Patent Description:
Modern Brayton and Ericsson cycle engines, including gas turbine engines for aircraft applications, continue to grow more complex. These engines require sophisticated control systems to handle increasing operational demands at reduced tolerances. Such engine control systems command engine actuators for control parameters such as estimated fuel-air ratio rate and variable engine geometries to achieve desired values of output parameters such as net thrust or engine rotor speed. A variety of control methods are currently used toward this end, including model-based control algorithms using predictive models that relate thermodynamic parameters such as flow rate, pressure, and temperature to input and output variables such as overall thrust, power output, or rotational energy.

Engine control systems are typically provided with a plurality of inputs including both current operating parameters and target parameters. Current operating parameters may include engine parameters such as rotor speeds, engine temperatures, and flow rates, as well as environmental parameters such as altitude and inlet total air pressure and air temperature. Some current operating parameters are directly measured, while others may be fixed at manufacture or estimated based on measured parameters. Target parameters may include desired rotor speeds or net thrust values specified according to desired aircraft activities.

In addition to achieving specified target parameters, engine control systems are expected to avoid engine trajectories resulting in engine states that unduly reduce component lifetimes or increase likelihoods of undesired events such as engine surge, compressor stall, or engine blowout. Lean combustor blowout, in particular, occurs when the fuel-air ratio (FAR) in the combustor of a gas turbine engine falls sufficiently that the combustor flame is extinguished. Conventional systems manage FAR indirectly, for example by limiting the fuel-sensed combustor pressure ratio, so as to avoid lean blowout conditions. Gas turbine engine control is disclosed in <CIT>, <CIT>, <CIT> and <CIT>.

The present invention is directed toward a gas turbine engine as defined by claim <NUM> and a method for controlling a gas turbine engine as defined by claim <NUM>.

<FIG> is a cross-sectional view of gas turbine engine <NUM>. Gas turbine engine <NUM> comprises compressor section <NUM>, combustor <NUM>, and turbine section <NUM> arranged in flow series between upstream inlet <NUM> and downstream exhaust <NUM>. Compressor section <NUM> and turbine section <NUM> are arranged into a number of alternating stages of rotor airfoils (or blades) <NUM> and stator airfoils (or vanes) <NUM>.

In the turbofan configuration of <FIG>, propulsion fan <NUM> is positioned in bypass duct <NUM>, which is coaxially oriented about the engine core along centerline (or turbine axis) CL. An open-rotor propulsion stage <NUM> may also be provided, with turbine engine <NUM> operating as a turboprop or unducted turbofan engine. Alternatively, fan rotor <NUM> and bypass duct <NUM> may be absent, with turbine engine <NUM> configured as a turbojet or turboshaft engine, or an industrial gas turbine.

In the two-spool, high bypass configuration of <FIG>, compressor section <NUM> includes low pressure compressor (LPC) <NUM> and high pressure compressor (HPC) <NUM>, and turbine section <NUM> includes high pressure turbine (HPT) <NUM> and low pressure turbine (LPT) <NUM>. Low pressure compressor <NUM> is rotationally coupled to low pressure turbine <NUM> via low pressure (LP) shaft <NUM>, forming the LP spool or low spool. High pressure compressor <NUM> is rotationally coupled to high pressure turbine <NUM> via high pressure (HP) shaft <NUM>, forming the HP spool or high spool.

Flow F at inlet <NUM> divides into primary (core) flow FP and secondary (bypass) flow Fs downstream of fan rotor <NUM>. Fan rotor <NUM> accelerates secondary flow FS through bypass duct <NUM>, with fan exit guide vanes (FEGVs) <NUM> to reduce swirl and improve thrust performance. In some designs, structural guide vanes (SGVs) <NUM> are used, providing combined flow turning and load bearing capabilities.

Primary flow FP is compressed in low pressure compressor <NUM> and high pressure compressor <NUM>. Some portion of primary flow Fp is diverted or bled from compressor section <NUM> for cooling and peripheral systems, and/or to avoid compressor stall. The remainder of primary flow Fp constitutes combustor airflow Fc, the airflow into combustor <NUM>. Combustor airflow Fc is mixed with fuel flow Ff in combustor <NUM> and ignited to generate hot combustion gas. Fuel flow Ff is controlled to avoid violating a lean fuel-air ratio (FAR) limit corresponding to lean blowout, as described in further detail below with respect to <FIG> and <FIG>. Ignited combustion gas expands to provide rotational energy in high pressure turbine <NUM> and low pressure turbine <NUM>, driving high pressure compressor <NUM> and low pressure compressor <NUM>, respectively. Expanded combustion gases exit through exhaust section (or exhaust nozzle) <NUM>, which can be shaped or actuated to regulate the exhaust flow and improve thrust performance.

