Auxiliary power units (APUs) provide energy on aircraft for functions other than propulsion. APUs often operate when the aircraft is on the ground while the aircraft's main engine or engines are powered off. APUs can provide power to start the main engines or provide power to other aircraft accessories, such as the cabin air circulation system or pre-flight check systems. After an APU is powered off, components in the “hot” section of the APU (typically, the combustor, turbine and exhaust silencer) remain at elevated temperatures. The hot components increase the temperature of adjoining and nearby components through conductive and convective heating. This event is often referred to as “hot soakback”. During flight, the APU can be cooled with ram air. On the ground, however, ram air is not available for cooling the APU and hot soakback must be addressed with other cooling solutions.
Hot soakback can cause a number of problems in an APU. First, fuel in the nozzles of the combustor and fuel lines can increase in temperature, causing the fuel to coke within the nozzles and lines and thereby interfere with proper combustion the next time the APU is operated. The coked fuel can also cause seals in the APU to fail prematurely. To remedy this situation, some APUs utilize fuel purge systems to purge fuel from the nozzles and lines during APU shutdown. Second, some APU aircraft compartments utilize composite materials on the outer skin to reduce the overall weight of the aircraft. Typically, these composite materials are unable to withstand the high temperatures experienced in the “hot” section of the APU. As a result, significant amounts of insulation are needed to insulate the hot section of the APU from components containing composite materials and reduce hot soakback—more than what is needed for merely operating the APU. Additionally, certain components in the hot section of the APU remain at elevated temperatures longer due to the presence of other hot components located nearby. For example, the rear bearing of the turbine is particularly susceptible. The rear bearing soaks heat from other turbine components and exhaust silencer. Prolonged thermal stress can cause this bearing to fail prematurely.
While fuel purge systems and insulation can reduce some of the hot soakback effects, each of these solutions adds weight to the aircraft and increases production costs.
A method for cooling a gas turbine engine includes supplying power to a motor to generate mechanical motion and translating the mechanical motion of the motor to a shaft of the gas turbine engine to rotate a compressor stage and a turbine stage after the gas turbine engine has been shutdown to circulate air within the engine and cool engine components.
A method for cooling an auxiliary power unit includes discontinuing fuel delivery to a combustor of the auxiliary power unit, supplying power to a starter to rotate a starter shaft and translating rotational motion of the starter shaft to a shaft of the auxiliary power unit to rotate a compressor stage and a turbine stage of the auxiliary power unit to circulate air within the auxiliary power unit until a temperature of the auxiliary power unit is below a low limit temperature threshold.
A system for preventing hot soakback in an auxiliary power unit includes a starter motor, a compressor having at least one stage, a turbine having at least one stage, a shaft connected to the at least one stage of the compressor and the at least one stage of the turbine, a gearbox for connecting the starter motor to the shaft, a temperature sensor and a controller. The controller receives information from the temperature sensor, instructs the starter motor to rotate when the temperature sensor senses a temperature above about a low limit temperature threshold. The gearbox translates rotation of the starter motor to the shaft to rotate the at least one stage of the compressor and the at least one stage of the turbine to circulate air within the auxiliary power unit in response to the controller when the temperature sensor senses the temperature above the low limit temperature threshold in order to reduce hot soakback.
The present invention provides a method and system for reducing hot soakback within an auxiliary power unit (APU). According to the present invention, the shaft of the APU is rotated after shutdown to circulate air within the APU. According to one embodiment of the invention, the APU starter/generator is used to rotate the APU shaft. Stages of the APU compressor and turbine are rotated to circulate air within the APU. This air circulation reduces hot soakback by expelling hot air from the APU through the exhaust, allowing the circulated air to cool the “hot” components of the APU before it exits. In one embodiment of a system for reducing hot soakback, a starter controller controls the rotation of the starter/generator and the APU shaft depending on certain conditions within the APU.
APU 10 also includes starter/generator 22, starter controller 24 and gearbox 26. Starter 22 converts power into mechanical motion (e.g., rotation) that is used to initiate rotation of compressor 12 and turbine 16 to start the main engine section of APU 10 (compressor 12, combustor 14 and turbine 16). Starter 22 can be an electric starter motor or an air turbine starter. Gearbox 26 translates motion from starter 22 to shaft 20 to rotate shaft 20 and compressor 12 and turbine 16. Starter controller 24 provides operational instructions to starter 22. Starter controller 24 dictates whether starter 22 is running and at what speed starter 22 rotates.
Starter 22 receives power from power supply 28. In embodiments where starter 22 is an electric motor, power supply 28 is a battery, the main aircraft engines, terminal connection power or a ground cart. In one embodiment, power supply 28 is a direct current power source. In embodiments where starter 22 is an air turbine starter, power supply 28 provides a source of compressed air for rotating the flow turbine of starter 22. The compressed air can be delivered from a ground cart or be bled from the main aircraft engines.
