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NATIONALADVISORYCOMMITTEE
FOR AERONAUTICS
TECHNICAL MEMORANDUM No. 1141 THE COMBINATION OF INTERNAL-COMBUSTION ENGINE AND GAS TURBINE By K. Zinner Zeitschrift des Vereines Deutscher Ingenieure May 13, 1944
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Washington April 1947 .
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31176014374483 , NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS —— TECENICAL MEMORANDUM NO. 1141 —— ,.. , ... ,..
TIE COMBINATION OF INTEI?ML-COMXJSTION ~NGI~ AN_J) GAS TURBINE* By K. Zinner While the gas turbine by itself has been apylied in particular cases for power generation and is in a state of promising develop- ment in this field, it has already met with considerable success in two cases when used as an bxhaust turbine :n connection with a cen- trifugal compressor, namely, ~.nthe supercharging of combustion engines and in the Ve.loxprocessl, which 5.sof particular applica- tion for furnaces.
In the present Taper the most important possi- bilities of combining a combustion engine with a gas turbine are considered. llcombination enginesttare compared with the These simple gas turbine on whose state of development a brief review will first be given.
The critical evaluation of the possibilities of development and fields of application of the various combus-tion- engine systems, wherever it is not clearly expressed in ‘the publications referred to, represents the opinion of the author.
The state of development of the internal-combustionengine is in its main features generally kno~rn. It is used predominantly at the present time for the pro:~ulsion of aircraft and road vehicles and, except for certain restrictions due to war conditions} has been used to an increasing extent in shills and rail cars and in some fields applied as stationary yower generators. In the Diesel engine a most economical heat engine with a useful efficiency of about 40 percent exists and in the Otto aircraft engine a heat engine of greatest power per vmit weight of about 0.5 kilogram per horsepower.
*lfDieVerbindung von Verbrennungsmotor und Gasturbine.ll VDI Zeitschrift, May 13, 1944, p. 245.
An article on the Velox supercharging process, which can be applied with advantage wherever the processes are accelerated by high pressure and where large quantities of heat are to be converted !> or exbh’anged, has recently ajpeared in’%his journal (rGference 20).
2 NACA TM No. 1141 The gas turbine - this term has come into use to denote a tur- bine working witlncombustion gas or hot air - is intei.ded to combine the oyerating advantages of a machine without reciprocating motion of masses with the operational and economic advantages of the internal-comb~wtionengine. The idea in itmlf is not new but has received ncw interest and a fresh outlook through the progress made in recent years in the fieSd of heat-resistant materials and in the aerodynamic research apnlied to the compmssors.
PRX3ENT STATT OF DEWLOPMENT OF THE GAS TURBINE A brief review will first be given of the state of development of the gas tu?bine for the direct production of power as based on (Among others known test results and publications of recent years.
see references 22 and 23.)
The dry gas turbine, which alone will be considered here, uses atmospheric air or the gases arising from the combustion of an air- fue,lmixture and consists essentially of a compressor, a combustion chamber, and a turbine. The efficiency of the system depends strongly on the temperature of the working substance at the inlet to the tur- bine and on the efficient design of the turbine and the compressor.
The maximum temperature in the gas-turbine cycle is, for reasons of ,strength,much lower than in the internal-combustionengine because, in the former case> the same structural parts are constantly subjected to high-temperature gas whereas in the latter the temperature peaks occur only for a short time and the structural parts therefore assuiie a mean tem~)erature lying far below the maximum value.
Depending on whether isothermal or adiabatic are desired during the comyession and the expansion, whether the heat is supplied at con- stant pressure or constant volume, whether and at what point of the cycle there is heat “exchange,etc., numerous modifications of the gas-turbine process are possible (references 6, 23, ’29,and 33). As in the case of the reciprocating engines, the gas turbines are divided intoconstant-vol..ume turbines and constant-pressure turbines according to the nature of the combustion process (heat supply after compression either at constant volume or at constant yressure). (See refer- ence 36.) This modeof classification has nothing to do with that of s-team turbines for which the terms constant pressure or high pressure refer to the pressures ahead of and behind the i?apeller.
NACA TM NO. 1)-41 TBE CONSTANT-VOLUMETURBINE In tine .conatmt-volllme. turbine. the"explosion,.chambergare filled w~tki air and fuel (combustion gas), a precornpression of the dlai-ge being requi~ed not for carrying out the process but to attain useful.
efficiencies. In the combustion chambers the mixture is electrically ignited and burns at constant, or approximately constant, volume.
Shortly before the end of the c~~bustioil the outlet valve opens and the hi~h-yressure hot gases flow into the turbine to deliver the use- ful Tower.
The efficiency of the ideal work cycle of’the constant-volume turbhe is, as seen from the T’-s” diagram (fig. 1) greater thaiithat of the constan-k-pressure turbine:?or eq~.lal precom.prbssion and equal maximum temperature. Becau8e the &atio of the adiabatic heat drop in the turbine to that in tke com~ressor is considerably greater ii~ the case of the constant-volume turbine, the i-atiois less deyendent on the comp~’essor efficiency, on th..? assumption of sirni~.ar turbir-e efficiency, than in the case of the constant-pressure turbine. At a time at which ‘~herewere as yet nc centrifugal compressors with sufficiently good efficiency, successful results were therefore first obtained in tests on the cozr.stant-voiume turbine, the develoym.ent of which is closely associated with the name Holzwarth (references 6, 9, and 36).
The periodically working explosion ckambers offer a nunber of technically difficuit prohle-ms. In the ca~e of several particularly critical structural parts, Water.cof>ling Must be used to a large eztezt, the combustion chambers thereby constituting a source of ccnr.siderable heat losses. Because the efficiency of an int)ermiktently acting turbine is also likewise below that or a continuously acting turbine, the actual efficiencies lie far ‘De]-Gw th~se of the ideal process. The ~onstant.~olu~e turbine is econo~.- icaq.only if the compressor is driven by a steam turbine whose boiler is heated with the exhaust heat of the gas turbine. Thereby the process becomes complicated and can be used with ad~rantage only for very definite limit~d fields of application. With recent install- ations of this type, an efficiency of 20 percent was attained (reference 23), whereas an increase to 25 gercent is expected (refer- ence 6).
THE CONSTANT-PRESSURE COMBUSTION TURBINE In its simplest form, the cycle of lineconstant-pressure turlirie !53 5.s formed of two adiabatic and-two isobars.(fig.l( b).). A setl~p.
working on “open circuit” that takes air fi-o~the surroundings and discharges the coml>ustior~ gas with still relatively high ~WI:(Jera.~LU?(3 — NACA.TM NO. 1141 into the air after expansion in the turbine has the advantage of very great simplicity (fig. 2) b~t the efficiencies attained. at preserit with this system are still 10-rin comparison with those of the constant-volume turbine (references 10 and 37).
The effective output of the gas turbj:neis the excess of the Because in the case of turbine output over the compressor output.
thq constant-pi”esoure turbine the compressor power forms a con- siderable :ymtion of the turbine pol:er~the tur:>ineis used to advantage only in connection with a high-efficier.cy compressor, which at present is avai?l-ab.le in the axial ~ompres~oia. For a perfect gas and for definite resumptions as to the highes~ maximum and minimum the effect of the indfl.vidual effici,~ncies of cycle tWlp~i”at.&l”eS2 compressor and turbine on the over-all efficiency neglecting lasses in the combustion chamber is shown in i“i~l~re .3. From this figure we see that, for a Gas inlet tem~erati.tre in the turbine of 60C0 C end individual efficiencies of 0.85, the over-all efficiency of the setup is still below 20 percent. With Present heat-resiotant steels, tem- peratures of 6000 C can today be used also z-orcontinuous operation.
A gas-turbine unit working on this process built by Brown Boverie power Geileratiagstation and Co., Baden, and.installed in an el@ctric-- in Neuchatel, Switzer,land2 as an emer~ency yower unit attained a brake efficiency of 18 percent at a power ctf4000 kilowatts (refer- ence 37). The inlet temperature of the combustion &as in the turbine was 5S2° C and the individual efficieilcies of the compressor and the turbine were 84.6 and 8U.4 ~ercent, respectively. The coml?ressor const’aed 73.5 percent of the turbine power.
