DETERMINATION OF FUEL DENSITY FOR METERING FUEL IN A FUEL SUPPLY CIRCUIT OF AN AIRCRAFT ENGINE

The invention generally relates to the fuel supply of aircraft engines, and in particular of turbomachines.

It concerns a method for metering fuel, as well as a fuel supply circuit for an aircraft engine.

General technical field and prior art

Aircraft engines are traditionally equipped with a specific fuel metering system, called FMU (for Fuel Metering Unit) or HMU (for Hydro Mechanical Unit) according to the Anglo-Saxon terminologies generally used.

This dosing system performs several functions. It is used to regulate the flow of fuel delivered to the combustion chamber, as well as to cut off the flow of fuel delivered to the combustion chamber (engine shutdown), in particular in the event of overspeed (engine speed exceeding the authorized speed. It ensures further pressurizing the engine fuel system.

Conventionally, a mass flow meter is interposed between this FMU metering system and the device for injecting fuel into the combustion chamber.

The information it provides is fed back to the engine's EEC control computer, but is not used in the engine control loops: it is transmitted by the EEC to the aircraft's computer for display. cockpit.

The metered flow information used in the control of the FMU metering device is generally obtained independently of this flow measurement supplied by the engine flow meter.

The metered flow rate is in fact usually reconstituted as a function of the position of the metering actuator, thanks to a position sensor of the LVDT (Linear Variable Displacement Transducer) type.

The dosed flow values ​​calculated in this way are, however, relatively inaccurate.

The most important inaccuracies are related to the variability of the type of fuel used for combustion, as well as the possible temperature range of use for the fuels.

This is illustrated in FIG. 1 in which different values ​​of fuel density for different types of fuel have been shown as a function of temperature.

As seen in this figure, at low temperatures, the density fluctuation from one fuel to another can be nearly 12.5 percent; it can be nearly 15 percent at high temperatures.

However, it is unusual to find only one type of fuel in all the tanks of an aircraft. The different airports or maintenance centers are not required to fill the tanks with the same fuel as the previous one introduced.

To avoid engine operability problems related to overdosing or underdosing of fuel, particularly during the start-up or deceleration phase, and more generally to avoid any risk of non-ignition, extinction or blockage, the engines are designed and certified to accept several types of fuel to be consumed in combustion, and this over the entire temperature range that the fuels can encounter. It is the same for the fuel circuits as well as the inherent equipment are also designed to be functional for the various possible fuels. They are adapted to the intrinsic properties (density, lubrication power, PCI) thereof.

Thus, the strong inaccuracies on the metered flow rates that variations in the types of fuel and temperature can generate, today require the oversizing of engines and their equipment.

The French patent applications references FR3053396A1 and FR3069021A1 are also known.

General presentation of the invention

A general object of the invention is to provide regulation of the fuel flow with greater metering precision.

Another object of the invention is to allow optimized sizing of the motor and of the power supply circuit and a gain in mass on both.

Another object of the invention is to make it possible to dimension the metering actuator and the elements of the engine independently of the nature and the temperature of the fuel.

An improvement in the metering precision has repercussions on a better dimensioning of the engine and in particular, for example, of the air compression module of the latter.

A more optimized compressor is more efficient and improves the specific fuel consumption of the engine.

This further decreases the amount of fuel to carry for a similar maneuver.

Also, the resultant gain in aircraft mass results in a reduction in the power to be delivered by the engines to provide thrust for the aircraft.

Apart from the reduced engine size, fuel consumption will be lower.

An improvement in metering precision also allows a reduction in the quantity of fuel recirculated in the fuel circuit, limited by the need for engine oil cooling.

Also, the fact that less fuel is recirculated results in a simplification of the fuel system and therefore weight gain.

Thus, according to one aspect, the invention proposes a method for metering fuel into a fuel supply circuit of an aircraft engine, said circuit comprising a metering device for an aircraft engine fuel circuit comprising, downstream of a fuel pumping system and upstream of injectors:

- a fuel inlet,

- a metering device and a cut-off device arranged in series,

- a regulating valve arranged on a fuel recirculation branch, so that fuel supplied in excess by the pumping system is pushed back into the fuel circuit,

At least one flow meter sensor is arranged on the recirculation branch and a density value of the metered fuel is determined as a function of the measurements of said sensor and the metering member is controlled as a function of the fuel density value thus determined.

