AND
FROM
POWERED
U. S. ENVIRONMENTAL PROTECTION AGENCY
. v
\
-------
GUIDE TO REDUCTION OF SMOKE AND
ODOR FROM DIESEL-POWERED VEHICLES
Southwest Researcn Institute
and
Office of Air Programs
ENVIRONMENTAL PROTECTION AGENCY
Office of Air Programs
Research Triangle Park, North Carolina
September 1971
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price 30 cents
Stock Number 6503-0028
-------
The AP series of reports is issued by the Office of Air Programs,
Environmental Protection Agency, to report the results of scientific
and engineering studies, and information of general interest in the
field of air pollution. Information reported in this series includes
coverage of Air Program intramural activities and of cooperative
studies conducted in conjunction with state and local agencies, re-
search institutes, and industrial organizations. Copies of AP reports
are available free of charge —as supplies permit —from the Office
of Technical Information and Publications, Office of Air Programs,
Environmental Protection Agency, Research Triangle Park, North
Carolina 27711
Office of Air Programs Publication No. AP-81
-------
CONTENTS
Page
Introduction 1
General Characteristics of Diesel Engines 3
Guidelines for Reducing Smoke 7
Guidelines for Reducing Odor 19
Summary of Recommendations 27
in
-------
GUIDE TO REDUCTION OF SMOKE AND
ODOR FROM DIESEL-POWERED VEHICLES
INTRODUCTION
In our air pollution-conscious country, the public is aware of all
pollution from motor vehicles especially that which they can see
or smell. Less than 1 percent of vehicles registered in this country
are diesel-powered and less than 5 percent of the fuel consumed by
highway vehicles is used by diesel-powered vehicles. Still, millions
of Americans each year have some contact with diesel vehicles.
Because such a large number of people are exposed to the smell and
smoke of diesel exhausts, adverse reactions or complaints have
reached significant proportions.
State and local governments at all levels continue to establish
and enforce regulations limiting smoke from motor vehicles, par-
ticularly the diesel. These regulations indicate that this country will
not tolerate excessive smoke from diesel-powered vehicles.
As of January 1970, engine manufacturers must certify that all
new diesel engines installed in highway vehicles of more than 6,000
pounds gross vehicle weight meet Federal smoke standards as given
in the Federal Register of June 4, 1968. Although no Federal
standard for controlling diesel odors exists now, such standards will
probably be set up once suitable ways of measuring odors are avail-
able. One state has established a standard for diesel odors that is
based on the subjective reactions of a panel of ten people who are
exposed to the exhaust fumes near the point of discharge. The
method will have to be improved before odor regulations based on
it can be enforced.
The purpose of this Guide is to help reduce the amount of
smoke and odor produced by diesel-powered vehicles. This reduc-
tion can only be accomplished by a team approach in which every-
body on the team is important. From the selection of the power
package and fuel through maintenance and driver performance.
-------
each person must do his part if smoke and odor emissions are to be
reduced.
The following pages contain practical information and recom-
mendations that can help in this task. These guidelines are the result
of a detailed study of technical literature and reports dealing with
diesel smoke and odor and of interviews with engine and vehicle
manufacturers, fuel and additive suppliers, fleet operators, and
enforcement agencies. This Guide discusses the general charac-
teristics of diesels, ways to reduce smoke, and ways to reduce odor,
each in separate sections.
REDUCTION OF SMOKE AND ODOR
-------
GENERAL CHARACTERISTICS OF
DIESEL ENGINES
Regardless of design, all diesel engines operate on the compres-
sion-ignition principle in which air is compressed and liquid fuel is
injected under high pressure. The high-temperature mixture ignites
spontaneously, resulting in power output from the engine. Both
two-stroke (one power stroke per cylinder for each engine revolu-
tion) and four-stroke (one power stroke per cylinder for every two
revolutions) are used. Diesel engines are either naturally aspirated or
turbocharged. In the first, air is taken in from the atmosphere with-
out external assistance. In turbocharged engines, exhaust energy is
used to power a turbine air compressor that increases the amount of
air inducted per engine stroke. In naturally aspirated engines, the
amount of air taken in depends on engine speed because there is no
throttle in the air inlet system in the diesel as there is in the gasoline
engine.
The amount of fuel injected determines the power output.
During idle, very little fuel is needed, but at high speeds (high
power output), of course, more fuel is needed. A convenient way to
think of the diesel is in terms of the air-fuel ratio:
air-fuel ratio = weight of air available for combustion
weight of fuel available for combustion
An air-fuel ratio of about 100:1 is present during idle, but at high
power output, the ratio is closer to 20:1. When the air-fuel ratio is
15:1, the chemically correct amounts of air and fUel are present for
complete combustion. Since the ratios in diesel engines are greater
than this, the diesel operatesa/r-rich. Generally speaking, if the fuel
supplied to the cylinders for combustion were kept constant, an
increase in air would result in decreased smoke production. If the
air supplied to the cylinders for combustion were kept constant, an
increase in fuel would produce an increase in smoke. In many diesel
engines, then, the amount of smoke produced is directly related to
the air-fuel ratio.
Sources of air pollution are either stationary (industrial or
domestic) or mobile (automotive). Mobile sources such as cars,
trucks, and buses produce about half of all the pollutants emitted
-------
into the atmosphere. In terms of the actual tons of air pollutants
produced by mobile sources, the diesel output is relatively minor,
but the smoke and odor from diesels are much greater than from
other types of surface transportation.
