The document discusses carburetion and carburetors. It describes carburetion as the process of preparing an atomized mixture of fuel and air before admission to the cylinder. A carburetor is defined as a device that supplies a metered spray of fuel mixed with the correct amount of air for efficient combustion in the cylinder under all operating conditions. It then discusses air-fuel ratios and mixture requirements for SI engines as well as factors that affect carburetion like engine speed, intake air temperature, fuel volatility, and intake manifold design.

1. By,
Parikshit Purandare
S.E. Mechanical Engg
VACOE, Ahmednagar

3. Carburetion: The process of preparation of mixture of atomized fuel
and air before admission to cylinder is called carburetion.
Carburettor: The device which supplies the metered spray of fuel
mixed with correct amount of air for efficient combustion in cylinder
at all operating conditions is called carburettor.
Air-Fuel Ratio (A/F) and Mixture requirements: In S.I.
Engines petrol fuel is used is mainly octane and has chemically
correct A/F is 15.12 : 1.
Rich mixture have A/F 11 to 15 : 1.
Lean mixture have A/F 16 to 18 : 1.

4. Speed
Temperature of inlet air
Volatility of fuel
Design of intake manifolds

5. Simple carburettor consist of
following parts:
1. Float chamber
2. Choke valve
3. Throttle valve
4. Venturi
5.Main nozzle

6. 1. Starting Choke: At the time of starting of engine rich mixture should
be provided during cold conditions and it is achieved by butterfly valve
called choke.
2. Main metering and Idling System: At no load running of the
engine, the engine requires rich mixture throttle valve is almost closed
airflow at venturi is greatly reduced so idling port is required.
3. Acceleration: Opening of throttle valve suddenly requires more fuel
and this is done by acceleration arrangement.
4. Meter Pin Method: In order to run the engine in maximum
economy meter pin method is used.

7. (continued)
Diagrams:
Acceleration
Idling
System

8. 1.Side draught carburettors
2.Updraught carburettors
3.Downdraught carburettors

9. 1.Compensating jet method: For maintaining constant
A/F ratio this method is used.
2.Air bleed method: For maintaining A/F ratio at all
speeds this method is used.
3.Auxiliary air valve method: By auxiliary air valve
constant A/F ratio is maintained.
4.Auxiliary port method: Extra port is provided for
maintaining constant A/F ratio.

10. (continued)
Air bleed
method

11. 1. Ice Formation:
 During atomization and vaporization process latent heat of vaporization
from the incoming air result in lower the temperature leads to formation
of ice at throttle valve.
 It can be avoided by heating intake air or by introduction of alcohol
which mixes with water and lowers is freezing point.
2. Vapour lock:
 Vaporization of fuel in pipes, fuel pump or in float chamber may cause
problem of vapour lock.
 If fuel pipe is of small cross-section and fuel flow is large, results high
velocity pressure drop formation of vapour bubbles at highest point and it
may interrupt the gasoline flow from fuel tank

12. To improve engine performance at
full power, the trend in automotive
design is to adopt more than one
carburettor on the same engine so
that each carburettor or barrel feeds
a limited number of cylinders, or
even a single cylinder: in this way,
volumetric efficiency (or combustion
chamber charge) is improved with
the added advantage that the fuel
feed to each cylinder, or group of
cylinders, is unaffected by the
intake stroke of the others, thus
ensuring a more uniformly blended
mixture distribution.