TECHNICAL FIELD 
       [0001]    This invention generally relates to aircraft auxiliary power units (APUs), and more particularly, this invention relates to a method and system for accurate oil quantity determination and indication for an APU. 
       BACKGROUND 
       [0002]    The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. Before the engines are to be turned, the APU is started. Once the APU is running, it provides the power to start the aircraft&#39;s main engines. 
         [0003]    APUs are also used to run accessories while the engines are shut down. This allows the cabin to be comfortable while the passengers are boarding before the aircraft&#39;s engines are started. Electrical power is used to run systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing crews to operate hydraulic equipment prior to main engine start up. This function can also be used, on some aircraft, as a backup in flight in case of the loss of main engine power or hydraulic pressure. 
         [0004]    APUs fitted to extended-range twin-engine operations (ETOPS) aircraft are an important backup system, as they supply backup electricity, compressed air and hydraulic power in the highly unlikely, yet postulated event of a loss of main engine power or a failed main engine generator. As such, operators flying ETOPS legs are required to track and record oil consumption rates to ensure the APU is always serviced with sufficient oil for the duration of the flight mission. To support that need, APU controllers will display oil quantity on a flight deck display. 
         [0005]    Certain control systems have the capability of displaying quantity in discrete units, e.g., quarts or liters, or volume; however, displaying a consistent and accurate oil quantity while the APU is running is problematic. This is because these systems inadequately account for APU startup or shutdown gulp. To acquire a consistent oil quantity measurement with sufficient accuracy to permit ETOPS operation requires the aircraft maintenance operators to shut down the APU. Unfortunately, shutting down the APU is not preferred due to the increased time, work and inconvenience to restart the APU, e.g., hook up ground power, get a ground cart, etc. Therefore, aircraft manufacturers and operators are consistently looking for improved APU oil quantity measurement systems to support maintenance activities and reduce aircraft downtime. 
         [0006]    Accordingly, it is desirable to provide methods and systems to determine and indicate an accurate oil quantity of an APU, particularly as the APU is running. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
       SUMMARY 
       [0007]    An APU oil quantity indication system and method establishes oil gulp on APU start up, during operation and at shutdown. As a result, it is possible to determine and indicate APU oil quantity whether the APU is running or off. Furthermore, stable oil quantity indication is possible during engine transients such as startup and rolldown after shutdown. 
         [0008]    In one of the herein described embodiments, an APU oil gulp value may be established at APU startup and shutdown, respectively. The gulp value is combined with a continuous oil quantity indication to provide a stable oil quantity indication under all operating conditions, including transient conditions. 
         [0009]    In still another embodiment, a method of and system for determining and indicating APU oil quantity includes establishing an APU oil gulp value at various phases of APU operation. An APU oil quantity is determined by combining the oil gulp value with a continuous oil quantity value. The method may further include indicating the APU oil quantity value. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The exemplary embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0011]      FIG. 1  is a functional block diagram of an APU lubrication system including an APU oil quantity indication system in accordance with various embodiments; 
           [0012]      FIG. 2  is a functional block diagram of an APU control unit including an oil quantity indication system in accordance with various embodiments; and 
           [0013]      FIG. 3  graphic depiction of APU oil quantity. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following detailed description is merely exemplary in nature and is not intended to limit the application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term system or module may refer to any combination or collection of mechanical and electrical hardware, software, firmware, electronic control component, processing logic, and/or processor device, individually or in any combination, including without limitation: application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
         [0015]    Embodiments of the invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number, combination or collection of mechanical and electrical hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various combinations of electrical components, e.g., sensors, integrated circuit components, memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that embodiments of the present invention may be practiced in conjunction with any number of mechanical and/or electronic systems, and that the systems described herein are merely exemplary embodiment of the invention. 
         [0016]    For the sake of brevity, conventional components and techniques and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the invention. 
         [0017]      FIG. 1  provides a simplified depiction of an APU  10  that includes a turbine engine  12  and a lubrication system  14 . The turbine engine  12  may be coupled to drive various equipment such as generators, pneumatic compressors, hydraulic pumps and the like generically indicated as drive  16 . The lubrication system  14  provides lubricating oil  18  from a sump  20  to the turbine engine  12  and the drive  16 . Lubricating oil  18  may be drawn from the sump  20  by a pump  22  and is communicated to the turbine engine  12  and the drive  16  via a cooler/filter  24 . From the turbine engine  12  and the drive  16 , oil may be returned to the sump  20  via scavenge pump  26  and a cooler/filter  28 . 
         [0018]    The lubrication system  14  includes an oil quantity sensor that provides an oil raw quantity indication in the form of an oil quantity value. For illustrative purposes, an oil quantity sensor  30  is shown disposed in association with the sump  20 . Oil quantity sensing during operation of an APU is known, and any suitable sensor arrangement may be used to provide the oil raw quantity indication. Suitable oil quantity sensing arrangements include capacitive or linear variable differential transformer oil quantity sensors that generate an oil quantity value that is proportional to oil quantity in discrete units such as quarts or liters or in volume. 
