Abstract:
Systems and methods for supplementing a power system to achieve consistent operation at varying altitudes are disclosed herein. A hybrid power system comprising a single power source driving multiple generators may implement a power recovery turbine to drive a supercharger compressor, which may provide compressed air at increased altitudes. The supplemental power system disclosed herein provides necessary shaft horsepower at high altitudes to drive a generator and produce cooling air.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of patent application Ser. No. 10/896,309, filed Jul. 21, 2004 now U.S. Pat. No. 7,111,462. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The U.S. Government has a paid-up license in this invention as provided for by the terms of contract No. F33615-03-2367 awarded by USAF/AFMC. 

   BACKGROUND 
   1. Field 
   The present disclosures herein relate to aircraft engines, and more specifically to systems and techniques for augmenting the power capability of aircraft engines in high altitude environments. 
   2. Background 
   Ongoing development and growth in the area of onboard aircraft electrical systems and electronic sub-systems has resulted in a desire to augment existing aircraft systems with supplemental electrical generating capability. However, standard aircraft production-design characteristics generally leave little, if any, room for significant electrical or cooling air systems expansion or modification. Thus, it is difficult for aircraft to accommodate post-production systems additions. The traditional approach of original equipment manufacturers with respect to expanding on-board power and electrical generating capability usually leads to extensive and costly aircraft and/or engine modifications. 
   Aircraft are often powered by gas turbine engines, which have a great power-to-weight ratio compared to internal combustion reciprocating engines. Gas turbine engines are commonly considered to be “over-powered” at low altitudes, because of their high power-to-weight ratio. However, at high altitude, when the air gets thinner, air-breathing internal combustion engines lose power. Even gas turbine engines can quickly become “under-powered” as an aircraft ascends. Unfortunately, power enhancement modifications to an aircraft engine often require costly structural alterations to the airframe itself. Thus, in addition to the main engines, aircraft often utilize additional small gas turbine engines that may be installed within the aircraft. These additional engines may generate electric power and provide pressurized air for power requirements while the aircraft is on the ground. Generally, these devices have their functions taken over in flight by the main engine. However, as electrical requirements for passenger amenities and other electronic needs have increased, these auxiliary power units have become correspondingly larger. In modern aircraft, auxiliary power units are often utilized in-flight. Although many auxiliary engines are now overpowered at sea level, they generally are only able to provide constant power up to altitudes of about 25,000 ft. (“FL25”), and have diminishing power as the increases beyond that. Gas turbine engines cannot easily be made any larger, as the increase in size and weight would require significant structural modification to the airframe itself. 
   In short, modern aircraft including military aircraft, which have high requirements for electrical power, suffer deficiencies when equipped with gas turbine engines, because they lose power at high altitude but cannot compensate with increased size due to airframe structural limitations. Thus, the in-flight power generating capability of aircraft is often significantly limited under prior art constructs. One result is that there is not currently a gas turbine power system capable of operating at high altitude with the ability to maintain the increasing demand for more horsepower to drive a generator and produce cooling air in sufficient quantity, without requiring significant modification to airframe structures. 
   SUMMARY 
   In one aspect of the present invention, a gas turbine power system for an aircraft includes a gas turbine engine having a sensor system configured to measure the air mass flow through the engine and an exhaust nozzle having a variable opening responsive to the sensor system, a power recovery turbine coupled to the variable opening in the gas turbine engine, a first compressor driven by the power recovery turbine and configured to deliver compressed air to the gas turbine engine, and a second compressor coupled to the gas turbine engine or the power recovery turbine. 
   In another aspect of the present invention, a method of regulating the power of a gas turbine power system installed on an aircraft includes measuring the air mass flow through a gas turbine engine having an air intake and an exhaust outlet, adjusting, as a function of the measured air mass flow, a variable opening nozzle coupled to the exhaust outlet of the gas turbine engine, directing exhaust from the gas turbine engine through the adjusted variable opening nozzle, driving a power recovery turbine with the exhaust, driving a first compressor with the power recovery turbine and routing compressed air generated by the first compressor to the air inlet of the gas turbine engine, and driving a second compressor with the gas turbine engine or the power recovery turbine. 