Low pressure shaft <NUM> and high pressure shaft <NUM> are mounted coaxially about centerline CL, and rotate at different speeds. Fan rotor (or other propulsion stage) <NUM> is rotationally coupled to low pressure shaft <NUM>. Fan rotor <NUM> may also function as a first-stage compressor for gas turbine engine <NUM>, and LPC <NUM> may be configured as an intermediate compressor or booster. Gas turbine engine <NUM> may be embodied in a wide range of different shaft, spool and turbine engine configurations, including one, two and three-spool turboprop and (high or low bypass) turbofan engines, turboshaft engines, turbojet engines, and multi-spool industrial gas turbines.

Operational parameters of gas turbine engine <NUM> are monitored and controlled by a control system including FAR control system <NUM>, described below with respect to <FIG>. FAR control system <NUM> monitors FAR in combustor <NUM>, and controls estimated fuel-air ratio Ff to minimize risk of lean blowout.

<FIG> is a schematic block diagram of a FAR control system <NUM>, comprising gas turbine engine <NUM> and electronic engine control <NUM> with engine model <NUM>, ratio block <NUM>, difference block <NUM>, model based control block <NUM>, and model correction <NUM>. As described above with respect to <FIG>, FAR control system <NUM> predicts and corrects FAR in combustor <NUM> to avoid lean blowout. The logic flow paths indicated in <FIG> reflect one time step in an iteratively repeating real time control process.

Electronic engine control system <NUM> is a digital controller that commands actuators of gas turbine engine <NUM> based on a specified FAR limit FARL, measured engine parameters MEP, and environmental parameters EVP. In particular, electronic engine control system <NUM> commands estimated fuel-air ratio FF via engine control parameters ECP. Model-based control system <NUM> also utilizes calibration parameters (not shown) which are set at manufacture or during maintenance, and which do not vary substantially during engine operation. Measured engine parameters MEP may, for instance, include rotor speeds and sensed pressures and temperatures at inlet <NUM> of LPC <NUM> and at the outlet of HPC <NUM> into combustor <NUM>.

Electronic engine control system <NUM> is comprised of five sections: engine model <NUM>, compressor ratio block <NUM>, difference block <NUM>, model based control block <NUM>, and model correction <NUM>. These logic blocks represent distinct processes performed by electronic engine control <NUM>, but may share common hardware. In particular, engine model <NUM>, ratio block <NUM>, model based control block <NUM>, and model correction <NUM> may be logically separable software algorithms running on a shared processor or multiple parallel processors of a full authority digital engine controller (FADEC) or other computing device. This device may be a dedicated computer, or a computer shared with other control functions for gas turbine engine <NUM>.

Engine model <NUM> is a logical block incorporating a model of gas turbine engine <NUM>. In some embodiments, engine model <NUM> may be a component-level model describing only compressor section <NUM>. In other embodiments, engine model <NUM> may be a system-level model describing the entirety of gas turbine engine <NUM>. Engine model <NUM> may, for instance, be constructed based on the assumption that specific heats and gas constants within gas turbine engine <NUM> remain constant over one timestep. Similarly, engine model <NUM> may incorporate simplifying assumptions that unaccounted pressure losses across gas turbine engine <NUM> and torque produced by cooling bleed mass flow are negligible. The particular simplifying assumptions used by engine model <NUM> are selected for high accuracy during normal modes of operation of gas turbine engine <NUM>, and may not hold during some exceptional operating conditions such as engine surge.

Engine model <NUM> produces a real time estimate of combustor airflow Fc based on environmental parameter EVP, engine measured engine parameters MEP, and engine control parameters ECP corresponding to a previous iteration of the logic process of compressor control system <NUM>. In some embodiments, engine model <NUM> may also estimate limit fuel-air ratio FARL, an optimal or proper FAR selected to avoid lean blowout based on current flight conditions, as described in greater detail below. In further embodiments, engine model <NUM> may concurrently be used to estimate other current state parameters gas turbine engine <NUM> for other (non-FAR) control applications.

