The Chemistry of Automobile Emissions

Polution by the exhaust gases of  a car

The automobiles are one of the major sources of air pollution . They release many gases which are degrading the quality of air we breathe in. These harmful pollutants are not only affecting the haman health, but also disturbing the overall environmental setup by causing disasters like global warming. According to a report of WHO , air pollution was responsible for 4.2 million deaths world wide in 2016. According to some other reports vehicular emissions account for about 15 percent of India's total air pollution. For India the concerns are at their peak, since it has some of the most polluted cities of the world.  Also, the Indian subcontinent has the highest particulate material concentration among all other regions of the world. In this post we will learn about the automobile emissions and their overall impact on the environment.

Types Of Automobile Emissions

Depending upon the mode of generation,  the vehicular emissions can be classified in three types.

1) Exhaust Emissions
When fuel is burnt inside the combustion chamber, many gases are produced . These gases are released in the atmosphere through the vehicle's exhaust system. They include gases like Carbon dioxide, Carbon Monoxide, Oxides of Nitrogen  and sulfur etc.

2) Evaporative Emissions
The vehicle fuels are generally very volatile. They evaporate very easily at room temperature.  The fuel in the tank of the vehicle keeps on evaporating always. The vapour thus former get collected inside the tank and may escape to the atmosphere whenever they get any passage. These gases consist of different hydrocarbons which are very harmful for us.

3) Crank case blowby
During the processes of combustion, some amount of unburnt charge leaks inside the crank case passing through the sides of the piston rings. These gases are called blow by gases. From the crank case these gases may be released to the atmosphere through ventilation port. Blow by gases contain hydrocarbons which are very harmful for our environment.

The Emission gases

Now let's look the different exhaust gases one by one and discuss their impact on the environment.

a) Carbon dioxide

Earth burning due to global warming autocurious

The major part of the exhaust emissions is carbon dioxide. This gas is formed due to the combustion of the fuel inside the engine. The fuels are mostly carbon compounds, hence on burning they release Carbon dioxide. CO2 is a non poisonous gas and hence is not responsible for any type of air pollution. But this gas is responsible for global warming which is a more severe concern. The CO2 forms a layer around our atmosphere, which entraps the solar radiations. This is causing a substantial increase in earth's temperature leading to global warming. Some other gases causing global warming are Methane and Ammonia. 

b) Carbon Monoxide
Carbon monoxide or CO forms when the fuel burns in insufficient air supply i.e. when the mixture is rich. This usually happens during the idling conditions. CO is poisonous for human and animals. The haemoglobin which acts as oxygen  carrier in our blood, has more affinity for CO than Oxygen. Carbon monoxide forms permanent bonds with the haemoglobin, preventing it from carrying oxygen through out the body. Prolong inhalation of CO can even cause death.

c) Nitrogen Oxides.
There are 7 oxides of nitrogen. Of these 7 oxides , NO and NO2 are very harmful for us. NO and NO2 are inter convertible at atmospheric temperature through oxidation and reduction. These two gases together are termed as NOx. They are formed due to the burning of fuel at temperature above 1100 degree Celsius. High temperature is a prerequisite condition for their formation. High temperature inside the engine occurs when either it is burning a lean mixture or during Knocking and misfiring. These gases are very harmful when inhaled.  They can cause breathing problems, eye irritation, lung problems etc. They also promote the formation of Ozone and particulate matter, which themselves are major pollutants. 

Effect of different pollutants  long human health

d) Hydrocarbons
The hydrocarbons are formed when flame  quenching occurs. This happens generally during the initial periods after the starting , when the engine is cold. The cold engine walls causes the flame quenching. Due to this ,  unburnt fuel comes out through the exhaust. These unburnt fuel particles are hydrocarbons. They are also generated due to evaporative emissions or crank cade blowby, about which we have discussed above. These gases are very harmful for humans and can cause problems like lung irritation , irregular heart beat etc.

e) Sulphur Oxides 
Sulphur based oxides like SO2 are produced when Sulphur containing fossil fuels are burnt. SO2 is very harmful if inhaled. It can cause asthma and various other respiratory diseases. It reacts with rain water to form Sulphuric Acid and cause acid rain. It also react with various gases and particles present in air to form sulphates. These sulphates are particulate in nature and are commonly known as Particulate Materials (PM) . Some of these PMs are so small, that they can enter our body through respiration and can cause various diseases related to the lungs and the heart. 

Technologies To Curb Vehicular Emissions. 

With increasing focus of engineers on reducing vehicular emissions, many technologies have been introduced. These technologies have been successful to a large extent in  limiting the emissions from vehicles. So let's discuss some of those technologies one by one. 

a)  Catalytic Convertors

Three  Way Catalytic Convertor autocurious

Catalytic converters are fitted in the vehicle's exhaust system. They reduce the level of Carbon Monoxide, NOx and Hydrocarbons by converting them to other non polluting gases. They utilize Platinum, Palladium and Rhodium based ceramic materials as catalyst for this process. 

b) Charcoal Canisters

Charcoal canister used in BS 4 vehicles for evaporative emission control
Charcoal Canister In A Motorcycle

Charcoal canisters are used to trap the evaporated fuel particle. The fuel particles evaporated in fuel tanks are carried to the charcoal canister, through pipes. The charcoal in canisters absorb them and hence prevent them from leaking to atmosphere. The fuel particles absorbed in canister are later supplied to the engine. 

c) Positive Crank Case Ventilation 
This system is utilised to remove blowby gases from crank case and supply them to the air intake system of the engine. This is results by a valve called PCV valve. This valve is operated by the pressure difference between the crank cases and the air intake system of the vehicle. 

d) Diesel Particulate Filters (DPF) 
DPF units have a structure very similar
to the mufflers. They are fitted in the exhaust system of Diesel vehicles. They contain very minute filters, which can screen the particles up to the size of one micron. They prevent the particulate material from being emitted to the atmosphere. 

e) Air Suction Valves (ASV) 

ASV connected With Exhaust Pipe

ASVs are connected between the exhaust manifold and the air intake unit of the engine. They supply fresh air from the air intake system to the engine exhaust manifold. The fresh air thus supplied helps in burning of any unburnt fuel particle or converting carbon monoxide in to CO2. 

f) Exhaust Gas Recirculation (EGR) 

Exhaust gas recirculation autocurious

EGR is fitted to the engines to reduce the NOx emissions. NOx is formed when the engine temperature becomes very high i.e. greater than 1100 degree celsius. To prevent this, EGR supplies some amount of exhaust gases to the intake manifold after cooling them. This dilutes the incoming charge in case of SI engines . The charge dilution reduces the engine temperature preventing the NOx formation. In case of CI engines, the recirculated exhaust gases occupy the space inside engine cylinder causing less amount of air to enter the cylinder. This reduces the combustion intensity, causing engine temperature to decrease. This further helps in preventing the NOx formation.

Friends, this was an article about vehicle emissions and the emission control technologies . I hope you enjoyed it. We welcome your valuable comments and suggestions . Please share your feedback in comment section. 
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