Patent Description:
Fuel metering of an aircraft involves measurement of fuel density, which can change in response to changes in conditions. Temperature, for example, affects the density of fuels used in aircraft engines. The amount of energy contained in a particular volume of fuel is dependent on the density of the fuel. Thus, aircraft fuel systems measure density of the fuel so that accurate metering of fuel to the engines can be performed. Temperature variations, pressure variations and vibrations that are experienced on an aircraft in flight can make accurate measurements of fuel density difficult. <CIT> discloses a pump used as a sensor for determining the density, solids consistency, volume rate of flow, and/or gas content of the liquid or slurry being pumped. Other measurements of fuel density are also disclosed in <CIT> and <CIT>.

A system according to claim <NUM> is provided.

A method according to claim <NUM> is provided.

Apparatus and associated methods relate to simultaneously pumping and measuring density of an aircraft fuel. The aircraft fuel is pumped by a centrifugal pump having an impeller. A rotational frequency of the impeller is determined while the centrifugal pump is pumping the aircraft fuel. Flow rate of the aircraft fuel through the centrifugal pump is sensed. Pressure of the aircraft fuel is measured at two different points within or across the centrifugal pump or a differential pressure is measured between the two different points while the centrifugal pump is pumping the aircraft fuel. Density of the aircraft fuel is determined based on an empirically-determined head-curve relation corresponding to the centrifugal pump. The head-curve relation is empirically determined during a characterization phase. The empirically-determined head-curve relation relates the density of the aircraft fuel to the rotational frequency, the flow rate, and the pressures at the two different points.

<FIG> is a schematic view of an aircraft engine configured to be supplied with aircraft fuel by a fuel system. In <FIG>, aircraft engine <NUM> includes fuel system <NUM>. Fuel system <NUM> includes fuel pump <NUM>, fuel metering unit <NUM>, Fuel densimeter <NUM>, and fuel control module <NUM>.

Fuel pump <NUM> receives fuel from a fuel inlet port connected to a fuel line and pumps the received fuel to an outlet port. Fuel densimeter <NUM> receives the fuel from the outlet port of fuel pump <NUM>, measures the density of the fuel, and provides fuel control module <NUM> a signal indicative of the measured density of the fuel. Fuel control module <NUM> controls the fuel metering unit <NUM>, based at least in part on the measured density of the fuel. Fuel metering unit <NUM> then meters the fuel provided to aircraft engine <NUM> as controlled by fuel control module <NUM>. Fuel densimeter includes a centrifugal pump and pressure sensors configured to measure fluid pressure at various radial distances from an impeller axis.

<FIG> shows cross-sectional views of a densimeter that measures density of aircraft fuel. In <FIG>, densimeter <NUM> includes centrifugal pump <NUM>, first and second pressure sensors <NUM> and <NUM>, and fuel density calculator <NUM>. Centrifugal pump <NUM> has pump casing <NUM> in which resides impeller <NUM>. Pump casing <NUM> has fuel inlet <NUM> through which the aircraft fuel is drawn. In some embodiments, pump casing <NUM> has a fuel outlet through which the aircraft fuel is pumped. The fuel outlet can be used to facilitate circulation of fuel through pump casing <NUM>. Impeller <NUM> is configured to rotate about impeller axis <NUM>. Impeller <NUM> has a plurality of blades 32A-32D. Impeller <NUM>, when rotated, causes the aircraft fuel to be circularly rotated within pump casing <NUM>, thereby creating a pressure differential between fuel located near impeller axis <NUM> and fuel located at a radial periphery of pump casing <NUM>. Impeller <NUM>, when rotated, causes a pressure difference between a first fuel pressure at a first radial distance from the impeller axis <NUM> and a second fuel pressure at a second radial distance from the impeller axis <NUM>.

First pressure sensor <NUM> is configured to measure the first fuel pressure, and second pressure sensor <NUM> is configured to measure the second fuel pressure. Second pressure sensor <NUM> is further configured to measure second fuel pressure in a radial direction at the pump casing. Density calculator <NUM> configured to calculate density of the aircraft fuel based on the first and second fuel pressures as measured by first and second pressure sensors <NUM> and <NUM>.

Densimeter is also depicted in <FIG> as having motor and speed sensor <NUM>. In some embodiments, motor and speed sensor <NUM> can be electrically coupled to densimeter calculator <NUM>, so that densimeter calculator <NUM> can control the rotational speed of impeller <NUM>.

