Automobile Emissions

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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

!he power to move a car comes from burning fuel in an engine. "ollution from cars comes from by#products of this combustion process $e%haust& and from evaporation of the fuel itself.

• Exhaust Emissions • Refueling Losses • Evaporative Emissions The Combustion Process 'asoline and diesel fuels are mi%tures of hydrocarbons, compounds which contain hydrogen and carbon atoms. (n a “perfect engine, o%ygen in the air would convert all the hydrogen in the fuel to water and all the carbon in the fuel to carbon dio%ide. )itrogen in the air would remain unaffected. (n reality, the combustion process cannot be “perfect, and an d automotive engines emit several types of pollutants. 2 of 4

Automobile Emissions: An Overview

“Perfect” Combustion *+E $hydrocarbons& - (/ $o%ygen and nitrogen& 0/B1) D(12(DE - water - unaffected nitrogen

Typical Enine Combustion *+E - (/ +)B+/)ED 34D/10/B1)5 - )(!/1'E) 12(DE5 - 0/B1) 61)12(DE - 0/B1) D(12(DE - water 

E!haust Pollutants " #$%&OCA&'O(S 3ydrocarbon emissions result when fuel molecules in the engine do not burn or   burn only partially. 3ydrocarbons react in the presence of nitrogen o%ides and sunlight to form ground#level ozone, a ma7or component of smog. 1zone irritates the eyes, damages the lungs, and aggravates respiratory problems. (t is our most widespread and intractable urban air pollution problem.  number of  e%haust hydrocarbons are also to%ic, with the potential to cause cancer. " ()T&O*E( O+)%ES ,(O!+nder the high pressure and temperature conditions in an engine, nitrogen and o%ygen atoms in the air react to form various nitrogen o%ides, collectively known as )1%. )itrogen o%ides, like hydrocarbons, are precursors to the formation of ozone. !hey also contribute to the formation of acid rain. " CA&'O( .O(O+)%E

 

0arbon mono%ide $01& is a product of incomplete combustion and occurs when carbon in the fuel is partially o%idized rather than fully o%idized to carbon dio%ide $01 &. 0arbon mono%ide reduces the flow of o%ygen in the bloodstream and is particularly dangerous to persons with heart disease. " CA&'O( %)O+)%E (n recent years, the +.5. Environmental "rotection gency $E"& has started to view carbon dio%ide, a product of “perfect combustion, as a pollution concern. 0arbon dio%ide 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 3ydrocarbon pollutants also escape into the air through fuel evaporation. 8ith 8ith today’s efficient efficient e%haust emission controls and today’s gasoline formulations, evaporative losses can account for a ma7ority of the total hydrocarbon pollution from current model cars on hot days da ys when ozone levels are highest. Evapo Evaporative rative emissions occur several ways9 3 of 4

Automobile Emissions: An Overview

 DIURNAL: 'asoline evaporation increases as the temperature rises during the  DIURNAL: day, heating the fuel tank and venting gasoline vapors.  RUNNING LOSSES:  RUNNING LOSSES: !he hot engine and e%haust system can vaporize gasoline when the car is running.  HOT SOAK: SOAK: !he engine remains hot for a period of time after the car is turned off, and gasoline evaporation continues when the car is parked.  REFUELING:  REFUE LING: 'asoline vapors are always present in fuel tanks. !hese vapors are forced out when the tank is filled with li:uid fuel.

/hat #as 'een %one to Control Automobile Emissions0 !he 0lean ir ct of ;<=> gave E" broad authority to regulate motor vehicle  pollution, and the gency’s gency’s emission control policies policies have become progressively more stringent since the early ;<=>’s. E" standards dictate how much pollution autos may emit but automakers decide how to achieve the pollution limits. !he emission reductions of the ;<=>’s came about because of fundamental improvements in engine design, plus the addition of charcoal canisters to collect hydrocarbon vapors and e%haust gas recirculation valves to reduce nitrogen o%ides. !he advent of “first generation catalytic converters in ;<=? ;< =? significantly reduced hydrocarbon and carbon mono%ide emissions. !he use of converters provided a huge indirect benefit as well. Because lead inactivates the catalyst, ;<=? saw the widespread introduction of unleaded gasoline. !his resulted in dramatic reductions in ambient lead levels and alleviated many serious environmental and human health concerns associated with lead pollution. !he ne%t ma7or milestone in vehicle emission control technology technolog y came in ;<@># @;. (n response to tighter standards, manufacturers e:uipped new cars with even more sophisticated emission control systems. !hese systems generally include a “three#way catalyst $which converts carbon mono%ide and hydrocarbons to carbonand dio%ide and water, wa ter, also helps reduceand nitrogen o%ides to elemental nitrogen o%ygen&, plus anand on#board computer o%ygen sensor.

 

!his e:uipment helps optimize the efficiency of the catalytic converter. Aehicle Ae hicle emissions are being further reduced by provisions of the ;<<> 0lean ir  ct. 6obile source provisions include even tighter tailpipe standards, increased durability, improved improved control of evaporative emissions, and computerized co mputerized diagnostic systems that identify malfunctioning emission controls. 4 of 4

Automobile Emissions: An Overview

/hat Emission Control .eant for Air1970 1uality0 Eforts#as by government and industry since have greatly reduced typical vehicle emissions. In those same years, however, the number o miles we drive has more than doubled. he increase in travel has ofset much o the emission control progress.  he net result is is a modest reductio reduction n in each automotive pollutant e e!cept !cept lead, or which aggregate emissions have dropped by more than 9" percent. #ith o$one continuing to present a persistent urban air pollution problem, uture vehicle emission control programs will emphasi$e hydrocarbon and nitrogen o!ide reductions. %arbon mono!ide control will remain critical in many cities, and limits on vehicle&generated vehicle&generate d carbon dio!ide may become important in the uture.

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 You can write to us at the th e E! "ational #ehicle #ehicle and $uel Emissions %aboratory& '()( lymouth *oad& !nn !rbor& M+ ,/(. Our phone number is 0112 ))-,111.

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