EGR is the abbreviation for Exhaust Gas Recirculation and is a nitrogen oxide emissions reduction technique used in petrol/gasoline and diesel engines.
How does it work?
EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. In a gasoline engine, this inert exhaust displaces the amount of combustible matter in the cylinder. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because nitrogen oxide forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reduces the amount of nitrogen oxide the combustion generates.
What will I gain out of this?
You will gain better fuel economy, more power and better driveability.