   In another aspect of the present invention, a gas turbine power system for an aircraft includes means for measuring the air mass flow through a gas turbine engine, means, responsive to the means for measuring, for variably opening an exhaust nozzle coupled to the gas turbine engine, means, coupled to the exhaust nozzle, for driving a first compressor, means for delivering a first portion of compressed air from the first compressor to the gas turbine engine, and means, coupled to the gas turbine engine or the means for driving the first compressor, for further compressing a second portion of the compressed air and routing it to an air conditioning system. 
   It is understood that other embodiments of the specific teachings herein will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only several embodiments of the teachings by way of illustration. As will be realized, the subject matter of the teachings herein is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of these teachings. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the disclosures herein are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein: 
       FIG. 1  is a schematic illustrating an exemplary system layout design; 
       FIG. 2  is a schematic illustrating aspects of the first system layout design illustrated in  FIG. 1 ; 
       FIG. 3  is a schematic illustrating a first alternative system layout design; 
       FIG. 4  is a schematic illustrating a second alternative system layout design; and 
       FIG. 5  is a schematic illustrating a third alternative system layout design. 
   

   DETAILED DESCRIPTION 
   The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the teachings herein and is not intended to represent the only embodiments in which the teachings herein may be practiced. The term “exemplary” used throughout this disclosure means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the teachings. However, it will be apparent to those skilled in the art that the teachings herein may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the teachings herein. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the teachings herein. The term “coupled” is used throughout this disclosure to indicate structural or functional cooperation between two components. In the case of structural cooperation, the components may be connected directly to one another or, where appropriate in the context, connected indirectly to one another, e.g., through intervening or intermediary devices or other means. In the case of functional cooperation, there may or may not be a physical connection between the two components. 
   In the following detailed description, various aspects of the teachings herein will be described in the context of a gas turbine engine that may comprise a commercially available off the shelf gas turbine engine. While these inventive aspects may be well suited for use with such an engine, those skilled in the art will readily appreciate that they are likewise applicable for use in various other exhaust-producing aircraft engines. Accordingly, any reference to a gas turbine engine is intended only to illustrate various aspects of the disclosures herein, with the understanding that such aspects have a wide range of applications. 
   The teachings herein apply to aircraft gas turbine engine, to augment its power capability so that it may produce sufficient power to run a generator and cooling systems for power and cooling demands of the aircraft at high altitudes. In an exemplary embodiment, certain modifications may be made to a gas turbine engine in order to increase its power. For example, a supercharger may be utilized to boost the pressure of ambient air at high altitudes and deliver the pressurized air to the gas turbine engine&#39;s air intake. Implementing the supercharger would maintain atmospheric pressure at the air intake of the gas turbine engine, enabling the engine to provide sufficient power to drive onboard electrical and cooling systems even as the aircraft flies at high altitudes. 
   Generally speaking, supercharging may be used to force compressed air into a gas turbine engine to achieve improved engine performance and fuel efficiency. The supercharger may be driven by a power recovery turbine, which in turn may be driven by exhaust gases from the gas turbine engine. The increase in air fed into the gas turbine engine by the supercharger may increase combustion force and power. This increase may compensate for thinner air at high altitudes, and prevent the engine from losing power as the aircraft climbs. The stabilized power produced by the gas turbine engine may be directed through an output shaft to drive rotors, compressors, ducted fans, or for any other intended use the system designer may have for such power. In accordance with the teachings herein, one or more superchargers may be driven by the gas turbine engine exhaust, or coupled to the power output shaft of the gas turbine engine itself. These general concepts will be explained in further detail below. 
