CarburetionPrinciples and TypesThe following lesson plan is courtsey of Virginia Tech University Agricultural Education, author unknown. It is reprinted here unedited.

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

Enabling ObjectivesGiven the instruction in class students will be able to define and describe the following terms with 80% accuracy:CarburetorAir-fuel mixtureVacuumAtmospheric PressureVenturi

Interest ApproachHas 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?

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

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

CarburetionThe carburetor must create an air fuel mixture that is correct for different circumstances such as:Cold or hot startingIdlingPart throttleAccelerationHigh speed operation

How does it work?

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

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.

Air-Fuel MixtureThe 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.

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

Pressure DifferencesCarburetors work on the principle of air pressure differences. When discussing pressure differences we will talk aboutVacuumAtmospheric PressureVenturi

VacuumAn 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 PressureAtmospheric 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 PrincipleA venturi is a restriction in an air passage that increases air speed or velocity.

VenturiThis 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.

VaporizationAlthough 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.

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.

SummarySince 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:VacuumAtmospheric PressureVenturi