Improvement jn Efficiency through Heat Recovery f~om the Esha!l.st Gases For the most economical ccupression ratio of the constant- pressure turbine according to the process of fi~ure l(b), the tem- of the combustion gas after expansion in the turbine perature T4 of the compressed air before remains hi@ler than the temperature T2 entrance ~nto the Combustion chambe~. The efficiency can therefore be improved by the heat tran~fer from the exhaust air to the compressed air before the entrance of the latter into the combustion chamber. The heat to be sul~plied to the air by conbu.stion decreases for constailt power by the quantity of heat exchanged (f’i,g. .1(c)). As shown in figure 4, considerable increases in the el?fi.ciency are thereby made possible althou~h the heat exchanger involveo an increase in the size of the system.
The Gas-Turbine Process with ‘ClosedCircuit A constant-preesum turbine %~ork--pmcess develoyed.hy Ackeret and Keller (references 1 and 11) and applied by the firm of Escher Wyss differs from the one deecribed prev3.ously in two respects: During compression an isothermal change of state is approximated through intercooling. and the work medium works with.higher density in a closed circuit (fig, 5).
As a result of the Iiiglier density, the dimensions of comyreseor, tl~~”bfne, ar~ heat exchanger for the heat recovery are smaller. A.s in the case of the boiler, however, the heat of combustion mid be sw~plied to the work medium (air) through an “air heater” consisting of combustion chamber and heat exchanger. The gas-turbine unit, working according to this process, is termed by the inventors an ‘taerod~amic heat engine.” The process is based on the so-called double.-isothermal cycle (fig. 6). The wo~kiLTIf?j ga~, with heat QO ~emoved, is isothermally compressed from state 1 to state 2.
Between sw.tes 2 and 5 the gas is heated at constar.t pressure f’romtb.elo~-erteuyerature T2 = T1 to the upper temperature T3 = T4, the totai quautity of heat of the gas flowirg back from the turbine to the cornpresoor being trans- ferred so that the t~m~eyature is again lowered to d?, . Only dur?.ng the isothermal expansion ie the heat Q suypliei ext&’rnal.Ly to the turbine. Inasmuch as in the case of the ideal process the heat added and the heat removed are throughout in a state of equilibrium, that is, occurs with the temperature difference zero, all individual processes are reversible and hence the efficiency is e~.~ivalenb to that of the Carnot cycle, The process can be only ap~roximately realized and, as seen in fi@_.n?e 7, would require a unit with a great many parts because not only the air ‘heatiel- and the heat exchanger bl.~t also the intercooler and the intersuperheaterare required between the individual compression and expansion stages. In order to simplify the unit, the intersuperheating is dispensed with in the process shown in fi~ure 8(b).
The advantage of isothermal as compared with adiabatic compres- sion in the case of the constant.pressure gas turbine for processes between equal temperatures and with the same yressures is readily seen from a comparison of the T-s diagram (fig. 8). The heat supplied Q is in both cases the same but the heat to be removed to the outside is in the case of the isothermal compression smaller and hence tineheat converted into worligreater. The lower temperature at the end of the iso:heyrnal compression requires no iacreased heat supply in the com- . . ..
bustion chamber because”a greatbti”lie~t”recovery from the gases leavj.ng the turbine is possible. The isothemal cornpressio n’naturally brings about greater efficiency only if a correspondingly larger heat exchanger is actually used to transfer the greater heat quantity.
The isothermal compression yroduced %y intercooling can naturally he used in the case of the o-pencircuit “but, due to the lower air also density.,Leads to e~sentially lar~er dimensions of the heat exchanger and intercooler than in the case of the closed circo.it. In G.rderto approximate the isotliermal expansion in”the turbine in the case of the ogen circuit, combustion in stages has beeriyroposed (refereiice 16) and this woL~ldrequire a subdivision of the combustion chamber and interconnectionbetween the individual turbine-stage groups.
Comparison between the Open and Closed Circuits Beca&se in the case of the closed circuit the comlmstion gases come in contact only with the air heater but not with the compressor and the turbine} the danger of corrosion and wear of the blades is avoided in using fuels containing tar and ash, Corrosion of the blades produces a considerable lowerin~ in the efficiency, whereas the wear prodnces in add;ition an impairment of the durability.
In the closed circuit, the power can be regulated for unchanged speed and unchanged position of the operating point in the character- istics of turbine and compressor, through the density (pressure head) of the working medium in the circuit alone, whereby, as compared with -.
the open circuit, better fractional load e~ficienci,es are yossikle.
However, the frequent yumping of the unit for ra??idload changes.
involves losses.
the closed circuit, heat exchangers are required as is not For the case Ior the oRen circuit: A precooler for cooling the circuit air before entry into the compressor, an air heater for transferring the ‘heatfrom the combustion Gas to the circuit air before entry into tho turbine, and an.air preheater for the combustion air to utilize the heat still remaining in the Combustion gas after exit from the air heater. Bscause the combustion gas, in the most favorable case, can of the circuit leave the air heater with the inlet temperature” T3 air, which, however, on account of the heat already ta’~enup in the heat exchanger is sufficiently high, there would be a considerable heat loss and hence decrease in efficiei~.cy with~ut the air preheater.
The closed circuit is more restricted in the application of high gas temperature than the open circuit because the hea-b-exchanCor tubes in the circuit-air heater assume a tern~erature above the maximum cycle temperature, whereas the turbine blades, which fo~m t~hecritical point in the case of the open circuit, remain below the maximum cycle tem- perature and in any given case may be held below it by coolil~g. The NACA TM No. 1141 7 requirement of heat-resfistant steels is I;mited in the case of open circuit to the turbine blades, whereas for the closed circuit also a ,..
part of the air-heater tubes must be “of-”high~~quality -matieriali With the need for application of the circuit-air heater and the combustion-air prehpater, the tinitwith closed circuit requires many parts and in spite of reduced dimensions of the turbine and the compressor becomes more bullqythan-in the case of the open circuit.
The circuit-air heater and combustion-air preheater are in their dimensions entirely comparable with the boiler units of steam-power installations because the heat transfer occurs on one (air heater) or on both sides (air preheater) only at atmosphetiic pressure.
The Velox process may be applied to this heat transfer (references 11 and 20) but in view of the tube temperatures of the air heater, the heat transfer on the side of the combustion gas may not be arbitrarily raised.
If heat recovery is dispensed with at the expense of lowered efficiency, a very simple gas-turbine unit from the structural and operational viewpoints is ~ossible with the open circuit, which also meets with high requirements as reqards bulk and weight inasmuch as with the open circ~it the combusti& chamber can be ~eld to small dimensions because of the greater air density under which combustion occurs.
The advantage of the closed circuit lies in the Greater ap@ica- bility of solid fuels and in the better possibility of utilizing the intercooling and heat recovery~ the spat= and weig~t requirements of which are less as compared with those of the air heater. This process is therefore in particular suitable for stationary units where it enters less in competition with the internal-combustionengine than with the stesm turbine.
COMBINATION ENGINES A combination engine is to be understood as a unit consisting of an internal-combustionengine, a turbine, and a compressor. Depending on where the power is taken off, the type of coupling of these three machines;and the degree of the supercharging, three cases may be distinguishe~: the exhaust-gas turbosupercharging, the propellant-gas proce”ss, and the high supercharging.
,,,, ,,, ,,,., ,,,,,, ,,,, ,, .-, .,,,,,,,,, , ,,., , ,, , ,, ,,. ,,, --.,,, NACA TM No. 1141 The exhaust-gas turbosupercharging has fo~”its object the utili- zation of the energy still reuainin~ in the woyking gafiexhaust. The adiabatic work recoverable by continuing the expansion from this state to the charging pressure mounts for the Diesel engine from 25 to 30 yercent and for the Otto engine from 35 to 40 percent (on account of the smaller expansion in the cylinder) of the indicated work of the engine (reterence 31).
