Dong co dot trong 1

TRNG I HC NHA TRANG

NTUi hc nha trang

1959

PGS. TS. Nguyn Vn Nhn

L thuyt

(Ti liu lu hnh ni b)

Nha trang - 2010

PGS. TS. Nguyn Vn Nhn - NG C T TRONG (1) - 2

Chng 1

TNG QUAN V NG C T TRONG

1.1. NH NGHA V PHN LOI NG C T TRONG ng c l mt loi my c chc nng bin i mt dng nng lng no thnh c nng. Tu thuc vo dng nng lng u vo l in nng, nhit nng, thu nng,v.v. ngi ta phn loi ng c thnh ng c in, ng c nhit, ng c thu lc,v.v. ng c t trong l mt loi ng c nhit, tc l loi my c chc nng bin i nhit nng thnh c nng. Cc loi ng c nhit ph bin hin nay khng c cung cp nhit nng t bn ngoi mt cch trc tip m c cung cp nhin liu, sau nhin liu c t chy to ra nhit nng. Cn c vo v tr t nhin liu, ngi ta chia cc loi ng c nhit thnh hai nhm : ng c t trong v ng c t ngoi. ng c t trong, nhin liu c t chy trc tip trong khng gian cng tc ca ng c v cng ti din ra qu trnh chuyn ho nhit nng thnh c nng. ng c t ngoi, nhin liu c t chy trong l t ring bit cp nhit cho mi cht cng tc (MCCT), sau MCCT c dn vo khng gian cng tc ca ng c thc hin qu trnh chuyn ho nhit nng thnh c nng.

Theo cch phn loi nh trn th cc loi ng c c tn thng gi nh : ng c xng, ng c diesel, ng c piston quay, ng c piston t do, ng c phn lc, turbine kh u c th c xp vo nhm ng c t trong ; cn ng c hi nc kiu piston, turbine hi nc, ng c Stirling thuc nhm ng c t ngoi. Tuy nhin, trong cc ti liu chuyn ngnh, thut ng "ng c t trong" (Internal Combustion Engine) thng c dng ch ring loi ng c t trong c in c c cu truyn lc kiu piston-thanh truyn-trc khuu, trong piston chuyn ng tnh tin qua li trong xylanh ca ng c. Cc loi ng c t trong khc thng c gi bng cc tn ring , v d : ng c piston quay (Rotary Engine), ng c piston t do (Free - Piston Engine), ng c phn lc (Jet Engine), turbine kh (Gas Turbine). Trong gio trnh ny, thut ng ng c t trong (vit tt : CT) cng c hiu theo quy c ni trn.

CT c th c phn loi theo cc tiu ch khc nhau (Bng 1.1-1). Cn c vo nguyn l hot ng, c th chia CT thnh cc loi : ng c pht ho bng tia la , ng c diesel , ng c 4 k v ng c 2 k.


Bng 1.1-1. Phn loi tng qut ng c t trong

Tiu ch phn loi Phn loi

Loi nhin liu - ng c chy bng nhin liu lng d bay hi

nh : xng, alcohol, benzol, v.v. - ng c chy bng nhin liu lng kh bay hi,

nh : gas oil, mazout, v.v. - ng c chy bng kh t .

Phng php pht ho nhin liu

- ng c pht ho bng tia la - ng c diesel - ng c semidiesel

Cch thc thc hin chu trnh cng tc

- ng c 4 k - ng c 2 k

Phng php np kh mi vo khng gian cng tc

- ng c khng tng p - ng c tng p

c im kt cu - ng c mt hng xylanh ; ng c hnh sao ;

hnh ch V, W, H, ... - ng c c xylanh thng ng, ngang, nghing

Theo tnh nng - ng c thp tc, trung tc v cao tc - ng c cng sut nh, trung bnh v ln

Theo cng dng

- ng c xe c gii ng b - ng c thu - ng c my bay - ng c tnh ti

ng c pht ho bng tia la (Spark Ignition Engine) l loi CT hot ng theo nguyn l : nhin liu c pht ho bng tia la c sinh ra t ngun nhit bn ngoi khng gian cng tc ca xylanh. Chng ta c th gp nhng kiu ng c pht

ho bng tia la vi nhng tn gi khc nhau, nh : ng c Otto , ng c carburetor, ng c phun xng, ng c t chy cng bc, ng c hnh thnh hn hp chy t bn ngoi , ng c xng, ng c gas, v.v. Nhin liu dng cho ng c pht ho bng tia la thng l loi lng d bay hi, nh : xng, alcohol, benzol , kh ho lng ,v.v. hoc kh t. Trong s nhin liu k trn, xng l loi c s dng ph bin nht t thi

k u lch s pht trin loi ng c ny n nay. V vy, thut ng "ng c xng"

thng c dng gi chung cc kiu ng c chy bng nhin liu lng c pht

ho bng tia la, cn ng c ga - ng c chy bng nhin liu kh c pht ho bng tia la.


ng c diesel (Diesel Engine) l loi CT hot ng theo nguyn l : nhin liu t pht ho khi c phun vo bung t cha khng kh b nn n p sut v nhit cao. Nguyn l hot ng nh trn do ng Rudolf Diesel - k s ngi c - xut vo nm 1882. nhiu nc, ng c diesel cn c gi l ng c pht ho

bng cch nn (Compression - Ignition Engine). ng c 4 k - loi CT c chu trnh cng tc c hon thnh sau 4 hnh

trnh ca piston.

ng c 2 k - loi CT c chu trnh cng tc c hon thnh sau 2 hnh trnh ca piston.


1.2. MT S THUT NG V KHI NIM THNG DNG 1) Tn gi mt s b phn c bn

1

23

14

4

5

6

7

8

13

9

11

12

10

H.1.2-1. S cu to ng c diesel 4 k 1- Lc khng kh, 2- ng np, 3- Xupap np, 4- Xupap x, 5- ng x,

6- Bnh gim thanh, 7- Np xylanh, 8- Xylanh, 9- Piston, 10- Xecmang, 11- Thanh truyn, 12- Trc khuu, 13- Cacte, 14- Vi phun nhin liu

2) im cht, im cht trn, im cht di im cht l v tr ca c cu truyn lc, ti d tc dng ln nh piston mt

lc ln bao nhiu th cng khng lm cho trc khuu quay. im cht trn (CT) l v tr ca c cu truyn lc, ti piston cch xa trc

khuu nht. im cht di (CD) l v tr ca c cu truyn lc, ti piston gn trc

khuu nht.

3) Hnh trnh ca piston (S) - khong cch gia CT v CD. 4) Khng gian cng tc ca xylanh - khong khng gian bn trong xylanh c

gii hn bi : nh piston, np xylanh v thnh xylanh. Th tch ca khng gian cng tc ca xylanh (V) thay i khi piston chuyn ng. 5) Bung t (VC) - phn khng gian cng tc ca xylanh khi piston CT.


6) Dung tch cng tc ca xylanh (VS) - th tch phn khng gian cng tc ca xylanh c gii hn bi hai mt phng vung gc vi ng tm ca xylanh v i qua CT , CD :

SDVS

=

4

2pi (1.1-1)

trong : D - ng knh ca xylanh; S - hnh trnh ca piston.

CTVC

CD

VS

V

H. 1.2-2. CT, CD v th tch khng gian cng tc ca xylanh

7) T s nn () - t s gia th tch ln nht ca khng gian cng tc ca xylanh (Va) v th tch ca bung t (Vc).

C

CS

C

a

VVV

VV +

== (1.1-2)

8) Mi cht cng tc (MCCT) - cht c vai tr trung gian trong qu trnh bin i nhit nng thnh c nng. nhng giai on khc nhau ca chu trnh cng tc, MCCT c thnh phn, trng thi khc nhau v c gi bng nhng tn khc nhau nh kh mi, sn phm chy, kh thi, kh st , hn hp chy, hn hp kh cng tc.

Kh mi - (cn gi l Kh np) - kh c np vo khng gian cng tc ca xylanh qua ca np. ng c diesel, kh mi l khng kh ; ng c xng, kh mi l hn hp khng kh-xng.

Sn phm chy - nhng cht c to thnh trong qu trnh t chy nhin liu trong khng gian cng tc ca xylanh, v d : CO2 , H2O , CO , SO2 , NOx , v.v.

Kh thi - hn hp cc cht c thi ra khi khng gian cng tc ca xylanh sau khi dn n sinh ra c nng. Kh thi ca ng c t trong gm c : sn phm chy, nit (N2) v oxy (O2) cn d.

Kh st - phn sn phm chy cn st li trong khng gian cng tc ca xylanh sau khi c cu x ng hon ton.


Hn hp chy (HHC) - hn hp ca nhin liu v khng kh. Hn hp kh cng tc - hn hp nhin liu - khng kh - kh st.

9) Qu trnh cng tc - qu trnh thay i trng thi v thnh phn ca MCCT trong xylanh din ra trong mt giai on no ca chu trnh cng tc.

10) Chu trnh cng tc (CTCT) - tng cng tt c cc qu trnh cng tc din ra trong khong thi gian tng ng vi mt ln sinh cng mt xylanh.

11) th cng - th biu din s thay i ca p sut ca MCCT trong xylanh theo th tch ca khng gian cng tc hoc theo gc quay ca trc khuu .

CDCT

p

Vc Vs V

z

c

cf

r

a

bpo

r

CT CD CT CD CT

00 1800 3600 a

cfc

pz

b r

5400 7200

c)

H. 1.2-3. th cng ca ng c 4 k


1.3. CC B PHN C BN CA CT Tuy c hnh dng bn ngoi, kch thc v s lng cc chi tit rt khc nhau,

nhng tt c CT u c cc b phn v h thng c bn sau y : 1) B khung 2) H thng truyn lc 3) H thng np - x 4) H thng nhin liu 5) H thng bi trn 6) H thng lm mt 7) H thng khi ng Ngoi ra, mt s ng c cn c thm h thng in, h thng tng p, h thng

cnh bo-bo v ,v.v.

1.3.1. B KHUNG CA NG C B khung bao gm cc b phn c nh c chc nng che chn hoc l ni lp t

cc b phn khc ca ng c. Cc b phn c bn ca b khung ca CT bao gm : np xylanh , khi xylanh , cacte v cc np y, m kn, bulng, v.v.

H. 1.3-1. B khung ca CT 1- Np xylanh, 2- Khi xylanh, 3- Cacte trn, 4- Cacte di

43

21


1.3.1.1. NP XYLANH Np xylanh l chi tit y kn khng gian cng tc ca ng c t pha trn v l

ni lp t mt s b phn khc ca ng c nh : xupap, n gnh xupap, vi phun hoc buji, ng gp kh np, ng gp kh thi, van khi ng, v.v.

Np xylanh thng c ch to t gang hoc hp kim nhm bng phng php c. Np xylanh bng gang t b bin dng hn so vi np xylanh bng hp kim nhm, nhng nng hn v dn nhit km hn.

ng c nhiu xylanh c th c 1 np xylanh chung cho tt c cc xylanh hoc nhiu np xylanh ring cho 1 hoc mt s xylanh. Np xylanh ring c u im l d ch to, tho lp, sa cha v t b bin dng hn . Nhc im ca np xylanh ring l kh b tr cc bulng lin kt np xylanh vi khi xylanh, kh b tr ng np v ng x hn so vi np xylanh chung.

H. 1.3-2. Np xylanh a) Np xylanh chung, b) Np xylanh ring

1.3.1.2. KHI XYLANH Cc xylanh ca ng c nhiu xylanh thng c c lin thnh mt khi gi l khi xylanh. Mt trn v mt di ca khi xylanh c mi phng lp vo np xylanh v cacte. Vch trong ca cc xylanh c doa nhn, thng gi l mt gng ca xylanh.

Vt liu c khi xylanh thng l gang hoc hp kim nhm. Mt s loi ng c cng sut ln c khi xylanh c hn t cc tm thp. Xylanh ca ng c c lm mt bng khng kh c cc cnh tn nhit tng kh nng thot nhit. ng c c lm mt bng nc c cc khoang trong khi xylanh cha nc lm mt.

a)

b)


1.3.1.3. LT XYLANH

H. 1.3-3. Lt xylanh a) Lt xylanh ca ng c 2 k,

b) Xylanh ca ng c c lm mt bng khng kh Lt xylanh l mt b phn c chc nng dn hng piston v cng vi mt di

ca np xylanh v nh piston to nn khng gian cng tc ca xylanh. Trong qu trnh ng c hot ng, mt gng ca xylanh b mi mn bi piston v xecmang. Tit din trn ca mt gng xylanh s b mn thnh tit din hnh bu dc v lm cho kn ca khng gian cng tc b gim st sau mt thi gian lm vic,. Bin php khc phc l doa li cho trn. Nu lt xylanh c c lin vi khi xylanh (H. 1.3-4a) th phi thay c khi sau vi ln doa khi ng knh xylanh qu ln v thnh xylanh qu mng. V vy, lt xylanh thng c ch to ring ri lp vo khi xylanh (H. 1.3-4b, c). C th phn bit 2 loi lt xylanh : lt xylanh kh v lt xylanh t.

Lt xylanh kh (H. 1.3-4b) khng tip xc trc tip vi nc lm mt. Phng n s dng lt xylanh kh c u im l khi xylanh cng vng hn, nhng yu cu chnh xc cao hn khi gia cng b mt lp rp ca lt v khi xylanh.

