Exhaust Gas Recirculation
Vehicles internal combustion engines produce harmful nitrogen oxide (NOx). The emission is a result of high temperatures caused by a higher compression ratio. Automotive manufacturers use Exhaust gas recirculation (ERG) technology to control and reduce Nitrogen oxide emissions.
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How it Works
Exhaust gas recirculation is a technique used to reduce nitrogen oxide emissions in petrol and diesel engines. This technique works by returning some exhaust gas into the engine cylinders. The exhaust gases are much hotter than fresh air sucked by the engine. Additionally, the exhaust gases dilute the oxygen content in the incoming stream of air. This leaves the exhaust gases with nothing to burn, as there is no oxygen or fuel to burn. This process results in a reduction of heat discharge and cylinder temperatures.
More importantly, it reduces nitrogen oxide formation and peak temperatures in the cylinders. Nitrogen oxide is formed when nitrogen and oxygen react under high temperatures inside the combustion cylinder. The production usually takes place during cylinder peak pressure.
In a diesel engine, the inert exhaust reduces the quantity of combustion charge altering the air-fuel ratio. In a petrol engine, the exhaust displaces excess oxygen in the pre-combustion mixture. High temperature is a prerequisite for nitrogen and oxygen to form nitrogen oxide.
By reducing the temperature of the combustion chamber, EGR reduces the amount of nitrogen oxide generated by the combustion process. Chemical composition, however, affects the usability of the EGR system. For instance, gasoline is less tolerant of methanol.
EGR valve controls exhaust entering the cylinders. It consists of vacuum diaphragm that is spring loaded. The valve is connected to a metered valve that controls exhaust gases passage. EGR vacuum chamber is linked to a ported vacuum that is located on top of a throttle valve. It is worth noting that the EGR impact is dependent on engine designs.
EGR system boosts gasoline engine efficiency by:
Reducing throttling process
The throttle plate has to be opened to allow additional exhaust gases into the intake system. This reduces throttling losses and increases pressure in the inlet manifold.
Lower heat rejection
Reduction of peak combustion temperatures reduces the formation of Nitrogen oxide. It also reduces the amount of heat lost to the surfaces of the combustion chamber. This leaves enough heat for use during the expansion stroke.
EGR also minimizes chemical dissociation
High load vehicles do not use EGR as it can lower the output of peak power. Similarly, it also not used in zero load or low speed because it would result in unstable combustion and hence rough idle.
EGR valves sometimes get clogged with carbon deposits which can hinder effective operation. While clogged valves can be cleaned, replacement is the best option if the valves are faulty.
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