Patent Publication Number: US-2012036866-A1

Title: Auxiliary power unit with multiple fuel sources

Description:
BACKGROUND 
     The present invention relates to an auxiliary power unit, and more particularly, to a fuel supply system for an auxiliary power unit that has two or more fuel sources. 
     Auxiliary power units (APUs) are a necessary part of most commercial and military aircraft. APUs are designed to meet aviation power needs during ground operations, when the main engines are not running. APUs provide power for electrical and instrumentation systems, hydraulic systems, and main engine startup, and supply cabin air to the environmental control system. More recently, aircraft have begun to use APUs not just for necessary ground operations but for in-flight functions. Thus, APUs are increasingly configured to operate as standalone sources of accessory power and cabin air, independent of the main engines. 
     A majority of aircraft emergencies that involve the failure of primary power source(s) (e.g., main engine driven hydraulic pumps and/or main engine driven electrical generators) are the result of the failure of the main engine due to jet fuel exhaustion or jet fuel contamination. Regulations require that aircraft have an emergency power source that is independent of the primary power source(s). The emergency power source is necessary to control an aircraft&#39;s flight surfaces in the event of a loss of the primary power sources. 
     Normally a ram air turbine, called a RAT, is used to provide emergency electrical power in the event of gas turbine engine failure. The RAT is an electrical generator or hydraulic pump equipped with a propeller that is commonly mounted within the body of the aircraft. In emergency conditions, the RAT is deployed into the air stream surrounding the aircraft to rotate and generate electrical or hydraulic power for the aircraft&#39;s systems. 
     One consideration associated with the RAT is the additional weight the unit adds to the aircraft. This additional weight may impose a fuel and performance penalty. Considering this, eliminating or reducing the size of the RAT could be advantageous. 
     SUMMARY 
     A system and method are disclosed for supplying fuel to an auxiliary power unit of an aircraft. The system includes a primary fuel source, a secondary fuel source, and a secondary valve. The primary fuel source is in fluid communication with the auxiliary power unit to provide a primary fuel thereto. Similarly, the secondary fuel source is in fluid communication with the auxiliary power unit to provide a secondary fuel thereto. The secondary valve regulates flow of the secondary fuel from the secondary fuel source to the auxiliary power unit. 
     The method provides a secondary fuel from a secondary fuel source to the auxiliary power unit if an emergency operating condition experienced by the aircraft results from either exhaustion or contamination of a primary fuel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of one embodiment of a fuel system for supplying an auxiliary power unit. 
         FIG. 2  is a schematic of another embodiment of the fuel system for supplying the auxiliary power unit. 
         FIG. 3  is a schematic of yet another embodiment of the fuel system for supplying the auxiliary power unit. 
         FIG. 4A  is a flow chart illustrating a method of supplying fuel to the auxiliary power unit. 
         FIG. 4B  is a flow chart illustrating another method of supplying fuel to the auxiliary power unit. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes a fuel supply system for an auxiliary power unit with both primary and auxiliary (secondary or backup) fuel sources. These dual fuel sources greatly increase the probability that fuel will be provided to the auxiliary power unit in the event of an emergency that results from fuel exhaustion or fuel contamination on an aircraft. In the case where the emergency results from fuel exhaustion or fuel contamination, the auxiliary power unit, supplied by auxiliary fuel, can power various crucial pumps and emergency generators that would allow the aircraft to descend and land safely. The fuel supply system with dual fuel sources is additionally beneficial because the weight of the aircraft employing such a system would be reduced by eliminating or reducing the size of the ram air turbine using the auxiliary power unit that already exists for ground operations. 
       FIG. 1  shows an embodiment of the fuel supply system  10 A for the auxiliary power unit  12  (hereinafter, “APU”) of an aircraft  8 . The embodiment of the fuel supply system  10 A shown in  FIG. 1  includes a primary fuel source  14 , a primary APU fuel line  16 , a combined APU fuel line  18 , a primary valve  20 , a primary pump  22 , an (auxiliary) secondary fuel source  24 , a secondary APU fuel line  26 , a secondary valve  28 , and a secondary pump  30 . The APU  12  includes fuel injectors  32 . The fuel supply system  10 A and APU  12  provide an emergency power system for the aircraft  8 . 
