EPA-450/3-77-039
'November 1977
MOTOR VEHICLE
EMISSIONS CONTROL
FUEL EVAPORATION
CONTROL SYSTEMS
....p*" ...•-N.-V-'..: .•:-:•.:.•;•...••:;•
'
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**•'•-...»•. . •• . - • ' . ^1 _ vH •
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
VT/
I
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EPA-450/3-77-039
MOTOR VEHICLE EMISSIONS CONTROL
BOOK FOUR
FUEL EVAPORATION
CONTROL SYSTEMS
by
B.D. Hayes, Project Director
M.T. Maness, Associate Project Director
R.A. Ragazzi, Principal Investigator
R.A. Barrett, Graduate Research Assistant
Department of Industrial Sciences
Colorado State University
Fort Collins, Colorado 80523
EPA Grants No. T008135-01-0 and T900621-01-0
EPA Region VIII Project Officer: Elmer M. Chenault
EPA Project Officer: Bruce Hogarth
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Waste Management
Office of Air Quality Planning and Standards
Control Programs Development Division
Research Triangle Park, North Carolina 27711
November 1977
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Copies of this publication are available free of charge to Federal employees,
current contractors and grantees, and nonprofit organizations - as supplies
permit - from the Library Services Office (MD-35), Environmental Protection
Agency, Research Triangle Park, North Carolina 27711; or, for a fee, from
the National Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22161.
This report was furnished to the Environmental Protection Agency by the
Department of Industrial Sciences, Colorado State University, Fort Collins,
Colorado, 80523, through Grants No. T008135-01-0 and No. T900621-01-0.
The contents of this report are reproduced herein as received from Colorado
State University. The opinions, findings, and conclusions expressed are
those of the authors and not necessarily those of the Environmental Protection
Agency. Mention of company or product names is not to be considered as
an endorsement by the Environmental Protection Agency.
Publication No. EPA-450/3-77-039
ii
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MOTOR VEHICLE EMISSIONS CONTROL
-- SERIES OF SEVEN BOOKS --
MOTOR VEHICLE EMISSIONS STAFF, COLORADO STATE UNIVERSITY
BOOK ONE - POSITIVE CRANKCASE VENTILATION SYSTEMS
BOOK TWO - THERMOSTATIC AIR CLEANER SYSTEMS
BOOK THREE - AIR INJECTION REACTION SYSTEMS
BOOK FOUR - FUEL EVAPORATION CONTROL SYSTEMS
BOOK FIVE - EXHAUST GAS RECIRCULATION SYSTEMS
BOOK SIX - SPARK CONTROL SYSTEMS
BOOK SEVEN - CATALYTIC CONVERTER SYSTEMS
111
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ACKNOWLEDGMENTS
The Motor Vehicle Emissions Control Staff of the Department
of Industrial Sciences at Colorado State University would
like to acknowledge the efforts extended by the Environmental
Protection Agency, Research Triangle Park, and Region VIII
Environmental Protection Agency, Manpower Development
Division.
A special thanks must be extended to the automotive vehicle
equipment and parts manufacturers for their cooperation and
assistance in the development of this training material.
iv
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INSTRUCTIONS FOR THE USE OF THIS BOOK
This book is one of a series designed specifically to teach the concepts
of automobile emissions control systems. Each book is designed to be
used as self-instructional material. Therefore, it is important that
you follow the step-by-step procedure format so that you may realize the
full value of the emissions system which is being presented. The topics
are taught in incremental steps and each topic treatment prepares the
student for the next topic. Each book is divided into sections which
include the introduction, purpose, function, inspection and testing of
the emissions system presented.
As you proceed through this series, please begin with book one and read
the following books in sequence. This is important because there are
several instances where material covered in a given book relies on
previously covered material in another book.
To receive the full benefits of the book, please answer the self-
evaluation statements related to the material. These statements are
separated from the text by solid lines crossing the page. The answers
to the statement can be found at the end of the book as identified by
the table of contents. You should check for the correct answer after
you respond to each statement. If you find that you have made a mistake,
go back through the material which relates to the statement or statements.
Fill-in-the-blank statements are utilized for self-evaluation purposes
throughout the material. An example statement would appear like this:
The American flag is red, white, and
You would write "blue" in the blank and immediately check your answer at
the end of the book.
v
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The material, statements and illustrations should be easy to follow and
understand. In several illustrations a small ghost named "VEC" (Vehicle
Emissions Control) has been used to make the picture easier to understand.
Upon completion of this series, you should be able to better understand
the emissions control systems and devices which are an integral part of
automobiles today. Your increased knowledge should help you keep these
"emissions controlled" vehicles operating as they were designed to
operate. Respectable fuel economy, performance and driveability, as well
as cleaner air, can be obtained from the automobile engine that has all
of its emissions systems functioning properly.
vi
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CONTENTS
Introduction to Emissions Control 4-1
Hydrocarbons 4-1
Carbon Monoxide 4-1
Oxides of Nitrogen 4-2
Formation of Hydrocarbons 4-2
Formation of Carbon Monoxide 4-3
Formation of Oxides of Nitrogen 4-3
Ignition Timing 4-3
Carburetion 4-5
System Introduction 4-7
System/Component Purpose 4-11
Sealed Gas Cap 4-12
Fuel Tank 4-13
Liquid Vapor Separator 4-14
Charcoal Canister 4-15
System/Component Function 4-17
Sealed Gas Cap 4-17
Fuel Tank 4-19
Liquid Vapor Separator 4-21
Liquid Check Valve 4-23
Overfill Limiting Valve 4-25
Three-Way Valve 4-26
Charcoal Canister 4-28
Anti-Perculator Valve 4-31
Variable Purge 4-33
Constant Purge 4-34
Constant and Demand Purge 4-35
System Inspection 4-39
System Testing 4-43
Summary 4-45
Answers 4-47
vii
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INTRODUCTION TO EMISSIONS CONTROL
As we all know emissions systems and devices have been installed on the
automobile engine because of the air pollution problem. In order for
you to understand these emissions systems and devices you should have a
background of the problem. All of the emissions control systems were
installed on the engine to reduce just three specific exhaust products.
