ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MOBILE SOURCE POLLUTION pps

701
MOBILE SOURCE POLLUTION
Mobile sources, alternatively called transportation or vehicular
sources, include cars, trucks, buses, ships and various aircraft.
Air pollutants emitted will vary, depending on the fuel being
combusted or reacted (in the case of fuel cells or batteries) and
the engine design of each vehicle.
THE AUTOMOBILE
The automobile’s discovery appears to satisfactorily com-
bine a human desire for rapid transportation with the desire
for independence and fl exibility. However, the increasing
vehicle population poses a series of threats to continued
physical and psychological well-being and to the future of
our environment.
As the automotive industry expands, other auxiliary
industries such as petroleum production and concrete and
tire manufacturing also grow, with additional potential for
pollution problems. Additional roads also have to be built,
with a negative impact on both ecology and landscape. A bal-
ance must be made between the right of an individual to use
his own car when and where he drives and the harm brought
upon society as a whole by his doing so. If it is accepted that
society needs to be protected, a number of legislative and
economic measures can be initiated to discourage automo-
tive usage.
Legislation has been enacted to limit the emission of HC
(hydrocarbons), NO


x

(oxides of nitrogen), CO (carbon mon-
oxide) and particulate matter including lead compounds.
However, other waste materials such as tires and the auto-
mobile itself (including the repugnant abandoned cars) must
be disposed of.
The broad approach to automotive pollution control is to
encourage alternative means of transportation. This would
include improvements in mass transit such as high-speed
trains, moving sidewalks, increased links and modernization.
In the city, bicycle riding and walking are low pollution, high
exercise alternatives; in rural areas car pools might be formed.
Cars have been built using different energy concepts
(ex. battery, turbine engine, sterling engine) and to run on
different fuels (ex. natural gas, alcohol). Special control
devices (ex. catalytic reactors, afterburners) placed at the
exhaust of internal combustion engines are the primary
means of reducing emissions of various materials. The total
solution to the problem will probably combine technical
and strategic methods.
The US EPA and US DOE produce the Fuel Economy
Guide to help car buyers choose from the best fuel gallons
per mile (mpg) ratings for both city and highway traffi c
modes. A pdf version may be found at />cgibin/epaprintonly.cgi
1

In the present discussion, we shall concentrate on the
major moving source of pollutants, the internal combus-
tion engine. In the automobile, evaporative losses of pol-
lutants occur from the fuel tank and carburetor (ca. 5%),

fumes from the crankcase (ca. 20%) and the exhaust system
(ca. 75%). The major offenders are unburned hydrocarbons,
carbon monoxide, nitrogen oxides and HC oxidants. Positive
Crankcase Ventilation (PCV) on all modern cars reduces the
emissions by pulling air and fumes into the engine by main-
taining a vacuum at the engine. Of course a highly effi cient
combustion process will eliminate the partially oxidized
substances. One reason for incomplete combustion is that as
the “fl ame front” generated from the spark moves toward the
relatively cool cylinder walls, a quenching action takes place
preventing further reaction, the kinetics of which are very
temperature sensitive. Other factors infl uencing incomplete
combustion are improper dilution by poor cycle timing and
less than the proper excess of oxygen admitted at the carbu-
retor. Standard engine cycles have been developed so that
pollutant guidelines might be drawn. The Federal Driving
Cycle initiated in 1972 in considered the standard of vehi-
cle testing. A federal short cycle Table 1 has been used for
convenience in some instances. Measured emission values
averaged for various test sites in representative US cities are
presented in Table 2 for steady state at various miles per hour
(mph) and for the short cycle. Carbon monoxide and NO


x



levels increase as the mph level increases. The federal 1975
proposed standards for CO, hydrocarbons and NO

x


are 11,
0.5 and 0.9 grams per mile, respectively, and 0.5%, 40 ppm
and 225 ppm, respectively. Their extent of enforcement is
increasing as of the early 1990s.
The CO standard is based primarily on tests which have
shown that the capacity of blood to carry oxygen is hampered
by CO absorption.
2
Brain function is retarded after an expo-
sure of 10 to 15 ppm CO for several hours. The HC and NO


x



levels are mainly based on their role in photochemical smog
using ambient levels. Shy et al.
3
found in their Chattanooga
study that “NO
2
alone and exposure to suspended particulate
matter alone appear to be the most probable explanation for
the observed excess in respiratory illness rates.” The data

are not completely convincing as pointed out by automobile
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702 MOBILE SOURCE POLLUTION
industry researchers.
4
However, the U.S. government took
the initiative for the welfare of the public in not waiting for
all the details to be perfectly established. Certainly there are
those among us (aged, with heart conditions, etc.) who will
be more severely affected than the average by typical ambi-
ent pollutant concentrations and will need protection.
At least two approaches to the removal of internal
combustion engine pollutants have come into wide accep-
tance. One is the improvement of the combustion process
itself. This includes reducing manifold vacuum (and hence
dilution of charge) between exhaust and intake steps;
increasing the cylinder wall temperature; and designing
for less maximum horsepower by minimizing the surface
to volume ratio.


