Carburetor air-fuel ratio calculations

Carburetor air-fuel ratio calculations

1. CARBURETTOR AIR FUEL RATIO
CALCULATIONS
UNIT-2
EME-505 I.C.ENGINES AND COMPRESSORS
(2013 batch)
BY
V.K.YADAV
MECHANICAL ENGG. DEPTT
G.L.B.I.T.M
20-May-21

2. The SIMPLE CARBURETTOR

3. CARBURETTOR A/F DETERMINATION
Can you write
STEADY FLOW ENERGY EQUATION
(SFEE) ??????
for section A-A and B-B
20-May-21

4. AIR-FUEL RATIO CALCULATIONS
Exact analysis (Practical case )
q-w = (h
2-h
1) + (C
2
2
-C
1
2
)/2
If we take compressibility of air into account, the air flow rate will change (but
the fuel flow rate will remain same as it is incompressible)
z

5. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21

6. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
So the C
2 expression will become

7. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
As P
1 v
1 ᵧ= P
2v
2ᵧ
V
1 is specific volume=
1/rho

8. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21

9. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
Eq. C.1
Theoretical mass flow rate

10. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
C.1

11. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
Note that due to petrol surface being lower
than the top of the jet by z meter , the
Pressure difference becomes (P
1-P
2-zgρ
f)
Instead of (P
1-P
2)

12. 20-May-21
AIR-FUEL RATIO CALCULATIONS Contd..

13. 20-May-21
AIR-FUEL RATIO CALCULATIONS Contd..
Note:
ToCalculatetheexactairflow,inadditiontopressuredrop,initialpressureP1and
initialtemperatureT1arerequired
Thecompressibilityofairissmallforthevelocitiesencountered.
ForathroatairVelocityof75m/sandacorrespondingpressuredifferenceof
25mmofHg,theairFuelratio,ascalculatedneglectingcompressibility
wouldrequireacorrectionofabout1.5%toincreasetherichness.
SizeoftheCarburettor:
Thesizeofthecarburettorisgenerallygivenintermsofdiameterofthroatinmm,and
jetsizeinhundredthsofamm,i.e.jetno50hasdiameterof50/100mm.
-Thesizeofventurithroatisabout30-35mm.Thejetdiameterisabout1/16of
VenturiDiameter
-(P1-P2)isabout50mmHg
-Velocityatthroatisabout90-100m/s
-ThecoefficientCd
aisusually0.85

14. 20-May-21
AIR-FUEL RATIO DETERMINATION
neglecting compressibility of air (Case II)
Approximate analysis (Assuming constant density of air)
Assuming initial velocity of air as negligible
C1

15. 20-May-21
AIR-FUEL RATIO DETERMINATION
neglecting compressibility of air Contd…

16. 20-May-21
AIR-FUEL RATIO DETERMINATION
CONCLUSIONS REGARDING A/F RATIO
From the expression of mass flow rate that we’ve derived it can be observed that

17. 20-May-21
AIR-FUEL RATIO DETERMINATION
CONCLUSIONS REGARDING A/F RATIO
Contd..
From the expression of mass flow rate that we’ve derived it can be observed that

18. 20-May-21
AIR-FUEL RATIO DETERMINATION
CONCLUSIONS REGARDING A/F RATIO
Contd..
From the expression of mass flow rate that we’ve derived it can be observed that

19. PROBLEM !!!
Theventuriofasimplecarburettorhasathroatdiameter
of20mmandthecoefficientofflowis0.8.Thefuelorifice
hasadiameterof1.14mmandthecoefficientofflowis
0.65.Thegasolinesurfaceis5mmbelowthethroat.
Calculate
(i)TheAirfuelratioforapressuredropof0.08barwhen
thenozzletipisneglected[CHECKSOLUTIONA/F
=15.15:1]
(ii)Theairfuelratiowhenthenozzletipistakeninto
account.[CHECKSOLUTIONA/F=15.19:1]
AssumeDensityofairandfuelas1.2Kg/m
3
and750
Kg/m
3