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Exhaust gas recirculation

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 engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines.[1] EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. 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.[2]

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.[3] 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 is more tolerant to EGR than gasoline.[4]

  1. ^ "Mazda's description of their hydrogen rotary engine". Archived from the original on 3 June 2021. Retrieved 4 June 2021.
  2. ^ Mazda’s Innovative 4-Cyl. Engine Pulls Like Big V-6. Ward's, 9 November 2017
  3. ^ "Exhaust Emissions and Driveability – Chrysler Corporation, 1973". Archived from the original on 26 July 2014. Retrieved 24 February 2011.
  4. ^ 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."

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