Centrifugal pump <NUM> has pump casing <NUM> in which resides impeller <NUM>. Pump casing <NUM> has fuel inlet <NUM> through which the aircraft fuel is drawn and, in some embodiments, a fuel outlet through which the aircraft fuel is pumped. In the depicted embodiment, fuel inlet <NUM> is aligned near or along impeller axis <NUM>. The fuel outlet, if present, is at a radially distal location of pump casing <NUM> as measured from impeller axis <NUM>. Impeller <NUM> is configured to rotate about impeller axis <NUM>. Impeller <NUM> has a plurality of blades 32A-32D. Impeller <NUM>, when rotated, causes the aircraft fuel to be drawn from fuel inlet <NUM> and to be expelled through the fuel outlet, if so equipped. Impeller <NUM>, when rotated, also causes a pressure difference between first fuel pressure P<NUM> at a first radial distance r<NUM> from the impeller axis <NUM> and second fuel pressure P<NUM> at a second radial distance r<NUM> from the impeller axis <NUM>.

As the impeller is rotated, a centrifugal pump imparts a rotational or circumferential component R to flow of the aircraft fuel being pumped. Because of this rotational component, a radial pressure gradient of the pumped fuel is produced. This radial pressure gradient varies for aircraft fuels of different densities. Therefore, such a pressure gradient can be indicative of the density of the aircraft fuel. For systems in which the first fuel pressure is measured along impeller axis <NUM> (i. e, the radial distance of first pressure sensor <NUM> from impeller axis <NUM> is zero: r<NUM> = <NUM>), such a relation between density D and measured pressures can be given by: <MAT>.

Here, P<NUM> is the first fuel pressure, P<NUM> is the second fuel pressure, r<NUM> is the radial distance of second pressure sensor <NUM> from impeller axis <NUM>, and ω is the rotation frequency of aircraft fuel. The rotational frequency of the aircraft fuel can measured and/or calculated based on a rotational frequency of the impeller as measured by motor and speed sensor <NUM>. In some embodiments, a relation between the rotational frequency of the impeller and the rotational frequency of the aircraft fuel can be based on aircraft fuel dynamics. In some embodiments, the aircraft fuel dynamics of the system are such that the rotational frequency of the impeller and the rotational frequency of the aircraft fuel are substantially equal to one another.

Various embodiments have first and second pressure sensors <NUM> and <NUM> located at various radial distances r<NUM> and r<NUM> from impeller axis <NUM>. For example a ratio of the distance r<NUM> to distance r<NUM> can be less than <NUM>. <NUM>, <NUM>, or it can be <NUM> when first pressure sensor <NUM> is aligned along impeller axis <NUM>.

In the embodiment in <FIG>, centrifugal pump <NUM> is a zero flow pump, having no fuel outlet. For such a zero flow pump, the impeller can be designed to direct the fluid in purely circumferential directions about impeller axis <NUM>. Such circumferential directed impellers can also be used for pumps designed for small flow rates - flow rates that corresponding to operation near zero flow rate as described above. The impeller of such a zero flow pump can have substantial axial mirror symmetry, thereby having to axial direction that is preferential.

In the embodiment depicted in <FIG>, impeller <NUM> is an open vane impeller. An open vane impeller has blades, such as blades 32A-32D extending from a central hub. In some embodiments, impeller <NUM> can be a semi-open vane impeller. A semi-open vane impeller has a plate, which in some embodiments can be substantially circular, affixed to one axial side of impeller blades 32A-32D. In some embodiments, impeller <NUM> can be a closed vane impeller, which has plates on both axial sides of impeller blades 32A-32D.

First pressure sensor <NUM> is configured to measure the first fuel pressure, and second pressure sensor <NUM> is configured to measure the second fuel pressure. Second pressure sensor <NUM> is further configured to measure second fuel pressure in a radial direction at the pump casing. Such a radial directive sensor can have a sensing membrane that has a normal vector aligned with a radial direction from the impeller axis. For example, the radial directive sensor can have a sensing membrane that is substantially conformal with an inside surface of pump casing <NUM>. Such a sensing membrane can deflect, in response to aircraft fuel pressure, in the radial direction that is parallel to the normal vector of the membrane. Fuel control module <NUM> can be configured to calculate density of the aircraft fuel based on the first and second fuel pressures as measured by first and second pressure sensors <NUM> and <NUM>.

In some embodiments, instead of first and second pressure sensors <NUM> and <NUM>, a differential pressure sensor measure a differential pressure between two different radial locations r<NUM> and r<NUM> from impeller axis <NUM>. For example, instead of pressures sensors <NUM> and <NUM> at the radial locations r<NUM> and r<NUM> from impeller axis <NUM>, a differential pressure sensor can be in fluid communication with ports located where pressures sensors <NUM> and <NUM> are depicted in <FIG>.

