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Re'search Memorandum c~~ififf$\? --A- CANC&LLtk, -j&$&+u 7&Czte-/ - &St , Au / 7 B ‘@. El4 3 5 is RE’SEARCH MEMORANDUM STUDY OF COMPRESSOR SYSTEMS FOR A GAS-GENERATOR ENGINE c .-? By Bernard I. Sather and Max J. Tauschek I I Lewis Flight Propulsion Laboratory i I’; Cleveland, Ohio Th~5oxulxa- -Lb-I EiE?ia p$g$p~gyg”~ym”& n&dmccdaat*Lnuj-mr. mM.a~-y yn 1L ,rO.Lblt.d m Irr. -wb!me sty I p.r.*ll. In th mm&y md mnl mwrvb.althabEd~.~ Ct*Ur omM~~~mycmarra~~ . SC- M*. aml Lil --.2iHmMMkDZD= L?hudd!szatl:r*d-munba NATIONAL ADVISORY COM M FOR AERONAUTICS WASHINGTON April 13, 1949 viJ!=s-! $ AC .c 3mkuy - --m!sv Awcf+AlrlliAL -km --L \ WV F-a-3 VI - --- --S~~l~~~~~~~lll~ NACA RM No. RCA28 ._..-___- -- .-.--- NATIONAL ADvIsaaY colNEEE Fak AERo~mIcs ByBernardI.SatheramdMaxJ.T'auschek Various methods of providing omessor-capacity and pressure- ratio control in the gas-generatm type of compound engine over a range of altitudes frcm sea level to 50,000 feet ax8 presented. The analythal results indicated that the best method of con- trol is that in which the first stage cf compression is carried out in a variable-speed supercharger driven by a hydmulio slip coupling. The second stage of oc~apression oould be either a rotazy constant-pressure-ratio-type cmpressor or a piston-type ccaspressor, both driven at oonstant speed. The a.n3lysis also iradioatedthat the variation Crp the value of the load coefficient for the first and second stages cd? the rotary constant-pressure-tgpe cqessor combination was within reasonable limits and that the valve timing . of the piston-type ccunpressor Gould be kept oonstarxt for the mnge of altitudes covered. With respect to engine performance, other control schemes 8180 appeared feasible. A variable-area turbine . nozzle was shown to be unnecessary for cruiehg operation of the =a-. I An analysis of anaircmft-propulsion system known as a piston- type gas-generator engine is presented in referenoe 1. En this power plant a two-stroke-cycle, omrpression-ignition engine drives a compressor, which in turn supplies air to the engine. No other shaft work is abstracted from th? engFne. The gases from the gen- erator comprising the ccmpessor-engine cmbination are then uti- Lazed in a turbine, which prcduces the net useful work of the cycle. A diagramatio sketch of this power plant is shown in figure 1. The analysis of reference 1 indicates that such an erg&e may have low specific weight cabined with low fuel consumption 09 the order CCF 0.32 pound per brake horsepow=-hour, whioh has been con- firmed, to a certain etient, by the experimental results reported 2 NACA RM No. E9A28 Inreference 2. The reference analysis is idealized, however, In that the problem of engine oontrol was not oonsidwed. In order for the performanoe cf an actual engine to approach that of the ideal ourves, at least three steps must be accomplished: (1) A ocmpres- sor that is capable cf operating over a range of air flows and pressure ratios must be obtained; (2) similarly, a turbine that will operate over the range cxf air flows and ~essure ratios must be obtained; and (3) satisfactory means of maintaining proper engine limits (peak cylinder lzessure and turbine-inlet temperature) must be evolved. The first c& the preceding steps, which pertains to the control of canpressor capacity and compression ratio, is treated herein. The speolfio objective of this investigation, which was conducted 8t the NACALewis laboratory, is to evaluate various comibinations cf corn- lessors and driving mechanisms with respect to engine performance as a function of altitude. Because It is currently impossible to make a oomplete evaluation with due oonsideratlon for such items as oanpressor weight, development problems, and control stability, the present analysis Is based on the degeneration cf the power output and the fuel eoonomy of the gas-generator ee In question when oe with the ideal engine. Some qualitative disoussion aP com- . pTessorweight,however, is included. METHOD OF ANALYSIS The various compressor-control systems that were investigated 0 inoluded the followlng ocunbWtions of eleanen-bs: (1 Constant-pressure-ratio ccmDt?essor with throttled inlet (2 i Multistage constant-pressure-ratio ocsapressor with first stage (supercharging stage) driven by three-speed gear (3) Multistage constant-pressure-ratio compressor with first stage (supercharging stage) driven by hydraulic slip ooupling 4) Constant-volume single-stage ocun~ssor I 5)Constant-volume c~essorwiththrottled inlet (6) Constant-volume ccmprressor tith means of varying volumetric capacity (7) Supercharged constant-volume cwessor cc%tprisFng constant- pressure-ratio first stage (supercharging stage) driven by hydraulic slip couplm followed by final stage aP compres- sion in constant-volume compressor In the preceding