Editing Exhaust gas recirculation (section)

{{Short description|NOx reduction technique used in gasoline and diesel engines}}
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[[Image:SaabHengine.jpg|thumb|right|300px|EGR valve the top of box on top of the inlet manifold of a [[Saab H engine]] in a 1987 [[Saab 90]]]]
In [[internal combustion engine]]s, '''exhaust gas recirculation''' ('''EGR''') is a nitrogen oxide ({{NOx|link=yes}}) emissions reduction technique used in [[petrol engine|petrol/gasoline]], [[diesel engine]]s and some [[hydrogen internal combustion engine vehicle|hydrogen engine]]s.<ref>{{cite web|url=https://www.mazda.com/en/innovation/technology/env/hre|title=Mazda's description of their hydrogen rotary engine|access-date=2021-06-04|archive-date=3 June 2021|archive-url=https://web.archive.org/web/20210603205101/https://www.mazda.com/en/innovation/technology/env/hre|url-status=dead}}</ref> EGR works by recirculating a portion of an engine's [[exhaust gas]] back to the engine [[cylinder (engine)|cylinder]]s.  The exhaust gas displaces atmospheric air and reduces {{O2}} in the combustion chamber. Reducing the amount of oxygen reduces the amount of fuel that can burn in the cylinder thereby reducing peak in-cylinder temperatures.  The actual amount of recirculated exhaust gas varies with the engine operating parameters.

In the combustion cylinder, {{NOx}} is produced by high-temperature mixtures of atmospheric nitrogen and oxygen, and this usually occurs at cylinder peak pressure. In a spark-ignition engine, an ancillary benefit of recirculating exhaust gases via an external EGR valve is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses. Mazda's turbocharged [[SkyActiv]] [[gasoline direct injection]] engine uses recirculated and cooled exhaust gases to reduce combustion chamber temperatures, thereby permitting the engine to run at higher boost levels before the air-fuel mixture must be enriched to prevent [[engine knocking]].<ref name=Wards>[https://www.wardsauto.com/engines/mazda-s-innovative-4-cyl-engine-pulls-big-v-6 Mazda’s Innovative 4-Cyl. Engine Pulls Like Big V-6]. ''[[Ward's]]'', 9 November 2017</ref>

In a gasoline engine, this inert exhaust displaces some amount of combustible charge in the cylinder, effectively reducing the quantity of charge available for combustion without affecting the air-fuel ratio. In a diesel engine, the exhaust gas replaces some of the excess [[oxygen]] in the pre-combustion mixture.<ref name=MTSC_73-7>{{Cite web |url=http://www.imperialclub.com/Repair/Lit/Master/308/page08.htm |title=Exhaust Emissions and Driveability – Chrysler Corporation, 1973 |access-date=24 February 2011 |archive-date=26 July 2014 |archive-url=https://web.archive.org/web/20140726140543/http://www.imperialclub.com/Repair/Lit/Master/308/page08.htm |url-status=dead }}</ref> 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}} that the combustion process generates. Gases 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. Chemical properties of different fuels limit how much EGR may be used. For example [[Methanol fuel|methanol]] is more tolerant to EGR than gasoline.<ref name=sileghem2011-1>Sileghem & Van De Giste, 2011. Quote: "The results on the Audi-engine indicate that methanol is more EGR tolerant than gasoline, due to its higher flame speed. An EGR tolerance of 27% was found when methanol was used. The efficiencies of the methanol-fueled engine obtained with EGR are higher to those obtained with throttled stoichiometric operation."</ref>