<FIG> is a graph depicting a relation between measured differential pressure and fluid flow and/or pump speed. In <FIG>, graph <NUM> includes horizontal axis <NUM>, vertical axis <NUM> and relations 46A, 46B, and 46C. Horizontal axis <NUM> is indicative of volumetric flow of a fluid being pumped by a centrifugal pump, such as centrifugal pump <NUM> depicted in <FIG>. Vertical axis <NUM> is indicative of measured differential pressure of the fluid being pumped by the centrifugal pump (for a given fluid density). Relations 46A-46Care indicative of relationship s between the measured differential pressure and the volumetric flow of the pumped fluid for different rotational speeds of the pump impeller, such as impeller <NUM> depicted in <FIG>.

As depicted in <FIG>, for a given rotational speed, the measured pressure differential is not constant. For embodiments in which no fluid flow is caused by rotation of impeller <NUM> (e.g., embodiments having no outlet port), such variable relations are not problematic. In such no-flow embodiments, the only operable point in the relation between measured density and volumetric flow is at the vertical axis where volumetric flow is zero. For embodiments that provide fluid flow via an outlet port, however, rate of fluid flow must be either measured (or otherwise be known), or must be controlled to within a certain range about a target operating point. For example, the flow rate can be maintained near zero by providing a small flowrate, as controlled, for example, by a pinhole orifice in the fuel outlet path. Such a low flow rate can facilitate fuel circulation, while maintaining fluid flow near the target operating point (e.g., near zero). Such a rate of fluid flow can be, for example, a flow rate corresponding to a measured differential pressure being within <NUM>%, <NUM>%, <NUM>%, or <NUM>% of the differential pressure measured for a zero fluid flow rate, for example.

In all embodiments of the invention, such as those which will be disclosed below with reference to <FIG> and <FIG>, a measured flow rate is used in the determination of density of the aircraft fuel. For example, using a known relation between differential pressure and fluid flow and pump speed, accurate determination of aircraft fuel density can be determined based on impeller speed, measured flow rate, and measured differential pressure. Various ways of determining such accurate determinations of aircraft fuel density can be performed using such measured metrics, as will be described below.

<FIG> is a block diagram of fuel system that simultaneously pumps and measures density of aircraft fuel using a single impeller. In <FIG>, fuel system <NUM> includes centrifugal pump <NUM>, first and second pressure sensors <NUM> and <NUM>, and fuel density calculator <NUM>. Fuel density calculator includes processor(s) <NUM>, aircraft interface <NUM>, and storage device(s) <NUM>, and sensor interface <NUM>. Processor(s) <NUM> can receive program instructions 54P from storage device(s) <NUM>. Processor(s) <NUM> can be configured to calculate fuel density, based on received pressure sensor signals and on program instructions 54P. For example, processor(s) <NUM> can be configured to receive pressure sensor signals, via sensor interface <NUM>, indicative of measured fuel pressures P<NUM> and P<NUM>. Processor(s) <NUM> can calculate fuel density based on the received pressure sensor signals and provide the calculated density to other aircraft systems via aircraft interface <NUM>.

As illustrated in <FIG>, fuel density calculator <NUM> includes processor(s) <NUM>, aircraft interface <NUM>, storage device(s) <NUM>, and sensor interface <NUM>. However, in certain examples, fuel density calculator <NUM> and/or fuel system <NUM> can include more or fewer components.

According to the present invention, fuel system <NUM> includes a tachometer configured to measure a rotational velocity of the impeller and optionally a rotational flow measurement sensor. The rotational velocity of the impeller as measured by a tachometer is indicative of the rotational frequency of the aircraft fuel. In some embodiments, fuel density calculator can include a flow regulator configured to regulate, based at least in part on the calculated fuel density, fuel flow of the aircraft fuel. In some examples, fuel density calculator <NUM> can be performed in one of various aircraft computational systems, such as, for example, an existing Full Authority Digital Engine Controller (FADEC) of the aircraft.

Processor(s) <NUM>, in one example, is configured to implement functionality and/or process instructions for execution within fuel density calculator <NUM>. For instance, processor(s) <NUM> can be capable of processing instructions stored in storage device(s) <NUM>. Examples of processor(s) <NUM> can include any one or more of a microprocessor, a controller, a digital signal processor(s) (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry. Processor(s) <NUM> can be configured to perform fuel density calculations.

