, there is provided a method of bowed rotor start mitigation for a gas turbine engine, the method comprising: receiving, by a controller, a speed input indicative of a rotor speed of a starting spool of the gas turbine engine; receiving, by the controller, a valve angle feedback from a variable position starter valve of the gas turbine engine; and controlling the variable position starter valve to continuously increase the rotor speed of the starting spool to approach a critical rotor speed gradually through a starter according to a dry motoring profile based on the rotor speed and the valve angle feedback while bowed rotor start mitigation is active, wherein the critical rotor speed refers to a major resonance speed of the starting spool, wherein the rotor speed is controlled to track to a target rotor speed profile of the dry motoring profile that drives the rotor speed to a set value and accelerates the rotor speed through the critical rotor speed, and wherein the target rotor speed profile in the dry motoring profile maintains a positive slope while bowed rotor start mitigation is active.

Optionally, embodiments may include monitoring a rate of change of the rotor speed, projecting whether the rotor speed will align with the target rotor speed profile at a future time based on the rate of change of the rotor speed, and adjusting a valve angle of the variable position starter valve based on determining that the rotor speed will not align with the target rotor speed profile at a future time.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include adjusting a slope of the target rotor speed profile in the dry motoring profile while the bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine is outside of an expected range.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include determining the dry motoring profile based on reading data from one or more temperature sensors of the gas turbine engine prior to driving rotation of the gas turbine engine while the bowed rotor start mitigation is active.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, in further embodiments the variable position starter valve is initially set to a valve angle of greater than <NUM>% open when the bowed rotor start mitigation is active.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include, based on determining that the bowed rotor start mitigation is complete, monitoring a vibration level of the gas turbine engine while sweeping through a range of rotor speeds including the critical rotor speed, and determining whether the bowed rotor start mitigation was successful prior to starting the gas turbine engine.

A technical effect of the apparatus, systems and methods is achieved by using a start sequence for a gas turbine engine as described herein.

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification.

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include using a variable position starter valve to precisely control a rotor speed of a starting spool of the gas turbine engine to mitigate a bowed rotor condition using a dry motoring process. During dry motoring, the variable position starter valve can be actively adjusted to deliver air pressure from an air supply to an engine starting system that accurately controls starting rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the variable position starter valve to follow a dry motoring profile. Embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually as thermal distortion is decreased and accelerate beyond the critical rotor speed to complete the engine starting process. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

A dry motoring profile for dry motoring can be selected based on various parameters, such as a measured temperature value of the gas turbine engine used to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The measured temperature value alone or in combination with other values can be used to calculate a bowed rotor risk parameter. For example, the measured temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring consistent with the dry motoring profile. In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed. As one example, if excessive vibration is detected as the rotor speed rises and approaches but remains well below the critical rotor speed, then the rate of rotor speed increases scheduled in the dry motoring profile can be reduced (i.e., a shallower slope) to extend the dry motoring time. Similarly, if vibration levels are observed below an expected minimum vibration level as the rotor speed increases, the dry motoring profile can be adjusted to a higher rate of rotor speed increases to reduce the dry motoring time.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequence can also or separately be used to assess the risk that a bowed rotor start has occurred due to some system malfunction and then direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor. Vibration data for the engine can also be monitored after bowed rotor mitigation is performed during an engine start sequence to confirm success of bowed rotor mitigation. If bowed rotor mitigation is unsuccessful or determined to be incomplete by the FADEC, resulting metrics (e.g., time, date, global positioning satellite (GPS) coordinates, vibration level vs. time, etc.) of the attempted bowed rotor mitigation can be recorded and/or transmitted to direct maintenance.

