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Procese și caracteristici ale motoarelor cu ardere internă Student:
Grupa:
Prof :
An universitar: 2013/2014
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Cuprins ............................................................................................................................................ ........................... 3 Date initiale: .................................................................................................................
Parametri constructivi si marimi de baza: ............................................................................................... 3 ................................................................... 4 Calculul procesului de admisie a motoarelor in patru timpi .................................................................... ........................................................................................................ 9 Calculul procesului de comprimare ......................................................................................................... ....................................................................................................................................... 9 Calculul arderii ........................................................................................................................................ ...................................................................................................... ................................................................................. ........... 14 Trasarea diagramei indicate. ................................. ............................................................................................... ... 17 Calculul parametrilor indicati si efectivi ............................................................................................
A.
Calculul parametrilor indicati ............................................................... .................................................................................................... ..................................... 17
B.
Calculul parametrilor efectivi ai motorului. motorului. ................................................................... .............................................................................. ........... 18
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Date initiale:
Turatia :
Numarul de cilindri: i=4 ; Alezaj: D=79mm ; Cursa : S=81.5mm; Raportul de comprimare :
Parametri constructivi constructivi si marimi de baza:
a)
Cilindreea unitara , 0,399486 dm Cilindreea totala, Volumul cilindrului, Volumul camerei de ardere , ; *+ Viteza medie a pistonului, 3
b)
c)
d)
e)
Pe baza acestui acestui criteriu s-a stabilit ca motorul face parte din categoria categoria motoarelor rapide la care
f) Valoarea raportului dintre cursa si alezaj ,
In baza clasificarii motoarelor cu ardere interna cu piston in functie de raportul raportul ψ (subpatrate, patrate si suprapatrate), motorul calculat este suprapatrat ( g) Raza manivelei arborelui cotit ,
h) Raportul dintre raza manivelei si lungimea bielei ,
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i) Unghiul de rotatie al arborelui cotit – reprezinta reprezinta unghiul facut de manivela cu axa cilindrului. Originea unghiului se alege corespunzator pozitiei pistonului in PMI . o cursa completa a a pistonului corespunde unui unghi de 180°RAC, iar pentru un unghi de 360°RAC arborele cotit efectueaza o rotatie completa in timp ce pistonul parcurge doua curse simple. j) Ciclul motor-reprezinta succesiunea proceselor care se repeta periodic in cilindrii motorului.
k) Timpul motor, -reprezinta partea din ciclul motor care se efectueaza intr-o cursa simpla a pistonului.
Calculul procesului de admisie a motoarelor in patru timpi Ipoteze de calcul: Fluidul proaspat si gazele reziduale sunt considerate gaze ideale. Dupa intrarea in cilindru, energia cinematica a incarcaturii proaspete se transforma integral in caldura. Conditii initiale de stare (au valori standardizate: STAS 6635-87):
Presiunea si temperatura gazelor reziduale.
Presiunea acestor gaze ,numite si gaze arse restante, la motoarele in patru timpi, se considera ca este egala cu c u presiunea medie din colectorul de evacuare. Valorile pentru calculele aproximative relativ la diferite tipuri de motoare se pot adopta din recomandarea urmatoare:
Presiunea gazelor reziduale variaza in functie de turatia motorului; pentru un calcul mai riguros, variatia se poate aprecia prin relatia:
In care:
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n
pr(n)
1900
0,105237
3300
0,108741
4700
0,114131
6100
0,121408
600
0,103673
2000
0,105425
3400
0,109063
4800
0,114588
6200
0,122
700
0,103736
2100
0,105622
3500
0,109396
4900
0,115055
800
0,103808
2200
0,105829
3600
0,109737
5000
0,115532
900
0,10389
2300
0,106046
3700
0,110089
5100
0,116018
1000
0,103981
2400
0,106272
3800
0,11045
5200
0,116514
1100
0,104082
2500
0,106508
3900
0,11082
5300
0,117019
1200
0,104193
2600
0,106753
4000
0,1112
5400
0,117534
1300
0,104313
2700
0,107008
4100
0,11159
5500
0,118058
1400
0,104443
2800
0,107273
4200
0,11199
5600
0,118593
1500
0,104583
2900
0,107547
4300
0,112399
5700
0,119136
1600
0,104732
3000
0,107831
4400
0,112817
5800
0,11969
1700
0,104891
3100
0,108125
4500
0,113246
5900
0,120253
1800
0,105059
3200
0,108428
4600
0,113684
6000
0,120826
0.125
0.12
0.115 ] a P M [ r P 0.11
0.105
0.1 0 Pr(n)
1000
2000
3000
4000
5000
6000
7000
n [rpm]
Temperatura gazelor reziduale depinde de tipul motorului, de raportul de comprimare
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[ ]
Coeficientii gazelor reziduale,
Cresterea Cresterea de temperatura temperatura a fluidului proaspat ,
, se definesste prin:
Fluidul proaspat se incalzeste in contact cu peretii sistemului de admisie si ai cilindrului precum si datorita gazelor reziduale. Cresterea de temperatura depinde in principiu de viteza fluidului proaspat, de durata admisiei, de temperatura peretilor si a fluidului proaspat.
