The present invention relates generally to control systems for gas turbine engines. More particularly, the present invention relates to an auxiliary power unit (APU) control system and an air management system.
Air management systems are regulatory systems commonly used in modern aircraft to control temperature, pressure, and air flow in aircraft cabins, but may sometimes also control air flow to other non-engine aircraft components. It is common for the air used by the air management system to be bled off of one or more of the aircraft's gas turbine engines. In particular, air is often bled off of auxiliary power units so as not to burden the aircraft's main engines.
Whether from an APU or a main engine, a portion of the air intake of at least one engine compressor is usually diverted into the air management system, rather than being fed to a combustor. Driving the compressor of a gas turbine generator requires power, and the efficiency of a gas turbine generator is limited, in part, by the energy expended to compress input gas. Diverting input gas from the compressor means that the compressor is burdened by the need to compress some amount of gas beyond what is actually used to generate power. This burden reduces the efficiency of operation of the gas turbine engine as a whole.
The air demands of air management systems may vary with time. Air management systems are designed to draw a sufficient flow of compressor air to match these varying demands. In past systems, a fixed fraction of compressor air was usually diverted from an engine compressor, a variable portion of which was used by the air management system. Any excess or unneeded portion of this bleed air was dumped overboard. This practice was inefficient, since the compressor would frequently draw and compress unneeded air.
One embodiment of the present invention is a system that includes a gas turbine engine, an air management system, and a turbine controller. The gas turbine engine is capable of driving an electric generator. The air management system regulates cabin pressure and temperature in an aircraft, and is connected to receive bleed air from the gas turbine engine. The turbine controller controls operation of the gas turbine engine according to air flow demand signals indicative of air flow required by the air management system.
A gas turbine engine and a method for controlling that engine based on inputs from an air management system of an aircraft are provided. By controlling the orientation of compressor inlet guide vanes and driving the turbine at the minimum speed required to meet the demands of the air management system, the gas turbine engine can be run at lower power when less than maximum bleed air is required by the air management system. In this way, average system efficiency is improved.
Air management system (AMS) 24 regulates cabin air pressure and temperature on the aircraft, and may include, for instance, cabin air conditioning systems. Air management system controller 28 controls the flow of air from load compressor 18 to air management system 26 via siphon 30. Air management system 26 may require varying amounts of airflow at different times. Air management system controller 28 determines the air flow required by air management system 26 at any particular time, and, using siphon regulation signal SR, accordingly controls the amount of air drawn from load compressor 18 by siphon 30. Air management system controller 28 also communicates the air flow required by air management system 26 to APU controller 24 via airflow demand signal AD. Airflow demand signal AD indicates the amount of bleed air to be drawn from load compressor 18 as a fraction of the maximum amount that load compressor 18 can provide.
APU controller 24 regulates the air intake of load compressor 18 and the fuel intake of combustor 14. In particular, APU controller 24 regulates compressor air intake by controlling the position of inlet guide vanes in load compressor 18 via compressor inlet guide vane position command IGVP, and regulates combustor fuel intake by controlling fuel system 20 via fuel flow rate command FR. Fuel system 20 may include a fuel supply with pumps, valves, or other components for metering fuel, but may otherwise take any form that is capable of delivering fuel at a commanded rate to APU 10. APU controller 24 will also receive additional inputs, including but not limited to compressor inlet temperature signal TCI, compressor inlet pressure signal PCI, exhaust gas temperature signal TEG, engine rotational speed signal SER, and compressor exit pressure signal PEXIT, which reflect corresponding engine parameters. In one embodiment these parameters are measured on the APU. In an alternative embodiment, some of these parameters may be measured elsewhere, or calculated from other measurements.
Within APU controller 24, baseline speed schedule 102 produces speed setpoint SPTS, and baseline pressure schedule 104 produces pressure setpoint SPTP. These setpoints correspond to desired engine speed and pressure, respectively, for a fixed value of bleed airflow demand. Baseline speed and pressure schedules 102 and 104 receive inputs including compressor inlet temperature signal TCI and compressor inlet pressure signal PCI, and determine speed setpoint SPTS and pressure setpoint SPTP by means of algorithms or lookup tables. Compressor inlet temperature and pressure are, for instance, either algorithm parameters or dimensions of lookup tables.