Referring now to <FIG>, a schematic illustration of a gas turbine engine <NUM> is provided. The gas turbine engine <NUM> has among other components a fan through which ambient air is propelled into the engine housing, a compressor for pressurizing the air received from the fan and a combustor wherein the compressed air is mixed with fuel and ignited for generating combustion gases. The gas turbine engine <NUM> further comprises a turbine section for extracting energy from the combustion gases. Fuel is injected into the combustor of the gas turbine engine <NUM> for mixing with the compressed air from the compressor and ignition of the resultant mixture. The fan, compressor, combustor, and turbine are typically all concentric about a central longitudinal axis of the gas turbine engine <NUM>. Thus, thermal deflection of the components of the gas turbine engine <NUM> may create the aforementioned bowing or "bowed rotor" condition along the common central longitudinal axis of the gas turbine engine <NUM> and thus it is desirable to clear or remove the bowed condition prior to the starting or restarting of the gas turbine engine <NUM>.

<FIG> schematically illustrates a gas turbine engine <NUM> that can be used to power an aircraft, for example. The gas turbine engine <NUM> is disclosed herein as a multi-spool turbofan that generally incorporates a fan section <NUM>, a compressor section <NUM>, a combustor section <NUM> and a turbine section <NUM>. The fan section <NUM> drives air along a bypass flowpath while the compressor section <NUM> drives air along a core flowpath for compression and communication into the combustor section <NUM> then expansion through the turbine section <NUM>. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment with two turbines and is sometimes referred to as a two spool engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. In both of these architectures the starting spool is that spool that is located around the combustor, meaning the compressor part of the starting spool is flowing directly into the combustor and the combustor flows directly into the turbine section.

The engine <NUM> generally includes a low speed spool <NUM> and a high speed spool <NUM> mounted for rotation about an engine central longitudinal axis A relative to an engine static structure <NUM> via several bearing systems <NUM>. It should be understood that various bearing systems <NUM> at various locations may alternatively or additionally be provided.

The inner shaft <NUM> is connected to the fan <NUM> through a geared architecture <NUM> to drive the fan <NUM> at a lower speed than the low speed spool <NUM> in the example of <FIG>. The high speed spool <NUM> is also referred to as a starting spool, as an engine starting system drives rotation of the high speed spool <NUM>. A combustor <NUM> is arranged between the high pressure compressor <NUM> and the high pressure turbine <NUM>.

The mid-turbine frame <NUM> includes airfoils <NUM> which are in the core airflow path.

A number of stations for temperature and pressure measurement! computation are defined with respect to the gas turbine engine <NUM> according to conventional nomenclature. Station <NUM> is at an inlet of low pressure compressor <NUM> having a temperature T2 and a pressure P2. Station <NUM> is at an exit of the low pressure compressor <NUM> having a temperature T2. <NUM> and a pressure P2. Station <NUM> is at an inlet of the combustor <NUM> having a temperature T3 and a pressure P3. Station <NUM> is at an exit of the combustor <NUM> having a temperature T4 and a pressure P4. Station <NUM> is at an exit of the high pressure turbine <NUM> having a temperature T4. <NUM> and a pressure P4. Station <NUM> is at an exit of the low pressure turbine <NUM> having a temperature T5 and a pressure P5. Measured temperatures in embodiments may be acquired at one or more stations <NUM>-<NUM>. For example, temperature T3 at station <NUM> can be used to as an engine rotor temperature measurement when there is no engine rotation. Alternatively, if available, temperature values at stations <NUM> (T4), <NUM> (T4. <NUM>), and/or <NUM> (T5) can be used as an engine rotor temperature. Temperature measurements can be normalized to account for hot day/cold day differences. For instance, temperature T2 can be used as an ambient temperature and a measured temperature (e.g., T3) can be normalized by subtracting temperature T2.

Although <FIG> depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations. For example, embodiments may be implemented in a configuration that is described as a "straddle-mounted" spool 32A of <FIG>. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in <FIG>. In over-hung mounted spool 32B, a bearing compartment 39B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as <NUM>% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from <NUM> to <NUM>. <FIG> also illustrate a starter <NUM> interfacing via a tower shaft <NUM> with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft <NUM> with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system.