Figure 1 gives a general comparison of the various pollutants
produced by gasoline- and diesel-powered vehicles. The diesel
emits less carbon monoxide (CO) and hydrocarbons (HC) than the
gasoline engine. The nitrogen oxides (NOX) emitted are about the
same for the two types of vehicles. Smoke and odor emissions,
however, are much greater from the diesel than from the gasoline
engine. Some diesels may produce smoke when operated with an
air-fuel ratio less than 25:1. Odor production from diesels, however,
does not seem to be directly related to the air-fuel ratio.
D
GASOLINE
DIESEL
CO
HC
NOX
ODOR
SMOKE
Figure 1. Relative levels of pollutants emitted by gasoline- and
diesel-powered vehicles.
TYPES AND CAUSES OF SMOKE
Smoke is usually the result of incomplete combustion. The
three types of smoke and their composition are:
1. Dark, black, or hot smoke, which consists of unburned car-
bon particles (soot) and is usually associated with operating
speeds, loads, and temperatures.
2. Blue smoke, which contains unburned engine oil that
reaches the combustion chamber because of worn piston
rings, cylinder liners, and/or valve guides. Some partially
burned fuel may also be present in this type of smoke.
3. White or cold smoke, which is made up of droplets of un-
burned liquid fuel and is usually associated with the startup
or idle of some engines.
REDUCTION OF SMOKE AND ODOR
-------
Black smoke, the most common type of exhaust smoke, is the
main concern of this Guide. Black smoke is always a result of
incomplete combustion. White (cold) smoke can be reduced by
reducing idle time, especially after initial startup, and by using fuel
that has the right ignition properties for the climate or prevailing
temperature. Cold smoke can also be reduced by parking the vehicle
indoors or by using engine-block heaters. Blue smoke can usually be
eliminated by engine maintenance or overhaul.
TYPES AND CAUSES OF ODOR
The chemical compounds in the exhaust that cause odor have
not yet been identified. Diesel odors are even hard to describe. In
fact, observers have to be trained for the job of judging kinds and
amounts of odors because no instruments now available can meas-
ure odor. Just as the methods of judging and describing odors are
far from satisfactory, the exact cause of odors are not well known.
It is known, however, that the design of a vehicle can change the
intensity of an odor and thus the detection of that odor by the
public.
Diesel Characteristics
-------
GUIDELINES FOR REDUCING SMOKE
The guidelines that follow are presented under the headings of
engine size and application, derating, maintenance, driver effects,
fuel, and control methods, and are given to help the purchaser,
mechanic, and driver of diesel-powered vehicles reduce smoke emis-
sions as much as possible.
EFFECTS OF ENGINE SIZE AND USE ON SMOKE
The choice of what diesei engine to use in a truck or bus is
often more art than science, even though engine manufacturers pro-
vide excellent guides for choosing engines for specific applications.
Unfortunately, the manufacturer's recommendations are sometimes
used just as rough guidelines, especially when vehicles are bought
through competitive bidding. In order to save $100 to $500 on a
vehicle costing $20,000 to $30,000, a buyer sometimes chooses a
slightly smaller engine than the one needed to do the job. A buyer
should always remember that:
Diesel-powered vehicles generally need and use all the power
available. Gasoline-powered passenger cars, on the other hand,
generally use only a fraction of the power available.
Too small an engine
often has no power margin
for the requirements of high-
er route speeds and greater
loads, or for the results of
deteriorations in engine per-
formance. In order to get
more power out of an engine
when he needs it, a driver
often overfuels. But since
overfueling causes more ex-
haust smoke, an operator
should never adjust the fuel
delivery beyond the specifi-
cations given by the manu-
facturer. As commonly believed, power and smoke are usually re-
lated. As shown in Figure 2, however, a small increase in power,
-------
HI
especially at the upper range,
produces a larger increase in
smoke.
Depending on the smoke
characteristics of an engine, an
increase in power or the
amount of overload can not
only produce too much smoke
but can possibly damage the
engine. If the engine is margi-
nal in size or is too small, it
will often be fully loaded,
with the net effect that more
smoke is produced.
The elevation at which a
vehicle must operate is an im-
portant consideration in
choosing an engine, especially
from the standpoint of smoke production. Some types of engines
are fairly insensitive to altitude but others produce much more
smoke at higher altitudes. Figure 3 shows the general effect of
POWER »-
Figure 2. Smoke versus power out-
put of typical naturally
aspirated engines.
50
40
30
Q.
O
111
¥
O
20
10
ilpK HIGH-SMOKE NATURALLY ASPIRATED
-
ift¥?
'.• • '.
':•:•'•
LOW-SMOKE NATURALLY ASPIRATED
TURBOCHARGED
g:
&.
II
•:•:£:
1
_
585
II
1
Sm
ill
**:;:
II;
88:
-
1000 3000
ALTITUDE, ft
5000
Figure 3. General effect of altitude on hot smoke pro
duction by diesel engines.
altitude on smoke production by naturally aspirated and turbo-
charged engines. As the figure indicates, a naturally aspirated engine
that produces very little smoke at sea level may produce only a
little more smoke at 5,000 feet. Another type of engine that
produces smoke of about 20 percent opacity at or near sea level
REDUCTION OF SMOKE AND ODOR
-------
may produce smoke of 40 percent opacity at 5,000 feet. The turbo-
charged engine is generally considered to be less sensitive to altitude
than many naturally aspirated engines.
