Formation and Control of Combustion Pollutants

in the proper proportions. The appropriate ratio of air to fuel is determined
by:

¢ 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:

145 1b air 107IbO, + 2331bO, + 11.11bN2
101b fuel = 013IbHo + O87JbC
15.5]b exhaust = 1.20IbH,O + 3.20IbCO,+ 1111bN;

However, even when the fuel is supplied at a rate that will
theoretically result in chemically correct proportions of air and fuel,
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:

16


Pollutant Control Approaches

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 of
oxygen and reducing agents in the exhaust are held close to their
stoichiometric values.

Figure 13. _ Lambda Sensor Operation.

800

_

700 —S

a

600

500

RICH <— > LEAN

300

SENSOR VOLTAGE - millivolts

200

100
0.97 0.98 0.99 1 1.01 1.02 1.03

LAMBDA

4.3. Interaction Complications

The application of the preceding 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

37


Pollutant Control Approaches

lean (slight excess of oxygen) and slightly rich (slight excess of reducing
agents) conditions.

Since exhaust composition depends on the composition of the ingested
mixture or the air/fuel ratio, the possibility exists of using exhaust gas
composition information to regulate fuel introduction. Regulating fuel
introduction 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 change in that signal indicating that the air/fuel ratio
needs to be adjusted.

3. adjusting the fuel flow rate to result in the desired air/fuel ratio.

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 Oz, 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 (O2) and carbon monoxide (CO) concentrations when the
stoichiometric (chemically correct) ratio for a specific fuel is 14.7.

Table 4. O2 & CO Concentration by Air/Fuel Ratio.

- Air/Fuel Ratio Qr co
(Lbs. air/lb. fuel) (Volume %) (Volume %)
13.9 (very rich) 0.3 14
14.6 (rich) 0.5 0.4
14.7 (stoichiometric) 0.6 0.3
14.8 (lean) 0.7 0.2
15.3 (very lean) 1.2 0.1

As this table indicates:

¢ the changes in O2 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 O2 and CO concentrations are very slight when the
air/fuel ratio deviates slightly from the stoichiometric ratio.

33


Formation and Control of Combustion Pollutants

air/fuel mixture which will produce the desired engine speed and power
output.

In compression-ignition 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 speed and load. The entrance of
atmospheric 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 spark-ignition 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 (C7)
has an octane number of 0 and isooctane (Cg) (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 compression-ignition engines, the required diesel fuels are
comprised of mixtures of hydrocarbons with distillation ranges of about 340 to
about 660°F. The majority of the hydrocarbon molecules have carbon atom
numbers that range from about Cy4 to Cig.

The spontaneous ignition characteristic of a diesel fuel is indicated by
its cetane number. In this rating system, heptane (C7) has a cetane number of
57 and hexadecane, or "cetane” (Cj¢), has a cetane number of 100. A cetane
number in the 40 to 50 range is usually adequate.

Fuel Introduction

In spark-ignition 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

42


Formation and Control of Combustion Pollutants

* molecules of nitric oxide (NO) which were formed when the
atmospheric nitrogen was subjected to the unique conditions
existing in the engine's combustion chambers.

¢ 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:
Fuel (HC) + Air (O2 & Nz) =
Energy + CO, + H.0 + N2 +H, +HC+CO+NO

The formation and control of the pollutants resulting from these imperfect
combustion conditions are discussed in the following chapters of this’

synopsis.

10


Formation and Control of Combustion Pollutants

Idehydi Ketones

H HH HOH H OHH
t= -t-ts -t- b-¢- -t-¢-¢-¢-
=a He 0 He a H-f-E- E61

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.

Carboxylic Acid:
HQ
0 H-¢-C-0H
H-C-OH
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
(NaCl) 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

ae: Hy 9
n-t-o-C-on HEE oe C-C-H
A A A

Methyl! Formate Ethyl Acetate

52


Formation and Control of Combustion Pollutants

a.

/ wi H
{

hE

Ethene Butene-1

HHA
H-¢-C=C-C-H HHH
h # os t-C=¢-H

Butene-2 , Butadiene

o-=
O-=
=-0-=
=-o-=
=

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 ne flame.

lsohexane-2 Cyclohexane Benzene


Formation and Control of Combustion Pollutants

The oxygenated organic compounds have one or more oxygen atoms
directly attached to the carbon atoms in their molecules (see figure). The
various types of oxygenated organic compounds 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 C,H2n + 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 C,Hz_ . When there is only one double
bond in the molecule, the general formula is Cy, Hon .