Turning now to <FIG>, a schematic of a starting system <NUM> for the gas turbine engine <NUM> of <FIG> is depicted according to an embodiment. The starting system <NUM> is also referred to generally as a gas turbine engine system. In the example of <FIG>, the starting system <NUM> includes a controller <NUM> which may be an electronic engine control, such as a dual-channel FADEC, and/or engine health monitoring unit. In an embodiment, the controller <NUM> may include memory to store instructions that are executed by one or more processors. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the engine <NUM> of <FIG>. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

The starting system <NUM> can also include a data storage unit (DSU) <NUM> that retains data between shutdowns of the gas turbine engine <NUM> of <FIG>. The DSU <NUM> includes non-volatile memory and retains data between cycling of power to the controller <NUM> and DSU <NUM>. A communication link <NUM> can include an aircraft and/or test stand communication bus to interface with aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand.

A motoring system <NUM> is operable to drive rotation of a starting spool (e.g., high speed spool <NUM>) of the gas turbine engine <NUM> of <FIG>. Either or both channels of controller <NUM> can output a valve control signal <NUM> operable to dynamically adjust a valve angle of a variable position starter valve <NUM> that selectively allows a portion of an air supply <NUM> to pass through the variable position starter valve <NUM> and a transfer duct <NUM> to an air turbine starter <NUM> (also referred to as starter <NUM>). The air supply <NUM> (also referred to as starter air supply <NUM>) can be provided by any known source of compressed air, such as an auxiliary power unit or ground cart. The variable position starter valve <NUM> is a continuous/infinitely adjustable valve that can hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the variable position starter valve <NUM> can be selected to meet dynamic response requirements of the starting system <NUM>. For example, in some embodiments, the variable position starter valve <NUM> has a response rate of <NUM>% to <NUM>% open in less than <NUM> seconds. In other embodiments, the variable position starter valve <NUM> has a response rate of <NUM>% to <NUM>% open in less than <NUM> seconds. In further embodiments, the variable position starter valve <NUM> has a response rate of <NUM>% to <NUM>% open in less than <NUM> seconds.

The controller <NUM> can monitor a valve angle of the variable position starter valve <NUM> using valve angle feedback signals <NUM> provided to both channels of controller <NUM>. As one example, in an active/standby configuration, both channels of the controller <NUM> can use the valve angle feedback signals <NUM> to track a current valve angle, while only one channel designated as an active channel outputs valve control signal <NUM>. Upon a failure of the active channel, the standby channel of controller <NUM> can take over as the active channel to output valve control signal <NUM>. In an alternate embodiment, both channels of controller <NUM> output all or a portion of a valve angle command simultaneously on the valve control signals <NUM>. The controller <NUM> can also monitor a speed sensor, such as speed pickup <NUM> that may monitor the speed of the engine rotor through its connection to the accessory gearbox which is in turn connected to the high spool via tower shaft <NUM> (e.g., rotational speed of high speed spool <NUM>) or any other such sensor for detecting or determining the speed of the gas turbine engine <NUM> of <FIG>. The starter <NUM> may be coupled to a gearbox <NUM> of the gas turbine engine <NUM> of <FIG> directly or through a transmission such as a clutch system (not depicted). The controller <NUM> can establish an outer control loop with respect to rotor speed and an inner control loop with respect to the valve angle of the variable position starter valve <NUM>.

In the example of <FIG>, the engine also includes a vibration monitoring system <NUM>. The vibration monitoring system <NUM> includes at least one vibration pickup <NUM>, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine <NUM> of <FIG>. Vibration signal processing <NUM> can be performed locally with respect to the vibration pickup <NUM>, within the controller <NUM>, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data <NUM>. Alternatively, the vibration monitoring system <NUM> can be omitted in some embodiments.

One or more temperature sensors <NUM>, such as thermocouples, can provide measured temperatures at associated locations of the gas turbine engine <NUM> to the controller <NUM>. For example, the temperature sensors <NUM> can be located at station <NUM> (T2), station <NUM> (T2. <NUM>), station <NUM> (T3), station <NUM> (T4), station <NUM> (T4. <NUM>), and/or station <NUM> (T5) as previously described with respect to <FIG>.

