Carburetion
What is
carburetion?
The process
of formation of a combustible fuel-air mixture by mixing the proper amount of
fuel with air before it is admitted into the engine cylinder.
Comes from
the words “car” and “burette” because the carburetor “meters” the appropriate
quantity of liquid fuel (like a burette) and mixed it with air before sending
the mixture into the engine cylinder.
Fuel system:
Factors affecting Carburetion:
•
Engine
speed.
In a 4-stroke engine running at 3000
rev/min, the intake will take about 10 ms during which the fuel has to
evaporate, mix with air and be inducted into the engine.
•
Vaporization
characteristics of the fuel.
Will require a high volatile fuel
for quick evaporation and mixing with air.
•
The
temperature of the in coming air.
Must be high enough to be able to
evaporate the fuel and yet not too high as to reduce mass of fresh charge. Can
be increased by heating the incoming air ( by using exhaust gas around
inlet manifold
•
Design
of the carburetor.
This will help in proper
introduction of fuel into the air stream and provide proper distribution of the
mixture to the various cylinders. (eg: Double Venturi, Double barrel and etc ..)
Carburetor System:
•
Fuel
tank
•
Fuel
supply pump
•
Fuel
line meter
•
Carburetor
•
Intake
manifold
Types of Carburetor:
Deficiencies
of the Elementary Carburetor:
1.
At low loads, the mixture becomes leaner; the engine
requires the mixture to be enriched at low loads. The mixture is richest at
idle.
2.
At intermediate loads, the equivalence ratio
increases slightly as the air flow rate increases; the engine requires an
almost constant equivalence ratio.
3.
As the air flow approaches the maximum (WOT) value,
the equivalence ratio remains essentially constant; the engine requires an
equivalence ratio of about 1.1 at maximum engine power.
4.
The elementary carburetor cannot compensate for
transient phenomena in the intake manifold. It also cannot provide a rich
mixture during engine starting and warm-up.
5.
It cannot adjust to changes in ambient air density
due to changes in altitude.
Modern Carburetors:
Modern
Carburetor Design:
The changes required in the elementary carburetor so that it provides the
equivalence ratio required at various air flow rates are as follows.
1.
The main metering system must be compensated
to provide a constant lean or stoichiometric mixture over 20 to 80% of the air
flow range.
2.
An idle system must be added to meter the
fuel flow at idle and light loads to provide a rich mixture.
3.
An enrichment system must be provided so that
the engine can get a rich mixture as WOT conditions is approached and maximum
power can be obtained.
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