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.

q HHH
nt W-t-t-H H-¢-C-¢-H

I hk |O AA

HH HH HHH HH
u-¢-¢-¢-¢-n =o -¢-¢-¢-¢-¢-n
HH AA HHA HA

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


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 nitric oxide (NO). 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 fuel combustion
co Inadequate oxygen . Incomplete fuel oxidation
NO Excessive temperature Nitrogen "fixation"

As Table 3 indicates, HC emissions result from portions of the air/fuel
mixture being 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 oxidation of the
molecules. Some of these 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 organic molecular structure and
are not fully oxidized to carbon dioxide (CO2) and water (HO).

When fuel hydrocarbon molecules completely react with the oxygen
molecules during the combustion process, the carbon atoms are oxidized to
carbon dioxide (CO2) and the hydrocarbon atoms are oxidized to water vapor.
However, a small fraction of the fuel molecules undergo less complete
conversions and different results are obtained. The carbon atoms are only
partially oxidized to carbon monoxide (CO). Instead of hydrogen atoms being
oxidized, they associate with each other to form diatomic hydrogen
molecules.

NO emissions are unique relative to CO and HC emissions in that NO
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 atmospheric nitrogen and oxygen

20


Formation and Control of Combustion Pollutants

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 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.

38


Introduction

The pollutant formation and control information provided in this
synopsis is specifically applicable to gasoline-powered, four-stroke,
reciprocating piston, spark-ignition 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 spark-ignition 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 ~

e 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 én 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 needed 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 that—

¢ 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),


Pollutant Control Approaches

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 present, passage through a "two-way" oxidation catalytic converter will
result in a significant fraction of the CO molecules being oxidized to CO2
molecules and the HC molecules being oxidized to CO2 and water (H20)
molecules .

The nitrogen atoms in the combustion ‘gases are in a polluting form
when they exist as nitric oxide (NO) molecules. When sufficient reducing
agents are present, passage through a reducing catalyst will result in a
significant fraction of the nitric oxide molecules 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/reducing 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 CO 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+2H20
2NO+2CO = N2+2COQ2
4NO+CH,g! = 2N2 +CO) +2H2,0

In connection with the above reactions, the following facts are worth
noting:

1 Where the methane molecule, CH,, represents all possible HC molecules.

31


Pollutant Formation—General Effects

* 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 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 in the intake
system. The decrease in pressure allows atmospheric pressure to force air into
the intake system via the air cleanerand 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 can be 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 multipoint 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


Pollutant Formation—Specific Effects

produced cause some "fixing" of atmospheric nitrogen. A small percentage of
the nitrogen molecules are "fixed" when they combine with oxygen in the
combustion chamber. The primary immediate reaction product is nitric oxide
(NO) which can undergo conversion into other oxides, such as nitrogen
dioxide (NO), during transit through the exhaust system or during
subsequent dispersion into the atmosphere.


Pollutant Formation—General Effects

The use of a pure hydrocarbon compound, such as isooctane, as a fuel
in lieu of gasoline will reveal the effectiveness of the combustion process in
promoting complex chemical reactions. When the exhaust of an engine
using such a pure fuel is analyzed, hundreds of different hydrocarbon
molecules can be identified. This complex variety reveals how extensively
the original isooctane molecules were chemically altered during their passage
through the combustion chamber.


Formation and Control of Combustion Pollutants

¢ all of the NO reduction conversions depend on the presence of
reducing agents, such as Hp, CO, and HC.

¢ the last two reactions result in the elimination of all three
pollutants, NO, CO, and HC.

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,

¢ 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 alone does not adequately lower the
concentrations of CO and HC, a downstream oxidizing catalyst (see
Figure 10) can be added 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 inhibits the 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

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.

32


Formation and Control of Combustion Pollutants

By varying the operating conditions of the test engine, two different
values, the research octane number (RON) and the motor octane number
(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 (MON8°8),

A typical gasoline sample will contain different types of hydrocarbons
with molecular carbon atoms predominately in the Cg to C12 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.

4


Formation and Control of Combustion Pollutants

As shown in Figure 2, the several inlet ports of the head ate 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.

Manifold : = __ 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 | Piston Stroke
Air/fuel mixture is ingested : Intake
Mixture is compressed Compression
Mixture is burned Power
Combustion products are expelled Exhaust

This "Otto cycle" or spark-ignition process is both similar and different
from that of a compression-ignition process . 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, four-stroke, spark-ignited motor vehicle
engine.


