GB2122398A - Engine stall early warning system - Google Patents

Abstract

To provide an aircraft pilot with an early and accurate warning of a nonrecoverable rotating stall in a gas turbine engine, a warning system detects engine speed and the gas stream temperature downstream of the combustor, calculates the ratio of speed to temperature, and signals the pilot when the ratio decreases to a predetermined fixed value which, for the engine being monitored, is indicative of a nonrecoverable stall. <IMAGE>

Description

SPECIFICATION Engine stall early warning system This invention relates to gas turbine engine compressor rotating stall warning systems.

Gas turbine engines experience two types of compressor stall: recoverable stall, which is known as "surge", and nonrecoverabie rotating stall, which is known as "stagnation".

These types of stall are well known in the gas turbine engine art and their causes need not be discussed herein. It is sufficient to say that these stalls usually occur during transient engine operation "i.e., during acceleration and deceleration); and are most likely to occur in engines which include augmentors, when those augmentors are brought into or are in operation.

Upon the onset of either nonrecoverable rotating or recoverable stall turbine temperatures and exhaust gas temperatures increase and compressor speed immediately or eventually decreases due to a disruption of the airflow through the engine. The engine may experience multiple recoverable stall surges before it recovers on its own and returns to normal operation. The pilot may experience a noticeable loss of power while this stall condition exists. On the other hand, a nonrecoverable rotating stall condition cannot automatically correct itself and requires the pilot to throttle back and ultimately turn off the engine before excessive damage is done to it by the ever increasing gas temperatures. The pilot must then restart the engine.Once the engine is shut off, the engine speed (i.e., compressor speed) must be reduced to within a predetermined range (unless it is already within that range) and the engine temperature (e.g., turbine inlet temperature) must be allowed to fall below a predetermined value in order that the pilot will have a reasonable chance of successfully restarting the engine.

The sooner the pilot realizes that the engine is in a nonrecoverable stall condition, the better will be his chances of guiding the engine into the preferred restarting region, and the better his chances of being able to restart the engine.

Engines not equipped with a nonrecoverable rotating stall detection system require that the pilot monitor the engine speed gage and the engine temperature gage to decide, based upon those readings and his judgment, whether or not he is in a nonrecoverable condition. Even if the pilot is looking at the gages at precisely the instant that a stall (either rotating or nonrotating) occurs, there will be a delay before temperatures and engine speeds change sufficiently to make him aware of the stall condition. The pilot will also have to wait an additional length of time to ensure that the stall is not of the recoverable type before he makes the relatively drastic decision to shut off the engine. This delay further reduces his chances for successfully restarting the engine.It is, therefore, necessary that any nonrecoverable stall detection system be able to discriminate between nonrecoverable rotating and recoverable stalls in order to avoid having the pilot unnecessarily shut down and restart the engine, which is a dangerous situation, at best.

U.S. Patent No. 3426322 describes a system for detecting a compressor stall, although the type of compressor stall is not discussed in the body of the patent. Basically, in that patent, whenever the exhaust gas temperature is above a predetermined value concurrently with the engine speed being between predetermined upper and lower limits, and that condition exists for a predetermined length of time (10 seconds is given as an example), then a warning signal is produced notifying aircraft personnel that the engine is in a compressor stall condition. Assuming the system is intended to warn of a nonrecoverable stall, it cannot be determined from the patent how well the system discriminates between nonrecoverable and recoverable stalls.One thing is certain, however, the system is not likely to be able to warn the pilot of a stall condition any sooner than the length of the time delay built into the system.

Several other patents which are representative of the state of the art in stall warning systems are U.S. Patents 3867717; 4060980; 4118926; and 4137710, the latter three having the same assignee as the present application.

One object of the present invention is a rotating compressor stall warning system which is able to discriminate between recoverable and non-recoverable compressor stalls.

A further object of the present invention is a nonrecoverable rotating compressor stall warning system which can more quickly and accurately detect the existence of a nonrecoverable stall condition than prior art systems.

According to the present invention, a ratio of the engine speed N to the engine temperature T is calculated, and an output signal is produced indicating a compressor nonrecoverable rotating stall condition when the ratio N T decreases to a predetermined value.

As used in the specification and in the claims, engine speed is the rotational speed of the compressor (either compressor in a twin spool engine), and the engine temperature is the temperature of the gas stream at some point downstream of the combustor.