In particular, a mass flow rate and a volume flow rate are determined as a function of the signals measured by the flow-metric sensor, the metered fuel density value being determined as equal to the ratio between the mass flow rate and the volume flow rate thus determined.

A metered flow can be calculated by subtracting the pumped flow from the recirculated flow seen by the flow meter sensor on the recirculation branch

As a variant, at least one flow-metric sensor is arranged downstream of the metering device and another density value of the metered fuel is determined according to the measurements of this sensor and the metering device is controlled according to this density value fuel, as well as the fuel density value determined according to the measurements of the sensor arranged on the recirculation branch.

The invention further relates to a fuel supply circuit for an aircraft engine.

It relates to an aircraft engine, in particular a turbomachine, comprising such a circuit and an airplane comprising such an engine.

Brief description of the drawings

Other characteristics and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and must be read with reference to the appended figures in which:

- Figure 1 is a graph on which are plotted, for different fuels, the curves of the densities of these fuels as a function of temperature;

- Figure 2 is a schematic representation of an example of general architecture already known fuel supply circuit of the combustion chamber of an aircraft engine;

- Figure 3 is a schematic representation of an FMU metering system associated with a sensor for the implementation of the invention;

- Figure 4 illustrates a general principle for an implementation of the invention;

- Figure 5 illustrates an implementation of the invention by an engine control computer.

Description of one or more modes of implementation and embodiment

Fuel System Reminders

The power supply circuit represented in FIG. 2 corresponds to a conventionally known general architecture. It comprises as standard, in the direction of fuel flow: a low pressure pump LP, a main heat exchanger FCOC or "Fuel OR Exchanger" using the fuel as a cold source, a fuel filter F, a high pressure pump HP, and the FMU fuel metering unit.

The high pressure pump HP is for example a gear pump whose fixed displacement is optimized on the engine speed of the turbomachine at takeoff.

In addition to supplying the combustion chamber, the high-pressure pump HP also supplies fuel to the "variable geometries" GV of the engine, which are equipment or turbomachine components which include moving parts, requiring variable hydraulic power to be drawn off to function.

These equipment or GV components can be of various natures, for example a jack, a servo valve, an adjustable compressor relief valve, a transient compressor relief valve, and/or an air flow adjustment valve for a system clearance adjustment at the top of rotor blades for low pressure turbine or high pressure turbine.

For this purpose, fuel is diverted from the fuel supply circuit, on a "variable geometries" supply branch B, which extends between a node E located between the HP pump and the metering unit FMU and a node C located between the low pressure pump LP and the high pressure volumetric pump HP.

At node E, the illustrated supply circuit includes a self-washing filter FA, to filter the diverted fuel flow fraction. This filter FA is washed by the flow of fuel circulating in the supply circuit towards the fuel metering unit FMU. Branch B may also include, upstream of the GV equipment, a heat exchanger ECT for temperature control of the derived fuel.

The combustion chamber supply circuit also comprises a recovery circuit RE, also called fuel recirculation branch, connecting the fuel metering unit FMU to the supply circuit, between the low pressure pump LP and the FCOC heat exchanger (node ​​C for example). The excess fuel flow supplied to the fuel metering unit FMU can thus be returned, through this recovery circuit RE, upstream of the heat exchanger FCOC, to the main fuel filter F and the pump HP high pressure.

Thus, in operation, the fuel coming from a tank R is sucked in by the low pressure pump LP and pumped into the supply circuit. In this supply circuit, it is first cooled in the main heat exchanger FCOC, and then filtered in the fuel filter F. Downstream of this filter F, it is sucked in by the high pressure pump HP, and pumped, under high pressure, to the connection (node ​​E), in which a fraction of the fuel flow is diverted from the supply circuit to the GV equipment and passes through the auto-washable filter FA.

The rest of the fuel flow passes through the auto-washable filter FA, towards the fuel metering unit FMU, cleaning said filter FA. The FMU unit ensures in particular the metering of the fuel flow supplied to the combustion chamber through the injectors I, via for example a flow meter DMT connected to the control computer EEC and injection filters Fl arranged upstream of the injectors i.

