Formation and Control
of
Combustion Pollutants
from
Gasoline-Fueled Otto Cycle
Motor Vehicle Engines
John W. Bozek
November 1990
Revised December 1991
Notice
The purpose of this document is to provide simple explanations regarding the
formation and control of combustion pollutants from gasoline-fueled Otto Cycle
motor vehicle engines. Formation phenomena are explained on the basis of
well-known cause and effect relationships. Control techniques are explained
on the basis of the operating characteristics of systems which are already in
widespread use. Consequently, this document takes the form of a synopsis of
information that is available in other formats in the applicable reference
literature. It should not be considered to be a technical report since it does not
(a) provide any new information, (b) supply any test results, (c) analyze new
regulatory issues, or (d) make any recommendations regarding EPA action.
Credits
Editor: Kathy E. Carter
Illustrator: Frank Lamitola
EPA-AA-CD-CPSB-90-01f\
Certification Division
Office of Mobile Source Air Pollution Control
U.S. Environmental Protection Agency
-------
Table of Contents
Acknowledgments iii
Introduction 1
Chapter 1. Gasoline-Fueled Otto Cycle Engines 5
Chapter 2. Pollutant Formation—General Effects 11
2.1. Fuel Complexity 11
2.2. Combustion Complexity 1 5
Chapter 3. Pollutant Formation—Specific Effects 1 9
3.1. Effect of Flame Quenching on HC Emissions 20
3.2. Effect of Oxygen Content on CO Formation 21
3.3. Effect of Temperature on NOx Formation 21
Chapter 4. Pollutant Control Approaches 23
4.1. Preventive Techniques 23
4.1.1. Combustion Chamber Redesign for HC Reduction 23
4.1.2. Fuel Control for CO Reduction 24
4.1.3. Temperature Control for NOx Reduction 24
4.2. Remedial Techniques 27
4.2.1. Supplementary Air Oxidation 27
4.2.2. Catalytic Conversion 29
4.3. Interaction Complications 36
Appendixes 39
Appendix 1. Comparison of Otto Cycle and DieselCycle
Engines 41
Speed and Power Output Control 41
Fuel Properties 42
Fuel Introduction 42
Combustion Initiation 43
Pollutant Formation 43
Appendix 2. Hydrocarbon Classes 46
Paraffin (Alkane) Hydrocarbons 47
Isoparaffin (Alkane) Hydrocarbons 47
Cycloparaffin (Alkane) Hydrocarbons 48
Olefin (Alkene) Hydrocarbons 48
Hydrocarbons with Six Carbon Atoms 49
Oxygenated Hydrocarbons 50
-------
List of Figures
Figure 1. Engine Block and Head (cross-section) 5
Figure 2. Engine Block and Manifolds 6
Figure 3. Otto Cycle Phases 7
Figure 4. Molecular Structure of Selected Hydrocarbons 11
Figure 5. Flame Quenching 20
Figure 6. Surface/Volume Ratio Changes 24
Figure 7. Internal Exhaust Gas Recirculation (Valve Overlap) 26
Figure 8. External Exhaust Gas Recirculation 26
Figure 9. Aspirated Air Injection 28
Figure 10. Forced Air Injection 29
Figure 11. Exhaust Composition 33
Figure 12. Closed Loop Fuel Flow Control System 34
Figure 13. Lambda Sensor Operation 35
List of Tables
Table 1. Relationship of Combustion Phase and Piston Stroke 6
Table 2. Hydrocarbon Compounds and Properties 12
Table 3. Exhaust Pollutants 19
Table 4. 02 & CO Concentration by Air/Fuel Ratio 32
-------
Acknowledgments
The preparation of this synopsis involved several revisions which
affected both form and content. The changes incorporated in the successive
drafts resulted in significant improvements; such as, (a) the addition of essential
material that was initially overlooked, (b) the clarification of some of the original
technical explanations, (c) the reduction of the number of pages in the original
text by the use an appendix for discussions that were judged to be too detailed
to be of general interest, and (d) the addition of technical drawings to clarify
material. These improvements resulted from comments and suggestions made
by several individuals who graciously agreed to review the early drafts. These
reviewers very generously devoted the time and energy which the detailed
reviews required with no motivation other than their desire to be helpful.
Consequently, an expression of appreciation for the efforts of the following
individuals is essential and most appropriate: R. J. Bergin, Gay MacGregor,
John L. Wehrly, and Kenneth L Zerafa.
In addition, thanks must be extended to the following individuals who
volunteered to review the later drafts and provided comments which permitted
the further refinement of this synopsis: Judy F. Carmickle, William B.
Clemmens, Charles D. Cole, John M. German, Barry S. Mclntyre, Jeff T. Prince,
and Clifford D. Tyree.
Lastly, thanks to editor Kathy E. Carter for the final refinements to the
document and to illustrator Frank Lamitola for creating the drawings which add
so much clarity to the text.
John W. Bozek
November 2, 1990
-------
Introduction
The pollutant formation and control information provided in this synopsis
is specifically applicable to gasoline-powered, Otto cycle motor vehicle engines.
This information is presented with the hopeful intention of providing the non-
technical reader with a basis for understanding the applicable chemical and
physical phenomena. Consequently, the discussion of these phenomena
emphasizes cause and effect relationships which may not be obvious to a
reader who does not have an engineering background and, hence, might be
overlooked by such a reader if such highlighting was not attempted.
For example, the discussion of Otto cycle engine construction and
operation in Chapter 1 calls attention to the correspondence between
• the four strokes of the cycle and the four phases of the associated
combustion process and
• the effects of the cycle and the process on the related pollutant
formation mechanisms.
Chapter 2 discusses pollutant formation on an overall basis by relating
two factors, fuel complexity and combustion complexity, to the generalized
effects on the energy generation process and on the pollution formation
mechanisms. Fuel complexity is discussed by relating the many different kinds
of hydrocarbon compounds present in gasoline to the effects of the physical
properties of the compounds on the combustion process. Combustion
complexity is discussed in Chapter 2 in terms of the variations in the structure
and operation of the engine. These variations affect the combustion of the fuel
by limiting the use of the chemical energy in the fuel for developing mechanical
energy need for operating the vehicle.
This chapter also provides an understanding of how one set of
complexities affects the other. The relationship between fuel complexity and
combustion complexity is shown by pointing out
• using a complex fuel (with different types of hydrocarbons), instead of
a simple fuel (with one type of hydrocarbon), complicates the
combustion process and
• using a complex combustion process (involving the discontinuous
burning of combustible air/fuel mixtures in an enclosed chamber),
instead of a simple process (involving the continuous open-flame
burning), complicates the effective utilization of the fuel.
The emphasis on relationships is continued in Chapter 3 which
discusses pollutant formation in detail by highlighting the differences between
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Formation and Control of Combustion Pollutants
the various cause and effect events accounting for the presence of
hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOX) in the
exhaust. This discussion stresses the following:
• The combustion of the fuel involves two successive events: the
disintegration of the HC molecules in the combustion chamber and,
under ideal conditions, the oxidation of the liberated hydrogen (H)
atoms and carbon (C) atoms to water vapor (H^O) molecules and
carbon dioxide (CC>2) molecules, respectively.
• The HC molecules in the exhaust are not products of the combustion
process but, rather, are fuel molecules which were isolated from that
process by a "flame quenching" phenomenon.
• The CO molecules in the exhaust are products of an incomplete
combustion process in which liberated carbon atoms are only partially
oxidized as a consequence of the operation of the engine under
oxygen deficient "rich" conditions which result from the addition of
excessive fuel. Such incomplete oxidation of the carbon atoms is
accompanied by the formation of hydrogen molecules (H2) which
result when the liberated, but unoxidized, hydrogen atoms associate
with one another.
• NOX molecules in the exhaust are not directly products of the
combustion process but, rather, are products of a high temperature
side reaction which involves the disassociation of some of the
atmospheric nitrogen molecules, the oxidation of the liberated nitrogen
atoms to nitric oxide (NO), and the subsequent conversion of the NO to
other nitrogen oxides, such as nitrogen dioxide (N02).
In a comparable way, Chapter 4 discusses pollution control by
associating several presently used techniques with the two possible control
approaches, preventive and remedial. Each technique is related to the
pollutant formation phenomena discussed in Chapter 3.
Engine combustion chamber redesign is discussed as an HC preventive
technique which reduces flame quenching and, hence, reduces the number of
fuel molecules that are isolated from the combustion process. Fuel flow control
is discussed as a CO preventive technique which controls fuel input to ensure
that the oxygen available in the ingested air is adequate to completely oxidize
the carbon atoms that are liberated from the fuel HC molecules. Exhaust gas
recirculation is discussed as an NOX preventive technique which limits the
combustion chamber temperature to minimize the oxidation of atmospheric
nitrogen.
Supplementary air injection is discussed as a remedial technique which
adds air to the hot exhaust gases so that the additional oxygen can react with
the HC and CO molecules that exit the combustion chamber. Catalytic
-------
Introduction
conversion is discussed as a remedial technique which oxidizes HC and CO
molecules and reduces NO molecules. This discussion is focused on closed
loop fuel control systems which use three-way catalysts, lambda sensors, and
electronic control units to maintain proportions of air and fuel close to the
chemically correct, "stoichiometric," ratio.
The final relationships highlighted in Chapter 4 are the possible
interactions between the various control techniques. This discussion illustrates
the fact that a technique which is implemented to control one pollutant can have
various undesirable side effects, such as promoting the formation of another
pollutant, degrading engine or vehicle performance, or causing a decrease in
the effectiveness of a remedial technique that is used to eliminate a different
pollutant.
Two appendixes are included. In Appendix 1, the Otto cycle engine
characteristics are briefly contrasted with those of Diesel cycle engines. The
differences discussed include the various operational parameters, such as the
means used to control speed and power output, the physical properties of the
fuels used, the means used to introduce the fuel into the combustion chambers,
the means used to initiate the combustion process, and the phenomena which
result in the emission of combustion pollutants.
