PGS. TS NGUYN DUY TIN

NGUYN L NG C T TRONG

236 trang, in giy BB 58

NH XUT BN GIAO THNG VN TI H NI - 2007

NLCT 3

Chnh sa gio trnh in t

Nguyn l ng c t trong

1. Thng tin v tc gi PGS.TS. Nguyn Duy Tin

CBGD: B mn ng c t trong- Khoa c kh- HGTVT H ni

Chuyn ngnh: ng c t trong

Hng khoa hc nghin cu:

- To hn hp v chy trong ng c t trong - K thut phun nhin liu trong ng c - Khai thc ng c t i mi trong iu kin nhit i - S dng mi trng sch thn thin mi trng sng

in thoi lin h: 0989376773 Tel: 0435564303

Email lin h: nguyenduytienPGS@gmail.com

2. Phm vi v i tng s dng gio trnh - Gio trnh s dng tham kho cho sinh vin ngnh c kh v c kh

ng lc khoa c kh trng i hc Giao thng Vn ti - Cng c th dng cho cc ngnh c kh ng lc hc nng, lm

nghip v thu li Kin thc yu cu ca mn hc trc:

- Nhit k thut 10 t kho tra cu:

4 NLCT

LI NI U

gp phn vo vic nng cao cht lng o to cn b ngnh C kh Giao thng vn ti, trong c mn hc ng c t trong, chng ti bin son gio trnh "Nguyn l ng c t trong" Gio trnh ch yu phc v cho vic hc tp v nghin cu ca sinh vin ngnh C kh chuyn dng thuc khoa C kh Trng i hc Giao thng vn ti. ng thi c th lm ti liu tham kho cho cn b k thut ang lm vic trong ngnh C kh giao thng.

Ni dung ca gio trnh gii thiu mt cch c h thng nhng vn c bn v nguyn l lm vic ca ng c t trong, tnh ton cc qu trnh nhit ng, cc thng s c bn v c tnh ca ng c t trong. ng thi gio trnh nu cu to v nguyn l hot ng thit k, tnh ton h thng cp dn nhin liu ng c xng v ng c diesel th h mi ang c s dng trong ngnh C kh giao thng Vit Nam hin nay.

Gio trnh c vit trn c s nhng bi ging c ging dy nhiu nm cho ngnh C kh chuyn dng - khoa C kh - Trng i hc GTVT, c b sung nhng kin thc mi v nhng cng trnh nghin cu khoa hc ca tc gi trong qu trnh ging dy v nghin cu khoa hc.

Tc gi chn thnh cm n tp th cn b ging dy b mn ng c t trong, khoa C kh, Trng i hc Giao thng vn ti ng gp cho gio trnh nhng kin qu bu.

Knh mong cc bn ng nghip, sinh vin v bn c ng gp kin xy dng cun gio trnh ny ln ti bn c hon thin hn.

Tc gi

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PHN I

CC QU TRNH C BN

CHNG 1

NGUYN L LM VIC CA NG C T TRONG 1.1. KHI NIM CHUNG

ng c t trong ni chung, ng c xng v ng c diesel ni ring kiu piston chuyn ng tnh tin thuc loi ng c nhit. Hot ng nh qu trnh bin i ho nng sang nhit nng do nhin liu b t chy ri chuyn sang c nng. Qu trnh ny c thc hin trong xylanh ca ng c. 1.2. PHN LOI

Theo nhin liu s dng: + ng c xng: ng c dng nhin liu xng. + ng c diesel: ng c dng nhin liu diesel. Theo phng php to ho kh v t chy: + ng c to ho kh bn ngoi, l loi ng c m hn hp nhin liu v

khng kh c to thnh bn ngoi xylanh nh mt b phn c cu to c bit (b ch ho kh - carbuarettor) sau c a vo xylanh v c t chy y bng tia la in (ng c xng dng b ch ho kh).

+ ng c to ho kh bn trong, l loi ng c m hn hp hi nhin liu v khng kh c to thnh bn trong xylanh nh mt b phn c cu to c bit (bm cao p v vi phun,...) v hn hp ny t bc chy do hn hp b nn nhit cao (ng c diesel).

Theo s k thc hin mt chu trnh cng tc: + ng c bn k (4 strokes): Chu k lm vic c hon thnh sau bn hnh

trnh ca piston hoc hai vng quay ca trc khuu; + ng c hai k (2 strokes): Chu k lm vic c hon thnh sau hai hnh

trnh ca piston hoc mt vng quay ca trc khuu. Theo qu trnh cp nhit v t s nn (): + ng c lm vic theo qu trnh cp nhit ng tch, loi ny bao gm nhng

ng c c t s nn thp ( = 512), nh ng c s dng xng, nhin liu cn v kh; + ng c lm vic theo qu trnh cp nhit ng p, loi ny bao gm nhng

ng c c t s nn cao ( = 1224), nh ng c phun nhin liu bng khng kh nn v t bc chy, ng c s dng bt than;

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+ ng c lm vic theo qu trnh cp nhit hn hp, loi ny bao gm nhng ng c c t s nn cao ( = 1224), nh ng c diesel.