<FIG> is a block diagram of a system <NUM> for bowed rotor start mitigation using the variable position starter valve <NUM> of <FIG> in accordance with an embodiment. The system <NUM> may also be referred to as a bowed rotor start mitigation system. In the example of <FIG>, the system <NUM> includes sensor processing <NUM> operable to acquire and condition data from a variety of sensors such as the valve angle feedback <NUM>, speed pickup <NUM>, vibration pickup <NUM>, and temperature sensors <NUM> of <FIG>. In the example of <FIG>, sensor processing <NUM> provides compressor exit temperature T3 and ambient temperature T2 to core temperature processing <NUM>. Alternatively or additionally, one or more temperature values from stations <NUM> (T4), <NUM> (T4. <NUM>), and/or <NUM> (T5) can be provided to core temperature processing <NUM>. Sensor processing <NUM> provides a valve angle <NUM> to a motoring control <NUM> based on the valve angle feedback <NUM>. Sensor processing <NUM> can provide rotor speed N2 (i.e., speed of high speed spool <NUM>) to the motoring controller <NUM> and a mitigation monitor <NUM>. Sensor processing <NUM> may also provide vibration data <NUM> to mitigation monitor <NUM>. The sensor processing <NUM>, core temperature processing <NUM>, motoring controller <NUM>, and/or mitigation monitor <NUM> may be part of controller <NUM>.

In some embodiments, a heat state of the engine <NUM> or Tcore is determined by the core temperature processing <NUM>. When the gas turbine engine <NUM> has stopped rotating (e.g., rotor speed N2 is zero), the compressor exit temperature T3 may be substantially equal to Tcore. In some embodiments, Tcore is set equal to T3 - T2 to adjust the temperature with respect to the measured ambient temperature of the gas turbine engine <NUM>. Further, temperature readings from other stations of the gas turbine engine <NUM> can be used to determine Tcore. Communication link <NUM> can provide the core temperature processing <NUM> with an indication <NUM> that a start sequence of the gas turbine engine <NUM> has been initiated. Once rotation of the gas turbine engine <NUM> begins, temperature readings can be collected for a predetermined period of time, such as ten to thirty seconds. The temperature readings, e.g., T3 or T3 - T2, can be averaged as core temperature Tcore before the temperature starts to change due to air flow from engine rotation. The core temperature processing <NUM> can determine a bowed rotor risk parameter that is based on the measured temperature using a mapping function or lookup table. The bowed rotor risk parameter can have an associated dry motoring profile <NUM> defining a target rotor speed profile over an anticipated amount of time for the motoring controller <NUM> to send control signals <NUM>, such as valve control signals <NUM> for controlling variable position starter valve <NUM> of <FIG>. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting the gas turbine engine <NUM> of <FIG>.

The bowed rotor risk parameter may be quantified according to a profile curve <NUM> selected from a family of curves <NUM> that align with observed aircraft/engine conditions that impact turbine bore temperature and the resulting bowed rotor risk as depicted in the example graph <NUM> of <FIG>. For instance, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting the gas turbine engine <NUM> of <FIG>. An anticipated amount of dry motoring time can be used to determine a target rotor speed profile in the dry motoring profile <NUM> for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions. An example of a target rotor speed profile <NUM> is depicted in graph <NUM> of <FIG> that includes a steep initial transition portion <NUM>, followed by a gradually increasing portion <NUM>, and a late acceleration portion <NUM> that increases rotor speed above a critical rotor speed, through a fuel-on speed and an engine idle speed. The target rotor speed profile <NUM> can be rescaled with respect to time and/or select portions (e.g., portions <NUM>, <NUM>, <NUM>) of the target rotor speed profile <NUM> can be individually or collectively rescaled (e.g., slope changes) with respect to time to extend or reduce the total motoring time. The target rotor speed profile <NUM> includes all positive slope values such that the actual rotor speed <NUM> is driven to essentially increase continuously while bowed rotor start mitigation is active. While the example of <FIG> depicts one example of the target rotor speed profile <NUM> that can be defined in the dry motoring profile <NUM> of <FIG>, it will be understood that many variations are possible in embodiments.

With continued reference to <FIG>, as used herein, motoring of the engine <NUM> in a modified start sequence refers to the turning of a starting spool by the starter <NUM> at a reduced speed without introduction of fuel and an ignition source in order to cool the engine <NUM> to a point wherein a normal start sequence can be implemented without starting the engine <NUM> in a bowed rotor state. In other words, cool or ambient air is drawn into the engine <NUM> while motoring the engine <NUM> at a reduced speed in order to clear the "bowed rotor" condition, which is referred to as a dry motoring mode.