Appendix 2 presents additional information regarding hydrocarbon
molecular structure. The discussion details the hydrocarbons present in
gasoline fuels and the reconfigured and partially oxidized hydrocarbons
produced when some fuel molecules pass through the combustion chamber of
an Otto cycle engine without the complete liberation of the molecular carbon
atoms.
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Formation and Control of Combustion Pollutants
-------
Chapter 1. Gasoline-Fueled Otto Cycle Engines
This synopsis focuses specifically on gasoline-powered, Otto cycle
engines. Such engines utilize a throttled, spark-initiated, internal combustion
process to convert the chemical energy in the fuel into the mechanical energy
required for the operation of the motor vehicle. This conversion process takes
place within several internal combustion chambers similar to the one shown in
Figure 1. The radial boundary of the chamber is determined by the cylindrical
cavity in the engine block. The engine head provides the fixed end to the
chamber. The top of the reciprocating piston provides the opposite movable
end. The combustible mixture of air and fuel enters the combustion chamber
via the inlet port and one or more intake valves. The combustion gases exit the
combustion chamber via one or more exhaust valves and the exhaust port.
Figure 1. Engine Block and Head (cross-section)
Spark Plug
Intake Valve
Exhaust Valve
Head
Intake
Port
Cylinder
Combustion
Chamber
Exhaust
Port
Head Gasket
Block
Crank and Crankshaft
Flywheel
Connecting Rod
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Formation and Control of Combustion Pollutants
As shown in Figure 2, the several inlet ports of the head are connected to
a common inlet by means of an intake manifold. A throttle valve regulates the
ingestion of air or an air/fuel mixture, depending on the design of the engine.
The exhaust ports are connected to a common exhaust outlet by means of an
exhaust manifold.
Figure 2. Engine Block and Manifolds.
Intake
Manifold
Inlet
Throttle
Valve
Exhaust Manifold
Outlet
The four-stroke combustion cycle, which was first put to practical use by
Nicolaus A. Otto in 1876, is shown in Table 1 and Figure 3. As Figure 3
illustrates, the four combustion phases do not start and stop at precise positions
of the piston, although they are affected by, or have an effect on, the four up-
and-down strokes of the piston.
Table 1. Relationship of Combustion Phase and Piston Stroke.
Combustion Phase
Air/fuel mixture is ingested
Mixture is compressed
Mixture is burned
Combustion products are expelled
Piston Stroke
Intake
Compression
Power
Exhaust
This Otto cycle process is both similar and different from that of a Diesel
cycle engine. Appendix 1 provides a brief contrast of the two types of engines
and the resulting pollutants. This synopsis focuses on pollutants formed by the
combustion process in a gasoline-powered, fuel-injected, Otto cycle motor
vehicle engine.
-------
Gasoline-Fueled Otto Cycle Engine
Figure 3. Otto Cycle Phases.
Intake Valve
Open
Air/Fuel
Mixture
1) INTAKE STROKE
Spark
Initiated
Flame
Both Valves
Closed
2)COMPRESSION STROKE
Exhaust
Gas
POWER STROKE
A) EXHAUST STROKE
When a naturally aspirated (unturbocharged or unsupercharged) Otto
cycle engine is in operation, the movement of each piston away from the
cylinder head during the intake stroke increases the available space in the
cylinder. The increase in volume results in a decrease in pressure. The
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Formation and Control of Combustion Pollutants
accumulative effect of the successive intake strokes of the several pistons is the
reduction of the intake manifold pressure to a negative (subatmospheric) value.
The extent to which the intake system pressure is reduced depends on the
speed of the engine and the position of the intake manifold throttle valve. The
speed of the engine in revolutions per minute determines the total number of
intake strokes per minute. The position of the throttle valve determines how
much obstruction is offered to the inflow of ambient air. The absolute pressure
in the intake manifold is the lowest (less than ^ atmospheric pressure) at idle
and highest (close to atmospheric pressure) at wide open throttle.
The subatmospheric pressure in the intake manifold facilitates the
ingestion of ambient air via the air cleaner and the flow of the air to the intake
ports of the several cylinders via the individual passageways or "runners" in the
intake manifold. At some time prior to the arrival of the air at each cylinder inlet
port, "atomized" fuel in the form of very fine droplets is added to the air. The fuel
flow control system is designed to add the fuel at a rate which will result in the
desired proportions of fuel and air.
At each cylinder inlet port, the subatmospheric pressure in the
combustion chamber (which is lower than the subatmospheric pressure in the
intake manifold) facilitates the entrance of the air/fuel mixture. This occurs when
an intake valve opens as the piston moves away from the cylinder head. This is
the first, or intake, stroke of the combustion cycle (see Figure 3).
The valve then closes and the piston moves toward the cylinder head
during the second, or compression, stroke. This movement compresses the
contained mixture and, as a consequence, heats it. An electric spark ignites
the compressed and heated air/fuel mixture sometime during the time interval
when the piston is advancing toward and then retreating from "top dead center"
(the position where piston is at its closest point to the cylinder head).
The high pressure developed as a result of this combustion process
forces the piston away from the cylinder head during the third (power) stroke of
the operating cycle. The exhaust valve opens when the piston is again moving
toward the cylinder head so that the combustion gases can be expelled. This is
the fourth and final (exhaust) stroke of the cycle.
This sequence of events is repeated over and over in each cylinder of the
engine and the combustion gases flow into the exhaust manifold and escape
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Gasoline-Fueled Otto Cycle Engine
into the atmosphere via the vehicle tailpipe(s). At any particular moment, the
position of the air inlet throttle valve determines the speed of the engine and the
power it is developing. When the valve is in its "closed" position, only enough
air is ingested to allow engine operation at the desired idle speed without the
generation of any power for vehicle operation. The small amount of fuel added
allows the generation of enough power to keep the engine running smoothly at
the idle speed. When the throttle valve is wide open and fuel is added at the
appropriate rate, the engine is able to develop its maximum power output. Then
its speed is determined by the load requirements placed on it by the operation
of the vehicle.
If a hydrocarbon fuel containing no impurities could be burned under
perfect conditions, the exhaust gases exiting the individual combustion
chambers would contain only:
• molecules of carbon dioxide (C02) (containing the carbon atoms
previously contained in the fuel molecules),
• molecules of water vapor (H20) (containing the hydrogen atoms
previously contained in the fuel molecules), and
• molecules of nitrogen (N2) (originally in the ingested air and not
involved in the combustion process).
This can be represented by the following equation:
Fuel (HC) + Air (O2 & N2) = Energy + C02 + H2O + N2
However, since most fuels contain some impurities and perfect
combustion conditions can not be realized in actual practice, the exhaust gases
may also contain varying percentages of:
• molecules of hydrogen (H2) which result when fuel hydrogen atoms
are not oxidized,
• fuel hydrocarbon (HC) molecules which were completely unchanged,
or only partially changed, during their passage through the combustion
chambers of the engine,
• molecules of carbon monoxide (CO) which are the result of the
incomplete oxidation of the fuel carbon atoms,
• molecules of oxides of nitrogen (NOX) which were formed when the
atmospheric nitrogen was subjected to the unique conditions existing
in the engine's combustion chambers, and
• molecules of inorganic compounds, such as sulfur oxides, which result
from reactions involving fuel or lubricating oil impurities.
This can be represented by the following equation:
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Formation and Control of Combustion Pollutants
Fuel (HC) + Air (02 & N2) =
Energy + CC>2 + H2O + N2 + H2 + HC + CO + NOX
The formation and control of the pollutants resulting from these imperfect
combustion conditions are discussed in the following chapters of this synopsis.
10
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Chapter 2. Pollutant Formation—General Effects
Two factors, fuel complexity and combustion complexity, have a general
effect on pollutant formation in that they preclude the complete oxidation of the
fuel. Fuel complexity results from the many different kinds of hydrocarbon
compounds present in gasoline. Combustion complexity results from the
various structural and operational parameters of the engine which affect the
various compounds in the fuel.
2.1. Fuel Complexity
The term "gasoline" is used to describe the portion, or fraction, of
petroleum distillate which has a boiling point range of about 75 to 415°F. This
portion is a complex mixture of a wide variety of hydrocarbon compounds. The
molecules of all of these compounds are comprised of only two kinds of atoms,
carbon and hydrogen. However, the four attachment points at each carbon
atom allow an almost limitless variety of molecular configurations. Some
examples of these molecular structures are shown in Figure 4 and more appear
in Appendix Two.
Figure 4. Molecular Structure of Selected Hydrocarbons.
H H
H-i-H H-C-H
H
"
H H H H H I H
H-C-C-C-C-H H-c—t-c-
HHHH "„ ' " ^ H
H-C-H H
H
Butane Isooctane (2,2,4-trimethyl pentane)
Gasoline hydrocarbons can be classified into three main types: paraffins,
olefins, and aromatics. The paraffin hydrocarbons are "saturated" in that each
molecule contains the maximum possible number of hydrocarbons atoms. The
olefin hydrocarbons are "unsaturated" or deficient in hydrogen atoms as a result
of double bonding between adjacent carbon atoms in the chains. The aromatic
hydrocarbons contain a unique structure, known as a benzene ring, consisting
of six carbon atoms in a closed chain or ring with single and double bonds
11
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Formation and Control of Combustion Pollutants
alternating between the adjacent carbon atoms. Each hydrocarbon class can
be found in gasoline and in exhaust emissions. The actual percentage of each
type of hydrocarbon depends on the type of gasoline.
The addition of oxygen atoms to hydrocarbon molecules further
increases the number of possible molecular configurations. In the case of the
alcohols, a hydrogen atom is replaced with a oxygen/hydrogen hydroxyl (-OH)
functional group. The two simplest alcohols, methanol and ethanol, can be
used alone as engine fuels or mixed with gasoline to form oxygenated fuels. In
the case of the aldehydes and ketones, a carbon atom is double bonded to one
oxygen atom in lieu of being single bonded to two hydrogen atoms. Such
oxygenated compounds are formed during certain reactions occurring in an
engine's combustion chamber.