Theo phng php np: + Ngi ta phn loi kh np c c nn trc khi np hay khng, tng

ng vi 2 loi c ng c tng p v ng c khng tng p. Theo t s S/D + ng c c hnh trnh ngn khi: S / D < 1 + ng c c hnh trnh di khi: S / D > 1 Theo tc ng c: Tu theo tc trt trung bnh ca piston:

.30mS nC , m/s (1-1)

+ Khi Cm = (3 6) m/s c gi l ng c tc thp; + Khi Cm = (6 9) m/s c gi l ng c tc trung bnh; + Khi Cm = (9 13) m/s c gi l ng c tc cao; + Khi Cm > 13 m/s c gi l ng c siu cao tc. Theo s lng v cch b tr xylanh: + S lng xylanh: ng c mt xylanh v ng c nhiu xylanh (ng c 2, 3,

4, 6, 8,.. xylanh); + Cch b tr xylanh: ng c c xylanh t thng ng, t nghing v nm

ngang; + Theo s hng xylanh: ng c 1 hng, ng c ch V v ng c hnh sao; +Theo s trc khuu: ng c mt, hai hoc ba trc khuu, thm ch c ng

c khng c trc khuu (nh ng c piston quay- Wallkel). Ngoi ra c th phn loi ng c theo cng dng, phng php lm mt v dung

tch lm vic... 1.3. NHNG THNG S C BN CA NG C

ng c bao gm cc b phn chnh sau y: + C cu trc khuu thanh truyn; + C cu phi kh; + H thng nhin liu; + H thng bi trn; + H thng lm mt; + H thng t ng iu chnh tc ng c; + H thng khi ng. ng c xng cn c thm h thng nh la.

1.3.1. Nhng thng s c bn ca ng c Nhng thng s cu to c bn ca ng c, hnh 1-1 gm c:

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im cht: im cht l im m piston i chiu chuyn ng. im cht trn (CT) l im xa nht ca piston so vi ng tm trc khuu. im cht di (CD) l im gn nht ca piston so vi ng tm trc khuu. Hnh trnh piston S (stroke) l khong cch t v tr cao nht ca piston (im

cht trn CT) n v tr thp nht ca ca piston (im cht di CD) khi piston dch chuyn. S = 2.R; trong R- l bn knh quay ca trc khuu.

Th tch lm vic ca xylanh Vh l th tch ca xylanh gii hn trong khong mt hnh trnh ca piston:

2. .

4hDV S ; (1-2)

Th tch lm vic ca ng c VH VH = Vh. i ; (1-3) Trong : i - l s xylanh ca ng c.

Hnh 1-1. Piston im cht trn v di Th tch bung chy Vc l th tch phn khng gian gia nh piston, xylanh v

np xylanh khi piston CT. Th tch cha ho kh (th tch ton b) Va l tng th tch lm vic ca xylanh Vh

v th tch bung chyVc. Va = Vh + Vc ; (1-4) T s nn ca ng c l t s gia th tch cha ho kh ca xylanh Va v th

tch bung chy Vc.

1

1

hcc

h

c

ch

c

a VVVV

VVV

VV

; (1-5)

T s nn biu hin ho kh (ng c xng) hoc khng kh (ng c diesel) b nn nh i bao nhiu ln khi piston dch chuyn t CD ln CT. T s nn c nh hng ln n cng sut cng nh hiu sut ca ng c.

T s nn ty thuc vo loi ng c v thng c tr s nh sau:

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ng c xng: = 3,5 11;

NG C DIESEL: = 13 22; 1.4. NGUYN L LM VIC CA NG C BN K 1.4.1. ng c xng bn k

Khi ng c lm vic hnh 1-2, trc khuu 1 quay (theo chiu mi tn) cn piston 3 ni bn l vi trc khuu qua thanh truyn 10, s chuyn ng tnh tin trong xylanh 2.

Mi chu trnh lm vic ca ng c xng bn k bao gm 4 hnh trnh l: np, nn, chy- gin n, thi, thc hin mt ln sinh cng (trong hnh trnh chy- gin n). thc hin c nh vy th piston phi dch chuyn ln xung bn ln tng ng vi hai vng quay ca trc khuu ng c (t 00 n 7200). Qu trnh din ra khi piston i t CD ln CT hoc ngc li c gi l mt k.

Chu k lm vic ca ng c xng bn k nh sau: .

1. trc khuu, 2. xylanh, 3. piston, 4. ng np, 5. b ch ho kh, 6. xupp np, 7. bu gi, 8. xupp thi, 9. ng thi, 10. thanh truyn

Hnh 1-2: Cc hnh trnh lm vic ca ng c xng 4 k Hnh trnh np: trong hnh trnh ny (hnh 1-2a), khi trc khuu 1 quay, piston 3

s dch chuyn t CT xung CD, xupp np 6 m, xupp thi 8 ng, lm cho p sut trong xylanh 2 gim v do ho kh b ch ho kh 5 qua ng np 4 c ht vo xylanh.