The other approach is controlling exhaust emissions
by further reaction, either after-burning or catalytic. After-
burning can be accomplished slightly downstream from the
exhaust valve by additional oxygen injection converting HC
and CO to CO
2
and water vapor. The after-burner has the dis-
advantage of not being able to remove NO

x


. In fact, because
of its high temperature, still more NO


x


is formed. The cata-
lytic approach is to fi nd a catalyst or set of catalysts and
temperature or set of temperatures which will completely
oxidize both CO and HC, but also will reduce the NO


x


. These
two approaches will be discussed in detail below.
Meteorological and vehicle persistence factors have been
developed for estimation of 1 hour carbon monoxide concen-
trations defi ned as worst-case total persistence factors.
33

IMPROVEMENT OF THE COMBUSTION PROCESS
As mentioned above the explosion of fuel accounts for

chemical pollutant formation. In the Rankine cycle engines,
liquids may continually be vaporized and recondensed, and
in the Sterling cycle engines a gas is repeatedly heated and
cooled. Both cycles are accomplished in a sealed container
and the heat used for the process comes from an open fl ame
external to the engine. Very little pollution is generated in
such a controlled rather than explosive fl ame.
Exhaust gas recirculation (EGR) is used on American
Motors, Chevrolet and Chrysler automobiles. This includes
a diaphragm-actuated fl ow control valve located between
the exhaust and intake manifolds. The valve is operated by
ported vacuum directed through hoses and a coolant temper-
ature override. Permitting metered amounts of exhaust gases
to enter the intake manifold, which are mixed with incoming
fuel mixtures, lowers the combustion temperatures within
the cylinders. Reducing maximum cylinder combustion
temperatures minimizes the creation of Oxide of Nitrogen
(NO


x


). EGR operation does not take place until engine oper-
ating temperature has reached a preset level and engine load
is suffi cient to permit proper EGR operation.
Almost all autos now contain a PVC system which
directs fi ltered air into the crankcase and channels vapors
but toward the manifold leading to the combustion chamber.
Fuel tank vapors are also concentrated by charcoal canisters

in American Motors vehicles for recycled combustion.
An H.E.W. report
5
discusses engine modifi cation sys-
tems, “Features shared by essentially all versions of the
engine modifi cation system include calibrated carburetors
that provide (a) relatively lean air–fuel mixtures for idle
and cruise operation and (b) higher engine idle speeds.
Refi ned control of spark timing is also used, and, in some
cases, regarded spark timing at idle is employed. In addition,
many engines are fi tted with special air cleaners and ducting
designed to supply heated air at nearly constant temperature
to the carburetor, to permit even leaner mixture settings.
Most versions also incorporate high-temperature radiator
thermostats to raise coolant temperatures, and thus improve
mixture distribution and promote complete combustion. In
some cases, higher capacity cooling systems are used to
handle the additional cooling load at idle that results from
wider throttle openings and retarded ignition timing during
this operating condition. In addition, combustion chamber
design attempts to avoid fl ame quenching zones where com-
bustion might otherwise be incomplete, and result in high
hydrocarbon emissions.”
Hydrocarbon and CO emissions are reduced by adjust-
ing the carburetor to a fuel-lean mixture during part throttle
TABLE 1
Federal short cycle
Mode
Average acceleration
rate (mph/sec)

No. Type Speed range Time in mode (sec) Average speed (mph)
1 Accel 0–16 6.0 8.00 2.67
2 Accel 16–29 23.0 22.50 0.57
3 Cruise 29 10.0 29.00 0.00
4 Accel 29–37 18.0 33.00 0.44
5 Accel 37–42 4.5 39.50 1.11
6 Decel 42–37 2.5 39.50 2.00
7 Decel 37–20 32.0 28.50 -0.53
8 Decel 20–0 7.5 10.00 -2.67
9 Idle 0 21.5 0.00 0.00
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MOBILE SOURCE POLLUTION 703
and idle operation. “Lean surge during cruise has been largely
overcome through improvement in manifolding (better mix-
ture distribution), better carburetor fuel-metering character-
istics, higher coolant temperatures, increased heating of the
air–fuel mixture, and, in some cases, provision for heating the
incoming air to the carburetor.
Exhaust emissions of CO and HC are particularly diffi cult
to control during engine idle and closed-throttle operation
(deceleration). Considerable effort has gone into design-
ing carburetor idle systems that will provide a lean air–fuel
mixture and minimize emissions during these periods. To
ensure that idle air–fuel mixture cannot be adjusted to be too
rich (which would tend to increase CO and HC emissions
appreciably), some means of limiting idle-mixture adjust-
ment is used on most carburetors. Such devices allow idle
mixture to be adjusted leaner than a predetermined value,
but not richer.”

The effects of charge dilution on the exhaust emission of
nitric oxide (NO) from a single-cylinder engine were evalu-
ated over a range of engine design and operating parameters.
6

Nitric oxide emission decreased as much as 70% as charge
dilution fraction was increased from 0.0065 to 0.164 due
to increased valve overlap, external exhaust recirculation,
and reduced compression ratio. NO emission was strongly
dependent on charge dilution fraction, but was independent
of the specifi c method used to change charge dilution. The
combined effects of increased charge dilution and 10 degree
spark retard reduced NO emission 90%. However, defi nite
limits of operation were observed on the single-cylinder
engine with high charge dilution.
The Ford Motor Company uses a system which reduces
the hydrocarbon and carbon monoxide content of exhaust
gases by continuing the combustion of unburned gases
after they leave the combustion chamber. This is achieved
by injecting fresh air into the hot exhaust stream leaving
the exhaust ports. At this point, the fresh air mixes with
hot exhaust gases to promote further oxidation of both the
hydrocarbons and carbon monoxide, thereby reducing their
concentration and converting some of them to carbon dioxide
and water.
TABLE 2
Summary of results, steady state tests and federal short cycle vehicle exhaust emissions in grams per vehicle mile
a
City Effluent Idle 15 mph 30 mph 45 mph 60 mph FSC
b