Naturally aspirated and turbocharged engines with known low-
smoke characteristics are the best diesel engines for operation at
higher altitudes. If a vehicle is consistently operated at a high alti-
tude and is used in extensive stop-and-go service, a low-smoke,
naturally aspirated engine, adjusted for the altitude, should be used.
Such an engine will probably produce less smoke overall than a
turbocharged engine, which has a greater smoke tendency during
initial acceleration. On the other hand, if a vehicle is to be used for
cruise-type operations, a turbocharged engine should be used be-
cause it will produce less smoke at the same altitude than the
naturally aspirated engine.
ENGINE DERATING CAN HELP REDUCE SMOKE
Engine derating is one of the most widely used methods for
cutting down the production of visible smoke. Derating, which is
the reduction of the available power or rating of an engine, is usu-
ally done by reducing the amount of fuel injected per stroke
throughout the operating speed range of the engine. Engine derating
generally involves injector replacement, fuel pump calibration, re-
duction of fuel delivery pressure, governor setting, or a combination
of these. If done in the field, this type of servicing should be accom-
plished at an authorized service center.
Because of design characteristics, some engines produce more
smoke with an increase in engine speed, especially at the higher end
of the speed range. The reverse is true for other engines. Engine
derating ordinarily reduces smoke output over the whole operating
speed range, especially for engines with moderate- to high-smoke
tendencies (Figure 4). For some types of engines, a reduction in the
maximum recommended or rated speed will result in less smoke,
largely because these engines produce more smoke when they are
operated near the upper end of their speed range. Derating by re-
ducing the maximum recommended speed simply gets rid of the
smoke that would otherwise be produced at higher engine speeds.
Derating by this method does not change the power available in the
rest of the operating speed range.
Fleet operators have found that it is a good practice to buy
vehicles with derated engines because they realize better fuel
economy and extended engine life, with less maintenance. The
slight increase in initial cost is more than compensated for on a
strict economic basis. Adding a turbocharger to certain engines
Guidelines For Reducing Smoke
-------
20
(J
6 10
LU
_
, DERATED 20 hp
Si::*::: DERATED 32 hp
-
•i r-n
ENGINE A
ENGINE B
ENGINE C
Figure 4. Effect of engine derating on exhaust smoke
from three moderate- to high-smoke engines.
makes it possible to get the same power from those engines but
with improved fuel economy and less smoke. When turbochargers
are added solely to increase power output, however, smoke is not
reduced.
FUEL SYSTEM MAINTENANCE AFFECTS SMOKE
One of the advantages of
the automotive diesel engine is
its ability to operate at higher
horsepower levels for long
periods of time with little or no
attention. Maintenance for the
diesel usually involves the fuel
injection or air induction sys-
tems (covered in the next sec-
tion). The condition of the fuel
injection system has a definite
effect on engine smoke output.
The fuel injection system is
probably abused and misused more than any other part of a diesel
engine. The fuel system includes the tank, delivery and return lines,
primary and secondary fuel filters, transfer and injection pumps,
and injectors. The injection system on engines with the "unit type"
injector includes a mechanical assembly to actuate the injector. An
injector often operates satisfactorily for well over 100,000 miles.
Injectors atomize liquid fuel by forcing it under high pressure
through small holes at a certain time in the cycle. Whatever happens
during operation to change the spray characteristics, or the start,
duration, or end of injection may affect engine performance and
10
REDUCTION OF SMOKE AND ODOR
-------
the amount of visible smoke produced. Sticky or worn injectors
that do not seal tightly, have lost their preload due to wear, or
otherwise do not permit clean injection with a full spray pattern
can increase smoke, odor, lubricating oil contamination, and engine
wear. Carbon deposits on the injector may also distort the spray
pattern and cause more smoke.
In order to minimize odor and smoke, some maintenance de-
partments have a rigorous program for periodic injector replace-
ment and repair. Bench test equipment is used to balance the flow
and match injector delivery rates, spray patterns, and penetration.
Bench test equipment is also used to check flow and to calibrate
injection pumps on a periodic basis. The net effect is to make sure
that fuel is injected into each cylinder in a similar way. The addi-
tional cost of maintenance has paid dividends in longer engine life,
lower fuel consumption, and reduced smoke and odor.
Some unit injection systems have a number of wear points in
the mechanism that actuates the injector. These systems operate
from the camshaft of the engine, and, as wear develops, the start of
injection takes place later than desired. The fuel is not delivered
into the combustion chamber at the optimum time for good com-
bustion. Late injection generally results in dense smoke. For some
engines, this problem is most pronounced at high engine speeds, but
for others, it is worse at lower engine speeds. Cleaning or replacing
the injectors is only part of the maintenance necessary to the injec-
tion system; the manufacturer's installation adjustment specifica-
tions should also be strictly followed.
In cases where injection timing becomes faulty more quickly
than the injector itself, timing and adjustments should be checked
more often, or the timing should be advanced slightly to compen-
sate for wear. Caution should be used in advancing the timing,
however, because of the resulting tendency to increase peak pres-
sure in the engine and reduce engine life.