Trn th cng hnh 1-3 ( th biu din mi quan h gia p sut v th tch lm vic ca xylanh ng vi mi v tr khc nhau ca piston), hnh trnh np c th hin bng ng ra (r-a).

Trong hnh trnh np, xupp np thng m sm trc khi piston ln im cht trn (biu th bng im d1), khi piston n CT (thi im bt u np) th xupp c m tng i ln lm cho tit din lu thng ln bo m ho kh i vo xylanh nhiu hn. Gc ng 1 vi on d1r c gi l gc m sm ca xupp np.

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Hnh 1-3. th cng Hnh 1-4. th phi kh ca ng c xng 4 k.

ng thi xupp np cng c ng mun hn mt cht so vi v tr piston CD (im d2) li dng chn khng cn li trong xylanh v lc qun tnh ca dng kh np, lm tng thm lng ho kh np vo xylanh (giai on np thm). Gc ng 2 vi on ad2 c gi l gc ng mun ca xupp np. V vy, qu trnh np khng phi kt thc ti CD m mun hn mt cht, ngha l sang c hnh trnh nn. Tuy nhin trong mt s ch tc thp do qun tnh ca dng kh np cn nh, (do pd2>p0) mt phn mi cht c np vo trong xylanh b lt ra ngoi trong giai on gc ng mun xupp np khi ngi ta gi l "hin tng thoi lui.

V vy, gc quay trc khuu tng ng ca qu trnh np l (1 +180 + 2 ) ln hn gc trong hnh trnh np 1800.

Cui qu trnh np, p sut v nhit ca ho kh trong xylanh l: pa = 0,8 0,9 kG/cm2 Ta = 350 4000 K. Hnh trnh nn: trong hnh trnh ny (hnh 1-2b), xupp np v xupp thi u

ng. Piston dch chuyn t CD ln CT, ho kh trong xylanh b nn, p sut v nhit ca n tng ln.

Hnh trnh nn c biu th bng ng ac (hnh 1-3), nhng qu trnh nn thc t ch bt u khi cc xupp np v thi ng kn hon ton, tc l lc m ho kh trong xylanh cch ly vi mi trng bn ngoi. Do thi gian thc t ca qu trnh nn (1800 - 2) nh hn thi gian hnh trnh nn l thuyt (1800 ).

Cui hnh trnh nn (im c hnh 1-3) bu-gi 7 ca h thng nh la phng tia la in t chy ho kh. Gc ng vi on cc (hnh 1-3) hay gc s (hnh 1-4) c gi l gc nh la sm ca ng c.

Cui hnh trnh nn, p sut v nhit ca ho kh trong xylanh l: pc = 11,0 15,0 kG/cm2 ; Tc = 500 7000 K.

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Hnh trnh chy gin n sinh cng: trong hnh trnh ny (hnh 1-2c), xupp np v thi ng. Do ho kh c bugi t chy cui hnh trnh nn, nn khi piston va n CT th tc chy ca ho kh cng nhanh, lm cho p sut ca kh chy tng ln rt ln trong xylanh v c biu th bng ng cz trn th cng. Tip theo qu trnh chy l qu trnh gin n ca kh chy (ng zb) piston b y t CT xung CD v pht sinh cng.

p sut v nhit ca kh chy ln nht trong xylanh l: pz = 40 70 kG/cm2 Tz = 2300 28000 K Hnh trnh thi: trong hnh trnh ny (hnh 1-2b), xupp np vn ng cn xupp

thi m. Piston dch chuyn t CD ln CT y kh chy qua ng thi 9 ra ngoi. Trc khi kt thc hnh trnh chy gin n sinh cng, xupp thi c m sm

mt cht trc khi piston ti CD (im b) gim bt p sut trong xylanh giai on gin n, do gim c cng tiu hao y kh ra khi xylanh. Ngoi ra khi gim p sut ny th lng sn phm chy cn li trong xylanh cng gim, do gim c cng trong qu trnh thi chnh v gim c lng kh st ng thi tng c lng ho kh np vo xylanh. Gc ng vi on bb hay gc 3 gi l gc m sm ca xupp thi.

ng thi thi sch kh chy ra khi xylanh, xupp thi cng c ng mun hn mt cht so vi thi im piston CT (im r). Gc ng vi on rr l gc 4 gi l gc ng mun ca xupp thi.

Do xupp thi m sm v ng mun nn gc quay trc khuu dnh cho qu trnh thi (3 +180 + 4 ) ln hn gc ca hnh trnh thi (180 ). p sut v nhit ca kh thi l:

pr = 1,0 1,20 kG/cm2 ; Tr = 900 12000 K Trn th cng on d1r biu th thi k trng ip ca xupp np v xupp thi,

tc l thi k m hai xupp cng m, gc ng vi on d1r l gc (1 + 4 ) (hnh1-4) gi l gc trng ip ca hai xupp.

Sau khi hnh trnh thi kt thc, th ng c xng 4 k mt xylanh hon thnh mt chu k lm vic v chuyn sang chu trnh tip theo. 1.4.2. ng c diesel bn k khng tng p

a) b) c) d)

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1. trc khuu; 2. xylanh; 3. piston; 4. ng np; 5. bm cao p; 6. xupp np; 7. vi phun; 8. xupp thi; 9. ng thi; 10.thanh truyn.