Lt xylanh t (H. 1.3-4c) tip xc trc tip vi nc lm mt. Phn di ca lt xylanh c cc vng cao su ngn khng cho nc lt xung cacte (H. 1.3-4d).

a)

b)


D d

h

m

H. 1.3-4. Thn ng c v lt xylanh a) Lt xylanh c lin vi khi xylanh, b) Lt xylanh kh,

c) Lt xylanh t, d) m cao su kn nc

1.3.1.4. CACTE Cacte l b phn bao bc v l ni lp t cc b phn chuyn ng ch yu ca ng c. Phn trn ca cacte (cacte trn) l ni lp t khi xylanh, trc khuu, trc cam, v.v. Phn di ca cacte (cacte di hay cacte nht) c chc nng y kn khng gian trong ng c t pha di v l ni cha du bi trn. a s ng c c nh v trung bnh c lm mt bng nc, c khi xylanh v cacte trn c c lin thnh mt khi gi l thn ng c (H. 1.3-4 ). mt s ng c c ln , cacte di va l ni cha du bi trn va l ni t trc khuu v cc b phn lin quan.

a) b)

c)

b)

d)


1.3.2. H THNG TRUYN LC H thng truyn lc c chc nng tip nhn p lc ca kh trong khng gian cng tc ca xylanh ri truyn cho h tiu th v bin chuyn ng tnh tin ca piston thnh chuyn ng quay ca trc khuu. Cc b phn chnh ca h thng truyn lc cng chnh l cc b phn chuyn ng chnh ca ng c, bao gm : piston, thanh truyn, trc khuu, bnh . Cc b phn c lin quan trc tip vi cc b phn chuyn ng chnh k trn cng c th c xp vo h thng truyn lc, v d : xecmang, cht piston, bc lt c chnh, bc lt c bin, v.v.

1

3

2

4

H. 1.3-5. C cu truyn lc 1- Piston , 2- Thanh truyn , 3- Trc khuu , 4- i trng

1.3.2.1. PISTON Piston l b phn chuyn ng trong lng xylanh. N tip nhn p lc ca MCCT ri truyn cho trc khuu qua trung gian l thanh truyn. Ngoi ra, piston cn c cng dng trong vic np, nn kh mi v y kh thi ra khi khng gian cng tc ca xylanh. Piston c c bng gang, hp kim nhm, v i khi bng thp. ng c cao tc thng c piston bng hp kim nhm nhm gim lc qun tnh v tng cng s truyn nhit t nh piston ra thnh xylanh do nhm nh v dn nhit tt hn gang. Piston c cc phn c bn l : nh piston, cc rnh xecmang, "vy" piston, cht piston v cc gn chu lc.

- nh piston c hnh dng kh a dng, tu thuc vo c im t chc qu trnh chy v qu trnh np - x, v d : nh lm to chuyn ng ri ca kh trong bung t ; nh li dn hng dng kh qut v kh thi ,v.v.

- Vy piston (piston skirt) c vai tr dn hng trong xylanh v chu lc ngang. - Rnh xecmang l ni t cc xecmang. Cc rnh xecmang kh c b tr pha

trn cht piston. Rnh xecmang du c th b tr pha trn hoc pha di cht piston.


4

3

2

1

H. 1.3-6. Piston 1- nh piston , 2- Phn rnh xecmang , 3- Phn vy piston , 4- cht piston

1.3.2.2. XECMANG Xecmang (cn c gi l bc piston hoc vng gng) ca CT l cc vng n

hi bng vt liu chu nhit v chu mi mn c lp vo cc rnh trn piston. Trn mt piston c 2 loi xecmang : xecmang kh v xecmang du.

Xecmang kh c chc nng lm kn bung t v dn nhit t nh piston ra thnh xylanh. Trn mi piston c t 2 n 4 xecmang kh . Xecmang kh trn cng c gi l xecmang la, mt ngoi ca xecmang ny thng c m crm tng bn.


Xecmang du c chc nng san u du bi trn trn mt gng ca xylanh v gt du bi trn t mt gng xylanh v cacte. Trn mi piston c t 1 n 2 xecmang du bi tr pha di xecmang kh.

p [bar]3020100

H. 1.3-7. Tc dng lm kn bung t ca xecmang kh

c)b)a)

H. 1.3-8. Hin tng xecmang bm du ln bung t (a, b) v tc dng gt du ca xecmang (c)

1.3.2.3. CHT PISTON Cht piston l chi tit lin kt piston vi thanh truyn. Cht piston thng c

khoan rng gim khi lng. C 3 phng n lin kt cht piston vi piston v thanh truyn nh sau :

- Cht piston c c nh vi thanh truyn v chuyn ng tng i vi piston (H. 1.3-9a).

- Cht piston c c nh vi piston v chuyn ng tng i vi thanh truyn (H. 1.3-9b).

- Cht piston chuyn ng tng i vi c thanh truyn v piston (H. 1.3-9c).


H.1.3-9. Cht piston v cc phng php nh v cht piston

1.3.2.4. THANH TRUYN Thanh truyn l b phn trung gian lin kt piston vi trc khuu v cho php bin chuyn ng tnh tin qua li ca piston thnh chuyn ng quay ca trc khuu. a s thanh truyn c ch to t thp bng phng php rn hoc dp. Thanh truyn c cu thnh t 3 phn : u nh, thn v u to. Thanh truyn ca ng c cng sut trung bnh thng c u nh, thn v na trn ca u to c rn lin thnh 1 chi tit, na di ca u to (cn gi l np thanh truyn) c lin kt vi na trn bng 2 4 bulng. Mt s ng c 1 xylanh loi nh c thanh truyn c dp hoc c lin . Thanh truyn ca ng c ln thng c cc phn c ch to ring bit ri lp vi nhau bng bulng. c th rt nhm piston-thanh truyn qua lng xylanh trong qu trnh sa cha, i khi phi ch to u to thanh truyn theo "kiu lch" gim chiu ngang ca thanh truyn (H. 1.3-11).

a) b)

c)


H. 1.3-10. Cc chi tit ca nhm thanh truyn 1- u nh , 2- Thn , 3- u to , 4- Np , 5- Bc c bin

6- Bulng thanh truyn , 7- Bc cht piston

H. 1.3-11. Thanh truyn vi u to kiu lch

1.3.2.5. TRC KHUU

Trc khuu l b phn c chc nng tip nhn ton b p lc ca kh trong xylanh ri truyn cho cc h tiu th ca bn thn ng c (v d : trc cam, bm du, bm nc,v.v.) v h tiu th bn ngoi (chn vt, my pht in, v.v.).

Phn ln trc khuu c ch to t thp bng phng php rn, sau tin hnh gia cng c kh (khoan cc l du, phay cc m khuu, tin v mi bng cc c khuu). Gi thnh ch to trc khuu chim mt t l ln trong gi thnh c ng c. gim gi thnh, ngi ta ang p dng ngy cng rng ri phng php c trc khuu bng gang hp kim.

Trc khuu ca ng c nhiu xylanh c cu thnh t cc khuu trc b tr lch nhau. Mi khuu trc c cc b phn sau y :

- c chnh lp trong chnh ca ng c, - c bin lp trong u to ca thanh truyn,


- m khuu lin kt c chnh vi c bin, - cc i trng cn bng lc qun tnh (i trng c th c c lin vi trc

khuu hoc c ch to ring ri lp vo mt u ca m khuu).

H. 1.3-12. Trc khuu a) Trc khuu, b) Bnh , c) Khuu trc

1- C chnh , 2- M khuu , 3- L dn du , 4- C bin.

1.3.3. H THNG NP - X H thng np -x (cn gi l H thng thay i kh hoc H thng trao i kh) c chc nng lc sch khng kh ri np vo khng gian cng tc ca xylanh v x kh thi ra khi ng c. Cc b phn c bn ca h thng np-x bao gm : lc khng kh, ng np, ng x, bnh gim thanh v c cu phn phi kh.

1.3.3.1. C CU PHN PHI KH C cu phn phi kh c chc nng iu khin qu trnh np kh mi vo khng gian cng tc ca xylanh v x kh thi ra khi ng c. Hu ht ng c 4 k hin nay c c cu phn phi kh kiu xupap . ng c 2 k khng nht thit phi c xupap ; trong trng hp khng c xupap, chc nng iu khin qu trnh np-x do piston, ca qut v ca x thc hin. ng c 2 k qut thng qua xupap x, kh mi c np vo xylanh qua ca qut trn thnh xylanh, cn kh thi c x ra ngoi qua xupap x ging nh ng c 4 k .

a) b)

c)


1

2

3

4

5

H. 1.3-13. C cu phn phi kh ca ng c 2 k qut vng (a) v ca ng c 4 k (b)

1- Trc cam ; 2- Con i ; 3- a y ; 4- n gnh ; 5- Xupap.

a) b) c)

H. 1.3-14. Cc kiu b tr v dn ng xupap

1.3.3.2. XUPAP Xupap l mt loi van c trng CT, c chc nng ng hoc m ng ng

np v x. Mi xylanh ca ng c 4 k thp tc v trung tc thng c 2 xupap : mt xupap np c chc nng ng v m ng ng np, mt xupap x c chc nng ng v m ng ng x. ng c cao tc c th c 3 hoc 4 xupap cho mi xylanh tng tit din lu thng ca kh ra, vo xylanh v gim ph ti nhit cho xupap, qua gim kh nng bin dng lm xupap khng ng kn. Xupap c th b tr theo kiu treo trong np xylanh (H. 1.3-14a, b) hoc kiu t trong thn ng c (H.1.3-14c). Trc cam cng c th c t trong thn ng c hoc trn np xylanh.

a)

Ca qut

Ca x

Ca np

b)


H. 1.3-15. Xupap c gn thm vt liu chu nhit v lm mt bng sodium nng chy

Trong qu trnh ng c hot ng, xupap x chu tc dng thng xuyn ca kh thi c nhit cao, nhit ca nm xupap x c th ti 600 - 700 0 C, cho nn n c ch to t thp hp kim cht lng cao. i khi t v phn cn ca nm xupap x c p thm vt liu chu nhit c bit (H. 1-18). Xupap np thng xuyn c lm mt bng dng kh mi nn nhit ca n khong 400 - 500 0C. Thng thng, xupap c lm mt bng cch truyn nhit ra vch ca np xylanh thng qua ng dn hng. i vi ng c cng ho cao, xupap x c lm mt bng cch cho cht Sodium (Na) vo khoang rng trong thn v nm xupap. Cht Na nng chy chuyn ng ln xung khi ng c hot ng c tc dng ti nhit t nm ln thn truyn ra phn dn hng.

1.3.4. H THNG BI TRN CT c rt nhiu chi tit chuyn ng tng i vi nhau. gim lc ma st v hao mn, ngoi vic chn vt liu, hnh dng v kch thc thch hp, nht thit phi bi trn cc b mt ma st ca chi tit. H thng bi trn ca ng c c chc nng lc sch ri a cht bi trn n cc cc b mt cn bi trn. C th phn bit 3 phng php bi trn c bn - Bi trn bng hi du, - Bi trn bng cch vung to du, - Bi trn di p sut. Phng php bi trn bng hi du c s dng cho ng c xng 2 k dng cacte lm bm qut kh . Trong trng hp ny khng th nht vo cacte ri bm i bi trn cc b phn c, m nht c ho trn vo xng vi t l 3 5 % c th n c cc b mt cn bi trn. Cht piston ca cc loi ng c khc cng c th c bi trn bng hi du.


Bi trn bng vung to l dng mt s chi tit chuyn ng ca ng c vung du ln cc b mt cn bi trn. Phng php ny n gin, nhng c nhc im c bn l du b lo ho nhanh, thi gian s dng du ngn. Ngoi ra, phng php ny c hiu qu thp trong mt s trng hp, v d : xe ln hoc xung dc, tu b nghing, lc, v.v.

H. 1.3-16. Bi trn bng cch vung to

a s ng c hin nay c trang b h thng bi trn di p sut. h thng ny, nht t y cacte hay bnh cha (H. 1.3-17) c bm nht nn ti p sut 1,5 - 8,0 bar ri y vo mch du chnh. T mch du chnh, nht theo cc l khoan trong cc chi tit ca ng c hoc theo cc ng du n bi trn cc c chnh, c bin ca trc khuu, trc cam, trc n gnh, v.v. Mt gng xylanh, piston, xecmang, v i khi c trc cam v cc bnh rng c bi trn bng nht phun ra t cc khe h hoc cc l c bit chnh v bin. Cht piston c th c bi trn bng nht i ln t bin qua cc l hoc ng dc thn thanh truyn hoc c bi trn bng hi du. Mt s chi tit ca ng c c th c bi trn bng cch khc, ngoi cc phng php gii thiu trn. V d : trc n gnh c th c bi trn bng cc bc thm du theo nh k ; mt gng xylanh ca mt s ng c kch thc ln c bi trn bng nht di p sut ln (ti 50 bar) do cc bm kiu piston cung cp qua cc l b tr ti cc v tr thch hp trn xylanh.


1 2 3 4

5

7

6

1 2 3

4

7

6

5

108

9

11

9

8

H. 1.3-17. H thng bi trn tun hon cacte t (a) v cacte kh (b) 1- Cacte du ; 2- Lc th ; 3, 11- Bm du bi trn ; 4- Lc tinh ;

5- Bnh lm mt du ; 6- Mch du chnh ; 7- p k du ; 8- Van iu p ; 9- Van an ton ; 11- Bnh cha du.

a)

b)


1.3.5. H THNG LM MT H thng lm mt c chc nng gii nhit t cc chi tit nng (piston, xylanh, np xylanh, xupap, v.v.) chng khng b qu ti nhit. Ngoi ra, lm mt ng c cn c tc dng duy tr nhit du bi trn trong mt phm vi nht nh duy tr cc ch tiu k thut ca cht bi trn. Cht c vai tr trung gian trong qu trnh truyn nhit t cc chi tit nng ca ng c ra ngoi c gi l mi cht lm mt. Mi cht lm mt c th l nc, khng kh, du, hoc mt s loi dung dch c bit. Khng kh c dng lm mi cht lm mt ch yu cho ng c cng sut nh. a s CT hin nay, c bit l ng c thu, c lm mt bng nc v n c hiu qu lm mt cao (khong 2,5 ln cao hn hiu qu lm mt ca du). C th phn loi h thng lm mt ca CT theo cc tiu ch sau y :

Theo mi cht lm mt : lm mt bng nc, lm mt bng khng kh, lm mt bng du v lm mt bng cc dung dch c bit.

