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 exhaust from the engine is re-circulated only when a
vacuum operated valve opens. The EGR valve is operated by the exhaust back
pressure. At a particular back pressure range, EGR valve allows some amount of
exhaust to be sent back to the combustion chamber.
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. EGR is not
used at engine idling speed to avoid unstable combustion. It is also not used at
high loads as we require high power output to gain high speeds and EGR would
just prevent high oxygen intake at high loads.
In a diesel engine, exhaust can be re-circulated as high as
50 % as diesel engine is not subjected to continuous flame front and knocking.
In the mid-1990s, pneumatic EGR valves became more popular
in 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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