The motoring controller <NUM> can use a dynamic control calculation in order to determine a required valve position of the variable position starter valve <NUM> used to flow an air supply or starter air supply <NUM> to the engine <NUM> in order to limit the motoring speed of the engine <NUM> due to the position (i.e., valve angle <NUM>) of the variable position starter valve <NUM>. The required valve position of the variable position starter valve <NUM> is determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, compressor horsepower requirements, parasitic drag on the engine <NUM> from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller <NUM> closes the loop for a motoring band of speeds according to the dry motoring profile <NUM>. Dynamic adjustments of the valve angle <NUM> are made to maintain the rotor speed (e.g., N2 or actual rotor speed <NUM>) along the target rotor speed profile <NUM> defined in the dry motoring profile <NUM>.

The example of <FIG> illustrates how a valve angle command <NUM> is adjusted between <NUM> to <NUM>% of a commanded starter valve opening to generate the actual rotor speed <NUM>. As the actual rotor speed <NUM> tracks to the steep initial transition portion <NUM> of the target rotor speed profile <NUM>, the valve angle command <NUM> transitions through points "a" and "b" to fully open the variable position starter valve <NUM>. As the slope of the target rotor speed profile <NUM> is reduced in the gradually increasing portion <NUM>, the valve angle command <NUM> is reduced between points "b" and "c" to prevent the actual rotor speed <NUM> from overshooting the target rotor speed profile <NUM>. In some embodiments, decisions to increase or decrease the commanded starter valve opening is based on monitoring a rate of change of the actual rotor speed <NUM> and projecting whether the actual rotor speed <NUM> will align with the target rotor speed profile <NUM> at a future time. If it is determined that the actual rotor speed <NUM> will not align with the target rotor speed profile <NUM> at a future time, then the valve angle of the variable position starter valve <NUM> is adjusted (e.g., increase or decrease the valve angle command <NUM>) at a corresponding time. In the example of <FIG>, the valve angle command <NUM> oscillates with a gradually reduced amplitude between points "c", "d", and "e" as the actual rotor speed <NUM> tracks to the target rotor speed profile <NUM> through the gradually increasing portion <NUM>. As dry motoring continues, the overall homogenization of the engine <NUM> increases, which allows the actual rotor speed <NUM> to safely approach the critical rotor speed without risking damage. The valve angle command transitions from point "e" to point "f" and beyond to further increase the actual rotor speed <NUM> in the late acceleration portion <NUM> above the critical rotor speed, through a fuel-on speed and an engine idle speed. By continuously increasing the actual rotor speed <NUM> during dry motoring, the bowed rotor condition can be reduced faster than holding a constant slower speed.

In reference to <FIG>, the mitigation monitor <NUM> can operate in response to receiving a complete indicator <NUM> to run a verification of the bowed rotor mitigation. The mitigation monitor <NUM> can provide mitigation results <NUM> to the motoring controller <NUM> and may provide result metrics <NUM> to other systems, such a maintenance request or indicator. Peak vibrations can be checked by the mitigation monitor <NUM> during the start processes to confirm that bowed rotor mitigation successfully removed the bowed rotor condition. The mitigation monitor <NUM> may also run while dry motoring is active to determine whether adjustments to the dry motoring profile <NUM> are needed. For example, if a greater amount of vibration is detected than was expected, the mitigation monitor <NUM> can request that the motoring controller <NUM> reduce a slope of the target rotor speed profile <NUM> of <FIG> to extend the dry motoring time before driving the actual rotor speed <NUM> of <FIG> up to the critical rotor speed. Similarly, if the magnitude of vibration observed by the mitigation monitor <NUM> is less than expected, the mitigation monitor <NUM> can request that the motoring controller <NUM> increase a slope of the target rotor speed profile <NUM> of <FIG> to reduce the dry motoring time before driving the actual rotor speed <NUM> of <FIG> up to the critical rotor speed.

<FIG> is a flow chart illustrating a method <NUM> of bowed rotor start mitigation using the variable position starter valve <NUM> of the gas turbine engine <NUM> in accordance with an embodiment. The method <NUM> of <FIG> is described in reference to <FIG> and may be performed with an alternate order and include additional steps. Before initiating bowed rotor start mitigation, a bowed rotor determination step can be performed to estimate a need for bowed rotor start mitigation. Examples include the use of models and/or stored/observed engine/aircraft state data, such as data received from DSU <NUM>, communication link <NUM>, and/or reading data from one or more temperature sensors <NUM> of the gas turbine engine <NUM>. For instance, a measured temperature can be determined based on reading one or more temperature sensors <NUM> of the gas turbine engine <NUM> for a predetermined period of time when a start sequence of the gas turbine engine is initiated (e.g., based on indication <NUM>). The measured temperature may be adjusted with respect to a measured ambient temperature of the gas turbine engine <NUM> (e.g., T3-T2, T4-T2, etc.).

At block <NUM>, controller <NUM> receives a speed input indicative of a rotor speed (e.g., N2/actual rotor speed <NUM>) of a starting spool of the gas turbine engine <NUM>. The speed input may be directly or indirectly indicative of the rotational speed of high speed spool <NUM>, for instance, derived from a rotational speed of gearbox <NUM>, from speed pickup <NUM>, or another source (not depicted).

At block <NUM>, the controller <NUM> receives valve angle feedback <NUM> from variable position starter valve <NUM> of the gas turbine engine <NUM>. Valve angle feedback <NUM> can be converted by sensor processing <NUM> into any desired engineering units (e.g., per cent open) as valve angle <NUM> for use by motoring controller <NUM>.

At block <NUM>, the controller <NUM> dynamically adjusts the variable position starter valve <NUM> (e.g., using valve control <NUM>/control signals <NUM>) to vary a rotor speed of the starting spool up to a critical rotor speed through starter <NUM> according to dry motoring profile <NUM> based on the rotor speed and the valve angle feedback <NUM> while bowed rotor start mitigation is active. The dry motoring profile <NUM> defines the target rotor speed profile <NUM> as continuously increasing (with varying slopes) to drive the actual rotor speed <NUM> to continuously increase while bowed rotor start mitigation is active.

The actual rotor speed <NUM> is controlled to track to the target rotor speed profile <NUM> of the dry motoring profile <NUM> that drives the actual rotor speed <NUM> to a set value and accelerates the actual rotor speed <NUM> through the critical rotor speed. As one example, the variable position starter valve <NUM> can be initially set to a valve angle of greater than <NUM>% open when bowed rotor start mitigation is active. The controller <NUM> can monitor a rate of change of the actual rotor speed <NUM>, project whether the actual rotor speed <NUM> will align with the target rotor speed profile <NUM> at a future time based on the rate of change of the actual rotor speed <NUM>, and adjust a valve angle of the variable position starter valve <NUM> based on determining that the actual rotor speed <NUM> will not align with the target rotor speed profile <NUM> at a future time.

Further dynamic updates at runtime can include adjusting a slope of the target rotor speed profile <NUM> in the dry motoring profile <NUM> while the bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine <NUM> is outside of an expected range. Adjusting the slope of the target rotor speed profile <NUM> includes maintaining a positive slope. Vibration levels may also or alternatively be used to check/confirm successful completion of bowed rotor start mitigation prior to starting the gas turbine engine <NUM>. For instance, based on determining that the bowed rotor start mitigation is complete, a vibration level of the gas turbine engine <NUM> can be monitored while sweeping through a range of rotor speeds including the critical rotor speed.

The mitigation monitor <NUM> may receive a complete indicator <NUM> from the motoring controller <NUM> when the motoring controller <NUM> has completed dry motoring, for instance, if the motoring time <NUM> has elapsed. If the mitigation monitor <NUM> determines that the bowed rotor condition still exists based on vibration data <NUM> collected, the motoring controller <NUM> may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics <NUM> for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU <NUM> based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete.