     FIG. 1  is an schematic illustrating an exemplary supplemental power system installation that may be used, for example, on aircraft. The exemplary supplemental power system may provide an aircraft with electrical power and compressed air for continuous ground and airborne operations. For example, it may provide for conditioning of the aircraft cockpit and passenger/cargo cabin areas, main engine starting and other electrical power requirements, at sea level and at high altitudes up to and beyond 40,000 feet (“FL40”). The exemplary supplemental power system may provide constant mass flow, variable inlet volumetric flow, and a variable compression ratio. That is, as the altitude of an aircraft increases, the exemplary supplemental power system may increasingly compress additional amounts of air, and feed this additional volume of compressed air to the air intake of the aircraft&#39;s gas turbine engine. As the aircraft increases altitude and encounters thinner air, the variably increased volumetric flow of air through the gas turbine engine will enable the gas turbine engine to experience a constant flow of pounds of air (“air mass flow”). In other words, larger amounts of compressed air at higher altitudes, where the air is thinner (i.e. has smaller mass), will approximate the air mass flow of smaller amounts of uncompressed (ambient pressure) air at lower altitudes, where the air is thicker (i.e. has larger mass). Thus, a gas turbine engine processing a greater volumetric flow of air at high altitudes may generate the same level of power as if it were processing a smaller volumetric flow of air at seal level, because it would actually be processing the same “air mass flow” in each case. 
   In an exemplary embodiment, a gas turbine engine  100  may include a compressor, a combustion area and a turbine. The compressor (not shown) may be located near the air intake of the gas turbine engine  100  and raise the pressure of incoming air to produce pressurized air. The high pressure air may then enter the combustion area (not shown), where fuel injectors may inject a stream of fuel, such as jet fuel in the case of an aircraft. In the combustion area, the air and fuel are mixed. The combustion area burns the fuel and produces exhaust, which may provide both power for use on board the aircraft, and thrust to cause the aircraft to move. The turbine (not shown) may be used to transmit power to other systems on the aircraft, by driving an output shaft  101 . The turbine may include a set of vanes placed in the exhaust stream, that “catch” the exhaust and cause the vanes to spin, like a windmill. The vanes may be attached to the output shaft, which will spin as the vanes in the exhaust stream spin. The output shaft  101  may then be used to drive other systems on the aircraft. In this manner, the turbine extracts energy from the high pressure, high velocity exhaust that flows from the combustion chamber, and transmits the extracted energy through the output shaft  101 . This energy may be used to provide power for electrical, cooling, and other systems on board the aircraft, that may be coupled directly or indirectly to the output shaft  101 . 
   As mentioned above, in addition to producing power the gas turbine engine  100  may provide thrust for causing the aircraft to move forward. A nozzle may be formed at the exhaust end of the gas turbine engine, to generate a high speed jet of exhaust gas. This high speed exhaust jet may provide thrust that causes the aircraft to move forward. Therefore, the gas turbine engine  100  may provide both thrust to move the aircraft forward, and additional power for driving various electrical and cooling systems on board the aircraft. The gas turbine engine  100  may comprise a commercial off the shelf (“COTS”) engine, as selected by a system designer. For example, the gas turbine engine  100  may comprise a Pratt &amp; Whitney PW127G turbine engine, capable of producing approximately 2600–2800 shaft horsepower (“shp”) at sea level static, standard day conditions. This is only one example of a gas turbine engine that may be utilized in the exemplary embodiment, and should not be read to limit the teachings herein. Those skilled in the art will recognize that any of a number of different engines may be used in conjunction with the teachings herein. 
   In accordance with the general principles disclosed herein, the gas turbine engine  100  may receive ambient air  102  at its air intake  128 . The ambient air may be compressed, mixed with fuel, and combusted in the gas turbine engine  100 , as explained above. The exhaust gases produced by the combusted fuel-air mixture may be used to rotate the output shaft  101 . The output shaft  101  of the gas turbine engine  100  may drive a power generator  106  that produces power as indicated at arrow  107 . Alternatively, the power generator  106  may be driven by a separate, power recovery turbine, as will be explained in further detail below. The power generator  106  may comprise a 1000 kilivolt ampere (“KVA”) generator, or any other generator that may be required to support various electrical, cooling, and other systems on board an aircraft to be fitted with the exemplary supplemental power system. 