Los Angeles HC 1.45 5.28 3.09 2.91 2.60 5.69
CO 17.02 69.11 29.50 24.60 25.51 48.53
CO
2
72.32 409.53 333.12 357.90 372.47 438.48
NO
x
0.08 0.51 1.69 3.78 5.51 3.10
Denver HC 1.47 6.06 3.74 3.94 3.87 7.07
CO 17.20 86.49 52.13 56.43 71.13 94.68
CO
2
60.54 345.65 297.92 330.09 368.91 348.85
NO
x
0.12 0.83 1.53 3.07 4.52 2.13
Chicago HC 1.30 5.47 3.48 3.59 3.62 5.74
CO 15.49 66.57 29.77 30.53 32.63 59.74
CO
2
63.85 358.70 318.61 357.65 417.11 392.28
NO
x
0.10 0.87 2.36 4.37 6.26 3.42
Houston HC 1.50 5.68 3.02 2.83 2.47 5.52
CO 18.74 77.29 35.11 32.38 31.71 63.16
CO
2
76.13 391.06 318.01 356.13 404.79 417.03
NO

x
0.14 0.83 1.49 3.47 5.70 2.83
St Louis CC 16.43 67.68 29.54 26.13 26.97 56.43
CO
2
64.28 334.24 301.64 315.26 362.26 378.28
NO
x
0.08 0.53 1.57 3.38 5.43 3.10
Washington HC 1.12 4.23 2.59 2.57 2.88 4.46
CO 13.25 56.10 26.19 25.40 25.79 48.11
CO
2
65.04 380.44 343.54 390.83 452.50 417.79
NO
x
0.13 1.04 2.88 6.04 8.90 4.23
All except HC 1.34 5.11 2.99 2.90 2.85 5.34
Denver CO 16.19 67.36 30.02 27.79 28.50 55.15
CO
2
68.35 374.23 323.03 355.55 401.60 408.96
NO
x
0.11 0.75 2.00 4.21 6.35 3.34
a
Idle results in grams per minute. NO
x
not corrected for humidity.
b

FSC—Federal Short Cycle.
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704 MOBILE SOURCE POLLUTION
EMISSION CONTROL DEVICES
Additional Combustion
An afterburner is an additional baffl ed tubular reactor in
which the gases are reignited and burned to completion. An
air pump provides the necessary oxygen-rich mixture and
the heat of reaction maintains a high temperature to speed
its completion.
It is not necessary in all cases to have a separate after-
burner. The aforementioned report
5
states, “An injection sys-
tems decrease exhaust CO and HC emissions by injecting air
at a controlled rate and at low pressure into each exhaust port.
Here, the oxygen in the air reacts with the hot exhaust gases,
resulting in further combustion of the unburned hydrocarbons
and CO that would otherwise be exhausted to the atmosphere.
Optimum reduction of emissions by this method depends on
proper air injection rates over a wide range of engine operating
conditions, carefully tailored air-fuel mixture ratios and spark
advance characteristics, and in some cases the use of heated
carburetor air. Some engines also provide for retarded ignition
timing during closed-throttle operation.
All air injection systems use essentially the same basic
air pump, a positive displacement rotary-vane type.
To guard against excessive temperature and back pres-
sure in the exhaust system resulting from high air delivery

rates at full throttle and high speeds, a pressure-relief valve is
installed in the pump housing. The valve opens to bleed off
some of the pump fl ow at a predetermined pressure setting.
Output from the air pump is directed through hoses and
an air distribution manifold (or two manifolds—one for each
bank on V-8 engines) to the air injection tubes located in
each exhaust port. A check valve between the air distribu-
tion manifold and the air pump prevents reverse fl ow of hot
exhaust gases in the event that pump output is interrupted.
A vacuum-controlled antibackfi re valve is used to pre-
vent fl ow of air to the exhaust ports during the initial stage
of closed-throttle acceleration. The high vacuum that occurs
during deceleration causes rapid evaporation of liquid fuel
from the intake manifold walls. The resulting rich mixture
creates a potentially explosive vapor in the exhaust manifold
if injected air is present.
As with engine modifi cation systems, most air injec-
tion systems also employ spark retard during idle or idle
and deceleration through use of ‘ported’ vacuum sources of
dual-diaphragm distributor-vacuum-advance mechanisms.”
Besides residual gas dilution and wall quenching, engine
variables have the most effect on the amount of hydrocarbons
in exhaust gases. Over sixty privately owned and operated
automobiles fueled with commercial leaded gasoline have
been tested and seven main engine variables were found
which changed the hydrocarbon concentration in the exhaust
gases. These results can be briefl y summarized as:
1) Air-fuel ratio: Low value for gas mixture results
in higher hydrocarbon concentration in exhaust
gases.

2) Ignition timing.
3) Speed: Increase in speed of engine decreases the
amount of hydrocarbon.
4) Air-flow rate: The effect of air-flow rate on the
total hydrocarbon concentration depends on the
air-flow ratio, ignition timing combination.
5) Compressor-ratio: Increasing the compression
ratio, by decreasing head-to-piston distance,
increases the total hydrocarbon concentration.
6) Exhaust Back Pressure: The amount of hydrocar-
bon in exhaust decreases with increasing exhaust
pressure.
7) Coolant Temperature: Increasing the coolant tem-
perature decreases the hydrocarbon concentration.
Catalytic Reactions
The engine exhaust gases may be passed through a
cylindrical shaped canister packed with catalytic particles.
Although this method has great potential, two problems
may arise. The long term stability of the catalyst (50,000
miles) is diffi cult to maintain since lead and other chemi-
cals in trace amounts poison the catalyst. The catalyst
structural stability is diffi cult to maintain under the infl u-
ence of varying gas fl ow rates and fairly high temperature.
Also, removing three or four pollutants simultaneously can
prove diffi cult for any single type of catalyst. The removal
of NO


x
, for instance, requires a reduction catalyst, whereas

CO requires an oxidation catalyst. For this reason dual stage
catalytic reactors have been proposed. The technical prob-
lems for this method are greater but the potential advantages
are even greater.
Nitric Oxide Removal A review of some chemical reac-
tion data which might be useful in automobile pollution con-
trol work has been presented by Shelef and Kummer.
7