Inasmuch as only a STMU fraction of this can be re~ained, the continuation of tho expansion in a second stage by applying an exhaust-gas turbine eloriewould not be of advantage unless a c<~mpressor is simultaneously used, that is, the compression is also made two-stage and an increased. chargu thus supplied to the engine.
For the exhaust-turbow~perckargin=‘system,the internal-combustion engine constitutes the princiyal machine trom Wtioseshaft the usefkl power is taken off, Between turbine and.centrifugal compressor (superchar~er), thei-emust be equality of output and this determines the ratio of the supercharger pressure to the pressure ahead of the turbine. The turbine impeller and the compressor wheei can, as a rule, be so designed that they run with tliesam speed and may therefore rotate on a cominonshaft. The cumbining of the exhaust-gas turbine and the compressor to form the exhaust-gas turbosupercharger or the superchar~er unit possesses the following principal advantages as compared with other types of supercharging: 1. The power for driving the supercharger is obtained from the energy of the exhaust gases.
certain freedom in the 2. No gearin~ is necessary and there is a arrangement of the supercharger unit.
In the case of the propellant-gas proce=s the useful power is taken from the tUrbine shaft so that the turklne becomes the principal machine . The internal-combustione~~inc drives a (piston) compressor, which supplies air for chargins the former and these two machines must The exhaust gases of the i@ernal- therefore have equal out~ut.
c>mbustion engine (Diesel) arc?raised to s,definite pressure and to~ether with the scavenging part of the air through the cylinder form the propellant gas for the turbine. The suyercharg~d Diesel its function as drive machine for the com- engine thus, in addition to pressor, takes over the ~;artof the “combustion chamber” for the gas turbine.
This combination of compressor and highly supercharged internal-combustioneng:.neis denoted a propellant-gas producer (references 33 and 39).
acd the In the process denoted high charging, both the com~rmscr turbine are coupled to the shaft of the internal-combustionengine as —-—. —. ,—.
— ,,.--, , .,, - ,-,-,, , -, ,,, , ,,,-,, ,,m_ ——.. . !...! , , , ,, ,, ,.,,,, ,,., , , ,- —, NACA TM No. 1141 a resvlt of which tlncreis sonewhat greater freedom in the dimensioning and i~.dfl.vidual powers of these three machines.
,. ..,,,.. . . . -, . .. . . . ,,, . ,,, .3 ..., .,, ‘---- -,. .. .
Proyosals for designing various combination engines after the rapid development of the com’ovstj.on and steam engines at the turn of the century, appear relatively early in patents and in the technical literature (references, 13, and40)i TEE EXIVK13T-GAS TURBOWl?ERCH/IRGING ON FOUR-STROKE DIESEL ENGIN?L5 The exiha~st-gas turbosuperchargingof four-stroke Diesel en@nes “B~chi Supercharging”) is at the present time carried out exclusively as the chief chaz”acteristic of ,whichis the exhaust-gas driven tur%o- supercharger with subdivided exhaust piping (rel?erence 3). This type of charging is the last stage of a long development in which the followi-ng steps, according to the yatents granted, may be traced: DIE’.No. 204,630, v. 16} Nov. 1905 describes a unit consisting of a centrifugal- cor3pressor, four-struke int,ernal-combustion engine, and t’.lrbine, all three machines working on a conmmn shaft, The air or the fuel-air uixture is compressed in the centi-ii’ugal coin~~es~or as far as possible isothermally to several atmospheres and a,fter being cooled is suyplied to +J1.m intemal-cornbusttonengine. In contrast to the designs prevalent until that time, not o@y a part but”the entire exhaust gases entered the turbine with greatly increased press-x-e.
Originally B~ch51had in mind yaising the presstme ahead of the turbine up to the pressure at the end of ex--sasion in the cylinder, i-orin cm.e eX~mple (re”ferenco 2), an isothelaal precompression in the cer,trlfugal compressor up to 3 to 4 atmospheres afida yressure ahead of the tur- bine of 16 atmospheres was assumed. The turbine was in this case tc take over the ~in lead and the internal-combustionengine was essen- tially to drive only the compressor.
There are two factors operating against the realization of a process with so high an increase of the exhaust-gas pressure above the charge pressure. ‘I’he residual gas with high pressure relnainiriin Vile coliipression space of the cylinder at the eraal.lst stroke will expand at the intaicestroke and hinder complete charging of 12ie cylinder witln fresh charge.
lib-en if the residual gas is expanded t-hxough a s~ecial valve to the surrounding ~pessure and a full charge thereby made ..atioof t,he work done during the exhaust stroke kY the possible, the .
piston against the high pressure in driving the ~as from the cylinder w,, ,..(area 4-1-7-6 in fig. 9) to,, j.tsexpansion work in the turbino (area 4- 5-6-7) is so unfav~rable that, with ~ccount taken”of the unavoidable throttling, friction, and heat losses, no appreciable gain would result.
The real~zation of the effect of the residual gas led B~;chito a charging process (DRP. No. 454,107, v. 27,3, 1921, priority, v. 2., Nov. 1915) in which the pressure in the cylinder d~iringthe exhaust stroke is made approximately equal to the pressure of the precom- pressed charge in--acertain lc~adrange and-even somewhat &er. The comiioncoupling of eng~.ne,turbine, and compessor is dieyensed with, the turbine driving the compressor. Moreover, the sirnultaneou.s opening of inlet and outlet valves (valve overlap) at the end of the exhaust stroke is provided for in order to be able to scavenge the rosid~~al gas out of the cylinder through the charging pressure, which lies a“bovethe exhaust pressr.re. In this way, without increasing the increase in the volume weight of the charge, there is also obtained an of the charge, which is greater the larger the “dead space,” that is, the lower the compression ratio,of the engine., Because the sce,vengin~ furthermore lowers the temperature of the charge and of the combustion- chamber walls, it becomes of greatest importance to make possible high supercharging.
The Impact Turbine Figure 10 shows the P-v diagram of the ideal work process of a combustion engine with exhaust-gas turke,~percharger for which the charging pressure is held above the pressure assumed constant ahead of the turbi”ne. An exhaust-gas turbine with an approximately constant inlet yressure above that of the surround?++?, which would be estab- lished in an exhaust piping acting as reservoir for a multicylinder engine~ is denoted as an impact turbine (references 8 and 31). A utilization of the-exhaust energy represented by the area 1~-4-5 is here dispensed with insofar as it does not contribute, through turbulence of the flow energy, to heating of the gas and hence to a The adiabatic work of volume increase from point 5 to point 8.
expansion corresponding to the area 6-7-8-9 must be greater by the amount of the losses in the turbine and the supercharger than the adiabatic work of compression corresponding to the area 0-1-10-6.
biT = i8 - i9, The adiabatic heat drop in the turbine amounts to where the symbols have the meaning indicated in figure 10, and the adiabatic pressure lieadex~ressed in heat units in the supercharger is compressed If ~ denotes the amount of charge to be AiL = il - io.
the supercharger efficiency, and GA that.of the exhaust gas, ~L and VT the turbine efficiency, then the condition for the equality of the exhaust-gae turbine and compressor output is (1) (il - ‘O) ~/~L = (i8 - ‘9) GA qT NACA TM No. 1141 Substituting the pressure ratios there is obtained -- ~A-l~ r ,.., ,..,... .
.. . , .._, ”L-~ . ..
[. ., .--. ., I I !
p?’i ‘L /p8\ ’<A ~ CPA GA ‘A -1=1 ___ .—. — .
(2) ~\P() ) ~[)[ ( L .1 ‘g .’ cm % ‘qL7T where are the specific heats at constant pressure,
%L and CPA
~L and KA the adiabatic exponents, TL= T1 the air inlet tempera- ture in the compressor, and TA = T8 the gas inlet temperature in the turline. For otherwise equal ratios, therefore the preesure ratio producible in the supercharger is directly p~oportional to the absolute gas temperature TA ahead of the turbine and to the turbine and the and inversely proportional to s~~percharger effici~cY ~T and ~L the air tem~erature TL ahead of the supercharger.
