Abstract:
An anti-flameout safety system for a gas turbine engine is disclosed which prevents engine flameout in the event of ingestion of hail and/or water through the engine intake. The anti-flameout safety system has a detector to detect the water concentration in the engine which generates a signal when the water concentration is at or above a predetermined level and an engine control means which is associated with the detector such that, when the detector generates the signal, the engine control means automatically increases the engine power output.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed toward an anti-flameout safety system for a gas turbine engine, particularly such a safety system which prevents engine flameout in the event of entry of hail or water through the engine intake. 
     Typical gas turbine engines comprise at least one upstream axial compressor which compresses the air taken in through the engine intake and supplies the compressed air to a combustion chamber to be mixed with suitable fuel and burned. Such axial compressors typically have front air intakes for the air flow required for good gas turbine engine operation. However, when operated in adverse atmospheric conditions, the gas turbine engines also may take in significant quantities of hail and/or water such as, for instance, when the aircraft on which the engine is mounted passes through storms or clouds. The hail and/or water taken into the gas turbine engine may cause malfunctioning of the engine. 
     If the gas turbine engine is operating at full power, the compressor raises the temperature of the air passing through it so that any water contained in the air is vaporized. Thus, under these operating conditions, the hail and/or water does not cause extinction of the flame in the combustion chamber, known as flameout. 
     When the aircraft is operating under low engine power conditions, such as, for instance, during a landing approach, the compression ratio of the compressor is small. In this instance, the increase in temperature of the compressed air as it passes through the compressor may be insufficient to vaporize water present in the air so that water may arrive, either in a liquid state or in the form of ice particles, at the combustion chamber and cause flameout of one or more burners, and possibly even of the entire combustion chamber, thereby causing engine flameout. Quite obviously, engine flameout has serious consequences during all aircraft operating conditions, especially during a landing approach. 
     The prior art has attempted to solve the problem of hail or water in the intake gases by placing mechanical obstacles in the path of the air. These devices have included centrifugal separators, scoops, or nose cowls which force the hail or water particles to undergo deflection such that they may be removed from the intake air. However, such devices have proven to be unduly complex and have fallen short of achieving their objectives. 
     Another solution has been to have the aircraft pilot manually operate the throttles to increase engine power when the aircraft passes through heavy rains or storms which may cause engine malfunction due to the high concentrations of water in the intake air. Such a solution has not proven to be effective, since it requires the manual intervention of the aircraft pilot. 
     SUMMARY OF THE INVENTION 
     An anti-flameout safety system for a gas turbine engine is disclosed which prevents engine flameout in the event of ingestion of hail and/or water through the engine intake. The anti-flameout safety system has a detector to detect the water concentration in the engine which generates a signal when the water concentration is at or above a predetermined level and an engine control means which is associated with the detector such that, when the detector generates the signal, the engine control means automatically increases the engine power output. 
     The present anti-flameout safety system is an automatic system which does not require aircraft pilot intervention and one which utilizes data from existing aircraft and engine sensors. The circuitry of the electronics involved in the anti-flameout safety system is simple and economical so as to provide a low cost, automatic, reliable system to prevent engine flameout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the temperature rise in a gas turbine engine compressor versus the rotational engine speed when the air is dry (10 and where it is water saturated (2). 
     FIG. 2 is a graph similar to FIG. 1 showing the combustion efficiency when the air is dry (4) and when the air is saturated with water (5). 
     FIG. 3 is a logic diagram of the anti-flameout system according to the present invention. 
     FIG. 4 is a schematic diagram of the anti-flameout system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The anti-flameout safety system according to the present invention comprises a first detector means for detecting the critical water concentration of the air in the engine based on the fact that the compressor raises the temperature of the air passing through it as shown by the characteristic curves 1 and 2 in FIG. 1. These curves represent the temperature difference ΔT between the compressor outlet temperature T 3  and the intake temperature T 2  as a function of the rotational speed N of the compressor. Curve 1 denotes these conditions when the intake air is dry, while curve 2 denotes the ΔT when the air contains a critical concentration of water. 
     The temperature difference ΔT also depends upon the relative humidity of the air taken in by the compressor. When the compressor compresses water laden air, the compressor partially or totally evaporates the water in the intake air with a ΔT less than if the air were dry. Thus, by measuring the ΔT, the water concentration in the air passing through the compressor can be determined. The anti-flameout safety system according to the present invention has a first detector based on measuring the compressor intake air temperature T 2  and the compressor outlet air temperature T 3  and computes ΔT which equals T 3  -T 2 . The acceptable ΔT will increase as the rotational speed of the engine increases, thus the determined operating point T 3  -T 2  =f(N 2 ). The critical value of the temperature difference, ΔT c , constitutes the minimum ΔT that will permit normal engine operation at a given rotational speed N. 