INDUCTION AND EXHAUST SYSTEM MAINTENANCE
AFFECTS SMOKE
Injectors are often blamed for poor performance and excessive
smoke when it is really the induction and exhaust systems of the
engine that need attention. Maintaining these parts of the engine is
generally straight-forward and is easier than maintaining the injector
system. Since the air-fuel ratio substantially affects the amount of
smoke produced by a diesel engine, air induction and exhaust
systems influence smoke production. The general effects of intake
and exhaust restrictions on smoke and power are shown in Figures
5a and 5b. Some engines,-especially naturally aspirated engines, are
Guidelines for Reducing Smoke 11
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INTAKE RESTRICTION
EXHAUST RESTRICTION
Figure 5. General effect of intake and exhaust restric-
tion on smoke and power.
quite sensitive to both intake and exhaust system clogging or re-
striction. Not only does the restriction of the air intake system
affect the smoke output of the engine, but it can mean an economic
loss resulting from the loss of power. A restriction of either the
intake or outlet tends to increase exhaust temperatures and makes
scavenging of the combustion chamber less efficient. Vehicle and
engine manufacturers must work together in the design and installa-
tion of the engine to make sure that intake and exhaust systems do
not limit engine performance.
If the air cleaner is too small or is partially clogged, the engine
simply can not take in enough air. In turn, decreased air intake
lowers the air-fuel ratio, increases exhaust temperatures and smoke,
and reduces power.
The induction system may include a turbocharger, which uses
energy in the exhaust gases to force air into the engine. When the
exhaust gases do not contain enough energy, such as during accele-
ration from idle, the turbocharger may not provide a full air charge.
Valving in the intake and exhaust systems also plays an impor-
tant part in the ability of an engine to breathe and, therefore, in the
smoke it produces. If valve underheads become loaded with de-
posits, the internal restriction that results could eventually require
the rebuilding of cylinder heads. Worn valve guides and bushings,
and piston rings and liners can result in excessive oil consumption,
which can produce blue smoke as well as black. If blue smoke
persists under cruise conditions, a major overhaul may be needed.
The service manuals supplied by engine manufacturers give a
good deal of information on how to control smoke through main-
tenance practices. This information ranges from troubleshooting
hints for determining the cause of smoke to detailed specifications
and tolerances on working parts. These manuals should be used for
establishing preventive maintenance programs, training diesel
mechanics, and assisting in day-to-jday repairs.
12
REDUCTION OF SMOKE AND ODOR
-------
EFFECT OF DRIVER TECHNIQUES ON SMOKE
Interviews with drivers, driver supervisors, and fleet operators
have shown the need to clear up a common misunderstanding about
the relationship between smoke and power:
Power is not proportional to smoke!
This is probably the most misunderstood fact about a diesel. It
is important to note again that a slight increase in available power
could result in a much greater increase in visible smoke, for when an
engine cannot take in more air to support combustion efficiently,
the small increase in horsepower is at a high cost in both fuel
consumption and smoke production (Figure 6).
Instead of making
road adjustments that
increase the amount of
smoke produced, a
driver should report any
complaints he has about
bad performance or
losses in power to the
maintenance and driver
supervisors, and should
keep at it until the
causes are found and
fixed. Chassis dyna-
mometers, which are
often used by service
garages, can be used to
check out losses in
power by measuring the power available at the rear wheels. Over-
fueling the engine by making certain adjustments — such as road
adjustments and changes in the fuel pumps, delivery pressure, or
injector backpressure — just puts off that much longer proper main-
tenance or the use of the right-sized engine for the job. In the
meantime, of course, overfueling results in more smoke. The fleet
operator who sees smoke coming from his diesels should realize that
more than smoke is going up the stacks because smoke can mean
poor performance, shorter life, added maintenance, bad public rela-
tions, and smoke citations — in short, money.
Most drivers have been warned that they should "keep the
engine speed up" to reduce smoke. Underspeeding the engine at full
load, called "lugging," may produce more smoke and can cause
ACCELERATOR POSITION
Figure 6. Smoke and power as affected by
accelerator position.
Guidelines for Reducing Smoke
13
-------
mechanical stress and eventual damage.
The driver should make it a habit to
drive within the manufacturer's recom-
mended speed range.
Driving habits can have a great deal
of influence on the amount of smoke
produced, depending on when and how
long maximum power is required. In
most cases, of course, this is fixed by
the route the driver has to travel. If the
driver will accelerate gradually, when
possible, he can reduce the duration
and amount of smoke production. On
truck-tractors with vertical stacks, some
fleets have installed a special mirror
that shows the tip of the exhaust pipe
and helps the driver minimize smoke by
"feathering" the accelerator pedal
during acceleration and cruise condi-
tions. When the driver of a loaded
vehicle is negotiating a hill or trying to
hold a minimum route speed and keep
traffic flow orderly, he has no choice
except to operate the vehicle at its
maximum power condition. For nonturbocharged engines, this may
be the maximum smoke condition as well.
EFFECT OF TYPES OF FUEL ON SMOKE
Diesel fuel is usually available as two types: Number 1, which is
a light or kerosene-like fuel, and Number 2, which is a heavier
fraction. Number 1 fuel is sometimes used by diesels involved in
stop-and-go, medium-duty operations in cities. Number 2, because
it costs less and has a higher heat content, is used almost entirely
for line-haul trucking and for other uses that require continuous,
high-power output. The main thing to remember in selecting diesel
fuel is that the fuel should conform to the specifications around
which the engine was designed.
