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QUESTION OF THE WEEK
Page 24
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What's the difference between SCR and EGR?
SCR stands for Selective Catalytic Reduction and EGR for Exhaust Gas Recirculation. EGR, as the name suggests, works by recirculating a proportion of the engine's exhaust gas back into the cylinders. This lowers the adiabatic flame temperature and increases the specific heat capacity. In plain English. because NOx formation progresses much faster at high temperatures, the lower temperature achieved by EGR limits the generation of NOx.
SCR makes use of a red uctant (AdBlue) and a catalyst to reduce the NOx content of the exhaust gas. The red uctant is injected into the exhaust gas before the mix is absorbed into the catalyst. In the catalyst the reductant reacts with the NOx to form water vapour and nitrogen gas, thereby reducing the NOx.
Currently, engines do not use both technologies together, although it is presumed that both will have to be used in conjunction at some point in the future to meet increasingly tough emission standards.