Storage device(s) <NUM> can be configured to store information within fuel density calculator <NUM> during operation. Storage device(s) <NUM>, in some examples, is described as computer-readable storage media. In some examples, a computer-readable storage medium can include a non-transitory medium. The term "non-transitory" can indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium can store data that can, over time, change (e.g., in RAM or cache). In some examples, storage device(s) <NUM> is a temporary memory, meaning that a primary purpose of storage device(s) <NUM> is not long-term storage. Storage device(s) <NUM>, in some examples, is described as volatile memory, meaning that storage device(s) <NUM> do not maintain stored contents when power to Fuel density calculator <NUM> is turned off. Examples of volatile memories can include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories. In some examples, storage device(s) <NUM> is used to store program instructions for execution by processor(s) <NUM>. Storage device(s) <NUM>, in one example, is used by software or applications running on fuel density calculator <NUM> (e.g., a software program implementing fuel density calculation).

Storage device(s) <NUM>, in some examples, can also include one or more computer-readable storage media. Storage device(s) <NUM> can be configured to store larger amounts of information than volatile memory. Storage device(s) <NUM> can further be configured for long-term storage of information. In some examples, storage device(s) <NUM> include non-volatile storage elements. Examples of such non-volatile storage elements can include magnetic hard discs, optical discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories.

Aircraft interface <NUM> can be used to communicate information between fuel density calculator <NUM> and an aircraft. In some embodiments, such information can include aircraft conditions, flying conditions, and/or atmospheric conditions. In some embodiments, such information can include data processed by fuel density calculator <NUM>, such as, for example, alert signals. Aircraft interface <NUM> can also include a communications module. Aircraft interface <NUM>, in one example, utilizes the communications module to communicate with external devices via one or more networks, such as one or more wireless or wired networks or both. The communications module can be a network interface card, such as an Ethernet card, an optical transceiver, a radio frequency transceiver, or any other type of device that can send and receive information. Other examples of such network interfaces can include Bluetooth, <NUM>, <NUM>, and Wi-Fi radio computing devices as well as Universal Serial Bus (USB). In some embodiments, communication with the aircraft can be performed via a communications bus, such as, for example, an Aeronautical Radio, Incorporated (ARINC) standard communications protocol. In an exemplary embodiment, aircraft communication with the aircraft can be performed via a communications bus, such as, for example, a Controller Area Network (CAN) bus.

<FIG> is a schematic diagram of a fuel system that simultaneously pumps fuel and measures fuel density based on a manufacturer's head-curve relation. In <FIG>, fuel system <NUM> includes centrifugal pump <NUM>, speed sensing arrangement <NUM>, flow-rate sensing arrangement <NUM>, pressure sensing arrangement <NUM>, and processor <NUM>. Centrifugal pump <NUM> includes an impeller (not depicted) configured to pump the aircraft fuel. Speed sensing arrangement <NUM> is configured to determine a rotational frequency of the impeller while the centrifugal pump is pumping the aircraft fuel. Flow-rate sensing arrangement <NUM> is configured to measure flow rate of the aircraft fuel through centrifugal pump <NUM>. Pressure sensing arrangement <NUM> is configured to measure pressure at two different points within or across centrifugal pump <NUM> or a differential pressure between the two different points while centrifugal pump <NUM> is pumping the aircraft fuel. Processor <NUM> is configured to calculate a density of the aircraft fuel based on the manufacturer's head-curve relation. The manufacturer's head-curve relation relates a normalized differential pressure to the rotational frequency and the flow rate. The normalized differential pressure is the differential pressure divided by the fluid density. Using this head-curve relation along with the differential pressure, the rotational frequency, and the flow rate, the fluid density can be determined.

Fuel system <NUM> depicted in <FIG> can determine a fluid density using a head-curve relation pertaining to the specific centrifugal pump <NUM>, using the measured parameters of differential pressure, impeller rotational frequency, and fluid flow rate.

According to an embodiment of the invention, measurements of impeller speed, fluid flow rate, and differential pressure are provided as inputs, and a density of the fluid is calculated using the head-curve relation pertaining to centrifugal pump <NUM>. These measured inputs are used to calculate a theoretical fluid density using the head-curve relation that relates such measured inputs to fluid density, such as, for example, a manufacturer-provided head-curve relation. In some embodiments, temperature of the fluid being pumped by centrifugal pump <NUM> can be measured and used in addition to the above cited measurement inputs for determining the fluid density.