The most important properties of fuels are volatility, gravity,
viscosity, ignition quality (cetane rating), and hydrocarbon compo-
sition. Volatility, or distillation range, is a measure of the vapori-
zation characteristics of a fuel. Generally speaking, less smoke will
be produced, without any fuel rate adjustment, if a low-boiling-
range, low-gravity Number 1 diesel fuel is used in place of the
14
REDUCTION OF SMOKE AND ODOR
-------
heavier, higher-boiling-range Number 2 fuel. The use of the lighter
fuel results in a loss in power, though, and is actually a type of
derating. Fuel Number 1 has less weight per volume than Number 2.
Since fuel is injected by volume, less weight is injected per stroke—
and fewer Btu's—so that less smoke and power are produced. If a
fuel has too high a specific gravity or too high a maximum boiling
point, it may cause excessive carbon deposits and exhaust smoke.
Viscosity is an important property because it influences the
flow and lubricating characteristics of fuels. Viscosity is critical in
the fuel delivery system, especially in the injectors, where it affects
the pattern of the fuel spray. If the fuel used has lower viscosity
than that recommended, the fuel may leak past the plunger during
the downstroke of some unit injectors. This leakage results in re-
duced fuel delivery, less power, and less smoke. Viscosity affects
the size of fuel droplets and volatility affects the rate of evapora-
tion of the droplets. Together, viscosity and volatility can affect
combustion efficiency and, therefore, the exhaust smoke produced.
The importance of ignition quality, or cetane rating, depends on the
design of the engine.
It goes without saying that the fuel should be free of foreign
matter such as water, dirt, or other particles, and that its quality
should be uniform and dependable. Although quality fuels are avail-
able from all major producers, contamination may occur in storage,
where moisture and corrosion can build up in fuel lines and tanks.
To protect the finely machined components of the fuel injection
system, there are usually at least two filter elements located in the
fuel supply system. A primary filter is usually located near the fuel
tank, followed by a secondary filter. Some types of injectors have a
screen in the body of the injector for final filtration. In addition to
routine cleaning or replacement of the fuel filters, routine cleaning
or draining of the fuel tanks is recommended. The suction pipes
through which the fuel is drawn from the vehicle tank should clear
the bottom of the tank so that water and sediment are not picked
up. Dispensing and storage tanks must be included in a successful
fuel cleanliness program.
There are a large number of diesel fuel additives on the market
that are intended for fuel storage stability, engine cleanup, and/or
combustion improvement. Additives have to be selected with care,
however, since they could possibly become pollutants themselves.
Fuel storage stabilizers are usually combustible enough that there is
little chance they will conttibute to pollution. In fact, if fuel stabi-
lizers help reduce engine malfunctions by keeping the quality of
fuel consistent, then they indirectly reduce air pollution.
Some additives are excellent dispersants and will suspend the
water and sediment in the.fuel that Sometimes tend to clog primary
Guidelines for Reducing Smoke 15
-------
filters. All tanks and supply lines must be thoroughly cleaned be-
fore such additives are used. Other additives have been shown to
extend injection life, so that maintenance costs and the malfunc-
tions that cause smoke problems are reduced.
Additives that are rated as combustion improvers frequently
contain metals, and there is growing concern that even though they
reduce smoke, they may produce metal-containing combustion
products that are toxic. Such fuel additives should not be used if
they release into the atmosphere any noxious or toxic materials
that are not ordinarily emitted by the engine.
EFFECT OF CONTROL DEVICES ON SMOKE
A number of devices and techniques have been proposed for
the reduction of smoke emissions. Turbocharging, discussed previ-
ously, is helpful, especially during cruise-type conditions. During
accelerations, rack limiters or aneroid controls can help maintain
the air-fuel ratio so that the engine performs well and smoke is
controlled. Turbocharging kits have been added by some operators
to existing engines. The kit permits the uprating of engine power,
but this is not usually done when the goal is to reduce smoke as
much as possible.
Dual fuel or fumigation systems using liquefied petroleum gas
(LPG) have found limited application in diesel trucks. These sys-
tems meter a small amount of LPG into the intake manifold to start
precombustion reactions that help the fuel burn better. Though the
use of LPG reduces smoke somewhat, the need for a dual fuel
supply limits the popularity of this system. Then too, the warranty
may be voided because overfueling can occur with the LPG fumiga-
tion system.
A number of methods have been tried for changing the appear-
ance of the smoke plume. Some operators have tried discharging the
smoke downward, near the drive wheels, to spread it out, only to
find that the smoke looked worse. In many cases, the downward-
directed exhaust smoke was not only more noticeable but obscured
the visibility of other motorists. Sometimes, depending on design,
the smoke entered the vehicle cab and bothered the driver. Some
operators of stop-and-go-type vehicles that are properly maintained
have found that an exhaust stack aimed toward the ground or
parallel to it, and near the ground, is better than a vertical stack for
keeping soot near the road level.
Other methods for changing the appearance of the exhaust
plume that have been tried include adding fresh air to the exhaust
pipe to dilute the smoke, or using a number of small pipes to break
16 REDUCTION OF SMOKE AND ODOR
-------
up the single plume. Pipes of different sizes, shapes, and arrange-
ments for inducting dilution air have also been tried. None of these
has been satisfactory and, more often than not, they made the
smoke more noticeable. Dilution is not really a control measure and
is not recommended because it does not reduce the amount of
particles given off by the exhaust, just the concentration.