The prodv.ction cf an effective scavenging pressure drop, that is, a mi’ficient pressure difference between the intake Fiping and the exhaust piping for scavenging the residual gases, offers difficulties on account o.fthe relatively low exhaust temperature of tke Diesel engine with the simple impact turbine. A ~ressure ratio of 1.5 in the ccxapressor, for example, for an exhaust temperature of 550° C, an ai::” iiile~ temperature into the compressor of 25° C, and a pressure raiic o:F1.3 in the turbine, requires an efficiency Of the cha~ging lunitof ~L l VT = 60 percent which, on account of the relatively small dimensions of the machines with the simple impact twbine, is not read~.lyattainable and was not att~inable at the start of the supercharging developmerh.
TlaeExhaust Turbine In a further stage of development) therefore, B~cki divided the exhaust ~i~ing in such a manner that only the cylinders with at least 24C0 crank-angle phase difference of ignition exhausted to the saine pipe (DIN? No. 568,855, v.19, Nov. 1926, Priority, v. 30, Nov. 1925).
(See fig. 11.) The individual pipe lines are led to scparat~ nozzle chambers of the exhaust-gas turbine.
The etiaust-gas ‘mrbocharging ofr,Dicsel engines through this process, which is today ‘knownas “Buchi turbocharging” first became practical.
The e~ihaustimpulses produce in the exhaust piping a strongly fluctuating pressure ,. variation of which the section of the low pressures was used for scavenging the cOrnbU&iOn s~ace under sirntiltaneou~ ‘openingof the inlet and outlet valves (references 3, 28, 30, and 34).
NACA TM No. 1141 Figure 12 shows as an example the oscillo@?aphically measured yressure variation in the exhaust piping of c Diesel engine with B&lli-charging. Elecauseth:sscavenging is cut off ‘beforethe pressure in the _pipinghas a~ain begun to rtoe due to the exhaust i.mpu.lse from the next cylinder, this process makes possible the use of piyes of If the small volume in which the pressure peaks are more marked.
imrbin.e-nozzle Ci-osssecticms are mailelar~~ enough so that the eshaust impulses may flow ofi iranediately wit)houtapp~eciable ~U.i~ding-up of pressure, the _h@)ine 5.udeno’bed as an exhaust tur- The conversion cf the energy takes place fi~ne(references ~ and ~1).
both over the presswre wavefitraveling “b.hro~lgh the piping, which are turbine nozzles, and over the firs’ti cenvcrted ir.tovelocity in the velocity already directly prodl.~ced in the out”l.et valve.
It is seen from f’igure12 that the scavenging pressure Qrop between the ckargin: pressure, which for ?.mlticylinderengines with common ihtake piping is subject to only slight fiuctoations, and the pressure valley during the scavenging period is considerably greater thaa wculd be the case for a constant, mean pressure in the exhaust piping. Because .the. high pressures cccur only in the neigh- borhood of tl~epistoiltop dead center, that i,+,over a period in which the piston motion is small, the exhaust stroke work to be yerformed 3Y the piston also remains small for this process.
Tl~eanalysis of t]l~energy conversion in the exhaust tu.r-KJine iS difficult because not only the yressure of the exhaust gas but also its temperature and its velocity are subject to strong fluctuations.
If the mean temperature and meailpreesure of the gas ahead of the turbine are used to compu’~e the adiatatic drop, apparm.t turbine efficiencies of over !.00percent, and hence ayparent over--all 80 percent may efficiencies of’thlesupei”charger unit,of eve. 60 to l~iththis mode of c.~mputation, there is be :!ound(~”eference 23).
not, on the cm han.djformed the correct mean value of tinestatic drop because for the premure peaks above ‘~h~ mean pressure) ~rea”ber exha.;st-gas quantities flow through the tvrbine and, on the other band, the kinetic energy nreduced,in the cuilet ‘Falvesis not taken T]lefi~~t ~Yfcct is generally considerably greater in+~ account.
because the velocity lCSSG.Sin the nwnerous changes in ci-o~s. seckion and direction of the exhaust piping are cons!.derable. Tne apyarent tul’’oj.ne eyficioncy becomes higllcrthe greaker the pro~port~.on of the pressure and velocity impulses in the ‘totaleneygy.
The charging pressures attaina-~le with the simple mha’~s”t trr- bine, for which the yressure in ihe exhaust piping must drop during the oxbaust process to the sumounding pressure, are of the order of NACA TM No. 1141 13 magnitude of several tenths atmospheres gage pressure.
This process, also at low exhaust temperature, makee possible a scavenging of the ., ., combustion space and charging. -- -...,,. ,., .
If ‘theresidual gases are completely scavenged, any further increase .inthe charge j.spossible only through an increase in the charge weight.
In order to attain the required higher charging pressure, an increase in the drop in tle turbine is required and thts leads to the application of the impact turbifie with utilization of the exhaust impacts.
The higher the charge pressure, the greater the decrease in the proportion of the pressure and the velocity impacts in the total energy.
Because for a greater charge not only mUst the residual gas be scaven~ed to reduce the thermal stressing of the engine but fresh air must be passed through the combustion space, a greater scavenging pressure drop is desirable.
The scavenging of the air results in an increase of the air consumption with increasing charge.
The”scavenging drop is higher the greater the individual efficiencies of blower and supercharger. Figure 13 shows the variation of charging pressure, mean pressure} and mean temperature of the exhaust gas aheadof the turbine plotted against the engine power for the case of a recent exhaust-gas turboblower for four-stroke Diesel engines.
Application of B~chi-Charging The B~chi system of sui>ercharging is variously applied for increasing the power of stationary, marinej and automotive engines.
The char~ing unit is mounted either on the front side of the engine over the coupling or, particularly in the case of V-engines, over the engine itself.
Because the space above the coupling is in most cases free anyway, the first arrangement as a rule requires no addi- tional space and leads to somewhat longer but stra~.ght exhaust piping.
The second arrangement is applicable only if space is available above the engine and leads to shorter but more shar@y bent piping., Figure 14 shows a secti;n through a Diesel.engine charging unit consisting of exhaust-gas turbine and supercharger.
The increase in power attained through B;chi-charging of Diesel engines amounts to 60 percent and more for an increase in weight of 5 percent and less. The fuel consumption referred to the output is loweked by 3 to 5 percent through the exhaust-gas turbocharging, whereas in using a supercharger directly driven by the engine ‘.
., (mechanical’charging)the fu~l consumption wouldbe-increased-by several hundredths.
I?ACATM NO. 134S OF OTTO ?i3?GIlW?S The power obtainable from unit weight of the charge is, in the case of the Otto engine, larger on account of the considerably smaller excess of the combustion air so that the blower power required for precomyressing the charge is therefore smaller in relation to the engine Tower than in the case of the Diesel engine. For otherwise equal conditions, that is; equal surrounding and charging pressures, the saving attainable in fuel consumption throu@ the use of an exhaust-gas turbosuperchar~er as compared with the mechanical super- charger is therefore smaller than in the case of the Diesel engine.
The exhaust-gas turbosuyercharging shows up to advantage only if high compression ratios are used.
Aircraft ~Ihgines Exhaust-gas turbosupercharging is of decided importance fOr the aircraft engine for attaining a large high-altitude output. In this case the exhaust-gas turbosupercharging is not so much for the purpose of raising the charging pressure on the ground (boosting) as to main- tain the pressure up to as high an altitude as possiblej as the out- side pressure drops with altitude. For this purpose, the exhaust-gas turbosupercharger is particularly well suited because it can operate well on the greatly expanded gas with decreasing air pressure and because the drop in the turbine increases as the supercharger power required increases with increasing altitude (reference 8), the super- charger unit thus being self-re~ulating to some extent.