   To enable it to provide sufficient power to drive generator  106  at high altitude, the gas turbine engine  100  may be supercharged. A power recovery turbine  112  coupled to the output of the gas turbine engine  100 , either directly or indirectly, may be used to drive a supercharger compressor  108 . The power recovery turbine  112  may be coupled to a variable area nozzle (“VAN”)  114  at the exhaust outlet of the gas turbine engine  100 . The VAN  114  may be opened or closed to change the size of the nozzle through which exhaust from the gas turbine engine  100  may be discharged. As the VAN  114  is opened, the nozzle area becomes larger, allowing exhaust to escape more easily. As the VAN  114  is closed, the nozzle area becomes smaller, partially blocking the path of exhaust from the gas turbine engine  100 . Thus, closing or partially closing the VAN  114  will increase backpressure at the exhaust outlet of the gas turbine engine  100 . 
   An exhaust duct  116 , coupled to the exhaust outlet of the gas turbine engine  100 , may include a bypass duct  118  for pressure relief. When the gas turbine engine  100  does not need to be supercharged in order to support the aircraft&#39;s on board systems, such as at lower altitudes, exhaust from the gas turbine engine  100  may pass through bypass duct  118 . However, when additional power is required, such as at higher altitudes, exhaust may be directed past the bypass duct  118  and through the power recovery turbine  112 . This re-direction of exhaust may be accomplished, for example, by closing or partially closing the VAN  114  and increasing the backpressure at the exhaust outlet of the gas turbine engine  100 . The details regarding how this may be accomplished will be explained in further detail below. In any case, as exhaust flows through the power recovery turbine  112 , it may rotate the power recovery turbine  112  having an output shaft  110  attached thereto. The output shaft  110  may be used to drive the supercharger compressor  108 , which may supercharge the gas turbine engine  100 . Supercharging the gas turbine engine  100  involves feeding additional volumes of air to the air intake  128  of the gas turbine engine  100 . This may sustain the level of air mass flow through the gas turbine engine and thus the power produced by the gas turbine engine  100 , even at the aircraft climbs to higher altitudes where the ambient air is thinner. 
   The supercharger compressor  108  may be either axial flow or radial, and may be a low-pressure compressor (LPC) which can be sized to produce additional volumes of air to supply air flowing either directly to air conditioning equipment or to another compressor for other cooling systems on board the aircraft. The supercharger compressor  108  may include inlet guide vanes (“IGV”)  109  to regulate the amount of ambient air  120  air that enters. The supercharger compressor  108  may compress the ambient air  120 , which results in compressed air  122 . The compressed air  122  may pass through an intercooler  124 , which may cool the compressed air  122  as well as ambient air  126 . This cooled air may be used to supplement the air that is received by the gas turbine engine  100 , as well as to support air conditioning and other cooling systems that may be on board the aircraft. Both applications will be explained in further detail below. 
   If the intercooler  124  generates cooled, compressed air in a quantity that exceeds system requirements imposed by the generator  106  and the air conditioning system, the excess air may be released at a bypass duct  132 . However, the majority of cooled, compressed air produced by the intercooler  124  may be used for both the gas turbine engine  100  and the air conditioning or other cooling systems. Cooling the compressed air  122  before it reaches the air intake  128  of the gas turbine engine  100  may preserve or increase the air mass flow of this compressed air  122 . Generally speaking, when air is compressed, such as by the supercharger compressor  108 , its temperature rises. If some or all of the air taken into a gas turbine engine has been supercharged, the benefit of supercharging (i.e. greater mass flow) may be reduced by this temperature rise. Thus, a cooling device such as the intercooler  124  may be employed to reduce the temperature of the supercharged air and preserve the greater air mass flow. Cooled air from the intercooler  124  may then be directed to the air intake  128  of the gas turbine engine  100 . The gas turbine engine  100  may thus receive both the ambient air  102  and the additional cooled air from the intercooler  124 . This additional amount of air from the intercooler  124  may increase pressure at the air intake  128 , until it approximates the backpressure at the exhaust outlet of the gas turbine engine  100 . Thus, if the backpressure had previously been increased, such as by closing or partially closing the VAN  114 , the additional air supplied to the gas turbine engine  100  via supercharging may cause an equivalent increase in pressure at the air intake  128 . 
