AND
  FROM
    POWERED
U. S. ENVIRONMENTAL PROTECTION AGENCY
     . v
\

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 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

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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

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                      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

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  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.

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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

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            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

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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

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    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

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          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,

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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.
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                        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

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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

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          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

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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

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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

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                         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

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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

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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

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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.
                               27

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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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