Explain Exhaust Gas Recirculation (EGR) and its types.

EGR reduces NOx emissions by recirculating a portion of exhaust gas back to intake, lowering combustion temperature (NOx formation increases exponentially above 1500°C). High-pressure EGR (HP-EGR) takes gas before the turbo and returns it before the intercooler. Low-pressure EGR (LP-EGR) takes gas after DPF and returns it before the compressor, providing cooler, cleaner gas but with higher pumping losses. Modern diesels use both systems coordinated by ECU.