These are known as products of combustion. The three products which the
emissions systems are designed to reduce are hydrocarbons, carbon monoxide
and oxides of nitrogen.
HYDROCARBONS
Gasoline, like all petroleum products, is made up of hundreds of hydro-
carbon compounds. The name "hydrocarbon" has been given to these com-
pounds because they are made up of hydrogen and carbon atoms. This is
also the reason hydrocarbons have the abbreviation (HC).
Hydrocarbons are gasoline vapors or raw gasoline itself. One reason
hydrocarbon emissions must be controlled is because it is one of the
major components of photochemical smog. Photochemical or "Los Angeles"
smog forms when hydrocarbons and oxides of nitrogen combine in the
presence of sunlight. In order to avoid this smog condition the hydro-
carbon emissions from automobiles must be controlled. Hydrocarbons
also act as an irritant to our eyes and some are suspected of causing
cancer and other health problems.
CARBON MONOXIDE
Another product of combustion that must be controlled is carbon monoxide.
Carbon monoxide has the abbreviation (CO). CO is also hazardous to our
health when it is mixed with the air we breathe. It can cause headaches,
reduce mental alertness and even cause death if enough of it is in the
air. Carbon monoxide is also a problem in that it speeds the formation
of photochemical smog. For these reasons CO emissions must be controlled.
4-1
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4-2
OXIDES OF NITROGEN
Oxides of nitrogen are the last harmful products of combustion we will
discuss. Nitrogen oxides have been given the abbreviation (NO ). As
/\
you already know, oxides of nitrogen and hydrocarbons combine to form
photochemical smog. The sunlight which triggers the formation of photo-
chemical smog has another effect on oxides of nitrogen. Some of the
oxides of nitrogen are broken down and a gas called ozone is formed.
Ozone is a lung and eye irritant and it also deteriorates rubber and
affects the growth of vegetation. Since the nitrogen oxides have these
effects they must also be controlled.
Now that you are familiar with the emissions which must be controlled
let's find out where they originate.
FORMATION OF HYDROCARBONS
Hydrocarbons, you will recall, are fuel vapors or raw fuel. For this
reason hydrocarbon emissions will result from any uncontained supply
of gasoline. Hydrocarbon emissions also come from the tailpipe. If
the automobile engine could achieve "complete combustion," all of the
unburned fuel or hydrocarbons would be used up. However, it is impos-
sible for today's automobile engines to achieve "complete combustion."
Any time the fuel mixture in the combustion chamber is not completely
burned, some hydrocarbons will be emitted from the tailpipe. The two
main reasons why hydrocarbons are not completely burned are because of
engine misfire and "quench areas." When an engine misfire occurs, none
of the raw fuel or hydrocarbons are burned. When this happens they are
simply exhausted directly to the atmosphere. Quench areas are places in
the combustion chamber where the flame goes out before the fuel is com-
pletely burned. Small cavities such as where the head gasket seals the
cylinder head to the block is a quench area. Another quench area is
located between the top of the piston and the first compression ring.
These areas are sources of hydrocarbon emissions.
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4-3
FORMATION OF CARBON MONOXIDE
Carbon monoxide is partially burned fuel. Carbon monoxide is formed in
the combustion chamber whenever there is not enough air to burn all the
fuel. This means that whenever a "rich" air/fuel mixture is pulled into
the combustion chamber carbon monoxide will be formed. After the flame
goes out the carbon monoxide is exhausted through the tailpipe and into
the air.
FORMATION OF OXIDES OF NITROGEN
Oxides of nitrogen are also formed in the combustion chamber. These
oxides result from the nitrogen which is contained in our air. In some
cases combustion temperatures in the automobile engine can exceed 4500°F.
At temperatures above approximately 2500°F, nitrogen oxides will start
forming. Therefore, if combustion chamber temperatures exceed 2500°F,
oxides of nitrogen will be produced and then exhausted to our atmosphere.
Now that you understand how these emissions are formed in the automobile
engine, we will see how changes in ignition timing and carburetor adjust-
ment affect the amount of these pollutants.
As you know, changes in timing and carburetion can have a large effect
on how an engine performs. These changes in timing and carburetion also
can have drastic effects on the amount of pollutants which are present
in the automobile's exhaust. The amount of hydrocarbons, carbon monoxide
and oxides of nitrogen which are present in the exhaust gases will vary
as timing and carburetion adjustments are changed.
IGNITION TIMING
Prior to emissions controlled automobiles, advancing the spark timing
was a common practice. Setting the spark timing this way caused the
spark plug to fire before the piston reached top dead center. This
advanced spark timing allowed the maximum amount of heat energy to be
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4-4
exerted on the piston. As a result the best performance and fuel econ-
omy could be obtained. Unfortunately, this also produced high hydro-
carbon and nitrogen oxide emissions levels.
In order to reduce emissions levels, ignition spark timing was retarded.
By firing the spark plug after the piston reaches top dead center, not
as much of the heat energy is converted to work on the piston. The
extra heat energy which is not used on the piston now passes through
the exhaust valve and into the exhaust manifold. This keeps the exhaust
gas temperatures higher. These higher exhaust temperatures allow burning
of the air/fuel mixture to continue in the exhaust manifold. This further
oxidation or burning in the exhaust manifold helps to reduce HC and CO
emissions.
Another advantage of retarded timing from an emissions standpoint is
that combustion temperatures are not as high. This is due to the fact
that the maximum combustion pressure will be lower. Since the combustion
temperatures will be lower and the formation of oxides of nitrogen de-
pends on temperature, a smaller amount of these pollutants will be ex-
hausted to the atmosphere.
There is one more advantage to using retarded spark timing. As you know,
when ignition timing is retarded the engine's idle speed will drop. This
decrease in idle speed occurs because less heat energy is applied to the
combustion chamber and more heat energy is being supplied to continue the
burning process in the exhaust manifold. In order to regain an acceptable
idle speed, the throttle plates must be opened wider. This wider throttle
plate opening allows more air to pass through the carburetor. This increase
in air flow will reduce the amount of residual exhaust gases in the cylinder.
This in turn will allow a more burnable mixture which can be made leaner.
Since the mixture can be leaner there will be more air in the combustion
chamber. As you know, the more air that is made available during com-
bustion the lower will be the HC and CO emissions.
