Page 438 - AWSAR 2.0

414 || AWSAR Awarded Popular Science Stories - 2019
continue to own an approximate share of 10% of the total GHG emissions pie, a fact that threatens to be the possible doom of automotive IC engines.
Nonetheless, considering the fact that there is no other competing and affordable technology today that can completely eliminate the use of IC engines in automobiles, engineers like me continue to work on technologies that can make IC engines cleaner. To this effect, I work on a technology called selective catalytic reduction (SCR) that aids in reducing the amounts of one of the most harmful pollutants emitted by a diesel engine NOx.
In diesel engines, nitrogen oxides, or NOx as they are commonly called, are predominantly produced by the reaction of nitrogen with oxygen at high temperatures created by the burning of diesel inside the combustion chamber (thermal NOx). Simply put, the process of combustion (or burning) is a chemical reaction between the
fuel and oxygen, accompanied
by the release of huge amounts
of energy in the form of heat.
While automobiles carry fuel
(diesel, petrol, etc.) in their fuel
tanks, the oxygen required for
combustion is drawn from the
air outside (21% oxygen and
78% nitrogen), and this air acts
as the major source of unwanted
nitrogen inside the combustion
chamber. Apart from thermal
NOx, NOx may also be formed
from the nitrogen in the fuel
itself (fuel NOx) and sometimes
by the rapid reaction of nitrogen with certain hydrocarbon radicals (prompt NOx).
These NOx emissions from IC engines are absolutely undesirable as they act as indirect greenhouse gases by producing tropospheric (the lowest layer of the earth’s atmosphere) ozone, which, in turn, can cause global
warming. NOx undergoes chemical reactions to form smog and can even cause acid rains. Additionally, inhalation of NOx in excessive quantities can lead to inflammations in the human respiratory system, and hence cause breathing diseases. Considering the negative impacts these gases have on human life and health, the permissible limit of NOx gases from an automobile engine has been restricted to less than 0.4 g/km according to the Bharat Stage VI regulations.
Exhaust gas treatment systems such as the SCR help automotive diesel engines to achieve the NOx gas targets mandated by such government regulations. In an SCR system, ammonia,whichisusedasthereducingagent, reacts with the NOx gases to produce nitrogen and water vapour. Since ammonia is toxic and relatively unsafe for transportation, urea- based SCR systems are used in automobiles. In a urea-based SCR system, a solution of
urea in water (32.5% urea and 67.5% water) is injected into the automobile exhaust gas in the form of a liquid jet. Due to the relative motion between the liquid jet and surrounding gases, the jet disintegrates into small droplets and forms a spray. The droplets in this spray undergo evaporation and thermal decomposition at temperatures above 133oC to produce ammonia and thereby act as the source of the required reducing agent.
Although the SCR technology in itself is many decades old, its adaptation to automobiles has brought about new challenges. Unlike stationary engines, which are used for power generation, automobile engines have limited space and offer limited time for the completion of the thermal decomposition of urea and
Over the years, they have become such an inevitable part of our lives, that, today, about 25% of the energy needs of the world are being met by the energy produced using IC engines operating on fossil fuels.