Architecture of an FMU system

The FMU metering system illustrated in FIG. 3 is placed in a fuel supply circuit of an engine, downstream of a fuel pumping system (HP pump) and upstream of the injectors I which it supplies.

A VR regulating valve is located at the inlet of the metering system, on the branch which ensures the fuel recirculation delivery.

This VR regulating valve ensures a constant pressure differential across the terminals of the FMU.

This regulating valve VR - classically called by-pass valve according to the Anglo-Saxon terminology - is a purely passive organ which, thanks to a back pressure of a spring, makes it possible to maintain a certain pressure differential between the inlet of the FMU and the outlet of the SOV .

The spring in the valve (example below) acts against a piston (spool) on either side of which fuel is at different pressures.

The metering of the flow is done by means of a metering device generally called FMV (for Fuel Metering Valve”). This device is controlled by the EEC control computer through a servo valve, which evaluates the metered mass flow Q by the following formula for calculating the flow through an orifice:

Q = Ks * Sjp * DR

where DR is the pressure differential, S the area of ​​the orifice allowing the fuel fluid to pass into the FMV, p the density of said fluid and Ks a parameter linked to the FMV.

Said FMV metering member conventionally comprises a movable drawer, associated with a linear position sensor LVDT (for Linear Variable Differential Transducer) - case illustrated in FIG. 2 - or rotary RVDT (for Rotary Variable Differential Transducer) .

The position of the spool as measured by the LVDT or RVDT sensor is transmitted to the EEC control computer which controls the movement of the spool via a servo valve (FMV EHSV in figure 2): the metered flow depends on the position of the mobile spool, since the pressure differential is kept constant.

At the outlet, the FMU has an HPSOV stop valve (for High Pressure Shut-Off Valve"), which on the one hand makes it possible to pressurize the fuel circuit and on the other hand makes it possible to cut off the injection flow (for example if motor overspeed is detected).

Like the metering device, the HPSOV stop valve comprises an LVDT or RVDT position sensor sending position information to the engine control EEC control computer. The movement of said HPSOV valve is controlled by the computer via an HPSOV EHSV servo valve.

Improved dosing accuracy

The FMU dosing system is also completed by a flowmeter sensor WFM1 placed upstream of the regulating valve VR, on the recirculation loop RE.

This WFM1 flowmeter allows both mass flow measurement and volume flow measurement.

These two pieces of information are processed by the EEC to determine the fuel density by simply dividing the measured mass flow by the also measured volume flow.

The WFM1 mass flow sensor is, for example, a sensor with two rotors of the type described in US patent 3,144,769, or even a drum and wheel sensor as described in patent EP 0,707,199 (Skull technology -https ://www.craneae.com/Products/Fluid/FlowmeterWorks.aspx).

As shown in Figure 4, in the case where the WFM1 sensor includes a drum output (DRUM) and an impeller output (IMPELLER), the rotational speed of the drum and the impeller is proportional to the volume flow Qv, while the time lag DT between the drum and the impeller is proportional to the mass flow Qm.

As shown in Figure 5, the EEC computer receives from the WFM sensor the signals from the solenoids of the latter which follow the rotation of the drum (DRUM) and the rotation of the impeller (IMPELLER).

After filtering (F IMPELLER; F DRUM) and amplification (A IMPELLER; A DRUM), these signals are digitized (A/D). The EEC consolidates the drum (DRUM) and impeller (IMPELLER) signals to derive the volume flow rate Qv, which passes through the WFM sensor. It compares the signals from the drum (DRUM) and the impeller (IMPELLER) to deduce the rotation time shift DT between one and the other, and the mass flow rate Qm (which is sent to the plane's computer (Plane ).

It then calculates the Qm/Qv ratio which corresponds to the fuel density.

Depending on the fuel density thus determined, as well as the opening S of the metering device supplied by the LVDT sensor, the EEC computer calculates the flow rate Q metered by the metering device using the formula:

Q = Ks * S ]p DR

already indicated above.

The EEC computer then determines a DC control current of the FMV EHSV servo valve, in order to regulate the movement of the valve of the FMV metering device so that it corresponds to the metering to be commanded.

