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 = (h2-h1) + (C2
2-C1
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 C2 expression will become

7. AIR-FUEL RATIO CALCULATIONS Contd..
20-May-21
As P1 v1 ᵧ = P2 v2 ᵧ
V1 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 (P1-P2-zgρf)
Instead of (P1-P2)

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

13. 20-May-21
AIR-FUEL RATIO CALCULATIONS Contd..
Note:
To Calculate the exact air flow, in addition to pressure drop, initial pressure P1 and
initial temperature T1 are required
The compressibility of air is small for the velocities encountered.
For a throat air Velocity of 75 m/s and a corresponding pressure difference of
25 mm of Hg, the air Fuel ratio, as calculated neglecting compressibility
would require a correction of about 1.5% to increase the richness.
Size of the Carburettor:
The size of the carburettor is generally given in terms of diameter of throat in mm, and
jet size in hundredths of a mm, i.e. jet no 50 has diameter of 50/100 mm.
- The size of venturi throat is about 30-35 mm. The jet diameter is about 1/16 of
Venturi Diameter
- (P1-P2) is about 50 mm Hg
- Velocity at throat is about 90-100 m/s
- The coefficient Cda is usually 0.85

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 !!!
The venturi of a simple carburettor has a throat diameter
of 20 mm and the coefficient of flow is 0.8. The fuel orifice
has a diameter of 1.14 mm and the coefficient of flow is
0.65. The gasoline surface is 5 mm below the throat.
Calculate
(i) The Air fuel ratio for a pressure drop of 0.08 bar when
the nozzle tip is neglected [CHECK SOLUTION A/F
=15.15:1 ]
(ii)The air fuel ratio when the nozzle tip is taken into
account. [CHECK SOLUTION A/F =15.19:1]
Assume Density of air and fuel as 1.2 Kg/m3 and 750
Kg/m3