It is well known that at the onset of most recoverable or nonrecoverable compressor stalls engine temprature begins to increase.

Engine speed may initially increase or decrease; but, if stall conditions persist it will eventually decrease to below its value at the time of stall onset. If the stall is of the recoverable type, the temperature will ultimately stop increasing and begin to fall and the engine speed will stop falling and will increase until, eventually, both return to normal levels. On the other hand, if it is a nonrecoverable rotating stall the temperature will continue to increase and the engine speed will eventually fall off relatively sharply and continue to decrease unless the pilot takes corrective action to eliminate this stall condition.

It has been determined through experimentation that, in a gas turbine engine, the ratio of engine speed to engine temperature N T will generally begin to fall upon the onset of either type of stall condition; but it will turn around and increase within several seconds if the stall is of the recoverable type. If the stall is of the nonrecoverable type, the ratio will continue to decrease further and faster. Because of the clear distinction between the recoverable and nonrecoverable N T signatures, it has been found possible to select a value of N T which is just below the lowest value which can be expected to occur during reoverable stall under all reasonably expected possibilities of operating conditions.When the monitored value of N T decreases to or falls below the selected value, it will be known that a non-recoverable rotating stall condition exists and an output signal can immediately be sent to warn the pilot. For Pratt 8 Whitney's F100 engine the warning system of the present invention is able to reliably detect a nonrecoverable stall condition no longer than about seven seconds from its onset at high altitudes, and considerably faster at lower altitudes where temperatures rise more rapidly. This characteristic provides the pilot with more time available to restart the engine when he needs it at low altitudes.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of the preferred embodiments thereof as shown in the accompanying drawing.

Figure 1 is a schematic and block diagram of a ducted twin spool turbofan engine incorporating the stall warning system of the present invention.

Figure 2 is a graph illustrating an advantage of the present invention over the prior art.

As hereinabove discussed, it is well known in the art that after the onset of either recoverable or nonrecoverable compressor stalls, engine speed N generally declines while the engine temperature T increases. Despite this similarity between these two types of stall, it has been found that the ratio of N T for each type of stall actually has a rather unique signature. More specifically, it has been found that the value of N T during any recoverable stall does not decrease beyond a certain value for N T and the value of N T for all non-recoverable stalls always decreases to below that same value. It has further been determined that, during recoverable stalls, engines reach their lowest value of N T within a very few seconds. From these dis coveries it was realized that a value X for N T may be selected just below the lowest value reached by recoverable stalls; and that value will be a positive indication of the existence of a nonrecoverable stall condition in the engine.

A diagrammatic representation of the stall warning system of the present invention is shown in Fig. 1, wherein a gas turbine engine is drawn schematically and is generally represented by the reference numeral 10. In this particular example the engine 10 is a twin spool augmented turbofan engine having a low compressor 1 2 followed by a high compressor 14. The low compressor 1 2 includes the fan, and is driven by the low turbine 1 6 to which it is connected by a shaft 1 8. The high compressor 14 is driven by a high turbine 20 to which it is connected through a shaft 22. A combustor 24, to which fuel is supplied, provides energy to drive the turbines 16, 20. An augmentor 26 is disposed within the exhaust duct 28 downstream of the turbine 16.The gases which pass through the turbines are expanded through a variable area exhaust nozzle 30.

Still referring to Fig. 1 and in accordance with the present invention, the engine speed N and the engine gas stream temperature T are measured, and signals indicative of the values of N and T are fed to a control 32 which calculates the value of N T and compares it to a predetermined value X.

When the value of N T decreases to the value of X it is then known that the engine is in a condition of nonrecoverable rotating stall, and a warning signal, which may be aural and/or visual is sent to the pilot so that he may take corrective action as soon as possible. The signal may also be sent to the engine control system for automatic corrective action where available. In this example the high compressor speed is used as the engine speed N and the inlet temperature to the low turbine is used as the engine gas stream temperature T.

A value for X of 5.6 was selected for use in the stall warning system designed for the F100 engine after careful analysis of considerable stall data of all types. As in the example of the drawing, the high compressor speed (in RPM'S) was used for the engine speed N, and the inlet temperature (in "F) to the low turbine was used for the engine gas stream temperature T. In one series of sea level engine tests which were conducted to evaluate the effectiveness of the stall detection system of the present invention one hundred four (104) nonrecoverable stalls were induced by various means over a wide variety of engine conditions. Every one of these stalls was detected at N = 5.6.