Pollutant Control Approaches

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 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

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 catalyst but upstream of an oxidation catalyst, to
promote catalytic oxidation reactions. In such systems a positive pressure

29


Formation and Control of Combustion Pollutants

retarding the spark. This change reduces the maximum temperature by
shortening the time available for 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 back flow —
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

26


pittoduction

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
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 spark-ignition
engine characteristics are briefly contrasted with those of compression-
ignition 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 organic compounds
produced when some fuel molecules pass through the combustion chamber
of a spark-ignition engine without the complete liberation of the molecular .
carbon atoms. :


Formation and Control of Combustion Pollutants

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 the various cause and effect events accounting for the presence of
hydrocarbons (HC), carbon monoxide (CO), and nitric oxide (NO) in the
exhaust. This discussion stresses the following:

¢ During the combustion process, most of the hydrogen and carbon
atoms in the fuel hydrocarbon molecules are separated from each
other and oxidized to water vapor (H2O) and carbon dioxide (COQ2),
respectively.

¢ Asmall fraction of the fuel molecules undergo less complete

. reactions which result in the carbon atoms being partially oxidized

to carbon monoxide and the hydrogen atoms associating with each
other to form diatomic hydrogen molecules.

¢ Another small fraction of the fuel molecules are more or less
excluded from the combustion process, primarily as a result of
flame quenching. These molecules pass through the combustion
chamber either completely unaltered, restructured in some way, or
converted into various forms of oxygenated organic compounds.

¢ NO 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 NO, and the subsequent conversion of the NO to
other nitrogen oxides, such as nitrogen dioxide (NO2), when the
exhaust gases are released to the atmosphere.

In a comparable way, Chapter 4 discusses pollution control by
associating several techniques presently used 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, minimizes 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 maximize the oxidation of the fuel HC molecules. Exhaust gas
recirculation is discussed as an NO preventive technique which limits the


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 a

spark-ignition 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

2.
3.

the continuous escape of hydrocarbon molecules from the liquid
surface. .

the heating of these molecules to their kindling temperature.

the reaction of these molecules with oxygen (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.

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.

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

15


Chapter 1. Gasoline-Fueled Spark-ignition Engines

This synopsis focuses specifically on gasoline-powered, spark-ignition
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 ~N opust Valve

Intake Valve
Head
Intake_\ Exhaust
Port’ Port
Head Gasket
Cylinder Block

Combustion
Chamber

Piston

Rings

Flywheel

Crank and Crankshaft

Connecting Rod


Pollutant Formation—Specific Effects

molecules react to form NO molecules which are subsequently converted to
other nitrogen oxides, such as nitrogen dioxide (NO2).

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 spark-ignition engine operates under
favorable conditions; i.e., close to a stoichiometric air/fuel ratio and with an
absence of malfunctions such as ignition misfire, the primary source of
hydrocarbons in the exhaust is the fuel molecules which were not adequately
involved in the combustion process. Some of these molecules may 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 asa 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.

Aired B+ S Wall Quenching
we / ey Crevis Quenching
Flame Front

21


Formation and Control
of
Combustion Pollutants
from
Gasoline-Fueled Spark-Ignition
Motor Vehicle Engines

John W. Bozek
November 1990
Revised November 1992

Notice

The purpose of this document is to provide simple explanations regarding
the formation and control of combustion pollutants from gasoline-fueled
spark-ignition 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.

Credits

Editor: Kathy E. Carter
Illustrator: Frank Lamitola

EPA-AA-CD-CPSB-90-01
Certification Division
Office of Mobile Source Air Pollution Control
U.S. Environmental Protection Agency


Formation and Control of Combustion Pollutants

The premixing phase can result in the subsequent formation of one or
more pollutants as a consequence of: (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 the maximum 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 the exhaust system. If the mixture is burnable but on the rich
side, the combustion of the fuel will be decreased 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


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


Gasoline-Fueled Spark-ignition Engine

Figure 3, Spark-ignition Phases.

Intake Valve Both Valves

Open —~___ p Closed

Air/Fuel
Mixture

@ INTAKE STROKE (@)COMPRESSION STROKE

Spark
Initiated
Flame

©@ POWER STROKE (EXHAUST STROKE

When a naturally aspirated (unturbocharged or unsupercharged)
spark-ignition 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


Formation and Control of Combustion Pollutants


Formation and Control of Combustion Pollutants

preclude such 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 during cold starting 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 oxidation conversion.

Figure 8, External Exhaust Gas Recirculation.