Variatia lui in functie de turatie, in vederea unui calcul ma i complet, poate fi stabilita orientativ cu relatia:
In care, este cresterea de temperatura corespunzatoare turatiei nomonale prezentata n fiind argumentul (variabila) functiei.
*Calculul si graficul sunt efectuate inegral in Microsoft Excel
n
∆T
2400
721,6
4400
496,1
600
924,55
2500
710,325
4500
484,825
700
913,275
2600
699,05
4600
473,55
800
902
2700
687,775
4700
462,275
900
890,725
2800
676,5
4800
451
1000
879,45
2900
665,225
4900
439,725
1100
868,175
3000
653,95
5000
428,45
1200
856,9
3100
642,675
5100
417,175
1300
845,625
3200
631,4
5200
405,9
1400
834,35
3300
620,125
5300
394,625
1500
823,075
3400
608,85
5400
383,35
1600
811,8
3500
597,575
5500
372,075
1700
800,525
3600
586,3
5600
360,8
1800
789,25
3700
575,025
5700
349,525
1900
777,975
3800
563,75
5800
338,25
, in relatia
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4300
507,375
1000 900 800 700
] K [ T
600 500 400 300 200 100 0 0 T(n)
1000
2000
3000
4000
5000
n [rpm]
Presiunea din cilindru la sfarsitul cursei de admisie ,
.
6000
7000
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-reprezinta densitatea fluidului proaspat. Pentru temperatura aerului t=20°C ,
densitatea fluidului proaspat la presiunea
este:
Temperatura Temperatura la sfarsitul cursei de admisie ,
Temperatura la sfarsitul cursei de admisie se poate determina din relatia bilantului termic aplicat fluidului proaspat inainte si dupa amestecare cu gazele arse. Daca se considera ca amestecarea se produce la presiune constanta , iar capacitatea calorica specifica a amestecului este egala cu a fluidului proaspat, temperatura se calculeaza cu relatia:
Unde:
sunt caldurile specifice la presiune constanta a gazelor
reziduale, respectiv a fluidului proaspat ; la motoarele cu formarea amestecului in exterior, valoare care se poate utiliza, cu aproximatie si la celelalte motoare.
denumit si randament al umplerii, coeficient de umplere, sau denumit Gradul de umplere, chiar randament volumetric este definit prin raportul dintre cantitatea (masica, gravifica, molara, volumica) de fluid proaspat retinut in cili ndru la sfarsitul admisiei si cantitatea posibila de a fi introdusa in cilindree , in conditiile de presiune si temperatura de la intrarea in motor, adica fara pierderi.
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Calculul procesului de comprimare Procesul si temperatura la sfarsitul comprimarii se pot calcula , aproximand comprimarea cu o evolutie cu exponent constant .
Exponentul reprezinta de fapt o valoare medie pe durata procesului de comprimare. Valorile exponentuluipolitropic mediu depind de schimbul de caldura dintre fluidul motor si peretii cindrului. Dintre toti factor ii de influenta, ponderea principala o are turatia; la cresterea turatiei ,mc are o crestere importanta. Motoarele racite cu lichid au un regim termic mai redus decat cele racite cu aer si, ca urmare, au valori pentru mai mici.
Determinarea exponentului mediu s-a facut pe baza exponentului adiabatic de comprimare . Valorile lui se determina din nomograma in functie de raportul de comprimare .
Calculul arderii
Ipoteze de calcul:
-caldurile specifice depind numai de temperatura
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care tine seama de caldura degajata prin ardere pana in punctul z si de pierderile de caldura aferente. Tab.1:Caracteristicile Tab.1:Caracteristicile combustibilului petrolier Combustibil Compozitia
Benzina
c h o 0.854 0.142 0.004 0.1065
0.5073
43529 43500
Aerul necesar arderii.
Combustibilii lichizi au urmatoarea compozitie elementara:
-c,h,o,s, sunt participantii masici de carbon, hidrogen, oxigen,sulf,etc. Cantitatea de aer teoretica necesara arderii sau cantitatea minima de aer necesara arderii complete, tinand seama de proportia volumica de oxigen in aer (21 procente), este:
] ( ) [ cantitatea reala de aer, disponibila pentru arderea unui kg de combustibil este :
(
Numarul de kmoli de substanta initiala care participa la reactia chimica este:
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in care:
valorile sale alegandu-se in functie de raportul al combustibilului.
Pentru combustibili petrolieri cu h/c=1,17...0,19 , inclusiv benzina, se aleg valori in intervalul 0,45...0,5 (valoarea aleasa in calcule a fost
Numarul total de kmoli de produse de de ardere:
Variatia molara.
