Understanding Carburetion in Small Engines: Principles and Operations, Slides of Applied Mechanics

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Carburetion

• Principles and Types

Performance Objectives

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

Interest Approach

• Has this ever happened to you?

It’s 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 carburetor’s primary purpose is to produce a mixture of fuel and air to operate the engine.

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.

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.

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

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.

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.

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.

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