Purdue University
Purdue e-Pubs International Compressor Engineering Conference
School of Mechanical Engineering
1992
Basic Study on Engine with Scroll Compressor and Expander E. Morishita University of Tokyo
Y. Kitora Mitsubishi Electric; Japan
M. Nishida Mitsubishi Electric; Japan
Follow this and additional works at: http://docs.lib.purdue.edu/icec Morishita, E.; Kitora, Y.; and Nishida, M., "Basic Study on Engine with Scroll Compressor and Expander" (1992). International Compressor Engineering Conference. Paper 849. http://docs.lib.purdue.edu/icec/849
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BASIC STUDY ON ENGINE liTH SCtDLL COMPRESSOR AND EXPANDtR Etsuo Morish ita
Yoshihi sa Kitora
Mitsuh iro Nishida
Associa te Profess or Dept. of Aerona utics Univ. of TokYo Tokyo, JAPAN
Manacer Central Researc h Lab. Mitsub lshi Electri c Anusa ki. JAPAN,
Chief Enginee r Fukuoka Works Mitsub ishi Electri c Fukuoka , JAPAN
ABSTRACT Scroll co1pres sors are beco•in s popular air conditi onior and refrige ration. This is priuri ly due to theirin higher efficie ncy and Jo, noise/v ibratio n charac teristic s. The scroll princip le can be appi ied also to the stea• expande r and the Brayton cycle engine, as shown in the past literat ure. The Otto cycle spark-l enition engine with a scroll co•pres sor 11nd expande r is studied in this report. The princip le ond basic structu re of the scroll entine are explain ed. and the enslne charac teristic s 11re calcula ted based and process es. A prototy pe 1odel hils been Proposeon the idealiz ed cycles d rotary type engine has always had a problem •ith and constru cted. The sealing . The scroll engine might overco• e this shortco •inr Jith .uch lower rubbinr speed co•Pare d to its previou s counte rparts, its and is therefo re worth investigatin~.
NOIENCLATURE
a :
Cv: F h N p
P q
r R0 : t :
T Y
I ~
Y
P ~
radius of a base circle specifi c heat at constan t volu•e force scroll height nu~ber of scroll turns - 1/4 scroll Pitch pressur e heat additio n per unit 1ass crank radius pressur e ratio (after ignitio n/ before isnitio n) scroll wrap thickne ss torque volume work per revolut ion involut e initial ansle on the base circle specifi c heat ratio volume tric ratio efficie ncy rotatin g angle pressur e ratio Involut e extensi on ~ngle
suffixe s a : av: c d e s 8
axial average colpres sor dischar ge expande r i-th chamber (increa se outward ) co•pres sor suction tansen tial end of co1pres sor dischar ge process
577
INTRODUCTION lower Scroll co1pres sors have a s1ooth torque variatio n. Th~ir cy, because efficien higher a realize to the• enables speed rubblns thP.llfY and the art •echanic al sealing can be effect ivel~ eaployed . Th~llLl~ producand of the scroll co1press or have alreadlP" been studied with the scroll tion is now in progress . This theory and exPerien ce scroll coapress or coapress or •aY be easily extended to an engine with a es the air-fuel and a scroll exPande r. The scroll coiPress or co•pressfrom the heat of aixture, and the scroll expander is·· used to get power ctional, which is the coabust ion. The flow is continuo us and uni-dire the Brayton differen t froa that of the reciProc ating ensine. Although a scroll expancYcle engine is possible with a scroll compres sor and coijld be extreme ly high der. the tempera ture of the scroll expande r scroll expandue to the continuo us coabusti on. The requirea ent of the scroll engine der aaterial aay beco1e ven severe. The spark-ig nition dard Otto is therefor e studied in this rePort, based on the air-stan be eased bY the cYcle. The thermal Probleas of the scroll expande r canchamber, although fresh air-fijel mixture charged into the combust ion high due to the the te11pera ture of the air-fuel 1ixture is already scroll ensine is the of volu•e •ent displace the Although sion. COIPres two pairs of halved co1pared to the reciproc atinc engine, we need t ur e w"il l be des c r o I I s • · Many con c e11 t s of t he s c r o 1 I · ens i n e s t r u cd,· part to cancel vel oped. A structur e of the scroll ensine is propose of the axial forces of the scroll expande r.