   In accordance with the teachings above, if the backpressure is adjusted, such as to approximate the typical backpressure that would be present at sea level conditions, the exemplary supplemental power system may be engaged as described above to increase the pressure at the air intake  128  so that it also approximates the typical air intake pressure that would be present at sea level conditions. By repeating this process of controlling the VAN  114  to increase backpressure at the exhaust  116 , then driving the supercharger compressor to feed more air into the air intake  128  and raise the pressure at the air intake  128 , the gas turbine engine  100  will be able to process a consistent air mass flow and produce a consistent level of power, even as altitude increases and the ambient air  102  becomes thinner. Further, this consistent air mass flow may be controlled such that it is approximately what the gas turbine engine  100  would experience at sea level, causing the gas turbine engine to operate at full-powered sea level conditions, even at high altitudes. 
   In addition to supplementing the air intake of the gas turbine engine  100 , cooled air from the intercooler  118  may be directed to a load compressor  104  that supports air conditioning and other cooling systems on board the aircraft. The load compressor  104  may produce compressed, cooled air  130  that may be used, for example, in on-board air conditioning, component cooling, or other types of conditioning systems that may be on board the aircraft. In an exemplary embodiment, the compressed, cooled air  130  may be approximately 800 pounds per minute, at 50 pounds per square inch absolute (“psia”). However, it will be recognized by those skilled in the art that exact specifications may be altered in accordance with the present teachings and tailored to fit the requirements of cooling systems on board various aircraft, as necessary. 
   The effect of supplying the gas turbine engine  100  with additional air from supercharger compressor  108  as described above, as well as mechanisms for controlling the amount of additional air, will now be explained. As illustrated in  FIG. 1  and described in reference thereto, the exemplary supplemental power system may utilize the residual power obtained from captured gas turbine engine exhaust to compress ambient air. The compressed air results in an increased volume of air, which may be used to preserve the air mass flow processed by the gas turbine engine  100  at increased altitudes, for example, where ambient air is thinner. This additional air may compensate for the lower air-weight volume ratios that occur at high attitude, which would otherwise reduce the power capabilities of gas turbine engine  100  when an aircraft flies at higher altitudes. Preserving the air mass flow as altitude increases may enable the gas turbine engine  100  to generate a consistent level of power, even as the aircraft climbs. The air mass flow may be preserved by controlling the compressed air that is fed to the gas turbine engine  100 , such that the gas turbine engine  100  receives air at a weight equal to that which it would receive at sea level. 
   A control system  134  may regulate the amount of compressed air that is fed to the air intake  128  of the gas turbine engine  100 , such that it receives an approximately consistent air mass flow between sea level and higher altitudes. The control system  134  may rely on a buildup of backpressure in the gas turbine engine  100  in order to produce sea level output even at high altitudes up to at least 40,000 feet (“FL40”). Pressure may be monitored by one or more sensors, such as an intake sensor  138  located at the air intake  128  of the gas turbine engine  100 , and an exhaust sensor  140  located in the exhaust duct  116  adjacent to the variable area nozzle  114 . The intake sensor  138  may include a plurality of sensors, and the exhaust sensor  140  may also include a plurality of sensors. The sensors may be pressure sensors or other appropriate sensors for measuring or determining the pressure at various areas within and around the gas turbine engine  100  in order to measure the mass air flow through the gas turbine engine  100 . The control system  134  may receive input from the pressure sensors  138  and  140 , as well as other input signals that will be described below, and correspondingly control the VAN  114  to regulate backpressure at the exhaust outlet of the gas turbine engine  100 . 
   The control system  134  may operate in conjunction with an aircraft&#39;s Full Authority Digital Engine Control (“FADEC”)  136  and a cooling air demand regulating system  138 . The FADEC  136  may control the outputs of the gas turbine engine  100  and power recovery turbine  112  as the aircraft is climbing through to FL40. The FADEC may, after receiving readings from an altimeter or other altitude sensing device, provide altitude information to the control system  134 . The FADEC may also provide information to the control system  134  regarding the power requirements of various systems on board the aircraft any a given point in time. By knowing the altitude or the power requirements, control system  134  may determine the power output requirement of generator  106 , and in turn determine the amount of power that must be generated by the gas turbine engine  100 . This information may be used by the control system  134  to control the supercharger  108  such that the gas turbine engine  100  is sufficiently supercharged to generate the necessary amount of power. The cooling air demand regulating system  138  may provide information to the control system  134  regarding the aircraft&#39;s demand for cooling air at a given time. This information may also be used by the control system  134  to control the supercharger  108  such that the load compressor  104  receives a sufficient amount of compressed air to supply the air conditioning and other cooling systems that may be on board the aircraft. 