Hnh 1-5. Cc hnh trnh lm vic ca ng c diesel 4 k Qu trnh lm vic ca ng c diesel bn k cng ging nh ng c xng 4 k,

ngha l piston cng phi thc hin bn hnh trnh np, nn, chy gin n, thi. Trong ng c diesel 4 k qu trnh np v nn mi cht l khng kh (m khng phi ho kh) v nhin liu t chy, do khng kh nn c nhit cao (m khng dng tia la in).

Chu k lm vic ca ng c diesel 4 k nh sau: Hnh trnh np: trong hnh trnh ny (hnh 1-5a), khi trc khuu 1 quay, piston 7

s dch chuyn t CT xung CD, xupp np 4 m, xupp thi 6 ng, lm cho p sut trong xylanh 2 gim, khng kh bn ngoi c np vo trong xylanh.

Cui qu trnh np, p sut v nhit ca ho kh trong xylanh l: pa = 0,8 0,9 kG/cm2 ; Ta = 330 3800 K. Hnh trnh nn: trong hnh trnh ny (hnh 1-5b), xupp np v xupp thi u

ng. Piston dch chuyn t CD ln CT, ho kh trong xylanh b nn, p sut v nhit ca n tng ln. Hnh trnh nn c biu th bng ng ac (hnh 1-6), nhng qu trnh nn thc t ch bt u khi cc xupp np v thi ng kn hon ton, tc l lc m ho kh trong xylanh cch ly vi mi trng bn ngoi. Do thi gian thc t ca qu trnh nn (1800 - 2) nh hn thi gian hnh trnh nn l thuyt (1800).

Cui hnh trnh nn (im c) vi phun 5 ca h thng nhin liu s phun nhin liu xylanh ho trn vi khng kh c nhit cao, ri t bc chy (ng c t chy). Gc ng vi im c (gc s) (hnh 1-4) c gi l gc phun nhin liu sm ca ng c.

Cui hnh trnh nn, p sut v nhit ca hn hp kh v nhin liu trong xylanh l:

pc = 40 50 kG/cm2 ; Tc = 800 9000 K. Hnh trnh chy gin n sinh cng: trong hnh

trnh ny (hnh 1-5c), xupp np v thi ng. Do nhin liu phun vo xylanh cui hnh trnh nn c chun b v t bc chy, nn khi piston n CT th nhin liu chy cng nhanh, lm cho p sut kh chy tng ln, ho kh chy cng nhanh, lm cho p sut trong xylanh tng ln rt ln v y piston t CT xung CD qua thanh truyn lm quay trc khuu v pht sinh cng.

p sut v nhit ln nht ca kh chy trong xylanh l:

pz = 60 80 kG/cm2 ; Tz = 1900 22000 K Hnh trnh thi: trong hnh trnh ny (hnh 1-5d),

xupp np vn ng cn xupp thi m. Piston dch chuyn t CD ln CT y kh chy qua xupp thi ra ngoi.

Hnh 1-6: th cng ng c diesel 4 k

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Trc khi kt thc hnh trnh chy gin n sinh cng, xupp thi c m sm mt cht trc khi piston ti CD (im b) gim bt p sut trong xylanh giai on cui qu trnh gin n, do gim c cng tiu hao y kh ra khi xylanh. Ngoi ra khi gim p sut ny th lng kh chy cn li trong xylanh cng gim, nh tng c lng ho kh np vo xylanh. Gc ng vi on bb hay gc 3 gi l gc m sm ca xupp thi.

ng thi thi sch kh chy ra khi xylanh, xupp thi cng c ng mun hn mt cht so vi thi im piston CT (im r). Gc ng vi on rr l gc 4 gi l gc ng mun ca xupp thi.

Do xupp thi m sm v ng mun nn gc quay trc khuu ng vi qu trnh thi (3 +180 + 4) ln hn ca hnh trnh thi (180). p sut v nhit ca kh thi l:

pr = (1,1 1,2) kG/cm2 ; Tr = (800 900) 0K Trn th cng on d1r biu th thi k trng ip ca xupp np v xupp

thi, tc l thi k m hai xupp cng m, gc ng vi on d1r l gc (1 + 4) (hnh1-4), gi l gc trng ip ca hai xupp.

Sau khi kt thc hnh trnh thi, ng c li lp li chu trnh lm vic tip theo. Trn hnh 1-6 l th cng ca ng c diesel bn k. th phi kh ca n

cng tng t nh ca ng c xng. Tm hiu nguyn l lm vic ca ng c xng v ng c diesel bn k ta c th

rt ra mt s nhn xt sau: Trong bn hnh trnh ca piston, ch c mt hnh trnh chy gin n sinh cng, ba

hnh trnh cn li l nhng hnh trnh chun b v c thc hin nh ng nng hay qun tnh ca cc b phn chuyn ng quay trn (trc khuu, bnh ) v mt phn cng sinh ra ca nhng xylanh khc i vi ng c nhiu xylanh.

