More Related Content



  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