Exhaust Manifold

EGR Valve

Control Signal

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 CO and HC in the exhaust gas. The necessary
additional oxygen can be introduced by means of a simple aspiration system

28


Appendix One

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 compression-ignition engines, the problems of the spark-ignition
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, compression-ignition engines are less subject to HC and CO
emission formation than spark-ignition engines. However, the compression-
ignition combustion process involves phenomena which can result in the
formation of NO and/or smoke emissions.

In both spark-ignition and compression-ignition engines, NO
formation is quite different than 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
atmospheric nitrogen acts as an unreactive diluent gas. However, the high
temperatures and pressures that are developed in the cylinders of the spark-
ignition, and to a greater degree in the cylinders of compression-ignition
engines, result in the oxidation of the atmospheric nitrogen.

In compression-ignition 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 spark-ignition
engines. In compression-ignition 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 spark-ignition engines when flame quenching occurs.
Furthermore, such continuous spontaneous combustion in cylinders of a
compression-ignition engine which originally ingested fuel-free air, rather
than an air/fuel mixture, eliminates CO formation occurring when an spark-
ignition engine is operated under rich conditions (excess fuel). A number of
factors unique to compression-ignition 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 compression-ignition engines

45


Pollutant Control Approaches

in some reduction in the maximum combustion temperature and, hence,
some reduction in NO formation. ;

Figure 7. Internal Exhaust Gas Recirculation (Valve Overlap).

When such inherent exhaust gas recirculation does not provide the
desired reduction in NO 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 NO while also minimizing any undesirable effects on
performance characteristics, such as a poor idle quality or reduced
performance during wide open throttle acceleration.

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 oxidize HC and CO
pollutants. The second oxidizes HC and CO and reduces NO pollutants.
4.2.1. Supplementary Air Oxidation

As discussed previously under preventive techniques, operating an
engine under very lean conditions can reduce CO but a number of factors

27


Formation and Control of Combustion Pollutants

Blank page


Formation and Control of Combustion Pollutants

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 factors are the locations of the spark plugs and the valves 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 conditions under which combustion occurs in
an internal combustion engine, fuel hydrocarbon molecules which are not
completely changed by the oxidation process are partially altered in various
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 which occur in the
combustion chamber, the relative proportion of paraffinic and aromatic
compounds is reduced, the relative proportion of olefinic compounds is
increased, and new oxygenated organic compounds are formed.

18


Pollutant Control Approaches

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 11. Exhaust Composition.

2
iS
3 CO
E
< a o5
2 + % He
9 »
8 a % HC
0 [ e ose od
097 0.98 0.99 1 1.01 1.02 1.03

LAMBDA

The detecting element of the lambda sensor is a tetravalent zirconium
oxide (ZrO4) 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 (Y2O3), 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 electrodes and as catalyzing
surfaces.

35


Formation and Control of Combustion Pollutants

Figure 12. _ Closed Loop Fuel Flow Control System.

AIR. | AIR FLOW |
IRELO ENGINE SWAY CATALYST = >

T LAMBDA SENSOR
! ' EXHAUST
1 ‘ GAS
: ‘
‘ 1
‘ 1 a
7 t

FUEL +>] INJECTION !
' K '
‘ 1 FUEL '
‘ 1 QUANTITY |
1 1 SIGNAL 1
t ! '
t '
t '
1 AIR
“quantity, ___,| ELECTRONIC 1

SIGNAL CONTROL aDA SiG N

UNIT LAMBDA SIGNAL

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.

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


Appendix Two

Oxygenated Organic Compounds

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
ul HH
H-¢-oH = H-¢-¢-08
a) | ha

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 Hu
H-t-0-€-H = y-t-o-¢-¢-n
ot i aes
Dimethyl ether Methyl ethyl 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 jis 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.

51


Chapter 4. Pollutant Control Approaches

Two general approaches for minimizing 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 minimize the formation of pollutants in
the combustion chamber, and

¢ the remedial approach which involves the treatment of the exhaust
gas to minimize 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 Minimization

Flame quenching results in some fuel hydrocarbon molecules exiting
unaltered, or partially altered, 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 design
changes that reduce 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.

24


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. Mcintyre,
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

iii


Appendix One

into each combustion chamber during the intake stroke of the cycle. The
mixture is subsequently compressed during the compression stroke.

In compression-ignition engines, fuel is injected into each combustion
chamber (or prechamber) after the air has 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 thousands of pounds per square inch, to force the fuel through very
small openings in the injector tips.