Thi im m v ng ca cc xupp np v thi khng trng vi thi im piston CT v CD c gi l thi im phi kh. y cng l mt c im c bn phn bit gia chu trnh lm vic thc t vi chu trnh lm vic l thuyt. Trong chu trnh lm vic l thuyt cc xupp thi khng m sm v ng mun nh ni trn.

Thi im phi kh cng nh cc gc ng vi thi gian m v ng ca cc xupp np v thi c biu th trn th phi kh.

Cc gc m sm v ng mun (gc phi kh) cng nh gc phun nhin liu hoc gc nh la cui hnh trnh nn c nh hng nhiu n cng sut, hiu sut v sut tiu hao nhin liu.

Thng thng cc gc ny c xc nh bng phng php thc nghim (bng 1):

Bng 1. Gc phi kh, gc phun nhin liu (gc nh la)

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Xupp np Xupp thi Loi

ng c M sm trc CT

ng mun sau CD

M sm trc CD

M sm trc CT

Gc phun nhin liu (gc nh la sm)

ng c xng 5

0 400 100 500 300 600 50 350

ng c diesel 10

0 300 450 750 300 600 50 300

100 300

1.5. NGUYN L LM VIC CA NG C HAI K (2 STROKES) Chu trnh lm vic ca ng c hai k cng bao gm bn qu trnh: np, nn,

chy gin n v thi, nhng khc vi ng c bn k l hon thnh mt chu trnh lm vic, trc khuu ca ng c hai k ch quay mt vng (3600) tng ng vi piston dch chuyn hai hnh trnh. Do , trong mi hnh trnh ca piston s c nhiu qu trnh cng xy ra.

ng c hai k thng dng hai kiu phi kh: loi c ca thi (ca np), ca thi (khng c xupp) v loi c ca thi v xupp thi. 1.5.1. ng c xng hai k , loi c ca thi v ca thi

ng c xng hai k, loi c ca thi v ca thi (khng dng xupp) c chu trnh lm vic nh sau:

Hnh trnh nn: trong hnh trnh ny (hnh 1-7a), khi trc khuu 2 quay, piston 5 dch chuyn t CD ln CT, khi ca thi 4 c piston ng kn, ho kh c sn trong xylanh 6 b nn, lm cho p sut v nhit ca n tng, n khi piston gn ti CT th n b t chy nh bugi 7 phng tia la in.

Khi piston i ln nn ho kh, pha di piston, trong ccte 1 p sut gim v ho kh t b ch ho kh, qua ng np v ca np c ht vo ccte chun b cho vic thi ho kh vo xylanh hnh trnh sau.

cui hnh trnh nn, p sut v nhit ca ho kh trong xylanh l: p = (6 10) kG/cm2 ; T= (400 600)0 K.

1. cc te; 2. trc khuu; 3. thanh truyn; 4. ca thi; 5. piston; 6. np xylanh; 7. xylanh; 8. ca thi ;

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9. ng thng. a) b)

Hnh 1-7. Nguyn l lm vic ng c xng hai k Hnh trnh sinh cng v thay kh: trong hnh trnh ny (hnh 1-7b), do ho kh

c t chy cui hnh trnh nn, nn khi piston n CT, th ho kh cng chy nhanh hn, lm cho p sut kh chy tng ln v y piston i xung CD qua thanh truyn 3, lm quay trc khuu 2 pht sinh cng.

Khi piston dch chuyn dn ti CD ca thi 4 m, ng thi sau ca thi 8 c chiu cao thp hn ca thi cng c m v ca np ng li. Do , kh chy sau khi lm vic, c p sut (3 - 4 kG/cm2) ln hn p sut kh tri (p0 = 1kG/cm2), c thi ra ngoi v ho kh di ccte b nn c p sut (1,2 1,3 kG/cm2) cao hn p sut ca kh chy cn li trong xylanh (1,1 kG/cm2) s theo ng 9 theo ca thi 8 vo xylanh pha trn nh piston, gp phn lm sch ho kh chy trong v to iu kin cho hnh trnh sau:

a) b)

Hnh 1-8. th cng v th phi kh ca ng c xng 2 k loi khng c xupp p sut v nhit ca kh chy trong xylanh l: p = (40 70) kG/cm2 ; T = (2000 2300)0 K. Sau hnh trnh sinh cng v thay kh, nu trc khuu vn quay th qu trnh lm

vic ca ng c xng hai k li lp chu k nh trn. 1.5.2. ng c diesel hai k, loi c ca thi v xupp thi

ng c diesel hai k c c im l khng dng ccte cha v thi kh m dng my nn kh ring thi kh trc tip vo trong xylanh.

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Chu trnh lm vic ca ng c ny nh sau: 1. trc khuu; 2. thanh truyn; 3. my nn kh; 4. xylanh; 5. vi phun; 6. xupp thi; 7. piston; 8. bung kh; 9. ca thi. a) b) Hnh 1-9. Cc hnh trnh lm vic ca ng c diesel hai k c xupp thi.

