internal combustion engines presentation
TRANSCRIPT
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CARBURETIONThe process of formation of a combustible fuel-air
mixture by mixing the proper amount of fuel with air
before admission to engine cylinder is called
carburetion.
The device which perform this job is called
carburetor.
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Function of Carburetor It breaks up or atomizes the fuel into a fine spray
and it mixes with air to make a mixture that willburn readily
It regulates the ratio of fuel to air
It regulates the amount of the fuel-air-mixture
going into the combustion chamber
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Factors Affecting Carburetion Engine speed
Vapourization characteristics of the fuel Temperature of the incoming air Design of the carburetor
Types of Mixtures Chemically correct mixture Rich mixture Lean mixture
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Ranges of Throttle Operation Idling
An idling engine is one which operates at no loadand with nearly closed throttle.under Idling conditions, the engine requires arich mixture.
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Cruising
In this range, the primary interest lies in obtainingthe maximum fuel economy.
High powerThis range is shown by the line CD in the above
figure.During peak power operation, the engine requires a
richer mixture due to the following reasons:a. To provide best powerb. To prevent overheating of exhaust valve and the
area near it.
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Principle of Carburetion The carburetor works on Bernoullis Principle: the faster
air moves, the lower its Static Pressure, and the higher
its dynamic pressure. The throttle (accelerator) linkage
does not directly control the flow of liquid fuel.
Instead, it actuates carburetor mechanisms which meter
the flow of air being pulled into the engine. The speed
of this flow, and therefore its pressure, determines the
amount of fuel drawn into the airstream.
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As the piston moves down a partial vacuum is created
in the cylinder
Atmospheric pressure pushes air through thecarburetor air intake to equalize this pressure
The air speed increases in the venturi (the narrow
passage in the air intake passage)
As the air speed increases the pressure is lowered.
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Because the pressure is lowered atmospheric
pressure in the fuel bowl pushes fuel through the
pipe to the venturi and into the air streamThe speed of the air in the venturi and the turbulence
past the venturi atomize the fuel and mix the tiny
droplets with air.Once the air & fuel are mixed, the next job of the
carburetor is to provide a means whereby the amount
of mixture that enters the cylinder can be controlled
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A butterfly (throttle) valve in the manifold controls
the fuel air mixture
If you want your engine to run fast open yourbutterfly (throttle) valve, the more fuel and air that
gets delivered the faster it will run
A Choke valve is used to aid in cold starts
It helps provide a better vaporization of the fuel air
and it provides more fuel-air (a richer mixture)
The choke valve is similar to the butterfly except it is
placed on the air-intake side of the carburetor.
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Operation of the Venturi Tube
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Simple Carburetor A carburetor basically consists of an open pipe through
which the air passes into the inlet manifold of the
engine. The pipe is in the form of a venturi: it narrows in
section and then widens again, causing the airflow to
increase in speed in the narrowest part.
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Fuel is introduced into the air stream through small
holes at the narrowest part of the venturi and at other
places where pressure will be lowered when notrunning on full throttle. Fuel flow is adjusted by
means of precisely calibrated orifices, referred to
as jets , in the fuel path.
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A simple Carburetor
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Parts of a Carburetor Fuel Strainer
Float Chamber
Main fuel metering and idling nozzles
Choke and throttle
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Strainer
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Float chamber
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Main Fuel Metering
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Types of Carburetors Depending on direction of flow of air
Updraught type
Downdraught type
Cross-draught type
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Constant choke carburetorThe air and fuel flow areas are always maintained to
be constant.Eg. Solex and Zenith carburetors
Constant Vacuum CarburetorAir and fuel flow areas are being varied as per the
demand on the engine, but vacuum is maintained tobe always same.
Eg. S.U. and Carter Carburetors
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