Patent Abstract:
Applicant&#39;s Differential-Pressure Torque Measurement System generates the torque signal from a differential gas pressure measured across the power turbine. The gas pressure differential is measured by using two pressure taps, the first tap taking the pressure reading of the expanding gas as the gas travels from the gas-generating turbine to the power turbine of the engine and the second tap taking the pressure reading of the gas as it escapes the engine through the exhaust port. The differential between the two pressure readings is determined. The pressure differential is then input to a processor which processes it in a pre-determined fashion along with the rotational speed signal of the power turbine, initial pressure and the temperature measurements of the air as the air is initially inlet into the engine. The result of the processing are various engine parameter indications including the torque.

Full Description:
DEDICATORY CLAUSE 
   The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalties thereon. 
   BACKGROUND OF THE INVENTION 
   A typical turbo shaft engine has a mechanical torque sensing device that drives a cockpit indicator so that the pilot or operator can know the power output of the engine. Torque is a critical parameter monitored by pilots and engine operators to control the aircraft or other engines and prevent damage to other drive train components. Most torque meters actually measure the twist in a drive shaft within the engine for torque indication. The accuracy of these torque measurements is affected by the shaft material properties, the temperature of the shaft, the frictional components that support the shaft and the torsional creep of the shaft itself. In addition, deficiencies in the accuracy, resolution, environmental response of the transducer, signal conditioning and computations have a large effect on the measurement accuracy. The cumulative effect of such deficiencies often is a torque indication that is unsatisfactory for smooth, safe and accurately reliable engine control. 
   One of the means to improve torque accuracy involves characterizing each torque shaft individually against a reference torque measurement system by entering the shaft-specific data into an electronic engine controller. With this shaft-specific data, the electronic engine controller can correct torque sensor signals to account for shaft material properties and operating conditions. Work has been done on improving the materials used for building torque shafts to achieve more uniform material characteristics. Low friction sleeves and bushings have been installed between reference shafts and load-carrying shafts to improve torque meter performance. 
   Another means for achieving accurate torque reading utilizes algorithms developed to adjust the torque readings to account for temperature variations in the torque meter shaft. Because a typical turbine engine is used to produce varying power output, the internal temperature of the engine changes constantly. This change in temperature causes a change in temperature of the torque meter. As is well-known, when a metal is subjected to changes in temperature, its material properties change which allows the metal to twist a different amount in response to the same applied torque. Corrective algorithms neutralize the effects of the temperature variations. But the use of corrective algorithms necessitates the added complexity of taking shaft temperature measurement or generating a synthesized (i.e. approximated) shaft temperatures and, as a result, reduces system reliability. 
   In providing torque indication for a helicopter engine, a single pressure tap in front of the power turbine has been used. But this positioning of the single tap cannot account for exhaust system losses or the effects of the dynamics of the helicopter, such as changes in the helicopter speed and the flight attitude that affect the backpressure to the engine. All these aspects tend to reduce the accuracy of the torque measurements. 
   Because of the general unreliability of many torque sensors, synthesized torque signals are often used by engine control systems as a backup torque signal. Synthesized torque signals are generated by using other engine parameters such as compressor discharge pressure, gas generator speed, turbine inlet temperature or combinations of these and pre-established engine characteristics. Such synthesized torque signals can give an approximate engine torque indications but are plagued with inaccuracies due to off-design operation, engine deterioration from wear and tear and even bleed air extraction in many turbine applications. 
   SUMMARY OF THE INVENTION 
   In applicant&#39;s Differential-Pressure Torque Measurement System, the torque signal is generated from a differential gas pressure measured across the power turbine. The gas pressure differential is measured by using two pressure taps, one tap positioned on either side of the power turbine. The first tap takes the pressure reading of the expanding gas as the gas travels from the gas-generating turbine to the power turbine of the engine while the second tap takes the pressure reading of the gas as it escapes the engine through the exhaust port. The two pressure readings from the two taps are then input to a differential pressure sensor which determines the differential between the two pressure readings. The pressure differential is, in turn, input to a processor which processes it in a pre-determined fashion along with the rotational speed signal of the power turbine, the initial pressure and the initial temperature measurements of the air as the air is inlet into the engine. The results of the processing are various engine parameter indications including the torque. 
   Unlike torque-reading methods that use only a single pressure tap, either in front of the power turbine or in the pre-combustion stage, applicant&#39;s differential pressure system compensates for any static and dynamic effects caused by the engine exhaust system and for any variation in power turbine speeds from the design operating speed. By using the pressure differential across the power turbine, the torque signal generated is consistent and accurate, because the power turbine deteriorates very little over the life of the turbine engine compared to other components such as the compressor and gas-generating turbine. Thus, the differential pressure system provides a simple, low-cost, lightweight, easy-to-install and accurate torque measurement system that can be used on helicopters, turboshaft-driven fixed-wing aircraft and industrial gas turbine engines. 