In this way, the control of the fuel metering is done with greater precision than in the case of a metered flow as conventionally reconstituted.

In an advantageous mode of implementation, the mass flow sensor downstream of the FMU is also used to access a second flow rate value. This second value provides redundancy.

It will be noted that such a solution allows a sustained throughput throughout the flight without exploration of very low throughputs and without rapid transients.

In particular, the calculated density is not disturbed by changes in engine speed and pressure variations.

Such an improvement in metering precision allows better sizing of the air compression module of the engine. Since the compressor is more optimized, the fuel consumption of the engine is improved. This results in a reduction in the quantity of fuel to be carried, as well as a gain in weight for the aircraft, which results in a reduction in the power necessary to be delivered to ensure the thrust of the aircraft.

Also, as the engine size is reduced, fuel consumption is lower. Better metering also allows a reduction in the amount of recirculated fuel, which leads to a simplification of the fuel system, and again, a gain in weight for the engine.

Also still, the more precise knowledge of the density of the fuel makes it possible to greatly simplify the design of the temperature compensation in the hydraulic block. In the power supply circuit of FIG. 2, the heat exchanger FCOC and the exchanger ECT can be dimensioned less significantly than in the case of previous solutions.

In an advantageous embodiment, the mass flow sensor downstream of the FMU is also used to access a second flow density value. This sensor (WFM2 in FIG. 2) is also of Skull or similar technology. The second density value that it makes it possible to obtain provides redundancy.

As a further variant, if the flow pumped by the pumping system downstream of the FMU is known to the computer EEC, the metered flow to be transmitted to the computer can be calculated by subtracting said pumped flow from the recirculated flow seen by the sensor WFM1. The WFM2 downstream sensor can then be removed.

CLAIMS

1. A method of metering fuel into a fuel supply circuit of an aircraft engine, said circuit comprising a metering device for an aircraft engine fuel circuit comprising, downstream of a fuel pumping system fuel and upstream of injectors:

- a fuel inlet (E),

- a metering device (FMV) and a cut-off device (HPSOV) arranged in series,

- a regulating valve (VR) arranged on a fuel recirculation branch, so that fuel supplied in excess by the pumping system is pushed back into the fuel circuit,

in which at least one flow-metric sensor (WFM1 ) being arranged on the recirculation branch, a density value of the metered fuel is determined as a function of the measurements of said sensor and the metering device is controlled as a function of the fuel density value thus determined.

2. Fuel metering method according to claim 1, in which a mass flow rate and a volume flow rate are determined as a function of the signals measured by the flow-metric sensor, the metered fuel density value being determined as equal to the ratio between the mass flow and volume flow thus determined.

3. Fuel metering method according to one of claims 1 or 2, wherein a metered flow is calculated by subtracting the pumped flow from the recirculated flow seen by the flow meter sensor on the recirculation branch.

4. Fuel metering method according to one of claims 1 or 2, wherein at least one flow-metric sensor (WFM2) being disposed downstream of the metering device, another density value of the fuel metered is determined as a function measurements of this sensor and the metering device is controlled as a function of this fuel density value, as well as of the fuel density value determined as a function of the measurements of the sensor arranged on the recirculation branch.

5. Fuel supply circuit of an aircraft engine, said system comprising a metering device for aircraft engine fuel circuit comprising, downstream of a fuel pumping system and upstream of injectors :

- a fuel inlet (E),

- a metering device (FMV) and a cut-off device (HPSOV) arranged in series,

- a regulating valve (VR) arranged on a fuel recirculation branch, so that fuel supplied in excess by the pumping system is pushed back into the fuel circuit,

- a computer for controlling the metering device and the regulating valve,

in which at least one flow-metric sensor (WFM1) is arranged on the recirculation branch, the computer being suitable for controlling the metering device by implementing the metering method according to one of Claims 1 to 3.

6. Supply circuit according to claim 5, in which at least one flow-metric sensor (WFM2) is arranged downstream of the metering device, the computer being adapted to control the metering device by implementing the metering method according to claim 4.

7. Aircraft engine, in particular turbomachine, comprising a supply circuit according to one of claims 5 or 6.

8. Aircraft comprising an aircraft engine according to claim 7.