The physical properties of a hydrocarbon compound are affected by the
number of carbon atoms in the molecule and the configuration of these atoms
within the molecular structure. The effect of these details on physical properties
is illustrated by Table 2.
The first twelve compounds in Table 2 illustrate how an increase in the
number of molecular carbon atoms results in an increase in boiling point and a
decrease in autoignition temperature. The decrease in the autoignition
temperature results from the increase in the susceptibility to thermal breakdown
which accompanies the increase in the length of the molecule.
The last two compounds in the table illustrate how the configuration of
molecular carbon atoms affects the boiling point and autoignition temperature.
The rearrangement of the six straight-chain carbon atoms in the paraffinic
hexane molecule into the aromatic benzene configuration results in about a
14% increase in boiling point and about a 50% increase in autoignition
temperature. The rearrangement of the eight straight-chain carbon atoms in the
paraffinic octane molecule into the branched-chain isooctane configuration
results in about a 18% decrease in boiling point, but about a 53% increase in
autoignition temperature.
Autoignition temperature is one of the most important physical properties
of a gasoline hydrocarbon because it is an indication of detonation tendency.
Detonation occurs when a portion of the air/fuel mixture in the combustion
chamber spontaneously ignites before the arrival of the spark-induced flame
12
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Pollutant Formation—General Effects
Table 2. Hydrocarbon
Compound
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
Undecane
Dodecane
Benzene
Isooctane3
Carbon
Atoms
1
2
3
4
5
6
7
8
9
10
11
12
6
8
Compounds
Physical
State
Gas
M
II
II
Liquid
M
it
it
it
it
M
Solid
Liquid
it
and Properties.
Boiling
Point
(°F)
-259
-127
-44
31
97
155
209
258
303
345
384
4212
177
211
Auto ignition
Temperature
m
1,346
1,050
995
961
933
909
893
880
871
866
—
—
1,363
1,350
Research
Octane
Number
1101
1041
100
92
61
25
0
-17
-45
—
—
—
110
100
1 Estimated ^ Melting point is 14°F 3 2,2,4-trimethyl pentane
front. Such detonation results in the generation of a second very rapidly moving
flame front. The resultant rapid increase in pressure in the combustion chamber
produces the characteristic "knocking" sound.
The octane number of a compound is an indication of its knocking
tendency, as can be seen in the proceeding table. The use of such numbers
provides a means for rating the knocking, or detonation, tendencies of gasoline
samples. This rating system is based on the knocking characteristics of normal
heptane (0 octane) and isooctane (2,2,4-trimethyl pentane) (100 octane) when
a mixture of these hydrocarbons is burned in a Cooperative Fuel Research
single cylinder engine.
The octane number of a gasoline sample indicates the relative amounts
(by volume) of isooctane and heptane in a mixture which exhibits a knocking
tendency comparable to that of the engine sample. For example, a gasoline
sample which has an octane rating of 80 has the same knocking tendency of a
mixture of 80% isooctane and 20% heptane. Fuel samples with a lower
detonation tendency than isooctane are rated with a different rating scale. This
scale involves the supercharging of the test engine to increase the inlet air
pressure when isooctane is used as the fuel.
By varying the operating conditions of the test engine, two different
values, the research octane number (RON) and the motor octane number
13
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Formation and Control of Combustion Pollutants
(MON), are obtained. The RON value, the lower of the two, is obtained by the
use of more severe test engine operating conditions, such as a higher speed, a
more advanced ignition timing, and a higher intake mixture temperature in the
intake manifold. The difference between the MON and RON values is called the
sensitivity of the sample. This value is used to indicate the tendency of a fuel to
preignite and produce knock when the conditions under which a vehicle engine
is operating becomes more demanding. The octane value shown on the
service station dispensing pump is the average of the MON and RON values
/MON + RON\
V 2 /•
A typical gasoline sample will contain different types of hydrocarbons
with molecular carbon atoms predominately in the Ce to 612 range. The carbon
atoms in these hydrocarbons can be arranged in different configurations. (The
tables in the appendix show a few hydrocarbon molecular configurations in
their simplest forms.) A specific gasoline sample can also contain some
hydrocarbons with fewer than six carbon atoms. For example, some butane
may be present, even though in the pure state this hydrocarbon is a gas at
standard temperature and pressure conditions. A gasoline sample may also
contain hydrocarbon molecules with more than 12 molecular carbon atoms, as
well as configurations involving various degrees of unsaturation and different
combinations of straight and branched chains and closed or aromatic rings.
Since the composition of "gasoline" fuels can vary over such a wide
range, the combustion characteristics of a particular gasoline sample is
dependent on the nature of the contained mixture of hydrocarbon compounds.
For this reason, EPA regulations specify that emission testing must be
performed with gasoline which has properties that fall within specified limits.
Furthermore, the test fuels must be either
• kept in sealed containers to prevent the preferential evaporation and
escape of the hydrocarbons with the lowest boiling points, or
• analyzed frequently enough to ensure that such preferential
evaporation or "weathering" does not result in changes which cause
physical properties, such as octane rating or vapor pressure, to exceed
the specified tolerances.
14
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Pollutant Formation—General Effects
2.2. Combustion Complexity
The conditions and consequences of combusting, or burning, gas in the
open are different from those when gasoline is burned in the cylinders of an
Otto cycle motor vehicle engine.
Burning in the open is a continuous process. The surface of the liquid
fuel is exposed to atmospheric oxygen. When a flame is initiated, it is
maintained by
1. the continuous escape of hydrocarbon molecules from the liquid
surface,
2. the heating of these molecules by the flame,
3. the breakdown of the molecules, and
4. the combining of the liberated carbon and hydrogen atoms with
atmospheric oxygen atoms, or oxidation.
Burning in an engine cylinder is an intermittent or batch combustion
process which differs from an open burning process in that:
1. oxidation occurs with a specific quantity of fuel and air. Air is mixed
with "atomized" fuel. This mixture is not perfectly homogenous in that
the size and distribution of the liquid droplets is not completely
uniform.
2. the air/fuel mixture is introduced into sealed combustion chambers
which have complicated internal surfaces. The mixture is then
compressed and heated before it is ignited.
3. the combustion process in each separate combustion chamber is
affected by numerous variables, such as:
(a) the amount of change in air/fuel mixture uniformity, such as
stratification which causes "top to bottom" variations in the
distribution of the fuel droplets.
(b) the amount of compression and, hence, the amount of heating the
charge experiences prior to ignition.
(c) the manner in which the spark-initiated flame front expands with
the combustion chamber.
(d) the degree to which the flame is extinguished as it approaches the
relatively cool walls of the combustion chamber.
(e) the amount of temperature and pressure rise that occurs during
the combustion process.
(f) the degree to which the limited time available for combustion
affects the completion of the process.
An essential first step in the oxidation of the gasoline hydrocarbon
compounds in the combustion chamber is the introduction of the air and fuel in
the proper proportions. The appropriate ratio of air to fuel is determined by:
15
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Formation and Control of Combustion Pollutants
• the number of carbon atoms in the average fuel molecule, and
• the ratio of hydrogen to carbon atoms in the average fuel molecule.
If a fuel which has an average carbon atom chain length of six and the
average hydrogen atom/carbon atom ratio of 1.86 is used, the chemically
correct or "stoichiometric" air/fuel ratio is 14.5 pounds of air to 1 pound of fuel.
Under perfect combustion conditions, the following results would be obtained:
14.5# air = 1.07#O2 + 2.33# 02 + 11.1#N2
1.0# fuel = 0.13#Ho + 0.87# C
15.5# exhaust = 1.20#H2O + 3.20# C02 + 11.1#N2
However, even when the fuel is supplied at a rate that will theoretically
result in a chemically correct air/fuel ratio, optimum engine operation may not
be realized. The desired results can be precluded by a number of inhibiting
factors inherent in the combustion process. One of these factors is the change
in the initial air/fuel ratio which occurs before the mixture is actually introduced
into the combustion chamber. The composition of this final mixture depends on
a number of details; such as, how well the fuel is "atomized," how well the fuel
and air are mixed, and how much exhaust remains in the combustion chamber
to mix with the incoming air and fuel.
A second limitation is the incomplete utilization of the fuel that does get
into the combustion chamber. As a consequence of these limitations, engine
operation for maximum power output may require richer (less than
stoichiometric) ratios whereas operation for maximum fuel economy may
require leaner (greater than stoichiometric) ratios.
A number of factors can adversely affect the combustion process even
when the proportion of air to fuel is chemically correct. One of the most
important of these factors is the manner in which the air and fuel are introduced
into the combustion chambers. A perfect induction system would:
• combine the fuel and air in a manner resulting in a perfectly
homogeneous mixture of individual hydrocarbon, oxygen, and
nitrogen molecules, and
• deliver the mixture in a manner resulting in a perfect distribution
between the individual combustion chambers.
Such perfection is impossible, but an actual induction system should minimize
fuel droplet size, maximize the mixing of the droplets in the air, and minimize
adverse effects when the mixture is distributed between the combustion
16
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Pollutant Formation—General Effects
chambers. Carburetor-based fuel systems present limitations to achieving this
goal.
When a carburetor-based system is naturally aspirated, subatmospheric
pressure is used in two different ways. The downward movement of the piston
during the intake stroke results in a pressure drop. The decrease in pressure
allows atmospheric pressure to force air into the intake system via the air
cleaner and the associated runner of the intake manifold. The flow of the
incoming air through one or more venturi in the carburetor results in another
pressure drop which allows atmospheric pressure to force fuel into the air
stream in the form of fine droplets. Such use of atmospheric pressure for fuel
induction places severe limitations on the reduction of fuel droplet size and on
the thorough mixing of these droplets with the incoming air.
