Stall Detector for a Gas Turbine Engine

This invention relates to means for detecting stall in a gas turbine engine and more particularly to a stall detection system that utilizes the existing engine fuel control together with another parameter.

As is well known, stall is a phenomenon that may occur in the compressor of a gas turbine engine which, if allowed to persist unabated, would impair engine performance and/or lead to the destruction of the engine. While the theory of stall is not completely understood, suffice it to say that stall is that effect occasioned when sufficient number of compressor blades stall and a momentary reversing of the airflow occurs through the compressor. This causes compressor discharge pressure to drop very rapidly and sometimes results in continual pressure oscillations until some corrective action is taken.

The art has seen a number of methods intended to either sense when stall is imminent and warn the pilot so that he can take corrective action or design the engine controls such that the area of engine operation where stall is likely to occur is avoided.

For example, fuel controls limit the amount of fuel admitted to the engine during acceleration so as to accelerate along a predetermined acceleration schedule that accounts for stall. Another method, which may be contemporaneously employed with this acceleration scheduling system, is to measure compressor discharge pressure and open compressor bleed valves whenever a predetermined compressor pressure change or rate of change occurs. And still another method which is described in U.S. Pat. No. 3,867,717 and granted to John Theodore Moehring and Vigil Willis Lawson on Feb. 18, 1975 is the utilization of computed compressor pressures and turbine or exit temperatures as a means for determining when stall is present.

While such stall detection and prevention means as described above may be effective for certain engines and/or their applications they are not always effective for other engines and/or their applications. For example, it may happen that under the same values of the computed compressor pressures or their rates and turbine temperatures or their rates another engine operation may occur which would lead to a false indication of stall; or the monitoring of the parameter may not be readily accessible or the inclusion of the sensing probes may interfere with the gas path and impair engine performance. Therefore the selection of the stall controller comes down to what stall system is best for that engine and its applications, what parameters are readily accessible, which system will provide the highest degree of accuracy, which one is fastest and a host of other considerations.

This invention contemplates monitoring the minimum stop position of the engine's fuel metering valve or other indicator of minimum fuel flow and another parameter, namely rate of change of burner or compressor discharge pressure, or the inlet or exit temperature of the turbine or the acceleration schedule produced by the fuel control.

These values will be computed so that two signals will be admitted to an "AND" gate that will produce the output signal indicative of stall whenever the fuel control calls for the minimum fuel flow and any one of the following conditions exist:

1. the decay rate of pressure of either the air discharging from the compressor or in the burner is at a predetermined value, or alternatively

2. the inlet or exit temperature of the turbine is at an abnormally high value, or alternatively

3. the fuel control is on the acceleration schedule.

The stall warning system requires both signals to be at a predetermined condition before the stall warning signal will be manifested.

In any stall control system that senses when stall is present the computed stall signal can either be utilized to provide a warning signal to the aircraft pilot, as by a visual or sound signal so that he can take corrective action as by retarding the power lever or the signal can be utilized to initiate corrective action in one of the following ways.

1. de-rich engine fuel flow

2. shutoff fuel

3. open compressor bleeds

4. change compressor stator vane angle

5. change aircraft inlet geometry

6. change engine outlet geometry

Furthermore the signal can be incorporated in a system that would initiate an automatic stall recovery sequence by shutting-off fuel, start ignition and reinitiate fuel flow in a timed sequence.

An object of this invention is to provide an improved stall warning system for an axial flow gas turbine engine.

A still further object of this invention is to provide a stall warning system that utilizes the minimum fuel flow schedule of the engine's fuel control together with another parameter solely when predetermined conditions of each signal is satisfied.

A still further object of this invention is to provide a stall warning signal upon satisfying the condition that the fuel metering valve of the engine's fuel control is at the minimum fuel stop position and any one of the following conditions is present, (1) the decay rate of P.sub.3 or P.sub.B is at a predetermined value, or (2) TIT or TET is abnormally high, or (3) the fuel control is on or near the acceleration schedule.

Other features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

FIG. 1 is a schematic representation, partially in section of a gas turbine engine and a schematic representation of a stall warning system connected thereto.

FIG. 2 is a graphical representation of a typical engine's operating line and its control functions it terms of W.sub.f /P vs N.

FIG. 3 is a block diagram typifying a fuel control system for providing the control according to the schedule of FIG. 2.

FIG. 4 is a schematic representation of a stall warning system and,

FIG. 5 is a schematic representation of another embodiment of a stall warning system.

