Exhaust Gas Recirculation is an effective method
for NOx control. The exhaust gases mainly consist of
carbon dioxide, nitrogen and water vapour. When a part
of this exhaust gas is re-circulated to the cylinder, it acts
as diluent to the combusting mixture. The specific heat
of the EGR is much higher than fresh air; hence EGR
increases the heat capacity (specific heat) of the intake
charge, thus decreasing the temperature rise for the
same heat release in the combustion chamber.
Re-circulated exhaust gas displaces fresh air entering
the combustion chamber with carbon dioxide and water
vapour present in engine exhaust. As a result of this air
displacement, the amount of oxygen in the intake
mixture is decreased. Reduced oxygen available for
combustion lowers the effective air–fuel ratio which
affects exhaust emissions substantially. In addition,
mixing of exhaust gases with intake air increases the
specific heat of intake mixture, which results in the
reduction of flame temperature. Thus combination of
lower oxygen quantity in the intake air and reduced
flame temperature reduces rate of NOx formation
reactions. The EGR (%) is defined as