Combustion Initiation

In spark-ignition engines, the ingested air/fuel mixture undergoes a
compression of about 8 to 1 during the compression stroke of the cycle. Asa
consequence, pressures of 120 to 400 pounds per square inch and temperatures
of around 900 °F 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 compression-ignition 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 about 1400°F. 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 spark-ignition 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.

43°


Appendix 1.
Comparison of Four-Stroke
Spark-ignition and Compression-ignition Engines

Four-stroke spark-ignition and compression-ignition engines are
similar in that they:

© 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 héat a separate working fluid.),

¢ burn liquid hydrocarbon fuels,
¢ ' incorporate reciprocating pistons, and

¢ use four piston strokes to complete each combustion cycle.

However, these two types of 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 spark-ignition 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

41


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 oxidation of these various compounds.

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.

nen nt

A HH H
H-c-C-C -¢ -H Ht iy

i A atl I
H

Butane lsooctane (2,2,4-trimethy! 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

u


Pollutant Formation—General Effects

induced flame 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.

Table 2. Hydrocarbon Compounds and Properties.

Compound Carbon Physical Boiling Auto Research
Atoms State Point ignition Octane
(°F) Temperature Number
(°F)
Methane 1 Gas -259 1,346 110!
Ethane 2 7 -127 1,050 1041
Propane 3 " -44 995 100
Butane 4 . 31 961 92
Pentane 5 Liquid 97 933 61
Hexane 6 " 155 909 25
Heptane 7 " 209 893, 0
Octane 8 " 258 880 -17
Nonane 9 "os 303 871 -45,
Decane 10 _ 345 866 _
Undecane 11 . 384 _ _
Dodecane 12 Solid 4212 — _
Benzene 6 Liquid 177 1,363 110
Isooctane3 8 " 211 1350 | 100
~ 1 Estimated 2 Melting point is 14°F 3 2,2,4-trimethyl pentane

The octane number of a compound is an indication of its knocking
tendency, as can be seen in the preceding 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 gasoline 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.

13


Figures

Figure 1. Engine Block and Head (cross-section)..........cssssssssscsseessnnnneeeseessen
Figure 2. Engine Block and Manifolds.
Figure 3. Spark-ignition Phases............
Figure 4. Molecular Structure of Selected Hydrocarbons.
Figure 5. Flame Quenching.
Figure 6. Surface/Volume Ratio
Figure 7.
Figure 8.
Figure 9. Aspirated Air Injection...
Figure 10. Forced Air Injection.
Figure 11. Exhaust Composition...
Figure 12. Closed Loop Fuel Flow Control System.

Tables

Table 1. Relationship of Combustion Phase and Piston Stroke aa)
Table 2. Hydrocarbon Compounds and Properties.
Table 3. Exhaust Pollutants........sccsssessssseessesseeesenes
Table 4. O2 & CO Concentration by Air/Fuel Ratio.

ii


Formation and Control of Combustion Pollutants

¢ when the air/fuel ratio is very rich (13.9), the concentration of Oy is
still about one half of the O2 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 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

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

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 reacttion 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 wihen the exhaust composition
is greater than stoichiometric (lean conditions with excessive oxygen).

34


Formation and Control of Combustion Pollutants

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 (see Figure
5).

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 molecules appear in the
exhaust in form of aldehydes and other partially oxidized compounds (see
Appendix 2. Oxygenated Organic Compounds.).

3.2. Effect of Oxygen Content on CO Formation

As was previously mentioned, some fuel hydrocarbon molecules do
not completely react with oxygen molecules during the combustion process
and the carbon atoms are only partially oxidized to carbon monoxide. The
likelihood of such results occurring decreases as the availability of oxygen
increases. However, even when relative proportions of oxygen and fuel
molecules are considerably in excess of the stoichiometric, or "chemically
correct” proportion, a very small fraction of the fuel carbon atoms are still
only partially oxidized to carbon monoxide molecules.

3.3. Effect of Temperature on NO Formation

Unlike HC and CO emissions, NO emissions do not arise as a
consequence of the incomplete oxidation of the fuel. NO 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

22


Contents

Acknowledgments....

Introduction
Chapter 1. Gasoline-fueled Spark-ignition Engines.........scccccssssssssssssesnneses 5

Chapter 2. Pollutant Formation—General Effects...
2.1, Fuel Complexity
2.2. Combustion Complexity

Chapter 3. Pollutant Formation—Specific Effects.
3.1. Effect of Flame Quenching. ;
3.2. Effect of Oxygen Content on CO Formation.
3.3. Effect of Temperature on NO Formation...