     The fuel supply system  10 A is in fluid communication with the APU  12  to provide fuel thereto. The general construction and operation of APUs for aircraft is well-known in the art, and therefore, a detailed discussion herein is unnecessary. As will be described subsequently, the fuel supply system  10 A provides fuel to the APU  12 , which operates to drive various actuators such as hydraulic pumps and/or electrical generators during emergency operation of the aircraft  8 . The APU  12  can also provide power to start the main turbine engines during startup operation of the aircraft  8  in a manner known in the art. 
     The fuel supply system  10 A has two or possibly more fuel sources. For convenience only a single emergency (auxiliary) fuel source  24  is illustrated in addition to the primary fuel source  14 . The primary fuel source  14  comprises the primary fuel tanks of the aircraft  8 . As shown in  FIG. 1 , the primary fuel source  14  communicates a primary fuel to the APU  12  via the primary APU fuel line  16  and the combined APU fuel line  18 , which connect together upstream of the APU  12 . Most commonly the primary fuel utilized for operation of the aircraft  8  comprises a jet fuel such as Jet A or Jet A- 1 . 
     The primary valve  20  is disposed in communication with the primary APU fuel line  16  and the pump  22  regulates the flow of the primary fuel from the primary fuel source  14  to the APU  12 . In one embodiment, the primary valve  20  is responsive to control signals from a Vehicle Management System (VMS)  21  to regulate the flow of the primary fuel. In other embodiments, the primary valve  20  can actuate as result of a loss of primary power thereto (as a result of e.g., a loss of power generation by the primary generator) to regulate primary fuel flow. For example, the primary valve  20  can comprise a ball valve that is actuated hydraulically or with a solenoid. When actuated in response to a loss of primary power thereto (or via control signals from the VMS  21 ), the ball valve has a component that moves from a first position, which allows the primary fuel to pass therethrough, to a second position which halts fuel flow from the primary fuel source  14  to the APU  12 . In other embodiments, the primary valve  20  can comprise any valve (including variable valves) known in the art for regulating fuel flow within an aircraft. 
     The VMS  21  electronically communicates with the APU  12 , the primary valve  20 , the primary pump  22 , the secondary valve  28 , the secondary pump  30 , and a sensor  23  that is disposed in the primary fuel source  14  or primary APU fuel line  16 . The sensor  23  measures if primary fuel is in the fuel system  10 A. The primary pump  22  is disposed in communication with the primary APU fuel line  14  and is responsive to control signals from the VMS  21  to deliver the primary fuel from the primary fuel source  14  to the APU  12 . Similar to the primary valve  20 , the primary pump  22  can comprise any pump known in the art for distributing fuel within an aircraft. In one embodiment, the primary pump  22  can comprise an electric positive displacement fuel pump. In another embodiment, the primary pump  22  can comprise a motive flow fuel pump. 
     The secondary fuel source  24  comprises one or more fuel tanks that are disposed within the aircraft  8 . The fuel tank(s) contain a dedicated supply of secondary fuel for the APU  12  and only supply it to the APU  12  during certain emergency operation conditions aboard the aircraft  8 . As shown in  FIG. 1 , the secondary fuel source  24  communicates a secondary fuel to the APU  12  via the secondary APU fuel line  26  and the combined APU fuel line  18 , which connect together upstream of the APU  12 . In one embodiment, the secondary fuel differs in composition from the primary fuel. In particular, the secondary fuel can comprise a synthetic hydrocarbon blend such as Jet Propellant 10 (JP-10) or a Fischer-Tropsch fuel with a longer storage life than the primary fuel. 
     The secondary valve  28  is disposed in communication with secondary APU fuel line  26  and regulates the flow of the secondary fuel from the secondary fuel source  24  to the APU  12 . Similar to the primary valve  20 , the secondary valve  28  can comprise a ball valve. The secondary valve  28  can be responsive to control signals from the VMS  21  to regulate the flow of the secondary fuel. In other embodiments, secondary valve  28  can be configured to actuate in response to certain emergency operating conditions such as a loss of power to the secondary valve  28  (resulting from e.g., a loss of power generation by the primary generator for the aircraft  8 ) to allow the secondary fuel to pass therethrough and flow from the secondary fuel source  24  to the APU  12 . The secondary valve  28  and the primary valve  20  can be configured to close and open in tandem. This can be in response to control signals indicating certain emergency operating conditions for the aircraft  8  including primary fuel exhaustion or primary fuel contamination. In such situations, the secondary valve  28  is responsive to control signals from the VMS  21  to open and provide flow from the secondary fuel source  24  to the APU  12  and the primary valve  20  is responsive to control signals from the VMS  21  to close and halt primary fuel flow from the primary fuel source  14  to the APU  12 . 
