1. Carburetion

2. 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

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

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

5. 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

6. 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.

7. 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

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

9. 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

10. 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.

11. 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.

12. 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.

13. 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.

14. 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