Vehicles emit exhaust gases as a result of the combustion of fossil fuels.
Various exhaust gases are emitted from the engine bases on various Conditions that develop within the combustion chamber.
These gases can either be treated and then sent to the atmosphere via tailpipe, Some amount being used in the purpose of exhaust gas recirculation (EGR) Or sent through the tailpipe without any treatment.
Gases such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), Particulate matter (PM), volatile organic compounds (VOC),
And in some cases smaller amount of sulphur dioxide (SOx) and ammonia (NH3).
Nitrogen oxides (NOx):
NOx refers to NO and NO2 and it is produced due to the combustion at very high temperatures.
At higher temperatures, the nitrogen molecule splits up and reacts with Oxygen forming NO and later on NO oxidizes to NO2.
Diesel engines produce more NOx than a gasoline engine because diesel engines have higher combustion temperatures.
Carbon monoxide (CO):
CO is formed as a result of incomplete combustion.
Ideally, only carbon dioxide (CO2) and water (H2O) should be formed as the products of combustion,
But lack of oxygen can result in an incomplete reaction of carbon with oxygen and thus CO is formed.
Gasoline engines emit more CO compared to diesel engines.
Particulate matter (PM):
Particulate matter is a mixture of solid and liquid particles suspended in the air, Such as dust, soot (fine carbon particles as dust as a result of incomplete combustion), Metal, acids, polycyclic aromatic hydrocarbons, and most of them being hazardous.
It has both coarse and fine particles.
The size of the coarse particles vary from 2.5 µm to 10 µm, whereas the finer particles’ size varies from 0.1 µm to 2.5 µm.
Finer particles are more in composition compared to coarse particles. Diesel engines emit more PM.
Volatile Organic Compounds (VOCs):
VOCs are formed due to partially burned or unburned fuel.
It is a combination of carbon compounds. It is also released due to the evaporation of fuel.
Gasoline engines emit more VOCs since gasoline is more volatile compared to diesel.
Hydrocarbons (HC):
Hydrocarbons are the unburned fuel which as a result of fuel not burning due to insufficient combustion temperature.
It is attributed to the presence of a lean air-fuel mixture inside the cylinder. In a lean air-fuel mixture, flame speeds may be too slow for complete combustion to take place.
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