Principles of Carburetion

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Principles of Carburetion

• Automotive Technology 1
• Mr. Wasacz

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Objectives

• You will be able to list and explain the
  principles of carburetion
• Identify the types of carburetion
• Explain the float carburetor system
• Define manual throttle controls
• List the basic functions of a governor
• Describe the purpose of an air cleaner

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Introduction

• In order for an engine to work, what must happen
  to the fuel??
• Liquid form gasoline will not allow the engine to
  run!

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Carburetors function

• The primary function is to produce an air fuel
  mixture to operate the engine
• This process can be done by using a simple mixing
  valve, however it is not as efficient
• The carburetor is a much more complex system that
  allows for the operation of the engine during
  these conditions

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Carburetion Cont

• Cold or Hot Starting
• Idling
• Part Throttle
• Acceleration
• High Speed Operation

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The Intake of Air

• Remember, atmospheric pressure draws air/fuel
  through the carburetor and into the cylinder
• This drawing in of air takes gasoline atoms with
  it creating an air fuel mixture

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Air / Fuel Mixture

• The amount of air needed for combustion is far
  greater than the amount of fuel needed
• The usual weight ration is 15 parts of air to 1
  part of fuel
• For example, if we had 1 cubic foot of gasoline,
  it would require 9000 cubic feet of air to meet
  this ratio
• This ration will change depending on engine
  operation

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Pressure differences

• A carburetor is a device that is operated by
  pressure differences
• These terms refer to those pressure differences
• Vacuum
• Atmospheric pressure
• Venturi Principles

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Vacuum

• An absolute vacuum is an area that is completely
  free of air or atmospheric pressure
• This is difficult to obtain and never reached
  within an engine
• Any pressure that is less than atmospheric
  pressure is referred to as a vacuum
• Basically, a vacuum is an empty space

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Atmospheric Pressure

• This is the weight of air molecules above earth
• This varies with altitude because they higher up
  you go, the less atmosphere is on you!
• If a vacuum is created, an equilibrium will want
  to be established, so the atmospheric pressure
  will fill that space (this is how engines work)

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Venturi Principles

• The carburetor creates a partial vacuum itself by
  using this principle
• By restricting the passage that the air is
  flowing through, the velocity of the air will
  increase
• This increase in velocity will decrease the
  pressure behind the venturi
• This low pressure will be filled by fuel
• Basically, a venturi is a restriction in an air
  flow passageway

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Venturi
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Types of Carburetors

• There are three different types of carburetors
• These carburetors are named after the direction
  of airflow from the outlet to the engine manifold
• They are as follows
• Natural (Side) draft
• Updraft
• Downdraft

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Natural Draft Carburetor

• Used to when there is little space in top of the
  engine
• Air flows horizontally into the manifold
• This type will be the most common on the small
  engines that we are working on.

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Updraft Carburetor

• Placed low on an engine
• Use gravity feed fuel supply
• Air / Fuel Mixture must be forced upward into the
  engine
• Needs high air pressure
• Uses smaller passages

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Downdraft Carburetor

• Operates with lower air velocities
• Uses larger passages
• Gravity assists the air fuel mixture flow into
  the engine
• Can provide large volumes of fuel for high speed
  and high power operation

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Float Carburetors

• A Float is a small sealed vessel made of brass or
  plastic.
• It maintains a constant level of fuel in the
  float bowl.
• The float rises and falls with the fuel levels
• As the float rises to the top, it pushes a needle
  valve closed
• This needs then shuts off fuel flow to the
  carburetor

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Float Bowl Carburetors
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Choke System

• Since cold fuel is hard to vaporize, the choke is
  used during cold engine starts to provide a rich
  mixture to the carburetor in order to get the
  engine started.
• The choke is a round disc mounted on a shaft
  located at the intake end of the carburetor.

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Throttle System

• The throttle is a round disc mounted on a shaft
  beyond the main fuel nozzle in the carburetor.
• It regulates the amount of air-fuel mixture
  entering the cylinder.
• The throttle can be connected to a governer
• The governer will open and close the throttle to
  regulate engine speed

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What it looks like all together
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Load Adjustment

• The amount of fuel entering the main discharge
  nozzle is sometimes regulated by a load adjusting
  needle.
• In some carburetors, there is no adjustment
  needle. These have a fixed flow that is designed
  for that specific engine

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Primer

• Many small engines have hand operated plunger
  called a primer. When depressed it forces
  additional fuel through the main nozzle prior to
  starting a cold engine.

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Diaphram carburetors

• This type does not have a float, rather the
  difference between atmospheric pressure and the
  vacuum created in the engine pulsates a flexible
  diaphragm
• The pulsation of the diaphragm takes place on
  every intake and compression stroke.

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Air Cleaners / Filters

• As an engine operates, it needs an enormous
  amount of air passing through the carburetor into
  the combustion chamber
• This incoming air needs to be clean (this means
  that all particulate needs to be removed!)
• If this grit were to enter the combustion
  chamber, it would cause rapid wearing and scoring
  of the precision machined parts

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Air Cleaners / Filters

• Under severe dust conditions, engine life can be
  reduced to minutes
• To keep this dust from entering the engine, we
  use different types of cleaners / filters
• Oil wetted
• Dry types
• Dual element

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Oil Wetted Air Cleaner

• Uses a filtering element (crushed aluminum and
  polyurethane foam) dampened with engine oil
• Air is drawn through the oil wetted filter
• The filter in return removes any debris or
  particulate
• This type of filter can be reused!
• Clean with soapy water
• Dry
• Re-oil

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Dry Type Air Cleaner

• Air passes through
• Treated paper
• Felt
• Fiber
• Flocked screen
• These are designed for 1 time use!
• You can only clean large particulate from the
  filter
• Most dust however will stay in between the fibers
  of the filter

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Dry Type Cleaner Cont

• A new style is the pleated paper design.
• This style uses a porous paper filter material to
  allow air flow to the engine, while allowing the
  filtration of debris
• This design provides great surface area to
  collect particles
• This is common to most automobiles

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Dual Element Air Cleaners

• These filters are used on engines that operate in
  dustier than normal conditions
• Can you guess why its called a dual element air
  cleaner?

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Dual Element Air Cleaners

• Its because they use both an oil wetted cleaner
  foam filter, and a dry type filter
• This offers more protection to dust and
  particulate
• The oil wetted filter acts as a pre cleaner
• The paper cartridge then cleans out any missed
  debris
• These come in many shapes and sizes, the most
  common is a cylidrical filter

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Summary

• What is the main point of the carburetor?
• The average weight ration of air to gasoline is
  151. What does this mean?
• How do carburetors operate? (think of the
  venturi!)
• What are the three types of carburetors, and what
  is the difference?

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Summary

• What are the two ways of regulating gasoline
  within the carburetor?
• What is the function of the throttle?
• What is the function of the choke?
• What is the function of the primer?
• What is the main purpose of a filtering system?