Hnh trnh nn: Trong hnh trnh ny (hnh 1-9a), khi trc khuu 1 quay, piston 7 dch chuyn t CD ln CT. Ca thi 9 c piston y kn v sau xupp thi 6 cng c ng li, khng kh c sn trong xylanh 4 b nn, p sut v nhit ca n tng ln cho n khi piston gn n CT, vi phun 5 ca h thng nhin liu s phun nhin liu vi p sut cao (100 140 kG/cm2) hnh thnh hn hp vi khng kh nn c nhit cao lm cho nhin liu ny t chy c.

Hnh 1-10. th cng v th phi kh ca ng c diesel 2 k, loi c xupp thi.

Cui hnh trnh nn p sut v nhit ca khng kh nn trong xylanh l: p = (40 50) kG/cm2 ; T = (800 900)0 K Hnh trnh sinh cng v thay kh: trong hnh trnh ny, do nhin liu c

t chy, nh khng kh nn c nhit cao cui hnh trnh nn, nn khi piston n

16 NLCT

CT, th nhin liu ny cng chy nhanh hn, lm cho p sut tng ln v y piston t CT xung CD, qua thanh truyn 2, lm quay trc khuu 1 pht sinh cng.

Khi piston dch chuyn gn ti CD, xupp 6 m, ng thi sau ca thi 9 cng c piston m ra. Do kh chy sau khi lm vic, c p sut (4-5 kG/cm2) ln hn p sut kh tri, c thi ra ngoi v khng kh mi bn ngoi, qua bnh lc, nh my nn kh 3, bung kh 8 v ca thi 9 c cung cp vo xylanh vi p sut khong (1,4,5) kG/cm2 ln hn p sut kh thi cn li trong xylanh (1,11,2 kG/cm2) gp phn lm sch kh chy trong v to iu kin cho hnh trnh sau.

p sut v nhit ca kh chy trong xylanh l: p = (80 100) kG/cm2 ; T= (1900 2100)0 K. Sau hnh trnh sinh cng v thay kh, nu trc khuu vn quay th qu trnh lm

vic ca ng c lp li nh trn. Tm hiu nguyn l lm vic ca ng c xng hai k v ng c diesel hai k, c

th rt ra mt s nhn xt sau: Trong hai hnh trnh ca piston, th ch c mt hnh trnh sinh cng cn cc hnh

trnh cn li c thc hin nh ng nng hay qun tnh ca cc b phn chuyn ng quay trn (trc khuu, bnh ) v mt phn cng sinh ra t nhng xylanh khc i vi ng c nhiu xylanh.

p sut ca ho kh hoc khng kh thi vo xylanh ln hn p sut kh tri. Do , phi dng bm thi kh hay my nn kh do trc khuu dn ng nn cng sut ng c cng phi gim i.

Trong qu trnh lm vic c mt phn hnh trnh ca piston dng thi v thi kh. Khi thi kh c mt phn nhin liu v khng kh mi theo kh thi ra ngoi.

p sut v nhit ca ho kh hoc khng kh cui qu trnh nn cng nh qu trnh chy v gin n ph thuc nhiu vo v tr ca ca thi, ca thi v t s nn ca ng c.

T s nn ca ng c hai k c tnh nh sau:

1'

cVhV (1-6)

Trong : Vh Th tch lm vit thc t ca xylanh, c tnh t lc piston bt u y kn ca thi hoc xupp thi ng, khi piston dch chuyn t CD ln CT cho n lc piston CT.

VC Th tch bung chy. Trong ng c hai k, qu trnh thi (np), nn, chy gin n v thi khng c

th hin r rng mi hnh trnh nh ng c 4 k. Do , ng c hai k, hnh trnh th nht cng c th l hnh trnh thi, thi v nn, cn hnh trnh th hai l hnh trnh sinh cng, thi v thi,v.v

1.6. SO SNH NG C 1.6.1. So snh ng c hai k vi ng c 4 k 1.6.1.1. u im

NLCT 17

ng c hai k c s hnh trnh sinh cng gp i ( khi cng s vng quay n) v c cng sut ln hn khong (5070)% (khi cng th tch lm vic Vh v s vng quay n) so vi ng c 4 k.

ng c hai k chy u v m hn ng c 4 k, v mi vng quay ca trc khuu c mt hnh trnh sinh cng. Do vi cc iu kin nh nhau (S,D,i v n), th ng c hai k c th dng bnh , lp trn trc khuu c kch thc v trng lng nh hn so vi ng c 4 k.

ng c hai k khng c xupp np v nu dng ccte thi kh vo xylanh, th cu to n gin v d s dng hn so vi ng c bn k 1.6.1.2. Nhc im

Hiu sut ca ng c hai k nh hn so vi ng c bn k, do c s hao ph nhin liu trong qu trnh trao i kh.

Nhit trong qu trnh lm vic ca ng c hai k ln hn so vi ng c 4 k, do c s ln sinh cng nhiu hn, lm cho ng c b t nng v c bit i vi ng c diesel d b bm mui than bung chy.v.v.