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4-5
CARBURETION
Adjustments made to the carburetor air/fuel ratio can also have a large
effect on the amount of pollutants which come from the automobile engine.
When idle mixture settings become richer there is less air present for
the combustion process. This lack of air results in an increase in hydro-
carbon and carbon monoxide emissions.
When the idle mixture screws are turned in, the amount of fuel is reduced
and the mixture becomes leaner. This leaner mixture contains more air
and therefore more oxygen is available for more complete burning of the
fuel. This results in lower HC and CO emissions levels.
As the idle mixture screws are turned in, the idle air/fuel mixture be-
comes leaner. If this mixture becomes too lean a "lean misfire" will
occur. A "lean misfire" will occur because the fuel is so diluted or
thinned out by the air that the mixture will not ignite. This leads to
a very large increase in hydrocarbon emissions. This happens because
the failure of the mixture to ignite results in that amount of raw fuel
being emitted to the atmosphere.
The carbon monoxide emissions decrease when a lean misfire condition is
present. Carbon monoxide is partially burned fuel. Since no combustion
takes place during a lean misfire condition no CO is formed and the total
amount of CO produced by the engine will be less.
A lean misfire usually occurs in one or more cylinders. This condition
may also move from cylinder to cylinder while the engine is running.
This is caused by the uneven distribution of the air/fuel mixture
delivered to each cylinder. This condition occurs mainly because of
problems with intake manifold design.
Now you should understand how changes in timing and carburetion adjust-
ment can affect emissions levels. With this knowledge you will be able
to understand how each emissions control system we will discuss helps to
reduce the air pollution caused by the automobile.
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FEC
4-7
SYSTEM INTRODUCTION
The fuel used in most of today's automobile engines is gasoline which is
made up of hydrocarbons. Gasoline is mixed with air to be burned in the
combustion chamber; therefore, it must be able to vaporize or evaporate
easily. Since gasoline is composed of hydrocarbons and evaporates
easily, it is a major source of air pollution.
It was found that 20% of all hydrocarbon emissions were caused by fuel
evaporation. There are two sources of hydrocarbon emissions caused by
evaporation. The two sources of fuel vapors on the automobile are the
3 SOURCES OF AUTO EMISSIONS
FUEL EVAPORATION -
TANK 8 CARBURETOR
20%
CRANKCASE
20%
EXHAUST
60%
FIGURE 4-1
fuel tank and the carburetor. The evaporation rate will increase as the
temperature increases. For this reason most hydrocarbon emissions from
the carburetor take place when the engine is shut off. After the engine
is turned off, air will no longer be circulating through the engine
compartment. The temperature will increase in this area and cause some
of the fuel in the carburetor to evaporate. Fuel will also evaporate
from the fuel tank as the temperature around it increases.
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4-8
In 1970 a system was installed on California automobiles to contain this
fuel evaporation. This system was called the Fuel Evaporation Control
system. It has been used on all US automobiles since 1971. This system
was designed to seal the entire fuel system from the atmosphere. This
prevents the hydorcarbons from escaping into the atmosphere.
The fuel evaporation control system has several different names. As you
can see in figure 4-2, all manufacturers use their own name for this
system. However, each one of these systems has the same purpose and
operates in the same manner.
rv t t i t t t t t i i FT
J NAMES OF SYSTEMS
A.M. - FTVC, FUEL TANK
VAPOR CONTROL
CHYL.-VAPOR SAVER
- EVAPORATION
CONTROL SYSTEM
- EEC, EVAPORATIVE
EMISSION CONTROL
FOREIGN - FUEL
VAPOR CONTROL
RECOVERY SYS.
j T
TI
FIGURE 4-2
This system is designed to trap the gasoline vapors in the fuel system.
In order to do this the fuel system had to be redesigned. The fuel tank
and carburetor were sealed from the atmosphere. This stopped the evapo-
ration of the fuel. This in turn stopped the hydrocarbon emissions.
Also a storage system was used to trap the vapors which were given off
by fuel in the carburetor and fuel tank. When the engine is first
started these vapors in the storage system are drawn into the engine and
burned. This empties the storage device to allow a place for any other
vapors to accumulate.
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FEC
4-9
This system also prevents wasting gasoline. Since the fuel tank has been
redesigned it is now impossible to overfill the gas tank. Fuel can no
longer escape to the atmosphere through the gas cap. This prevents loss
of fuel when the temperature increases or the vehicle is parked on a hill,
With the use of this system it is possible to control the evaporative
loss and the HC emissions from this source. In using this system 20% of
all the HC emissions from the automobile are eliminated.
1. Before the use of the FEC system of all hydro-
carbon emissions were caused by fuel evaporation.
2. The evaporation of fuel causes hydrocarbon emissions.
The two sources of fuel vapors on the automobile are the
and the
3. As the temperature , the evaporation
rate of gasoline will increase.
4. Since the evaporation rate of gasoline increases as the
temperature increases, most hydrocarbon emissions from
the take place when the engine is
shut off.
5. The fuel evaporation control system is designed to seal
the entire fuel system from the .
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4-10
6. The storage system is used to ^^ the vapors
which are given off by the fuel in the carburetor and
fuel tank.
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FEC
4-11
SYSTEM/COMPONENT PURPOSE
As you will recall from the introduction the fuel evaporation system
contains the vapor given off by raw gasoline. This stops the HC
emissions which were a problem before this system was used.
The purpose of the fuel evaporation system is to trap these gasoline
vapors and return them to the fuel system. Once these vapors are
returned to the fuel system they may be burned in the combustion chamber.
The fuel evaporation control system is made of several components. Not
all manufacturers use the same components in their system. However, all
of the systems do have the same purpose. A typical fuel evaporation
system is shown in figure 4-3.
FUEL EVAPORATION SYSTEM
PURGE
LINE
FUEL TANK
VENT LINE
\
PRESSURE
VACUUM
SAFETY
FILLER CAP
CHARCOAL
CANISTER
OVERFILL
LIMITING
VALVE
LIQUID
VAPOR
SEPARATOR
FIGURE 4-3
All manufacturers use similar components in their individual systems.