   The procedure by which the control system  134  may control the supercharger  108  will now be explained in further detail. The control system  134  may be designed to advantageously use the “balanced” design of a gas turbine engine such as gas turbine engine  100 . Gas turbine engines are typically balanced by design, such that they operate with the same atmospheric pressure at the engine exhaust as at the air intake. Thus, as altitude increases and the ambient air pressure decreases, the control system  134  may adjust the backpressure of gas turbine engine  100  to compensate for the pressure decrease. Specifically, the control system  134  may cause the VAN  114  to close or partially close, which will reduce the cross section of the exhaust outlet of the power recovery turbine  100  and cause an increase in backpressure, accordingly. The backpressure may be increased to the level of backpressure that is normally experienced at sea level, by closing the VAN  114  an appropriate amount. This amount may be pre-determined based on altitude and pressure values, and programmed into the control system  134 . The control system  134  may be designed to produce the increase in backpressure at a threshold altitude or at a series of threshold altitudes, which may be determined by the system designer. The increased backpressure may also be initiated based on readings received from pressure sensors  138  and  140 . The control system may utilize algorithms to determine the ratio of altitude to pressure, and activate the VAN  114  at certain threshold ratios. Appropriate timing, based on altitude and pressure ratios, may be determined by a system designer who is skilled in the art, and programmed in the control system  134 . 
   Once the VAN  114  is activated, the increased backpressure at the exhaust of the gas turbine engine  100  will result in a pressure drop between the exhaust outlet of the gas turbine engine  100  and the power recovery turbine  112 . This pressure differential will create a vacuum, causing exhaust from the gas turbine engine  100  to flow through and activate the power recovery turbine  112 . Driven by the power recovery turbine  112 , the supercharger compressor  108  may augment air pressure at the air intake  128  of the gas turbine engine  100 , such that it becomes equivalent to the increased backpressure at the exhaust outlet of the gas turbine engine  100 . This procedure of increasing backpressure to sea level conditions, which instigates an increase in pressure at the air intake  128  such that the air intake  128  is also at sea level conditions, may enable the gas turbine engine  100  to receive the same constant flow of air mass flow per minute that it was designed to receive at sea level. It is to be understood that the gas turbine engine  100  may be any suitable gas turbine engine, and that the control system  134  may be programmed to achieve air mass flow conditions appropriate for whatever gas turbine engine is selected for a particular application. 
   Under control of the control system  134 , the backpressure of the turbine engine  100  may be incrementally increased to approximate typical sea level backpressure conditions as the aircraft gains altitude. Controlling the VAN  114  to create sea level backpressure will in turn control the amount of exhaust that is directed to the power recovery turbine. This may be accomplished under control of the control system  134 , described above. The increase in backpressure at each increment may instigate or augment compressor and turbine stages of supercharger compressor  108 , to supercharge the gas turbine engine  100 . In this manner, i.e. by increasing the amount of exhaust the power recovery turbine  112  it receives from gas turbine engine  100 , the power recovery turbine  112  may be controlled. The control system  134  may variably apply the incremental compressor and turbine stages of the supercharger compressor  108  to the gas turbine engine  100 , adding more and more air volumetrically while producing additional power by use of the power recovery turbine  112 . When power recovery turbine  112  is not in use, such as at sea level, it can be removed from operation of the overall system such as by the bypass duct  118 . 