     The tandem opening and closing of the secondary valve  28  and the primary valve  20  can also be the result of a loss of power to both of the valves  20  and  28  from primary power sources. In this case, the loss of power to the primary valve  20  would cause the primary valve  20  to close and halt the flow of the primary fuel, while the loss of primary power to the secondary valve  28  would cause the secondary valve  28  to open and allow the secondary fuel to flow to the APU  12 . 
     The secondary pump  30  is disposed in communication with the secondary APU fuel line  26  and is responsive to control signals from the VMS  21  to deliver the primary fuel from the secondary fuel source  24  to the APU  12 . The secondary pump  30  can comprise any pump known in the art for distributing fuel within an aircraft including an electric fuel pump. 
     The combined APU fuel line  18  directs either the primary fuel or the secondary fuel (depending on the operating state of the aircraft  8 ) to the fuel injectors  32  within the combustor section of the APU  12 . After passing through the fuel injectors  32  the primary fuel or the secondary fuel is ignited to drive the APU  12 . During initial emergency operation of the aircraft  8  prior to and during initial startup of APU  12 , actuation/operation of the primary valve  20 , the primary pump  22 , the secondary valve  28 , and the secondary pump  30  can be powered by alternative power source(s) known in the art including the aircraft&#39;s emergency batteries, a small ram air turbine, and/or emergency generators driven by the primary turbine engines in a windmill condition. As discussed previously, in some embodiments the primary valve  20  and the secondary valve  28  can be configured to actuate to regulate the primary and secondary fuel by the loss of power generation by the main generators of the aircraft  8 . 
     Startup of APU  12  at high altitude can be accomplished in a manner known in the art. In particular, APU  12  or fuel supply system  10 A can be outfitted with an incendiary device as disclosed in U.S. Pat. No. 4,965,995 to Vershure, Jr. et al. which is incorporated herein by reference. Startup of APU  12  could also be accomplished by a jet fuel starter or by utilizing chemical means such as those disclosed in U.S. Pat. Nos. 3,722,217, 3,800,534 and 4,033,115, which are incorporated herein by reference. 
       FIG. 2  shows another embodiment of the fuel supply system  10 B for the APU  12  of the aircraft  8 . The fuel supply system  10 B operates in a manner similar to and has many components identical to that of the fuel system  10 A shown in  FIG. 1 . However, the secondary fuel source  24  of the fuel supply system  10 B comprises a pressurized fuel tank  25 P. 
     In the event certain emergency operating conditions for the aircraft  8  including primary fuel exhaustion or primary fuel contamination, a pressure differential in the system  10 B that results from the pressurized fuel tank  25 P delivers the secondary fuel along the secondary APU fuel line  26  and the combined APU fuel line  18  from the pressurized fuel tank  25 P to the APU  12 . In particular, the secondary fuel within the fuel tank  25 P can be pressurized in a manner know in the art including via pneumatic pressure that pushes upon a bladder carrying the secondary fuel within the fuel tank  25 P or by a spring loaded piston. The pressure exerted upon the secondary fuel within the tank  25 P will depend on the design of the APU  12 . The pressure should be selected to optimize the fuel flow rate into the combustion chamber of the APU  12 . In one embodiment, the pressure should be between about 50 and 100 psi (0.345 and 0.70 MPa) above the pressure within the combustion chamber of the APU  12 . 
     Employing the pressurized fuel tank  25 P eliminates the need for a pump to deliver secondary fuel from the secondary fuel source  24  to the APU  12  in the fuel supply system  10 B. As discussed previously, the secondary valve  28  can be actuated by control signals from the VMS  21 , or by a combination of a loss of primary power to the secondary valve  28  coupled with the pressure in the secondary fuel source  24 . 
       FIG. 3  shows yet another embodiment of the fuel supply system  10 C for the APU  12  of the aircraft  8 . The fuel supply system  10 C operates in a manner identical to and has many components similar to that of the fuel system  10 A shown in  FIG. 1 . However, the secondary APU fuel line  26  and the primary APU fuel line  16  of the fuel supply system  10 C are entirely separated from each other. This eliminates the combined APU fuel line  18  shown in  FIG. 1 . 