Each fuel evaporation system will have a redesigned fuel tank and gas cap,
a liquid vapor separator, a charcoal canister, and an overfill device.
The purpose of each one of these components will now be discussed.
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4-12
Before the fuel system was sealed to prevent hydrocarbon emissions auto-
mobiles were equipped with vented gas caps. A typical vented gas cap is
shown in figure 4-4. These vented gas caps allowed raw fuel vapors to
VENTED GAS CAP
OUTSIDE AIR
VENTED
CAP
FUEL TANK
FIGURE 4-4
escape to the atmosphere. Under some conditions these gas caps would
even allow liquid gasoline to escape. For these reasons, the gas caps
were redesigned when the fuel evaporation system was developed.
SEALED GAS CAP
The new gas caps which are used on the emissions control equipped cars
SEALED GAS CAP
OUTSIDE AIR
SEALED
CAP
FUEL TANK
FIGURE 4-5
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FEC
4-13
are not vented. The purpose of these new caps is to prevent any liquid
gasoline or vapors escaping from the tank.
This prevents hydrocarbons emissions from the fuel tank under most condv
tions.
7. The main purpose of the fuel evaporation system is to
prevent gasoline HC vapors from escaping into the
8. Automobiles were equipped with
gas caps
before the use of the FEC system.
FUEL TANK
In addition to redesigning gas caps, the fuel tank has been modified.
The fuel tanks found on today's cars now have some type of expansion
space built into them. This additional space is to allow room for the
FUEL TANK FILL CONTROL
10-12%
EXPANSION
SPACE
FILLER
CAP
FUEL
TANK
FUEL
FIGURE 4-6
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4-14
gasoline as it expands when the temperature increases. The expansion
space is usually about 10-12% of the total tank volume. This additional
space in the gas tank combined with the redesigned gas cap prevents fuel
from escaping from the tank. At this point you know what has been done
to contain the raw gasoline but what happens to the vapors?
LIQUID VAPOR SEPARATOR
As you can see in figure 4-7 a device called a liquid vapor separator is
connected to the fuel tank by several vent lines. The purpose of this
liquid vapor separator, as the name implies, is to separate the raw fuel
LIQUID VAPOR
SEPARATER
TO CHARCOAL
CANISTER
VAPOR
VENT
LINES
LIQUID
RETURN
LINE
FUEL
TANK
FIGURE 4-7
from the vapors. This is done so the liquid or raw fuel may be returned
to the fuel tank and the vapors sent to a storage device.
9. The additional space provided in the new fuel tanks is to
allow room for the gasoline as it expands when the
temperature .
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FEC
4-15
10. The expansion space in the new fuel tanks is usually
about of the total tank volume.
11. The liquid vapor separator is normally located near the
fuel tank. Its job is to separate the fuel vapor from
the fuel.
CHARCOAL CANISTER
A vapor line from the liquid vapor separator runs forward to the storage
device. The liquid vapor separator is normally located near the fuel
tank. The vapor storage device is normally located in the engine compart-
ment and is referred to as a charcoal canister. However, 1971 and 1972
Chrysler automobiles used the engine crankcase for storing the gasoline
vapors. The purpose of the storage device or charcoal canister is to
contain the fuel vapors given off from the fuel tank and the carburetor.
The vapors are held in this device until the engine is started. At this
time the vapors are drawn out of the charcoal canister and burned in the
combustion chamber.
Since the charcoal canister is designed to store vapors, it is important
to keep liquid gasoline out of the canister. This is why an overfill
device is put in the vapor line somewhere between the liquid-vapor
separator and the charcoal canister. This overfill device serves the
purpose of stopping any liquid gasoline which might get past the liquid
vapor separator and into the vapor line. This saves the charcoal canister
from becoming saturated with raw fuel.
The various vacuum and vapor hoses complete the fuel evaporation system.
These are connected between the components to operate the system. The
positioning and location of these connecting lines will be discussed
further in the system function section of this book.
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4-16
12. The vapor storage device which is usually referred to as
a canister, is normally located in the
engine compartment.
13. Fuel vapors are held in the charcoal canister until the
engine is .
14. An device is used in the vapor line
between the liquid/vapor separator and the charcoal
canister. This device is used since the charcoal canis-
ter is designed to store vapors and it is important to
keep liquid gasoline out of the canister.
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FEC
4-17
SYSTEM/COMPONENT FUNCTION
In this section we shall talk about the function of each component in the
fuel evaporation system. Then we will see how these components work
together to make the whole system function.
SEALED GAS CAP
The new fuel tank caps which are used on the emissions control equipped
automobiles are not vented. Any new fuel tank cap has two built-in
safety relief valves. As you can see in figure 4-8, a safety pressure
relief valve and a safety vacuum relief valve are now part of the automo-
bile fuel tank cap. Any time an excessive pressure condition is present
PRESSURE-VACUUM RELIEF CAP
TANK PRESSURE 1/2 - I PSI
SEALING
GASKET
OUTER SHELL
PRESSURE
SPRING
PRESSURE
RELEF
VALVE (OPEN)
VACUUM RELIEF
VALVE (CLOSED)
LOCKING
LIP
VACUUM SPRING
FIGURE 4-8
in the gas tank, the pressure relief valve will open and bleed to the
atmosphere. For example, this might happen after the cool gasoline is
pumped from the underground storage tank filling the automobile's fuel
tank. As the temperature of the fuel rises it will expand and any
excess pressure that may build in the tank will be relieved through the
cap.
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4-18
In figure 4-8, you can see the parts of the gas cap. As this pressure
builds in the tank, the pressure spring will be compressed. This will
move the pressure relief valve off its seat allowing the excess pressure
to be vented to the atmosphere. This will occur anytime the pressure in
the tank exceeds 1/2 to 1 psi.
The gas cap is also designed to relieve vacuum conditions in the tank.
Since the new caps are sealed whenever fuel is drawn from the tank by
the fuel pump a vacuum will be created in the tank. As you can see in
figure 4-9, when a vacuum is present in the tank the vacuum spring will
CAP RELIEVING TANK VACUUM
VACUUM I/*"- 1/2" HG
SEALING
GASKET
PRESSURE
SPRING
OUTER SHELL
\
PRESSURE
RELIEF
VALVE
(CLOSED)
VACUUM RELIEF
VALVE (OPEN)
LOCKING
LIP
VACUUM
SPRING
FIGURE 4-9
be compressed and the vacuum relief valve will open. This will allow air
into the tank and return the tank to atmospheric pressure. A vacuum of
1/4 to 1/2" Hg is required to open this valve.
