Transcript Principles of Carburetion

Principles of Carburetion
Automotive Technology 1
Mr. Wasacz
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

Introduction
In order for an engine to work, what
must happen to the fuel??
 Liquid form gasoline will not allow the
engine to run!

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
Carburetion Con’t
Cold or Hot Starting
 Idling
 Part Throttle
 Acceleration
 High Speed Operation

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

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

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
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

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)

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

Venturi
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)
 Updraft
 Downdraft
draft
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.

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
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
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

Float Bowl Carburetors
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.

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

What it looks like all together
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

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.
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.

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

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
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
 Dry
 Re-oil
with soapy water
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

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

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?

Dual Element Air Cleaners



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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
Summary
What is the main point of the
carburetor?
 The average weight ration of air to
gasoline is 15:1. What does this
mean?
 How do carburetors operate? (think of
the venturi!)
 What are the three types of
carburetors, and what is the
difference?

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?
