What EGR Means:
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines.
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. This means the heat of combustion is less, and the combustion generates the same pressure against the piston at a lower temperature. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.
Because NOx formation progresses much faster at high temperatures, EGR reduces the amount of NOx the combustion generates. NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature.