Patent Publication Number: US-2020283138-A1

Title: Method and system for engine windmilling control

Description:
TECHNICAL FIELD 
     The application relates generally to engine control systems, and more specifically to systems for addressing engine windmilling. 
     BACKGROUND OF THE ART 
     Many aircraft have auxiliary power units (APU), typically mounted in a tailcone of an aircraft separate from other aircraft engines, which serve to supply electricity to various aircraft systems and to provide compressed air for the aircraft. The APU has a lubricating system which circulates a lubricating fluid, for example oil, between components of the APU. 
     In certain stages of operation of the aircraft where the APU is non-operational, air circulation in the vicinity of the APU can cause rotatable components of the APU to windmill. This, in turn, can cause lubricating fluid to unnecessarily circulate through the APU, which can affect fire safety, and can cause unnecessary wear and tear to the APU. 
     As such, there is room for improvement. 
     SUMMARY 
     In accordance with a broad aspect, there is provided a method for controlling windmilling in an engine. A determination is made regarding whether the engine is in a windmilling state. When the engine is in the windmilling state, a circuit element is commanded to apply a DC signal to an electric starter motor which is coupled to the engine. The DC signal applied to the electric starter motor is modulated to control a level of rotational motion of the engine. 
     In accordance with another broad aspect, there is provided a system for controlling windmilling in an engine. The system comprises an electric starter motor coupled to the engine, a circuit element coupled to the electric starter engine and to a DC signal source, and a control system coupled to the engine and to the circuit element. The control system is configured for: determining whether the engine is in a windmilling state; when the engine is in a windmilling state, commanding the circuit element to apply a DC signal to the electric starter motor; and modulating the DC signal applied to the electric starter motor to control a level of rotational motion of the engine. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of an example engine, in accordance with one or more illustrative embodiments; 
         FIG. 2  is a block diagram of an example system for controlling engine windmilling, in accordance with one or more illustrative embodiments; 
         FIG. 3  is a flowchart of an example method for controlling engine windmilling, in accordance with one or more illustrative embodiments; and 
         FIG. 4  is block diagram of an example computing device for implementing the method of  FIG. 3 , in accordance with one or more illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is illustrated a gas turbine engine  100 . Note that while engine  100  is a turbofan engine, the methods and systems described herein may be applicable to turboprop, turboshaft, and other types of gas turbine engines, or combustion engines generally. In addition, the engine  100  may be an auxiliary power unit (APU), an auxiliary power supply (APS), a hybrid engine, or any other suitable type of engine. In addition, although the foregoing discussion relates to a singular engine  100 , it should be understood that the techniques described herein can be applied substantially concurrently to multiple engines. 
     The engine  100  generally comprises in serial flow communication: a fan  120  through which ambient air is propelled, a compressor section  140  for pressurizing the air, a combustor  160  in which the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases, and a turbine section  180  for extracting energy from the combustion gases. Axis  110  defines an axial direction of the engine  100 . In some embodiments, a low pressure spool is composed of a low pressure shaft and a low pressure turbine. The low pressure shaft drives the propeller  120 . A high pressure spool is composed of a high pressure turbine attached to a high pressure shaft, which is connected to the compressor section  140 . It should be noted that other configurations for the engine  100  are also considered. 
     Control of the operation of the engine  100  can be effected by one or more control systems, for example an engine controller  210 . The engine controller  210  can modulate a fuel flow rate provided to the engine  100 , the position and/or orientation of variable geometry mechanisms within the engine  100 , a bleed level of the engine  100 , and the like. 
     In the course of normal operation, there may be periods of time during which the engine  100  is inoperative. For instance, if the engine  100  is an APU of an aircraft, there may be certain times during which the operator of the aircraft, or an aircraft-level control system, commands the engine  100  to be shut off or otherwise rendered inoperative. If the engine  100  is shut off during a flight operation, or during high wind conditions, the engine  100  can begin to experience a phenomenon known as windmilling, in which one or more rotatable components of the engine  100  exhibit rotational motion due to airflow from an external source. Windmilling can occur in a variety of circumstances, and due to a variety of factors. In some cases, windmilling occurs during flight because an inlet to the engine  100  is open, causing the engine  100  to be subjected to a flow of air from the inlet. In other cases, windmilling occurs during flight because the engine inlet is permanently open. In still other cases, windmilling occurs when the aircraft is on the ground and exposed to high winds or similar weather conditions. It should be noted that in certain cases, the engine  100  is substantially unpowered when experiencing windmilling. Other causes of windmilling are also considered. 
     With reference to  FIG. 2 , there is shown a system  200  for controlling engine windmilling, for instance the engine  100 . In some embodiments, the system  200  is implemented in the context of an aircraft  105 , and is therefore coupled to an aircraft system  250 . In other embodiments, the system  200  is implemented in different contexts, such as an industrial context, or in the context of another vehicle, and the system  200  can be coupled to different control systems, as appropriate. 
     The system  200  is coupled to the engine  100  and is composed of an engine controller  210 , a starter motor  220 , and a circuit element  222 . The engine controller  210  serves to control the operation of the engine  100 , and can be composed of any suitable hardware and software for controlling operation of the engine  100 . In some embodiments, the engine controller  210  is implemented via a full-authority digital engine controller (FADEC) or similar device. In embodiments in which the engine controller  210  is used in the context of the aircraft  105 , the engine controller  210  is configured for controlling operation of more than one engine  100  substantially concurrently. The engine controller  210  is also configured for logging various information about aircraft usage and operating conditions, including engine speed, engine operating state, and the like, and to log the occurrence of certain events, for example fire events. 
     The engine controller  210  is coupled to the aircraft system  250  for receiving therefrom various instructions, for instance as provided by an operator of the engine  100 . The engine controller  210  is also connected to the engine  100 , the starter motor  220 , the circuit element  222 , and to a fuel module  230 . The engine controller  210  is configured for providing instructions to the engine  100 , the starter motor  220 , the circuit element  222 , and the fuel module  230 , for instance to alter the operation thereof, in accordance with the techniques disclosed herein. It should be noted that although the engine controller  210  is shown here as a single element, in certain embodiments the engine controller  210  can be composed of multiple controllers working collaboratively. For example, the engine controller  210  can be composed of separate devices for controlling the engine  100  and the starter motor  220 . Other examples are also considered. 
     In addition, the engine controller  210  is configured for acquiring various information from the elements to which it is connected, including the engine  100 , the starter motor  220 , the circuit element  222 , the fuel module  230 , and the aircraft system  250 . To this end, the engine controller  210  can be provided with, or be coupled to, a variety of sensors to allow the engine controller  210  to monitor operating conditions of the engine  100  and/or other components to which the engine controller  210  is coupled. For example, the engine  100  can include one or more sensors of various types which collect information about the operating conditions of the engine  100 , and which is provided to the engine controller  210 . In another example, the fuel module  230  can include one or more sensors, such as a fuel flow rate sensor, and can provide information regarding rate of fuel flow toward the engine  100  to the engine controller  210 . Other sources, and types, of information are considered. 
     The starter motor  220  serves to assist with the engine start sequence  100 : when the engine is inoperative, the starter motor  220  can be coupled to the engine  100  to facilitate, or initiate, the ignition process for the engine  100 . The starter motor  220  can be any suitable type of electrical motor, for instance powered by a direct current (DC) source  224 . In addition, the starter motor  220  can be used as a generator in certain circumstances: when the engine  100  is operational, the starter motor  220  can be mechanically coupled to the engine  100  and caused to rotate with the engine  100 . In this fashion, the starter motor  220  can be used to produce electricity, for instance for use within the aircraft  105 . 
     The circuit element  222  serves to manage the electrical coupling between the starter motor  220  and the DC source  224 , and optionally with one or more additional electrical sources and/or systems. For example, the circuit element  222  includes a switch or other similar element which can be moved between an open state and a closed state by the engine controller  210 . In another example, the circuit element  222  includes one or more additional switches for connecting the starter motor  220  to various electrical systems to allow the starter motor  220  to provide the electrical systems with electrical power, for instance when the starter motor  220  is acting as a generator. 