PRINCIPLE in Fig.!. The operatin g principl e of the scroll engine is shown compres sor and The scroll engine consists of two co11pone nts, the scroll the same geometthe scroll expande r. The ele•ents are assumed to have so. ric di•ensio ns and to be synchro nized via a proper mechani or. The coapress Fis.la) shows the seal-o.f f position of the scroll peripher y. This air-fuel mixture is taken into the co11pres sor from the correspo nds to the suction process. mixture After several degrees of rotation , the compres sed air-fuel as shown in Fig.lb). The volume of discharg e is to comaence at e". r is minimum at the coabusti on chaaber (innerao st) of the scroll expande shape sholl'n in the scroll this angle. The volume is actually zero when port of the scroll compres sor and the Fis.l is employed . The discharg e d via a dischar! e combust ion chamber of the scroll expande r are connecte to the coobusti on red transfer is •ixture air-fuel ed co1press The valve. The volume Fig.lc). in shown is This r. expande scroll the of chamber during the transchange rate of the two connecte d chamber s is the same to the scroll fer process. The discharg e fro• the scroll compres sor and the volume of the coabusti on chamber of the expande r ends at e
co•Press ot and the expander rotatins angles. This is given bN (3)
Eq,(3) shows that the volume of the combustio n chamber (i=l) of the scroll expander takes a aini1u1 value when the air-fuel mixture discharge commence s fro• the scroll compress or. to The volu.e of chubers is shown in Fig,2 when Eq.(3) is applied =~~:/5 the scroll engine, where IJ •. ,=11:/2 (IJ •.• =37r/2l,IJ ,'=3~~:/2 (IJ .'•ll'/2l,a calcuThe sue condition s are applied to the followin£ E "8.90. and of lations and fi;ures. The overlappi nt: zone of the minimum chambers mixture the scroll compresso r and expander corresPon d to the air-fuel is transfer process fro• the compresso r to the expander . This Process t. There assumed to occur at a constant pressure in the ideal treatmenthe dead appears a discontin uitN in volume change. which is due to the invovolume of the innermost chaaber. The dead volume exists bewhen avoided when It can lute of a circle is used for the wrap shape. the shape shown in Fir.! Is e1ploYed . Pressure therThe Pressure of each cha1ber Is obtained assuming the proper presThe modynamic process, and the isentropi c process is considere d. by sure ratio of each chamber of the scroll compresso r is riven l:iiii::iiiN+l Eq.(4) is based on the co1presso r suction pressure pressure ratio of the scrol I expander is riven by p,,,=p,,
i
(8,)
l::iiii::iiiN+I
(4)
Ps(=PN+l ).
The
(5)
P5 • The exact Eq. (5) is also based on the colpresso r suet ion pressure Eq.(S) actualexpressio ns for Eqs.(4) and (5) are given in Appendix B. ly contains R0 • This is given by R
c-
p., I (8 •.• ) " " " S,I. PIJ. 1 (8 ... ,d b•/cr• s_ r.
and indicates the pressure ratio after the spark-ig nition.
in
(6)
the co11bustio n chamber before
and
angle Fi,.3 shows the pressure of the scrol I ensine vs the rotating for R0 =3 •
.t:1. 0 j u r am The P-V diarram in the idealized cYCle and processes is obtained and fig.3, and for the scroll eosine by combining the results of fig.2 kinks in the is shown in Fis.4 in non-dime nsional for• for R0 = 3. The The P-V diagram are due to the dead volume in the innermost chambers. of the area covered by the P-V diasra• is the theoretic al output work because scroll engine. The theoreti~al output work is zero when Rc~l drive the the output of the scroll expander is the same as the power to is also scro-ll compress or. The theoretic al output work per revolutio n for calculate d from the Otto cycle theory, and this is riven as follo•sthe Otto cycle scrol I engine: where
(7)
w_=2?r (2N-l) __ P.prh V,,
.:=
v •.
1
(8)
11 (Q) (e •.• )
r techThis is called built-in voiUie ratio in the field of compresso expansion nolon. This is also ter•ed as the compress ion ratio and the ratio at the ratio in the enrine field. It correspon ds to the volume ting botto• dead center and at the top dead center In the reciproca al engine. £q.(7) simply tel Is us that the scrol I en~ine theoreticthe i.e., output work is proportio nal to the nu•ber of scroll turns, 580
displacem ent volume. The Fi~.
T~V
5 for
diagram is also obtained fro•
9 0 =3.
p~v
dia~rams
and,
is shown
in
The combustio n process is handled as the.~eat addition process at a constant volume in the idealized cYcle. The heat q added Per unit mass of the cycle gas, i.e., air. is estiu.,ted ·from th-e followins equation:
R :::I+ q/ (C,T,)
(9)
~J'-1
c:
H0
is used as a paraaeter in this report, although H0 Is estimated from the lower heo.t of coabustio n of the fuel in the real engine. The relation of Eq.(9) is shown in Fig.S for • = 8.90. Effjcien q .2.1
Otto~
k.u.U
~
The efficienc y of the Otto cycle scrol I e~~~ne is exactly the same as that of the conventio nal Otto cycle engine , and is given as fol~ I o ws:
1 77=1-e .,.-1
(I 0)
The compressi on ratio • is a function of Y~ J(8,.• ). i.e., 8, .• This relation is shown in Fi,.7. • is var1ed by changing the discharge timing fro~ the co•presso r into the exPander.
The theoretic al oijtput Hrk of the scroll engine is given by Eq. ( 7), and therefore the o.verage torque T._v of the scroll engine is derived from the relation f•21ffav together with Eq.(7),
~= P,prh
(2N-l)
(R,-I)
The avera~e torque Tav is also turns like the engine output.
(e'~'-l)
(II)
proportio nal
to
the
nu1ber
of
scroll
The torque variation during a rotation iS calculate d as the difference between the scroll exp•nder and the scroll co1presso r torque. a.nd is ( 12) The scroll co•Presso r theory is applied to calculate Eq.(l2), Eq.(3) is used fora,. The torque of the scroll expander is obtained and by reversing the rotatio~ of the scrol I co~pressor. The resijlt is obtained as follows. by using Eq.(l): T,(
8
,)
P,prh
=t.• , (Zi-2+~>r )
{P •. ,(8,)
- p •. ,.,(8,)}
(13)
The torqije of the scroll coapresso r is given by (I) T, (8,) ~ 8, p,prh
=,-:,
(2i--;-) {Pt-4a) J =A,,, (Zn-8,)
3-
(
[Zj -.5] -8,)
3
l
i=l 0:$8,