Theo phng php lm mt : lm mt bng nc bay hi, lm mt bng i lu t nhin, lm mt cng bc.

Theo c im cu to ca h thng lm mt : h thng lm mt trc tip (h thng lm mt h) v h thng lm mt gin tip (h thng lm mt kn). H thng lm mt trc tip bng nc thng c p dng cho ng c thu hoc ng c t c nh ti khu vc gn sng, h. h thng lm mt trc tip (H. 1.3-18), nc t ngoi mn tu c bm vo lm mt trc tip ng c ri c x ra ngoi tu.

H thng lm mt gin tip bng nc c p dng rng ri nht cho CT s dng trong nhiu lnh vc khc nhau. ng c thu, nc ngt sau khi lm mt ng c s c dn n bnh lm mt nc-nc. Sau khi c lm mt bng nc bin, nc ngt c bm tr li tip tc lm mt ng c (H.1.3-19). ng c t - nc ngt lm mt trc tip ng c, cn khng kh lm mt nc ngt trong bnh lm mt nc - khng kh (H. 1.3-20). H thng lm mt trc tip c u im l cu to n gin, gi thnh thp, hot ng tin cy. Tuy nhin, so vi h thng lm mt kn, h thng h c nhng nhc im sau y :

Cc khoang lm mt ca ng c b ng cn v b n mn nhanh do nc bin cha nhiu loi mui ho tan. hn ch n mn, ngi ta gn cc cc km trong khoang lm mt ; cn hn ch ng cn, phi duy tr nhit nc ra khi ng c khng cao hn 55 0C .

Sut tiu hao nhin liu ca ng c c lm mt trc tip bng nc bin cao hn do phn nhit truyn t kh trong xylanh ra nc lm mt nhiu hn.


4a)

2

1

3

61

3

b)

4

5

2

H. 1.3-18. H thng lm mt trc tip 1- Lc, 2- Bm lm mt ng c, 3- Bnh lm mt du bi trn, 4- ng nc lm mt ra khi ng c, 5- Kt nc cn bng,

6- Bm nc t ngoi mn tu.


5

61

2

3

4

7

H. 1.3-19. H thng lm mt gin tip ca ng c thu 1- Lc, 2- Bm lm mt ng c, 3- Bnh lm mt du bi trn, 4- ng nc

lm mt ra khi ng c, 5- Kt nc cn bng, 6- Bm nc t ngoi mn tu, 7- Bnh lm mt nc-nc

1

2

3

6

4

5

7

H. 1.3-20. H thng lm mt gin tip bng nc ca ng c t


1.3.6. H THNG NHIN LIU CA NG C DIESEL 1.3.6.1. CHC NNG V CC B PHN C BN H thng nhin liu ca ng c diesel c chc nng lc sch ri phun nhin liu

vo bung t theo nhng yu cu ph hp vi c im cu to v tnh nng ca ng c.

D c cu to v nguyn l hot ng kh a dng, nhng tuyt i a s h thng nhin liu thng dng ca ng c diesel u c cu thnh t cc b phn c bn sau y (H. 1.3-21) :

Thng nhin liu bao gm thng nhin liu hng ngy v thng nhin liu d tr. Thng nhin liu hng ngy cn c dung tch m bo cha s nhin liu cho ng c hot ng lin tc trong mt khong thi gian nh trc.

Bm thp p l bm c chc nng ht nhin liu t thng cha hng ngy ri y n bm cao p. H thng nhin liu c th khng c bm thp p nu thng cha nhin liu hng ngy c t v tr cao hn ng c nhin liu t chy n bm cao p (H. 1.3-21b).

Lc nhin liu - Trong h thng nhin liu ca ng c diesel c cc b phn c ch to v lp rp vi chnh xc rt cao, nh : u phun, cp piston-xylanh ca bm cao p, van trit hi. Cc b phn ny rt d b h hi nu trong nhin liu c tp cht c hc. Bi vy nhin liu phi c lc sch trc khi n bm cao p.

ng dn nhin liu gm c ng cao p v ng thp p. ng cao p dn nhin liu c p sut cao t bm cao p n vi phun. ng thp p dn nhin liu t thng cha n bm cao p v dn nhin liu hi v thng cha.

Bm cao p (BCA) c cc chc nng sau y : - Nn nhin liu n p sut rt cao ( khong 100 - 1500 bar) ri y n vi phun

(chc nng nn). - iu chnh lng nhin liu cung cp vo bung t ph hp vi ch lm

vic ca ng c (chc nng nh lng). - nh thi im bt u v kt thc qu trnh phun nhin liu (chc nng nh

thi). Vi phun nhin liu - i a s vi phun nhin liu ng c diesel ch c

chc nng phun nhin liu cao p vo bung t vi cu trc tia nhin liu ph hp vi phng php t chc qu trnh chy . mt s h thng nhin liu c bit, vi phun cn c thm chc nng nh lng v nh thi.


2

109

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7

63

4

1

8

1

5

4

6

3

10

9

H. 1.3-21. S cu to h thng nhin liu ca ng c diesel 1- Thng nhin liu ; 2- Bm thp p ; 3- Lc nhin liu ; 4- Bm cao p ; 5- ng cao p ; 6- Vi phun ; 7- B iu tc ; 8- B iu chnh gc phun sm ; 9- ng thp p ; 10- ng du hi.


K h e h g i a c c c h i t i t c a c p s i u c h n h x c

6 - 1 0 m 3 ,5 - 6 ,5 m 1 ,5 - 2 ,5 m

H. 1.3-22. Khe h gia cc cp siu chnh xc v kh nng lc Ton b h thng nhin liu ca ng c diesel c th chia thnh hai phn c

qui c gi l : phn cp nhin liu v h thng phun nhin liu . Phn cp nhin liu - cn gi l Phn thp p , bao gm thng cha nhin

liu, bm thp p, lc nhin liu v ng thp p. Chc nng ca phn cp liu l lc sch nhin liu ri cung cp cho h thng phun di p sut xc nh .

H thng phun nhin liu ( HTPNL ) - cn gi l Phn cao p , bao gm bm cao p, vi phun , ng cao p v cc b phn iu chnh-hiu chnh. HTPNL thc hin hu nh tt c cc yu cu t ra i vi qu trnh phun nhin liu v c nh hng quyt nh n cht lng qu trnh to hn hp chy ng c diesel.

Kch thc tp cht [m] 40 - 60 20 - 30 8 - 10 4 -5 2

Phn t lc bng n

Kh nng lc

Phn t lc bng giy

B lc kp hin i


1.3.6.2. PHN LOI H THNG PHUN NHIN LIU Bng 1.3-1. Phn loi tng qut h thng phun nhin liu ca ng c diesel

Tiu ch phn loi Phn loi

Phng php phun nhin liu 1) H thng phun nhin liu bng khng kh nn 2) H thng phun nhin liu bng thu lc

Phng php to v duy tr p sut phun

1) H thng phun trc tip 2) H thng phun gin tip

Phng php iu chnh qu trnh phun

1) H thng c iu chnh kiu c kh 2) H thng c iu chnh kiu in t

Cch thc t hp cc thnh t ca h thng phun

1) H thng phun c in 2) H thng phun vi BCA-VP lin hp 3) H thng phun vi BCA phn phi 4) H thng phun c bit.

Loi vi phun 1) H thng phun vi vi phun h 2) H thng phun vi vi phun kn

1) H thng phun nhin liu bng khng kh nn thi k u pht trin ng c diesel, ngi ta dng khng kh nn di p

sut 50-60 bar phun nhin liu vo xylanh ng c. Phng php ny khng yu cu phi c cc chi tit siu chnh xc m vn m bo cht lng ho trn nhin liu vi khng kh kh tt. Tuy nhin, ng c phi lai my nn kh nhiu cp, va cng knh va tiu th mt phn ng k cng sut ca ng c ( cng sut do my nn kh tiu th bng khong 6 8 % cng sut ca ng c, trong khi h thng phun nhin liu bng thu lc tiu th khong 1,5 3,5 % ) ; ngoi ra, vic iu chnh lng nhin liu chu trnh cng phc tp v kh chnh xc, nn kiu h thng phun nhin liu bng kh nn ng c diesel c thay th hon ton bi h thng phun nhin liu bng thu lc.

2) H thng phun nhin liu bng thu lc h thng phun nhin liu bng thu lc, nhin liu c phun vo bung t do

s chnh lch p sut rt ln gia p sut ca nhin liu trong vi phun v p sut ca kh trong xylanh. Di tc dng ca lc kch ng ban u trong tia nhin liu v lc cn kh ng ca kh trong bung t , cc tia nhin liu s b x thnh nhng ht c ng knh rt nh ho hi nhanh v ho trn vi khng kh.

3) H thng phun trc tip HTPNL trc tip l mt loi HTPNL bng thu lc, nhin liu sau khi ra

khi BCA c dn trc tip n vi phun bng ng dn cao p c dung tch nh. u im ca HTPNL kiu ny l : kt cu tng i n gin, c kh nng nhanh chng thay i cc thng s cng tc ph hp vi ch lm vic ca ng c. Nhc im c bn ca HTPNL trc tip l : p sut phun gim khi gim ca tc quay ca ng c , iu


hn ch kh nng lm vic n nh ca ng c tc quay thp. Mc d cha p ng hon ton cc yu cu t ra, nhng HTPNL trc tip vn c s dng ph bin nht hin nay cho tt c cc kiu ng c diesel.

3

1

2

24

1

2

3

1

2 3

a)

d)

b)

c)

1 + 2 + 3

H. 1.3-23. Phn loi h thng phun nhin liu theo cch thc t hp cc thnh t c bn

1- Bm cao p , 2- ng cao p , 3- vi phun , 4- b phn phi a) HTPNL c in vi BCA n b) HTPNL c in vi BCA cm c) HTPNL vi bm cao p phn phi d) HTPNL vi BCA-VP lin hp


4) H thng phun nhin liu gin tip h thng phun gin tip (cn gi l h thng tch phun), nhin liu t BCA

khng c a trc tip n vi phun m c bm n ng cao p chung. Thng thng, ng cao p chung c dung tch ln hn nhiu ln so vi th tch nhin liu c phun vo bung t trong mt chu trnh, nn p sut phun hu nh khng thay i trong sut qu trnh phun . iu m bo cht lng phun tt trong mt phm vi rng ca tc quay v ti . m bo yu cu nh lng v nh thi, h thng tch phun c kt cu kh phc tp. V vy n thng ch c s dng cho nhng ng c diesel c yu cu cao v cht lng phun nhin liu nhng ch ti nh.

5) H thng phun nhin liu vi Bm cao p - Vi phun lin hp

H. 1.3-24 BCA-VP lin hp ca hng GM

HTPNL vi Bm cao p-Vi phun lin hp (BCA-VP) l mt hnh thi bin tng ca HTPNL c in . loi HTPNL kiu ny, bm cao p v vi phun c t hp thnh mt cm chi tit gi l BCA-VP lin hp, thc hin chc nng ca c 3 b phn : bm cao p, vi phun v ng cao p. Trong BCA-VP, nhin liu sau khi c nn n p sut rt cao v c nh lng s c a trc tip vo vi phun m khng cn c ng dn nhin liu cao p.


1.3.6.3. H THNG PHUN NHIN LIU C IN H thng phun nhin liu c in l l tn gi qui c ca loi HTPNL trc tip

c nhng c im c bn sau y : Ton b HTPNL c t hp t cc " tiu h thng phun " hon ton ging nhau . Mi tiu h thng phun c cu thnh t mt phn t bm, 1 ng cao p v 1 vi phun nhin liu (H. 1.3-25). ng c c bao nhiu xylanh th c by nhiu tiu h thng phun. Cc tiu h thng phun hot ng c lp vi nhau .

1

Phan t bm

Voi phun nhien lieu

ong cao ap

3

4

5

6

N

B

7

F

2

C

H. 1.3-25. Cu to tiu h thng phun ca HTPNL vi BCA Bosch c in 1- Cam nhin liu, 2- Con i , 3- L so kh hi, 4- Piston, 5- Vnh rng v

thanh rng iu khin, 6- Xylanh, 7- Van trit hi , N- Khoang np, B- Khoang bm, C- Khoang cao p, F- Khoang phun


1 ) Bm cao p Bm cao p (BCA) l cm chi tit quan trng nht ca HTPNL c in ni ring

v ca HTPNL c kh ni chung v ngi ta thng phn loi HTPNL cn c vo c im ca BCA. BCA c th c phn loi theo nhng tiu ch khc nhau. Nu cn c vo phng php nh lng, tc l phng php iu chnh lng nhin liu chu trnh (gct) , c th phn bit 3 loi BCA c in : BCA iu chnh bng cch thay i hnh trnh c ch ca piston, BCA iu chnh bng cch thay i hnh trnh ton b ca piston v BCA iu chnh bng van tit lu.

a) BCA nh lng bng cch thay i hnh trnh c ch ca piston BCA nh lng bng cch thay i hnh trnh c ch ca piston do hng Bosch

thit k v ch to ln u tin, n hot ng theo nguyn l thay i hnh trnh c ch ca piston thay i lng nhin liu thc t c bm n vi phun . Trong nhiu ti liu chuyn mn, BCA loi ny thng c cc tn gi khc nhau, nh : BCA Bosch c in ( phn bit vi cc loi BCA khc ca Bosch), BCA iu chnh bng rnh cho trn piston, BCA Bosch kiu piston-ngn ko, v.v.

