Abstract:
Methods and apparatus for reducing temperature overshoot in an engine, such as in a gas turbine aircraft engine, are described. In an exemplary embodiment, an unmeasured temperature to be regulated in an engine is determined by measuring a temperature in the engine wherein the measured temperature being related to the unmeasured temperature, determining a bias of the measured temperature wherein the bias being an amount estimated to be a difference between the measured temperature and a steady state measured temperature and adding the bias to the measured temperature to restore the relationship between measured and unmeasured temperature so that the unmeasured temperature may be properly regulated. The bias is determined using a heat transfer model.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     The government has rights in this invention pursuant to Contract No. F33657-97-C0016 awarded by the Department of the Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine engines and, more particularly, to reducing temperature overshoot in such engines. 
     In a closed system, an unmeasured temperature sometimes is regulated by a related and measured temperature. The characteristics of the unmeasured temperature differ from the measured temperature as a result of differences in thermal constants, flow fields, and thermal time constants of the measured temperature medium. As a result, the unmeasured temperature may exceed a pre-defined maximum temperature or fall below a pre-defined minimum temperature. 
     To maintain an unmeasured temperature within a predefined range, anticipation methods can be utilized. Generally, anticipation methods attempt to reduce temperature overshoot following a rapid change in temperature. Such anticipation methods sometimes are referred to as rate-based lead-lag anticipation. An amount, or magnitude, of measured temperature anticipation is dependent upon the rate at which the measured temperature signal changes. Thermal states are not taken into account when determining an amount of anticipation. Therefore, the same anticipation is utilized for both cold and warm thermal states. 
     As a result, too much anticipation may be provided for a warm thermal state and too little anticipation may be provided for a cold thermal state. For a cold thermal state, the anticipation decays in a matter of seconds when the temperature overshoot can last for a much greater time (e.g. longer than one minute). 
     As one specific example, and in at least one known gas turbine aircraft engine, a gas temperature T 41  overshoot occurs when the gas temperature T 41  is controlled by a measured high pressure turbine (HPT) metal temperature T 4 B. The gas temperature T 41  overshoot following a cold burst from idle to full power decays over a span of one minute. The T 41  overshoot characteristic is caused by a changing relationship between the measured T 4 B metal temperature and actual T 41  gas temperature. The relationship between the T 41  and T 4 B temperatures is altered as a result of the greater cooling effectiveness of HPT blades when the engine bore is cool (heat-soak) at idle as compared to when the rotor is warm at full power. When the engine is cool, the HPT cooling air releases heat to various metals as it passes through the engine bore. The cooled air biases the T 4 B measurement low and allows the unmeasured T 41  gas temperature to increase. Component life can be extended, and life cycle cost can be reduced, by reducing such overshoot. 
     BRIEF SUMMARY OF THE INVENTION 
     Methods and apparatus for reducing temperature overshoot in an engine, such as in a gas turbine aircraft engine, are described. In an exemplary embodiment, an unmeasured temperature to be regulated in an engine is regulated by measuring a temperature in the engine wherein the measured temperature being related to the unmeasured temperature, determining a bias of the measured temperature wherein the bias being an amount estimated to be a difference between the measured temperature and the measured temperature without cooling air beat-soak (e.g. steady state), and adding the bias to the measured temperature to restore the relationship between measured and unmeasured temperature so that the unmeasured temperature may be properly regulated by the measured temperature and estimated steady state measured temperature. The bias is determined using a heat transfer model. By using a heat transfer model to determine the bias, and then adjusting the measured temperature based on the bias, temperature overshoot can at least be reduced to facilitate extending component life and reducing life cycle costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of a simplified anticipation algorithm; 
     FIG. 2 is an example flow diagram of an anticipation algorithm for determining a cooling factor due to heat-soak; and 
     FIG. 3 is a flow diagram for determining P3CALC, T3CALC, and W25CALC values used in the flow diagram shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a flow diagram of an exemplary anticipation system  10 . System  10  includes a heat transfer model  12 . A delta cooling temperature  14  is generated by multiplying  16  an output of model  12  by negative one. Delta cooling temperature  14  is then added  18  to the measured temperature  20  to generate a regulated temperature  22 . Regulated temperature  22  is then input to a temperature regulator (not shown) and utilized to maintain the unmeasured temperature within a selected range. 
     Model  12  is configured to generate a heat transfer factor. The heat transfer factor is generated using total airflow  24 , cooling airflow  26 , and cooling airflow temperature  28  as inputs. Specifically, total airflow  24  is supplied to a lookup table  30  to determine a cooling air effectiveness  42 . Cooling airflow  26  is multiplied  32  by a constant  34 , and the product is then multiplied  36  by a tuning constant  38  to get an airflow product  44 . Tuning constant  38  is lumped heat transfer coefficients. Cooling airflow temperature  28  is multiplied  40  by airflow product  44  and the resulting product is multiplied  16  by cooling air effectiveness  42  to generate a metal temperature bias. The metal temperature bias is also multiplied  16  by negative one as described above. 
     Although one specific embodiment of a heat transfer model is described above, many different heat transfer models could be utilized depending upon the particular cooling/heating adjustment to be made to a measured temperature relative to an unmeasured temperature. Generally, by using a heat transfer model to determine a delta cooling temperature, and then adjusting the measured temperature based on the delta cooling temperature, temperature overshoot can at least be reduced to facilitate extending component life and reducing life cycle costs. 
     FIG. 2 illustrates a flow diagram of a specific heat transfer model  50  for determining a DT 4 B cooling factor due to heat-soak. The DT 4 B cooling factor is then added to the measured T 4 B temperature for input to a temperature regulator. 
     More specifically, the logic of model  50  predicts cooling air beat-soak and determines the anticipation for the T 4 B regulator. The HPT blade temperature (T 4 B) is used to regulate the flow path gas temperature (T 41 ). The relationship between T 4 B and T 41  is changed by the cooling air heat-soak, or bias. Estimating the cooling air heat-soak bias improves the T 4 B anticipation and decreases T 41  overshoot. The logic of model  50  generates an anticipation that accounts for the cooling air heat-soak. Also, with the heat-soak based anticipation logic, thrust overshoot for a cold rotor burst or undershoot when too much anticipation is present such as a hot rotor burst are reduced and thrust quickly settles in at a steady state level. 
     The logic of model  50  also provides cold-rotor anticipation while eliminating over anticipation on a hot-rotor re-burst. Specifically, a cold-rotor burst occurs during take-off and go around. During this time, the bore of the engine is cool. The cooling air for the HPT turbine blade passes through the cool engine bore and gives up heat to the exposed metal. The cooled air biases the T 4 B temperature low. 
     A hot-rotor re-burst occurs while the bore of the engine is still warm when power level angle is increased. Because there is little difference between the bore metal and cooling air temperature, the relationship between T 41  and T 4 B is only affected slightly. Logic of model  50  recognizes this situation and applies only a small anticipation. As a result, the overshoot is reduced to a spike that very quickly fades to almost steady state. With T 41  properly regulated, thrust overshoot and “engine rollback” are reduced, providing near steady-state levels. 
     Calculated values supplied to model are W25RCALC, PIT3CALC, and T3CALC. A model  100  for determining these calculated is described below. Model utilizes these calculated values, as well as a selected value, i.e., PT25SEL, to determine the DT 4 B cooling factor. 
     To reduce complexity and fault accommodation considerations, it is desirable to reduce complexity of model  50 . Boxes  52  and  54  in FIG. 2 illustrate processing that can be eliminated through simplification. For example, the complex cooling airflow calculation may be replaced with the engine total airflow multiplied by a scalar. Use of this simple expression eliminates the T 3 /P 3  functionality and the need for the calculated value for P 3 . 
     Also, Taudisk  56  is a time constant for the bore metal such as the turbine shafts, rotors, and other metals in the cooling flowpath, and the complex expression is a function of the heat transfer coefficients of air, density of air, cooling airflow, and effective masses. A simplified method of making the calculation reduces to just a function of cooling airflow, which reduces computational complexity by eliminating fractional exponents. 
     Further, tuning constant  38  where hA/mc p  equals approximately 0.55 is used in determining the temperature of the cooling air (where h is a convection heat transfer coefficient, A is heat transfer area, m is mass, and c p  is specific heat.) This constant has the largest influence of any curve or constant. This constraint may be used to adjust T 4 B anticipation to balance between T 41  overshoot, acceleration times, and thrust rollback. 
     FIG. 3 is a flow diagram for determining P3CALC, T3CALC, and W25CALC values used in the flow diagram shown in FIG. 2. A box  102  in FIG. 3 identifies processing that can be eliminated through simplification of model shown in FIG.  2 . 
     In one embodiment, the models described above are implemented in an on-board engine control computer including a processor. The processor is programmed to execute each step as described above. The engine control computer also includes a non-volatile memory (NVM). Adjustments for tuning parameters can be stored and easily adjusted in the NVM. 
     For engines installed in the field, and once the models are loaded into the engine control computer, the original anticipation algorithms can remain in the computer and a master disable can also be provided in the NVM so that the control laws can revert to existing algorithms in the event that such reversion is required or in the event of input signal faults that make the anticipation model invalid. Providing a NVM switch to choose between the above described models and the existing logic is beneficial in that if needed, the existing logic can be used without a software build. In addition, thrust asymmetry issues can be eliminated by disabling or enabling the above described models to better balance the thrust of the aircraft. 
     Also, and with respect to a core speed (NG) sensor failure, T 41  gas temperature increases and then settles to a steady state value equivalent to a good sensor. An intermittent NG signal and a T 41  transient upon a failed NG signal is undesirable and may cause thrust perturbations. Therefore, the NG signal should be latched to a default value once a failed signal is detected to prevent a NG signal from being used if it becomes good again 
     Further, a core speed (NG) sensor failure has an undesired result under transient conditions and sensor failure. Upon a cold-rotor burst from idle to max power, the T 41  overshoot increases for a brief period and steady-state T 41  is concurrent with a good sensor. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.