   
     DESCRIPTION OF THE DRAWING 
       FIG. 1  is a diagram of tire preferred embodiment of the Turbine Engine Differential-Pressure Torque Measurement System. 
       FIG. 2  details the process performed by the processor to generate engine parameters. 
       FIG. 3  is a graphic depiction of a representative engine characteristic for the process detailed in FIG.  2 . 
       FIG. 4  shows an alternate, simpler process performed by the processor to generate engine parameters. 
       FIG. 5  is a graphic depiction of a representative engine characteristic for the process detailed in FIG.  4 . 
       FIG. 6  is a diagram of yet another alternate process performed by the processor, taking into consideration differential pressure across the exhaust system. 
       FIG. 7  is a graphic depiction of a representative engine characteristic for the process detailed in FIG.  6 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawing wherein like numbers represent like parts in each of the several figures, solid lines with arrowheads indicate signal paths and broken lines with arrowheads indicate optional signals and paths, the Turbine Engine Differential-Pressure Torque Measurement System is explained in detail. 
   To facilitate the description of the structure and operation of the Torque Measurement System, the following terms and definitions are used:
         DP 1 =P 4 -P 5 : first differential pressure measured, in psi, with differential pressure sensor  27 .   DP 2 =P 5 -P 0 : second differential pressure, in psia, across the exhaust system.   P 0 : ambient air pressure, in psia, measured with ambient air pressure sensor  28 .   P 1 : initial pressure of the inlet air measured, in psia, with pressure sensor  16 .   P 4 : first pressure reading.   P 5 : second pressure reading. 
         Delta   =       P1   14.696     ⁢     :         ⁢               
 
correction factor where 14.696 is in units of psia and is a normalization constant corresponding to standard day sea level pressure.
   NP: rotational speed signal, in RPM or %, measured with speed sensor indicating the rotational speed of power turbine  20  and output shaft  21 , where NP=100% represents a specific, pre-determined rotational speed. The conversion from RPM to % rotational speed is engine model-specific and is established by the engine manufacturer.   T 1 : initial temperature of inlet air measured, in degrees Rankine (Degree R), with temperature sensor  15 . 
       Theta   =       T1   518.7     ⁢     :           
 
correction factor where 518.7 is in R and is a normalization constant corresponding to standard day sea level ambient air temperature. 
       NPC   =       NP     theta       ⁢     :           
 
NP corrected to T 1 . 
       DP1C   =       DP1   Delta     ⁢     :           
 
DP 1  corrected to pressure P 1 . 
       PR1   =       DP2   P5     ⁢     :           
 
pressure ratio  1 , the backpressure to the engine caused by the engine exhaust system
   SHP: power delivered from the engine to the load (i.e. any device that is powered by the engine), typically in units of shaft horsepower. 
       SHPC   =       SHP       (   Delta   )     ⁢       (   Theta   )     0.50         ⁢     :           
 
SHP corrected to P 1  and T 1 . 
       Q   =           (   SHP   )     ⁢     (   5252.1   )       NP     ⁢     :           
 
torque delivered from the engine to the load, typically in units of foot-pounds (ft-lbs.).
   S 1 : referring collectively to SHP, SHPC and Q.   5252.1: a standard conversion constant used in converting shaft horsepower to torque, Q, based on the rotational speed of the shaft, NP.       

     FIG. 1  illustrates the Differential-Pressure Torque Measurement System which operates in conjunction with a typical gas turbine engine  10  that has a shaft ouput  21  and provides power to load  40  that is driven by free power turbine  20 . Power turbine  20  is free because it does not drive compressor  11 , even if it is physically connected to the compressor by, say, bearings. Load  40  can be any controllable device such as an aircraft gearbox that transmits power to rotorblades of a helicopter or the propeller of a propeller-driven fixed-wing aircraft. The device may also be an electrical generator or any other industrial hardware. 
   In operation of the Differential-Pressure Torque Measurement System, outside air is let into compressor  11  through inlet  18 . Adjacent to the inlet are temperature sensor  15  that provides the inlet air temperature measurement T 1  and pressure sensor  16  that provides the inlet air pressure measurement P 1 , both measurements being input to processor  50 . The inlet air is compressed by compressor  11  and forwarded to combustor  14  which communicates with the compressor and where fuel is added and ignited. The expanding gasses that result from this combustion turn gas-generating turbine  12  which, in turn, drives connecting shaft  13 . Since the connecting shaft connects the gas-generating turbine and the compressor, the action of driving the connecting shaft also drives the compressor. Thus, the compression and combustion cycle is maintained as long as inlet air and fuel are combined at an appropriate ratio to sustain combustion. 
   The excess expanding gas that remains after the the requirement for compression-combustion sustainment is met leaves gas-generating turbine  12  and enters power turbine  20 . On its way, the gas passes first pressure tap  22  which provides first pressure reading P 4  to differential pressure sensor  27 . Meanwhile, in response to the incoming expanding gas, power turbine  20  turns output shaft  21  to drive load  40 . The rotational speed of the power turbine is measured by speed sensor  25  which provides speed signal NP and inputs it directly to processor  50 . After the expanding gas departs power turbine  20 , it exits engine  10  through exhaust port  26  and duct  30 . As the gas exits, its pressure is read by second pressure tap  24 , thus providing a second pressure reading P 5 . P 5  is input to differential pressure sensor  27  and may further be input to processor  50 . In response to P 4  and P 5  inputs, the differential pressure sensor produces first differential pressure signal DP 1  and inputs DP 1  to processor  50 . 
     FIG. 2  details the process executed by processor  50  to generate engine output parameters S 1 . The processor can be a subset of an electronic engine controller, a data acquisition system, a facility/system controller/monitor or even a stand-alone electronic device. It may be comprised of analog circuitry, digital circuitry or a combination of both types of circuitry and may be configured in any fashion that may occur to one skilled in the art as long as it is sufficient to perform the process illustrated in FIG.  2 . 
   As represented by  FIG. 2 , processor  50  comprises a plurality of dividers and product blocks, as well as a means for calculating SHPC, the corrected shaft horsepower value. In operation of the processor, Delta is produced by first divider  121  from the initial pressure measurement P 1  of the inlet air as the numerator and first pre-determined normalization constant, 14.696 psia, as the denominator. The Delta value is input to second divider  122  and second product block  126 . In turn, second divider  122  utilizes first differential pressure signal, DP 1 , as the numerator and the Delta as the denominator and produces DP 1 C, corrected differential pressure signal, and inputs this result to calculating means  124 . Third divider  123  utilizes the initial temperature measurement T 1  of the inlet air as the numerator and second pre-determined normalization constant, 518.67 R, as the denominator to produce Theta value. The Theta value is input to first product block  125  and second product block  126 . Both the Delta and Theta values are standard correction factors used to correct or refer engine parameter data to a pre-defined condition: in this case, sea level standard atmospheric day conditions of 14.696 psia and 59 degree F. or 518.67 degree R. However, depending on the particular environment in which the Turbine Engine Differential-Pressure Torque Measurement System is to be used, different pre-determined normalization constants that correspond to that particular environment should be used to calculate the Delta and Theta values. 