These particular limitations have been largely overcome by the use of an
injector which operates at a pressure in excess of atmospheric pressure and
sprays the fuel into the air stream. However, the use of such a "throttle body"
injection system does not eliminate adverse distribution effects. These adverse
effects can be caused by differences between the intake manifold's individual
runners or branches in regard to parameters such as temperature, total length,
and aerodynamic flow resistance. Such differences can result in the individual
combustion chambers receiving air and fuel in somewhat different proportions.
Such unequal distribution effects can be minimized by the use of a multipoint
system in which a separate fuel injector is located in the intake port of each
combustion chamber.
In such systems, only the incoming air flows though the throttle body and
the intake manifold runners. Each individual injector is operated by an
electronic control unit in a manner which tends to result in the correct amount of
fuel being delivered to the intake port of the associated engine cylinder. Some
cylinder-to-cylinder differences in the air/fuel ratios can result from distribution
problems which can affect the flow of the air through the runners of the intake
manifold. Nevertheless, multipoint injection systems largely eliminate the
distribution problems that are associated with single point injection systems and
carburetted systems.
A number of engine configuration and timing details can affect the
combustion process. In regard to configuration details, some of the more
important orientation details are the locations of the spark plugs and the valves
17
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Formation and Control of Combustion Pollutants
in the chamber and the locations of the rings on the piston. In regard to timing
details, some of the most significant parameters are the opening and closing of
the intake and exhaust valves and the firing of the spark plugs. The movement
of the intake and exhaust valves relative to the movement of the piston
determines the movement of the air/fuel mixture and the exhaust gas, including
the reverse flow of some of the exhaust gas when an "overlap" condition results
in the intake and exhaust valves being open at the same time. The timing of the
spark generation determines when the ignition of the air/fuel mixture is initiated
and, hence, how much time is available for the completion of the combustion
process. The ignition timing therefore has an effect on a number of variables
such as the maximum pressure, the mean effective pressure, and the thermal
efficiency.
As a result of the unique combination of conditions under which
combustion occurs, the fuel hydrocarbon molecules which are not completely
disintegrated are altered in different ways. Long chains can be severed and
some of the resultant fragments can be rejoined to form new molecular
structures. The new structures may have a greater or lesser degree of
unsaturation or ring formation than the original compounds in the fuel. In
addition, new compounds, such as aldehydes (see Appendix 2), can be formed.
Consequently, the proportion of a particular class of hydrocarbons in the
exhaust will be quite different to the proportion in the original gasoline fuel. As
a result of the reactions occurring in the combustion chamber, the proportion of
paraffins and aromatics will be reduced while olefins and oxygenated
hydrocarbons are formed.
Using a pure hydrocarbon compound, such as isooctane, as a fuel in lieu
of gasoline reveals the effectiveness of the combustion process in promoting
chemical reactions. When the exhaust of an engine using such a pure fuel is
analyzed, a variety of hydrocarbon molecules are found. This complex variety
reveals how extensively the original isooctane molecules were chemically
altered during their passage through the combustion chamber.
18
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Chapter 3. Pollutant Formation—Specific Effects
In general, concern regarding the emission of combustion pollutants is
concentrated on three specific pollutants; i.e., hydrocarbons (HC), carbon
monoxide (CO), and oxides of nitrogen (NOX). These three kinds of pollutants
differ in regard to the cause and effect phenomena which result in their
presence in the exhaust (see Table 3).
Table 3. Exhaust Pollutants.
Pollutant Type Cause Effect
HC Flame Quenching Absence of combustion
CO Inadequate oxygen Incomplete oxidation
NOX Excessive temperature Nitrogen oxidation
As Table 3 indicates, HC emissions result from portions of the air/fuel
mixture which were partially or completely excluded from the combustion
process. The hydrocarbons in the exhaust are the residual fuel hydrocarbons
which passed through the combustion chamber without the destruction of the
molecules. Some of these unused fuel hydrocarbons remain in their original
form. Some have their molecular structures changed as a result of the
replacement of hydrogen atoms by oxygen atoms or as a result of changes in
the carbon chain configuration. In all cases, the carbon and hydrogen atoms
remain bound in some hydrocarbon or oxygenated hydrocarbon molecular
structure and are not liberated for oxidation to carbon dioxide (CO2) and water
(H20).
The CO emissions result when the amount of oxygen present in the
ingested air/fuel mixture is inadequate. The total number of oxygen atoms is too
low to combine with all the carbon atoms liberated from the fuel molecules
which are involved in the combustion process. As a result these carbon atoms
are not completely oxidized to C02 molecules.
The NOX emissions are unique relative to CO and HC emissions in that
their formation is not directly related to the combustion process. The oxidation
or "fixation" of the atmospheric nitrogen would occur without the presence of
any hydrocarbon fuel. At the high temperatures which occur in the combustion
chamber, a fraction of the nitrogen and oxygen molecules disassociate and the
liberated atoms combine primarily in the form of nitric oxide (NO).
19
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Formation and Control of Combustion Pollutants
Details regarding the three phenomena causing each of these conditions
are presented in the following sections.
3.1. Effect of Flame Quenching on HC Emissions
When a gasoline-fueled Otto cycle engine operates under favorable
conditions; i.e., close to stoichiometric air/fuel ratio and with an absence of
malfunctions such as ignition misfire, the primary source of hydrocarbons in the
exhaust is fuel molecules which were not adequately involved in the
combustion process. Some of these molecules may have been involved
enough to have undergone some structural changes, such as a reduction in
carbon-chain length or a replacement of some of the hydrogen atoms by
oxygen atoms. The remainder of the uninvolved molecules will have passed
unchanged through the combustion chamber as a consequence of being
isolated from the combustion process as a result of "flame quenching."
When a flame in the combustion chamber is initiated at the spark plug, it
expands very rapidly in all directions. However, quenching, or extinction, due to
heat transfer can occur at the boundaries of the expanding ball of flame.
Wall quenching occurs when the flame front approaches the physical
boundaries of the combustion chamber where a sharp drop in temperature
occurs (see Figure 5). The fuel in close contact with these boundaries has a
lower temperature as a result of its proximity to the cooler metal surfaces. When
the temperature is too low to support combustion, the flame is quenched.
Figure 5. Flame Quenching.
Flame Front
Unbumed
Air/Fuel
Mixture
Wall Quenching
— Crevis Quenching
20
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Pollutant Formation—Specific Effects
Restriction, or crevis, quenching occurs when the flame front reaches a
restriction which impedes further propagation. Typically, the flame encounters
such a restriction when it reaches any very small volume in the combustion
chamber. One such flame quenching volume is the cavity that is formed by (a)
the upper surface of the topmost piston ring, (b) the outside diameter of the
piston, and (c) the inside diameter of the cylinder.
Regardless of the cause, when the flame is quenched, the fuel in the
volume beyond the boundary where quenching occurs is more or less isolated
from the combustion process. One fraction of this isolated fuel leaves the
combustion chamber essentially in its original unaltered state. Another fraction
leaves in an altered state, as a result of low temperature preflame reactions in
the quenched zones. These altered hydrocarbons appear in the exhaust in
form of aldehydes and other partially oxidized compounds (see Appendix 2.
Figure 6.).
3.2. Effect of Oxygen Content on CO Formation
During the oxidation process within the combustion chamber, the fuel
hydrocarbon molecules are disintegrated and the carbon atoms separate from
each other and from the attached hydrogen atoms. The liberated carbon and
hydrogen atoms then combine with the oxygen atoms in the chamber. When
the total number of oxygen atoms is inadequate, some of the carbon atoms
combine with single oxygen atoms and carbon monoxide molecules are
formed. The hydrogen atoms combine with each other to form diatomic
hydrogen molecules.
3.3. Effect of Temperature on NOX Formation
Unlike HC and CO emissions, NOX emissions do not arise as a
consequence of the incomplete oxidation of the fuel. NOX emissions result from
a side reaction that occurs under the unique conditions produced in the
combustion chamber when the fuel is oxidized. These conditions are quite
different than those that prevail when gasoline is burned in the open.
During open burning conditions, the nitrogen molecules in the air are not
affected by the combustion process and atmospheric nitrogen acts as a non-
reactive diluent. However, when gasoline is burned in the cylinders of an
internal combustion engine, the high temperatures and pressures that are
produced cause some "fixing" of atmospheric nitrogen. A small percentage of
21
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Formation and Control of Combustion Pollutants
the diatomic nitrogen molecules are disassociated and the liberated nitrogen
atoms are "fixed" when they combine with oxygen atoms in the combustion
chamber. The primary immediate reaction product is nitric oxide (NO) which
can undergo conversion into other oxides, such as nitrogen dioxide (NC>2),
during transit through the exhaust system or during subsequent dispersion into
the atmosphere.
22
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Chapter 4. Pollutant Control Approaches
Two general approaches for reducing the emission of pollutants from the
tailpipe of a motor vehicle may be taken:
• the preventive approach which involves the modification of the
combustion conditions to reduce the formation of pollutants in the
combustion chamber, and
• the remedial approach which involves the treatment of the exhaust gas
to reduce the concentrations of the pollutants which exit the
combustion chamber.
Each approach uses a variety of techniques. This chapter discusses each of
these techniques and the possible interactions and complications of these
techniques.
4.1. Preventive Techniques
The three specific preventive techniques are
• changes in engine design (combustion chamber redesign),
• changes in engine operation (fuel flow controls), and
• changes in combustion conditions (temperature reduction).
These techniques will affect hydrocarbon, carbon monoxide, and nitric oxide
emissions, respectively.
4.1.1. Combustion Chamber Redesign for HC Reduction
Flame quenching results in some fuel hydrocarbon molecules exiting
unaltered from the combustion chamber (see Chapter 3). Combustion chamber
redesigns can minimize the effects of this phenomenon. Quenching due to wall
effects can be reduced by any design change reducing the surface area of the
combustion chamber relative to the clearance volume (piston is at top dead
center). Combustion chamber design changes that can be implemented to
accomplish the desired decrease in the surface to volume (s/v) ratio include:
• reducing depressions and protrusions in the chamber.