One approach to solving the stability problems has been
to avoid leaded fuel in automobiles containing converters.
The catalytic approach to conversion for American Motor
cars is to use a pellet-type of catalyst with a monolithic-type
warm-up feature for California and high altitude cars. The
warm-up converter is separately mounted ahead of the cata-
lytic converter. Chrysler Corporation and Chevrolet use a cat-
alyst support coated with platinum, palladium and rhodium.
Hydrocarbons, CO and NO


x
are all reduced by this three
component catalyst. An extensive literature exists on more
economical active phases
11,13,16,22
which are not as effective
converters. The air to exhaust ratio in catalytic converters is
computer controlled in American Motor cars.
For this reason, reactions which involve combination
with rather than decomposition of NO

2
are being studied
very carefully. The equilibrium constants in terms of par-
tial pressure are given in Table 3 for NO combination with
hydrogen, CO and methane at various exhaust temperatures.
The thermodynamic conditions (large K
p
values) are gener-
ally favorable for conversion (reduction). A Monel (nickel–
copper alloy) catalyst has been found reasonably successful
for removing NO by combining it with residual CO in the
exhaust stream. The Monel dissociates the oxygen from NO
and then oxides CO to the harmless dioxide.
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In this system, when the engine operation is too rich (too
little oxygen) nitrogen oxide reduction is found to be excel-
lent, but carbon monoxide conversion poor. The nitrogen
oxide will readily combine with Monel, forming Monel oxide
and nitrogen since there is little oxygen to compete with the
nitrogen oxide for active sites on the Monel. However, since
the CO:NO ratio is so large, there will be insuffi cient Monel
oxide formed to give up its oxygen to the carbon monoxide.
On the other hand, if the engine operation is too lean, NO
reduction will be poor and CO conversion excellent since the
reaction becomes signifi cant and oxygen will compete with
the nitrogen oxide for sites on the Monel.
Some typical conversion data
1

at space velocities of
50,000 v/vhr indicated that conversion of NO in synthetic
gas mixture (2% CO, 1000 ppm NO) was over 95% if the
temperature was above 700ЊC, but fell off sharply at temper-
ature below 650ЊC. Problems associated with this technique
are the formation of NH
3
if any residual H
2
is present and
back pressure buildups with the current catalyst structure.
In addition, dusts form which are emitted as particulates. It
is essential for effi cient performance of this catalyst that it
warms up to operating temperatures very rapidly. Lead in the
fuel reduces the chemical activity and ultimately increased
the rate of deterioration of the catalyst.
Another catalyst which showed promise for the same
reaction but at lower temperature (200 to 350ЊC) has also
been mentioned.
18
Despite the fact that conversions of better
than 90% were reported equal amounts of CO and NO were
used. Automotive exhaust have about 16 times more CO than
NO. Activated carbon has been used
19
successfully with H
2

gas at 600ЊC. However, activated carbon lost a considerable
portion of its activity after only 7 hours service.

CO and Hydrocarbon Removal Major automotive and
petroleum companies have combined efforts in the develop-
ment of an inexpensive multi-thousand mile catalytic pack-
age for reducing CO and HC exit concentrations. Since the
input of gasoline and hence the effl uent gases are rarely at
steady state, any study of a catalytic reactor must consider
the dynamic situation. To give some idea of magnitude, the
exhaust gas fl ow in standard cubic feet per minute (SCFM)
is roughly twice the miles per hour equivalent of an auto-
mobile and the temperatures of the exhaust gas change from
that of the ambient to well over 600ЊC. Figure 1
shows typi-
cal CO and temperature levels in the exhaust stream after
engine startup in a Federal cycle.
For balancing pollution problems with no catalytic or
afterburner control an air–fuel ratio of about 16 is recom-
mended. Above this level the NO


x


level increases markedly
and below it the amount of unburned HC and CO substan-
tially increases. With catalytic devices this ratio may no
longer be optimum, since the catalyst selectivity may be
greater towards removal of one of the pollutants than any of
the others.
Wei
20

has noted that the kinetics of CO oxidation over
an egg-shell catalyst turn out to be fi rst order for CO and
0.2 order for O
2
in the range of 1 to 9% oxygen. The curva-
ture in the Arrhenius plot (Figure 2) is believed to be caused
by a pore diffusional phenomena. As the catalyst ages and
activity falls the reaction rate becomes controlling and the
Arrhenius plot becomes a straight line. The catalyst of the
fi gure is best above 350ЊC, but a lower operating tempera-
ture may be preferred for longer catalyst life. Wei
20
found
that, “As the catalyst lost 90% of its activity, the emission
rose by only 30%; but the last 10% of activity loss would
result in a precipitous rise of carbon monoxide emission.
A catalyst with 50% reduction in heat capacity performs much
better; a reactor with 50% reduction in volume performs better
when the catalyst is fresh and worse then it is aged.” His phi-
losophy is, “It is our engineering goal to produce a low-cost
and convenient solution. However, any solution requires some
inconvenience and cost from everyone. Quick warm-up is no
problem if we are willing to sit and wait in the car for 2 min
for an auxiliary heater to warm up the catalyst bed before the
car moves. We have ninety million cars on the road, and a
$100 device will cost us nine billion dollars. How much are
we willing to pay for 90% cleaner air? These decisions belong
to the public, not the engineers. For the sake of everyone we
hope to be able to present to the public an economical and
convenient solution in the near future.”

