What is EGR and how does it reduce NOx emissions?

Exhaust Gas Recirculation (EGR) redirects a portion of exhaust gas back into the intake to reduce combustion temperatures. The inert exhaust gas displaces fresh oxygen and absorbs heat, lowering peak combustion temperatures where NOx formation occurs (above ~1800C). External EGR uses a valve and cooler; internal EGR uses variable valve timing to retain exhaust gas. EGR rate varies with operating conditions (0-50%), controlled by the ECU based on engine maps and emissions requirements.