Depending on the fuel that is used, internal combustion engine vehicles release variable amounts of so-called point source pollutants, carbon dioxide, and water vapor.
The polluting emissions are mostly made up of nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), ozone (O3), particulate matter (PM), and unburned hydrocarbons. These emissions can cause various pathologies as soon as they reach a certain atmospheric concentration.
Emissions of carbon dioxide, the now famous CO2, result from the combustion of hydrocarbons, whether from a combustion engine or other sources. It's not strictly speaking a pollutant, but rather a greenhouse gas that contributes to global warming.
Air pollution emissions
Considerable progress has been made in reducing atmospheric pollution since the 1980s, both in large cities in countries with a long history of car ownership, and more recently in some cities in emerging countries. However there is still great cause for concern, particularly in the urban areas of emerging countries where atmospheric pollution is responsible for 1 million premature deaths per year and as many prenatal deaths (according to experts from the United Nations Environment Programme). Children and the elderly are the most severely affected. Yet the population of people aged 60 and up is likely to triple between now and 2060, and most of them will live in cities. In addition to the impact on human health, vegetation, infrastructures, and buildings are also affected. Transportation is not the only culprit, far from it, but it undeniably contributes to this point source pollution, especially in countries where fuels are still of mediocre quality.
Regulation of air pollution emissions
In the 1960s, governments in Europe, the United States, and Japan became aware of the toxicity of certain emissions and established anti-pollution regulations for cars and then commercial vehicles.
The regulation established a limit expressed in grams of pollutant per kilometer. Measurements are taken in a laboratory on a chassis dynamometer running on a standardized cycle that includes different phases of braking, acceleration, stable speeds, and deceleration.
For example, the first European ECE (Economic Commission for Europe) cycle simulated a mostly urban situation. Since 1992, a new phase simulating road and highway driving has been added (EUDC - Extra Urban Driving Cycle). The current NEDC (New European Driving Cycle) procedure is even more realistic as it takes into account the cold starts, which generate even more polluting emissions.
The evolution of American and Japanese regulations is comparable to these European examples. China is now well aware of high pollution levels in its megacities and has also implemented rigorous regulations.
What can be regulated?
- Fuel quality
The addition of catalytic converters to gasoline engines required the elimination of lead starting in the 1980s in the USA, and then in Europe. This met the dual objective of improving both public health and after-treatment catalytic technologies. By the early 2000s all countries around the world had eliminated lead.
Benzene should also be reduced to close to 1 % by the early 2010s.
In Europe and the US, sulfur levels in diesel fuel were divided by 300 to reach 10 and 15 ppm respectively in the early 2010s. In addition to the almost total elimination of sulfur oxides from emissions, this measure led to the development of very effective particulate filters and systems to eliminate sulfur oxides.
- Pollution emissions per kilometer (from European regulations)
Carbon monoxide (CO) is a toxic gas that is a byproduct of incomplete combustion. It has long been seen as a problem, and was limited to 35 g/km in 1970. The 2006 European Euro 4 emissions standards established a limit of 1 g/km for gasoline, LPG, and NGV vehicles, and 0.5 g/km for diesel vehicles.
Nitrogen oxides (NO and NO2= NOx) are produced by the combination of nitrogen and oxygen under the influence of very high temperatures in combustion gas. When nitrogen monoxide comes into contact with air, it changes more or less completely into nitrogen dioxide NO2. Standards apply to the total called NOx. For diesel engines the limit went from 250 mg/km for Euro 4 in 2006 to 80 mg/km for Euro 6 in 2014. For gasoline engines that produce fewer NOx emissions, the limits went from 80 to 60 mg/km between 2006 and 2011. No changes are planned for 2014.
Unburned hydrocarbons (HC) were unable to combine with oxygen (because of proximity to combustion chamber wall, for example), and are released as is.
Standards first included the totality of HC and NOx emissions, since they are difficult to measure separately. In the early 1970s, the limit was established at 11 g/km, a high level that car manufacturers were able to reach through appropriate adjustments to carburation and ignition.
Since the 2000s, standards relative to HC only refer to spark-ignition engines, since combustion is more complete on principle. Since 2006, Euro 4 has established the limit for unburned hydrocarbons at 100 mg/km. Particulate matter (PM) emitted by diesel engines has been regulated since the early 1990s. The limit went from 140 mg/km for the Euro 1 in 1992 to 5 mg/km for the Euro 5 in 2011.
Regulations for other countries
North America and Japan have taken the same steps as Europe. China is also following in their tracks. However, it's difficult to compare standards from different countries or continents, since testing procedures and limits are different. The United Nations is attempting to harmonize world vehicle regulations as part of its WP.29, particularly in regards to safety.
Non-regulated pollutants and ozone
Exhaust gas contains a number of substances that are likely to affect air quality, such as benzene, butadiene, formaldehyde, acetaldehyde, and polycyclic aromatic hydrocarbons (PAH). Fortunately, these by-products are eliminated by catalytic after-treatments and only traces are emitted through the exhaust.
Catalyzed gasoline engines emit 10 g/km of CO, 5 mg/km of benzene, and 1 mg/km of butadiene, while catalyzed diesel engines emit 2 µg/km of PAH.
Carbon dioxide emissions - CO2
Combustion of any kind of hydrocarbon releases CO2 emissions and water vapor, not just internal combustion engines.
Combustion of one molecule of gasoline or diesel fuel produces one molecule of CO2. The simplified equation is (CH1.8)n = (CO2)n + (0.9H2O)n. Therefore the molecular mass is:
The mass of CO2 that is produced is equal to 3.2 times the mass of the fuel that is burned.
Carbon dioxide is not a pollutant in itself, but rather a greenhouse gas whose increasing accumulation in the atmosphere affects the climate by increasing the average temperature. Transportation methods are not solely responsible for global warming, but they have an undeniable impact that varies by country. No matter where it is produced, CO2 is released into the atmosphere: we call this "nonpoint source pollution" even though the gas doesn't adversely affect human health, because it can disrupt the functioning of the biosphere.
Large industrial plants can capture CO2, but it would be difficult for each individual car to do so.
The planet is of course reacting to the increase in atmospheric CO2, as seen in the increase in the rate at which plants are growing for example. But the time constant necessary for the climate system to return to equilibrium is undoubtedly close to 100 years. It is thus absolutely critical that we reduce anthropogenic CO2 emissions everywhere in order to limit the impact on climate.
Whether we are measuring air pollution emissions or CO2 emissions, evaluating the impact of combustion requires taking into account all of the emissions that are directly or indirectly tied to gasoline or diesel fuel: extraction, transportation to the refinery, the refining process itself, distribution, and finally combustion.
Greenhouse gases (other than CO2) that are emitted through the production chain are also taken into account as a "CO2 equivalent," according to their potential to contribute to global warming (global warming potential and how long they remain in the atmosphere). That's what we define as the well-to-wheel CO2 equivalent.
CO2 eq. from well-to-wheel = (CO2 eq. from well-to-tank + CO2 eq. from tank-to-wheel) x vehicle efficiency
g CO2 / km g CO2 / kWh kWh / km
Despite all the progress made thus far, reducing both air pollution emissions and carbon dioxide emissions remains a major concern for both manufacturers and equipment suppliers. Research into new and promising solutions continues.