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Acceleration Control for a Gas Turbine Engine with Duct Pressure Loss Compensation

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Acceleration Control for a Gas Turbine Engine with Duct Pressure Loss Compensation Europaisches Patentamt European Patent Office © Publication number: 0 401 152 A2 Office europeen des brevets © EUROPEAN PATENT APPLICATION © Application number: 90630111.4 © Int. CIA F02C 9/28, F04D 27/02 © Date of filing: 30.05.90 © Priority: 30.05.89 US 359186 © Applicant: UNITED TECHNOLOGIES 30.05.89 US 359451 CORPORATION United Technologies Building 1, Financial @ Date of publication of application: Plaza 05.12.90 Bulletin 90/49 Hartford, CT 061 01 (US) © Designated Contracting States: © Inventor: Smith, Jesse Walter DE FR GB IT 4186 S.E. Fairway Court Stuart, Florida 34997(US) © Representative: Waxweiier, Jean et al OFFICE DENNEMEYER S.a.r.l. P.O. Box 1502 L-1015 Luxembourg(LU) © Acceleration control for a gas turbine engine with duct pressure loss compensation. © A control to control the acceleration mode for a gas turbine engine so as to allow the engine to accelerate rapidly with adequate stall margin by sim- ulating a compressor stall limit. The stall limit is attained by generating a limiting ratio of burner pres- sure and another engine pressure as a function of corrected compressor rotor speed assuming that the stall margin is independent of compressor bleed, power extraction and degradation of engine efficien- cy. Control logic means to compensate for fan by- pass duct pressure losses in a system that controls the acceleration mode for a twin spool gas turbine engine so as to allow the engine to accelerate rap- idly with adequate stall margin by simulating a com- CM pressor stall limit. The stall limit is attained by gen- -5I8.7' FIG. I < erating a limiting ratio of burner pressure and an- other engine pressure as a function of corrected CM LO compressor rotor speed assuming that the stall mar- gin is independent of compressor bleed, power ex- traction and degradation of engine efficiency. Com- pensation is by generating an engine pressure ratio signal as a function of corrected low pressure com- pressor rotor speed and using the difference be- tween the signal and actual engine pressure ratio for 0L modifying the simulated compressor stall limit. LU Xerox Copy Centre =P 0 401 152 A2 ACCELERATION CONTROL FOR A GAS TURBINE ENGINE WITH DUCT PRESSURE LOSS COMPENSATION This invention relates to gas turbine engines for provide open loop schedules with sufficient stall Dowering aircraft and more particularly to that por- margins for avoidance of stall in all contemplated :ion of the control system designed to control the operations of the engines. For details of accelera- jngine's operation during acceleration which many tion controls, reference should be made to the further include means to compensate for variations 5 aforementioned control models. n the fan operating line when scheduling fuel flow Such control systems manifest a control pa- for acceleration. rameter that is indicative of Wf /PB (where Wf is As is well-known, gas turbine engines that uti- fuel flow rate in pounds per hour and Pb is burner lize axial flow compressors are subject to stall and pressure in pounds per square foot absolute). This surge. Stall may occur in the compressor when the fo parameter varies as a function of compressor angle of attack and other conditions are such that speed (either the low compressor Ni or the high the boundary layer of the air adjacent to the com- compressor N2) in a twin spool engine and other pressor blades separates inducing a pressure pul- engine parameters selected to correct the speed to sation. If the pulsation does not subside and is a baseline value and is multiplied by actual burner allowed to propagate to other blades, the entire 15 pressure (Pb) or its equivalent to schedule the compressor will surge which could lead to an en- proper fuel flow to the engine for engine accelera- gine malfunction. The industry has attempted to tion. eliminate surge or provide means for insuring that Other engine control schemes may utilize a N1 surge will not ensue, and if so, in situ, a remedy is or N2 (rate of change signal) to provide the same designed to obviate the condition. 30 function as the Wf /PB parameter. But, in either Historically, fuel controls are designed to pro- instance or by a combination of the two, the stall vide an open loop schedule that has sufficient margin is excessive and/or inherently provides slow surge margin to assure that the engine can be accelerations when not operating under worst case accelerated without incurring surge. The accepted conditions. Such inadequacies of these systems philosophy for such schedules is to provide suffi- 25 are acerbated even further when engine operations cient margin between the engine operating line and deviate from the norm due to power extraction, the surge line at the worst operating condition so compressor bleed and engine efficiency degrada- that no matter what engine condition is