Water-cooled exhaust gas recirculation (EGR)
At high combustion temperatures, unwanted oxides of nitrogen are formed in all internal combustion engines. Most of them can be avoided by the exhaust gas recirculation process. The EGR process in Audi TDI engines feeds most of the exhaust gas back into the combustion chambers. This reduces the proportion of oxygen-rich fresh air, and the combustion temperatures fall.
EGR was introduced in Audi’s first 2.5 liter TDI during its 1994 evolution phase. To boost its effect, water-cooled EGR is nowadays used in all models, featuring a cooler governed by a mapped characteristic in the return exhaust line to the engine.
The new three- and four-cylinder TDI engines combine high and low pressure EGR. The uncooled high pressure EGR is activated after cold-starting and at very low load. The cooled low pressure EGR, covering most driving, is very compact and designed to ensure low flow losses.