Cp piston-xylanh ca BCA kiu Bosch c in Cp piston-xylanh ca BCA gm 2 chi tit : xylanh v piston (H. 1.3-26). Trn

thnh xylanh c l np, l x v l nh v. L np nhin liu t khoang np (khng gian cha nhin liu thp p trong BCA) i vo khoang bm (khng gian cng tc ca xylanh c gii hn bi nh piston, van trit hi v thnh xylanh ca BCA). L x nhin liu thot t khoang bm ra khoang np . L nh v c nh xylanh vi v BCA. Mt l trn xylanh c th ch thc hin mt chc nng (np, x, nh v) hoc thc hin ng thi 2 hay c 3 chc nng. Trn phn u ca piston c rnh dc, rnh cho v rnh ngang. Rnh dc cho nhin liu t khoang bm thot v khoang np sau khi rnh cho thng vi l x. Mp vt c tc dng lm thay i hnh trnh c ch ca piston , qua iu chnh lng nhin liu chu trnh khi piston c xoay trong lng xylanh. to ra c p sut rt cao ca nhin liu trc khi phun vo bung t, khe h hng knh gia piston v xylanh phi rt nh (khong 0,015 0,025 mm) . Cp piston-xylanh l b phn quan trng nht ca BCA v l mt trong cc cp lp ghp siu chnh xc trong h thng phun nhin liu ca ng c diesel.

Trn th trng hin nay c kh nhiu kiu cp piston-xylanh ca BCA iu chnh bng cch thay i hnh trnh c ch ca piston. Thc cht chng u l nhng bin tng ca cp piston - xylanh kiu Bosch do cc hng khc nhau ch to (H. 1.3-26b, c, d) .

Mi cp piston-xylanh ca BCA c th c t trong mt v ring to thnh BCA n (H. 1-30a) hoc nhiu cp piston-xylanh c t trong mt v chung to thnh BCA cm (H. 1.3-27b).


b) c) d)

H. 1.3-26. Cp piston-xylanh ca BCA c in a) kiu Bosch , b) F& M , c) CAV , d) SIMMS

L np L x + nh v

Xylanh

Piston

Rnh dc

Mp vt

Rnh cho

Rnh ngang

a)


Nguyn l hot ng BCA Bosch c in hot ng theo kiu chu k. Mi chu trnh cng tc ca n

c hon thnh sau 1 vng quay ca trc cam nhin liu, tng ng vi 2 hnh trnh ca piston BCA , c gi l hnh trnh np v hnh trnh bm. Hnh trnh np ca piston BCA (piston BCA i t im cn trn n im cn di) c thc hin nh tc dng ca l xo kh hi ; cn hnh trnh bm (piston BCA i t im cn di n im cn trn) do cam nhin liu y. ng c 4 k, mt vng quay ca trc cam nhin liu tng ng vi 2 vng quay ca trc khuu v 4 hnh trnh ca piston ng c ; cn ng c 2 k - tng ng vi 1 vng quay ca trc khuu v 2 hnh trnh ca piston ng c.

H. 1.3-27. Chu trnh cng tc ca BCA Bosch c in a) Piston im cn trn , b) Np nhin liu vo khoang bm ,

c) Piston im cn di , d) Bt u bm hnh hc , e) Kt thc bm hnh hc , g) Kt thc chu trnh cng tc (piston ca BCA tr li im cn trn)

giai on u ca hnh trnh np, nhin liu trong khoang bm va dn n va thot ra khoang np qua rnh dc. Khi piston m l np, nhin liu t khoang np trn vo khoang bm (H. 1.3-27b). Sau khi c l xo kh hi ko v im cn di, piston ca ca BCA s khng chuyn ng trong mt khong thi gian tu thuc vo cu to ca cam nhin liu v tc ca ng c. Hnh trnh bm c thc hin nh tc dng y ca cam nhin liu (H. 1.3-27c, d, e). giai on u ca hnh trnh bm, khoang np v khoang bm vn c thng vi nhau. Qu trnh nn nhin liu trong khoang bm c bt u t thi im piston ng hon ton l np v l x trn xylanh ca BCA. Nhin liu bt u c bm vo khoang cao p (khng gian cha nhin liu trong rcco cao p, ng cao p v vi phun nhin liu) khi lc tc dng ln kim van trit hi t pha di (FB) c to ra bi p sut trong khoang bm t ti tr s bng lc tc dng t pha trn (FC) c to ra bi lc cng ban u ca l so van trit hi v p sut d trong ng cao p. Qu trnh phun nhin liu vo bung t bt u khi lc tc dng ln mt cn nng ca kim phun (Ff) c to ra bi p sut ca nhin liu trong khoang phun (khng

c)b)a) d) e) g)


gian cha nhin liu trong u phun ca vi phun) thng c lc cng ban u ca l xo vi phun (F0). Qa trnh phun nhin liu vo bung t ko di cho n khi rnh cho trn piston c thng vi khoang np (H. 1.3-27g), khi nhin liu di p sut cao t khoang bm v khoang cao p s thot ra khoang np qua rnh dc. Qu trnh phun nhin liu kt thc ti thi im p sut trong khoang cao p gim xung n tr s, ti Ff = F0 . Sau thi im kt thc phun, piston tip tc i ln kt thc hnh trnh bm ti im cn trn kt thc chu trnh cng tc ca h thng phun nhin liu .

Tt c cc kiu BCA iu chnh bng rnh cho trn piston u hot ng theo mt nguyn l chung l :

- y piston nn nhin liu bng cam. - Kh hi piston bng l xo. - Hnh trnh ton b ca piston khng i ( h0 = const ) - iu chnh lng nhin liu chu trnh ( gct ) bng cch xoay piston thay i

hnh trnh c ch ( he = var ).

H. 1.3-28. Nguyn l iu chnh gct a) V tr stop (khng cp nhin liu), b) Cp liu trung bnh, c) Cp liu cc i

a) b)

c)


b) BCA iu chnh bng cch thay i hnh trnh ton b ca piston 4

3

2

1

10

9

5

6

7

8

H. 1.3-29. BCA iu chnh bng cch thay i hnh trnh ton b ca piston 1- Piston, 2- Xylanh, 3- u ni ng cao p, 4- ng cao p, 5- Van trit hi, 6- Van np, 7- L xo kh hi, 8- Con i, 9- Cam nhin liu, 10- Thn bm

c) BCA iu chnh bng van tit lu

11

12

10

9

8

7

6

5

4

3

2

1

H. 1.3-30. Bm cao p iu chnh bng van tit lu 1- Piston, 2- Xylanh, 3- u ni ng cao p, 4- ng cao p, 5- Van tit lu,

6- L np, 7- Chm iu chnh gc phun sm, 8- L so kh hi, 9- Thn BCA, 10- Con i, 11- Cam nhin liu, 12- Cn bm tay.


2) Vi phun nhin liu Bng 1.2-2. Phn loi tng qut vi phun nhin liu

Tiu ch phn loi Phn loi c im cch ly khoang phun vi bung t

1. Vi phun h 2. Vi phun kn

c im cu to u phun 1. Vi phun kiu cht 2. Vi phun kiu l 3. Vi phun kiu van

Phng php to lc p ban u ln kim phun

1. p kim phun bng l xo 2. p kim phun bng thu lc

Theo phng php iu khin 1. Vi phun iu khin c kh 2. Vi phun iu khin in t

Vi phun h l loi khng c b phn ngn cch khng gian cha nhin liu trong vi phun vi khng gian trong bung t ca ng c. u phun c th ch c nhng l phun bnh thng (H. 1.3-31a) hoc c cu to c bit to ra cu trc tia nhin liu thch hp, v d : c cc rnh cho chm tia nhin liu c hnh qut phng (H. 1.3-31b).

a) b)

H. 1.3-31. Vi phun h

Vi phun h c cu to rt n gin v hot ng tin cy. Nhc im c bn nht ca vi phun h l khng c kh nng loi tr hin tng phun rt vo nhng thi im cui ca qu trnh phun. Vo nhng thi im , khi p sut phun gim ng k, cc ht nhin liu c phun ra c kch thc ln v vn tc nh, kh chy hon ton v rt d b cc ho.

Vi phun h tng c s dng cho mt s kiu ng c diesel cao tc vi p sut phun rt ln. Hin nay n c thay th gn nh hon ton bng cc kiu vi phun kn.


10

9

1

2

3

4

5

6

7

8

H. 1.3-32. Vi phun kn

1.3.6.4. CC THNG S CNG TC C TRNG CA HTPNL C IN

Cc thng s cng tc ca HTPNL nh hng trc tip n cht lng qu trnh phun nhin liu bao gm : p sut bm (pb), p sut phun (pf), p sut m vi phun (pfo), hnh trnh kim phun (hk), cu trc tia nhin liu, quy lut phun, lng nhin liu chu trnh (gct), nh lng khng ng u (g), gc phun sm (), nh thi khng ng u (). 1) p sut bm (pb) - p sut ca nhin liu c o ti khoang bm ca BCA ( khoang bm l khng gian trong cylindre ca BCA c gii hn bi piston, mt di ca van trit hi v thnh xylanh).

2) p sut phun (pf) - p sut ca nhin liu ti khoang phun ( khng gian cha nhin liu trong u phun ca vi phun).

3) p sut m vi phun (pfo) - p sut phun ti thi im kim phun bt u c nng ln khi b .

4) Hnh trnh ca kim phun (hk) - chuyn v ca kim phun trong qu trnh phun nhin liu. Quy c ly hk = 0 ng vi v tr ng ca kim phun , tc l khi kim phun cn tip xc vi b .


p sut bm (pb) v p sut phun (pf) thay i theo gc quay ca trc khuu ng c (H. 1.3-33). c im bin thin v tr s ca pb , pf ph thuc vo hng lot yu t, nh : c im cu to v tnh trng k thut ca h thng phun, ch lm vic ca ng c, cc hin tng thu ng din ra trong khoang np, khoang bm v khoang cao p, v.v. p sut m vi phun (pfo) l mt thng s iu chnh quan trng, n nh hng trc tip n cu trc tia nhin liu v cht lng qu trnh to HHC. Bi v khi ta thay i p sut m vi phun cng c ngha l ta thay i ng thi p sut ng vi phun v p sut phun trung bnh trong qu trnh phun nhin liu. p sut m vi phun cao hay thp ph thuc vo c im k thut ca ng c, trong cu to bung t v tc quay l hai yu t c vai tr quyt nh. Thng thng, pfo = 100 220 bar ; tr s nh dng cho ng c c bung t ngn cch vi vi phun kiu cht, tr s ln - bung t thng nht vi vi phun kiu l. i vi mt kiu ng c c th, p sut m vi phun c iu chnh theo tr s do nh ch to quy nh.

Thi im bt u hnh trnh bm ca piston BCA c k hiu bng im A trn th. Thi im bt u bm hnh hc (thi im piston ng hon ton l x) c k hiu bng im 1. Nhin liu bt u c bm vo khoang cao p khi p sut trong khoang bm cn bng vi p sut d trong ng cao p v thng sc cng ca l xo van trit hi (im 2). Khi p sut ca nhin liu trong khoang phun t ti tr s pfo (im 3), kim phun c nng ln khi v tr tip xc vi b . chnh l thi im thc t bt u phun nhin liu.

Giai on tnh t thi im 2 n im 3 c gi l giai on chm phun (23). Tng thng 23 = 2 - 15 0gqtk .

Gc quay trc khuu tnh t thi im thc t bt u bm (im 2) v thc t bt u phun (im 3) n thi im piston ca ng c ti CT c gi tng ng l gc bm sm (bs) v gc phun sm (). Vic xc nh thi im thc t bt u phun nhin liu i hi nhng trang thit b kh phc tp, bi vy thay v phi kim chnh gc phun sm , chng ta thng kim chnh gc bm sm . R rng l, vi cng mt tr s gc bm sm , giai on chm phun cng ln th gc phun sm cng nh.

Thi im kt thc phun hnh hc (thi im rnh cho trn piston bt u thng vi khoang np) c k hiu bng im 5 trn th. Trong mt thi gian rt ngn sau thi im 5, nhin liu t khoang bm thot ra khoang np vi vn tc rt ln lm cho p sut trong khoang bm v khoang cao p gim xung t ngt. Kim phun bt u hnh trnh ng ti thi im p sut trong khoang phun t ti tr s nh hn pfo mt t (im 6). Thi im kt thc qu trnh phun thc t (thi im kim phun tip xc tr li vi b ) v thi im kt thc chu trnh cng tc ca h thng phun (thi im piston BCA tr li im cn trn) c k hiu tng ng bng im 7 v im B.


CT

0

0

0

g ct

pd

gct.x

dgct/d

hk

pf

pb

pd

7

3

A

73

B

B

3 7

4

5

6

2

3

1

6

54

3

2

1A

a)

b)

c)

d)

e)

6

H. 1.3-33. th biu din qu trnh phun nhin liu


Giai on ko di t thi im bt u phun thc t (im 3) n thi im kt thc phun hnh hc (im 5) c gi l giai on phun chnh (II). Giai oan phun chnh di hay ngn ph thuc vo ti ca ng c v c th hin bng hnh trnh c ch ca piston BCA. Giai on t im 5 n im 7 c gi l giai on phun rt (III). Giai on phun rt din ra trong iu kin p sut phun gim nhiu nn cu trc cc tia nhin liu khng m bo yu cu i vi qu trnh to HHC. Rt nhiu BCA hin nay c trang b van trit hi-gim ti rt ngn giai on phun rt. Thi gian phun thc t (f) c tnh t thi im bt u phun thc t (im 3) n thi im kt thc phun thc t (im 7).

