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Exhaust Gas Recirculation | Working Principle , Design and Benefits
Diesel engines tend to emit higher Nitrogen Oxide (NOx) which is harmful to humans. This is because of high temperatures in the engine cylinders because of higher compression ratio. To control and decrease the NOx, manufacturers employ ‘Exhaust Gas Recirculation’ technology in engines.
The term EGR stands for Exhaust Gas Recirculation. It is a part of modern-day diesel engine vehicles which helps to decrease the Nitrogen Oxide (NOx) emissions. Exhaust Gas Recirculation is the technique used for reducing the nitrogen oxide in both the internal combustion diesel engines as well as petrol engines.
The exhaust gas, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature.
In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and NOx emissions.
A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:
• Reduced throttling losses.
The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
• Reduced heat rejection.
Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
• Reduced chemical dissociation.
The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC (Top Dead-Center), rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.
EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle.
Since the EGR system recirculates a portion of exhaust gases, over time the valve can become clogged with carbon deposits that prevent it from operating properly. Clogged EGR valves can sometimes be cleaned, but replacement is necessary if the valve is faulty.
A vacuum controlled EGR valve regulates the amount of exhaust gases admitted into the cylinders. It consists of a spring-loaded vacuum diaphragm. It links to a metered valve which controls the passage of the exhaust gases. Ported vacuum from a calibrated signal port located above the throttle valve connects to the EGR vacuum chamber.
At idling, the EGR valve is in the closed position because of the spring pressure and lower ported vacuum. The engineers designed it so because if the exhaust gases recirculate at the idling, then it would cause rough/erratic idling. Upon opening of the throttle applies the ported vacuum and gradually opens the tapered valve. This causes the exhaust gas to flow into the intake manifold.
However, when the throttle opens fully, there is no vacuum in the intake manifold. So, it closes the tapered valve and stops the exhaust gases from entering the intake manifold.
The Exhaust Gas Recirculation system recirculates a part of the exhaust gas back into the engine cylinders through the combustion chamber. The logic behind the EGR system is very simple. The exhaust gas is hotter than the fresh air sucked by the engine. So, the exhaust gas significantly reduces the contents of the cylinder for combustion. Because of the absence of oxygen (O2), the exhaust gases have nothing to burn as they contain neither fuel nor oxygen particles.
Thus, it results in lower heat discharge and cylinder temperatures. It reduces the formation of nitrogen oxide (NO2) as well. The dormant exhaust gas present in the cylinder also limits the peak temperatures. It also reduces the loss that arises because of throttling in petrol engines while improving the engine life by reducing the cylinder temperatures. The three-stage catalytic converter further reduces the NOx to acceptable levels.
The engineers designed the EGR system in such a way that it recirculates the exhaust gases only when the engine forms the Nitrogen Oxide (NOx). Thus, the EGR system DOES NOT affect the ‘Full-Load’ operation. The Exhaust Gas Recirculation system also has a thermal control valve in the vacuum line which prevents the operation of EGR at lower engine temperatures. This system is useful especially in the diesel engines where the catalytic converter cannot stimulate the chemical reduction due to high oxygen contents. So, the NOx emission remains the same in such conditions.
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