Because, in the case of the Otto engine, the exhaust temperatures due to the smaller expansion in the cylinder, are considerably higher than in the case of the Diesel engine, there is available for equal compression ratio a greater heat drop and therefore a greater power in the tui”bine. In the case of the Otto en~ine, it is therefore easier to keep the charging pressure above the pressure ahead of the turbine and thereby produce the required pressure drop for scavenging the residual gases. The exhaust-~as turboswpercharger of the aircraft engine draws further advanta~e from the fact that the air temperature decreases with increasil~ al-titudeso that the supercharger power becomes smaller, equation (2). The pressui-esin aircraft-engine superchargers and therefore the supercharger powers become so great at high altitudes that they cannot be attained with an exhaust turbine but only with a high drop produced by a building-up of the inlet pressure. The energy of the exhaust impulses depends essentially only on the engine Tower and therefore remains approximately unchanged with altitude. Its ratio to the total turbine power decreases with NACA TM NO. 1141 increasing heat drop in tke turbine so that for aircraft engines a subdividing of the exhaust pipin~ may generally be-dispensed with.
The difficul”:ies of the exhaust-gas turbosugerchargim~ of ai.r- ‘“cm.ft’ engines are chtefly”-”of a structural nat”ure. “Becausethe aircratt engine represents the extreme case of light construction, very high peripheral speeds of turbine and blower wheel must %e chosen in order to realize smaller dimensions.
The high stresees that then arise can be controlled,,ho~?ever, only at not too high temperatures of -thestructural i)az%s. The exhaust gases of aircraft engines whose temperature is cf the order of ma~mitude of 1000° C are therefore either cooled before entry into the turbine (~as coolin~) or a direct cooling of the crit”ical parts of the tiurbine(structural part cooling) is provided (reference 32).
The develo~ent of exl%aust-uas turbosupercharging of aircraft engines was u~-dertaken in France by Rateau as far back as the first world ~A~ar (refei”ences 4, 21, and 22). It is worth noting that the first aircraft en~ine with an exhaust-gas turbosupercharger already pos~essed an intake-air cooler for c~oling the chai”ge hea-ted“Dythe corqression in the blower (fige. 15 and 16)0 Automotive llqines Recent attempts have been made to apply the exhaust-gas turbo- supercharger also for raisi~ the pwer of atltOmObile O“Ltoengines using generator gas. In reconverting for generator gas, there occurs what is known as a considerable power drGp to be ascribed tG the low heat value of the generator gas-air mixture and to the high suction in the intake piping and hence low volumetric efficiency of the cylinder due to the resistances in the intake-gas generator and in the gas cleaning apparatus.
Through the exhaust-gas turbosuper- charger, which can be mounted without =Lly changes in the en~ine, the resistances are overcome and therefore the volumetric efficiency and power or the engine i-ncreased. Of particular promise is a super- charger process applied by Brown Boveri and Co. in which the blower compresses only air that divides into a stream leading to the gas generator and into a stream leading directly to the mixing apparatus (references 12 and 3S). In the mixin~ nozzle, compressed air and gas are mixed and led to the engine under a pressure slightly above alznospheri c.
NACA TM No. 1141 CHARGING OF TiJO-ST!ROKE DIESEL ENGINES The ex~aust-gas tur-oosuperchareing of t~o-s~ro~e engines (refer- en.ce41) differs frou that & the f’owr-strokeengine ~iilly in the following two resyects: 1. The two-stroke,enghe requires e greater quantity of ail-yer unit powe? Yor scaver,ging the com’!usticm Saaes Yron the cylinder than the four-stroke engine. Tile‘tiem~eretuue uf the exhaust gases is ablo h the tu.rbiile smal].er than -thereforelower and the energy avail.
in the case of tinefoul’-strolw engine.
2. In the case of the two-stroke engine, in order ‘thatchal-gii~~ exist for each load of the cyli~.der should occur at all, thare zmst co-ndi-tion a positive ~cavenging y~essure dmp between the inJ~ake and exhaust piping. TMs positive drop cannot bo supplied by the e.xhaust- gas turbosupereharger in starbin~ and ut low mgiae loads.
“fiereasin the case of the sv.pcrcharged four-stroke engine the greatest part cf the combustion gases is .rmovetl by the yiston and oniy the residual gas in the dead sya.ceis removed by the scavenging gas, in the case oi”the two-siro;:e engine the scave”n~in,g aii”Ymust take over tifle entire work of removai or the coiilusiion gases. The four-stroke engine draws in fresh cb-arge al,sowhen, with decreasir.g load, the charginS p~essure. becomes S-nnller than the pressure a~~ead con~ition cannot be maintained of the turbine; Whereas such o~erat~.ng in the case of the two-stroke engirm.
The magnitude of the scavengin~ pressure drop in the two-stroke en~ine depends OP.the type of tilescave~g!.ng system (thlwughflow, cross-flew, and reverse Y1OW scavengin~j etc.), the dimensions of the :30rts,the l“esistencesin the scavenging and outlet passages, the ratio of the air dicck-arged to the displacement Volumej and tdiepiston s~eed. In nGst engines the scavenging pressv,re?trop:or tho reqtiired LXLrticularly on a-mou.ni cf air to be ~asscd through is so high-tfi.at accoian.t of the relatively low temperature of ‘~hecdxaust gases it cmnot bc produced even a% “fuliload ly the exhaust-gas turbosU@er- chargcr alone.
~ The engine retains ‘Lhcmechanically driven scavenging blcwer -.
t~~r-~ost:porcl:.ai-;qcr is comected to which in addition the md:.u,ust-gas (Curtis Patent DR2. NO. 545,907, Priorj.ty T. 2, ?-2,and 24).
F
NACA TM No+ 1141 17 2. The compressor (or a piston blower) of dimensions corre- sponding tO the required charging is driven WJ “the engine shaft and the exhaust-gas,,t.u rbine,also d,elivezysits yower to the qngine shaft.
ThroU@ the fixed speed ratio between the engin”e and blower) ‘t~le latter always yroduces a sufficient charging i~ressure. This com- bination leads to tiletype of charging denoted alove as hi~h chaqgin.g.
l131e Power increase attainable through exhau~t-gas turboin~per- ch.ar~ingis smaller in the case of the two-stroke engine on account of its lower etiaust-.gas terayeratures than for the four-stro’ke engine, whereas the dimensions of the char~ing unit are Larger on account of the .greate~air consumption per unit power of the two-stroke engine.
Exhaust-gas turbosupei-charging has therefore not yet been commonly adopted for tiletwo-stroke engine, the application of the turbosuper- charger renaining limited to particular cases.
It has been used with great ]?racticalsuccess only in the case of the Junkers aircraft Diesel engine (references 21 and 22).
This engine is a through-flow scaven~ed two-stroke Diesel engine with opposed pistons with divided- shaft construction, one piston controlling the inlet ports, the other the outlet ports. The exhaust-gas turbosupercharger is connected to a blower driven by the engine.
Recent tests have been conducted by Sulzer, Winterthur, on opposed-piston Diesel engines with through-flow scavenging (refer- ence 24). A piston compressor that delivers the scavenging and charging air was directly connected to the engine crankshaft. The exhaust &ases are slowed behind the engine and drive an exhaust turbiile geared to the crankshaft (fig. 17). The charge can thereby be raised to very high Bean effective pressures.
The following table shows the relation between the mean effective pressure and charging preesure where the latter denotes the pressure in the exhaust yi~i~ between the engine and the exhaust-gas turbine: ~ Charging pressure, 236 atmospheres absolute Mean effective pressure, 6 12 15 1!3 kilograms per square centimeters piston area The yower required by the piston compressor over a wide .chargin~ range is not greater than the power delivered by the exhaust-gas turbine.
By this process, the highest knownmean effective. pressures of two- stroke engines have been attai-ned.
. . . . .
. ,, , .. ———-...,.. . .
,. ,,... --.-, ---,-...
—,,1, ,!,--- , !!! !.. 1. I I ..
18 NACA TM No. 1141 The particular suitability of ‘~heopposed-piston engine l?or supercharging together with the high degree of scavenging hrou~ht about hy the throuGh-flow t~~e of scavenging is chiei’ly a consequence’ pIWSSUZW dr~~ made possille by ~lie large of the ‘mall scavenging inlet an-doutlet yorts.
