Exhaust Gas Recirculation (EGR) is an emission control system used to;
Reduce NOX emissions in particular.
It redirects some amount of exhaust back to the Combustion Chamber of an engine to dilute the oxygen in the intake stroke and reduce the combustion temperature.
NOX is produced at peak engine temperature and the emission is higher
When the oxygen content in the intake is high (lean mixture).
At high temperatures, Nitrogen combines with oxygen to form various oxides of nitrogen (Collectively known as NOX).
NOX is responsible for the formation of smog in cities.
Design and Operation of EGR:
The engine exhaust only recirculates when a vacuum operated valve opens.
The EGR valve is driven by backpressure exhaust.
The EGR valve allows some amount of exhaust to be sent back to the combustion chamber at a restricted backpressure range.
The exhaust gas is an inert gas, hence it cannot be burned.
It dilutes the fresh air in the inlet manifold, thus reducing the oxygen content.
The inert gases also absorb the Combustion chamber heat to reduce the peak temperature.
The exhaust gases pass through an EGR cooler to reduce its temperature.
Reducing the temperature of exhaust helps in reducing the peak temperature of the cylinder.
In an S.I. engine, EGR re-circulates 5 % to 15 % of the exhaust.
Excess amount of recirculation can compromise engine efficiency.
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At engine idling speed EGR is not used to avoid uncontrolled combustion.
It is also not used at high loads, as high power output is needed to achieve high speeds and EGR will simply prevent high oxygen intake at high loads.
Also, read:
In a diesel engine, Exhaust can be re-circulated as high as 50 % as the diesel engine is not subjected to Continuous flame front and knocking.
In the mid-1990s, pneumatic EGR valves became more popular in the EGR system.
It consists of an electromagnetic valve which is operated by vacuum.
Vacuum is usually generated by the pressure difference between Inlet manifold and exhaust manifold.
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