Variatia numarului de kmoli in urma arderii este:
coeficientul chimic de variatie molara este exprimat prin raportul
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[ ] Pentru combustibilii petrolieri din Romania, sunt date orientativ compoziitiile lor necesare arderii complete, precum si puterile calorice inferioare (vezi Tab.1).
In cazul in care motorul functioneaza cu lipsa de aer, arderea fiind incompleta, in produsele de ardere apar CO si H2.. Astfel, caldura degajata fiind mai mica , puterea calorica pentru arderea incompleta se determina cu relatia:
[ ]
Calculul temperaturii maxime de ardere.
Temperatura la sfarsitul arderii se determina cu ajutorul relatiei de ardere care reprezinta de fapt bilantul energetic al procesului respectiv. Temperatura
va fi solutia pozitiva a urmatoarei ecuatii de gradul doi:
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Calculul presiunii maxime a ciclului rotunjit (corectat).
La nivelul ciclului rotunjit, valoarea presiunii maxime, ,difera de valoarea determinata la nivelul ciclului nerotunjit , fiind mai mica. Corectarea ciclului teoretic te oretic conduce la valori ale presiunii maxime reprezentand circa (0,85...0,92) .
;
Calculul destinderii.
In conditiile unui permanent schimb de caldura intre gazele c are evolueaza in cilindru si exterior, pentru evaluarea procesului, destinderea este asimilata unei transformari politropice, cu exponent politropic constant, egal cu valoarea medie.
Relatia de calcul pentru presiunea si temperatura de la sfarsitul cursei de destindere sunt urmatoarele:
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Trasarea diagramei indicate. Diagrama indicata a motorului cu ardere interna se construieste pe baza calculului proceselor de lucru. Se traseaza mai intai diagrama nerotunjita, apoi se rotunjeste in raport cu cotele de reglaj ce se adopta. Trasarea se face in coordonate p-V. Se alege o scara a presiunilor si o scara a volumelor.
Trasarea liniilor de evacuare si admisie se face prin cate o izobara de valoare .
si
In ceea ce priveste politropele de destindere si comprimare, deoarece in abscisa apare si cursa pistonului, se traseaza variatia presiunilor in functie de cursa pistonului. Pentru politropa de comprimare:
( ) ( )
Pemtru politropa de destindere:
unde:
presiunea corespunzatoare deplasarii x a pistonului in timpul cursei de comprimare;
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Intimpul unui ciclu de functionare , arborele cotit efectueaza doua rotatii complete, deci variaza intre 0 si 720°. Legatura dintre variatia unghiului de rotatie a arborelui motor si procesele de lucru este data explicit in rumatorul tabel:
Tab.2
Procesul de lucru Legea de variatie a presiunii 0°- 180° Admisie 180° - 360° Comprimare 360° - 540° Ardere si destindere 540° - 720°
Pentru trasarea diagramei indicate nerotunjite, se calculeaza urmatoarele marimi:
*Calculul si graficul sunt efectuate inegral in Microsoft Excel
180
-1
81,5
0,088826
195
-0,96593
80,45269
0,090091
210
-0,86603
77,31397
0,09409
225
-0,70711
72,11148
0,101479
240
-0,5
64,94531
0,113567
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Diagrama indivata nerotunjita
] r a b [ p 4
z
3.5
3
2.5
2
c
1.5
1
0.5 pr pa 0
d a 0
10
20
30
40
50
60
70
80
PME
90
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Iași Universitatea Tehnică “Gheorghe Asachi”, Iaș Facultatea de Mecanica Departament:IMAR
Calculul parametrilor indicati si efectivi A. Calculul parametrilor indicati 1. Presiunea medie indicata pe ciclul este o marime conventionala si poate fi definita astfel: -presiunea constanta ca marime, care actionand a ctionand asupra pistonului in timpul cursei de destindere ar produce un lucru mecanic egal cu lucrul mecanic indicat al cuclului -inaltimea unui dreptunghi avand baza egala cu cilindreea si aria cu lucrul mecanic indicat.
lucrul mecanic al diagramei de presiune inalta;lucrul mecanic de presiune joasa, sau lucrul mecanic de pompaj, se considera in consumul propriu de lucru mecanic al motorului, sau in randamentul mecanic al motorului.
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Iași Universitatea Tehnică “Gheorghe Asachi”, Iaș Facultatea de Mecanica Departament:IMAR
3. Randamentul indicat, combustibil.
;
, se defineste prin caldura disponibila a unitatii de cantitate de
* +
4. Consumul specific indicat, este cantitatea de combustibil consumata de motor in unitatea de timp , pentru unitatea de putere dezvoltata de motor.
B. Calculul parametrilor efectivi ai motorului. 1. Presiunea medie efectiva , este o presiune conventionala, constanta ca marime ,