APU controller 24 receives airflow demand signal AD, as previously noted, from air management system controller 28. Speed correction block 106 calculates speed correction factor SCF arithmetically from airflow demand signal AD, compressor inlet temperature signal TCI, and compressor inlet pressure signal PCI. Pressure correction block 108 calculates pressure correction factor PCF arithmetically from airflow demand signal AD, compressor inlet temperature signal TCI, and compressor inlet pressure signal PCI. Speed correction factor SCF and pressure correction factor PCF are used to scale speed setpoint SPTS and pressure setpoint SPTP at speed calculator block 110 and pressure calculator block 112, respectively. Speed calculator block 110 multiplies speed setpoint SPTS by speed correction factor SCF to produce factored speed setpoint FSPTS, which reflects the desired engine speed corrected to account for the airflow demand from air management system controller 28. Factored speed setpoint FSPTS reflects the desired engine speed for a given airflow demand signal AD. Similarly, pressure calculator block 112 multiplies pressure setpoint SPTP by pressure correction factor PCF to produce factored pressure setpoint FSPTP, which reflects the desired engine pressure for a given airflow demand signal AD.
In an illustrative embodiment, baseline speed schedule 102 produces a speed setpoint SPTS corresponding to optimal engine speed if 100% of maximum bleed air were requested by air management system controller 28. In this example, if airflow demand signal AD indicated that air management system 26 needed only 50% of maximum bleed air, speed correction block 106 would produce a scaling value less than one as speed correction factor SCF, reflecting the fact that a lower engine speed would be adequate to supply all of the required bleed air (and that fueling a higher engine speed would therefore be wasteful). Speed setpoint SPTS would then be reduced according to speed correction factor SCF to provide a factored speed setpoint FSPTS corresponding to an engine speed that would enable sufficient bleed to satisfy the demands of air management system 26 with minimum fuel expenditure.
Factored speed setpoint FSPTS and factored pressure setpoint FSPTP are used by proportional integral feedback algorithms to continuously regulate the rate of fuel flow into combustor 14 and the position of inlet guide vanes in load compressor 18, respectively. Speed control proportional integral algorithm 114 combines the weighted integral and weighted sum of the difference between factored speed setpoint FSPTS and engine rotational speed signal SER to produce fuel flow rate command FR. This regulation results in a feedback loop: engine rotational speed will vary as more or less fuel is fed into combustor 14 in accordance with fuel flow rate command FR, and this rotational speed change will be reflected by a change in engine rotational speed signal SER. Thus, speed control proportional integral algorithm 114 constitutes a feedback algorithm which will continuously adjust the fuel supply rate to combustor 14 so as to reduce the difference between the factored speed setpoint and the actual engine speed.
Pressure control proportional integral algorithm 116 operates analogously to speed control proportional algorithm 114. Factored pressure setpoint FSPTP is combined with compressor exit pressure signal PEXIT to produce inlet guide vane control command IGVP, which represents a correction to inlet guide vane position. This feedback loop will continuously adjust inlet guide vane position until desired pressure is achieved.
Exhaust gas temperature signal TEG operates as a limit on the normal functioning of the aforementioned system. Extremely high temperatures can be harmful to the continued functioning and long-term durability to the APU. To prevent damage, APU controller 24 computes a ceiling to fuel flow rate command FR and restricts inlet guide vane position command IGVP, based on exhaust gas temperature TEG, so as to prevent overheating.
Speed control proportional integral algorithm 114 continuously monitors engine rotational speed via engine rotational speed signal SER (Step 210a). Pressure control proportional integral algorithm 116 continuously monitors pressure at the exit of load compressor 18 via compressor inlet vane position signal PEXIT (Step 210b). In order to regulate engine operation, speed control proportional integral algorithm 114 then takes the difference between factored speed setpoint FSPTS and engine rotational speed signal SER to produce a speed error value (Step 212a). The weighted sum and integral over time of this speed error value are combined to produce fuel flow rate command FR (Step 214a). Similarly, pressure control proportional integral algorithm 116 takes the difference between factored pressure setpoint FSPTP and compressor inlet guide vane position signal PEXIT to produce a pressure error value (Step 212b). The weighted sum and integral over time of this pressure error value are combined to produce compressor inlet guide vane position command IGVP (Step 214b).
Finally, fuel flow rate command FR is transmitted to fuel system 20 to control fuel flow, and inlet guide vane position command IGVP is transmitted to load compressor 18 to control air intake (Step 216). Rotational speed, airflow, and pressure in APU 10 will vary in response to these signals, giving rise through a feedback loop to continuous changes in fuel flow rate command FR and inlet guide vane position command IGVP, thereby continuously zeroing in on desired engine operation conditions.
In the past, air management systems have controlled the rate of air bleed from a turbine compressor, but have not been involved in the control of engine operation parameters such as engine fuel and air intake. By controlling fuel and air flow into APU 10 in accordance with airflow demand AD, the present invention is able to minimize waste air compression without interfering with electrical power generation. This improves the average fuel efficiency of APU 10.
While the invention has been described with reference to an exemplary embodiment(s), 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 embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.