Improved component matching and the wider usage of auto-
matic transmissions are potential methods for reducing odor and
smoke. The heavy-duty automatic transmission is expected to
improve driveability, route speed, and maintenance, and it may
reduce odor and smoke considerably by preventing the engine from
being overloaded throughout its speed range.
Passing the exhaust through afterburners or catalytic mufflers
has been considered, but these devices are not technically promising
for smoke control because they need very high temperatures to
oxidize the carbon particles in the exhaust. The electrostatic precip-
itation of carbon particles has been tried with diesel exhaust, but is
not practical because the carbon particles quickly coat the inside
and short-circuit the system. Mechanical filters and separators have
also been suggested for removing smoke from the exhaust stream
but these methods usually increase exhaust backpressure beyond
allowable limits, or they are just too inefficient and involve too
much cleaning and repair.
Federal smoke requirements for 1970 diesel engines have been
met, however, without external devices or equipment such as those
mentioned. The test procedure includes a 1,000-hour run to show
that the engine will continue to meet the smoke limits over a long
time. Actually, the need for external devices for controlling smoke
from engines qualified under the 1970 Federal smoke standards
should not be necessary if engine use is in line with manufacturer's
recommendations. The critical requirements for reducing smoke
are: (1) the use of an engine large enough to do the job, (2) the use
of the appropriate fuel for the engine selected, and (3) an emphasis
on the maintenance necessary to keep the engine running properly.
Guidelines for Reducing Smoke 17
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GUIDELINES FOR REDUCING ODOR
Guidelines for reducing diesel odor are much harder to formu-
late than those for smoke because less is known about the causes of
and factors influencing diesel odor. The guidelines are given under
the same headings used in the smoke section, but they are much
briefer because of the level of current knowledge.
EFFECTS OF ENGINE SIZE AND USE ON ODOR
The selection of a diesel engine that produces a minimum of
odor involves the entire vehicle design and goes beyond engine type
or size. Broadly speaking, incomplete combustion brought about by
the wrong air-fuel ratio, inadequate fuel injection, inadequate time
for the burning process, insufficient temperature to promote com-
plete combustion, incorrect fuel, incorrect speed, etc., are all
factors that may influence the odor produced by engines.
Experience indicates that engines operated in highway-type uses
in vehicles equipped with vertical stacks put out odors that are less
noticeable than those from diesel-powered vehicles equipped with
horizontal exhaust pipes. An engine will usually produce less odor
when operated under highway-type conditions than in city-type,
stop-and^go service. Although not all the reasons for this are under-
stood, engine temperature and engine speed may have some influ-
ence. Engine size—that is, displacement and power output—does not
influence exhaust odor as much as it affects smoke. At low or high
idle (zero power output), however, some engines produce a notice-
able odor but no visible smoke. Odor levels from four-stroke turbo-
charged engines are not generally different from those from natu-
rally aspirated four-stroke engines.
The position of the exhaust pipe, however, has a gross effect on
odor. The quantity or quality of theodors given off is not reduced,
but the gases are dispersed and are usually less noticeable to the
public. For this reason, less perceptible odors result from vertical
stacks than from low-to-the-ground, or horizontal, exhausts.
CAN ENGINE DERATING HELP REDUCE ODOR?
Field and laboratory experience indicates that there is little
advantage in derating an engine for the sole purpose of reducing
19
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odor. The effect of a moderate amount of derating in a well-
maintained, properly adjusted engine is usually difficult to see.
Speed and power variations within the operating range of the engine
seem to have only a slight effect on odor.
FUEL SYSTEM MAINTENANCE AFFECTS ODOR
Engine maintenance can have an effect on odor. Depending on
the type of engine, fuel injection characteristics can have as much
effect on odor as they do on smoke. In most cases, improving
combustion reduces both smoke and odor.
Most manufacturers of automotive diesel engines have active
programs aimed at reducing exhaust smoke, odor, and other emis-
sions and periodically they put out improved products that can be
incorporated into older engines. As an example, the crown-type S
fuel injector used in older Detroit Diesel 71-E series engines can be
replaced with needle-type N injectors, a change that reduces smoke
and odor. Recently, an improved version of the N-type injector was
put into production and is currently furnished in new engines.
Almost the same reduction in odor from older engines can be ob-
tained by replacing, at a nominal cost, the S-type injector used in
many of the existing engines with the new N-type injector. The
reduction in odor possible with this change is shown in Figure 7.
o
f—j STOCK-USED
60S INJECTORS
pjNEW 60N
^INJECTORS
CRUISE IDLE IDLE- DECELER-
ACCELER- ATION
ATION
Figure 7. Effect of injector type on odor.
20
REDUCTION OF SMOKE AND ODOR
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An injector with deteriorated spray pattern and cutoff may
cause significant levels of odor. For example, if fuel is injected into
the combustion chamber after the normal cutoff point, it may be
only partially burned. During deceleration of certain engines, a
fairly pungent odor occurs if the injectors are badly worn and do
not positively cut off the fuel. The effect of late injection on odor
production has yet to be investigated; however, it is not expected to
be significant. Diesels often produce higher odor levels in the winter
than in the summer, or in cold climates relative to warm climates.