5) Cu trc tia nhin liu Cu trc tia nhin liu l thut ng c s dng biu t khi nim bao hm

c im ca cc tia nhin liu c hnh thnh trong bung t trong qu trnh phun. Cu trc tia nhin liu bao gm cu trc v m v cu trc vi m.

Cu trc v m c c trng bng s lng, v tr tng i v kch thc ca cc tia. Vi phun nhin liu thng c t 1 n 8 l phun v to ra s tia nhin liu tng ng. Kch thc mi tia nhin liu c c trng bng chiu di (Lf) v gc nn (f). Cc tia nhin liu c th c phn b i xng hoc khng i xng qua ng tm ca vi phun (H. 1.3-34).

Cu trc vi m c c trng bng phun nh v phun u

Lf

f

f /2 f/2

f 2

1

H. 1.3-34. Cu trc tia nhin liu Lf - chiu di tia , f - gc nn tia ;

f , - cc gc xc nh v tr cc tia trong trng hp u phun nhiu l Cu trc tia nhin liu c nh hng trc tip n ng nht ca HHC v quy

lut hnh thnh HHC. Chiu di tia nhin liu qu ln s lm cho mt phn nhin liu ng trn vch bung t, iu khng nhng lm nhin liu chy rt nhiu m cn gia tng cng hao mn chi tit do mng du bi trn trn mt gng xylanh b tn hi.


Cc tia nhin liu qu ngn v phn b khng hp l, kch thc cc ht nhin liu khng nh u nh hng xu n ng nht ca HHC.

Cn lu rng, trong mt s trng hp, ngi thit k ch phn b cc tia nhin liu khng i xng hoc nhin liu c phun sao cho hnh thnh cc mng nhin liu lng trn vch bung t nhm to ra quy lut hnh thnh HHC c li nht .

6) Quy lut phun nhin liu Quy lut phun nhin liu l khi nim bao hm thi gian phun v c im phn

b tc phun. C th biu din quy lut phun di dng vi phn hoc di dng tch phn.

dgct/d

gct.x

g c

t

0cf cfe

0cf cfe

a)

b)

cx

gct.x

H. 1.3-35. Quy lut phun nhin liu - gc quay trc khuu , cf - thi im bt u phun , cfe - thi im kt thc phun ,

dgct/d - tc phun nhin liu , gct.x - lng nhin liu c phun vo bung t tnh t thi im bt u phun n thi im cx .

Quy lut phun di dng vi phn l hm s th hin c im thay i tc phun tc thi theo gc quay trc khuu trong qu trnh phun (H. 1.3-35a).

Quy lut phun di dng tch phn l hm s th hin c im thay i theo gc quay trc khuu i ca lng nhin liu c phun vo bung t tnh t thi im bt u phun (H. 1.3-35b).

Quy lut phun nhin liu c nh hng quyt nh n quy lut hnh thnh HHC, c bit l i vi phng php to HHC kiu th tch, qua nh hng n hng lot ch tiu cht lng ca ng c diesel. Vic la chn quy lut phun nhin liu nh th no l tu thuc vo tnh nng ca ng c v cch thc t chc qu trnh chy .


1.4. NGUYN L HOT NG CA NG C T TRONG 1.4.1. NGUYN L HOT NG CA NG C DIESEL 4 K

a)

c)

b)

d)

H. 1.4-1. Chu trnh cng tc ca ng c diesel 4 k a) Np , b) Nn , c) N , d) X


b1

X

Np

r1d1

cf

a1

CT

CD

Chy v

Sinh cng

Nncf

r

z

Vs

CT CD

V

p

ab

c

a)

b)

r

CT CD CT CD CT

00 1800 3600 a

cfc

pz

b r

5400 7200

c)

H. 1.4-2. Cc th biu din chu trnh cng tc ca ng c 4 k a) th cng , b) th gc, c) th cng khai trin

cf - thi im bt u phun nhin liu ( ng c diesel) hoc thi im buji nh la ( ng c xng), z- thi im p sut chy t gi tr cc i,

b1 - thi im xupap x bt u m, r1 - thi im xupap x ng hon ton, d1 - thi im xupap np bt u m, a1 - thi im xupap np ng hon ton.


ng c 4 k l loi CT m mi chu trnh cng tc ca n c hon thnh sau 4 hnh trnh ca piston.

1) Hnh trnh np Trong hnh trnh np, piston c trc khuu ko t CT n CD. Xupap np m, xupap x ng. Khng kh c ht vo xylanh qua xupap np.

2) Hnh trnh nn Trong hnh trnh nn, piston c trc khuu y t CD n CT. C 2 loi xupap (np v x) u ng. Do b piston nn, p sut v nhit ca kh trong xylanh tng dn. Khi piston ti gn CT (im cf), nhin liu bt u c phun vo bung t v t bc chy lm cho p sut v nhit trong xylanh tng ln t ngt.

3) Hnh trnh sinh cng Trong hnh trnh sinh cng, piston c kh trong xylanh c p sut cao y t CT n CD v lm trc khuu quay. C 2 loi xupap vn ng. Qu trnh chy nhin liu vn tip tc din ra giai on u ca hnh trnh sinh cng.

4) Hnh trnh x Trong hnh trnh x, piston b trc khuu y t CD n CT. Xupap np ng,

xupap x m. Kh thi trong xylanh b piston y ra ngoi qua xupap x.

Bng 1.4-1. Tm tt chu trnh cng tc ca diesel 4 k

Hnh trnh NP NN SINH CNG

X

Chuyn v ca piston CT CD

CD CT

CT CD

CD CT

Xupap np M ng ng ng Xupap x ng ng ng M Kh mi i vo xylanh Khng kh - - - Vi phun nhin liu ng M ti cf ng ti

ckf

ng

Mi cht cng tc KK + Kh st

KK + Kh st

Hn hp Kh thi

Gc quay trc khuu 0 180 0 180 3600

360 5400

540 7200


1.4.2. NGUYN L HOT NG CA DIESEL 2 K

Khng kh

Kh thi

H. 1.4-3. Nguyn l hot ng ca ng c diesel 2 k a- Dn n sinh cng; b- X; c- Qut; d- Lt kh; e- Nn

a)

e)

b)

c) d)


c)a)

d1n

b1n

Qutb1x

X

d1x

CT

CD

Chy v

Sinh cng

N ncf

d1xb1x

b

p

V

CDCT

VS

c

z

z

p

cfc

b d1nb1x d1x

b

00 1800 3600CD CT CD

b)

H. 1.4-4. Cc th biu din chu trnh cng tc ca ng c 2 k a) th cng ch th , b) th cng khai trin, c) th gc


ng c 2 k l loi ng c t trong m mi chu trnh cng tc ca n c hon thnh sau 2 hnh trnh ca piston.

1) Hnh trnh sinh cng Trong hnh trnh sinh cng, piston c kh c p sut v nhit cao (do nhin

liu chy t chu trnh trc) y t CT n CD v lm trc khuu quay. giai on u ca hnh trnh sinh cng, cc qu trnh din ra trong xylanh cng tng t nh ng c 4 k. Bt u t thi im piston ti mp trn ca ca x (im b1x), khng gian cng tc ca xylanh c thng vi ng x v kh thi t thot ra ngoi qua ca x do c p sut cao hn p sut kh tri. Bt u t thi im piston ti mp trn ca ca qut (im d1X), khng gian cng tc ca xylanh c thng vi khoang cha khng kh qut c p sut cao hn p sut kh tri ; kh qut c thi vo xylanh qua ca qut y phn kh thi cn li ra ngoi, ng thi np y khng gian cng tc ca xylanh. Qu trnh, trong kh thi c x ra khi xylanh v kh qut i vo xylanh din ra ng thi c gi l qu trnh qut.

Trong hnh trnh sinh cng ng c 2 k din ra cc qu trnh sau y : Chy, Dn n -Sinh cng , X , Qut .

2) Hnh trnh nn Trong hnh trnh nn, piston c trc khuu y t CD n CT. giai on

u ca hnh trnh nn, qu trnh qut vn tip tc cho n thi im piston ng hon ton ca qut (d1n). Bt u t thi im piston ng hon ton ca x (b1n), khng gian cng tc ca xylanh c ng kn v khng kh trong xylanh b nn lm cho p sut v nhit ca n tng dn. Khi piston ti gn CT (im cf), nhin liu bt u c phun vo bung t v t bc chy lm cho p sut v nhit trong xylanh tng ln t ngt.

Trong hnh trnh nn ng c 2 k din ra cc qu trnh sau y : Qut , Lt kh (giai on d1n b1), Nn , Chy.

So snh ng c 4 k v 2 k : 1) V nguyn l hot ng, ng c 4 k v 2 k khc nhau c bn qu trnh

np v x. Qu trnh np-x ng c 4 k ko di hn 360 0 gc quay trc khuu (hn 2 hnh trnh ca piston) v c iu khin bng c cu phn phi kh kiu xupap. Qu trnh np-x ng c 2 k ch din ra khi piston gn CD, trong khong thi gian < 180 0 gc quay trc khuu.

2) Nu c cng dung tch cng tc (i. VS) v cng tc quay (n) th ng c 2 k c cng sut ln hn khong 1,7 ln cng sut ca ng c 4 k.

3) Thng thng, ng c xng 2 k c sut tiu th nhin liu cao hn so vi ng c xng 4 k ; ng c diesel 2 k v diesel 4 k c sut tiu th nhin liu gn nh nhau.

4) ng c 2 k c cu to n gin hn so vi ng c 4 k. 5) ng c 4 k c tui bn cao hn so vi ng c 2 k, nu cc iu kin khc

nh nhau.


Bng 1.4-2. Cc thng s c trng ca chu trnh cng tc

TT Cc thng s ng c diesel ng c xng

1 T s nn () 12 - 20 (30) 6 - 12 2 p sut cui hnh trnh nn (pc) , [bar] 30 - 50 7- 20 3 Nhit cui hnh trnh nn (Tc) , [0C] 700 - 900 400 - 600 4 p sut chy cc i (pz) , [bar] 50 - 100 (150) 40 - 60 5 Nhit chy cc i (Tmax ) , [0C] 1600 - 2000 2100 - 2600 6 p sut cui qu trnh dn n (pb) , [0C] 2,0 - 4,0 3,5 - 5,5 7 Nhit cui qu trnh dn n (Tb) , [0C] 800 - 1200 1300 - 1500 8 Sut tiu hao nhin liu ( ge ), [g/kW.h] 220 - 245 300 - 380


Chng 2

CHU TRNH CNG TC V TNH NNG K THUT CA NG C T TRONG

2.1. CHU TRNH CNG TC CA NG C T TRONG CT, s bin i ho nng ca nhin liu thnh c nng c tin hnh thng qua hng lot qu trnh l - ho din ra theo mt trnh t nht nh v lp li c tnh chu k. Mi chu k hot ng ca CT c gi l mt chu trnh cng tc. Chu trnh cng tc (CTCT) ca CT l tng cng tt c nhng s thay i v nhit , p sut, th tch, thnh phn ho hc,v.v. ca MCCT tnh t thi im n c np vo cho n khi c x ra khi khng gian cng tc ca xylanh. Mi CTCT tng ng vi mt ln sinh cng trong mt xylanh.

2.1.1. CC CH TIU CHT LNG CA CHU TRNH CNG TC nh gi cht lng ca CTCT v phng din nhit ng, ngi ta thng dng hai i lng : hiu sut nhit v p sut trung bnh.

1) Hiu sut nhit ca chu trnh Hiu sut nhit ca chu trnh () c xc nh bng t s gia phn nhit c bin i thnh c nng (sau y gi tt l cng chu trnh - Wct) v tng s nhit lng cp cho MCCT trong mt chu trnh (gi tt l nhit lng chu trnh - Q1).

= Wct / Q1 (2.1-1) Hiu sut nhit l i lng nh gi chu trnh v phng din hiu qu kinh t.

Vi cng mt lng nhit cp cho MCCT, chu trnh no c hiu sut nhit cao hn th s c nng c sinh ra nhiu hn.

2) p sut trung bnh ca chu trnh p sut trung bnh ca chu trnh (ptb) c xc nh bng t s gia cng chu trnh (Wct) v dung tch cng tc ca xylanh (VS).

ptb = Wct / Vs (2.1-2) p sut trung bnh l i lng nh gi chu trnh v phng din hiu qu k

thut. Vi cng mt dung tch cng tc, chu trnh no c p sut trung bnh cao hn th cng c sinh ra trong mt chu trnh ln hn.

Tu thuc vo cch xc nh cng chu trnh (Wct) , chng ta s c cc khi nim hiu sut nhit v p sut trung bnh khc nhau, nh : hiu sut l thuyt, hiu sut ch th, p sut l thuyt trung bnh, p sut ch th trung bnh, p sut c ch trung bnh (xem mc 2.2).


2.1.2. CHU TRNH L THUYT CA NG C T TRONG 2.1.2.1. KHI NIM CHUNG

Chu trnh cng tc CT thc t bao gm hng lot qu trnh nhit ng, kh ng, ho hc v c hc rt phc tp. Din bin ca cc qu trnh ny chu nh hng ca rt nhiu yu t, nh :

- Kt cu ca ng c (hnh dng v kch thc ca bung t, t s nn, kch thc ca xylanh, v.v.).