Stein et al.
21
have evaluated the effectiveness of possible
catalysts by a microcatalytic technique based on gas chroma-
tography. The technique which is described in detail allows
a large variety of hydrocarbons and catalysts to be rapidly
tested over a wide range of temperatures (100–600ЊC). In
general oxides of cobalt, chromium, iron, manganese, and
nickel are the most effective catalysts. The higher molecular
weight hydrocarbons are more easily oxidized than the lower
TABLE 3
The equilibrium constants for some reduction reactions of nitric oxide
17
log Kp
Reactions 600 700 800 900 1000K
NO ϩ 5H
2
→ 2NH
3
ϩ 2H
2
O 49.0 48.1 32.13 26.44 21.8
NO ϩ 2H
2
→ N
2
ϩ 2H
2
O 51.7 43.4 37.1 32.2 28.3
NO ϩ CO → ½N

2
ϩ CO
2
27.3 22.6 19.7 16.46 14.29
NO ϩ CH
4
→ ½N
2
ϩ 2H
2
19.59 18.1 17.6 16.2 15.5
NO ϩ Hat → HNO 12.4 9.77 7.79 6.24 5.0
NO ϩ ½ H
2
→ HNO Ϫ3.7 Ϫ3.6 Ϫ3.52 Ϫ3.46 Ϫ3.42
Temperature
600
7
CO %
0
0
Time, min.
7
0
°
C
FIGURE 1
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and hydrocarbons of a given carbon number increased in reac-
tivity according to the series: aromatic Ͻ branched paraffi n
or alicyclic Ͻ normal paraffi n Ͻ olefi nic Ͻ acetylenic. The
olefi nic hydrocarbons, generally considered the most unde-
sirable, are relatively easy to remove. Other results could be
summarized in the following manner:
1) With most of the catalysts tested, some crack-
ing occurs during the oxidation of hydrocarbons.
Oxidation without CH
4
formation was possible
with the oxides of Co, Cr, Cu, Mn and Ag only.
Zirconium oxides are unique in that they pro-
duce only cracking products, mainly methane and
smaller amounts of intermediate hydrocarbons.
2) Complete removal of all hydrocarbons was not
attained with some of the catalyst, even at 600ЊC.
The oxidation is not a simple function of temperature.
The potential of copper oxide-alumina catalyst for air pol-
lution control has been studied by Sourirajan and Accomazzo.
22

They stated that, “the simultaneous removal of hydrocarbons
and carbon monoxide present in the auto exhaust gases has
been tested making use of a six-cylinder Chevrolet engine
run on leaded gasoline fuel. The hydrocarbon and carbon
monoxide concentrations encountered in these studies varied
in the range 170–17,000 ppm and 1–7%, respectively. It was
found that the minimum initial temperature of the catalyst
bed required for the complete removal of both hydrocarbons

and carbon monoxide, simultaneously, was 226ЊC under no
load condition, 342ЊC, under an engine load of 2.5hp, 400ЊC,
under an engine load of 5.1hp or higher, and 236ЊC under
deceleration conditions. The catalyst showed no deterioration
in performance even after 100 hours of continuous service in
conjunction with the above auto exhaust gases. The extent of
removal of hydrocarbons from the exhaust gases was found
to depend on the initial temperature of the catalyst bed and
the engine load condition.
It is realized that a successful 100 hour run does not
constitute a life test on the catalyst, but it does indicate the
potential applicability of the catalyst in air pollution control
devices. The engineering design of the suitable converter for
any particular practical application of the catalyst should
naturally take into account the heat liberated during oxi-
dation. Instantaneous catalyst temperatures of the order of
900ЊC have been encountered in this work with no deleteri-
ous effect on the subsequent effectiveness of the catalyst . . .
the heat liberated during the reaction can be advantageously
used to maintain the full effectiveness of the catalyst under
all conditions of engine operation encountered in normal
practice.”
When contaminants are passed through a Hopcalite
(unsupported coprecipitate of copper and manganese oxides)
catalyst burner, the results vary from almost complete oxida-
tion of some organics to very slight oxidation of the lower
molecular weight aliphatic hydrocarbons
23
at some 300ЊC.
Nitrogen compounds form N

2
O when oxidized and haloge-
nated compounds indicate a strong acid reaction when the
reactor effl uent is tested with detector paper.
An interesting example of the use of exhaust gas recy-
cle and catalysts has been presented by the Esso Research
Group. In Figure 3, typical hot cycle traces of CO, O
2
and
NO are presented for cases before the catalyst, after Monel
catalyst and after a 2nd stage Platinum-alumina catalyst,
for instance, with and without recycle. The major benefi cial
effect of recycle is on the NO concentration.
The combustion of gasoline is more or less incomplete
regardless of the quantity of excess air used. About 1% of the
exhaust gas is composed of harmful products chiefl y carbon
monoxide (CO), oxides of nitrogen (NO


x


) and hydrocarbons
(HC). A signifi cant variable affecting each of these pollutant
concentrations is the air to fuel ratio (ATFR). The stoichio-
metric value, (ATFR)
STO
is about 14.7:1.0 on a weight basis.
Using a catalytic three way converter, more than 90%
of the pollutants can be converted to harmless substances.