   Actuating the VAN  114  from full open, to closure in stages, may be the principal means by which power for power recovery turbine  112  is controlled. Closing the VAN  114  produces greater backpressure and thus more power, opening the VAN  114  reduces backpressure and the resultant power. As an aircraft climbs and altitude increases, the VAN  114  may progressively close, which would impose increased backpressure or resistance to the exhaust gas flow from the engine. This in turn would cause a pressure drop from the engine exhaust outlet to the power recovery turbine exit. As explained above, the pressure drop would create a vacuum effect, which would draw exhaust through the power recovery turbine  112 . When the IGV  109  are open or partially open, power recovery turbine  112  may drive supercharger compressor  108  to generate additional air for the gas turbine engine&#39;s air intake  128 . By appropriate scheduling according to altitude and air mass flow conditions, algorithms for which may be programmed into control system  134 , the pressure at the engine exit can be maintained at sea level conditions or incrementally adjusted to sea level conditions, providing incremental pressure drops for the power recovery turbine  112  to produce incrementally larger amounts of power from the remainder of the energy available at a particular altitude. 
   In another exemplary embodiment, the power recovery turbine  112  may comprise a two-stage design. For a two-stage power recovery turbine, both stage nozzles may be VANs for maintaining more favorable pressure drops through the stages and, thus, better overall turbine efficiency. The two VANs may be coupled or individually controlled. Input pressure at high altitude may be sensed by a transducer and sent to the control system  134 . The control system  134  may then open or close the variable area nozzles by pre-programmed amounts, selected as a function of altitude. Because the goal is to keep the engine inlet conditions similar or equivalent to sea level conditions, control logic of the control system  134  may include the coupling effect of the power recovery turbine  112  and its influence on the gas turbine engine&#39;s backpressure and power. 
   In an exemplary embodiment of the control system  134 , control algorithms based upon pre-determined altitude conditions may be programmed in the control logic to adjust VAN  114  (or a combination of two VANs) in relation to altitude. The control logic may be implemented according to the algorithms in order to engage supercharger compressor  108  sufficiently to maintain sea level inlet conditions of gas turbine engine  100 . An example of such conditions for the control logic is provided below. However, it is to be understood that these conditions and instructions may be modified as appropriate to ensure satisfactory operations of the system. 
   In the exemplary scenario, at sea level altitude the IGV  109  may be fully closed, and first and second variable area nozzles may be fully opened. The gas turbine engine  100  and the load compressor  104  may be fed through the bypass duct  118  as the gas turbine engine  100  operates at sea level conditions. At 10,000 feet, the IGV  109  may remain fully closed. The first VAN may be partially closed, keeping sea level conditions at the gas turbine engine exit. The power recovery turbine  112  produces partial power, and the gas turbine engine  100  operates at nearly full load. At approximately 15,000 to 20,000 feet, the IGV  109  may open partially. The first VAN may close further, and the second VAN may also partially close, creating sea level conditions at the engine exit. The supercharger compressor  108  may begin to operate, circulating gas through the intercooler  142  to the gas turbine engine  100  and the load compressor  104 . The power recovery turbine  112  produces partial power. At 20,000 feet, the IGV  109  may fully open. The first VAN may be sufficiently closed to produce sea level conditions at the gas turbine engine exit, and the system may be fully operational at this point. Between 20,000 and 40,000 feet, the system may be fully operational. During this altitude range, the second VAN may progressively close to maintain the optimum pressure drop through the power recovery turbine  112 . Again, it is to be understood that the conditions and instructions described above may be modified as appropriate to ensure satisfactory operations of the system, and that the teachings of control system  134  herein are not to be limited to the exemplary control logic instructions provided. 
   The exemplary control system embodiment described above may be implemented in a variety of system layout designs. As will be recognized by those skilled in the art, changes may be made to the particular examples provided in the foregoing descriptions when constructing a system according to the teachings herein. For example, the supercharger compressor  108  may be downsized to need, producing only sufficient air to supercharge the engine itself. Alternatively, it may be upsized where unabsorbed power produced by an added power recovery turbine  112  is re-directed to the gas turbine engine&#39;s primary power output shaft by coupling the power recovery turbine  112  to the gas turbine engine  100  itself. The power recovery turbine  112  and the supercharger compressor  108  may be allowed to “wind mill” with only a minor power penalty. Alternatively, the power recovery turbine  112  may be uncoupled by means of a clutch enabling it to uncouple from the gas turbine engine  100  at low altitudes. When the power recovery turbine  112  is not in use, it can also be bypassed by ducting such as exhaust duct  118 . Various system layout designs may utilize a gas turbine engine in a turbo-shaft configuration to provide power required for the generator  106  and/or load compressor  104 . The gas turbine engine  100 , a turbine engine, may be either specifically designed for a particular application, or may comprise a COTS turbine engine. The power recovery turbine  112  may be a low pressure turbine, the supercharger compressor  108  may be a low pressure compressor, and the load compressor  104  may be a high pressure compressor. These turbines and compressors may be arranged in various configurations with an intercooler and electrical generator to meet specific systems application requirements at varying high altitudes. 
