Carburetion

1
Carburetion
The following lesson plan is courtsey of Virginia
Tech University Agricultural Education, author
unknown. It is reprinted here unedited.

• Principles and Types

2
Performance Objectives

• Students will be able to list and explain the
  principles of operation pertaining to small
  engine carburetors.

3
Enabling Objectives

• Given the instruction in class students will be
  able to define and describe the following terms
  with 80 accuracy
• Carburetor
• Air-fuel mixture
• Vacuum
• Atmospheric Pressure
• Venturi

4
Interest Approach

• Has this ever happened to you?
• Its spring, and you get you mower out of the
  garage to give the lawn that first grooming.
• The gas tank still has gas in it so you try to
  fire it up. You crank the engine several times
  but nothing happens. What is wrong with this
  mower that worked last year?

5
What is a Carburetor?

• A carburetors primary purpose is to produce a
  mixture of fuel and air to operate the engine.

6
Carburetion

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

7
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

8
How does it work?

• Air enters the top of the carburetor and is mixed
  with liquid fuel.

9
How does it work?

• The air fuel mixture is forced into the intake
  manifold by atmospheric pressure and burned in
  the combustion chamber of the engine.

10
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.

11
Air-Fuel Mixture

• At normal operating speed a small engine will use
  an air-fuel mixture of about 15 pounds air to 1
  pound of gasoline

12
Pressure Differences

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

13
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

14
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.

15
Venture Principle

• A venturi is a restriction in an air passage that
  increases air speed or velocity.

16
Venturi

• 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.

17
Vaporization

• Although the venturi breaks the fuel into fine
  particles, it is further vaporized by the heat of
  the engine in the intake manifold and by the
  swirling action of the air in the combustion
  chamber.

18
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.

19
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