Chapter 4. Pollutant Control Approaches
4.1. Preventive Techniques.

4.1.1. Combustion Chamber Redesign for HC

Minimization

4.1.2. Fuel Control for CO Minimization.

4.1.3. Temperature Control for NO Minimization

4.2. Remedial Techniques ...........s:ssceee

4.2.1. Supplementary Air Oxidation .

4.2.2. Catalytic Conversion....... +30

4.3. Interaction Complications

Appendixes... secssecssessesrnsesersneeccnnneeenneeess
Appendix 1. Comparison of Four-stroke Spark-ignition and
Compression-ignition Engines...
Speed and Power Output Control...
Fuel Properties......
Fuel Introduction.
Combustion Initiatio
Pollutant Formation...
Appendix 2. Hydrocarbon Classes ..
Paraffin (Alkane) Hydrocarbons.
Isoparaffin (Alkane) Hydrocarbons.
Cycloparaffin (Alkane) Hydrocarbons
Olefin (Alkene) Hydrocarbons........
Hydrocarbons with Six Carbon Atoms...
Oxygenated Organic Compounds...



Formation and Control of Combustion Pollutants

emitting fuel carbon in the form of elemental carbon particles, rather than in
the form of unburned fuel or CO as is the case with spark-ignition engines.
Under certain conditions, such as the severe overfueling of a heavy-duty
compression-ignition engine, the emission of carbon particles becomes visibly
evident as a plume of black smoke.


Formation and Control of Combustion Pollutants

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 warmup 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.

Figure 10. Forced Air Injection.

Intake

Manifold Exhaust Manifold

Diverter
Three-wa}
Filter Air Pump Valve Catalyst

Oxidation
Catalyst

4.2.2. Catalytic Conversion

Another aftertreatment of the exhaust gases, catalytic conversion,
involves promoting chemical reactions which involve the CO, HC, and NO
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

30


Appendixes

39


Pollutant Control Approaches

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 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-f- O | [J
66 6.4

SV 80 72

4.1.2. Fuel Control for CO Minimization

As was mentioned in section "3.2. Effect of Oxygen Content on CO
Formation,” an increase in the availability of oxygen tends to decrease the
number of fuel carbon atoms that are only partially oxidized to carbon
monoxide. Such increase in oxygen availability can be accomplished by
reducing fuel input to increase the air-fuel ratio beyond the stoichiometric
ratio so that combustion occurs under lean conditions.

However, such a lean approach to minimization 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 NO Minimization (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


Gasoline-Fueled Spark-ignition Engine

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 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 (CO2) (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).

¢ molecules of nitrogen (N2) (originally in the ingested air and not
involved in the combustion process).
This cari be represented by the following equation:
Fuel (HC) + Air (O2 & Nz) = Energy + CO, + H20 + 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 (Hz) which result when fuel hydrogen atoms
* are not oxidized and associate to form diatomic molecules.

¢ 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.


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 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 carbon atoms
connected in closed rings. The olefins are rare in "straight run" gasoline
which are produced by the simple distillation of raw petroleum. However,
olefins are present in considerable quantity in highly "cracked" gasoline
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.

47


Appendix Two

carbon atoms is necessary for chain branching. The two octane isomers show
how configurations can vary when large molecules are involved.

H H
HHH HHH HH WH H-¢-H HCH
H-C-C-C-H H-C-C-C-C-C-C-C-H | Hl
H | 4a H | HHa Wd ! Hy]
H-C—C—C-cC—c-4
H-C-H H-C-H H rm |
1 1
H H H-C-H H
u
Isobutane (2-methyl propane) Isooctane (2-methy! heptane) lsooctane (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-C-H
I H
H-C—C\,

H of Hl a

AY

H’ y H” 6H
Cyclopropane Methyicyclobutane

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.

49


Formation and Control of Combustion Pollutants

ring with single and double bonds 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 (CH3OH) and ethanol
(CH3CH2OH), can be used alone as engine fuels or mixed with gasoline to
form oxygenated fuels. When aldehydes and ketones are formed during the
combustion of the fuel compound, a carbon atom is double bonded to one
oxygen atom in lieu of being single bonded to two hydrogen atoms.

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 auto ignition
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 auto ignition
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 auto
ignition temperature. The rearrangement of the eight straight-chain carbon
atoms in the paraffinic octane molecule into the branched-chain isooctane
configuration (see Figure 4) results in about a 18% decrease in boiling point,
but about a 53% increase in auto ignition temperature.

Auto ignition 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-

12