According to the invention, an empirically-determined head-curve relation is used, one that is calibrated for the specific centrifugal pump <NUM> used for each aircraft. The specific head-curve relation is determined during a calibration phase and such measured input parameters are used to determine aircraft fuel density. <FIG> is a schematic diagram of a fuel system that simultaneously pumps fuel and measures fuel density based on a head-curve relation determined by such a calibration step. In <FIG>, fuel system <NUM> includes centrifugal pump <NUM>, speed sensing arrangement <NUM>, flow-rate sensing arrangement <NUM>, pressure sensing arrangement <NUM>, processor <NUM>, and computer-readable memory <NUM>, which contains data indicative of an empirically-determined head-curve relation that is specific to centrifugal pump <NUM>. Centrifugal pump <NUM> includes an impeller (not depicted) configured to pump the aircraft fuel. Speed sensing arrangement <NUM> is configured to determine a rotational frequency of the impeller while the centrifugal pump is pumping the aircraft fuel. Flow-rate sensing arrangement <NUM> is configured to measure flow rate of the aircraft fuel through centrifugal pump <NUM>.

Pressure sensing arrangement <NUM> is configured to measure pressure at two different points within or across centrifugal pump <NUM> or a differential pressure between the two different points while centrifugal pump <NUM> is pumping the aircraft fuel. Processor <NUM> is configured to calculate a density of the aircraft fuel based on the calibrated head-curve relation. The calibrated head-curve relation relates a normalized differential pressure to the rotational frequency and the flow rate of the aircraft fuel. The normalized differential pressure is the differential pressure divided by the fluid density. Using this calibrated head-curve relation along with the differential pressure, the rotational frequency, and the flow rate, the fluid density is determined.

Computer-readable memory <NUM> is used to store data indicative of the calibrated head-curve relation. Computer-readable memory <NUM> can be a smart card, for instance, which can contain data indicative of the empirically-determined head-curve relation specifically pertaining to centrifugal pump <NUM>. The calibrated head-curve relation defines a relation between density of the aircraft fuel and flow rate, speed, and either the pressures of two different points within or across centrifugal pump <NUM> or a differential pressure between such two different points. In the <FIG> embodiment, the differential pressure is measured across centrifugal pump <NUM> between an input port and an output port.

Using the empirical data that characterizes the head-curve relation pertaining to centrifugal pump <NUM> can provide accurate density determinations regardless of the flow rate of the aircraft fuel. Such head-curve relations can be determined during a characterization or calibration phase of fuel system <NUM>. For example, during installation or maintenance of fuel system <NUM>, a head-curve relation can be empirically determined. Such empirical head-curve relations can be tabulated as look-up tables, cubic spline curves, tables, etc..

Empirically determining the head-curve relation of a centrifugal pump can be performed during an installation phase, during a maintenance phase, or during a calibration phase, for example. In some embodiments, a fuel system that simultaneously pumps and measures density of aircraft fuel using a single impeller, such as fuel system <NUM> depicted in <FIG> can be used to empirically determine the head-curve relation of centrifugal pump <NUM>. For example, fluids of various densities can be pumped at various flow rates using various rotational frequencies. The pressures of these various fluids can be measured at the two different points. These data - the measured pressures, flow rates, rotational frequencies, and known fluid densities can be used to determine the specific head-curve relation corresponding to or characterizing centrifugal pump <NUM>. Processor(s) <NUM> can receive the measured pressures, flow rates, rotational frequencies, and known fluid densities and use these to determine a specific head-curve relation that characterized centrifugal pump <NUM>. In other embodiments, a pump characterization system can be used to determine the head-curve relation of centrifugal pump <NUM>.

Claim 1:
A system for simultaneously pumping and measuring density of an aircraft fuel, the system comprising:
a centrifugal pump (<NUM>) including an impeller (<NUM>) configured to pump the aircraft fuel;
a speed sensing arrangement (<NUM>) configured to determine a rotational frequency of the impeller while the centrifugal pump is pumping the aircraft fuel, the speed sensing arrangement comprising:
a tachometer configured to measure a rotational velocity of the impeller, the rotational velocity being indicative of the rotational frequency of the aircraft fuel;
a flow-rate sensing arrangement (<NUM>) configured to measure flow rate of the aircraft fuel through the centrifugal pump;
a pressure sensing arrangement (<NUM>) configured to measure pressure at two different points within or across the centrifugal pump or a differential pressure between the two different points while the centrifugal pump is pumping the aircraft fuel;
one or more processors (<NUM>); and
computer-readable memory (<NUM>) containing data indicative of an empirically-determined head-curve relation corresponding to the centrifugal pump and determined during a characterization phase, the head-curve relation defining a relation between density of the aircraft fuel and flow rate, speed, and either the pressures of the two different points or the differential pressure between the two different points, the computer-readable memory further containing instructions that, when executed by the one or more processors, cause the system to:
calculate, using the empirically-determined head-curve relation during an operation phase, a density of the aircraft fuel based on the rotational frequency, the flow rate, and either the pressures of the two different points or the differential pressure between the two different points.