• changing the nature of the surfaces that provide the clearance volume
(see Figure 6).
• making the cylinder geometry more favorable.
Restriction quenching can be reduced by any chamber redesign which
eliminates or reduces any small volume that impedes the propagation of the
flame front. For example, if the topmost piston ring is moved up toward the end
of the piston (see Figure 1), a volume reduction is realized in the annular cavity
23
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Formation and Control of Combustion Pollutants
which is formed around the top of the piston. Consequently, the amount of fuel
which is isolated from the oxidation process is reduced.
Figure 6. Surface/Volume Ratio Changes.
Combustion Chamber Shape Changes
Cylinder,
Head
Piston -H- O
S/V
8.0
7.2
6.6
6.4
4.1.2. Fuel Control for CO Reduction
Sometimes the number of oxygen atoms in the combustion chamber is
not great enough for the oxidation of all of the carbon and hydrogen atoms in
the fuel. When this occurs, some of the carbon atoms combine with individual
oxygen atoms to form carbon monoxide molecules and the hydrogen atoms
remain unoxidized in the form of hydrogen molecules. This undesirable result
can be minimized by "lean" operating conditions that occur when air/fuel ratios
greater than stoichiometric are used. Such lean operating conditions ensure
that enough oxygen atoms are present to allow two oxygen atoms to combine
with every carbon atom that is released when the fuel is burned.
However, such a lean approach to reduction of carbon monoxide
formation is limited by several possible undesirable side effects, such as rough
engine operation, increased nitrogen oxide formation, and adverse effects on
downstream exhaust aftertreatment devices. Moreover, beyond a specific
ultimate limit in lean operation, the proportion of fuel molecules to oxygen
molecules becomes too low for efficient combustion. At that point, the mixture of
air and fuel fails to ignite and "lean misfire" occurs.
4.1.3. Temperature Control for NOX Reduction (Exhaust Gas Recirculation)
Fixing atmospheric nitrogen within the combustion chamber can be
inhibited by any mechanism that reduces the maximum temperature developed
during the combustion process. As was previously mentioned, one means for
accomplishing the necessary temperature reduction is retarding the spark. This
change reduces the maximum temperature by shortening the time available for
24
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Pollutant Control Approaches
the completion of the combustion process. However, retarding ignition has
disadvantages, such as the loss of power output that results from the reduction
in the maximum pressure in the cylinder.
A more satisfactory means of lowering the maximum combustion
temperature is to increase the heat capacity of the gases in the combustion
chamber by diluting the new charge of air/fuel mixture with a gas that does not
take part in the combustion process. The exhaust gas is ideally suited for this
purpose because:
• it is readily available and can be easily recirculated back into the
combustion chamber.
• it has a high concentration of carbon dioxide and water molecules
which have higher heat absorption capacities than the molecules of a
diatomic gas, such as atmospheric nitrogen.
Some dilution of the new mixture charge by exhaust gas is inherent in
the four-stroke combustion cycle. At the completion of the exhaust stoke, some
of the exhaust gas, which is at a pressure in excess of atmospheric pressure, is
left behind in the clearance volume which remains when the piston is top dead
center. When the intake valve opens at the start of the intake stroke and
exposes the cylinder to the subatmospheric pressure in the intake system,
pressure equalization occurs. As a result, a fraction of the residual exhaust
backflows through the open intake valve and mixes with, and dilutes, the
entering new charge of air/fuel mixture. Further dilution of the mixture is caused
by the fraction of the exhaust which did not backflow out through the intake
valve and remained in the cylinder.
Such inherent dilution is considerably enhanced when there is a "valve
overlap" condition in which the exhaust valve is still open when the intake valve
opens (see Figure 7). The reduction in cylinder pressure which results from the
downward movement of the piston during the intake stroke results in a fraction
of the exhaust gas which was forced out of the combustion chamber into the
exhaust port during the exhaust stroke reentering the cylinder via the open
exhaust valve. The primary purpose of such valve overlap is to produce the
desired engine performance characteristics. Nevertheless, the resultant
recirculation of exhaust gas results in some reduction in the maximum
combustion temperature and, hence, some reduction in NOX formation.
25
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Formation and Control of Combustion Pollutants
Figure 7. Internal Exhaust Gas Recirculation (valve Overlap).
; Both valves open
Air/Fuel mixture
Exhaust
When such inherent exhaust gas recirculation does not provide the
desired reduction in NOX formation, additional exhaust gas flow must be
provided by means of some routing external to the combustion chamber. Such
an "add-on" exhaust gas recirculation (EGR) emission control system can
consist of (a) a connecting passageway between the intake and exhaust
manifolds of the engine and (b) a valve which controls the flow of exhaust gas
through the passageway (see Figure 8). The EGR system control mechanism is
designed to open and close the valve as needed to minimize the formation of
NOX while also minimizing any undesirable effects on performance
characteristics, such as a poor idle quality or reduced performance during wide
open throttle acceleration.
Figure 8. External Exhaust Gas Recirculation.
Intake
Manifold
Exhaust Manifold
Control Signal
26
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Pollutant Control Approaches
4.2. Remedial Techniques
Two techniques are used remedially to remove pollutants which exit the
combustion chamber:
• thermal oxidation in the exhaust manifold
• catalytic conversion in the exhaust stream
The first of these techniques uses supplemental air to reduce HC and CO
pollutants. The second affects HC, CO, and NOX.
4.2.1. Supplementary Air Oxidation
As discussed previously under preventive techniques, operating an
engine under very lean conditions can reduce CO. A number of factors
preclude this lean operation. However, atmospheric oxygen can be added to
the exhaust stream after the exhaust gases leave the combustion chamber.
Such oxygen enrichment facilitates the continuation of the oxidation reactions
that were initiated in the combustion chamber. The secondary air is injected
directly into the exhaust ports of the cylinders. The carbon monoxide and
hydrocarbons in the very hot exhaust gases are thermally oxidized without the
production of a visible flame. One side benefit of such secondary air injection is
that the increase in exhaust gas temperature occurring as a result of the
oxidation reactions reduces the time required for the downstream catalytic
converter to reach the temperature necessary for efficient conversion.
Depending on the purpose for the introduction of additional air into the
system, negative or positive pressures (below or above atmospheric pressure)
can be used to provide the necessary differential pressure. For example, a
negative pressure system can be used when the purpose of secondary air
introduction is to reduce emissions during idle operation, particularly when a
cold engine is started and is warming up. During such engine operation, the
concentration of oxygen in the exhaust manifold is too low to effectively oxidize
the carbon monoxide and hydrocarbons in the exhaust gas. The necessary
additional oxygen can be introduced by means of a simple aspiration system
which takes advantage of the negative pressure pulses in the exhaust manifold.
In the simplest form of such a system, the necessary air flow control
functions are performed by a one-way aspirator valve located in a line
connecting the exhaust port air injection tubes with an inlet for atmospheric air,
usually in the air cleaner (see Figure 9). The aspirator valve allows the
entrance of atmospheric air, but prevents the escape of exhaust gas. When the
27
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Formation and Control of Combustion Pollutants
engine is idling and a large negative (below atmospheric) pressure pulse is
produced at an exhaust port as the result of the opening of the associated
exhaust valve, the higher atmospheric pressure forces the aspirator valve
open. Atmospheric air flows into the system and exits at the exhaust port via the
air injection tube. As soon as the exhaust valve closes and the pressure at the
exhaust port rises above atmospheric pressure, the higher pressure closes the
aspirator valve and the escape of exhaust gases into the atmosphere is
prevented.
Figure 9. Aspirated Air Injection.
Intake
Manifold
Exhaust Manifold
Air
Cleaner
Aspiration valve
In some systems, secondary air is injected into the exhaust ports to
promote thermal oxidation reactions and into the exhaust system, downstream
of a three-way reduction catalyst but upstream of an oxidation catalyst, to
promote catalytic oxidation reactions. In such systems a positive pressure
system must be used. The required higher than atmospheric pressures are
provided by an engine-driven mechanical air pump (see Figure 10). When the
engine warm process has been completed, the output of the pump is switched
from the exhaust manifold to the injection point in the exhaust system upstream
of the oxidation catalyst. The added air ensures that sufficient oxygen is in the
converter for the required conversion efficiency.
28
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Pollutant Control Approaches
Figure 10. Forced Air Injection.
Intake
Manifold
Exhaust Manifold
Air
Filter
Air Pump
Diverter
Valve
Three-way
Catalyst
Oxidation
Catalyst
4.2.2. Catalytic Conversion
Another aftertreatment of the exhaust gases, catalytic conversion,
involves promoting chemical reactions in the carbon monoxide, hydrocarbons,
and nitrogen oxides emissions. These polluting forms of carbon and nitrogen
are then changed into non-polluting forms. The use of such exhaust
aftertreatment as a remedial pollution control is made possible by catalytic
converters which have the required performance characteristics. Such
converters must be able to selectively promote the chemical reactions
producing the desired results and to suppress other possible competitive
reactions which either lower effectiveness or yield undesirable by-products.
Chemical Reactions—The carbon atoms in the combustion gases are in
a polluting form when they exist in CO and HC molecules. If sufficient oxygen is
29
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Formation and Control of Combustion Pollutants
present, passage through a "two-way" oxidation catalytic converter will result in
a significant fraction of the CO molecules being oxidized to C02 molecules and
the HC molecules being oxidized to C02 and water (H20) molecules .
The nitrogen atoms in the combustion gases are in a polluting form when
they exist in oxides, such as nitric oxide (NO) or nitrogen dioxide (NO2). When
sufficient reducing agents are present, passage through a reducing catalyst will
result in a significant fraction of the oxides being reduced to atmospheric
diatomic nitrogen molecules (N2).
The desired reactions, the oxidation of CO and HC and the reduction of
NO, can occur more or less simultaneously when the exhaust gas is passed
through a "three-way" oxidizing converter if the upstream composition of the
exhaust is appropriately controlled. Such control of exhaust gas composition is
necessary because the reduction of the NO to diatomic nitrogen (N2) is
somewhat more complex than the oxidation of CO and HC to C02 and H20.
