What is EGR and how does it reduce emissions in diesel engines?

Exhaust Gas Recirculation (EGR) redirects a portion of exhaust gases back into the intake manifold to reduce NOx emissions. The inert exhaust gas (mainly CO2 and N2) dilutes the fresh charge, lowering peak combustion temperatures which are primary cause of NOx formation. EGR rates typically range from 5-30% depending on engine load. While effective for NOx reduction, EGR can increase particulate emissions and reduce efficiency, requiring careful calibration with other emission control systems.