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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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