Carburetion

Objectives Terms
 Students will be
able to list and
explain the
principles of
operation
pertaining to small
engine carburetors.
 Carburetor
 Air-fuel mixture
 Vacuum
 Atmospheric
Pressure
 Venturi

What is a Carburetor?
 A carburetor’s
primary purpose is
to produce a
mixture of fuel and
air to operate the
engine.

Carburetion
 Gasoline engines cannot run on liquid
gasoline. It must be vaporized and
mixed with air in the proper
proportions for varying conditions.

Carburetion
 The carburetor must
create an air fuel
mixture that is
correct for different
circumstances such
as:
 Cold or hot starting
 Idling
 Part throttle
 Acceleration
 High speed
operation

How does it work?
 Air enters the top of
the carburetor and is
mixed with liquid fuel.
 The air fuel mixture is
forced into the intake
manifold by
atmospheric pressure
and burned in the
combustion chamber
of the engine.

Air-Fuel Mixture
 The mixture will vary
depending on the
conditions. The proportion
is given as the number of
pounds of air compared to
the number of pounds of
gasoline.
 At normal operating speed
a small engine will use an
air-fuel mixture of about
15 pounds air to 1 pound
of gasoline

Pressure Differences
 Carburetors work
on the principle of
air pressure
differences. When
discussing pressure
differences we will
talk about:
 Vacuum
 Atmospheric
Pressure
 Venturi

Vacuum
 An absolute vacuum is
an area completely
free of air or
atmospheric pressure.
 Although an absolute
vacuum is not reached
in a small engine, any
pressure less that
atmospheric pressure
is generally referred to
as a vacuum

Atmospheric Pressure
 Atmospheric
pressure is the
pressure produced
by the weight of air
molecules above
the earth.
 A partial vacuum is
produced by the
piston on the
intake stroke.
When the intake
valve opens,
atmospheric
pressure forces air
through the
carburetor to fill it.

Venture Principle
 A venturi is a
restriction in an air
passage that increases
air speed or velocity.
 This increase in
velocity reduces
pressure causing fuel
to be drawn into the
air stream
 Particles of fuel are
vaporized by air
rushing through the
venturi.

Vaporization
 Although the venturi
breaks the fuel into
fine particles, it is
further vaporized by:
 The heat of the
engine in the intake
manifold
 The swirling action of
the air in the
combustion chamber.

Combustion
 Cold fuel is difficult to vaporize, this is
why we choke or prime a cold engine
to help get it started.
 Over choking or priming can cause
raw fuel to be pulled into the
combustion chamber resulting in
bypass or a condition known as
flooding.

Summary
 Since a gasoline engine does not
operate on liquid fuel, it is the
responsibility of the carburetor to
provide it with an air-fuel mixture.
The carburetor operates on the
principles of the following:
 Vacuum
 Atmospheric Pressure
 Venturi