Fuels and Emissions 101

Fuels and Emissions 101


Polluting emissions from vehicles and their relationship to fuels1


Vehicles powered by internal combustion engines produce two main types of greenhouse gas emissions: evaporative and tailpipe emissions.


La evaporación y emisiones, Source: INE, SEMARNAT, 2005


1) Evaporative Emissions are caused by the evaporation of fuel; they can occur when the vehicle is in motion or in rest. The evaporation process depends on the characteristics of the vehicle, geography and meteorological conditions (such as altitude and temperature), and the vapor pressure of the fuel. When ethanol is used to oxygenate fuels it swells the vehicles’ gaskets, increasing fugitive emissions, even if the vapor pressure is maintained constant. Some of the processes that cause evaporative emissions in vehicles are:

  • Diurnal emissions generated in the vehicle’s fuel system are caused by changes in temperature throughout the day.
  • Emissions from the fuel system when a hot engine is turned off occur because of the residual heat volatilization factor of fuels.
  • Evaporative emissions while driving occur when the engine is in its normal operation.
  • Emissions at rest with a cold engine occur mainly because of the permeation of the fuel system components.
  • Evaporative emissions during refueling consist of vapor leaks from the fuel tank during the charging process; these occur while the vehicle is at the service stations.3


2) Tailpipe Emissions are the product of fuel combustion (gasoline, diesel or other liquefied fuel or biofuel) and comprise a number of pollutants such as carbon monoxide and carbon dioxide, hydrocarbons, nitrogen oxides, particulate matter, and certain pollutants like sulfur. Tailpipe emissions depend on the characteristics of the vehicle, like the technology used, the emission control system, the maintenance and other operational factors (such as velocity, frequency and intensity of accelerations), and fuels composition.4


Pollutant Characteristics How are they produced Impacts to health
Hydrocarbon (HC)1 There are a variety of hydrocarbons or volatile organic compounds (VOC). Caused by an incomplete combustion process. VOCs are ozone precursors and some (such as benzene, formaldehydes and acetaldehydes) are highly toxic to humans.
Carbon monoxide (CO)2 CO is a colorless, odorless gas. Caused by incomplete combustion and when carbon in fuels are partially oxidized. CO adheres easily to hemoglobin in blood and reduces the flow of oxygen into the bloodstream causing alterations in the nervous and cardiovascular systems.
Carbon dioxide (CO2)3   Caused when combustion is completed in the presence of sufficient oxygen to oxidize fuel’s entire carbon. Carbon dioxide poses no threat to health but it is a greenhouse gas with a significant heat-trapping factor and contributes considerably to global warming.
Nitrogen oxides (NO)4 Nitrogen oxides, like hydrocarbons, are ozone precursors. In the presence of humidity, NO are converted into nitric acids, thus contributing to acid rain. When both the pressure and temperature in the engine are high, the nitrogen atoms and oxygen react to form NO, nitrogen dioxide (NO2) and other nitrogen oxides.  Acute exposure to NO2 can increase respiratory diseases, especially in children and asthmatics. Chronic exposure to this pollutant can reduce defenses against respiratory infections.
Sulfur dioxide (SO2)5 SO2 is a colorless gas with a strong odor. In the presence of humidity, it becomes sulfuric acid, another critical precursor of acid rain. Caused by the oxidation of sulfur during fuel combustion. It irritates the eyes, nose and throat, and exacerbates asthma and bronchitis symptoms. Sustained exposure to sulfur dioxide reduces lung functioning and cause respiratory diseases. 
Particulate matter (PM)6 They are solid or liquid particles suspended in the air. The particles may be formed from a complex mixture of acids and heavy hydrocarbons, including dust.There are fine particles, ultrafine and nanoparticles, less than 2.5 in diameter (PM2.5); and PM10, smaller than 10 microns in diameter. Caused by fuel combustion. This pollutant has bigger impacts on human health; it has been associated with increased symptoms of respiratory diseases, decreased lung functioning, aggravation of asthma, and premature death caused by respiratory and cardiovascular diseases. 


1 This document was developed by the Mexican Center for Environmental Law (CEMDA), from data available in the the following site: respiramexico.org.mx
2 INECC, Los vehículos automotores como fuentes de emisión, Consultado en: http://www2.inecc.gob.mx/publicaciones/libros/618/vehiculos.pdf
3 Ibíd., pp. 26 y 27.
4 Ibíd., pág. 27.
5 Ibíd., pág. 28.
6 Ibíd., pág. 28.
7 Blumberg O. Katherine, Walsh P. Michael y Pera Charlotte. Gasolina y Diesel de bajo azufre: la clave para disminuir las emisiones vehiculares, The international council on clean trasnportation, 21 de mayo de 2003, pág. 61. Consultado en: http://www.theicct.org/sites/default/files/Bajo_Azufre_ICCT_2003.pdf
8 Op. cit., Los vehículos automotores como fuentes de emisión, pág. 28.
9 Ibíd., pág. 29.
10 Op. cit., Gasolina y Diesel de bajo azufre: la clave para disminuir las emisiones vehiculares, pág. 59.

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