15.
safety relief valve(s) are built into all new
fuel tank filler caps.
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FEC
4-19
16. Whenever an excessive pressure condition is present in
the gas tank, the pressure relief valve will open and
bleed to the
17. The safety pressure relief valve built into the present
fuel tank filler cap opens when pressure in the fuel
tank exceeds approximately •
18. The new safety relief cap is also designed to relieve
conditions in the tank.
FUEL TANK
Now we shall take a closer look at the redesigned fuel tanks used on the
emissions controlled automobile. As you will recall, an expansion space
of 10% to 20% is provided in these new tanks. There are several designs
used by automobile manufacturers to provide this expansion space.
By far the most popular method of providing an expansion space in the
FUEL TANK FILL CONTROL
10-12%
EXPANSION
SPACE
FILLER
CAP
FUEL
TANK
FIGURE 4-10
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4-20
fuel tank is shown in figure 4-10. The filler neck is located in such
a way that an air pocket will be trapped in the top of the tank. This
will provide the needed expansion space.
Another means of accomplishing this expansion space is shown in figure
4-11. This tank uses a fill control tube to shut off the gas pump
nozzle. The gas pump nozzle will shut off when all but-10-12% of the
AIR SPACE PROVIDED FOR
FUEL EXPANSION
10-12% OF TANK VOLUME
FUEL
TANK
FUEL
FIGURE 4-11
tank is full. This will provide the necessary expansion space in the
tank. The fill control tube is designed to feed gasoline back to the
filler neck when the tank is full except for its expansion space. At
this time the fuel will rise through the fill control tube until it
contacts the gasoline pump nozzle. The fuel when contacting the nozzle
will trigger it to its off position. This will prevent any more gasoline
from being added to the tank. The 10-12% expansion space will remain in
the tank.
Another means of providing the needed expansion space is through the use
of an external expansion tank. This is pictured in figure 4-12. This
works much like the method just described. A fill control tube will
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EXTERNAL EXPANSION CHAMBER
EXPANSION
TANK \
TO CHARCOAL
CANISTER
FUEL VAPOR LINES
FUEL
TANK
TO FUEL PUMP
FIGURE 4-12
FEC
4-21
shut off the gasoline pump nozzle when the main tank is full. The expan-
sion space will be provided in the external tank. This external tank is
located near the main fuel tank and looks like a miniature fuel tank.
LIQUID VAPOR SEPARATOR
You will recall from the purpose section of this unit that the vapors are
separated from the raw fuel and sent on to the storage device. This process
of separating vapors and liquid fuel is done with a liquid vapor separator.
LIQUID VAPOR
SEPARATER
TO CHARCOAL
CANISTER
VAPOR
VENT
LINES
LIQUID
RETURN
LINE
FUEL
TANK
FIGURE 4-13
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4-22
Figure 4-13 shows the fuel tank, vent lines and liquid vapor separator.
Automobiles equipped with the fuel evaporation system will have several
vent lines connected to the corners of the fuel tank. These vent lines
run from the tank to the liquid vapor separator.
There is also a liquid return line which connects to both the tank and
the separator. The remaining line which connects to the liquid vapor
separator runs to the storage device or charcoal canister and provides
a way to remove the vapor from the tank and separator. A cut-away view
of the liquid vapor separator is shown in figure 4-14. Now we will look
HORIZONTAL MOUNTED LIQUID
VAPOR SEPARATOR
•TO CHARCOAL
CANISTER
MOUNTING
HOLE'
FIGURE 4-14
at how the separator functions. The separator shown in figure 4-14 has
three vapor vent lines and a liquid return line. The vapor vent lines
provide a means for the vapor to leave the tank. Some amount of liquid
gasoline will also enter the separator through these lines. The liquid
return is either a shorter line or it will have a hole cut in it near
the bottom of the separator. The shorter effective length of the liquid
return line will drain the liquid which enters the separator back to
the tank. Since the separator is mounted slightly higher than the gas
tank the liquid gasoline will naturally flow back to the tank. The
-------
FEC
4-23
vapor which enters the separator is much lighter than the gasoline. For
this reason it will stay in the upper portion of the separator. As the
vapor comes from the tank it will push any vapors in the separator out
through the highest line in the separator. This line connects to the
storage device and will usually have a small orifice where it joins the
separator. The orifice is to aid in preventing liquid gasoline from
entering this line. From this you can see how the separator can return
the liquid gasoline to the tank and allow the vapors to move on to the
charcoal canister.
19. The fuel tanks on most cars are prevented from being
filled 100% because of the expansion space of -
designed into the tank, or the design of the filler neck
on the fuel tank.
20. Most automobiles equipped with the fuel evaporation
system will have several vent lines connected to the
corners of the
LIQUID CHECK VALVE
A few automobiles do not use a liquid vapor separator in their fuel
evaporation system. Instead, a valve is used to allow the vapors to
pass from the tank to the charcoal canister. The liquid check valve is
shown in figure 4-15. This valve is built into the tank on some models
and mounted outside the tank on others. The vent lines from the tank
enter the valve at the bottom. The line to the vapor storage device
is located at the top of the valve. Figure 4-15 shows the liquid check
valve in the open position. In this position, fuel vapors from the
-------
4-24
LIQUID CHECK VALVE (VALVE OPEN)
VALVE
SEAT
TO
CHARCOAL
CANISTER
BOWL
CHAMBER
n
FUEL VAPOR LINES
FROM FUEL TANK
FIGURE 4-15
tank can enter the bowl chamber. These vapors will pass between the
needle valve and the valve seat and move on to the charcoal canister.
