Abstract:
A control disclosed herein for providing fuel to an auxiliary power unit (“APU”) includes a constant speed electrical motor, a first pump driven by the motor; and, a second pump driven by the motor wherein the electric motor, the first pump and the second pump provide fuel at sufficient pressure/flow capacity to run the APU.

Description:
BACKGROUND 
       [0001]    An auxiliary power unit (“APU”) creates pneumatic power or electrical power to run the air conditioning system, start the main engines and run other accessories on an aircraft. An APU is important to use because the aircraft is not required to use ground power for aircraft air conditioning, to provide electrical power or start the main engines. 
         [0002]    APU Fuel Controls Units (FCUs) are typically shaft driven from the APU gearbox and the fuel is metered based on the APU load by a metering device (i.e., servo valve). Some APU FCUs are driven by a variable speed electrical motor that attempts to meter the fuel demanded by the APU by changing the speed of the pump motor. 
       SUMMARY 
       [0003]    An example control disclosed herein for providing the fuel to an auxiliary power unit (“APU”) includes a constant speed electrical motor, a first pump driven by the motor; and, a second pump driven by the motor wherein the electric motor, the first pump and the second pump provide fuel at sufficient pressure to start the APU. 
         [0004]    According to a further example provided herein a method for providing fuel to an APU includes providing a constant speed electrical motor; providing a first pump driven by the motor; providing a second pump driven by the motor; and driving the electric motor at a constant speed such that the first pump and the second pump provide fuel at sufficient pressure to start and to operate the APU. 
         [0005]    These and other features of the present disclosure can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a schematic view of a prior art APU fuel system. 
           [0007]      FIG. 2  is a schematic view of a simple and reliable APU fuel system as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Referring to  FIG. 1  a prior art, fuel control unit (“FCU”)  10  for an APU  15  is shown. Generally, fuel must be pumped from a fuel tank  17  to a first pressure and then a higher pressure for use at APU fuel nozzles  20 . The higher pressure is required to provide proper atomization for fuel entering the nozzles  20 . 
         [0009]    Fuel passes from the fuel tank  17  to a boost stage pump  25 . The boost stage pump  25  is driven off an APU gearbox (not shown) and pressurizes the fuel and sends it to a fuel filter  30  via lines  35  and  40 . After passing through the fuel filter  30 , the fuel is delivered to the high pressure pump  45  via line  50 . The boost stage pump  25  and the high pressure pump  45  are ganged together by shaft  55 . After passing through the high pressure pump  45 , the fuel is at a proper pressure for delivery to the nozzles  20 . The fuel then passes through a servo valve  60  which is controlled by a controller  65  to meter flow through lines  70  and  75 . A valve  67 , also controlled by controller  65 , is disposed downstream of the servo valve  65  and acts as an emergency shut off. The fuel lines  70  and  75  include a flow divider  90  to apportion fuel to a simplex nozzle  80  or duplex nozzle  85  within the APU  15 . The flow divider  90 , as is known in the art, uses a ball valve  97  that gives way according to the fuel pressure against the spring  95  to provide fuel to the simplex nozzle  80  or to the duplex nozzle  85 . 
         [0010]    Fuel passing through the high pressure pump  45  may be diverted if the pressure becomes too high through the pressure relief valve  100 , which again is a standard ball valve to recirculate fuel through the fuel filter  30 . If the fuel filter  30  becomes clogged and pressure backs up there, fuel may be diverted around the fuel filter  30  through a pressure relief valve  105  via lines  40 ,  110  and  115 . 
         [0011]    During start up of the APU  15 , the shaft driven FCU  10  needs to generates enough flow capacity with relatively high pressure for good atomization at the nozzles  20 . However, when an FCU  10  is physically sized for the start condition, such an FCU  10  will generate considerably more flow capacity than needed when the APU  15  is running at normal operating speed. This excess fuel flow which is typically about 300%-500% of what is needed, is recirculated back to the high pressure pump  45  and the boost stage pump  25  through the pressure relief valve  100  back through lines  35 ,  40  and  50 . The amount of fuel recirculation back into the inlet of the pump increases even more when the APU  15  is operating at no load at high altitude conditions. This in turn may cause fuel overheating that may be difficult to resolve. Shaft driven FCUs also contain seals (not shown) at the gearbox interface. In time due to rubbing action, these seals will wear and can cause external oil and/or fuel leakage that impact the safety and reliability. An FCU  10  with external leakage will then need to be replaced. 
         [0012]    Note that the FCU  10  shown in  FIG. 1  may provide fuel flows that are 300% to 500% higher than required by the APU  15 , making it an inefficient design when it comes to power consumption. 
         [0013]    Further, prior art systems (not shown) have been designed with variable speed electrical motors (not shown) that drive pumps (not shown) such that fuel flow is metered by speeding and slowing the motor driving the pump. However, such systems require expensive motors and sophisticated motor controllers for precise motor speed control with very fast response time to be able to manage the rapid required transient response necessary for an APU. 
         [0014]    Referring now to  FIG. 2 , instead of driving the shaft  55  off an APU gearbox (not shown), the shaft  55  is now driven by a constant speed electric motor  200 . The motor  200  may be semi-hermetic with no dynamic seals to wear. Since during APU starting, the electric motor  200  of FCU  210  is at 100% speed and independent of the actual APU speed, the physical size of the pump  25 ,  45  can be substantially smaller as compared to the shaft driven FCU  10 . The FCU  210  is typically sized to deliver the maximum fuel demanded by the APU  15  plus a slight margin for engine/pump deterioration. This margin may be as high as 20% or more. The electric motor  200  may be AC induction, DC brushless, switch reluctance or other types. The electric motor  200  may be single speed or a multiple speed motor. The electric motor  200  may be low voltage or high voltage and might be powered during the APU  15  start by the aircraft battery or the APU generator and may be designed to have its input power switched to another source such as an APU driven alternator or other external power. Prior to APU  15  cranking for the start, the electric motor  200  starts to full speed within seconds and generates the proper fuel pressure and flow demanded by the APU controller  65  for proper combustion ignition through the nozzles  85 . During the normal operation, the excess flow will be recirculated similar to the existing FCUs. However, this quantity of recirculation is substantially less than the existing mechanically driven pumps  25  and  45  as shown in  FIG. 1 . Moreover, this FCU  210  does not require a complicated variable speed motor or a motor controller with very fast response time for precise motor speed control. If the APU  15  is running by using FCU  10  that utilizes the gearbox driven boost stage pump  25  and high pressure pump  45 , the excess fuel flow may be over 300% to 500% of fuel flow needed, which is not only inefficient but also may cause fuel overheating when the fuel demand is low. 
         [0015]    In contrast, the FCU  210  that uses the constant speed electrical motor  200 , the maximum over pumping at the same operating conditions is about 20%. As such, the drain on the APU  15  to drive the electric motor  200  is less than the power required to drive the shaft driven FCUs. The electric motor  200 , which is independent of APU speed, provides higher start reliability, better energy efficiency due to little recirculation during full APU speed, and no dynamic seals for enhanced reliability and safety. There is less drag on the APU gearbox during cold starts which increases APU start torque margin. 
         [0016]    Although preferred embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.