In the oxidation conversions, new molecules are formed when their
precursor "raw material" component parts combine with each other. However,
the reverse does not occur in connection with the NO reduction. The process is
not as simple as the breakup of the NO molecules with the release of both the
nitrogen and oxygen atoms back to the atmosphere. The reduction of NO
depends on the interaction of NO with other exhaust reducing components in
various reactions, such as the following:
2NO + 2H2 = N2 + 2H2O
2NO + 2CO = N2 + 2C02
4NO + CH41 = 2N2 + C02 + 2H2O
In connection with the above reactions, the following facts are worth
noting:
• all of the NO reduction conversions depend on the presence of
reducing agents, such as CO and HC.
• the last two reactions result in the elimination of all three pollutants,
CO, HC, and NO.
System Requirements—The actual hardware that can be used to
accomplish the desired conversion reactions depends on the requirements of a
particular system. For example:
• if NO emissions are not a problem, a two-way oxidizing converter can
be used to oxidize CO and HC,
1 Where the methane molecule, CH4, represents all possible HC molecules.
30
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Pollutant Control Approaches
• when NO emissions are a problem, a three-way catalyst can be used
to simultaneously reduce NO and oxidize CO and HC, or
• if a three-way catalyst does not adequately lower the concentrations of
CO and HC, a downstream oxidizing catalyst (see Figure 10) can be
used to further lower the concentrations of these pollutants.2
The performance of catalytic converters is affected by the chemical
composition of the exhaust. In the case of oxidizing converters, the relationship
between composition and performance is fairly straight-forward. When the
oxygen concentration falls to a level which precludes the complete oxidation of
CO and HC, performance is degraded.
In regard to three-way converter operation, the relationship between
composition and performance is more complex . Such a converter will yield the
desired results only when the concentrations of all of the several reactants
remain within a limited range. For example, if the concentration of oxygen
remains too low relative to the concentrations of CO and HC (as a result of
continuous engine operation under rich conditions), adequate oxidation
conversion of these pollutants will not be possible. Similarly, if the
concentrations of reducing agents, such CO and HC, remain too low relative to
the concentration of NO (as a result of continuous engine operation under lean
conditions), adequate reduction conversion of NO will not be possible. The
solution to the problem which results from these conflicting requirements is to
rapidly oscillate the exhaust composition between slightly lean (slight excess of
oxygen) and slightly rich (slight excess of reducing agents) conditions.
Since exhaust composition depends on air/fuel ratio, the possibility exists
of using exhaust gas composition information to regulate fuel consumption.
Regulating fuel consumption can then maintain the air/fuel ratio within the range
that maximizes engine and converter performance. The operation of such a
closed loop "feedback" fuel control system involves:
1. generating some output signal that is dependent on the exhaust gas
composition.
2. detecting any signal change which indicates that the current air/fuel
ratio needs to be changed.
3. adjusting the fuel flow rate to result in the desired air/fuel.
2 The three-way catalyst and the oxidizing catalyst can be housed in one container or they can be
housed in two separate containers. If necessary, air can be injected upstream of the oxidizing
catalyst to ensure an adequate concentration of oxygen in the oxidizing catalyst.
31
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Formation and Control of Combustion Pollutants
The successful operation of such a system is dependent on its ability to make
effective use of the small changes in exhaust composition which occur when
air/fuel ratio deviations are held within the desirable narrow tolerance band.
The concentrations of all of the exhaust components, such as HC, CO,
NOX) and 62, undergo very gradual changes when the air/fuel ratio varies over
a very wide range. This effect is illustrated in the following table of typical
oxygen (62) and carbon monoxide (CO) concentrations when the stoichiometric
(chemically correct) ratio for a specific fuel is 14.7.
Table 4. O? & CO Concentration by Air/Fuel Ratio.
Air/Fuel Ratio
(Lbs. air/lb. fuel)
13.9 (very rich)
14.6 (rich)
14.7 (stoichiometric)
14.8 (lean)
15.3 (very lean)
02
(Volume %)
0.3
0.5
0.6
0.7
1.2
CO
(Volume %)
1.4
0.4
0.3
0.2
0.1
As this table indicates:
• the changes in ©2 and CO concentrations are very gradual when the
air/fuel ratio varies over a wide range from very rich (excess fuel) to
very lean (excess air) or vice versa.
• the changes in 02 and CO concentrations are very slight when the
air/fuel ratio deviates slightly from the stoichiometric ratio.
• when the air/fuel ratio is very rich (13.9), the concentration of 02 is still
about one half of the 02 concentration when the air/fuel ratio is
stoichiometric.
• when the air/fuel ratio is very lean (15.3), the CO concentration is still
about one third of the CO concentration when the air/fuel ratio is
stoichiometric.
A fuel control system that is dependent on such small changes in the
concentration of a particular exhaust gas component would have to meet or
exceed very demanding performance criteria in regard to:
• the amount of air/fuel ratio deviation occurring before the need for a
change in fuel flow rate is indicated, and
• the time delay between the fuel flow rate change and the elimination of
the excessive air/fuel ratio deviation.
Such system requirements would be much more demanding under the
non-steady state conditions that typify the operation of motor vehicle engines.
During the operation of such engines, fuel flow requirements are greatly
32
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Pollutant Control Approaches
complicated by the numerous and rapid changes in engine speed and load that
occur.
A more promising approach is to make use of the relative changes in the
exhaust gas components. As Figure 11 shows, the concentration of oxygen
decreases and the concentrations of reducing agents, such as hydrogen and
CO, increase as the composition of the exhaust changes from slightly lean to
slightly rich. The exhaust gas parameter which would have to be monitored
would be the shift in the lambda of the exhaust, from rich (lambda < 1) to lean
(lambda > 1), or vice versa, relative to the stoichiometric value.3
Figure 11. Exhaust Composition.
.% CO
• ° 2
.%H2
%HC
0.97
0.98
0.99
1.01
1.02
1.03
LAMBDA
3 A "lambda" value indicates how the actual proportions of oxygen and reducing agents in an
exhaust gas mixture compare with the chemically correct or stoichiometric proportions that
would be necessary for the complete reaction of the oxygen with the reducing agents. Lambda
is less than one when the exhaust composition is less than stoichiometric (rich conditions with
excessive reducing agents) and more than one when the exhaust composition is greater than
stoichiometric (lean conditions with excessive oxygen).
33
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Formation and Control of Combustion Pollutants
Closed Loop Fuel Control System—Such a "stoichiometric shift"
approach to exhaust composition control is utilized in the closed loop fuel
control system that is pictured in Figure 12. As is shown, the lambda sensor is
installed upstream of the three-way catalyst to provide a signal which is used by
the electronic control module (ECM). Depending on the "feedback" signal
which is generated by the lambda sensor, the ECM makes the changes in the
fuel flow rate necessary to keep the exhaust composition close to stoichiometric.
The control "loop" is "closed" when the sensor reacts to the change in exhaust
gas composition resulting from the change in the fuel flow rate.
Figure 12. Closed Loop Fuel Flow Control System.
AIR
ENGINE
FUEL-
LAMBDA SENSOR
3 WAY CATALYST
INJECTION
FUEL
QUANTITY
SIGNAL
AIR
QUANTITY
SIGNAL
ELECTRONIC
CONTROL
UNIT
EXHAUST
GAS
LAMBDA SIGNAL
The detecting element of the lambda sensor is a tetravalent zirconium
oxide (Zr04) ceramic thimble. This thimble is installed in the exhaust pipe so
that the interior of the thimble is in contact with the outside atmosphere and the
exterior is exposed to the exhaust gases. The zirconium oxide thimble is
impermeable to atmospheric oxygen or any gas which is in atomic or molecular
form. However, a very small amount of a lower valent oxide, such as the
trivalent yttrium oxide (¥203), is dispersed in the zirconium oxide. The yttrium
oxide provides anion vacancies which allow oxygen ions (with two negative
charges) to diffuse through the walls of the thimble. Portions of the interior and
exterior of the thimble are covered with porous platinum coatings which function
as conductive electodes and as catalyzing surfaces.
34
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Pollutant Control Approaches
In this application, the lambda sensor functions as a voltaic cell which
generates a signal voltage. This voltage is generated when the oxygen which
migrates through the sensor thimble reacts with reducing agents, primarily
hydrogen and carbon monoxide, in the exhaust. When the air/fuel mixture
ingested by the engine is lean, the exhaust contains more oxygen than
necessary for reaction with the reducing agents present. Under these
conditions, very little opportunity exists for the reaction of the reducing agents
with the oxygen migrating through the sensor thimble and the output voltage
remains at about 0.1 volt.
When the air/fuel mixture is rich, the exhaust contains less oxygen than
needed for the reaction with the reducing agents. Under these conditions, the
reducing agents can react with oxygen which migrates through the sensor
thimble and a voltage potential of about 0.9 volts is developed. The switch-like
operation of the lambda sensor is illustrated in Figure 13.
Figure 13. Lambda Sensor Operation.
800
o
E
i
UJ
cr
O
UJ
O)
0.97
0.98
0.99
1.01
1.02
1.03
LAMBDA
35
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Formation and Control of Combustion Pollutants
In the closed loop fuel flow control system shown in Figure 12, the air/fuel
ratio continuously oscillates between conditions that are slightly rich or lean
relative to the stoichiometric value. By optimizing the system, both the
magnitude of the deviations of the air/fuel ratio from the stoichiometric value and
the duration of the deviations can be reduced. Such optimization reduces HC
and CO tailpipe emissions by
• reducing the emission of the pollutants from the individual combustion
chambers as a result of the operation of the engine with an air/fuel
ratio close to stoichiometric, and
• reducing the concentrations of the pollutants in the exhaust stream as
a result of the operation of the catalytic converter at the maximum
effectiveness which results when the concentrations or oxygen and
reducing agents in the exhaust are held close to their stoichiometric
values.