     FIG. 2  is a schematic illustrating aspects of the system layout design illustrated in  FIG. 1 . A generator  200  and a high pressure compressor (“HPC”)  202  may be driven by a turbo shaft gas turbine engine  204 . A gearbox  206 , included with the gas turbine engine  204 , may be used to provide optimum revolutions per minute (“rpm”) for either the generator  200  or the HPC  202 . A supercharger low pressure compressor (“LPC”)  208  may be driven on a separate shaft by a power recovery low pressure turbine (“LPT”)  210 . The LPT  210  may receive full exhaust from the gas turbine engine  204 . An intercooler  212  may provide cooler inlet temperature air for the gas turbine engine  204 , thereby increasing its power capability and efficiency. Air may enter the LPC  208 , pass through the intercooler  212  and then split to supply both the gas turbine engine  204  and the HPC  202 . Compressed air from the HPC  202  may be discharged to air conditioning equipment  214 . Air leaving the gas turbine engine  204  may pass through the LPT  210 , which in turn may drive the supercharging LPC  208 . The LPT exhaust gas is then discharged to ambient, as illustrated at  216 . 
     FIG. 3  is a schematic illustrating a first alternative system layout design. In this configuration, a generator  300 , LPC  302 , LPT  304  and HPC  306  may all be on a low-pressure spool and driven by the turbine engine exhaust gas from a gas turbine engine  308 . The gas turbine engine  308  may supply the LPT  304  with high pressure and high temperature exhaust gas. Thus, the LPT may provide a moderate to high load driving the generator  300  and the two compressors  302  and  306 . LPT exhaust gas may be discharged to ambient, as illustrated at  310 . An intercooler  312  may cool air produced by the LPC  302 . Cooled air from the intercooler  310  may then split to produce air intake for the gas turbine engine  308 , and compressed air, from the HPC  306 , for air conditioning equipment  314 . 
     FIG. 4  is a schematic illustrating a second alternative system layout design. In this configuration, a generator  400 , LPC  402  and LPT  404  may again be on a low-pressure spool and driven by the turbine engine exhaust gas from gas turbine engine  406 . However, a HPC  408  may be on the gas turbine engine shaft. Gas from a gas turbine engine  406  may drive the generator  400  and the LPC  402  on the low pressure spool. Air from the LPC  402  may pass through an intercooler  410  and then split to supply both the gas turbine engine  406  and the HPC  408 . Compressed air from the HPC  408  may be discharged to air conditioning equipment  412 . Air leaving the gas turbine engine  406  may pass through the LPT  404 , which in turn may drive the supercharging LPC  402 . LPT exhaust gas is then discharged to ambient, as illustrated at  414 . 
     FIG. 5  is a schematic illustrating a third alternative system layout design. In this configuration, a generator  500  may be placed on the shaft of a gas turbine engine  502 . LPT  504  may drive HPC  506  and LPC  508  on the low pressure spool. Air from the LPC  508  may pass through an intercooler  510  and then split to supply both the gas turbine engine  502  and the HPC  506 . Compressed air from the HPC  506  may be discharged to air conditioning equipment  512 . Air leaving the gas turbine engine  502  may pass through the LPT  504 , which in turn may drive the supercharging LPC  508 . LPT exhaust gas is then discharged to ambient, as illustrated at  514 . 
   The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the teachings herein. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the teachings disclosed herein. Thus, the scope of the disclosures herein is not intended to be limited to the embodiments shown and described, but is to be accorded the widest scope consistent with the general principles and novel features disclosed herein.