  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. exhaust gas is routed back into the combustion chamber because the exhausted air is much hotter than the intake air. egr works by diluting the n2 and providing inert gases to combustion (co2 primarily) to act as an absorbent of combustion heat to reduce peak in cylinder temperatures. nox is produced in a narrow band of high cylinder temperatures and pressures. 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.exhaust emissions and driveability â chrysler corporation, 1973 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 (though at some loss of engine efficiency). gasses re-introduced from egr systems will also contain near equilibrium concentrations of nox and co; the small fraction initially within the combustion chamber inhibits the total net production of these and other pollutants when sampled on a time average. most modern engines now require exhaust gas recirculation to meet emissions standards. == history == the first egr systems were crude; some were as simple as an orifice jet between the exhaust and intake tracts which admitted exhaust to the intake tract whenever the engine was running. difficult starting, rough idling, and reduced performance and fuel economy resulted. by 1973, an egr valve controlled by manifold vacuum opened or closed to admit exhaust to the intake tract only under certain conditions. control systems grew more sophisticated as automakers gained experience; chrysler's "coolant controlled exhaust gas recirculation" system of 1973 exemplified this evolution: a coolant temperature sensor blocked vacuum to the egr valve until the engine reached normal [[operating
Air_pollution_control_systems Chemical_process_engineering Engine_technology NOx_control