I?lSTONT-TYW PROPELLANT-GAS GENERATOR The Wcrk Process The fu-ndarnental idea of tinepropellant-gas process, that is, ~he charging of a combustion eqgine to such a degree that the i“equired coiipressor power must le covered by the engine cylinders and tke useful power is taken off the tiurhine,is not v.ewand the testing and de~elopment work comaected with various modii’icakions already can look back on a relatively. long history (reference 40). The principal advantages d the >ropellant-gas process are to be found iilthe possibility of a spatial separation of the propellant-gas generator !lj:omb-u.s.~i~n c]l~~iber” ) fro~ the turbine and in the (tb.e “boiler” or possibility of attaining efficiencies resuiting fz’omthe combination of the piston engine, which is suited for high pressures and iemper- atures~ with the tLn.’bine, which is suited for low pressures and her,ce l’neP-v diagram of the process has flmdamentally the large volumes.
same appearance as that for the combusti-on engine ~~itk. the exhaust- $-asturbosupercharger (fig. 10).
Because the co-aqpressor is driven by the internal-combustim engine, the WOrliarea of the engine 1-2-3’-3-4 must be greater by -thelomses in Viieengine and conpi-essor tlnanthe ~~oiak area OF the compressc.r 0-1.-12-6. The deg>-eeof attainable charging pressure is determii~ed ky this ~o~d~-ti~n~ Also with the propellant.gas process here des~i’ibed, there is o“otainedthiiou@ the expar.sion of the exkaust ~ases a loss in work corresponding to tlw area 1.. ’-4-5(fig. 10).
In the case of the ~~f:nst:.pel-charged. two-stroke eriine, it is possible tm Utiliz:> ike Udx3ust energy for scavenging ?rithoutusing a scavenging blower (re;crence 5) but the utilization of this ener~y becamos relative].y small as soon as the free outflow is obs-tiruutod. the propellant-gas ~rocess The indicated effici@lcY of . - tl?erc?ore differs littlo from that of tho piston engine. For driving a highly charged four-stroke engine oi”“bwo-stro’ke the compressor engine may be used lut the two=-stroko process is prei’erable because of tts greater output per unit displacumcnt volwmc and becalusoof the l~wt<%z’ possibility of scavc:yji~+ largo qm.ntities of air thlwough the ungine as is necessary in order ‘Lalo~?~i- the ex’aaust-~as tmpcre.t-.nm and the thermal loading of the engine.
NACA TM No. 1141 19 The Crank-Type Propellant-Gas Generator ,..
If’ the compressorpiston is drivenover a,.crankshaft, by.the Diesel piston, a “crank-type propellant-gas generator” (fig. 18) is being dealt with. An’installation of the @ta Works, G&eburg (reference 7) operating on this process uses a slowly rupning, simply acting, two-stroke engine with through scavenging, the inlet ports being controlled by the piston and an outlet valve in the cover through cams and rods.
In order to assure ignition in starting and at low loads without having the combustion pressure in the cylinder,too high at high load, the compression is regulaked by the closing-time point of the exhaust valve as a function of the propellant-gas pressure bekrindthe engine, that is, by the load.
The actual compression amour,ts~ at starting and at 10TT loads, (propellant-gaspreseure up to 0.5 atmosphere gage), to 85 percent of the piston stroke and at high loads with 3 atmospheres gage to on~J 40 percent of tbe ;)istonstroke.
As the lowest fuel consump- tion for this installation, the value of 185 grams per horsepower hour is given, which for an average lower heating value of the gas oil of ,10,100kilogram calories per kilogram correspond~ to a b~ake efficiency of 33.8 ?ercenk.
The supercharged two-stroke en~ine has a mean effective pressure of 7.5 to 8 atmospheres; the weight of the entire installation i~ 20 kiSograms per horsepower.
The process is exclusively intended for marine propulsion, as would be expected from the choice of the large sSowly running engine and has already been put in practical operation in many installations.
The Floating-Piston-TypePropellant-Gas Generator The raising in pressure of the exhaust gases and the high- -pressurescavenging wi-ththe entire compression-air lead in the case of the floating-piston compressor (references 19 and 25) to the “floating-pistonpropellant-gas generator” in which, as in the case of the floating--piston compressor, the Diesel engine and compressor pistons are directly coupled to a crankshaft, whereby a particularly simple ty~e of construction of the propellant-gas ~enerator is obtained.
Figure 19 shows the type of construction according to the patents of Pescara and Junkers.
Through combustion of the charge in the Diesel engine, the opposed pistons are thrust outward and compress air in the compressor cylinder or cylinders, which take up the energy of motion of the pistons.
Only a part of the air is displaced out of the compressors into the receiver formed by the engi’l~d housing, the remainder of the ahj’ through its expansion, again throwing the pistons toward the bottom dead center and thereby compressing the new charge.
In the Junkers design the energy of the back motion of the pistons is stored up only in the suitably 20 I?ACATM NO. 1141 dimensioned dead spaces of the compressor, whereas in the design of Pescara two air cushions, which are additionally built in for regu- lating purposes, take over a part of the energy. As soon as the yistons in their top-dead-center positions expose the ports in the cylinder walls, the combustion gases are scavenged into the propellant- gas piping and the Diesel cylinder is filled with the ~recompressed air.
Test installations with floating-yiston propellant-gas generators have been developed by I?escara(reference 26) and Sulzer (refer- ence 24). The efficiency of the proTe3-le.nt-@s process for the Sulzer installations is given as 35 to 40 ~er~ent rindthe propellant-gas pressure between engine and tu?hine as 5 to 6 atmospheres. For an .
increase of the mean effeetive pressure in the Diesel cylinder to three times that of the nonsupercharged engine, the buil-b-uppressure behind the engine (propellant-gasyressure) nustbe raised sixfold.
In crder to attain still higher charging pressures aildhence still smaller dimensions,in partic~?lar of the pis-toncompressor, two-stage ~ the charge air may be used, the first stage being precompression 04 formed by a centrifugal blower driven by an aux:.liary turbine.
In order to evaluate the perfoimancc of thu propellant-gas rat-loand the behavior in process, a knowledge of’the weight..y..~;er opei-ationis necessary in addition to the efficiency and fuel con- sumption. As follows from test data thus far published (refer- ences 24 and 26) and from theoretical investigations (reference 39), the useful efficiency of the Diesel engine may thereby, in the most favorable case, be attained but not exceeded. .Fundamentally) through ‘theexhaust-gas turhosupercha,rging as conpared with the nonsuper- charged engine, an improvement in the efficiency is obtained that again decreases] howeverj with the present-da:}’ separate efficiencies of corn:?ressor and turbine in the range of very high charging pres- sures such as are required for the yropel.lant-gas process.
As also regards weight, the crank-type propellant-gas Senerator can hardly compete with the Diesel en~ine particularly if the super- used for comparison. The floating-piston- charged Diesel engine % type propellant-gas generator, beca~o.se the crankshaft is missing and the force transmission between the Diesel engine piston and the compressor piston is simpler~ is as regards bulk and weight much superior to the crank-type propellant-gas generator. This advantage, however, does not show up to the same degree for multicyl.inder installations because the distance between cylinders of independent floating-piston-type propellant-gas generators cannot be made as small as in the case of the crankshaft engines.
NACA TM No. 1141 21 In the case of the floating-piston propellant-gas generator, regulation constitutes a particularly difficult problem. As there qre nQ,,UEltOrmlY moving masses, the piston ener~ stored by the corn- ..- .-. .
pressed air rnus~be bi~ed on the’’working stroke of thepiston, that is,on the load. In addition to this adjustment of the piston energy to be stored in the dead space of the compressor or in the air cushion to the variable propellant-gas pressure and hence the variable volume of the combustion space, a number of other regulating functions must be fulfilled (as the adjustment of the propellant-Gas quantity and pressure to the load of the turbine and the determination of the piston stroke for the most favorable operating values), which require a sufficiently complicated regulating apparatus on the reliable functioning of which the operation of the entire installation depends.
Because the propellant-gas generator, both the crankshaft- and floating-piston types, does not have atiJspecial scavenging pump, the scavenging being carried out with the ,high-pressureair of the com- pressor, the engine cannot idle.
In order that the work cycle be maintained, air must constantly be compressed into the cylinder and propellant-gas constantly generated.