For the purposes of this description, the gas turbine engine illustrated in FIG. 1 typifies any number of different types of engines where this invention may be utilized. Such engines may include, as for example, the JT-3D, JT-8D, JT-9D, JT-12, TF-30 manufactured by the Pratt & Whitney Aircraft Division of United Technologies Corporation, but are not limited thereto. Suffice it to say that this invention is applicable where stall is a problem which is generally the case in any axial flow compressor. Also, the fuel control for the purpose of this invention may include any type, be it electronic, hydromechanical or the like that serves to meter fuel to the engine. Such fuel controls may include, for example, the JFC-12, JFC-25, JFC-60, JFC-68 manufactured by the Hamilton Standard Division of United Technologies Corporation or the AJ-H1, CJ-G5, CJ-G7, CJ-G8 manufactured by the Bendix Energy Controls Division of the Bendix Corporation but are not limited thereto.

Essentially, the gas turbine engine exemplified by FIG. 1 is a twin spool axial flow gas turbine engine having an engine casing 10 in which the low pressure compressor 12 and driving turbine 14 and high pressure compressor 16 and its driving turbine 18 are rotary mounted. The burner section 20 burns fuel metered thereto by fuel control illustrated by the block 22 and the generated gases after driving the turbines are exhausted to the afterburner 24 which may or may not be included. In this scheme fuel control 22 which responds to power lever 26 and other engine operating parameters in a well known manner supply fuel through lines 28 and 30 to the burner section of the core engine and the afterburner.

Depending on the particular engine and its application it may include a variable geometry inlet duct 32, variable compressor stators generally illustrated by reference numeral 34, and/or compressor bleeds, generally illustrated by reference numeral 36.

A typical fuel control, as those mentioned above, or the one described in U.S. Pat. No. 2,822,666 granted to S. G. Best and assigned to the same assignee, all of which are incorporated by reference herein, sense certain control parameters and compute them so as to manifest the control schedules depicted by the graph in FIG. 2. As noted, this graph which is a plot of W.sub.f /P.sub.3 or W.sub.f /P.sub.B vs. N.sub.1 or N.sub.2 shows the acceleration line A which serves to prevent the engine from exceeding a predetermined temperature to assure that the engine parts will not exceed their temperature limits, and that the compressor will not operate in the stall region. Also, the control provides the typical deceleration schedule and the minimum W.sub.f represented by the lines B + C, respectively.

In the fuel controls noted above, the acceleration schedule is produced by a three-dimensional cam that responds in one direction to speed of the compressor and in another direction to compressor inlet temperature. The minimum W.sub.f is generally manifested by a stop which prevents the main fuel metering valve from closing-off. Since the pressure drop across the metering valve is held constant, the fuel flow, when on the stop, will be constant.

This best can be seen by referring to FIG. 3 which shows by block diagram the major functions of a typical W.sub.f /P.sub.B type of fuel control. As noted, the speed request function generator 40 responding to the position of the power lever 26 (PLA) which may be biased by total compressor inlet temperature (T.sub.T2) and/or total compressor inlet pressure (P.sub.T2) produces a compressor speed (high compressor N.sub.2) demand signal. Comparator 42 compares the actual N.sub.2 and produces a delta (.DELTA.) N.sub.2 signal indicative of the difference between the demand and actual.

While the controls mentioned above are droop governing this is not a limitation, as isochronous governing may be equally employed without affecting the invention. The gain or droop of the signal produced by the governor and illustrated by box 44 produces an output signal that is related in terms of a W.sub.f /P.sub.B desired schedule signal. This corresponds to the abscissa of the graph illustrated in FIG. 2.

Concomitantly, the computing section of the fuel control monitoring N.sub.2 and T.sub.T2 produces an acceleration schedule 46 and as mentioned above in the case of the hydromechanical controls noted above this is the function of the three-dimensional cam. Both the steady-state W.sub.f /P.sub.B represented by the output of droop slope 44 and the scheduled W.sub.f /P.sub.B acceleration represented by the output of 46 are passed through the minimum selector 48 allowing the lower of the two to pass to the multiplier 50.

Multiplier 50 in a well known manner multiplies either the W.sub.f /P.sub.B droop signal or W.sub.f /P.sub.B acceleration signal by actual P.sub.B to produce a W.sub.f demand signal. The W.sub.f demand signal, in turn, positions the metering valve 52 to meter the amount of fuel dictated by the computer of the fuel control. As noted in box 52 the travel of fuel metering valve is limited by a stop represented by the dash line E.

The description above for both a typical engine and a typical fuel control do not form a part of this invention, except for the fact that existing components may be utilized to execute this invention. Hence, a detailed description of both the engine and fuel control are omitted for the sake of convenience and simplicity, but for further details reference should be made to the literature of the engine and the fuel controls already mentioned herein.