To avoid catalyst contamination lead free gasoline must
be used. In the closed loop electro-mechanical control of
(ATFR)
STO
described by Robert Bosch,
34
1985, an oxygen
sensor in the exhaust gas transmits a signal which is used
to correct ATFR deviations. This control method is particu-
larly effective on fuel injection engines because they do not
have the additional delay times of carburetor engines. For
catalytic converter operation, the optimum ATFR range is
X
X
X
X
X
X
100
50
k
SEC
–1
INTRINSIC
120 SCFM
ACCELERATION
60
30
needed for
80%

conversion
10 IDLE
500 400 300 200
C°
AGED
FRESH
10
5
1
0.5
.25mm
1.5mm
I
/ T °K
FIGURE 2 Carbon monoxide activity of an eggshell catalyst.
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© 2006 by Taylor & Francis Group, LLC
MOBILE SOURCE POLLUTION 707
between 99 and 100% of (ATFR)
STO.
Above this range NO
x

levels increase markedly as ATFR is increased; below this
range CO and HC levels increase as ATFR is decreased.
Electric vehicles (EV’s) operated by high energy high
powered batteries are making great strides toward commer-
cialization. The near term goal is to provide over 100 miles
per recharge at accelerations capable of matching the internal
combustion engine. The California Air Resources Board has

issued a technical document in December of 1995 supporting
the concept of such vehicles. The key performance parameter
for EV’s is its specifi c energy (or energy density), measured
in watthours per kilogram, with a near term goal of 80–100
wh/kg, suggested by the US Advanced Battery Consortium
(USABC).
38
Another important measure of performance is the
peak specifi c power (or power density), which gives us an idea
of an EV’s acceleration. It is measured in units of watts per
kg with an USABC near term goal of 150 w/kg that can be
sustained for 30 seconds during discharge down to 80% depth
of discharge. A comparison of different chemical system per-
formance parameters is presented in Figure 4.
All of the bat-
teries are expected to achieve the USABC’s midterm goals for
these EV parameters, which coincide with lasting about fi ve
years (-600 cycles) and costing no more than $150 per kwhr of
battery capacity. Hybrid power electric-diesel engines became
available in 1995. They alternate between diesel power opera-
tion at 2000–2600 RPM where its effi ciency is best to battery
power at other RPM.
RELATED TRANSPORTATION PROBLEMS
Diesel Exhaust Odors
Diesel engines are found in buses, trucks, off-road vehicles
and power applicators and increasingly in automobiles. Public
reaction to diesel-engine exhaust odors provides the impetus
for controlling effl uents of that type of fuel combustion.
24

A list of oily kerosene and smokyburnt odor compound iden-
tifi ed by A.D. Little, Inc. is included in Table 4. Exhaust odor
and smoke from diesel engines are more objectionable than
those from spark ignition. CO emissions are generally less
serious but NO


x


is troublesome (4 to 10 g/mile). Improved
fuel injection and afterburners are considered to be the most
promising of the existing control methods. The injection dif-
fers from the internal combustion engine in that fuel does not
enter the cylinder as a mixture with air, but is injected under
high pressure into the chamber in exact quantities through
low tolerance nozzles. For NO


x


removal the basic approaches
Without
Recycle
With
Recycle
Without Recycle
With Recycle
Without Recycle

With Recycle
CO
= 0
30 60 90 120 0 30
TIME, SECONDS
60 90 120 0 30 60 90 120
1000
4
2
4
2
60
40
20
PPM NO
% O
2
% COMPH
BEFORE CATALYSTS AFTER MONEL AFTER BOTH CATALYSTS
~
FIGURE 3
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© 2006 by Taylor & Francis Group, LLC
708 MOBILE SOURCE POLLUTION
SPECIFIC ENERGY
USABC long-term goal
USABC midterm goal
Watthours per kilogram
Sodium-
nickel

chloride
Sodium-
sulfur
Lithium-
ion
Nickel-
metal
hydride
Nickel-
cadmium
Lead-acid
1998
1996
1998
2001
2001
1998
1998
2004
1996
250
200
150
100
50
0
PEAK SPECIFIC POWER
USABC long-term goal
USABC midterm goal
Sodium-

nickel
chloride
Sodium-
sulfur
Lithium-
ion
Nickel-
metal
hydride
Nickel-
cadmium
Lead-acid
500
400
300
200
100
0
1998
2004
2004
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© 2006 by Taylor & Francis Group, LLC
MOBILE SOURCE POLLUTION 709
are engine refi nement, fuel additives, catalytic conversion
and reduction of peak combustion temperature.
Gas Turbines
The exhaust of gas turbines contains perhaps less pollutants
than that of any other internal combustion process. The
high airport traffi c density makes the problem a real one,

however. Sulfur dioxide emissions are low, but the smoke
and odor producing compound levels are high. For auto-
mobile use, the total mass of gas turbine exhaust is many
times greater than that for the gasoline engine of equivalent
power. Hydrocarbon and CO mass emissions are known to
be low and diffi cult to reduce. A greater deal of the pollu-
tion control work underway is in the engine modifi cation
area. In aircraft the turbofan engine is largely replacing the
turbojet. Turbofans bypass some of the air past the engine
and rejoin it with the burner gases at the exhaust tailpipe.
Modern dry NO


x


combustion systems can obtain emissions
of 25 ppm at 15% O
2
.
36,37

The airplane is a much more effi cient carrier (pollu-
tion wise) than the automobile, on a people × miles basis.
Aircraft engine research has been concerned primarily with
smoke reduction. Fuel type, fuel additives and combustion
chamber design have been the primary variables studied.
PARTICULATE EMISSIONS
The particulate matter emitted from automobiles has been
characterized