With decreasing load of the turbine therefore, insofar as several gas generators work on one turbine, the individual machines must be successively disconnected, that is, for bridging over the individual partial load stages and for idling of the individual units, by_passvalves”around the turbine must be opened through which the propellant gas may escape without performing work. This regulation results in an impairment of the ~~artialload efficiency.
FIELDS OF APPLICATION OF THE VARIOUS PROCESSES In concluding, a brief discussion will be given of the fuel consumption, weight-power ratio, and several other determining characteristics of the different installations considered.
With the propellant-gas process, it is possible to operate a turbine with very good efficiency but the process does not have the operational simplicity of the simple gas turbine.
Inasmuch as the propellant-gas process is to be considered as a particular case of the exhaust-gas turbosupercharged Diesel engine and uses the same fuels, it is to be compared chiefly with the Diesel engine. The most imyortant characteristic of the propellant-gas process is the force transmission by means of compressed air and the consequent possibility ,,,. ,,, . ,, ..., of transforuin&the speed and torque and, ~y connecting together several units, of combining a large power’in ‘one “tu’i~bi’ner ‘Andtherefore on one shalt. Because the propellant-gas process shows no advantages .
22 NACA TM No. 1141’ as conqmred with th~ Diesel engine with regard %0 fuel consun.~tion, mode of o~eration, or space requirements, it will find application only i:ncams where the ad.van’cage of the syecd and torquo conversion ilsa~.important factor.
gas %lurbinefinds the following For direct yower gencraticn, the fields of avmlicatior$: ..
in particular.”, forms of aircraft.
1. Propulsion of large vehicls~, NACA TM No. 1141 3. Stationary Power Generation.
For this application only an installation using solid fuels or . . . .
&-aseous fuels’obtained from the former with as extensive-heat recovery as possible has any prospects of competing with other stationary power generators. With such installations, which would have, however, considerable space requirement, it is possible to exceed the efi’i- ciency of the steam turbine.
For an increase in the efficiency of from 1!3to 26 yercent, there is required for the open circuit a heat- exclxmge area of about 1 meter squared per horsepower, increasing the weight of the installation by about 10 kilograms per horsepower.
Each further increase in the efficiency makes necessary an increasingly larger heat exchanger and therefore a heavier installation (refer- ence 27). For the application of solid ftie.ls and for the attainment of high effici&cy, the closed-circuit system is ?nor@suitable. The latter possesses, it is tru~, an air heater corresponding in its dimensions to the Steam boiler but the apparatue for raising the efficiency, as intercooler and heat cxch~mger, are smaller and may be designed with smaller temperature and pressure losses than for the open-circuit system.
The advantages of the gas turbine as compared with the internal- conbustion engine increase with the power.
In the first place, the efficiencies of the gas turbine improve with increasing size; secondly, the weight-power ratio does not increase with the power, whereas in the case of the internal-combustionengine large outputs are attainable only through greater weight-power ratio (larger cylin- der dimensions) or through a large number of cylinders.
From the preceding discussion, it may be inferred in what fields the internal-cornhustion engine i~illcontinue to maintain its position: In the first place, in the field of small outputs up to about 1000 horsepower where the gas turbine, on account of the lower efficiency of flow machines at small sizes, becomes too uneconomical and where the high-speed combustion engines can still use few cylin- ders.
In the second place, also in the field of medium and large outputs wherever small space and weight and small fuel consumption are simultaneously required.
In particular the exhaust-gas turbo- supercharging,which likewise will continua to benefit from the further development of flow engines, makes possible not only a considerable lowering in the weight-power ratio but also a certain improvement in the efficiency, for example, through better utiliza- tion of the exhaust energy.
As”to how far the simple gas turbine will succeed; mainly by way of an increase in the temperature, in entering the efficiency range N.&34TM No. 114.1 of the internal-combustion engine also for small.space and weight conditions, ~.ofinal answer can at present be given.
.
‘Translation’by S. I@iss, National Advtecry Co’tiittee for Aeronautics.
REF.WNCES Aerodynamische W&mekraftmaschine mit 1; Ackeret, J., u. Keller, C.: Z. ‘KOIBd. 95 (1941) S. 491/500 U.
geschllossm.om Kreislauf.
Escher-Wyss-Mitt. Bd. 15/16 (i942/45) S. 5/19.
2. b~chi, A.: her Verhrennun@kraf’tmaschinen. Z. ges. Tv.rb.-Wes.
Bd. 6 (1909) S. 313; Auszug in [61.
3. B~chi, A.: Die eiztscheidenden Mer’kmaleder B~chi-Abgastur’oinen- Aufladung von Verbrennnngsrcotoren.Mot.-Teclln. Z. Bd. 1 (1939) s. 19!3/99.
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. . . . . . . . . .
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13.
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14.
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15.
Leist, K.: Problemo des Ab,gast~~rbinenba~les. Luf%f.-Forsc@.
Bd. 15 (1939) S. 451/94 1,1.
Z. VIIIBd. 33 (1939) S. 1206/07.
16.
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Bd. 119 (1942) S. 229/33 U. 241/42.
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21.
v. d. Ntill, W.: Abgasturbolader f%r Flugmotoren. Z. VDI Bd. % (1941) s. !347/57 .
22.
v. d. Ntill, W.: Stratosph&enfflugzeugund H&entriebwerk.
Luftwissen Bd. 10 (1943) S. 212/21 u. 247/53.
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Autonol. Bd. 7.0(1937) S. 423/33.
La combustion clansle~ chanbres ~ volume variable.
26. Pescara, R.: Chal. et Ind. Bd. 20 (1939) S. 145/50; G&.&.teur >.pistons litires et turbine k gaz. Chal. et Ind. Bd. 20 (1939) S. 211/14.
Z\lsammenwirken von Motor und Gebl~se lei Avflade- 29. Pflatxm, W.: VI)Z-Hauptversmlung, Dieselmaschinen. Be~icktshefi 74.
Berlin 1.936, VDI-Verlag, S. 252/60.
Bauarten, Entwicklm.gsstand DIe Verbre~~~-ui~stu~-bine, 29. l?lening, W.: 3d. 22 (1’340) S. 19/Z3.
und Aussichten. Arch. W&mew.
Leist~lnZs~teige~un~ von Viertalrt-Dieselmaschinen 30. Reu’ter,H.: clurch Aui’lade-Gebl~se und Abgasturb:ne. Mot.-techn. Z. Bd. 3 (~gl$l ‘ .-) s. 335/!39.
~~erbrenn:~ng sno%ol”en..
Berlin 1939, Spri.nger- 31. Schmidt, 1’.A. F.: Verlag.
Unterstichun~ ~! .iber die 3eherrschunG H.oher 32. Sch8rner, Chr.: Abgastenperat>;ren- bei Abgas”b~~l-%oa.~;z”;.adung durch InnenkfiY.lung.
Jb. dtsch. Luftf.-Forsch~. Toil 11, Mfinchen-Bfir3.in 1935, Vei”lag s. 21-9/23.
Oldeabourg, Der He~~ti~eSt~~-ddes Gast’u.rbtp.enbaues.
Z. VIII 33, schtitte, A.: Bd. S4 (1.940) 8. 609/15.
Da~m@’-und Gastur’oinen. 6. Al~fl.Beialin1924, 36. Stodola, A.: s“ Iprlnger.
NACA TM No. 1141 37. Sto&ola, A,: Leistun&versuche an.einer Verbrenn.mgsturbine.
Z. VDIBd. “84 (1940) S. 17/20- ,.
39. Tree.sch, .M.:_. WistunGss%eigg&UEg_y.onHo%%a~~toren durch Abgasturbo]-ader der A. G. Brown Boverio & Cie., Baden (Schweiz)* ~4g/44 l Mot,-Techn. Z. Bd. 5 (1943) S.
3S. Zinner, K..: Die Gasturbine -mitKolbontreibgaserzeuger. Mot.- techn. Z. Bd. 5 (1943) S.,81/S0.
Die Gasturbine. Berlin 1913, Volklnarln. AUSZUg 40. Zs41yi, A.: darau~ in c6~.
41. Zernan~J.: Die neuere Entwicklung iiesZweitaktnotors, I. Verfahren 57 (1943) und Einrichtungen f~r don I,adungswechsel. Z. YDIBd.
s. 7/14.