4.3. Interaction Complications
The application of the above control techniques is greatly complicated by
the complex interactions that these techniques have on each other. The use of
one technique to accomplish a specific change, such as the reduction of one
type of pollutant, may result in another change which produces an undesirable
effect. For example, a change in engine operating conditions which reduces
the formation of one type of pollutant can increase the formation of another type
of pollutant. The change in engine operation may also have other undesirable
affects, such a degradation of performance, an increase in fuel consumption, or
an exhaust temperature decrease which may have an adverse effect on a
downstream aftertreatment device.
This tendency of an engine operating change to simultaneously produce
desirable and undesirable results is particularly troublesome in regard to the
hydrocarbons and nitric oxides in the exhaust. This dilemma results from the
fact that an increase in combustion efficiency tends to decrease exhaust
hydrocarbons but increase exhaust nitric oxides. The increase in combustion
efficiency results in a more complete oxidation of the fuel and, hence, a
decrease in exhaust hydrocarbons. However, the increase in combustion
efficiency results in an increase in the maximum temperature in the combustion
chamber and, hence, an increase in exhaust nitric oxide as a consequence of
the increase in the fixation of atmospheric nitrogen. Changes which decrease
combustion efficiency and, hence, decrease nitric oxide formation, such as rich
engine operation (excess fuel) and spark retardation, tend to increase the
36
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Pollutant Control Approaches
concentration of hydrocarbons in the exhaust and decrease engine efficiency
and fuel economy. In these situations, a compromise is made to provide a
balance between positive and negative effects.
37
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Formation and Control of Combustion Pollutants
38
-------
Appendixes
39
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Formation and Control of Combustion Pollutants
40
-------
Appendix 1.
Comparison of Otto Cycle and Diesel Cycle Engines
Otto cycle and Diesel cycle engines are similar in that they:
• bear the names of the German innovators who introduced the first
successful engines in the last half of the nineteenth century,
• use an intermittent internal combustion process in which the high
pressure combustion gases are directly used as the working fluid.
(External combustion engines, such as steam engines, use a
continuous combustion process to heat a separate working fluid.),
• burn liquid hydrocarbon fuels,
• incorporate reciprocating pistons, and
• use four piston strokes to complete each combustion cycle.
However, Otto cycle and Diesel cycle engines differ in regard to:
• the means by which engine speed and power output are controlled,
• the physical properties and combustion characteristics of the required
fuels,
• the manner in which the fuel is introduced into the combustion
chamber,
• the technique which is used to initiate the combustion process, and
• the mechanisms which result in the formation of combustion pollutants.
The significance of these differences between the two types of engines
are briefly discussed in the following paragraphs. This discussion excludes
engines which use superchargers or turbochargers to blow air into the engine
intake systems. Furthermore, the discussion excludes two-stroke cycle engines.
At the present time, two-stroke, gasoline-fueled, spark-ignition engines are not
used to power light-duty motor vehicles and only one engine manufacturer
produces two-stroke, diesel-fueled, compression ignition engines for use in
heavy-duty vehicles.
Speed and Power Output Control
In Otto cycle engines, air flow is the primary control parameter. The
accelerator is connected to a throttle valve which regulates the rate at which
atmospheric air enters the intake system. Prior to the flow of the air into each
combustion chamber, fuel is added in an amount resulting in an air/fuel mixture
which will produce the desired engine speed and power output.
In Diesel cycle engines, fuel flow is the primary control parameter. The
accelerator is connected to fuel injectors which inject into each combustion
chamber subsequent to the ingestion of the intake air, the amount of fuel
needed for the desired engine spped and load. The entrance of atmospheric
41
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Formation and Control of Combustion Pollutants
air into the engine intake system is uncontrolled. Consequently, when the
engine is not operating at maximum speed and power output, the amount of air
ingested is in excess of that needed for the complete combustion of the fuel.
Fuel Properties
In Otto cycle engines, the required gasoline fuels are comprised of
complex mixtures of hydrocarbon compounds. These mixtures have distillation
ranges of about 75 to 415°F. The majority of the hydrocarbon molecules in
these mixtures have carbon atom numbers that range from six to twelve in
number.
The preignition ("detonation") or knocking tendency of a gasoline fuel is
indicated by an octane number. In this rating system, normal heptane (Cy) has
an octane number of 0 and isooctane (CQ) (2,2,4-trimethyl pentane) has an
octane number of 100. An octane number close to 90 is normally adequate for
an engine which does not require a premium grade fuel.
In Diesel cycle engines, the required diesel fuels are comprised of
mixtures of hydrocarbons with distillation ranges of about 340 to about 6GO°F.
The majority of the hydrocarbon molecules have carbon atom numbers that
range from about Cu to 613.
The spontaneous ignition characteristic of a diesel fuel is indicated by its
cetane number. In this rating system, heptane (67) has a cetane number of 57
and hexadecane, or "cetane" (C-ie), has a cetane number of 100. A cetane
number in the 40 to 50 range is usually adequate.
Fuel Introduction
In Otto cycle engines, "atomized" fuel in the form of minute droplets is
added to the incoming air by means of a carburetor or one or more low pressure
injectors. Atmospheric pressure forces the resulting mixture into each
combustion chamber during the intake stroke of the cycle. The mixture is
subsequently compressed during the compression stroke.
In Diesel cycle engines, fuel is injected into each combustion chamber
(or prechamber) after the a/'rhas been ingested during the intake stroke of the
cycle and then compressed during the compression stroke. The fuel is
introduced into the chamber in the form of a spray of very small droplets. This
spray results from the use of injectors, operating at pressures of several
42
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Appendix Two
thousands of pounds per square inch, to force the fuel through very small
openings in the injector tips.
Combustion Initiation
In Otto cycle engines, the ingested air/fuel mixture undergoes a
compression of about 8 to 1 during the compression stroke of the cycle. As a
consequence, pressures of 120 to 400 pounds per square inch and
temperatures of 400 to 600°C are developed in the combustion chambers. The
increase in temperature is not great enough to cause the spontaneous ignition
of the fuel. Therefore, the combustion of the air/fuel mixture is initiated by
means of an electric spark generated between the electrodes of one or more
spark plugs. The flame then expands to fill the combustion chamber as the fuel
in the mixture is progressively oxidized.
In Diesel cycle engines, the ingested air undergoes a compression of
about 15 to 1. As a consequence, the pressure of the air is increased to about
400 to 800 pounds per square inch and its temperature is increased to 700 to
900°C. At such elevated air temperatures, the fuel undergoes spontaneous
ignition almost as soon as it emerges from the holes in the injector tips.
Pollutant Formation
In Otto cycle engines, pollutant formation problems include (a) the fuel
wetting of the walls of the intake system, (b) the introduction of rich mixtures into
the combustion chamber, and (c) the quenching of the flame after it is initiated.
Two different phases of the combustion process can affect pollutant formation.
The first phase is the premixing of the fuel and air prior to ingestion into the
combustion chamber. The second phase is the burning of the mixture within
this enclosed chamber.
At the premixing phase, one or more pollutants may be formed when (a)
the air/fuel ratio of the mixture entering the chamber deviates from the ratio
needed for complete combustion, (b) a localized deviation occurs in part of the
mixture, or (c) deviations occur between cylinders. These three phenomena are
discussed further in the following paragraphs.
The air/fuel ratio of the mixture entering the engine may deviate from the
ratio that is necessary for complete combustion of the fuel. In the most extreme
case, combustion may not occur because the mixture is either too rich
(excessive fuel) or too lean (excessive air) to burn, and raw fuel is emitted from
43
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Formation and Control of Combustion Pollutants
the exhaust system. If the mixture is burnable but on the rich side, the
combustion of the fuel will not be complete and CO emissions will be increased.
If the mixture is burnable but on the lean side, NO emissions may be increased.
The mixture that actually enters the combustion chamber may not be
homogeneous in that it varies in regard to the size and/or the distribution of the
fuel droplets. Such a lack of homogeneity may result in localized rich or lean
zones that have different combustion characteristics.
Some of the fuel droplets may separate out of the mixture as a result of
being deposited on and "wetting" the interior surfaces of the intake system. In
carburetted or single point injection systems, wetting can occur in the individual
runners of the intake manifold. In multipoint injections systems, wetting can
occur at the intake port of each cylinder. Such deposition of liquid fuel can
result in mixtures with differing air/fuel ratios being delivered to the several
cylinders in the engine. The extent of such wetting depends on a number of
variables, one of the most important being the temperature of the intake system
interior surfaces. When the amount of fuel being deposited is increasing, the
mixture entering the combustion chamber will tend to be lean. When the
amount of deposited fuel is decreasing, the mixture entering the combustion
chamber will tend to be rich. These changes in composition will tend to lead to
the formation of the emissions that are associated with each condition.
During the second phase, or burning of the mixture, undesirable
quenching phenomena can result in the emission of unburned fuel. The
expanding ball of flame can be quenched, or extinguished, as a result of the
approach of the flame front to the relatively cool walls of the combustion
chamber or a restriction which impedes further propagation. In either case, the
quenching of the flame results in the emission of unburned fuel from the
combustion chamber.
In Diesel cycle engines, the problems of the Otto cycle engine are
avoided by the direct injection of the fuel into a volume of high temperature air
and the resultant spontaneous ignition of the fuel. Consequently, Diesel cycle
engines are less subject to HC and CO emission formation than Otto cycle
engines. However, the Diesel cycle combustion process involves phenomena
which can result in the formation of NO and/or smoke emissions.
44
-------
Appendix Two
In both Otto cycle and Diesel cycle engines, NO formation is uniquely
related to HC and CO formation in that NO is not a by-product of fuel
combustion. In internal combustion engines, NO formation is a result of the
"fixation" of atmospheric nitrogen. In open type burning, such as occurs with an
oil-fired furnace burner, atmospheric nitrogen acts as an unreactive diluent gas.