25
as consisting of “lead salts, alkaline earth
compounds, iron oxides, soot carbonaceous material, and
tars. This material ranges in size from large fl akes to submi-
cron particles and varies in consistency from hard and brittle
particulate to oil mists. Some of the particulate material is
generated in the engine combustion chamber and nucleated
and agglomerated in the vehicle exhaust system before it
passes out the tail pipe. On the other hand, a large portion of
the particulate material generated in the engine subsequently
deposits on various surfaces of the exhaust system. At some
later time, this deposited material fl akes off and becomes
reentrained in the exhaust gas to be emitted from the vehicle.
Obviously, opportunities exist for various types of chemi-
cal and physical processes to take place and, as a result, the
overall particulate emission process for a vehicle is quite
complex and diffi cult to defi ne.”
CYCLE LIFE
2004
2000
2000
1998
2002
USABC long-term goal
USABC
midterm
goal
Lead-acid
Nickel-
cadmium

Nickel-
metal
hydride
Lithium-
ion
Sodium-
sulfur
Sodium-
nickel
chloride
2500
2000
1500
1000
500
0
FIGURE 4: Performance Parameter Comparison for Electric Vehicles as a Function of Advanced Battery Type (After Moore
38
).
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710 MOBILE SOURCE POLLUTION
Much of the particulate is in the form of polynuclear aro-
matic (PNA) hydrocarbons and tars. As for gaseous products,
standard federal cycles have been designed to test for particu-
late emissions.
Lead particulate matter is emitted at a rate of about
0.16g/mile. The average particle size emitted is slightly less
than a micron, but sizes are substantially distributed over
the 0.10–10 micron range. In Table 5, the effect of mileage

on particle distribution may be observed. As the number of
miles driven increases, the segment of particles greater than
9 microns also increases, probably because of deposition
buildup on exhaust pipes.
Using 500 microns as a divider, about 30% of the lead
burned over the lifetime of a car is in the form of fi nes 40%
in the form of coarse particulate.
Research studies
27
showed that a relationship exists
between traffi c volume, proximity to the highway engine
acceleration vs . constant speed, wind direction and the
amount of lead in the air. Lead values can be expressed as
a quadratic function of time and a linear function of traf-
fi c volume. Ter Haar
28
and coworkers found that for Federal
7-mode cycle conditions (Z) cars vary widely in the amounts
and composition of their particulate emissions; (2) cold cycle
operation gives 2–8 times more particulate than hot engine
operation; (3) lead compounds represent less than one-third
of total particulates, the remainder being carbon compounds
along with ammonium and nitrate ions and unknown mate-
rials; (4) carbon emission for stabilized cars using leaded
gasoline varies widely but averages about 35% of the total;
(5) suspended particulate emissions are nearly equal with
new cars whether or not lead is present; (6) exhausted lead
varies with the condition of the exhaust system and ranges
between 7 and 30% of the lead consumed by the engine;
(7) fuel additives affect the amount of emitted particulates;

(8) probe sampling techniques underestimate by a large
factor of amount of particulates emitted by vehicles; and
(9) trapping systems offer potential for greatly reducing the
emission of suspended total particulates. Nonleaded-fuel
cars were found to exhaust 0.165g of particulate per mile
while leaded-fuel cars exhausted 0.152g/mile. The reason
for this is that nonleaded fuel is more aromatic in nature
and that the percentage emission for aromatic hydrocarbons
is considerably greater than that for paraffi nic material. An
increase in the volume of aromatics in the fuel from 10 to
70% doubles the amount of carbon particulates emitted. It
has been claimed
29
that a reduction of 80–90% of the lead
emitted is possible by trapping systems.
Two simple traps have been compared for particulate
removal and the results presented in Table 6.
More complex traps are likely to be very successful in
reducing both lead and total particulate matter.
A more detailed chemical analysis of particulates has
been described under Federal emission test conditions.
30
This
investigation found that a decrease in the total mass of partic-
ulates especially particles of very small size, occurred when
using nonleaded fuel. The discrepancy with the aforemen-
tioned work
28
may be a result of differences in exhaust depo-
sition for the two systems. Particles emitted in the vehicle

exhaust varied in size from 0.01 to 5000 microns, the latter
for such products as rust scale. Typical chemical composi-
tions for the particulate exhausts are presented in Figure 5.

The ratio of Pb to Br is relatively constant 2.1:1. The
ratio on impactor plates was somewhat lower, indicating
that the chemical composition of lead salts may be related
to particle size. Considering some of the differences in the
fi ndings for different particulate matter studies it is critical
that such investigations be perfected and intensifi ed. An
interesting additional discussion of the characterization of
particulate lead in vehicle exhaust may be found in the work
of Habibi
31
and Mueller.
32