—.
; ,,
P
J NACA TM NO. 1141 Fig. I !-, .. . . . . . .. .
,.. . . ..— ..j .— .
: s + a L w * a .
VI : Entropy, a~ a: Exhauat turbine Area 2-3-6-5 = heat supplied Area 1-2-3-4 = work available b: Conatant-preasure turbine without heat recovery Area 2-3-6-5 = heat supplied Area 1-2-3-4 = work available c: Conatant-preasure turbine with exhauat heat recovery Area 3~-3-8-6 = heat supplied Area I-2-3-4 = work available Area 2-3~-6-5 = 4~-4-8-l = exchanged heat Figure 1.
- Ideal work Cycle of the gas turbine, T-s diagram.
z o .
-4
. —.
a. Turbine d. Current generator b.
Air compressor e.
Starting motor c. Combustion chamber n -.
I Figure 2?. - Constant-pressure gas turbine with open circuit without exhaust heat recovery. ~
I
N — N ACA Ttvf NO.
IJ41 Fig. 3 .
.— . ... .—% .30 qG = 0.9; ~~ - 0,9 I I ,~G - 0.S5; mT - 0.85 .20’ ~G ‘IJ.8; ?IT= 0.8 . 10 #- ~G = 0.75; ?)T = 0,75 w I 2 3 4 5 6 7 8 9 10 II Compreeelon ratio, p2/pl ideal ges, lowest cycle temperature t, - 200 c; highest cycie temperature t3 = 600° C.
no loss in combustion chamber.
Figure 3. - Effect of compressor efficiency qG and of turbine efficiency ~T on efficiency of constant-pressure gas turbine without exhaust heat recovery.
— .
NACA TM No. 1141 Fig. 4 4 ‘ / .30 ‘ / / P .20 v . 10 /’ I o~ 1 I I I 2 3 4 8 9 5 6 7 Compression ratio, p2/pl loss; compressor efficiency no heat and pressure Ideal gas, turbine efficiency ~G = 0.85.
= 0.85; ~G Efficiency without exhaust heat recovery –---–-–Efficiency for transfer of 70 percent of heat from exhaust gases to compressed air before entrance into combustion chamber Figure 4. - Efficiency of constant-pressure gas turbine as ratio and gas inlet temperature a function of compression in the turbine.
‘3 Fig. 5 NACA TM NO. 1141 m , — .,. . . . .
-!_
f d p[ / a.
Compressor e. Circuit air heater b.
Turbine f. Heat exchanger c.
Current generator Pump for regulating air in circuit 9.
d. Combustion-air preheater h.
Precooler Figure !3. - work process of constant-pressure gas turbine with closed circuit according to Ackeret and Kelier.
,--- I Fig. 6 NACA TM NO. 1141 Q )4 Entropy, s~ Dashed arrows denote best exchsnge between compressed and ex- psnded working gas.
Area 3-4-8-7 = heat supplied Q Area I-2-5-6 = heat removed QO diagram.
Double isothermal Cycle represented in T-s Figure 6. - ‘1 z b b b o .
lat+H at-
T II
Lk
a.
Compressor-stage groups e. Air heater b. Turbine-stage groups f.
Intersuperheater c.
Precooler Heat exchanger 9.
d.
Intercooler h. Current generator Figure 7. - Installation for approximate double isothermal cycle according to Ackeret and Keiter.
n -.
m .
I NACA TM NO. 1141 Fig. 8
Al
a Entropy, s ~ Q = heat supplied, equal in both cases for equal pressures and compression ratios Q. = heat removed Heat Area 1-2-3-4 corresponds to heat transformed into work.
recovery in exchanger is represented by dashed arrows.
* Ideal work process of constant-pressure gas Figure 8. - turbine with exhaust heat recovery for adiabatic and isothermal compression.
— NACA Ttd NO.
Fig. 9 ~e ,— 2( I Loss due to incomplete expansion I Volume ~ Area 1-2- 3! -3-4 = work obtainable in engine cylinder Area 1-4-5 = loss due to incomplete expansion in cylinder Area 1-4-7-6 = work during exhaust stroke when pressure at in cylinder end of expansion is P4 Area 4-5-6-7 = work obtained in turbine by using total expansion Figure 9. - Ideal work cycle represented by p-v diagram ot internal-combust ion engine.
Fig. 10 NACA TM NO. 1141 3’
I ( ‘
: : Ill al k l.OSS due to incomplete expansion (exhaust energy) b a Specific volume ~ e. Charging pressure b. Exhaust pressure aheed of turbine c. Surrounding pressure Area 1-2-31-3-4 - work obtainable in en91ne cylinder Area 6-7-8-9 - work obtainable in turbine Area o- I-IO-6 = work to be exPended in compressor Area 11-4-5 = loss due to incomplete exPans i On in cylinder Diagram not drawn to scale Figure 10. - Work cycle of combination engine with charging pressure above expansion exhaust pressure.
—.., ,.,. . . . . ,,, ,, ,,, , ,, , ,, 1141 Fig. II NACA TM NO.
,..
.
Pd e
/ \
‘II.( “-
Jii5f
‘f i4i?-
3.4 Crank arrangement a. c. Charging unit with turbine impel]er d Engine cy I inders b. Exhaust piping and blower disk e f.
Charge-air piping ese eng,ine with Figure II. - Six-cylinder four-stroke D Buch i-charging.
mmmmmmm ,m,, mum Im.-—M 888 . 8 ---—- s-. -- .. ,, . , .--———- m M n —m— Fig. 12 NACA TM NO.
U.T.6 u.T.2 u.T.3 u.T.6
I I
I
b. Mean charging pressure e. Pressure difference between c. Mean pressure in exhaust piping mean charging pressure and d. Time marks mean exhaust pressure f.
Scavenging time Black bars = opening time of outlet valve White bars = opening time of inlet valve T, scavenging time Figure 12. - Pressure variation [aJ in exhaust piping of a Diesel engine with Buch i-charging.
NACA TM NO.
Fig. 13 .
A .60 .+M .40 d p / //fl pL P , 200 .20 k I I ~ ~ F 4 T 4 T Engine power Figure 13. - Mean exhaust temperature tat charging pressure .
and pressure PA pL, in exhaust’ piping, measured with an inert manometer, of a four-stroke Diesel engine with Buc hi- charging.
Surrounding pressure about I atmosphere.
NACA TM NO. 1141 Fig. 14 . ., ——.— - :. .... ,— - .. ... ... .— -.— a. Gas-inlet housing f. Air-inlet channel lined b. Guide apparatus with sound-muffling material c. Turbine impeller Gas-outlet housing 9.
d. Blower disk e. Pressure spiral - Section through supercharger unit.
Figure 14.
z o .
f.
a. Exhaust-gas outlet to Scoop for intake air b. Intake-air piping turbine c.
Dry air cooler go Pressure carburetor n -.
d. h.
Compressor Exhaust piping (n e.
Exhaust-gas turbine i. Regulating vaive (n .
Figures 15 and 16. - oldest aircraft engine with exhaust-gas turbosupercharger of Rateau type [ reference 22).
z o .
a. Compressor c. Exhaust-gas turbine b. Diesel engine d.
Gear n -.
m Figure 17. - eight-cylinder opposed-piston engine supercharged to 2.5 at- ‘ Sulze r-type, mospheres absolute [ reference 24).
NACA Tlyf NO. 1141 Fig. 18 .
e Id,
‘T’lllllri
a.
Compressor cylinder b.
Diesel cylinder c.
Inlet port d.
Exhaust valve e. Propellant-gas piping f.
Gas turbine Figure 18.
- Propellant-gas process with crank-type propellant- gas generator.
————.
NACA TM No. ‘1141 ., Fig. 19 f
Q
c
-E
J2iF=-” “, e. Gear wheel a. Diesel engine f. Propellant-gas piping b. Diesel piston Receiver c. Compressor piston 9.
h.
Gas turbine d. Rack Figure 19. - Propellant-gas process with floating-piston- type, propellant-gas generator.
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