Exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in diesel engines. EGR works by recirculating a portion of an engine's exhaust gasses back to the engine cylinders. In a diesel engine, the exhaust gasses replace some of the excess oxygen in the pre-combustion mixture. Because NOx 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 NOx the combustion generates.