   The Theta value is used, along with NP (the NP being input simultaneously to the first product block  125  and third product block  127 ), by the first product block to produce NPC according to a formula above mentioned. NPC, in turn, is input to calculating means  124 . The calculating means may be a function, either a look-up table or a mathematical equation, that generates the engine parameter SHPC from DP 1 C and NPC.  FIG. 3  graphically depicts the function, showing the SHPC along the vertical axis as a function of DP 1 C along the horizontal axis. A collection of SHPC v. DP 1 C curves is shown in terms of NPC. The value of SHPC is input to second product block  126 . 
   SHP is yielded by second product block  126  as a product of the equation, (SHPC)(Delta)((Theta) 0.50 ). This equation represents the typical conversion from SHPC to SHP used by engine manufacturers. However, some variations can and do exist. Some engine manufacturers adjust the exponent of Theta to represent the conversion more accurately for their specific engine. For example, a manufacturer may use SHP=(SHPC)(Delta)((Theta) 0.537 ) for its conversion. By adjusting the exponent of Theta, the manufacturer can more accurately refer its engine&#39;s performance data to a wider range of ambient conditions for a specific model of engine. The equation for a specific engine model may vary thusly, but the process remains the same as long as the engine model-specific equation is inserted in second product block  126 . The value of SHP is input to third product block  127 . 
   The third product block outputs the torque value, Q, of engine  10  according to a formula set forth above. The output, S 1 , of processor  50  enables the operator of load  40  to gauge the capacities of the engine accurately and consequently maintain a precise control of the engine for optimum support of the controllable device. 
   An alternate, simpler process that can replace the process illustrated in  FIG. 2  is shown in FIG.  4 . This alternate process is identical to that depicted in  FIG. 2  except that it eliminates first product block  125  while still generating SHPC, SHP and Q. The alternate process can be used on engines that run at a constant NP or have torque characteristics insensitive to the allowable changes in NP or on engines where the desired system accuracy can be achieved without compensating for variations in the rotating speed of power turbine  20 . As seen in  FIG. 5 , the alternate process reduces the amount of upfront engine characterization data required. Like the graph in  FIG. 3 , the graph in  FIG. 5  depicts SHPC along the vertical axis as a function of DP 1 C along the horizontal axis. A single curve of SHPC v. DP 1 C is shown. 
     FIG. 6  shows yet another alternate process that may be performed by processor  50  to generate the engine parameters. This second alternate process is also identical to the process described in  FIG. 2  except for the adder  141  and fourth divider  142 . The adder combines P 0  and P 5  to produce second differential pressure signal, DP 2 , which, then, is input to the fourth divider. The fourth divider utilizes DP 2  as the numerator and P 5  as the denominator to provide PR 1 . PR 1  is input to calculating means  124  wherein it is processed along with other inputs to produce SHPC. This embodiment may be used on engines that are run at variable NP speeds and when the engine is further susceptible to a variation in exhaust backpressure. When an engine is installed in an aircraft or put to an application with a complex exhaust system, the exhaust backpressure can vary with engine power output. A typical case is an aircraft fitted with engine exhaust infrared suppressors. The variation in backpressure changes the delta pressure across the power turbine and can reduce the accuracy of the embodiments depicted in  FIGS. 2 and 4 . The embodiment of  FIG. 6  compensates for the changes in backpressure at the exit of the power turbine.  FIG. 7  shows a collection of SHPC v. DP 1 PC curves in terms of NPC and a given percent backpressure. 
   Although a particular embodiment and form of this invention has been illustrated, it is apparent that various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention should be limited only by the claims appended hereto.

Technology Classification (CPC): 5