     The curves 1 and 2 of FIG. 1 are characteristic of each particular gas turbine engine and are empirically determined. Curve 2 determines the maximum water concentration at which the engine will function normally. If the measured operating point, based on ΔT=f(N 2 ), falls below curve 2, in the shaded area 3 of FIG. 1, the measured water concentration will be such that it will effect normal engine operation. In this instance, the detector transmits a signal to an engine control means which raises the engine power which, in turn, increases the rotational speed N and thereby increases the ΔT. 
     A second detector may be operatively interposed between the first detector and the engine control means to provide an extra measure of data input to the anti-flameout safety system. The second detector senses the critical water concentration independently of the first detector and provides confirmation of the information provided by the first detector. 
     The second detector is based upon the fact that beyond a critical water concentration, water which is not vaporized by the compressor will enter the combustion chamber, thereby decreasing the combustion efficiency of the engine. The combustion efficiency is lowered since part of its energy must be provided to vaporize the water. Thus, the second detector measures the combustion efficiency, for instance, by monitoring the relationship: 
     
         T.sub.49 -T.sub.3 =f(Wf/P.sub.3 -kN.sup.2) 
    
     where: 
     T 49  =temperature at turbine outlet; 
     T 3  =temperature at combustion chamber inlet (also at the compressor outlet); 
     Wf=the flow of fuel by weight; 
     P 3  =compressor outlet pressure; 
     N=rotational speed of the turbine and compressor; 
     k=constant. 
     FIG. 2 illustrates this relationship when the gas turbine engine intake air is dry, by curve 4, and when the intake air contains a critical concentration of water, curve 5. Once the operational parameters Wf, P 3 , N, T 49  and T 3  have been measured the values of (Wf/P 3  -kN 2 ) and (T 49  -T 3 ) are then calculated to determine the operational point of the engine. This operational point is compared with the critical value (T 49  -T 3 )c corresponding to the measured value (Wf/P 3  -kN 2 ). If the calculated operational point lies in the shaded area 6, the second detector confirms the signal from the first detector and the engine control means increases the engine power. As can be seen, the second detector provides a redundant, back-up to the first detector to increase the reliability of the system and to prevent erroneous signals from increasing the engine speed. 
     A logic diagram for the anti-flameout safety system according to the present invention is illustrated in FIG. 3 and comprises the first detector 12, the second detector 13 and the engine control means 14. A more detailed schematic diagram of the system is illustrated in FIG. 4. As can be seen in that figure, a signal 8 of a reference rotational engine speed N 0  is determined by the position of the throttle 7, operated by the pilot. The engine computer provides the required information to the fuel metering valve 9 to deliver a fuel flow Wf to match the desired engine operation. The actual rotational speed N is measured and compared with the reference speed N 0  by a comparator 10 to determine the difference ε between the two speeds (N and N 0 ) and to correct the fuel flow Wf in order to achieve an actual engine speed N equal to the reference speed N 0  and thereby to provide regulation of the fuel flow. 
     The temperatures T 3  at the compressor outlet and T 2  at its intake, as well as the engine speed N are measured and provide the input data for the antiflameout safety system according to the present invention. Computer 15 calculates ΔT which is the difference T 3  -T 2  and compares the operating point ΔT=T 3  -T 2  =f(N 2 ) with the critical operating point ΔT c  corresponding to the measured speed N. When ΔT is less than ΔT c  for the measured engine speed N, the computer calculates a Δ corresponding to the difference between the measured ΔT and ΔT c  and then relates this value of Δ to a ΔN which represents the required speed increment of the engine to arrive at ΔT c . 
     This ΔN is compared with ε by a known maximum preponderance circuit 11, which is also called &#34;highest win&#34;, which, in turn, provides the data of the largest value among ε and ΔN to the fuel metering valve 9 to correspondingly increase the fuel flow Wf to increase the engine speed N. 
     The safety system according to the present invention may also include a second detector for detecting the critical water concentration by using the combustion chamber efficiency. In this instance, the second detector operates as a double check to the signal generated by the first detector and eliminates false alarms from any malfunctions of the first detector. 
     The present invention offers the advantages of using parameters already detected by engine and aircraft sensors and which are used for other control purposes of the gas turbine engine, in particular the FADEC electronic control. In order to implement the invention, appropriate preliminary tests will define the critical limits which are characteristic of each engine and the actual engine operation will be compared to these critical limits. 
     Known means for preventing engine hunting and stalling may also be incorporated into the anti-flameout safety system according to the present invention. 
     The foregoing description is provided for illustrative purposes only and should not be construed as in any way limiting this invention, the scope of which is defined solely by the appended claims.