U.S. ENVIRONMENTAL PROTECTION AGENCY EPA 400-F-92-007

OFFICE OF MOBILE SOURCES

FACT SHEET OMS-5

August, 1994

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.

 

• Exhaust 

Emissions

• Refueling Losses

• Evaporative Emissions

The Combustion Process

Gasoline and diesel fuels are mixtures of hydrocarbons, compounds which contain 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.
2 of 4

Automobile Emissions: An Overview

“Perfect” Combustion

FUEL (hydrocarbons) + AIR (oxygen and nitrogen)

CARBON DIOXIDE + water + unaffected nitrogen

Typical Engine Combustion

FUEL + AIR UNBURNED HYDROCARBONS + NITROGEN OXIDES

 + CARBON MONOXIDE + CARBON DIOXIDE + water

Exhaust Pollutants

• HYDROCARBONS

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 irritates 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.

• NITROGEN OXIDES (NOx)

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

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.

• CARBON DIOXIDE

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

warming

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:
3 of 4

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 reduced hydrocarbon and carbon monoxide emissions. The use of converters provided 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 hydrocarbons to carbon dioxide and water, and also helps reduce nitrogen oxides to elemental 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 diagnostic systems that identify malfunctioning emission controls.
4 of 4

Automobile Emissions: An Overview

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 vehicle emission control programs will emphasize hydrocarbon and nitrogen oxide reductions. 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.

Engine

Carburetor or

Fuel Injector

Computer

Catalytic

Converter

Muffler

Tailpipe

Oxygen Sensor

Typical Catalyst System for Exhaust Emissions

Basic Controls for Exhaust and 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 

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