T Forty seven stalls of the recoverable type were also induced. Only one recoverable stall was falsely detected as a nonrecoverable stall because the value of N T during that particular stall reached a low value of 5.53. That single false detection was not considered a sufficient basis for lowering the value of X, because the conditions imposed on the engine to cause such a stall were more extreme than any which might be expected to occur during actual flight.

In a series of altitude tests forty-four (44) low power and four (4) high power nonrecoverable stalls were induced, and all were detected using a value of X = 5.6. Sixteen (16) low power and fifty-four (54) high power recoverable stalls were also induced, and none were falsely detected as nonrecoverable stalls.

Furthermore, there were no false detections during any normal engine transients, all of which maintained a value of N T well above the detect limit of 5.6.

In all cases tested, it took no longer than seven seconds from stall onset to detect the non-recoverable stalls, with the average time being much less. During the sea level tests discussed above, the average detection time was about one second. These detection times may be compared very favorably to the time it takes a pilot to detect the existence of a nonrecoverable stall in an aircraft having engines which do not incorporate the warning system of the present invention. In such instances the pilot must rely on his temperature and speed gages and his judgment to determine the existence of a nonrecoverable stall.

The time it takes, from stall onset, for the pilot to be convinced that the engine is in a nonrecoverable stall may vary between about 10 and 40 seconds, depending upon a variety of conditions. It is estimated that the warning system of the present invention, when used in the F100 engine, will inform the pilot of the existence of a non-recoverable stall anywhere from about 5 to 35 seconds before he would have recognized that condition himself.

Fig. 2 is a graph which, in a simplified manner, illustrates the advantage which the pilot obtains by receiving an early warning of the existence of a nonrecoverable stall condition. Gas stream temperature T is plotted on the horizontal axis and increases from left to right. Engine speed N is plotted on the vertical axis and increases from the bottom to the top of the page. The heavy solid line labeled A has the constant value N = X.

T The shaded area labeled B represents the engine restart window. If an engine is shut down during flight, its speed and temperature must be brought within the window B for the pilot to have a reasonable chance of restarting the engine. In this graph the window is bounded by a maximum temperature T' and minimum and maximum speeds N', N", respectively. The curve C represents engine conditions after the onset of a nonrecoverable rotating stall, which occurs at point m. In this simplified depiction of a stall, speed decreases and temperature increases as the stall condition continues. At point p the value of N T equals X and a stall warning signal is sent to the pilot in accordance with the teachings of the present invention. In this particular instance the pilot reacts to this warning signal by shutting down the engine as at point s.

Engine temperature immediately begins to fall, along with engine speed, as depicted by the curve D. When the engine conditions cross into the restart window region B, as at the point t, the pilot can attempt to restart restart the engine. Crossing into the region B through the rotor speed line N" gives the pilot the maximum amount ot time to restart the engine and accelerate, such as along the phantom line Q, to a normal engine operating condition.

Curves E and F illustrate what may happen if there were a further delay in shutting down the engine after the onset of stall, such as might occur with no stall warning system or a stall warning system of the prior art. For example, if the pilot did not recognize that he was in a nonrecoverable stall until engine conditions had reached point w, he might not be able to shut off the engine until reaching point z. The pilot must then wait for rotor speed and engine temperature to fall to within the restart region B. In this illustration the delay in recognizing the stall condition almost causes the pilot to miss the restart region completely.

Although the invention has been described in connection with detecting nonrecoverable rotating stalls in an augmented ducted fan twin spool engine, it is useful in any gas turbine engine comprising a compressor, combustor, and turbine in serial relation, whether or not augmented. Also, it is not critical that turbine inlet temperature be used as the gas stream temperature T for the ratio N T , Turbine exit temperature could also be used, as well as any other gas stream temperature downstream of the combustor. It is preferred to measure temperature at a location which responds fastest to the stall condition.With only that criteria in mind, in a twin spool engine it would be most preferred to use the high spool turbine inlet temperature; however, the somewhat lower inlet temperatures to the low turbine spool in are easier to measure and also work well in the present invention.

