EPA 400-F-92-007
   Automobile Emissions: An  Overview
Cars and Pollution
Emissions from an individual car are generally low, relative to the smokestack
image many people associate with air pollution. But in numerous cities across
the country the personal automobile is the single greatest polluter, as emissions
from millions of vehicles on the road add up. Driving a private car is probably a
typical citizen's most "polluting" daily activity.
Sources of Auto Emissions
The power to move a car comes from burning fuel in an engine.  Pollution from
cars comes from by-products of this combustion process (exhaust) and from
evaporation of the fuel itself.
      Evaporative Emissions
                                            Refueling Losses
The Combustion Process
Gasoline and diesel fuels are mixtures of hydrocarbons, compounds which con-
tain hydrogen and carbon atoms. In a "perfect" engine, oxygen in the air would
convert all the hydrogen in the fuel to water and all the carbon in the fuel to
carbon dioxide. Nitrogen in the air would remain unaffected. In reality, the
combustion process cannot be "perfect," and automotive engines emit several
types of pollutants.
                                                    FACT SHEET OMS-5
                                                      August, 1994

  Automobile Emissions: An Overview
"Perfect" Combustion
 FUEL (hydrocarbons) + AIR (oxygen and nitrogen)
                 CARBON DIOXIDE + water + unaffected nitrogen

Typical Engine Combustion
Exhaust Pollutants
  Hydrocarbon emissions result when fuel molecules in the engine do not burn or
  burn only partially  Hydrocarbons react in the presence of nitrogen oxides and
  sunlight to form ground-level ozone, a major component of smog. Ozone irri-
  tates the eyes, damages the lungs, and aggravates respiratory problems. It is
  our most widespread and intractable urban air pollution problem.  A number of
  exhaust hydrocarbons are also toxic, with the potential to cause cancer.
  Under the high pressure and temperature conditions in an engine, nitrogen
  and oxygen atoms in the air react to form various nitrogen oxides, collectively
  known as NOx. Nitrogen oxides, like hydrocarbons, are precursors to the
  formation of ozone. They also contribute to the formation of acid rain.
  Carbon monoxide (CO) is a product of incomplete combustion and occurs when
  carbon in the fuel is partially oxidized rather than fully oxidized to carbon
  dioxide (CO ). Carbon monoxide reduces the flow of oxygen in the bloodstream
  and is particularly dangerous to persons with heart disease.
  In recent years, the U.S. Environmental Protection Agency (EPA) has started to
  view carbon dioxide, a product of "perfect" combustion, as a pollution concern.
  Carbon dioxide does not directly impair human health, but it is a "greenhouse
  gas" that traps the earth's heat and contributes to the potential for global

  Evaporative Emissions
Hydrocarbon pollutants also escape into the air through fuel evaporation. With
today's efficient  exhaust emission controls and today's gasoline formulations,
evaporative losses can account for a majority of the total hydrocarbon pollution
from current model cars on hot days when ozone levels are highest. Evaporative
emissions occur  several  ways:
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  Automobile Emissions: An Overview
DIURNAL: Gasoline evaporation increases as the temperature rises during the
day, heating the fuel tank and venting gasoline vapors.
RUNNING LOSSES: The hot engine and exhaust system can vaporize gasoline
when the car is running.
HOT SOAK: The engine remains hot for a period of time after the car is turned
off, and gasoline evaporation continues when the car is parked.
REFUELING: Gasoline vapors are always present in fuel tanks. These vapors
are forced out when the tank is filled with liquid fuel.
What Has Been Done to Control Automobile Emissions?
The Clean Air Act of 1970 gave EPA broad authority to regulate motor vehicle
pollution, and the Agency's emission control policies have become progressively
more stringent since the early 1970's.

EPA standards dictate how much pollution autos may emit but automakers
decide how to achieve the pollution limits. The emission reductions of the 1970's
came about because of fundamental improvements in engine design, plus the
addition of charcoal canisters to collect hydrocarbon vapors and exhaust gas
recirculation valves to reduce nitrogen oxides.

The advent of "first generation" catalytic converters in 1975 significantly re-
duced hydrocarbon and carbon monoxide emissions.  The use of converters pro-
vided a huge indirect benefit as well.  Because lead inactivates the catalyst, 1975
saw the widespread introduction of unleaded gasoline. This resulted in dramatic
reductions in ambient lead levels and  alleviated many serious environmental
and human health concerns associated with lead pollution.

The next major milestone in vehicle emission control technology came in 1980-
81. In response to tighter standards, manufacturers equipped new cars with
even more sophisticated emission control systems. These systems generally
include a "three-way" catalyst (which converts carbon monoxide and hydrocar-
bons to carbon dioxide and water, and also helps reduce nitrogen oxides to el-
emental nitrogen and oxygen), plus an on-board computer and oxygen sensor.

This equipment helps optimize the efficiency of the catalytic converter.
Vehicle emissions are being further reduced by provisions of the 1990 Clean Air
Act.  Mobile source provisions include  even tighter tailpipe standards, increased
durability, improved control of evaporative emissions, and computerized diagnos-
tic systems that identify malfunctioning emission controls.
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   Automobile Emissions: An Overview
 Basic Controls for Exhaust and Evaporative Emissions
                             Typical Catalyst System for Exhaust Emissions
  Typical Canister System for Evaporative Emissions
                           gas vapors from the engine and fuel
                           tank flow into the canister

                           vapors in the canister flow back into
                           the engine when the car is running
What Has Emission Control Meant for Air Quality?
Efforts by government and industry since 1970 have greatly reduced typical vehicle
emissions. In those same years, however, the number of miles we drive has more than
doubled. The increase in travel has offset much of the emission control progress.
The net result is a modest reduction in each automotive pollutant except lead, for which
aggregate emissions have dropped by more than 95 percent.
With ozone continuing to present a persistent urban air pollution problem, future ve-
hicle emission control programs will emphasize hydrocarbon and nitrogen oxide re-
ductions.  Carbon monoxide control will remain critical in many cities, and limits on
vehicle-generated carbon dioxide may become important in the future.
For More Information:
The Office of Mobile Sources is the national center for research and policy on air
pollution from highway and off-highway motor vehicles and equipment.  You
can write to us at the EPA National Vehicle and Fuel Emissions Laboratory,
2565 Plymouth Road, Ann Arbor; MI 48105. Our phone number is (313) 668-4333.
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