TABLE 4
Partial list of odorifous compounds
General classification Compounds
Indans and tetralins Methyl indan
Tetralin
Dimethyl indan
Methyl tetralin
Dimethyl tetralin
Trimethyl tetralin
Alkyl tetralin
Alkyl-substituted naphthalenes Methylnaphthalenes
Dimethylnaphthalenes
Indenes, acenapthenes, and Alkyl-substituted indenes

benzothiophenes Dimethylbenzothiophenes
Acenaphthene
General class Carbon range
Alkenone C5 to C11
Furan C6 to C10
Diene/ C9 to C12
Furfural C6 to C7
Methoxy benzene C8 to C9
Phenol C7 to C12
Benzaldehyde C7 to C10
Benzofuran C8 to C9
Indanone C6 to C10
Indenone C9 to C12
Naphthol C10 to C14
Naphthaldehyde C11
TABLE 5
Lead particulate emissions as a function of size and mileage
25
Average
mileage
Average lead salt emissions, g/mile
>9 microns <1.0 microns <0.3 microns
5,000 0.04 0.07 0.05
16,000 0.06 0.05 0.04
21,000 0.06 0.05 0.03
28,000 0.10 0.03 0.02
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MOBILE SOURCE POLLUTION 711
In addition to the fuel and exhaust line, additional

sources of particulates are to be found in catalysts for gas-
eous pollutant removal and from the disintegration of tires.
The monel catalyst, described previously
1
for NO

x

and Co
removal, deteriorates with time at about 0.019 g/mile. The
presence of lead in the fuel substantially increases the rate of
Monel deterioration.

Sheih
35
estimated that the increase in diesel emissions
concentration in the Chicago area would be predominantly
for particles having diameters smaller than one micron. At
diameters in the vicinity of 0.2 microns the concentration
would increase 10 fold.
One other source of vehicle particulate emissions which
might be mentioned is that from jet aircraft. By weight, more
10.0
1.0
0.1
0.01
0.001
0.0001
5
3

2
.5
.3
.2
.05
.02
.005
.003
.03
.002
.0005
.0003
.0002
1 2 3 4 6 7 10 15 20 25 35 50 70 100 150 200
TOTAL PARTICULATE MASS
LEAD
BROMINE
IRON
ZINC
FTP CYCLE NUMBER
PARTICULATE EMISSION (GRAMS/MILE)
FIGURE 5: Particulate mass emissions from car operating on Indolene-30, as measured by the Total Volume System, versus
FTP driving cycle number.
30
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712 MOBILE SOURCE POLLUTION
than fi fty per cent of the total particulate matter emitted from
aircraft engines appears to be due to unburned and partially
oxidized fuels. More detailed analysis of this constituent

must await development of improved measurement tech-
niques. About 90 per cent of the non-hydrocarbon material
present is elemental carbon (i.e., smoke), while the remainder
is material eroded from the engine, fuel contaminants, or dust
ingested by the engine. Fuel additives containing metallic
compounds are seldom used for normal aircraft operations.
The emission of such metallic particles does not appear to
constitute a problem.
It has been demonstrated that fuel-rich pockets in the pri-
mary combustion zone of the burner were the source of the
smoke-producing carbon particles and that the smoke could
be reduced by “leaning out” the fuel-rich areas. Redesign of
the burner is believed to be the most effective approach to
smoke reduction.
The author wishes to acknowledge the assistance of
Mr. George Musser, Exxon Research and Engineering
Company, for his discussion on particulate matter emis-
sions, of Dr. William Peterson, formerly of the Polytechnic,
for supplying test procedure information, and of Mr. John
Pinto of the New York City Department of Environmental
Protection for his review.
The particulate problem most often cited for diesel engines
in PM-2.5 emissions.
39
The 2.5 refers to the particle aerody-
namic diameter, d, in microns, ␮ m and the PM refers to the
concentration in ␮ m/m
3
of particulate matter of d Յ 2.5 ␮ m.
An Environment Defense report estimated that in New York

Harbor alone, ferries, tugboats, container ships and other
commercial vessels produce as much smog-forming con-
tamination over the course of a year as do 300,000 automo-
biles.
40
These sources were largely unregulated as of 2005. It
has been noted by the International Maritime Organization
(IMO) that in terms of emission intensity—emissions per
ton of cargo transported—marine shipping is one of the most
environmentally effi cient methods of shipping. According
to the IMO, between 85–90% of all commodities are trans-
ported by ships, and diesel engines power the majority of
these vessels.
41
The diesel engines have become more effi -
cient in recent years. New air pollution rules expected to be
in effect will require diesel and gasoline engine emission
level equivalence by 2010.
REFERENCES
1. US EPA and US DOE, “2005 Fuel Economy Guide”, www.
fueleconomy.gov
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(1967).
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TABLE 6
Particulate trapping—federal 7-mode cycle
28
Car, year, and make Miles Exhaust systems Particulates, total g/mile Pb
1970 make D 6,000 Standard 0.169 0.046
6,000 Anchored vortex 0.139 0.039
% Reduction 19 16
1970 make B 6,000 Standard 0.274 0.071
6,000 Anchored vortex 0.227 0.056
% Reduction 17 21
1970 make A 6,000 Standard 0.471 0.114
6,000 Anchored vortex 0.189 0.043
% Reduction 60 66
1970 make D 0 Standard 0.116 —
0 Agglom inertial # 1 0.037 0.006
% Reduction 68
16 cars (1966–1969) 30,000ϩ Standard 0.339 0.060
1965 make D 24,000 Agglom inertial # 2 0.125 0.036
% Reduction 63 39
1969 make D 12,000 Agglom inertial # 3 0.108 0.034
% Reduction 67 43
1970 make D 12,000 Agglom inertial # 4 0.098 0.019
% Reduction 71 68
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© 2006 by Taylor & Francis Group, LLC
MOBILE SOURCE POLLUTION 713

12. Sakaida, R.R. et al., A.I.Ch.E.J. 7, 658 (1961).
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Dept. of Tech. Info., Stuttgart FDG, Sept., 1985.
35. Sheih, C.M., Wat. Air & Soil Poll. 32, 417 (1987).

36. Jeffs, E., Gas Turbine World, May–June (1989), p. 20.
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EDWARD N. ZIEGLER
Polytechnic University
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