     The system  200  is also connected to the DC source  224  and the fuel module  230 . The DC source  224  can be any suitable source of DC current, including a battery, a capacitor or capacitor-based system, an inverter connected to an alternating current (AC) source, or the like. For example, the DC source  224  can include a capacitor bank which can be charged during operation of the engine  100  or another engine. In this fashion, if other elements within the DC source  224  fail due to a fire event or other event, the capacitor bank can provide DC current to the starter motor  220 . The fuel module  230  serves to control the flow of fuel to the engine  100 . The operation of the fuel module  230  can be controlled by the engine controller  210 , for example based on instructions received from the aircraft system  250 , or based on algorithms and/or schedules available to the engine controller  210 . 
     In operation, the engine controller  210  can monitor the operating state of the engine  100 , for example to detect undesirable behaviour, including windmilling. If the engine  100  is found to be windmilling, the engine controller  210  can use the starter motor  220  to control the level of windmilling of the engine  100 , including reducing the level of windmilling, stopping the windmilling, and/or preventing the windmilling of the engine  100 . 
     The engine controller  210  can rely on different factors to determine whether the engine  100  is windmilling. The engine controller  210  can evaluate one or more of the factors when assessing whether windmilling is occurring, or likely to occur. It should be noted that the foregoing discussion of various factors indicative of windmilling in the engine  100  presents various factors in particular combinations, but the use of each of the factors individually, or in any suitable combination, is also considered. 
     In some embodiments, the engine controller  210  determines whether the aircraft  105  is airborne. The engine controller  210  can perform this evaluation using any suitable techniques. For example, the engine controller  210  can use readings from a pressure sensor to determine whether the atmospheric pressure in the vicinity of the aircraft  105  is indicative of the aircraft being airborne. In some instances, the engine controller  210  can determine or estimate an altitude for the aircraft  105 . If the engine  100  and/or the aircraft  105  are designed with an inlet duct to the engine  100  being permanently open, the engine controller  210  can determine that windmilling of the engine  100  is occurring on the basis of the aircraft  105  being airborne, or on the basis of the aircraft  105  being airborne above a certain altitude. 
     In some embodiments, the engine  100  and/or the aircraft  105  are designed with the inlet duct of the engine  100  being selectively closable, for instance via a mechanically-actuatable element. The engine controller  210  can determine whether the inlet duct of the engine  100  is open as part of determining whether the engine  100  is windmilling. For example, the engine controller  210  can query a sensor coupled to the inlet duct of the engine  100  to determine whether the inlet duct of the engine  100  is open. If the inlet duct of the engine  100  is open and, for example, the aircraft  105  is airborne, the engine controller  210  can determine that windmilling of the engine  100  is occurring. 
     In some embodiments, the engine controller  210  determines whether an operating state of the engine  100  corresponds to an “engine off” state. The engine controller  210  can query the aircraft system  250  to determine a commanded engine operating state, for instance as requested by an operator of the engine  100 . Alternatively, or in addition, the engine controller  210  can receive instructions from the aircraft system  250  to set the engine operating state to the “engine off” state, or can automatically set the engine operating state to the “engine off” state based on algorithms or other control schemes operated by the engine controller  210 . If the commanded engine operating state of the engine  100  is the “engine off” state, and the engine controller  210  determines that the inlet duct to the engine  100  is open and/or determines that a rotational speed of one or more components the engine  100  is exceeds a predetermined threshold, the engine controller can determine that windmilling of the engine is occurring. For example, a “windmilling threshold” can be established by a manufacturer, operator, or other authority. When the engine  100  is in the “engine off” state, and when the engine  100  is rotating above the windmilling threshold, the engine controller  210  can determine that windmilling is occurring. 
     Other approaches for determining whether windmilling is occurring are also considered. The engine controller  210  can employ any suitable number and combination of approaches for assessing windmilling, as appropriate. In addition, the engine controller  210  can, responsive to determining that a fire event is occurring in the vicinity of the engine  100 , proactively take measures to prevent windmilling of the engine  100 . 
     Once the engine controller  210  has determined that the engine  100  is windmilling, the engine controller  210  can then use the starter motor  220  to control the rotational motion of the engine  100  caused by the windmilling. This can include stopping the rotational motion, reducing the rotational motion, decelerating the rotational motion, or performing any other suitable type of control of the rotational motion of the engine  100 . Optionally, upon determining that the engine  100  is windmilling, the engine controller  210  can first perform one or more safety tests to determine whether the starter motor  220  can safely be used to control the windmilling of the engine  100 . For example, the engine controller  210  can perform a safety test to determine whether the starter motor  220  is functioning adequately, and can be safely used to control the windmilling of the engine  100 . 
