Our gas-guzzling automobiles are the main source of air pollution and efforts are underway to reduce emission of polluting gases, through the exhaust. There are two main courses of action, to reduce air pollution. One is changing the source fuel by opting for electric and hydrogen cars and the second approach is to modify existing petroleum fuel based automobile technology, to reduce emissions. One such measure is aimed at reducing nitrogen oxide (NOx) emissions and is called the exhaust gas recirculation (EGR) technology.
EGR valves are fitted in gasoline (petrol) and diesel engine assemblies of most cars today. They are an important modification made to car engines, in an effort to make them eco-friendly. Most importantly, this technology reduces emissions, without compromising the engine efficiency substantially.
Need For the Technology
EGR technology was developed to counter the growing threat of mono-nitrogen oxide (NOx) production, which occurs during combustion in engines. When emitted in the air, these gases (NO and NO2) react with other volatile organic compounds, to create photochemical smog, which has a harmful effect on the respiratory system.
These compounds react with moisture in the atmosphere, to produce nitric acid, which is a major component of the precipitation, that occurs in the form of acid rain. The combustion engines are ideal sites for reaction of nitrogen and oxygen due to the high temperatures that exist inside, reaching and exceeding 1600° Celsius.
So, the EGR technology was developed to counter this threat of NOx emissions from fuel combustion, which contributes substantially, to air pollution. The key to counter it, was to reduce the temperature at which the combustion occurs in car engines.
These valves, first introduced in 1972, were used to feed back a portion of the exhaust gases, emitted by the car engine, back to the combustion chamber. The addition of this volume of gases reduces the quantity of oxygen available for combustion. Also, the addition of exhaust gases, made to the air-fuel mixture, increases its specific heat capacity.
The dearth of oxygen and increase in heat capacity leads to the combustion of gases, at a lower temperature. This effectively reduces the amount of mono-nitrogen oxide gases, that are created and released. Generally, 5% to 15% of the exhaust gases are fed back in spark-ignited engines. In diesel engines, the exhaust gases are cooled before they are fed back to the engine.
The efficiency of the engine does get reduced but there is a switching mechanism in place to take care of that. When the engine has a high RPM demand (Wide Open Throttle Conditions) and is in full acceleration mode, the EGR valve action is stopped and exhaust gases are no longer fed back to the engine.
This ensures that the engine efficiency is not compromised, when the RPM demands are high. It is also turned off when the engine is idling, as it can destabilize it. Only in the region of mid-acceleration, where the engine is between the two extreme conditions of full throttle and idle, does the EGR valve come into play. In modern cars, the action of the valve is controlled by an electronic engine control mechanism. Such improvements in technology, on a larger scale, will improve the quality of air we breathe and make an impact on the global effort to reduce air pollution.