- Cc thng s iu chnh ca ng c (gc phun sm nhin liu, gc nh la sm, thnh phn hn hp chy, v.v.).

- Ch lm vic ca ng c (tc , ti, nhit , v.v.). c th thit lp c c tnh v mc nh hng ca cc thng s v ca

cc qu trnh nhit ng n cc ch tiu cht lng ca chu trnh, qua c th ra c phng hng v bin php nng cao cng sut v hiu sut ca ng c thc t, ngi ta tm cch thay th cc qu trnh nhit ng thc t phc tp bng cc qu trnh n gin hn. Chu trnh l thuyt ca CT l chu trnh nhit ng c xy dng trn c s nhng gi nh n gin ho cc qu trnh thc t vi mc ch ni trn. Mc n gin ho c la chn tu thuc vo mc ch nghin cu. V d : c th gi nh MCCT l kh l tng vi nhit dung ring l hng s hoc l khng kh vi nhit dung ring ph thuc vo thnh phn ca sn phm chy; qu trnh chy thc t c th c thay bng qu trnh cp nhit t mt ngun nng bn ngoi ng c hoc thay bng qu trnh chy c thc hin trong nhng iu kin l tng ho, v.v. Vi nh hng nghin cu khai thc k thut CT, chng ta gi nh nh sau :

1) MCCT l khng kh vi nhit dung ring l hng s. Lng MCCT khng thay i trong thi gian thc hin mt chu trnh nhit ng. 2) Qu trnh nn v dn n l nhng qu trnh on nhit, tc l trong qu trnh nn v dn n khng c s trao i nhit gia MCCT trong khng gian cng tc ca xylanh vi mi trng xung quanh. 3) Qu trnh chy c t chc thc hin trong iu kin khng hn ch v thi gian v hn hp chy l ng nht. 4) Qu trnh x din ra trong iu kin ng tch. 5) B qua mi dng tn tht do ma st, lt kh, bc x, v.v. Cn c vo c im qu trnh cp nhit cho MCCT, c th phn bit 3 kiu chu trnh l thuyt ca CT : - Chu trnh cp nhit ng tch - Chu trnh cp nhit ng p - Chu trnh cp nhit hn hp.


a a a

CT

Vc VVs

CD CT

Vs Vs

CDV

CT CDV

c

a)

p

c z

b

z

b)

p

b

c

b

c)

pzy

H. 2.1-1. Chu trnh l thuyt ca CT a) Chu trnh cp nhit ng tch b) Chu trnh cp nhit ng p c) Chu trnh cp nhit hn hp K hiu :

a - thi im u qu trnh nn. c - thi im cui qu trnh nn. y - thi im p sut chy t n tr s cc i. z - thi im kt thc qu trnh chy. b - thi im kt thc qu trnh dn n. pa , pc , py , pz , pb - p sut trong khng gian cng tc ca xylanh ti cc im c trng ca chu trnh , [N/m2]. Va , Vc , Vy , Vz , V b - th tch ca khng gian cng tc ca xylanh ti cc im c trng ca chu trnh, [m3]. Q1 - lng nhit chu trnh (tng s nhit nng cp cho MCCT trong mt chu trnh) , [J]. Q1V - phn nhit nng cp cho MCCT trong iu kin ng tch , [J]. Q1P - phn nhit nng cp cho MCCT trong iu kin ng p [J]. Q2 - phn nhit nng do MCCT truyn cho ngun lnh , [J].

c

a

VV

= - T s nn

c

z

pp

= - T s tng p sut

c

z

VV

= - T s dn n ban u.


2.1.2.2. CHU TRNH CP NHIT HN HP Chu trnh cp nhit hn hp (cn c gi l chu trnh Sabathe' ) c cu thnh t

cc qu trnh nhit ng sau y : - Nn on nhit (ac) - Cp nhit ng tch (cy) - Cp nhit ng p (yz) - Dn n on nhit (zb) - Nh nhit ng tch (ba)

1) Hiu sut v p sut trung bnh ca chu trnh cp nhit hn hp Vi nhng gi nh t ra trn, c th xc nh lng nhit chu trnh (Q1) ,

lng nhit truyn cho ngun lnh (Q2) v cng ca chu trnh l thuyt (Wt) nh sau : Q1 = Q1V + Q1P = M. cv. (Ty - Tc) + M. cp. (Tz - Ty) (2.1-3) Q2 = M. cV. (Tb - Ta ) (2.1-4) Wt = Q1 - Q2 = M. cv. [(Ty - Tc) - (Tb - Ta)] + M. cp. (Tz - Ty) (2.1-5)

trong cc cng thc trn : M - lng MCCT c trong khng gian cng tc ca xylanh trong mt chu trnh, [kmol]; cv , cp - nhit dung ring ng p v ng tch ca MCCT, [J/kmol.K]; Ta , Tc , Ty , Tz , Tb - nhit ca MCCT ti cc im c trng ca chu trnh, [K]. Trn c s phng trnh ca cc qu trnh nhit ng c bn (qu trnh ng tch, ng p, on nhit), c th biu din nhit ca MCCT ti cc im c trng thng qua nhit ti im u qu trnh nn nh sau :

11

=

=

ka

k

c

aac TV

VTT (2.1-6)

=== 1kacc

ycy TTp

pTT

(2.1-7)

=== 1kayy

zyz TTV

VTT (2.1-8)

ka

k

z

k

b

zzb TTV

VTT

==

=

11

(2.1-9)

T cc phng trnh trng thi ti im a v im c :

pa . Va = M. R . Ta ; pc . Vc = M . R . Tc

ta c :


( )111 =

==

a

av

c

a

a

c

a

a

caS pTk

cMpp

TT

pTRM

VVV

(2.1-10) trong cc cng thc trn :

k - ch s on nhit, ph thuc vo tnh cht ca MCCT, k = cP / cV R - hng s ph bin ca cht kh, [J/kmol.K]

R = cp - cv = cv. (k - 1) Thay Tc , Ty , Tz v Tb t cc cng thc (2.1-6) , (2.1-7) , (2.1-8), (2.1-9) vo cc

cng thc (2.1-3) , (2.1-4) , (2.1-5) v sau khi rt gn ta c : Q1 = M. cv . Ta . k - 1 . [ -1 + k. ( - 1) ] (2.1-11) Q2 = M. cv . Ta . (. k - 1) (2.1-12) Wt = M. cv . Ta . {[ - 1 + k . . ( - 1)] . k - 1 - ( . k - 1)} (2.1-13) Thay Q1 , Q2 , Wt v VS t cc cng thc (2.1-11) , (2.1-12) , (2.1-13), (2.1-10)

vo cc cng thc (2.1-1), (2.1-2) v sau khi rt gn ta c : Hiu sut l thuyt ca chu trnh cp nhit hn hp (t.C) :

( )11111 1

+

=

k

k

kCt (2.1-14)

p sut l thuyt trung bnh ca chu trnh cp nhit hn hp (pt.C) :

( )[ ] ( ){ }11111

1.

+

= kka

Ct kkpp

(2.1-15a) hoc

( )[ ] Ctak

Ct kkpp

..

1111

+

= (2.1-15b)

Khi nhng i lng M, cv , Ta , , k v Q1 c gi tr khng i, t cng thc (2.1-11) ta c :

( )[ ] constAkTcM

Qk

av

==+=

1111

(2.1-16)

Trong trng hp ny, cc cng thc (2.14a) v (2.15b) c th vit nh sau :

111

= k

k

Ct A

(2.1-17)

Cta

k

Ct Akpp

.. 11

= (2.1-18)


2) Nhng yu t nh hng n tC v ptC T cng thc (2.1-14), (2.1-15), (2.1-16), (2.1-17) v (2.1-18) chng ta thy rng :

hiu sut l thuyt ca chu trnh cp nhit hn hp ph thuc vo nhng yu t sau : - T s nn (). - Lng nhit v phng php cp nhit cho MCCT (A, , ). - Tnh cht ca MCCT (k).

p sut c ch trung bnh ca chu trnh cp nhit hn hp ph thuc vo nhng yu t sau

- T s nn (). - Lng nhit v phng php cp nhit cho MCCT (A , , ). - Tnh cht ca MCCT (k). - p sut ca MCCT u qu trnh nn (pa). - Hiu sut nhit ca chu trnh (t. C).

T cng thc (2.1-16), c th xem A nh l mt i lng c trng cho nhit lng cp cho MCCT. , c trng cho phng php cp nhit, trong c trng cho lng nhit cp trong iu kin ng tch, - cp trong iu kin ng p. Mi quan h gia v vi cc tr s khc nhau ca A c th hin trn H. 2.1-2. Cc hnh 2.1-3 , 2.1-4, H. 2.1-5 v H. 2.1-6 biu din nh hng ca t s nn (), lng nhit chu trnh (A hoc Q1) v phng php cp nhit ( , ) n hiu sut nhit (t. C) v p sut trung bnh (pt. C) ca chu trnh cp nhit hn hp.

=1,0

= 2,6

12 1513 14

0,66

0,62

0,58

0,54A = const

=1,8

=2,4

= 1,6

= 1,0

tC

H. 2.1-3. nh hng ca , v A n tC


= 1,0

= 2,02,7

4,0

0,64

0,60

0,56

0,52

3,02,0 4,01,0

tC

1,4

1,8

2,4

k = 1,4

= 1,4

To = 293 K

H. 2.1-4. nh hng ca , v v A n tC

1,0 2,0 3,0 4,0

10

14

18

A = 4,06

= 1,0

p tC1,4

1,8

2,6

A = 3,38

A = 2,71

A = 2,03 k = 1,4

po = 1 bar

= 1,4

[bar]

H. 2.1-5. nh hng ca , v A n ptC


2,52,01,51,0 3,0

9

10

11

12

p tC[bar]

= 1,0

1,8

1,4

=16 =14 =12

A = const

H. 2.1-6. nh hng ca , v n ptC - Khi nhit lng chu trnh khng i (A = const), tC s tng khi tng (tng

phn nhit cp iu kin ng tch ng thi gim phn nhit cp iu kin ng p); tC = tC.max khi = max v = 1 ; tC = tC.min khi = 1 v = max . Tng cng lm cho pt. C tng theo t l thun vi t. C . Tuy nhin, khi tng s lm cho p sut cc i (pz) tng, gy nn ph ti c hc ln ng c thc t. Vi = const, nu tng (bng cch tng A) cng lm cho tC tng cht t (H. 2.1-3). - T s nn () c nh hng tch cc n t. C v pt. C (H. 2.1-4 v H. 2.1-6). C t. C v pt. C u tng khi tng, nhng pt. C tng chm hn so vi t. C . Khi tng t 12 ln 16 th t C tng khong 6 % vi = 1 ; khong 10 % vi = 1. Mc tng t. C gim dn theo chiu tng ca . Trong thc t, t s nn ca ng c diesel c quyt nh ch yu bi yu cu m bo s t bc chy ca nhin liu ; cn ca ng c xng-yu cu khng b kch n .

- Nu tng lng nhit chu trnh (tng A) bng cch gi = const v tng th p sut trung bnh s tng nhanh, cn hiu sut nhit s gim (H. 2.1-5).

2.1.2.3. CHU TRNH CP NHIT NG TCH Chu trnh cp nhit ng tch (cn c gi l chu trnh Otto) c cu thnh t

cc qu trnh nhit ng sau y (H. 2-1a) : - Nn on nhit (ac). - Cp nhit ng tch (cz). - Dn n on nhit (zb). - Nh nhit ng tch (ba). Bng phng php trnh by trn i vi chu trnh cp nhit hn hp hoc bng

cch thay = 1 vo cc cng thc (2.14), (2.15) v (2.17) ta c c cc cng thc biu din mi quan h gia hiu sut l thuyt (t.V) v p sut l thuyt trung bnh (pt. V) ca chu trnh cp nhit ng tch nh sau :


111

= kVt

(2.1-19)

( ) ( )1111

1.

=ka

Vt kpp

(2.1-20a)

( ) Vtak

Vt kpp

..

111

= (2.1-20b)

Cc cng thc trn cho thy rng : hiu sut ca chu trnh cp nhit ng tch (t.V) ch ph thuc vo t s nn () v tnh cht ca MCCT (k) ; cn p sut trung bnh (pt. V) ph thuc vo :

- T s nn (). - Tnh cht ca MCCT (k). - Lng nhit chu trnh (). - p sut u qu trnh nn (pa). - Hiu sut ca chu trnh (t. V).

2.1.2.4. SO SNH CC CHU TRNH L THUYT CA CT Chu trnh cp nhit ng tch l chu trnh l thuyt ca ng c xng , cn chu trnh

cp nhit ng p l chu trnh l thuyt ca ng c diesel dng khng kh nn phun nhin liu vo bung t. Sau khi pht minh v lm ch cng ngh ch to thit b phun nhin liu bng thu lc, loi ng c diesel vi cch phun nhin liu bng khng kh nn b loi dn. ng c diesel hin nay u hot ng trn c s chu trnh cp nhit hn hp.

Di y chng ta s so snh chu trnh cp nhit ng tch v cp nhit hn hp v phng din hiu qu kinh t, tc l so snh hiu sut l thuyt ca chng.

Vi t s nn () v lng nhit chu trnh (Q1) nh nhau th lng nhit thi ra (Q2) chu trnh cp nhit ng tch nh hn so vi chu trnh cp nhit hn hp mt lng tng ng vi phn gch cho (bc - bv - m - n - bC) trn H. 2.1-9a. iu c ngha l phn nhit lng c bin i thnh c nng chu trnh cp nhit ng tch ln hn hay hiu sut nhit ca n cao hn.