     To control the windmilling of the engine  100 , the engine controller  210  can cause the starter motor  220  to act as a brake or load on the engine  100 , and thereby reduce or prevent windmilling of the engine  100 . The engine controller  210  causes the starter motor  220  to be mechanically coupled to the engine  100 , and then causes a DC signal to be applied to the starter motor  220 . The DC signal applied to the starter motor  220  causes the starter motor  220  to rotate, or to attempt to rotate, in a direction opposite to the rotation of the engine  100 , thereby counteracting the windmilling of the engine  100 . 
     In some embodiments, the circuit element  222  includes a circuit which reverses the polarity of the DC signal usually supplied by the DC source  224 . For example, the DC source  224  can provide a “positive” DC signal, used by the starter motor  220  when assisting in the ignition or start-up sequence for the engine  100 . When the inverter circuit of the circuit element  222  is activated, the positive DC signal can be inverted, thereby reversing the direction of motion of the starter generator. Other approaches are also considered, for example the use of a variable DC source  224 . 
     In some embodiments, the engine controller  210  is configured for modulating the DC signal provided to the starter motor  220 , for instance by controlling the operation of the circuit element  222 . The DC signal can be modulated to alter the level of rotational motion of the engine  100 , for instance resulting from windmilling. For example, a first DC signal can be used to substantially counteract the windmilling in the engine  100 ; that is to say, to substantially prevent any rotational motion in the engine  100 . In another example a second DC signal can be used to reduce the level of rotational motion of the engine  100 , for instance a predetermined safe level. The safe level can correspond to any suitable amount of rotational motion. 
     For instance, the safe level of rotational motion can be established based on a corresponding rotational speed for the engine  100  below or at which a lubricant system of the engine  100  is inoperative. In the event of a fire event or other unexpected event during flight of the aircraft  105 , preventing the flow of lubricant within the engine  100  can lead to reduced risks of failure of the engine  100  and/or within the aircraft  105 , and/or to reduced risk of other catastrophic failure. Thus, the starter motor  220  can be used to halt the flow of lubricant within the engine  100 , and reduce or mitigate part or all of the risk associated with a fire event. In some embodiments, the system  200  is configured to first determine whether a fire event is occurring in the vicinity of the engine  100 , and then to detect and, if necessary, reduce or prevent windmilling of the engine  100 . 
     In some other embodiments, the engine controller  210  is configured for varying the modulation of the DC signal over time. For example, the DC signal can be modulated to gradually reduce the level of rotational motion of the engine  100  over a predetermined time period. Gradually reducing the level of rotational motion of the engine  100  may reduce the risk of damage to the engine  100  and/or to the starter motor  220 . In another example, the windmilling of the engine  100  may complicate ignition of the engine  100 , for instance by reducing the starting envelope for the engine  100 . The DC signal to the starter motor  220  can be modulated to first reduce the level of rotational motion of the engine  100  to a level suitable for ignition, and then to maintain the level of rotational motion within a range around the level suitable for ignition until the engine  100  is successfully ignited. Other approaches are also considered. For instance, the engine controller  210  can substantially continuously monitor the level of rotational motion of the engine  100 , and adjust the DC signal to respond to changes in the level of rotational motion, as appropriate. 
     In some embodiments, the engine controller  210  is also configured for modulating the DC signal based on the capabilities of the DC source  224 . For instance, if the DC source  224  is an alternator which generates DC current from the operation of a separate engine, the engine controller  210  may evaluate the capability of the DC source  224 , including the fuel cost associated with the operation of the separate engine, and the like, versus the requirement to reduce windmilling in the engine  100 , before causing the DC signal to be applied to the starter motor  220 . Other considerations are also envisaged. 
     It should also be noted that in some instances, the DC signal may be a 0 V signal. That is to say, the circuit element  222  may serve to short-circuit the terminals of the starter motor  220 , thereby causing the starter motor to act as a mechanical load on the engine  100 . In these embodiments, the windmilling of the engine  100  is reduced or prevented due to the mechanical load of the starter motor  220 . In some instances, the engine controller  210  can monitor the level of rotational motion and apply a non-zero DC signal if the level of rotational motion goes beyond a predetermined threshold. Other control approaches are also considered. 