It is apparent from the foregoing that this invention contemplates utilizing the minimum fuel flow of the fuel metering valve of the fuel control. This can be accomplished in one of several ways, as by illustration without limitation, measuring the physcial position of the metering valve, as say when it bears against the minimum fuel flow stop or measuring the servo pressure of the metering valve actuating servo which would also be an indication of when the valve is at the minimum fuel flow, or measuring flow itself. The particular means utilized to obtain this signal will be generally dictated by the particular fuel control model, but it is to be understood that whatever means are employed, this invention contemplates utilizing an existing and accessible signal which either is readily available or can be made available with only minor modifications.

As can be viewed in FIG. 1, the "AND" gate 56 which is commercially available serves to produce an output signal indicative of a stall condition solely when two input parameters are present. One of the parameters, as mentioned immediately above is a signal indicative of when the fuel control is metering the minimum fuel flow or is on minimum flowstop as represented by curve C of the graph of FIG. 2.

Suitable temperature sensor, whether it be thermocouples, pyrometers or other well known means serve to measure TIT or TET and a signal generator 58 responsive thereto will produce an output signal when it senses a value indicative of an abnormally higher temperature which signal will be imposed as the other input signal of "AND" gate 56. The temperature sensor can be located upstream of the turbine (TIT) or downstream (TET) thereof. Some engines come equipped with sensors which can likewise be utilized for this purpose.

FIG. 4 represents still another embodiment wherein the minimum fuel flow stop signal is compared to an excessive decay rate of the compressor discharge pressure (P.sub.3 or P.sub.B). In a twin spool engine as shown, P.sub.3 would be the pressure intermediate the low compressor and high compressor and P.sub.B would be indicative of the pressure of the air discharging from the high compressor. In this embodiment a pressure sensor would be converted to a signal that would measure its rate of change. A suitable rate of change computer represented by box 60 would, in turn, produce an output signal solely when the negative rate shows the pressure decay rate is excessive. When both these conditions exist, that is, the fuel control is on the minimum fuel flow stop and the pressure decay rate is excessive, the "ANd" gate will generate an output signal indicative of a stall.

It may be desirable to hold the stall signal a predetermined time so as to prevent, for example, the stall signal from deactivation if for instance the power lever is pulled to idle or cut-off or speed decay rate stabilizes. A suitable commerically available timer and release mechanism schematically represented by box 57 serves this purpose.

FIG. 5 is another embodiment where the minimum fuel flow schedule or stop position of the fuel control is utilized as a control parameter for the stall detector. Referring to FIG. 5 where like numerals designate previously described elements, "AND" gate 56 receives a minimum fuel flow stop signal from the fuel control.

An on acceleration signal also manifested by the fuel control is the other control parameter and when predetermined conditions of both of those parameters are evident they are presented as inputs to "AND" gate 56, and it in turn, will produce an output signal indicative of when stall is imminent or present.

As can be seen of FIG. 3 inasmuch as the fuel control schedules acceleration means are incorporated to readily adapt existing fuel controls to produce a signal indicative of when it is on or in proximity to the acceleration schedule. For example any of the types of controls noted above, can easily be modified by sensing when the cam follower of the three-dimensional cam is on the acceleration schedule profile or in proximity thereto. As schematically represented in FIG. 3 comparator 66 compares the schedule acceleration signal which is in terms of W.sub.f /P.sub.B and the W.sub.f /P.sub.B signal being imparted to multiplier 50 (the output of minimum selector 48) and when both values are equal will be indicative of when the fuel control is on the acceleration schedule.

It will be apparent from the foregoing that whenever the fuel control is on the acceleration schedule and the metering valve is on the minimum fuel flow stop, an abnormal condition during normal engine operation, stall or imminent stall will result.

In the event the engine shuts off during flight and a restart is made it would be necessary to decouple the stall detector during this operation. Preferably this would be pilot operated by energizing suitable latching and unlatching mechanism represented by box 76.

By virtue of this invention stall detection is manifested by monitoring the metering valve or other means indicative thereof and producing an output signal indicative of when the fuel control is on the minimum fuel flow schedule and monitoring another engine operating parameter and computing it so that whenever the temperature of the turbine inlet or exit is abnormally high, or when the fuel control is on the acceleration schedule or the rate of decay of the compressor pressure is excessive a stall warning signal will be produced.

In certain engines the minimum fuel flow may be set too high under certain operating conditions and the stall signal may be utilized to re-set the minimum fuel flow to a lower value when the compressor is in stall. Otherwise, the amount of fuel permitted by the minimum fuel flow schedule before it was changed could cause the engine to overheat, even to an extent of burning it out.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit or scope of this novel concept as defined by the following claims.