In a twin spool engine nonrecoverable rotat ing stall it is occurs in the high compressor. For this reason it is preferred to use the high spool speed as the measure of engine speed.

Generally speaking, however, low spool speed could be used. In some twin spool engines, however, there may be the small possibility of a recoverable rotating-type stall occurring in the a low compressor; and such a stall could trigger a false nonrecoverable stall warning if low spool speed were being used as the measure of engine speed N. Thus, in engines which exhibit such behavior it may not be desirable to use low spool speed in the stall warning system of the present invention. The ultimate objective is to provide the earliest possible stall warning without any possibility of error.

Of course, the detection of a nonrecovera- ble rotating stall is only one of any number of abnormal engine to conditions which may be desirable or necessary to detect. For example, it is desirable to warn the pilot of an engine 4 overtemperature which may or may not be associated with a nonrecoverable stall. It may also be desirable to warn the pilot when the engine reaches an abnormally low speed in- dicative of impending engine die-out, which also may or may not be associated with may be a nonrecoverable stall. Thus there may be a turbine inlet temperature Tmax represented by the dotted line G in Fig. 2 above to which some corrective action must be taken to avoid tur- bine damage. And there might be a minimum speed NQ represented by the dotted line H in Fig. 2, below which some corrective action might also have to be taken. The occurrence of these abnormal conditions may be the signaled to the pilot independently of the stall warning signal, or a system logic may be set up which warns the pilot of the first to occur of these conditions (i.e., engine die-out, engine overtemperature, or nonrecoverable stall).

It is well known in the art to provide inflight warning systems with deactivation means to prevent false warning signals during certain non-flight situations such as start-up.

Such deactivation means may be used with the warning system of the present invention, but is not considered to be a feature of the present invention.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that other various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims (14)

1. Stall detection apparatus for detecting nonrecoverable rotating stall in a gas turbine engine of the type having a compressor, combustor and turbine in serial relation comprising: means for detecting engine speed N and for providing a signal indicative thereof; means for detecting the gas stream temperature T within the engine downstream of the combustor and providing a signal indicative thereof; and means for receiving said speed and temperature signals and for calculating the value of N T and for producing an output signal when N T decreases to a predetermined value which is indicative of the existence of nonrecoverable rotating stall in the compressor.

2. The rotating stall detection apparatus according to claim 1 wherein the temperature T is a turbine inlet temperature.

3. The rotating stall detection apparatus according to claim 1 wherein the temperature T is a turbine exit temperature.

4. Stall detection apparatus for detecting nonrecoverable rotating stall in a gas turbine engine of the type having a low compressor, a high compressor, a combustor, a high turbine connected to said high compressor, a low turbine connected to said low compressor, and an augmentor, in serial flow relation, comprising: means for detecting the speed N of one of said compressors and for providing a signal indicative thereof; means for detecting the gas stream temperature T within the engine downstream of the combustor and providing a signal indicative thereof; and means for receiving said speed and temperature signals and for calculating the value of N T and for producing an output signal when N T decreases to a predetermined value which is indicative of the existence of nonrecoverable rotating stall in the compressor.

5. The rotating stall detection apparatus according to claim 4 wherein T is the high turbine inlet temperature.

6. The rotating stall detection apparatus according to claim 4 wherein T is the low turbine inlet temperature.

7. The rotating stall detection apparatus according to claim 5 or 6 wherein N is the speed of the high compressor.

8. A method for detecting nonrecoverable rotating stall in a gas turbine engine having a compressor, combustor, and turbine in serial flow relation comprising the steps of: detecting compressor speed N; detecing gas stream temperature T downstream of said combustor; calculating N T and producing a signal when N T decreases to a predetermined value indicative of nonrecoverable rotating stall.

9. The method according to claim 8, wherein said predetermined value of N T is less than the values of N T which may be generated during recoverable stall conditions.

10. The method according to claim 9 wherein the temperature T is detected at a location between the turbine inlet and the turbine exit.

11. The method according to claim 9 or 10 wherein the engine includes an augmentor.

1 2. The method according to claim 11 wherein the engine is a twin spool augmented gas turbine engine.

1 3. The method according to claim 1 2 wherein the compressor speed N is a high compressor speed.

14. The method according to claim 1 3 wherein the gas stream temperature T is measured at the inlet to the low turbine.