     With reference to  FIG. 3 , there is shown a method  300  for controlling windmilling in an engine, for instance the engine  100 . At step  302 , a determination is made regarding whether the engine  100  is in a windmilling state. The determination can be based on any suitable combination of factors, including commanded engine operating state, engine speed, an airborne status of an aircraft of which the engine  100  is a part, an altitude of the aircraft, a status of an inlet duct to the engine  100 , and the like. 
     At decision step  304 , when the engine  100  is in the windmilling state, the method  300  proceeds optionally to step  306 , or to step  310  if step  306  is not performed. When the engine  100  is not in the windmilling state, the method  300  can return to step  302 . 
     Optionally, at step  306 , a safety test is performed to assess whether a DC signal can be applied to an electric starter motor coupled to the engine  100 , for example the starter motor  220 . The safety test can be any suitable type of safety test. At decision step  308 , when the safety test indicates that the DC signal cannot safely be applied, the method  300  returns to some previous step, for instance step  306 , where the safety test can be performed again. Repeated failures of the safety test may cause the method  300  to terminate. When the safety test indicates that the DC signal can be applied safely, the method  300  proceeds to step  310 . 
     At step  310 , a circuit element, for example the circuit element  222 , is commanded to apply the DC signal to the starter motor  220 , which is coupled to the engine  100 . The DC signal causes the starter motor  220  to counteract the windmilling of the engine  100 , for example to reduce or prevent windmilling of the engine  100 . 
     At step  312 , the DC signal is modulated to control the level of rotational motion of the engine  100 . The DC signal can be set to any suitable value to reduce and/or prevent windmilling of the engine  100 . In some embodiments, the modulation of DC signal can be varied over time, for instance to gradually reduce the level of rotational motion in accordance with a predetermined schedule or to account for changes in the exterior causes of windmilling of the engine  100 . 
     With reference to  FIG. 4 , the method of  FIG. 3  may be implemented by a computing device  410  as an embodiment of the engine controller  210 . The computing device  410  comprises a processing unit  412  and a memory  414  which has stored therein computer-executable instructions  416 . The processing unit  412  may comprise any suitable devices configured to implement the functionality of the engine controller  210  such that instructions  416 , when executed by the computing device  410  or other programmable apparatus, may cause the functions/acts/steps performed by the engine controller  210  as part of the method  300  and as described herein to be executed. The processing unit  412  may comprise, for example, any type of general-purpose microprocessor or microcontroller, a digital signal processing (DSP) processor, a central processing unit (CPU), an integrated circuit, a field programmable gate array (FPGA), a reconfigurable processor, other suitably programmed or programmable logic circuits, custom-designed analog and/or digital circuits, or any combination thereof. 
     The memory  414  may comprise any suitable known or other machine-readable storage medium. The memory  414  may comprise non-transitory computer readable storage medium, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. The memory  414  may include a suitable combination of any type of computer memory that is located either internally or externally to device, for example random-access memory (RAM), read-only memory (ROM), compact disc read-only memory (CDROM), electro-optical memory, magneto-optical memory, erasable programmable read-only memory (EPROM), and electrically-erasable programmable read-only memory (EEPROM), Ferroelectric RAM (FRAM) or the like. Memory  414  may comprise any storage means (e.g., devices) suitable for retrievably storing machine-readable instructions  416  executable by processing unit  412 . 
     It should be noted that the computing device  410  may be implemented as part of a FADEC or other similar device, including electronic engine control (EEC), engine control unit (ECU), engine electronic control system (EECS), and the like. In addition, it should be noted that the techniques described herein can be performed by the engine controller  210  substantially in real-time. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the present disclosure. Still other modifications which fall within the scope of the present disclosure will be apparent to those skilled in the art, in light of a review of this disclosure. 
     Various aspects of the systems and methods described herein may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. Although particular embodiments have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects. The scope of the following claims should not be limited by the embodiments set forth in the examples, but should be given the broadest reasonable interpretation consistent with the description as a whole.