Trong ng c xng hai k, nu dng ccte cha du bi trn thi kh, th d lm hng du bi trn.

Cn c vo nhng u im trn, ng c xng hai k thng c dng ng c c cng sut nh. V d ng c ph my ko, ng c my phun thuc v mt s ng c mt xe my,.. Cn ng c diesel hai k li c dng nhiu ng c c cng sut trung bnh v ln, v d ng c t, tu thu, u my xe la, my xy dng v my pht in. 1.6.2. So snh ng c xng v ng c diesel 1.6.2.1. u im

Hiu sut ca ng c diesel ln hn ng c xng, do hao ph nhin liu t v t s nn cao. V d, nu ng c xng c sut tiu hao nhin liu l ge= (150240)g/kW.h th ng c diesel l ge= (110190)g/kW.h, ngha l lng nhin liu tiu hao ng c diesel l t hn ng c xng khong (30-35)%.

Nhin liu dng trong ng c diesel l du diesel r tin v t gy chy hn so vi xng dng trn ng c xng.

H thng nhin liu ca ng c diesel (bm cao p, vi phun) t b h hng v d dng hn h thng nhin liu ca ng c xng (dng b ch ho kh, hoc h thng phun xng in t,..).

1.6.2.2. Nhc im Kch thc v trng lng ca ng c diesel ln hn ng c xng v p sut

kh chy trong ng c diesel ln. Do trng lng ring ca ng c diesel (trng lng trn mt n v cng sut tnh bng kW) ln hn trng lng ring ca ng c xng (40-70)%.

ng c diesel, c bit l h thng nhin liu, ch to kh hn ng c xng. Do , gi thnh ca ng c diesel thng cao hn ng c xng.

18 NLCT

ng c diesel dng nhin liu nng kh chy v phng php to ho kh gia nhin liu phun sng vi khng kh khng tt nn kh khi ng hn ng c xng. Do , cng sut ca ng c diesel, thc t coi nh bng cng sut ca ng c xng (khi cng th tch cng tc v s vng quay mc d hiu sut ca ng c diesel cao hn). 1.7. NHNG THNG S LM VIC C BN CA NG C

Nhng thng s lm vic c bn ca ng c bao gm: cng sut, hiu sut v sut tiu hao nhin liu. Nhng thng s ny c chia ra lm hai loi: Thng s ch th (hoc thng s tnh ton) c trng cho chu trnh lm vic ca ng c v thng s hu ch hoc thng s s dng c trng cho kh nng lm vic thc t ca ng c. 1.7.1. Thng s ch th 1.7.1.1. Cng sut ch th

Mun xc nh cng sut ch th cn phi xc nh p sut ch th, l p sut gi thit khng i tc dng ln piston trong mt hnh trnh lm vic sinh ra mt cng bng cng ch th ca kh chy trong mt chu trnh lm vic ca ng c.

Khi c th cng hay th ch th thc t (hnh 1-11), c th xc nh c p sut ch th trung bnh nh sau:

mLF

ip . (1-7)

Trong : pi- p sut ch th trung bnh (N/m2). F- Din tch ca th cng hay th ch th, c gii hn gia ng cong nn v chy gin n, (mm2). L- Chiu di ca th cng (mm). m- T l xch p sut ca th cng (N/m2/mm).

Tr s ca p sut ch th trung bnh pi chnh l chiu cao ca hnh ch nht ABCD c din tch bng din tch ca th cng hay th ch th.

Cng sut ch th l cng do kh chy thc hin c xylanh ca ng c trong mt n v thi gian.

Cng ch th do kh chy thc hin c xylanh ca ng c sau mt chu trnh lm vic s l:

Li = pi.Vh; Nm/chu trnh. Trong : pi - p sut ch th trung

bnh (N/m2). Vh- Th tch lm vic ca

mt xylanh (m3). Nu gi l s k ca ng c hay s

hnh trnh ca piston sau mt chu trnh lm vic, th cng sut ch th do kh chy thc hin c xylanh sau thi gian mt giy s l:

NLCT 19

(Nm/s) ;.60

2..

nhVipiL

(1-8) Trong : pi - p sut ch th trung bnh (N/m2). Vh- Th tch lm vic ca mt xylanh (m3). n S vng quay ca ng c (vg/ph). - S k ca ng c. Cng sut ch th ca ng c nhiu xylanh, khi s xylanh l i, c dng:

30

... inhVipiN ; (kW) (1-9)

hay ;.07,22

...

inhVipiN (ml) (1-10)

1.7.1.2. Hiu sut ch th

Hiu sut ch th i l t s gia nhit lng bin i thnh cng ch th ca chu trnh so vi nhit lng ca nhin liu tiu hao:

HQnlG

iLi . ; (1-11)

Trong : Li Cng ch th (J); Gnl Lng nhin liu tiu hao (m3,kg); QH Nhit tr ca nhin liu (J/m3, J/kg). Hiu sut ch th thng c gi tr nh sau: ng c xng i = 0,250,35 ng c diesel i = 0,380,50

1.7.1.3. Sut tiu hao nhin liu ch th Tnh kinh t ca ng c cng c th nh gi bng sut tiu hao nhin liu cho

mt kW ch th trong mt gi.