However, if liquid should happen to pass through the tank vent lines,
the liquid check valve will close. Figure 4-16 shows the liquid check
valve's operation when liquid gasoline enters this device. As you can
LIQUID CHECK VALVE (VALVE CLOSED)
VALVE
SEAT
BOWL
TO
CHARCOAL
CANISTER
NEEDLE
VALVE
FLOAT
GUIDE PIN
FUEL VAPOR LINES
FROM FUEL TANK
FIGURE 4-16
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FEC
4-25
see, the liquid will enter the bowl chamber through the vent lines. As
the gasoline level rises in the bowl chamber, the float will remain
above the gasoline. The needle valve is connected to the float. As the
float rises the needle valve contacts the valve seat. This will close
the line to the charcoal canister. No more fuel will be allowed to enter
the valve. As gasoline is used the fuel in the liquid check valve will
return to the tank and the valve will again pass vapors to the canister.
Now you can see how this liquid check valve acts as a liquid vapor
separator and a control to prevent the canister from receiving liquid
fuel.
OVERFILL LIMITING VALVE
The overfill limiting valve used on some Chrysler and Ford vehicles works
much the same as the liquid check valve. Figure 4-17 shows an overfill
VENT HOSE-
TO CANISTER
VALVE
SEAT
OVER-FILL
LIMITING VALVE
-VALVE
STOP
,VENT HOSE
TO TANK
FIGURE 4-17
limiting valve. This valve is usually quite small and is located in the
vapor line -which leads to the canister. Its function is to prevent any
liquid which might pass through the separator from reaching the canister.
If liquid reaches this device the valve will float on the fuel until it
contacts the valve stop. This will close the line and prevent the fuel
from reaching the canister. In normal operation the vapors will pass
-------
4-26
between the valve and the valve seat. They will then pass on to the
cansiter.
THREE-WAY VALVE
Some Ford vehicles did not use a pressure-vacuum fuel tank cap. For this
reason a device had to be used to vent any excess pressure or vacuum from
the system. A three-way valve is used on these automobiles. This valve
is also located between the liquid vapor separator and the charcoal
canister. Figure 4-18 shows the three-way valve in its pressure venting
condition. This valve will be in this position whenever a vapor pressure
OPERATION OF 3-WAY VALVE
FUEL TANK TO CANISTER
TO
ATMOSPHERE
DIAPHRAGM
VALVE
HOUSING
DIAPHRAGM
TO
ATMOSPHERE
SPRING
FIGURE 4-18
condition exists in the tank. This pressure acts upon the upper diaphragm
and compresses the upper spring. In this position the vapors will move on
to the storage canister. When a vacuum condition is present in the fuel
tank the upper diaphragm will seat and the atmosphere bleed will open.
This condition may be seen in figure 4-19. The atmosphere bleed valve
will remain open until the vacuum condition is removed from the tank.
The bleed valve will then close. The lower spring and diaphragm are used
as a safety pressure relief valve to protect the gas tank. If the vent
line to the canister or the canister itself became blocked, it would be
necessary to vent the pressure in the system to the atmosphere.
-------
FEC
4-27
OPERATION OF 3-WAY VALVE
ATMOSPHERE TO FUEL TANK
OPEN TO
ATMOSPHERE
DIAPHRAGM
VALVE
HOUSING
DIAPHRAGM
TO
CANISTER
TO
ATMOSPHERE
SPRING
FIGURE 4-19
Figure 4-20 shows the three-way valve in this position. The upper spring
and diaphragm will open when a small vapor pressure condition occurs. If
a blockage in the vent line is present the vapor pressure will increase.
OPERATION OF 3-WAY VALVE
FUEL TANK TO ATMOSPHERE
DIAPHRAGM
TO
ATMOSPHERE
VALVE
HOUSING
TO
ATMOSPHERE
SPRING
FIGURE 4-20
This higher pressure will push the lower diaphragm and spring off its
seat and vent this pressure to the atmosphere. This will prevent the
tank from deforming due to the high pressure.
-------
4-28
21. The purpose of the liquid check valve's bowl and float
is to allow vapors to flow through the valve but stop
any liquid from entering the vapor line to the
22. A valve is used on some Ford vehicles
which did not use a pressure-vacuum gas tank cap.
23. If the vent line to the canister or the canister itself
became blocked in a three-way valve system, it would be
necessary to vent the pressure in the system to the
At this point, you should understand how the liquid gasoline and the
gasoline vapors are separated. You have seen how the liquid is returned
to the tank and how the vapors are sent to the storage device. We have
also looked at several devices which aid in preventing any liquid from
reaching the canister. We shall now look at the canister itself and its
connecting lines.
CHARCOAL CANISTER
Figure 4-21 shows a cut-away view of a charcoal canister. This storage
canister uses activated charcoal granules to store the fuel vapors. The
typical canister contains about 1 1/2 pounds of charcoal. This is enough
charcoal to store about one cup of fuel in its vapor form. The canister
shown in figure 4-21 has two connecting hoses. One line brings the
vapors from the tank to the canister. The other line connects to the
carburetor, PCV hose or air cleaner. This line is called the purge line.
-------
FEC
4-29
CHARCOAL CANISTER
HOSE
CARBURETOR
OR
AIR CLEANER
CHARCOAL
GRANULES
HOSE
TO
FUEL TANK
VENT
CANSTER
CASE
FBERGLAS
FILTER
OUTSIDE AIR
FIGURE 4-21
The bottom of the canister is open to the atmosphere. It also has a
fiberglass filter which covers the open bottom of the canister. This
filter is simply an air filter used to clean the air which is drawn
through the canister.
24
Activated
granules are used to store
the fuel vapors in the charcoal canister.
25. A typical canister contains about 1 1/2 pounds of char-
coal. This is enough charcoal to store about __
cup(s) of fuel in vapor form.
When the engine is started, a vacuum condition will be created in the air
cleaner and the carburetor. Figure 4-22 shows what will happen after the
engine is started.
-------
4-30
CANISTER PURGING (ENGINE RUNNING)
VAPOR LINE
TO FUEL
TANKv
OUTSDE AIR
FIGURE 4-22
This vacuum will draw fresh air through the filter in the bottom of the
canister. This air will then move through the charcoal granules removing
the gasoline vapors. This is called the evacuation or purge cycle. The
air and gasoline vapors, after being purged from the canister, are drawn
into the intake manifold and burned in the combustion process.
The charocal canister in figure 4-23 is of a slightly different design.
This design has an open space above and below the granules.