Ratio block <NUM> produces estimated fuel air ratio FARE. FARE is the ratio of fuel flow Ff to combustor airflow Fc. As shown in <FIG>, fuel flow Ff may be a commanded fuel flow specified by model based control block <NUM>. Alternatively, fuel flow Ff may be a sensed quantity from among measured engine parameters MEP. Difference block <NUM> takes the difference between estimated fuel-air ratio FARE and a commanded limit fuel-air ratio FARL to produce error E. In some embodiments, FARL may be estimated by engine model <NUM> as shown in <FIG> and described above. In other embodiments, FARL may be retrieved from a lookup table indexed by engine state variables predicted by engine model <NUM> and/or included in measured engine parameters MEP.

Model based control block <NUM> commands actuators of gas turbine engine <NUM> via engine control parameters ECP. Engine control parameters ECP reflect a plurality of engine operating parameters, including fuel flow Ff. In some embodiments, engine control parameters ECP may also include actuator commands for inlet guide vanes, bleed valves, and variable geometry stator vanes to adjust combustor airflow Fc, thereby providing an alternative or additional route to correct combustor fuel-air ratio. Model based control block <NUM> may perform other functions in addition to lean blowout avoidance via FAR control, in which case engine control parameters ECP may include a wide range of additional actuator commands. Model based control <NUM> determines engine control parameters at least in part based on error signal E. In particular, model based control <NUM> specifies commanded fuel flow FF so as to correct for any fuel excess or deficiency indicated by FARE. If estimated fuel-air ratio FARE falls below limit fuel-air ratio FARL, model based control <NUM> will respond to resulting positive error E by adjusting fuel flow Ff upward via engine control parameters ECP.

Engine control parameters ECP are also received by engine model <NUM> in preparation for a next timestep. Model correction <NUM> updates engine model <NUM> for the next timestep, correcting for gradual drift due and deterioration of gas turbine engine <NUM>. With the aid of model correction block <NUM>, the approximation provided engine model <NUM> converges on actual engine behavior sufficiently quickly to ensure that the model remains a good predictor of actual engine values, but sufficiently slowly to avoid tracking noise in measured engine parameters MEP and environmental parameter EVP.

<FIG> is a flowchart of control method <NUM>, an exemplary method carried out by FAR control system <NUM> to avoid lean blowout. Control method <NUM> may be repeated many times during operation of FAR control system <NUM>. Method <NUM> differentiates between first and subsequent passes. In the first iteration of method <NUM>, engine model <NUM> is initialized using measured engine parameters MEP and control values corresponding to a default actuator state of gas turbine engine <NUM>. In subsequent iterations of method <NUM>, engine model <NUM> is updated using engine parameters ECP produced in previous iterations. Engine model <NUM> estimates combustor airflow FC in real time. Ratio block <NUM> computes estimated fuel-air ratio FARE by dividing fuel flow Ff by estimated combustor airflow Fc. Fuel flow Ff may be measured directly, or may be specified by model based control <NUM>. Difference block <NUM> produces error E as a means of comparing estimated fuel-air ratio FARE with limit fuel air ratio FARL. Error E is the difference between estimated fuel air ratio FARE and limit fuel-air ratio FARL. Model-based control block <NUM> computes engine control parameters ECP including fuel flow Ff to correct for error E. Finally, engine control parameters ECP are used both to actuate fuel flow and other engine parameters (Step S8).

FAR control system <NUM> meters fuel flow Ff based on an estimate of FAR derived from a current or previous-iteration value of fuel flow Ff and a realtime model-based estimation of combustor airflow Fc. Model-based estimation of combustor airflow Fc allows improved precision in FAR estimation over prior art indirect management of fuel air ratio FAR by means of pressure sensors. This improved accuracy in turn allows improved transient capability and reduced emissions of gas turbine engine <NUM> by enabling leaner operation of combustor <NUM> without risk of lean blowout.

Claim 1:
A gas turbine engine comprising:
a compressor (<NUM>, <NUM>), combustor (<NUM>), and turbine (<NUM>, <NUM>) in flow series; and
an electronic engine control system (<NUM>) configured to estimate combustor fuel-air ratio based on a realtime model-based estimate of combustor airflow, and command engine actuators to correct for a difference between the estimated combustor fuel-air ratio and a limit fuel-air ratio selected to avoid lean blowout, wherein estimating combustor fuel-air ratio comprises dividing combustor fuel flow by the realtime model-based estimate of combustor airflow, wherein the electronic engine control system generates the limit fuel-air ratio in real time from an engine model, and the engine model is corrected for changes of the gas turbine engine.