In addition overcooling may. increase odor. High-temperature
thermostats, thermostatically controlled radiator shutters, or vari-
able-speed, thermostatically controlled fans, if properly maintained,
are all devices that will keep engine temperatures at an optimum
level.
INDUCTION AND EXHAUST SYSTEM MAINTENANCE
AFFECTS ODOR
The induction system has a significant effect on odor in some
uses and in some engine types. A seriously restricted air intake may
result in exhaust products with highly objectionable odors.
Many fleet maintenance departments have internal maintenance
procedures, preventive maintenance schedules, and service policies
that are based on manufacturers' recommendations and service
manuals as well as on their own direct experience. The result is an
individualized approach, tailored closely to the fleet application and
need. Service bulletins concerning parts and procedural changes are
published by the manufacturer and should be incorporated into the
fleet maintenance procedures. In addition, detailed procedural
instructions to the mechanic are usually effective in keeping odor as
low as possible through proper engine maintenance. If smoke and
odor control are not presently emphasized, a continuing program of
information, education, and training that advocates a clean, clear
exhaust is recommended. When it is carefully used, an incentive
program or some other recognition can show management's concern
about exhaust emissions and can result in better odor and smoke
control.
EFFECT OF DRIVER TECHNIQUE ON ODOR
Just as he can influence the amount of smoke from his vehicle,
the driver is able to reduce odor under certain conditions. Road
adjustments of the engine fueling system made to increase power
also increase the smoke output and can increase odor as as well. A
Guidelines for Reducing Odor 21
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double reason exists, then, for not making such adjustments. Fur-
thermore, keeping the engine speed within the manufacturer's
recommended operating speed range helps minimize odor and
smoke. It is also important to keep the engine temperature up,
particularly during cold weather.
Another way in
which the drivt" and
only the driver can
reduce odor is to
minimize idle time
and light-load opera-
tion. Unburned fuel
and products of com-
bustion from some
engines condense in
the exhaust, particu-
larly during warmup
or shortly after en-
gine startup, and a
more intense odor
may result. A good
rule is to start the en-
gine and get it under load as soon as practical. Idling the engine
generally does not do much to warm up the engine; it only loads
the combustion chamber with unburned fuel, which may dilute the
lubricating oil. Local delivery operations are particularly bad in this
respect because this type of operation usually includes a great deal
of idle time, either at the end of the run or at delivery points.
Actually, the engine should be shut off instead of being allowed to
idle for long periods. Some localities have regulations that restrict
the period of engine idle.
Any program for reducing vehicle odor and smoke should in-
clude driver training and education. Drivers should be encouraged to
follow the recommended driving procedures described in hand-
books published by most vehicle manufacturers. Management can
improve the appearance of a fleet and reduce complaints or cita-
tions by encouraging drivers attitudes and habits that result in less
smoke and odor. A driver should be constantly and habitually
aware of the part he can play in reducing odor and smoke. Manage-
ment has successfully increased driver awareness by asking drivers
on the road to watch other trucks in their fleet for excessive smoke.
If drivers cooperate and maintenance crews are alert during opera-
tion near depots, they can be an unbeatable team. Management
must take the initiative in obtaining the cooperation of both
22
REDUCTION OF SMOKE AND ODOR
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operation and maintenance.
Scheduling should not be so
critical, or replacement units so
few and overworked, that a
"smoker" or a malodorous vehi-
cle cannot be brought in for a
check after being reported.
EFFECTS OF TYPES OF FUEL ON ODOR
In general, engines are optimized by the manufacturer to
operate with either Number 1 or Number 2 fuel. An engine opti-
mized for Number 1 fuel may produce a more intense odor when it
is operated on a Number 2 fuel. For exam-
ple, a heavier, burnt-smoky, and sometimes
more acrid odor has been reported from a
municipal bus optimized for Number 1 but
operating on Number 2 fuel. The usual kero-
sene-type fuel odor was less noticeable, how-
ever. Studies of fuels with widely different
properties and impurities have not pinpoint-
ed specific cause and effect relationships.
The additives used for controlling odor
are either maskants or counteractants. A
maskant has an odor that covers up the
original odor; a counteractant reacts to form
a new odor or to destroy the odor originally
present. Some operators have found that the
use of odor additives, mainly maskants, has
helped reduce complaints. Laboratory tests
have not shown that additives reduce the in-
tensity of an odor; however, they do change
the quality of the odor so that it sometimes
becomes more acceptable. No unusual en-
gine difficulties or maintenance problems
Guidelines for Reducing Odor
23
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have been reported when the maskant-type additives have been used.
They do have certain disadvantages, though, including their cost, the
fact that they may have or produce an odor that is more unpleasant
to some people than the original odor, and the fact that they may
create more toxic material in the exhaust. For these reasons, their use
is not recommended.
Most diesel manufacturers have specific fuel recommendations
for satisfactory engine operation. In addition, marketers of diesel
fuel can help match the fuel specification to the particular engine
and job application. The advice of both the engine manufacturer
and the refiner should be obtained when fuel specifications are
being determined, especially for abnormal climatic conditions or
unusual duty cycles. Some engine manufacturers specifically warn
against the use of fuel additives other than those they have
approved; in some cases, the engine warranty may be jeopardized if
additives are used.