OUTSDE AIR
HOSE TO
FUEL TANK
HOSE TO
AIR CLEANER
CHARCOAL
GRANULES
FIGURE 4-23
-------
FEC
4-31
It also has a tube which extends from the bottom of the canister out
through the top. The purge line of this system will connect to the air
cleaner or the PCV line. When the car is started a vacuum condition
will result in the purge line. This vacuum will draw fresh air down
through the center tube. The air will then pass up through the granules
removing the gasoline vapors.
The engine crankcase is another area which has been used to store gasoline
vapors. This system is shown in figure 4-24. When the engine is started
FUEL TANK
FIGURE 4-24
the stored vapors are drawn into the engine intake system through the
PCV valve. This "purges" the stored vapors from the crankcase so it will
be ready to store more vapors when the engine is shut off.
ANTI-PERCULATOR VALVE
Before we had emissions controlled automobiles, the carburetors were
vented to the atmosphere. This was a source of hydrocarbon emissions.
For this reason, the carburetor float bowl is now vented to the charcoal
canister. An anti-perculator valve is used to control this venting
operation. As can be seen in figure 4-25, the throttle plate is closed.
-------
4-32
CARBURETOR
THROTTLE
CLOSED
FIGURE 4-25
This will open the anti-perculator valve and vent the vapors into the
charcoal canister. When the throttle plate is open, as in figure 4-26,
the anti-perculator valve will close and seal the float bowl from the
canister. In this way, any excessive vapors which are given off after
the engine is shut off will be vented to the charcoal canister.
TO CANISTER
TO CARBURETOR
LINKAGE
CLOSED ANTI
PERC MU.VE
CARBURETOR
BOWL
THROTTLE
OPEN
FIGURE 4-26
-------
FEC
4-33
26. After the air and gasoline vapors are purged from the
canister, they are drawn into the ___________
and burned in the combustion process.
27. In the crankcase storage system when the engine is
started, the vapors are drawn into the engine intake
system through the valve.
28. The anti-perculator valve assures that any excessive
vapors which are given off from the carburetor after the
engine is shut off will be vented to the
Now you should understand the vapor storage devices. It is time to look
more closely at the various ways to purge these components of their
stored vapors.
VARIABLE PURGE
One method of purging the storage canister is shown in figure 4-27. This
\
AIR
CLEANER
TANK
CARBON
CANISTER
FIGURE 4-27
-------
4-34
system has its purge line connected to the air cleaner snorkel. As intake
air passes through the snorkel a small vacuum will be created in the
purge line. When the speed of the air passing through the snorkel
increases the purge vacuum increases. This will cause the purging
action to increase. The amount of purging will vary with throttle plate
position. For this reason it is called a "Variable Purge."
The purging system shown in figure 4-28 is much the same as the one just
discussed. This system of purging, however, uses a second purge line which
connects inside the air cleaner. This line, however, is on the carburetor
side of the air filter. For this reason, at low air velocity conditions
the purging is done through this second line. As the air flow speeds up,
the purging is done through the snorkel purge line as in the variable
purge system.
AIR CLEANER
CARBON-
CANISTER
PURGE
AIR
LLU
FIGURE 4-28
CONSTANT PURGE
Figure 4-29 shows another type of purging system. As you can see, this
system uses a variable purge system. It also shows a "constant" purge
line. This purge line is connected to the PCV valve line. There is a
-------
FEC
4-35
CONSTANT
PURGE
TO
PCV
TO GAS
TANK
VARIABLE
PURGE
CONSTANT
PURGE
FIGURE 4-29
small orifice located in the canister which limits the amount of air
through this line. This purge line will evacuate the cansiter all the
time the engine is running. For this reason, it is called a "constant1
purge.
CONSTANT AND DEMAND PURGE
The system shown in figure 4-30 is called the "constant and demand"
PORTED
VACUUM
\
RESTRICTED
ORIFICES
PURGE VALVE
PCV
^C
CARBURETOR
BOWL VENT
CONSTANT a DEMAND
PURGE SYSTEM
FROM
FUEL
TANK
FIGURE 4-30
-------
4-36
purge system. This system has a constant purge line connected into the
PCV line. This constant purge has a small orifice to control the amount
of purging. The charcoal canister also uses a purge valve to control the
purging modes. This valve is triggered by a ported vacuum signal. When
the throttle plates are opened and the engine begins to gain speed, a
vacuum condition will build in the purge valve. This will open an addi-
tional area in the canister to the PCV line. When this happens, the
amount of purging will increase. This is the "demand" purge. This
system allows a small amount of purging at idle, but when the engine
speed increases, a greater amount of purging will occur. This "demand"
purging is designed to occur during engine conditions which will least
affect performance and driveability.
You should now understand the various ways of purging the charcoal
canister.
29. When the purge rate is controlled by the air flow enter-
ing the air cleaner, the system is called the
___ system.
30. The purging system which evacuates the canister whenever
the engine is running is known as the
system.
31. The purging system which is designed to occur during
engine conditions which will least affect performance
and driveability is called the purge.
-------
FEC
4-37
As a review, we will look at the components of the fuel evaporative
system again: a fuel tank filler cap which seals the system, a special
fuel tank designed to allow space f^r fuel expansion, and a venting
VAPOR S4VER SYSTEM
PURGE
UNE VENT
LJNE
FUEL TANK
VENT UNE
CHARCOAL
CANISTER
OVERFLL
LIMITING
VftLVE
PRESSURE
VACUUM
SAFETY
FILLER CAP
LIQUC VAPOR
SEPARATOR
FIGURE 4-31
system to carry vapors from the fuel tank to the charcoal canister and
into the engine for burning. With the use of these components, the
hydrocarbon emissions caused by fuel evaporation are eliminated.
-------
FEC
4-39
SYSTEM INSPECTION
An inspection of the fuel evaporative system should be made periodically.
This inspection will require no tools or instruments and takes only a few
minutes. Many problems may be avoided or corrected by these steps.
1. Check to see that all components are properly installed
on the engine and no modifications have been done.
Canister, separator and all purge lines should be in
place.