Trong thc t, ng c xng ch c th hot ng vi t s nn thp (X = 6 12), trong khi ng c diesel phi hot ng vi t s nn cao hn nhiu (D = 14 20). V vy, mc d hot ng trn c s chu trnh nhit ng c hiu qu kinh t thp hn, nhng ng c diesel li c hiu sut nhit cao hn ca ng c xng.


c

a)

p

S

y

zv

zc

bcbv

a

h m n nmh

a

bvbc

zczv

y

S

p

b)

cv

cc

H. 2.1-9. So snh chu trnh cp nhit ng tch v chu trnh cp nhit hn hp trn th T - S

a) Cng t s nn () v lng nhit chu trnh (Q1) b) Cng p sut cc i (pz) v lng nhit chu trnh (Q1)

Trn quan im thc t, phi so snh cc chu trnh trong iu kin p sut cc i (pz) v lng nhit chu trnh (Q1) l nh nhau. H. 2-9b th hin cch so snh nh vy. Chng ta thy rng chu trnh cp nhit ng tch (a - cv - zv - bv - a) c t s nn nh hn v lng nhit nh ra (tng ng din tch bv - a - h - n - bv) ln hn so vi chu trnh cp nhit hn hp (a - cc - y - zc - bc - a). Nh vy, nu c p sut cc i v lng nhit chu trnh nh nhau th chu trnh cp nhit hn hp c hiu sut nhit cao hn ca chu trnh cp nhit ng tch.


2.2. TNH NNG K THUT CA NG C T TRONG Tnh nng k thut ca ng c l thut ng c dng biu t mc v

hiu qu thc hin chc nng ca ng c. C th nh lng tnh nng k thut ca CT bng 3 nhm thng s sau y : tc , ti v hiu sut.

2.2.1. TC CA NG C 1) Tc quay (n) - l s vng quay ca trc khuu trong mt n v thi gian. n v thng dng ca tc quay l vng/pht , vit tt l [vg/ph ] hoc [rpm]. Tc quay ca CT thng thay i trong qu trnh ng c hot ng, tu thuc vo iu kin lm vic hoc yu cu ca ngi vn hnh ng c. Cn phn bit mt s khi nim lin quan n tc quay sau y :

Tc quay danh ngha (nn) - tc quay do nh ch to nh ra v l c s xc nh cng sut danh ngha, tnh ton cc kch thc c bn ca ng c, la chn ch lm vic hp l, v.v.

Tc quay cc i (nmax) - tc quay ln nht m nh ch to cho php s dng trong mt khong thi gian xc nh m ng c khng b qu ti.

Tc quay cc tiu (nmin) - tc quay nh nht, ti ng c vn c th hot ng n nh.

Tc quay ng vi cng sut cc i (nN) Tc quay ng vi momen quay cc i (nM) Tc quay ng vi sut tiu th nhin liu nh nht (ng) Tc quay khi ng (nk) - tc quay nh nht, ti c th khi ng

c ng c. Tc quay s dng (ns) - tc quay c ngi thit k t hp ng c -

my cng tc khuyn co s dng va pht huy ht tnh nng ca ng c va m bo tin cy v tui bn cn thit.

nN

Me

nMnminnnng

ge

NeMege N e

nmax nnmaxT

H. 2.2-1. Cc im c trng trn c tnh tc ca CT


Bng 2.2-1. Tc quay thng gp ca ng c t

Tc quay [rpm] ng c xng ng c diesel nn 3000 - 6000 2000 - 4000

nmin 300 - 600 350 - 700

nmax (1,05 1,10) nn (1,05 1,07) nn nM (0,4 0,6) nn ( 0,5 0,7 ) nn n2 (1,7 2,0) nn ( 1,4 1,6 ) nn

2) Vn tc trung bnh ca piston ( Cm )

30nSCm

= (2.2-1)

trong : Cm - vn tc trung bnh ca piston, [m/s]; S - hnh trnh ca piston, [m]; n - tc quay ca ng c, [rpm]. Tc ca CT l thng s tnh nng nh gi s chu trnh cng tc c thc hin trong mt n v thi gian v c trng cho "tnh cao tc" ca ng c, trong bao hm hng lot tnh cht vn hnh, nh : cng lm vic, cng hao mn cc b mt ma st, ph ti c v ph ti nhit, v.v. Cn c vo tc , CT c phn loi thnh : ng c thp tc, trung tc v cao tc. C tc quay (n) v vn tc trung bnh ca piston (Cm) u c th c dng lm tiu ch nh gi tnh cao tc. Tuy nhin, Cm c coi l ch s nh gi tnh cao tc ca ng c mt cch chnh xc hn, v n c lin quan mt cch trc tip hn n cc tnh cht vn hnh ni trn. Cn lu rng, vic nh ra gii hn tc xp mt ng c c th vo loi thp, trung hoc cao tc ch mang tnh cht tng i, v d : mt ng c thu c tc quay l 2000 rpm thuc loi cao tc, nhng mt ng c t cng vi tc quay th thuc loi trung hoc thp tc. i vi ng c thu, c th tham kho cch phn loi nh sau :

- ng c thp tc : nn 240 rpm - ng c trung tc : 240 < nn 1200 rpm - ng c cao tc : nn > 1200 rpm Tc danh ngha ca CT cao hay thp l tu thuc trc ht vo mc ch s

dng. Nhng yu t khc c nh hng n vic la chn tc danh ngha l : hiu sut, tui bn, tin cy, cng ngh ch to, v.v. Hu ht ng c chnh ca tu thu trng ti ln l loi thp tc. Ngc li, xung gn my, tu thuyn nh c trang b ch yu bng ng c cao tc. ng c trung tc thng c s dng lm ngun ng lc cho tu ko, ph, tu c xa b, my pht in, my nn lnh, v.v. Phn ln ng c t thuc loi cao tc v xu hng chung trong cng nghip t l tng tc ca ng c.


Trong khi tc quay danh ngha (nn) ca CT do nh ch to nh ra th tc quay cc tiu (nmin) li khng ch tu thuc vo mong mun ch quan ca ngi thit k, ch to hoc khai thc k thut ng c. tc quay qu thp, cht lng qu trnh hnh thnh hn hp chy s rt km , p sut v nhit ca MCCT trong xylanh s khng cao do lng kh lt qua khe h gia piston - xylanh - xecmang v lng nhit truyn qua vch xylanh ln. Kt qu l nhin liu s khng bc chy c hoc chy khng n nh. Tm li, tc quay cc tiu ca CT c quyt nh bi iu kin m bo cho qu trnh chy din ra mt cch n nh tc quay thp. iu ph thuc vo cht lng thit k, ch to, lp rp ng c v tnh trng k thut ca n. Ngoi ra, i vi ng c tng p bng t hp turbine kh thi - my nn kh, tr s tc quay cc tiu cn b gii hn bi "hin tng bm" xut hin khi tc quay ca ng c gim xung qu thp so vi tc quay thit k. ng c xe c gii ng b c nmin nh s c tui bn cao hn v tiu hao t nhin liu hn v thi gian hot ng ch tc quay cc tiu ca loi ng c ny chim t l ng k trong tng thi gian vn hnh ng c. ng c thu c nmin nh s m bo tnh an ton v tin cy cao hn khi vn hnh tu thu trong iu kin khng thun li, nh : trong khu vc cng, trn cc on sng cht hp, v.v.

2.2.2. TI CA NG C

Ti l i lng c trng cho s c nng m ng c pht ra trong mt chu trnh cng tc hoc trong mt n v thi gian. Cc i lng c dng nh gi ti ca CT bao gm : p sut trung bnh, cng sut, momen quay.

1) p sut trung bnh ca chu trnh p sut trung bnh ca chu trnh (ptb) l i lng c xc nh bng t s gia cng sinh ra trong mt chu trnh (gi tt l cng chu trnh) v dung tch cng tc ca xylanh .

S

cttb V

Wp =

(2.2-2a)

Tu thuc vo vic cng chu trnh c xc nh nh th no, c th phn bit :

p sut l thuyt trung bnh : s

tt V

Wp = (2.2-2b)

p sut ch th trung bnh : s

ii V

Wp =

(2.2-2c)

p sut c ch trung bnh : s

e

e VW

p = (2.2-2d)

trong : Wct - cng ca chu trnh, [J]; Wt - cng l thuyt ca chu trnh, [J]; Wi - cng ch th ca chu trnh, [J]; We - cng c ch ca chu trnh, [J] .


Cng ch th (Wi) - l cng do MCCT sinh ra trong mt chu trnh thc t, trong bao gm c phn tn tht c hc. C th xc nh cng ch th nh sau :

Wi = Q1 - Qi = Q1 - (Qm + Qx + Qkh + Qcl ) (2.2-3) trong : Q1 - lng nhit chu trnh (lng nhit sinh ra khi t chy hon ton lng nhin liu a vo bung t trong mt CTCT); Qi - tng nhit nng b tn tht trong mt chu trnh nhit ng thc t; Qm - tn tht do lm mt (phn nhit nng truyn t MCCT qua vch xylanh cho mi cht lm mt); Qx - tn tht theo kh thi (phn nhit theo kh thi ra khi khng gian cng tc do s khc bit v nhit , nhit dung ring v lu lng ca kh thi so vi kh mi); Qcl - phn nhit tn tht khng tnh chnh xc c vo cc dng tn tht k trn, v d : tn tht do lt kh qua khe h gia piston v xylanh, lt kh do xupap khng kn, tn tht do bc x nhit t cc chi tit nng ca ng c, v.v.

Cng tn tht c hc (Wm) - l cng tiu hao cho cc "hot ng mang tnh cht c hc" khi thc hin mt CTCT. Cc dng tn tht nng lng sau y thng c tnh vo cng tn tht c hc :

- Tn tht do ma st gia cc chi tit ca ng c chuyn ng tng i vi nhau .

- Phn nng lng tiu hao cho vic dn ng cc thit b v c cu ca bn thn ng c, nh : bm nhin liu, bm du bi trn, bm nc lm mt, c cu phn phi kh, ...

- " Tn tht bm" (phn c nng tiu hao cho qu trnh thay i kh). Cng c ch (We) - l cng thu c u ra ca trc khuu. l phn c

nng thc t c th s dng c dn ng h tiu th cng sut.

We = Q1 - Qe = Wi - Wm (2.2-4) trong Qe l tng ca tt cc cc dng tn tht nng lng khi thc hin mt chu trnh cng tc thc t.

Bng 2-2. Tn tht c hc ng c t

Tr s tng i [%] Loi tn tht c hc ng c xng ng c diesel

Tn tht do ma st - Ma st gia piston-xylanh-xecmang - Ma st trong cc chnh v bin

Tn tht bm Tn tht cho dn ng thit b v c cu ca ng c - Dn ng c cu phn phi kh - Dn ng cc loi bm

44 22 20

8 6

50 24 14

6 6

Tng cng 100 100


2) Cng sut ca ng c Cng sut l tc thc hin cng. Tr s cng sut ca ng c cho ta bit ng c "mnh" hay "yu". Cng sut ca CT thng c o bng n v kilowatt (kW) hoc m lc ( HP , hp - Horse power ; cv - Chevaux ; PS - Pferdestarke).

1 kW = 1 kJ/s 1 HP = 75 kG.m/s 1 PS = 1 metric HP = 0,735 kW 1 hp = 1,014 PS.

Cn phn bit cc khi nim cng sut sau y ca CT : Cng sut ch th (Ni) - l tc thc hin cng ch th ca ng c. Ni cch

khc, cng sut ch th l cng sut ca ng c, trong bao gm c phn tn tht c hc.

Cng sut c ch (Ne) - Cng sut ca ng c c o u ra ca trc khuu.

T nh ngha ca cng sut, p sut trung bnh ca chu trnh v tc quay ta c cc cng thc xc nh cng sut ch th v cng sut c ch di y :

z

inVpN Sii

= (2.2-5)

z

inVpN See

= (2.2-6)

trong : i - s xylanh ca ng c; z - h s k ; z = 1 i vi ng c 2 k ; z = 2 i vi ng c 4 k.

Cng sut danh ngha (Nen) - Cng sut c ch ln nht m ng c c th pht ra mt cch lin tc m khng b qu ti trong nhng iu kin quy c.

Cc iu kin c bn c quy c khi xc nh cng sut danh ngha ca CT bao gm :

- iu kin kh quyn tiu chun . - Tc quay danh ngha, - Loi nhin liu v cht bi trn xc nh, - Trang thit b ph tr cho ng c khi o cng sut , v.v.

Bng 2-3 gii thiu quy nh ca mt s nc v iu kin kh quyn tiu chun khi xc nh cng sut ca CT.

Vn trang b cho ng c khi o cng sut cng c quy nh khng hon ton ging nhau. V d : Tiu chun ca c (DIN), ca Ba lan (PN) v nhiu nc chu u khc quy nh : khi o cng sut, ng c phi c trang b y cc b phn ph , nh lc khng kh , bnh tiu m, qut gi, ... ging nh khi n lm vic trong thc t. Trong khi , tiu chun ca Lin x () v ca M (SAE) cho php o cng sut


ca ng c vi cc b phn ph k trn l trang thit b tiu chun ca phng th nghim. Chnh v iu kin th nghim khng hon ton ging nhau nn cng sut danh ngha ca cng mt ng c cng khc nhau. V d : cng sut ng c c o theo SAE (Society of Automotive Engineers) ln hn khong 10 25 % so vi cng sut o theo DIN; nu o theo tiu chun CUNA (Commisione tecnica di Unificazione nell Automobile) ca Italy th ln hn 5 10 %.