     The secondary APU fuel line  26  communicates secondary fuel from the secondary fuel source  24  to secondary fuel injectors  32 S within the APU  12 . Similarly, primary APU fuel line  16  communicates primary fuel from the primary fuel source  14  to primary fuel injectors  32 P within the APU  12 . Thus, the primary fuel and the secondary fuel follow entirely separate flow paths to the primary fuel injectors  32 P and the secondary fuel injectors  32 S, respectively. The configuration of fuel supply system  10 C eliminates the probability that contaminants from the primary fuel will clog all the fuel injectors of the APU  12  making it difficult or impossible to inject secondary fuel into the APU  12  during emergency operation of the aircraft  8 . 
       FIG. 4A  is a flow chart illustrating a method  100  of supplying fuel to the APU  12 . The method  100  can be part of the logic used by the VMS  21  of the aircraft  8  to determine if the APU  12  requires secondary fuel from the secondary fuel source  24  ( FIGS. 1-3 ). 
     The method  100  starts at block  102  and proceeds to query block  104 . In start block  102 , the APU  12  runs with primary fuel. A query block  104  determines whether any emergency operating condition for the aircraft  8  exists. This condition can be ascertained by methods known in the art, some of which include the pilot manually indicating an emergency, the main turbine engine shaft speed fluctuating in a manner associated with a windmill condition, and/or the main engine generators not spinning, and therefore, not producing electrical power. 
     If no emergency condition exists, the method  100  returns to the start block  102 . If an emergency condition exists, the method  100  proceeds from the query block  104  to a query block  106 . The query block  106  ascertains whether the primary fuel in the primary fuel source  14  is exhausted. This can be accomplished using the sensors  23  disposed in or adjacent the primary fuel source  14 . If the query block  106  determines that the primary fuel is still available, and therefore, primary fuel is not exhausted, then the method  100  moves to a query block  108 . If the query block  106  ascertains that the primary fuel has been exhausted, method  100  proceeds to a block  110 . 
     The query block  108  determines if the primary fuel is contaminated. This is determined by, for example, abnormally large or increasingly large fuel pressure drops through the various components of the fuel supply system. Additionally, contamination of the primary fuel could be ascertained by assuming that if the primary fuel is available in the system yet no fuel is reaching the APU  12  then there must be a blockage in the primary portion of the system (e.g., in the primary APU fuel line  16 , the combined APU fuel line  18 , the primary valve  20 , the primary pump  22 , or the fuel injectors  32 ). If the block  108  determines that the primary fuel is not contaminated, the method  100  returns to the start block  102  where the APU  12  runs with primary fuel. 
     If the query block  106  determines that the primary fuel is exhausted, or if the query block  108  determines that the primary fuel is contaminated, then the method  100  moves to a block  110 . In the block  110 , the fuel supply system operates in the manner previously described with reference to  FIGS. 1-3  to provide the secondary fuel to the APU  12 . After the aircraft  8  ceases emergency operation, the block  110  moves to a block  112 , which returns the method  100  to the start block  102  where the APU  12  runs with primary fuel. 
       FIG. 4B  is a flow chart illustrating a second method  200  of supplying fuel to the APU  12 . In some respects, the method  200  operates in a manner similar to the method  100 . The method  200  proceeds from a start block  202 , where the APU  12  is supplied with primary fuel, to a query block  204 . The query block  204  determines whether any emergency operating condition for the aircraft  8  exists. This can be ascertained in the manner previously described. 
     If no emergency condition is determined to exist, the method  200  returns to the start block  202  where the APU  12  is supplied with primary fuel. If an emergency condition exists, the method  200  proceeds from the query block  204  to a block  206 . The block  206  continues to attempt to supply the APU  12  with the primary fuel from the primary fuel source  14  ( FIGS. 1-3 ). The method  200  moves from the block  206  to a query block  208 . The query block  208  determines if a predetermined period of time has elapsed without the APU  12  starting. In one embodiment, the predetermined time period can be between 2 and 5 seconds. If the predetermined time period has not elapsed, the method  200  returns to the block  206 . If the predetermined time period has elapsed and the APU  12  has not started, the method  200  proceeds to a block  210 . In the block  210 , the fuel supply system operates in the manner previously described to supply the secondary fuel to the APU  12 . After the aircraft  8  ceases emergency operation the block  210  moves to a block  212 , which returns the method  200  to the start block  202  where APU  12  is supplied with primary fuel. 
     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.