         [0019]      FIG. 2  illustrates an APU controller  32  that includes oil quantity sensor  30  and gulp value logic  34 . The output of the sensor  30 , as noted, is a raw oil quantity value  40 . The output of the gulp value logic  34  is a gulp value  42  determined at various stages of APU operation. The raw oil quantity value  40  and the gulp value  42  are combined, e.g., summed, at combining logic  36  to provide an indicated oil quantity value  38 . The indicated oil quantity value  38  may be in quarts, liters, volume or any other suitable set of units. 
         [0020]    In accordance with various embodiments of the invention, the gulp value logic  34  calculates an engine gulp on APU start, while running and at shutdown providing, for example, first, second and third gulp values. For purposes of discussion, and with reference to  FIG. 2 , these values may be designated  42   ST ,  42   L  and  42   SD . Combining the appropriate gulp value for the given operating state with the continuous raw oil quantity value  40  results in an oil quantity indication  38  that is substantially the same value when the APU is off, starting, running or rolling down after shut down, and which furthermore is an accurate indication of actual APU oil quantity. 
         [0021]    When the APU is started, the gulp logic  34  determines a start up gulp value,  42   ST , as a difference between the last oil quantity value  38  and the raw oil quantity value  40  at start up. This value is then combined, e.g., added, into the continuously updated raw oil quantity value  40 . In this regard, the gulp logic  34  will store or reference the last oil quantity value  38 , e.g., as determined at shut down of the APU  10 , and use this value to determine a startup gulp value,  42   ST  corresponding to initial APU  10  startup. During start up and until the APU  10  has reached governed operational speed, the gulp logic  34  will periodically determine the gulp value  42   ST  as a difference between the last oil quantity value  38  and the raw oil quantity value  40 . With the gulp value  42   ST  continuously updated and added back to the raw oil quantity  40 , a smooth and consistent oil quantity display is achieved as the APU goes from off to governed speed. 
         [0022]    Once the APU  10  achieves governed operating speed, i.e., the turbine engine  12  reaches its normal continuous operating speed, the gulp logic  34  latches the gulp value,  42   L , to prevent the gulp term from continuing to increase as the APU  10  consumes oil, i.e. the oil quantity display would not show the effects of oil consumption. Moreover, because it is expected that the APU  10  will consume some amount of oil during operation, the APU controller  32  will latch the oil quantity  38  at APU shutdown and use this oil quantity to calculate the gulp term,  42   SD , during shutdown. This will result in smooth and consistent oil quantity display as the APU  10  goes from governed speed to off. 
         [0023]      FIG. 3  graphically depicts the values  38 ,  40  and  42   ST ,  42   L  and  42   SD  in accordance with there herein described embodiments and operation of the gulp logic  34 . In addition, the speed  46  of the APU turbine engine  12  is depicted to highlight the several states of APU  10  operation. The vertical black line is intended to represent a break in time so that the graphs may better represent the effect of oil consumption due to normal operation. 
         [0024]    At initial APU  10  start up, raw oil quantity indication  40  illustrates a sharp drop corresponding to start up gulp. Correspondingly, the gulp value  42   ST  rises based upon the calculated difference between the oil quantity value,  38 , and the raw oil quantity value,  40 . Once the APU reaches governed operation, the gulp value,  42   L , is latched. With constant gulp value  42   L , the indicated oil quantity  38  decreases with time corresponding to oil consumption during use. After the APU  10  has completed its preparation to shutdown, the gulp value is unlatched and a shutdown gulp value,  42   SD  is established during APU  10  shutdown. 
         [0025]    The gulp logic  34  may be modified to allow the gulp value  42   SD  to increase during APU shutdown to account for the effects of reduced scavenge efficiency. Such a modification may improve the stability of the indicated oil quantity  38  during shutdown processes. Suitable logic would be required to account for an aborted shutdown process. The gulp logic  34  may additionally compensate for temperature expansion, and may further bracket all gulp values, i.e., truncate the gulp value at low and high limits, to prevent anomalous results due to data errors or other transient conditions. The skilled person will further appreciate that filtering and other data smoothing techniques may also be employed. 
         [0026]    In general, there is likely to be some variance between indicated oil quantity value  38  when the engine is running and just post engine shutdown. The quantity of oil that returns to the sump  20 , and thus reflected in the raw oil quantity value  40 , is dependent on oil temperature, age of the oil, de-oil capability, and other factors. If these factors are not accounted for, there may be an indicated oil quantity with some variation.  FIG. 3  shows this condition in which more oil returns to the sump than was gulped on startup and some acceptable variation in indicated oil quantity  38  at shutdown, especially as the variation is greatly reduced as compared to existing systems and methodologies. 
         [0027]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.
An auxiliary power unit (APU) oil quantity indication system and method provides a stable and accurate oil quantity indication during startup, mission duration and shutdown. The system determines a gulp value at various stages of operation, which is combined with a raw oil quantity indication to provide an indicated oil quantity.