Bng 2.2-3. iu kin kh quyn tiu chun

iu kin kh quyn tiu chun

Tiu chun - Nc p0 [mm Hg]

t0

[0C] 0

[ % ] TCVN 1684-75 (Vit Nam) 760 20 70 PN-78/S-02005 (Ba lan) 750 25 DIN 70 020 (c) 760 20 - ECE (Economic Commision for Europe) 750 25 -

Cng sut quy i (Neq) - Cng sut ca ng c c hiu chnh theo cc iu kin tiu chun.

Chng ta bit rng, cng sut v mt s ch tiu khc ca ng c, nh : momen quay, sut tiu hao nhin liu, lng tiu hao nhin liu gi,v.v. chu nh hng ng k ca iu kin mi trng xung quanh, c bit l p sut v nhit . c th so snh c kt qu th nghim c tin hnh trong nhng iu kin mi trng khc nhau, cn phi quy i kt qu o thc t theo cc iu kin tiu chun.

TCVN 1685-75 quy nh cch quy i cng sut ca ng c diesel khng tng p nh sau :

293273746 t

PBNN

n

eeq+

= (2.2-7)

trong : Neq - cng sut quy i, [kW]; Ne - cng sut o c khi th nghim, [kW]; B - p sut kh quyn trong khi th nghim, [mm Hg]; Pn - phn p sut ca hi nc trong khng kh m trong iu kin nhit v m tng i ca khng kh ti phng th nghim, [mm Hg]; t - nhit ca khng kh trong phng th nghim c o khong cch 1,5 m t ming ht khng kh ca ng c, [ 0C].

Tiu chun PN-78/S-02005 ca Ba lan khuyn ngh cch quy i cc kt qu th nghim nh sau :

eeq NKN = 0 (2.2-8a)


eeq MKM = 0 (2.2-8b) eeq gKg = 0 (2.2-8c) eeq GKG = 0 (2.2-8d)

i vi ng c diesel khng tng p : 5,065,0

0 298100

=

TB

K (2.2-8e)

i vi ng c xng : 5,0

0 298100

=

TB

K (2.2-8g)

trong cc cng thc trn : Neq , Meq , geq , Geq - cng sut, momen quay, sut tiu hao nhin liu v lng tiu hao nhin liu gi quy i ; Ne , Me , ge , Ge - cng sut, momen quay, sut tiu hao nhin liu v lng tiu hao nhin liu gi o c trong th nghim ; K0 - h s quy i; B - p sut kh quyn trong khi th nghim, [kPa]; T - nhit khng kh trong phng th nghim o c ti khu vc gn lc khng kh ca ng c, [ 0K].

Cng sut cc i (Ne.max) - Cng sut c ch ln nht m ng c c th pht ra trong mt thi gian nht nh m khng b qu ti.

TCVN 1684-75 quy nh cng sut cc i ca ng c phi t 110 % Nen trong khong thi gian 1 gi. Tng s thi gian lm vic ch cng sut cc i khng qu 10 % tng thi gian lm vic ca ng c. Khong thi gian lp li ch cng sut cc i khng c nh hn 6 gi.

Cng sut s dng (Nes) - Cng sut c ch do ngi thit k t hp ng c - h tiu th cng sut khuyn co s dng va pht huy ht tnh nng ca ng c va m bo tui bn, tin cy cn thit.

2.2.3. HIU SUT Trong tng s nhit nng a vo ng c, ch c mt phn c "ch bin" thnh

c nng c ch, phn cn li b tn tht nhng cng on khc nhau trong qu trnh ch bin. Hiu sut l i lng nh gi hiu qu bin i nhit nng thnh c nng ca ng c. nh gi mc tn tht trong tng cng on ca c qu trnh bin i nng lng, ngi ta a ra cc khi nim hiu sut sau y : hiu sut l thuyt, hiu sut ch th, hiu sut c hc, hiu sut c ch.

1) Hiu sut l thuyt (t) - l hiu sut nhit ca chu trnh l thuyt. 2) Hiu sut ch th (i) - l hiu sut nhit ca chu trnh nhit ng thc t.

111

11 QQQQQ

QQ

QW clkhXmii

i+++

=

==

(2.2-9a)

C hiu sut l thuyt (t) v hiu sut ch th (i) u l hiu sut nhit, tc l i lng nh gi mc hon thin ca ng c v phng din nhit ng. Chng khc


nhau ch, trong hiu sut ch th ngi ta tnh n tt c cc dng tn tht nhit nng c th tn ti khi thc hin mt chu trnh nhit ng ng c thc; cn hiu sut l thuyt ch bao gm dng tn tht nhit nng theo quy nh ca nh lut nhit ng II - nhit nng phi truyn cho ngun lnh c th thc hin mt chu trnh nhit ng lc. Bin i cng thc (2.2-9a) ta c :

ittt

tiii W

WQW

QW

=== 11

(2.2-9b)

i lng t-i = Wi / Wt c gi l h s din tch th cng, n c trng cho mc khc nhau gia din tch th cng ch th v th cng l thuyt. 3) Hiu sut c hc (m) - l i lng nh gi mc tn tht c hc trong ng c, tc l nh gi mc hon thin ca ng c v phng din c hc. N c xc nh bng cng thc :

i

m

i

em W

WWW

== 1 (2.2-10)

Hiu sut c hc chu nh hng ca rt nhiu yu t cu to v vn hnh khc nhau, v d :

- Vt liu ch to, - Cht lng thit k, ch to v lp rp, - Cht bi trn v ch bi trn, - T s nn, tc , ti, v.v.

m

n pe

m

m

H. 2.2-2. c im thay i ca m theo n , pe v

4) Hiu sut c ch (e) - l i lng nh gi tt c cc dng tn tht nng lng trong qu trnh bin i nhit nng thnh c nng c ch ng c.

mittmie

e QW ===

1 (2.2-11)

5) Sut tiu th nhin liu (ge) - Hiu qu bin i nhit nng thnh c nng ca CT cng ng ngha vi khi nim " tnh tit kim nhin liu" ca n. Trong thc t khai thc , ngi ta t dng hiu sut m thng dng i lng th hin lng nhin liu


do ng c tiu th nh gi tnh tit kim nhin liu. Lng nhin liu do ng c tiu th trong mt n v thi gian c gi l lng tiu th nhin gi (Ge). Lng nhin liu do ng c tiu th sinh ra mt n v cng sut c ch trong mt n v thi gian c gi l lng tiu th nhin liu ring c ch (gi tt l sut tiu th nhin liu - ge).

e

ee N

Gg = (2.2-12)

trong : ge - sut tiu hao nhin liu c ch; Ge - lng tiu th nhin liu gi; Ne - cng sut c ch ca ng c

n v thng dng ca Ge l [kg/h] hoc [lt /h] ; n v thng dng ca ge l [g/kW. h ] hoc [g/HP. h ].

2.2.4. CNG LM VIC CA NG C Cng lm vic ca CT c th nh gi bng nhiu thng s khc nhau.

Di y gii thiu 2 ch tiu tng hp nh gi cng lm vic ca ng c.

Cng sut lt - z

pnCiV

NN eVS

eV

=

= (2.2-13)

Cng sut piston - z

pCCiA

NN emPp

eP

=

= (2.2-14)

trong : CV, CP - cc h s ph thuc vo th nguyn ca cc i lng c trong cng thc; AP - din tch ca nh piston.

n v thng dng ca NV l [kW/dm3] hoc [HP/dm3] ; ca NP l [kW/dm2] hoc [HP/dm2].

NP < 15 kW/dm2 - ng c cng ho thp 15 NP < 45 kW/dm2 - ng c cng ho trung bnh NP 45 kW/dm2 - ng c cng ho cao


Chng 3

NHIN LIU V MI CHT CNG TC

3.1. PHN LOI NHIN LIU DNG CHO CT Nhin liu l cht chy c v sinh ra nhiu nhit khi chy.

Bng 3.1-1. Phn loi tng qut nhin liu

Tiu ch phn loi Loi nhin liu

Trng thi tn ti iu kin p sut v nhit kh quyn

- Kh t : kh m, kh l ga, kh thp, kh l cao, kh ho lng, v.v.

- Nhin liu lng : xng, du ho, gas oil, benzol, cn, du solar, du mazout, v.v.

- Nhin liu rn : than , than bn, ci, v.v. Nguyn liu sn xut nhin liu

- Nhin liu gc du m : xng, du diesel, du ho, v.v. - Nhin liu thay th : xng tng hp, cn, hydro, v.v.

Mc ch s dng

- Nhin liu dng cho ng c pht ho bng tia la : xng, cn, kh t, v.v.

- Nhin liu diesel : gas oil, mazout, kh t, v.v. - Nhin liu my bay : xng my bay, nhin liu phn lc.

Cng ngh sn xut

- Xng chng ct trc tip - Xng cracking - Xng reforming - Nhin liu tng hp

Theo nhit tr - Nhin liu c nhit tr cao : xng, du diesel, v.v. - Nhin liu c nhit tr thp : kh l ga, kh l cao, v.v.

1) Kh m - cn gi l kh t nhin (natural gas) - l hn hp cc loi kh c khai thc t cc m kh t hoc m du trong lng t. Kh m c th c phn loi thnh : kh ng hnh, kh khng ng hnh v kh ho tan.

Kh ng hnh l kh t do c trong cc m du. Kh khng ng hnh l kh c khai thc t cc m kh t trong lng t

v khng tip xc vi du th trong m du. Kh ho tan l kh ho tan trong du th c khai thc t cc m du. Thnh phn ca kh m c th rt khc nhau tu thuc vo v tr a l m kh m

c khai thc (Bng 3-2), tuy nhin chng u cha ch yu l methane (CH4), ethane (C2H6) v mt lng nh cc cht khc nh dioxide carbon (CO2), nit (N2), helium (He), v.v.


Ngoi cng dng lm nhin liu cho CT ni ring v nhin liu ni chung, kh m cn c s dng lm nguyn liu sn xut phn ho hc, vt liu tng hp, xng, v.v.

2) Kh lc-ho du l cc loi kh thu c trong qu trnh ch bin du m, v d : chng ct trc tip, nhit phn, cracking, v.v.

3) Kh l ga (producer gas) l kh t thu c bng cch kh ho cc loi nhin liu rn nh : than , than nu, than ci, g,v.v. nhit cao bng mt loi thit b c tn l l sinh kh. H. 3.1-1 gii thiu s l sinh kh v mt s thng s cng tc trong qu trnh kh ho than .

Nguyn l hot ng ca l sinh kh nh sau : khng kh c thi vo l t pha di. Ngay pha trn ghi l, than c t chy theo phn ng to nhit : C + O2 = CO2 + 406000 kJ/kmol (3.1-1) Khu vc din ra qu trnh chy ni trn c gi l tng chy. Khu vc pha trn tng chy l tng kh. Ti tng kh din ra 2 loi phn ng thu nhit di y : CO2 + C 2CO - 176000 kJ/kmol (3.1-2) H2O + C CO + H2 - 132000 kJ/kmol (3.1-3) Phn ng (3.1-2) v (3.1-3) l cc phn ng 2 chiu. T s CO/CO2 c hnh thnh phn ng (3.1-2) v H2/H2O phn ng (3.1-3) ph thuc trc ht vo nhit ti khu vc din ra phn ng. nhit 700 0C, CO/ CO2 1 v H2/ H2O 2,3 ; nhit 1000 0C, CO/ CO2 165 v H2/ H2O 103 .Trong trng hp sn xut kh l ga t than , ngi ta thng thi mt lng nht nh hi nc vo trong l cng vi khng kh nhm mc ch gim nhit tng chy bo v cc b phn ca l tip xc trc tip vi than v tro c nhit cao. Nu khng c hi nc, nhit ti khu vc ngay

1300 0C

900 - 1100 0C

500 - 900 0C

150 - 500 0C

KK + H2O

Kh l ga

5

4

3

2

1

H.3.1-1. S l sinh kh 1- Tng sy, 2- Tng chng ct, 3- Tng to kh, 4- Tng chy, 5- Phn cha tro.


trn ghi l c th t ti 1700 0C. Ngoi ra, hi nc cng c tc dng lm tng cht lng ca kh l ga nh tng hm lng H2 c hnh thnh t H2O. Tu theo chiu cao ca l, nhit ti tng kh dao ng trong khong 900-1100 0C. Pha trn tng kh l tng chng ct c nhit c duy tr trong khong 500 900 0C. Ti y, hu ht nhng thnh phn d bay hi ca nhin liu rn thot ra v c ht ra ngoi cng vi cc thnh phn khc ca kh l ga. Kh l ga c sn xut bng phng php c in c cc thnh phn chnh vi hm lng trung bnh nh sau : 27 % CO, 7 % H2 , 2 % CH4 , 4 % CO2 , 58 % N2 . Ngoi ra, trong kh l ga cn c mt lng nh hi nc v mt s loi hydrocarbon [6]. Kh l ga c s dng lm nhin liu cho ng ng c ga, turbine kh, cc ngnh luyn kim, thu tinh, gm, v.v. N c u im l c s octane kh cao (RON 100), nhng c nhit tr thp ( H 5650 kJ/m3 ) v cha nhiu N2 .

4) Kh thp (illuminating gas) l kh t c sn xut quy m cng nghip t cc loi nhin liu rn hoc lng nh : than , than nu, du, v.v. Cc loi kh thp thng dng l kh t (water gas), kh du (carbureted water gas) v kh than (coal gas). Kh t thu c bng cch thi hi nc qua mt lp than hoc cc (coke) nng. Thnh phn ch yu ca kh t l CO v H2. Kh du v Kh than thu c bng cch nhit phn du hoc than. Thnh phn ch yu ca chng l H2, CH4 , C2H4 v CO.

5) Kh

Dong co dot trong 1

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