However, the high temperatures and pressures that are developed in the
cylinders of the Otto cycle, and to a greater degree in the cylinders of Diesel
cycle engines, result in the disassociation of a small fraction of the atmospheric
nitrogen diatomic molecules and the oxidation of the liberated nitrogen atoms.
In Diesel cycle engines, any incomplete oxidation of the carbon atoms in
the fuel molecules tends to result in particulate emissions rather than HC and/or
CO emissions as is the case with Otto cycle engines. In diesel engines, the
continuous spontaneous combustion of the fuel minimizes the possibility of fuel
hydrocarbon molecules being completely excluded from the combustion
process, as is the case with Otto cycle engines when flame quenching occurs.
Furthermore, such continuous spontaneous combustion in cylinders which
originally ingested fuel-free air, rather than an air/fuel mixture, minimize the
possibility of the CO formation occurring when an Otto cycle is operated under
rich conditions (excess fuel). A number of factors unique to Diesel cycle
engines, such as the longer carbon chains in the fuel hydrocarbons, the higher
initial temperatures and pressures in the combustion chambers, and the
spontaneous ignition of fuel that is injected into a volume of hot air, result in
Diesel cycle engines emitting fuel carbon in the form of elemental carbon
particles, rather than in the form of unburned fuel or CO as the case with Otto
cycle engines. Under certain conditions, such as the severe overfueling of a
heavy-duty vehicle Diesel cycle engine, the emission of carbon particles
becomes visibly evident as a plume of black smoke.
45
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Appendix 2. Hydrocarbon Classes
Various distillation and extraction processes can be used to separate the
hydrocarbons in raw petroleum into fractions with specific uses. All these
hydrocarbon compounds are comprised of only two kinds of atoms, carbon and
hydrogen. Each carbon atom has four positions for attachment of hydrogen
atoms or other carbon atoms. As a consequence, the carbon atoms can be
connected together in a great many ways.
The paraffin (alkane) hydrocarbons are saturated in that their molecules
contain the maximum possible number of hydrogen atoms. The paraffin
hydrocarbon atoms can be arranged in three ways. Normal paraffins have the
carbon atoms connected to each other in straight chains (see figure).
Isoparaffins have isomeric structures in which side chains of carbon atoms are
attached to primary chains (see figure). Cycloparaffins have the carbon atoms
arranged in closed chains or rings (see figure). The three types of paraffins
comprise about 50% or more of the the hydrocarbons in gasoline and about
35% of the hydrocarbons in exhaust gases.
The alkene or olefin hydrocarbons are unsaturated in that double bonds
between two adjacent carbon atoms eliminate two sites for the attachment of
hydrogen atoms (see figure). Like the paraffins, the olefins can have branched
chain isomers. The olefins can also have the carbons atoms connected in
closed rings. The olefins are rare in "straight run" gasolines which are
produced by the simple distillation of raw petroleum. However, olefins are
present in considerable quantity in highly "cracked" gasolines which are
produced by refining processes that break or crack hydrocarbon molecules with
long carbon chains. Olefins comprise as much as 10% of the hydrocarbons in
gasoline and about 45% of the hydrocarbons in exhaust gases.
The aromatic hydrocarbons have one or more benzene rings in their
molecular configuration. The unique benzene molecule is in the form of a
closed ring in which six carbon atoms are connected by alternating single and
double bonds. Aromatic hydrocarbons are found in crude petroleum and are
the predominant hydrocarbon class in coal distillation products. Aromatic
hydrocarbons comprise about 40% of the hydrocarbons in gasoline and 20% of
the hydrocarbons in exhaust gases.
46
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\ Appendix Two
The oxygenated hydrocarbons have one or more oxygen atoms directly
attached to the carbon atoms in their molecules (see figure). The various types
of oxygenated hydrocarbons are alcohols, ethers, aldehydes, ketones,
carboxylic acids, and esters.
If the carbon atoms are connected together to form straight open chains,
the general formula is CnH2n + 2, where n is the number of carbon atoms in the
chain. If carbon atoms in side chains are present, as in the paraffin isomers or
isoparaffins, the same general formula is applicable. If the carbon atoms are
arranged in closed chains or rings, as is the case with the cycloparaffins, the
general formula is CnH2n • When there is only one double bond in the molecule,
the general formula is CnH2n.
Paraffin (Alkane) Hydrocarbons (Saturated Straight Chain)
The first five hydrocarbons in the series are shown. Each terminal
carbon atom has three attached hydrocarbon atoms and each interior carbon
atom has two attached hydrocarbon atoms. In their pure isolated state, the first
four members of the series; methane, ethane, propane, and butane, are in a
gaseous state at standard temperature and pressure conditions.
1 H H H H H
H—C—H H-C-d-H H-C-C-C-H
II H A H H ^
Ethane Propane
Methane
H H H H H H H H H
H-C-ti-C-C-H H-(!-C-t-C-C-H
i i i i i i i i i
HHHH H H H H H
Butane Pentane
Isoparaffin (Alkane) Hydrocarbons (Saturated, Branched Chain)
Three different isomers are shown which have branched chains and at
least one carbon atom in the molecule is attached to three other carbon atoms.
Isobutane, the first member in the series, shows that a minimum of four carbon
47
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Formation and Control of Combustion Pollutants
atoms is necessary for chain branching. The two octane isomers show how
configurations can vary when large molecules are involved.
H H
" V V "M « V F P « H-C-H H-C-H
H-C-C-C-C-C-C-C-H
H-C-C-C-H
i
H
H
H-C-H
i
H
Isobutane (2-melhyl propane)
i
H
H
H
H
H-C-H
i
H
Isooctane (2-rnethyl heptane)
H
H
H
H-C— C— C-C— C— H
C
I
H
H-C-H
H
Isooctane (2,2,4-trimethyl pentane)
Cycloparaffin (Alkane) Hydrocarbons (Saturated, Closed Ring)
Two closed ring cyclohydrocarbon molecules are shown. The
cyclopropane molecule shows that a minimum of three carbon atoms are
necessary. The methylcyclobutane molecule shows how another structure,
such as a straight chain, can be attached to a closed ring.
H-rj-H
,. ' 'H
'
M
H, /*\ H
C C
H' ]\ H'
Cyclopropane Methylcyclobutane
Olefin (Alkene) Hydrocarbons (Unsaturated)
Four alkene Hydrocarbon molecules which are "unsaturated" in that
double bonding between two adjacent carbon atoms eliminates two hydrogen
atoms from the molecule are shown. The two different butene molecules show
how the location of the double bond can occupy different locations within a
molecule. The butadiene molecule shows how a molecule can contain more
than one double bond between carbon atoms.
48
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Appendix Two
H H H H
H-C=C-C-C-H
H H H H
Ethene Butene-1
H H H H
H-C-C=C-C-H H H H H
H H H-C=C-C=C-H
Butene-2 Butadiene
Hydrocarbons with Six Carbon Atoms
Five of the ways are shown in which six carbon atoms can be arranged in
isomeric molecular configurations. The benzene molecule, with its combination
of single and double bonds, is a unique configuration. It has distinctive
characteristics, such as an aromatic odor and a tendency to burn with a smoky-
flame.
H H H H H H H H H H H H
H-C-C-C-C-C-C-H H-C=(i-C-
-------
Formation and Control of Combustion Pollutants
Oxygenated Hydrocarbons
Two ways are shown in which oxygen atoms can be directly attached to
the carbon atoms in a chain.
The alcohols have a hydroxyl (-OH) functional group attached to a
terminal or intermediate carbon atom in the chain. The two simplest alcohols,
methanol ("wood alcohol") and ethanol ("grain alcohol"), are shown in the
figure.
Alcohols
H H H
H-t-OH H-i-t-OH
A A A
Methanol Ethanol
The ethers have an oxygen atom which serves as the connecting link
between two carbon chains. The two simplest ethers, dimethyl ether and methyl
ethyl ether are shown in the figure.
Ethers
H H H H H
H-i-0-t-H H-C-0-i-C-H
A A A A A
Dimethyl ether Methy, e%| ether
In some molecules, an oxygen atom replaces two hydrogen atoms by
double bonding with a carbon atom to form a carbonyl (C=O) functional group.
When the carbonyl group is attached to a terminal carbon atom at the end of a
chain, the resulting compound is called an aldehyde. When the carbonyl group
is attached to a carbon atom that is connected to two other carbon atoms, the
resulting compound is called a ketone. The simplest forms of aldehydes and
ketones are shown in the figure.
50
-------
Appendix Two
Aldehydes
H H H H H H HOHH
C=0 H-£-C=0 H-i-C-i-H
A A A A A A A
Formaldehyde Acetaldehyde Acetone Methy ethyl ketone
The carboxylic acids can be considered to be hybrids of alcohols and
aldehydes in that a terminal carbon atom has a carbonyl group and a double
bonded oxygen atom attached to it. The two simplest acids, formic acid and
acetic acid, are shown in the figure.
Carboxvlic Acids
H 0
jj H-i-C-OH
H-C-OH A h
Formic Acid Acetic Acid
An ester has two carbon chains joined by a oxygen atom and one of the
carbon atoms attached to the oxygen atom also has a double bonded oxygen
attached to it. An ester can be synthesized by combining a molecule of an
carboxylic acid and a molecule of an alcohol in a manner which results in the
liberation of a molecule of water. In this respect, an ester can be considered to
be the organic equivalent of an inorganic salt, such as sodium chloride (NaCI)
which is formed when an acid, such as hydrochloric acid (HCI) reacts with a
base, such as sodium hydroxide (NaOH). Consequently, the methyl formate
molecule in the figure can be considered to be the end product when a
methanol molecule and formic acid molecule combine. The ethyl acetate
molecule in the figure can be considered to be the end product when an
ethanol molecule and an acetic acid molecule combine.
Esters
SB 5 v B v
H-C-O-C-OH H_£_r-o-c-C-H
A A A A
Methyl Formate Ethyl Acetate
51
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Index
52
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