EFFECT OF CONTROL DEVICES ON ODOR
Measures for controlling diesel odor include catalytic mufflers
and many of the methods proposed for smoke reduction. Catalytic
mufflers of the precious-metal type have been effective in reducing
odor. Because of the dependence of reaction rates on exhaust
temperature, the universal application of even the best catalytic
muffler is not practical. In some engine uses, such as stop-and-go
city deliveries, catalytic mufflers are only partially helpful in reduc-
ing odor. They are more effective with the high exhaust tempera-
tures and nearly constant exhaust flow that are typical of a station-
ary engine or line-haul vehicle. Most fleet operators have thought
that the initial cost of equipment and installation (as much as $500
on some vehicles) and the costs of operating and maintaining them
do not justify the use of catalytic mufflers except when extended
operation in confined, unventilated areas is required. Even then,
exhaust odors and irritants may be reduced less than 50 percent.
Some designs require the periodic or even continuous burnoff of
contaminants to renew the surface of the catalyst and maintain
performance. This requires additional equipment and operating
expenses; furthermore, in some applications, the regeneration
process could present a safety problem. Further research and
development are required to determine the full potential of cata-
lytic mufflers as well as to make them usable on a large variety of
motor vehicles.
As mentioned earlier, the position and the direction of vehicle
exhaust stacks can have an important effect on the odor noticed by
24 REDUCTION OF SMOKE AND ODOR
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the pedestrian or motorist.
More than just diluting the
exhaust fumes, such stacks
can direct odorous gases
away from the observer.
This does not reduce the
total amount of odor given
off, of course, but the
nuisance effect can be re-
duced. Three to ten times
the atmospheric dispersion
can be achieved by using a
vertically directed stack
ending 12 feet above the
road instead of a horizon-
tal pipe 1 foot off the
road. To reduce the no-
ticeability of odor, straight
vertical stacks ending as
high as practical above the
road are recommended.
Some operators have
found that in certain stop-
and-go operations, the use
of throttle-opening delays
or dash pots to control the rate of acceleration has reduced odor
and smoke emissions. Fuel injector design seems to have an impor-
tant bearing on odor and can potentially reduce both smoke and
odor futher. New, improved fuel injectors, for example, were found
to reduce odor and smoke by aboyt one-third in several low-
compression two-stroke engines, and the cost per injector is nomi-
nal.
The use of other control devices such as afterburners and wet
scrubbers has been suggested. Direct flame incineration, though
successful in controlling odor in some stationary applications, has
not been applied to mobile engines. Variations in exhaust flow and
the differences in engines and uses are seen as disadvantages, along
with probable cost and safety factors. Wet scrubbers are even less
practical because of the weight and space needed to handle large
quantities of high-temperature exhaust without developing exces-
sive backpressure.
Guidelines for Reducing Odor
25
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SUMMARY OF RECOMMENDATIONS
The purpose of this Guide is to help minimize odor and smoke
from diesel-powered vehicles. A team effort by everyone involved in
the purchase, operation, maintenance, application,and driving of
diesel-powered vehicles is needed to achieve the goal of minimum
odor and smoke. The suggestions offered are summarized below:
1. Since the air-fuel ratio has a greater effect on smoke emis-
sions from diesel engines than other factors, anything that
tends to reduce this ratio, such as excess fuel or insufficient
air, will lead to more smoke and should, therefore, be
avoided.
2. Engine derating, the method usually employed to reduce
smoke, is recommended and should be considered good pro-
curement policy.
3. Vehicle and engine maintenance programs must include a
continuing campaign against faulty fuel injectors, restricted
air cleaners, and incorrect adjustment of injectors.
4. Turboeharging can be used to minimize visible smoke during
operation at high elevations. New engines can be procured
with this feature, and kits may be obtained from some
manufacturers to modify existing naturally aspirated en-
gines.
5. Odor and smoke from certain buses and trucks powered by
two-cycle engines can be reduced by the use of improved
needle-type unit injectors in place of the original crown-
type injectors. Use of the needle injector in the older
E-series engine may reduce odor and smoke to the level
emitted from the newer N-series engine. This recommended
conversion can be accomplished at a nominal cost per
injector.
6. Drivers should develop those driving habits that are known
to minimize odor and smoke emissions. Drivers should, for
instance, avoid excessive engine idle and underspeeding
(lugging). In addition, they should use a rear-view mirror
aimed at the exhaust outlet so that they are aware of exces-
sive smoke emissions. An active and continuing driver
education program should be instituted to encourage drivers
to operate their vehicles in a manner that produces the least
possible amounts of smoke and odor.
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7. The location and direction of the vehicle exhaust stack or
pipe can make a significant difference in the noticeable and
potentially objectionable aspects of smoke and odor. To
reduce the noticeability of odor, the use of a straight verti-
cal stack terminating as high above the ground as possible
should be considered. In some stop-and-go applications in
public areas, the use of a horizontal, low-to-the-ground
exhaust has been found necessary to reduce fallout of soot.
8. The fuel selected should conform to the specifications
recommended by the manufacturer of the engine.
9. Fuel additives for the reduction of diesel odor and smoke
should not be used if they cause the emission into the
ambient air of any noxious or toxic matter that is not ordi-
narily emitted by the engine.
28 REDUCTION OF SMOKE AND ODOR
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