Referring now to <FIG>, a graph <NUM> illustrating examples of various vibration level profiles <NUM> of an engine, such as gas turbine engine <NUM> of <FIG> is depicted. The vibration level profiles <NUM> represent a variety of possible vibration levels observed before and/or after performing bowed rotor mitigation. Critical rotor speed <NUM> is the speed at which a vibration peak is expected due to amplification effects of a bowed rotor condition along with other contributions to vibration level generally. A peak vibration <NUM> at critical rotor speed <NUM> may be used to trigger different events. For example, if the peak vibration <NUM> at critical rotor speed <NUM> is below a maintenance action threshold <NUM>, then no further actions may be needed. If the peak vibration <NUM> at critical rotor speed <NUM> is above a damage risk threshold <NUM>, then an urgent maintenance action may be requested such as an engine check. If the peak vibration <NUM> at critical rotor speed <NUM> is between the maintenance action threshold <NUM> and the damage risk threshold <NUM>, then further bowed rotor mitigation actions may be requested, such as extending/restarting dry motoring. In one embodiment, a maintenance request is triggered based on the actual vibration level exceeding maintenance action threshold <NUM> after completing an attempt of bowed rotor mitigation.

The lowest rotor vibration vs. speed in <FIG> (vibration profile 502D) is for a fully homogenized rotor, where mitigation is not necessary (engine parked all night long, for example). The next higher curve shows a mildly bowed rotor and so on. The maintenance action threshold <NUM> is a threshold for setting a maintenance flag such as requiring a troubleshooting routine of one or more system elements. The damage risk threshold <NUM> may be a threshold to trigger a more urgent maintenance requirement up to and including an engine check. As dry motoring is performed in embodiments, the gas turbine engine <NUM> may shift between vibration profiles. For instance, when a bow rotor condition is present, the gas turbine engine <NUM> may experience vibration levels according to vibration profile 502A, if mitigation is not performed. As dry motoring is run, the gas turbine engine <NUM> may have a vibration profile that is gradually reduced from vibration profile 502A to vibration profile 502B and then vibration profile 502C, for example. By checking the current vibration level at a corresponding rotor speed with respect to time, the controller <NUM> can determine whether adjustments are needed to extend or reduce the slope of the target rotor speed profile <NUM> of <FIG> depending on an expected rate of bowed rotor reduction. In embodiments, a slope of the target rotor speed profile <NUM> in the dry motoring profile <NUM> can be adjusted and maintains a positive slope while bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine <NUM> is less than a targeted maximum range <NUM>, which may define a safe level of vibration to ensure that no risk of a maintenance action or damage will likely occur if the actual rotor speed <NUM> is increased faster than previously planned.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a "bowed rotor" condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the controller <NUM> may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the controller <NUM> and/or DSU <NUM> comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the controller <NUM> and/or DSU <NUM> may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the controller <NUM> and/or DSU <NUM> may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

Claim 1:
A bowed rotor start mitigation system comprising:
a gas turbine engine (<NUM>) comprising an air turbine starter (<NUM>);
a variable position starter valve (<NUM>) fluidly connected to the air turbine starter (<NUM>); and
a controller (<NUM>) configured to dynamically adjust the variable position starter valve (<NUM>) to deliver a starter air supply (<NUM>) to the air turbine starter (<NUM>) to drive rotation of a starting spool (<NUM>) of the gas turbine engine (<NUM>) according to a dry motoring profile that continuously increases a rotor speed of the starting spool (<NUM>) to approach a critical rotor speed gradually while bowed rotor start mitigation is active, wherein the critical rotor speed refers to a major resonance speed of the starting spool (<NUM>), wherein the controller (<NUM>) monitors the rotor speed (<NUM>) and dynamically adjusts a valve angle (<NUM>) of the variable position starter valve (<NUM>) to maintain the rotor speed (<NUM>) along a target rotor speed profile (<NUM>) defined in the dry motoring profile that drives the rotor speed to a set value and accelerates the rotor speed through the critical rotor speed; and wherein the target rotor speed profile (<NUM>) in the dry motoring profile maintains a positive slope while bowed rotor start mitigation is active.