APU 10 also includes temperature sensor 30. Temperature sensor 30 can measure the temperature of exhaust gases within APU 10 (e.g., an exhaust gas temperature sensor), the temperature of air within the APU but outside of the APU's gas flow path, or the temperature of a surface of APU 10 (e.g., a skin temperature sensor). In the embodiment illustrated in
In some embodiments, APU 10 includes an inlet duct and a door for allowing air external to APU 10 to enter the inlet duct. As shown in
The operation of APU 10 to reduce hot soakback will now be described.
In step 54, starter controller 24 determines whether the temperature of APU 10 is above a low limit temperature threshold indicating that APU 10 must be cooled to prevent hot soakback effects. Starter controller 24 receives information from temperature sensor 30 concerning the temperature of APU exhaust gas or an air or surface temperature within APU 10. If the temperature sensed by temperature sensor 30 is below a low limit temperature threshold, the APU requires no additional cooling to prevent serious hot soakback effects and power is not delivered to starter 22 or is discontinued. If the temperature sensed by temperature sensor 30 is above a low limit temperature threshold, the process continues to step 56. In exemplary embodiments, the low limit temperature threshold is between about 177° C. (350° F.) and about 260° C. (500° F.), and more preferably between about 204° C. (400° F.) and about 232 ° C. (450 ° F.). In one particular embodiment, the low limit temperature threshold is about 204° C. (400° F.).
In step 56, starter controller 24 delivers power (e.g., electric or pneumatic power) from power supply 28 to starter 22. In step 58, starter controller 24 instructs starter 22 to rotate. Starter controller 24 controls the speed at which starter 22 rotates. As starter 22 rotates, gearbox 26 transmits power from the rotation of starter 22 to shaft 20. In some embodiments, starter controller 24 can also control the rate at which power is converted from starter 22 to shaft 20 by gearbox 26. In step 60, gearbox 26 engages with shaft 20 to rotate shaft 20, thereby rotating at least one stage of compressor 12 and at least one stage of turbine 16 to circulate air within APU 10.
In one embodiment, the supply of fuel to combustor 14 is shut off during step 60, to prevent further combustion (and heat formation) within APU 10. Once the supply of fuel to combustor 14 has been discontinued, shaft 20 can rotate at virtually any speed to circulate air within APU 10. In exemplary embodiments, shaft 20 rotates at a speed between about 25% and about 50% of nominal operation speed. In an alternate embodiment, shaft 20 is rotated below the light-off speed of APU 10 during step 60. The light-off speed is the rotation speed at which APU 10 will begin burning fuel and can efficiently run on its own. Light-off speeds for APUs are typically between about 10% and about 40% of nominal operation speed. While
By rotating at least one stage of compressor 12 and at least one stage of turbine 16 in step 60, air is circulated through APU 10 and eventually exits through exhaust pipe 19. The circulating air absorbs heat from the hot components within APU 10 (e.g., combustor 14 and turbine 16) and exits through exhaust pipe 19, thereby carrying high temperature air away from APU 10 to reduce or eliminate the effects of hot soakback within APU 10.
As indicated in
By balancing the amount of power delivered to starter 22 and the speed at which starter 22 rotates, starter controller 24 can provide adequate cooling of APU 10 while minimizing the amount of power drawn from power supply 28. For example, starter controller 24 can minimize power draw by rotating starter 22 at a lower speed for a slightly longer length of time when power supply 28 is a battery. When power is supplied by a terminal connection and power draw is a lesser concern, starter controller 24 can rotate starter 22 at a higher speed for a shorter length of time to cool APU 10 more quickly.
Method 44 provides a method and system for reducing hot soakback effects within APU 10. Other methods of reducing hot soakback come with disadvantages. Fuel purge systems used to purge fuel lines, fuel injectors and fuel nozzles add additional cost and weight to APU 10. Fuel purge systems also do not provide benefits to APU components besides the fuel system (e.g., they provide no benefit to composite components). Providing additional insulation within APU 10 also adds weight and cost. Additional insulation also does not provide significant benefits to the fuel system. By utilizing method 44, the effects of hot soakback can be reduced without adding significant weight or additional costs to APU 10. Method 44 utilizes existing components of APU 10 to provide a method for reducing hot soakback. Only minor changes and additions are necessary. Due to increased use during method 44, a more robust starter/generator 22 than one used only to start APU 10 may be warranted. The addition of starter controller 24 adds some cost and weight, but pales in comparison to a fuel purge system and additional insulation.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.