lfst, the terms %x&ant volume" and "constant pressure" refer to compressors exhibiting these oharacteristios at NACARMNo. E9A28 3 const8nt speed or effective speed. Thus the oentrifug8l0npl~ssor Y . or the mixed-flow cunpressor (reference 3) would most nearly rep- resent the constant-presslule-ratio group. Theconstant-volme class would lnolude any.of the positive-displacement c~essors, such 8~ the piston-type campressor or Roots blower. The d&l-flow ccm- pressor would 8180 fit into this olass ff suitable means were avail- 2 8ble for broadening its operating range so as to maint8fnhIgh eSfi- 4 cienoy over 8 tide range of pressure ratios and air flows. Schfrmatic diagrams cfthe various systerma are showninfigure 2. These possible oaubinations were Berived by setting up the ideal requiraments for the ccmpressor for the gas-generator engine 8nd then selecting the owessor systsPns most likely to fulfill these demands. The ideal requfrennents for the ccm@ressor (fig. 3) were computed frcuz reference 1. Engine speed was not included 8s a control means because aP the necessfty for keeping the scavenge ratio within reasonable limits (reference 1). Thus, (8) If means . were provided for keep- the soavenge lgtio constant, such as a variable-area turbine nozzle, the reduction in engine speed neces- saryto decrease the ocmpressor-pressure ratio to the required c value as altitude is deoreased (fig. 3) would result in greatly reducedairwefghtfUw8ndhence reduced 8ngZne power output; and (b) If no means are wovided for controlling eoavenge ratio at . will., as tithe c8se at' the fixed-area turbine nozzle, reduction in engine speed would result in inoreasIng the scavenge ratio and hencebm mixture r8tio beyond the usable range. All the ccpnbin8tions were investigated with the an8lysis of reference 1 used as a basis. The changeerequiredintheanalysis 8s 8 r8SUlt of the use of8 8pedfl.O canpr8ssor are given in the &pPand-88. All the combilrrtions were analyzed with a feed-area turbine nozzle and, In addition, eystams 2 to 6 were also treated with a variable-are8 nozzle. In analyzing the various ocmbinations, the design cotitions of the compessors were set to provide,sufficient camcity and pressure ratio for engine operatfon 8t 8n altitude of 20,000 feet. At this altitude, the engine w8s 888lrmed to operate with design operating limits of peak cylinder pressure of 1600 pounde per square inoh, turbine-inlet temperature of 16KI°F,8ndsc8venge ratio of 1.0. For caloulationa of operation at higher altitudes, the turbine-inlet temperature and the oqesaor speed were held constant and the peak cylinder pressure w8s allowed to decrsase. At lower altitudes, the generator-inlet pressure was so varfed 8s to maintain both engine limits, unless the characteristics of the c~essor system precluded this possibility, in which case the 4 NACA RI4 Ho. E9A29 oykbd8r pressure was 8llCWed to vary. when turbine-nozzle area was fixed, it was impossible to hold the soavenge ratio to 8 con- stant value of 1;O at altitudes other than the design altitude because under choked oonditions the area of the turbine nozzle determines the gas flow through the engine; however, the value of soavenge ratio did not vary mtly from 1.0 over the range of alti- tudes consiijered. When the ar8a of the turbine nozzle wa6 made va;riabl8, the saavenge ratio was kept son&ant at a value af 1.0. The scavenge ratio of the Ideal gas-generator engine was 1.0 for 811 8ltitud88. Systsans 2, 3, and 7, which utilize a rotary sup8roharging compr8ssor, were so arranged that the supercharger operated at rated Speed at an altitude of 20,000 fe8t and idled 8t Sea 18V81. In th8 eystam Irrvolvlng the three-speed gear wbination, the eup8rCharger Was aSSum8d to idle 8t et p3XSSUr8 ratio of l&t alti- tudes below 10,000 feet a& to operate in the low-speed gear ratio at altitudes b8tw88n 10,000 and 20,000 feet. Above 20,000 feet, the supercharggr op8X'ated in high Speed. The over-811 efficiency c& a.ny ocmbination of campressorswas made equal to 0.85 at an altitude of 20,000 feet. Although thie prsotioe resulted in rather high stage efficiencies, it was neoes- earJithatth8 dat&ag?.%ewith that cxfr8f81~1~elat thiSbaSiC oonditfon. Th8 lack cxf 8CCurate data on the affiCien0188 of ConSt~t-VO;lum8 O~SSOZ?S, as, for erample, that of the axial- flow o~essor at off-design WtiitioXIS or th8 piStoPtype cam- pressor at high piston speeds, pwluded oomDe,rison of the oone~-pressureandoonetant-volzmw compr888ors onan efficiency b8sis. Th8 most reliable hAgA data on 8ffiCienCy of piston-type ccm~8ssor8, however, indloate values in the ra,nge from 0.85 to 0.95, whioh is entirely oompatibl8 with the general assumption. No chang88 In sffioienoy with ohanges in the specific f&w for the constant-pressure compr8ssoTs were considered; Instead, the pres- .
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