Nguyn l hot ng ca h thng nh la trn t - 19/12/2006 8h:59

ng c t trong c lch s pht trin hn 100 nm qua. T khi ra i cho n nay, cc nh thit k lun tm cch ci tin, tng hiu sut lm vic, gim mc tiu th nhin liu v gim mc c hi trong kh x ng c.

ng c t trong l mt c my c nhiu h thng ph tr nh h thng nhin liu, h thng lm mt, h thng phn phi kh, h thng tng p... Ring i vi ng c xng th h thng nh la l mt trong nhng thnh phn quan trng nht. N c tc dng bin dng in mt chiu in p thp (6-12V, 24V) thnh cc xung in cao p (12.000-40.000) to nn tia la in bugi t chy ha kh vo ng thi im quy nh theo mt th t nht nh.

Hnh1: S nguyn l m phng nguyn l hot ng ca h thng nh la

Ch thch: A, D: Dy cao p; B: Np chia in; C: Con quay; E: V chia in; F: Cam chia in; G: Cm bin nh la; H: IC nh la; I: B-bin; J: Bugi nh la.

Trong bi vit ny cp n h thng nh la ca ng c. Bt u l thi im nh la, sau chng ta hy xem tt c nhng thnh phn to ra tia la nh nn in (bugi), cc cun tng p (bbin) v b chia in. Cui cng, chng ta s lt qua mt h thng nh la lp trnh hin.

Ti sao phi nh la sm?

H thng nh la trn chic xe ca bn cn phi lm vic ph hp vi cc h thng khc ca ng c. N cn pht ra tia la chnh xc mt thi im nht nh t chy hn hp kh dn n trong xi-lanh pht huy ht cng sut. Nu nh la sai thi im th cng sut ng c b gim i, tiu hao nhin liu v lng cht c hi trong kh x tng ln.

Hnh2: Bugi nh la trc khi piston ln ti im cht trn

Ch thch: 1: K np; 2: K nn; 3: K n; 4: K x; 5: Bugi; 6: im cht trn

Khi khng kh v nhin liu ho trn trong xi lanh b t chy, nhit tng ln v nhin liu b chy thnh kh x. iu ny dn n p sut trong xi lanh tng ln t ngt v y piston i xung.

tng cng sut v m-men ng c, cn thit phi tng p sut trong xi lanh trong thi k chy. p sut ln nht s cho hiu sut ng c cao v iu ny hon ton ph thuc vo thi im sinh tia la in t chy hn hp kh.

S c mt thi gian tr k t khi bu-gi pht tia la n khi hn hp kh b t chy hon ton v p sut trong xi-lanh t cao nht. Nu tia la xut hin khi piston chm n im cht trn ca

k nn, piston sn sng di chuyn xung trc khi p sut trong xi lanh t n tr s cao nht. y khng phi l thi im ti u.

s dng trit nng lng ca nhin liu, tia la cn xut hin trc khi piston t im cht trn ca k nn n khi piston i xung ng lc p sut trong xi lanh t tr s cao nht.

Ta bit rng: Cng = lc * khong cch; V trong xi lanh: Lc = p sut * din tch nh piston; Khong cch = hnh trnh piston. V vy: Cng = p sut * din tch nh piston * hnh trnh piston. i vi mt ng c c th th ng knh piston v hnh trnh l hng s, v vy ch cn cch l tng p sut tng cng sut ng c.Thi gian nh la rt quan trng, v thi im nh la sm ln hay mun i cn tu thuc vo cc iu kin khc. Thi gian hn hp chy gn nh l khng i, nhng tc ca piston s tng ln khi tc ng c tng. Ngha l, tc ng c cng cao th thi im nh la cng phi sm ln.

Ngoi vic tng cng sut, ta hy xt nhng mc tiu khc, v d nh ti thiu ho cc cht c hi trong kh x. Thi im nh la mun i (tc l thi im nh la gn thi im piston n im cht trn hn), p sut ln nht trong xi lanh v nhit c th gim i. Nhit gim s lm lm gim lng xit ni t NoX (mt cht c hi trong kh x). nh la mun cng lm gim ting g trong my (mt vi loi xe hin nay c cm bin ting g ng c d tm ting g ng c t ng).

Cc thnh phn chnh ca h thng nh la

Bugi: v l thuyt th kh n gin, n l cng c ngun in pht ra h quang qua mt khong trng (ging nh tia st). Ngun in ny phi c in p rt cao tia la c th phng qua khong trng v tia la mnh. Thng thng, in p gia hai cc ca nn in khong t 40.000 n 100.000 vn.

Hnh3: Bugi t chnh gia 4 van ca c cu phi kh

Bugi phi cch ly c in th cao tia la xut hin ng theo v tr nh trc ca cc in cc ca nn, mt khc n phi chu ng c iu kin khc nghit trong xilanh nh p sut v nhit rt cao, hn na n phi c thit k cc bi than khng bm li trn cc b mt in cc trong qu trnh lm vic.