310.iN

nlGig ; (g/kW.h). (1-12)

hay: 3

10..36,1 iNnlG

ig ; (g/ml.h) (1-13)

Trong : Gnl Lng nhin liu tiu hao trong mt gi (kg/h); Ni Cng sut ch th (kW). Sut tiu hao nhin liu ch th thng c gi tr sau:

Hnh 1-11. th cng

20 NLCT

ng c xng: gi = 140 180; g/kW.h; hay: gi = 190 250; g/ml.h; ng c diesel: gi = 96 125; g/kW.h; hay: gi = 130 160; g/ml.h;

1.7.2. Thng s c ch 1.7.2.1. Cng sut c ch

Cng sut ch th pht sinh trong xylanh ng c khng bin i hon ton thnh cng hu ch, m mt phn s b tiu hao khc phc ma st gia cc b mt lm vic ca nhng chi tit (xylanh v piston, trc khuu v trc,) gia nhng chi tit chuyn ng v khng kh (trc khuu, thanh truyn v bnh ,). Mt phn khc dn ng cc c cu v h thng ph (bm, qut gi, my pht in, my nn kh, ). Do , cng sut c ch trn trc khuu ca ng c Ne s nh hn cng sut ch th mt gi tr bng cng sut dng khc phc nhng tr lc trn gi l cng tn tht c hc Nm.

Ne = Ni Nm ; (kW). (1-14) Cng sut tn tht c hc, tng t cng sut ch th, c th xc nh nh sau:

30

... inVpN hmm ; (kW) (1-15)

Trong : pm - p sut tn tht c hc trung bnh, l mt phn ca p sut ch th trung bnh c tiu hao cho tn tht c hc (N/m2).

Nu ly p sut ch th trung bnh pi tr i p sut tn tht c hc trung bnh th cn li mt phn p sut khng i tc dng ln piston sinh ra mt cng bng cng c ch trn trc khuu ng c. Tr s ny l p sut c ch trung bnh pe.

pe = pi pm; (N/m2 ). (1-16) p sut c ch trung bnh pe ca cc ng c, khi lm vic vi cng sut nh

mc, c tnh nh sau:

30

... inVpN hee ; (kW) (1-17)

hay: ;.07,22...

inVp

N hee (ml). (1-18)

nh gi nhng tn tht c hc, thng dng hiu sut c hc m , l t s gia p sut c ch trung bnh pe v p sut ch th trung bnh.

i

m

i

mi

i

ei p

pp

pppp

1

; (1-19)

Hoc cng c th biu th hiu sut c hc bng cng sut c ch Ne v cng sut ch th nh sau:

;1 N

mi

m

i

i

i

em N

NN

NNN

(1-20)

NLCT 21

Nh vy, khi tng ph ti ca ng c, m vn gi nguyn s vng quay, th cng sut tn tht c hc Nm hu nh khng thay i. Do , hiu sut c hc m tng ln. Nhng khi ng c chy khng ti, tc l cng sut c ch bng khng (Ne = 0) th hiu sut c hc cng bng khng (m = 0) v lc ny ton b cng sut ch th dng tiu hao cho tn tht c hc, ngha l cng sut ch th bng cng sut tn tht c hc Ni = Nm.

Hiu sut c hc ph thuc vo loi ng c v cht lng ch to ng c, ngoi ra hiu sut c hc cn ph thuc vo cc iu kin s dng. Do o, nu iu kin s dng khng tt, th hiu sut c hc ca ng c cng gim.

Trong iu kin lm vic bnh thng, hiu sut c hc ca ng c nh sau: m = 0,70 0,85 ;

1.7.2.2. Hiu sut c ch

Hiu sut c ch e l t s gia nhit lng bin i thnh cng c ch trn trc khuu ng c so vi nhit lng ca nhin liu tiu hao:

Hnl

ee QG

L.

; (1-21)

Trong : Le Cng c ch (J). Gnl Lng nhin liu tiu hao (m3, kg). QH Nhit tr thp ca nhin liu (J/ m3,J/kg). Hiu sut c ch thng c gi tr nh sau: ng c xng e = 0,18 0,30 ; ng c xng e = 0,27 0,42 ;

1.7.2.3. Sut tiu hao nhin liu c ch Sut tiu hao nhin liu c ch tng t nh sut tiu hao nhin liu ch th c th

xc nh nh sau:

e

nle N

Gg

310. ; (g/kW.h) (1-22)

hay: 310..55,173 e

nle N

Gg ; (g/ml.h). (2-23)

Trong : Gnl: Lng nhin liu tiu hao trong mt gi (kg/h). Ne Cng sut hu ch (kW). Sut tiu hao nhin liu hu ch thng c gi tr nh sau: ng c xng: ge = 150 240; g/kW.h; hay: ge = 210 280; g/ml.h; ng c diesel: ge = 110 150; g/kW.h; hay: ge = 160 210; g/ml.h;

1.8. NG C NHIU XYLANH