\APOR SAVER SYSTEM
PURGE
LINE VENT
LINE
FUEL TANK
VENT LINE
PRESSURE
VACUUM
SAFETY
FILLER CAP
CHARCOAL
CANISTER
OVERFILL
LIMITING
VALVE
LIQUID VAPOR
SEPARATOR
2.
FIGURE 4-32
In order for ihe fuel evaporative system to function, all
components must be installed correctly. If any of these
components are missing, the system will operate inefficiently
or possibly not at all.
Check the gas cap gaskets and relief valves. The gaskets
should be inspected for deterioration and any signs that
they may be leaking. The relief valves should be checked
to see that they operate correctly.
-------
4-40
PRESSURE-VACUUM RELIEF CAP
TANK PRESSURE 1/2 - I PSI
SEALNG
GASKET
OUTER SHELL
PRESSURE
SPRING
PRESSURE
RELIEF
VALVE OPEN)
VACUUM RELIEF
VftLVE (CLOSED)
UUM SPRING
FIGURE 4-33
If either the gaskets or the relief valves are in poor
condition, the gas tank will not be sealed from the
atmosphere and vapors will escape from the tank.
3. Inspect the liquid check valve, separator, and connecting
lines which may be damaged or leaking. If any of these
components are not in good condition, leaks may occur.
This will allow vapors to escape from the system into the
atmosphere.
4. Inspect the filter in the bottom of the canister. If
this filter is excessively dirty, replace the filter.
Manufacturer's specifications should be checked to see at
what intervals this should be done.
If the filter in the bottom of the canister is dirty,
the purging of the canister will be hampered. This will
restrict the evacuation of vapors from the canister.
-------
FEC
4-41
BOTTOM OF CANISTER
BERGLAS
FILTER
FIGURE 4-34
32. The gas tank will not be sealed from the atmosphere and
vapors will escape from the tank if either the gaskets
or the are in Poor condition.
33. The only part.of the fuel evaporation system which
required scheduled replacement is the
used for the charcoal canister.
-------
FEC
4-43
SYSTEM TESTING
The fuel evaporation control system is usually very troublefree. For
this reason no testing procedures have been included in this book. If
any problems arise with the fuel evaporation control system, the manu-
facturer's recommendations for testing and repair should be consulted.
-------
FEC
4-45
SYSTEM SUMMARY
PURPOSE
The purpose of the evaporative emissions control system is to control the
release of hydrocarbons (HC) to the atmosphere that results from raw fuel
vapors escaping from the fuel tank and carburetor vents.
MAIN COMPONENTS
Fuel Tank - A sealed unit for storing the fuel that has a built-in air
space to allow for fuel expansion due to temperature increases.
Fuel Tank Filler Cap - Seals the fuel tank and acts as a pressure relief
valve to protect tank from excessive pressure or vacuum.
Vapor Vent Lines - Allows vapors to escape from the fuel tank and pass to
a vapor-liquid separator.
Vapor-Liquid Separator - Prevents passage of liquid fuel to the charcoal
canister by means of either a float valve that seals the outlet when
liquid fuel enters or by means of vapor lines. Vapor lines are set at
different heights to allow liquid fuel to return to the tank and only
vapors to escape.
Charcoal Canister - Contains activated charcoal that traps and stores fuel
vapors. When the canister is purged with fresh air the fuel vapors are
removed and vented to the carburetor.
Purge Line - Allows passage of fuel vapors from the charcoal canister to
the carburetor or air cleaner when the engine is running.
SYSTEM FUNCTION
When the fuel tank is full and subjected to an increase in temperature,
the fuel expands and is taken up in the built-in air space. Vapors are
released; they pass through the vent lines to the vapor-liquid separator
which allows the vapors to pass through, but prevents any liquid fuel from
-------
4-46
passing. The vapors travel to the activated charcoal canister where they
are trapped and stored. When the engine is started, the canister is
purged via a purge line that runs to the carburetor or air cleaner.
-------
FEC
4-47
ANSWERS
1. 20%
2. fuel tank, carburetor
3. increases
4. carburetor
5. atmosphere
6. trap or contain
7. atmosphere
8. vented
9. increases
10. 10-12%
11. liquid
12. charcoal
13. started
14. overfill
15. two
16. atmosphere
17. one-half (1/2) psi.
18. vacuum
19. 10-12%
20. fuel tank
21. canister
22. three-way
23. atmosphere
24. charcoal
25. one
26. intake manifold
27. PCV
28. charcoal canister
29. variable purge
30. constant purge
31. demand
32. relief valves
33. filter pad
-------
TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before completing!
1. REPORT NO.
EPA-450/3-77-039
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
Motor Vehicle Emissions Control - Book Four
Fuel Evaporation Control Systems
November 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
B.D. Hayes
M.T. Maness
s. PERFORMING ORGANIZATION REPORT NO.
R.A. Ragazzi
R.A. Barrett
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Industrial Sciences
Colorado State Ujiiversity
Fort Collins, Colorado 80523
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
T008135-01-0
T900621-01-0
12. SPONSORING AGENCY NAME AND ADDRESS
Control Programs Development Division
Office of Air Quality Planning and Standards
Office of Air and Waste Management
U.S. Environmental Protection Agency
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
EPA 200/04
15. SUPPLEMENTARY NOTES Research Triangle Park, North Carolina Z7T11
16. ABSTRACT
This book is one of a series designed specifically to teach the concepts of auto-
mobile emissions control systems. It is intended to assist the practicing mechanic
or the home mechanic to better understand the Fuel Evaporation Cdhtrol Systems
which are an integral part of automobiles today. The mechanic's increased know-
ledge should help him keep "emissions controlled" vehicles operating as designed.
Respectable fuel economy, performance and driveability, as well as cleaner air, can
be obtained from the automobile engine that has all of its emissions systems func-
tioning properly.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Photochemical
Fuel Evaporation
Intake Manifold
System Inspection
lydrocarbons
Carbon Monoxide
Oxides of Nitrogen
Carburetor
Fuel Tank
Atmosphere
Vacuum
Charcoal Canister
Variable Purge
Conetant Purge
13. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report/
Unclassified
21. NO. OF PAGES
54
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
*U.S. GOVERNMENT PRINTING OFFICE: 19 7 8 - 7 k 5 - 2 2 Hr
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