Bugi s dng loi s cch in cch ly ngun cao p gia cc in cc, n phi m bo tia la phng ra ng hai u ca in cc ch khng phi bt c im no thuc hai cc. Ngoi ra cht s ny cn c tc dng khng cc bi than bm vo trong qu trnh s dng. S l vt liu dn nhit rt km, v vy vt liu rt nng trong qu trnh lm vic. Sc nng gip lm sch bi than khi in cc.

Hnh 4: Cu to ca bugi

Mt s xe i hi phi s dng loi bugi nng. Loi bugi ny c thit k c cht s bao bc tip xc vi kim loi t hn do vy vic trao i nhit km hn v nn nng hn v lm sch bi bn tt hn. Bugi lnh th ngc li, thit k vi vng trao i nhit ln hn v vy s ngui hn khi hot ng.

Nh thit k la chn nhit lm vic ca nn in ph hp cho mi loi xe. Mt s chic xe c hiu sut cao s sinh nhiu nhit hn do vy phi s dng nn ngui hn. Nu nn in qu nng, n s lm cho hn hp chy trc khi tia la pht ra, v vy cn la chn chnh xc loi nn in ph hp cho mi loi xe.

Hnh 5: Bugi nng (tri), bugi ngui (phi)

Bbin (hnh 6): l b phn sinh ra cao p to ra tia la. Rt n gin, in th cao c sinh ra do cm ng gia hai cun dy. Mt cun c t vng c gi l cun s cp (mu vng), cun xung quanh cun s cp (mu en) nhng nhiu vng hn l cun th cp. Cun th cp c s vng ln gp hng trm ln cun s cp.

Dng in t ngun in chy qua cun s cp ca bbin, t ngt, dng in b ngt i ti thi im nh la do m vt (ang ng kn mch in th t ngt m ra). Khi dng in cun s cp b ngt i, t trng in do cun s cp sinh ra gim t ngt. Theo nguyn tc cm ng in t, cun th cp sinh ra mt dng in chng li s thay i t trng . Do s vng ca cun th cp ln gp rt nhiu ln s vng dy cun s cp nn dng in cun th cp c in p rt ln (c th n 100.000 vn). Dng in cao p ny c b chia in a n nn bugi qua dy cao p.

Hnh 6: Bbin tng p

B chia in (hnh 7): c mt s chc nng nh sau: th nht, n chia ngun in cao p t tng in n cc xi lanh. iu ny c thc hin bi trc b chia in v con quay gn u. Cun th cp ca tng in c kt ni vi con quay, np b chia in c cc u ni vi cc dy cao p n cc xi lanh. Khi con quay quay vng trn n s chia ngun in cao p cho cc xi lanh theo mt t t nht nh.

Hnh 7: B chia in

Ch thch: A: Dng cao p n t b-bin nh la; B: Con quay; C: Np chia in; D: Dng cao p ti cc xi lanh.

B chia in i c hn (s dng m vt) (hnh 8) c hai phn, phn trn l b chia cao p nh va nu, cn pha di l b phn ngt dng in s cp ca bbin. u tip t ca tng in c ni vi m vt ca b chia in.

Mt trc cam trung tm b chia in s lm cho phn ng ca m vt tch khi phn tnh ti thi im nh la. iu ny l gii ti sao dng in ca cun dy s cp li b mt i t ngt v sinh ra xung cao p.

Hnh 8: B chia in i c s dng cam, m vt v t in

Ch thch: A: Dy ni vi b-bin nh la; B: M vt; C: Vt chnh thi im nh la sm; D: Cam dn; E: Cam quay; F: T in.

Vi nm gn y, chc bn c nghe v cc xe mi ch cn iu chnh v bo dng sau 100.000 dm. Mt trong nhng cng ngh ko di c thi gian bo tr l h thng nh la khng c b chia in, thng gi l h thng nh la lp trnh ESA. H thng ny khng ch c mt bbin tng p m mi mt xi lanh u c mt tng in ring. Khi ECU trung tm s quyt nh ton b thi im nh la chnh xc cho cc xi lanh. u im ca h thng nh la ESA chnh l: th nht, khng c b chia in; th hai, khng cn dy cao p; v cui cng l thi im nh la c t ng iu chnh theo chng trnh lp sn. iu ny lm tng hiu sut ng c, gim tiu th nhin liu v cc cht c hi trong kh x ng thi lm tng cng sut tng th ca ng c.

iu khin thi im nh la (thi im m m vt), ngi ta s dng h thng lm sm chn khng hoc h thng lm sm ly tm. Nhng h thng c kh ny iu khin sm la theo ti trng v theo tc ng c.

Thi im nh la ng vai tr rt quan trng i vi hiu sut ca ng c, v vy hin nay cc xe thng s dng cc cm bin nh la thay cho m vt. Cc cm bin ny s bo cho khi ECU chnh xc v tr ca piston, my tnh trn xe s quyt nh khi no m hoc ng dng in trong cun dy s cp.

Hnh 9: H thng nh la khng dng b chia in, mi bugi u c b tng p ring

H thng nh la ca cc xe hin i c rt nhiu iu th v v ng quan tm. Nu bn l ngi yu thch xe hi, hy thng xuyn cp nht nhng thng tin v cc h thng mi trn xe t.

c Hng (Bin dch)

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