encoun- tion. tered, surge will be avoided. The margin provided It has been found that one can provide a con- using this philisophy is a compromise between the 30 trol scheme that assures the optimum acceleration rate of acceleration which could be achieved under (most rapid) without risk of compressor stall with the safest operating conditions and the surge mar- any combination of bleed, power extraction and gin required for the worst operating condition. engine condition. This invention contemplates a Since acceleration time is always sacrificed in favor closed loop system which provides an acceleration of avoiding surge, accelerations are not as rapid as 35 control which generates a simulated compressor desired or possible when operating at conditions stall limit signal which is converted to a desired other than the worst possible combination. Of burner pressure limit. This limit is calculated by course, it is ideal to accelerate the engine as selecting a desired engine pressure ratio as a rapidly as possible, so that in this scenario any function of corrected high pressure compressor means that will assure avoidance of surge while 40 rotor speed and closing the loop on actual burner allowing rapid acceleration at all operating con- pressure to control fuel flow to the burner. The ditions is a desirable objective in this art. error between the actual burner pressure signal Since the surge margin required for accelera- and simulated compressor stall limit signal deter- tion is normally dictated by the most severe opera- mines the rate of fuel flow during acceleration, tion, the engine may encounter (even though that 45 properly accounting for compressor bleed, power situation hardly arises, if ever ), it is quite apparent extraction and degradation of engine efficiency. that the engine operation can be enhanced at most The desired engine pressure ratio may be a operating conditions merely by ignoring the worse limiting ratio of burner pressure and another engine case scenario. Obviously, such is an unaccceptable pressure. Such a control mode relies on the use of solution to the problem, since surge must be avoid- 50 a function generator which utilizes a ratio of burner ed at all operating conditions to assure flight safety. pressure and other engine pressure to simulate As is well-known, fuel controls such as the high pressure compressor pressure ratio. There are JFC-12, JFC-60 and JFC-68 manufactured by the numerous engine stations where the measured Hamilton Standard Division of United Technologies pressure correlates well with high compressor inlet Corporation, the assignee of this patent application, pressure. Those locations include, but are not limit- 2 3 EP0 401 152 A2 ed to, total and static pressure measurements at pressure ratio and burner pressure limits. any point along the fan bypass duct, augmentor Another feature of this invention is to provide a inlet pressures, and pressures at the fan discharge. closed loop acceleration control that simulates a As pressures near the aft end of the fan bypass compressor stall limit by generating a limiting ratio duct are used to simulate compressor inlet pres- 5 of burner pressure and other engine pressure as a sure, duct losses in the fan duct have an influence function of corrected high pressure compressor on the correlation between sensed pressure and rotor speed and closing the loop on actual burner the simulated compressor inlet pressure. The im- pressure by controlling fuel flow to the burner. pact of fan duct losses on the correlation between A still further feature of this invention is to compressor pressure ratio and the selected control io provide an acceleration control that can be imple- variable is greatest when the ratio of burner pres- mented electronically, either by analog or digital sure and augmentor inlet pressure (P6) is used to controls, while utilizing current state-of-the-art tech- simulate high compressor pressure ratio. When us- nology including the use of existing sensed param- ing this variable (Pa/Pe) as a control parameter, eters and control systems. changes in duct loss due to fan operating line 15 A further object of this invention is to provide variations can alter the correlation between this an improved correlation schedule for a gas turbine parameter and pressure ratio across the high pres- power plant that utilizes PB/P6 or similar control sure compressor. When using the ratio of burner parameter as the control parameter for engine ac- pressure and other engine pressures forward of the celeration by including control logic to compensate augmentor inlet as a control parameter, the impact 20 for variations in fan duct losses. The Pb/Ps ratio of variations in fan op-line on duct pressure losses includes a pressure indicative of burner pressure and the subsequent impact on the correlation be- and a pressure which correlates with compressor tween compressor pressure ratio and the selected inlet pressure ( in this case, augmentor inlet pres- control parameter is less, but still impacts the cor- sure Pe).
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