SCR vs. EGR Technology
In 2000, the EPA proceeded with its rule to make heavy-duty trucks and buses run cleaner with the "2007 Highway Rule." According to the EPA, "Beginning with the 2007 model year, the harmful pollution from heavy-duty highway vehicles will be reduced by more than 90 percent."
For 2010, the US Environmental Protection Agency requires that diesel truck emmissions contain a 97 percent reduction in their sulfer content. Engine manufacurers have come up with two advanced pollution control technology options for cars, trucks, and buses which include:
Selective Catalytic Reduction (SCR) is an exhaust aftertreatment system that injects a small amount of a chemical called Diesel Exhaust Fluid (DEF) into the exhaust. DEF is mixed with exhaust in the presenc e of a catalyst turning NOx (oxides of nigrogen - a harmful pollutant that contributes to smog and acid rain) into harmless nitrogen and water vapor.
Exhaust Gas Recirculation (EGR) is an other way to reduce NOx formation. In an EGR system, engine exhaust is recycled back through the engine to dilute the oxygen. Almost all engine manufacturers use a form of EGR, as it takes both EGR and SCR to achieve near-zero NOx emissions.
Majority of the engine manufacturers have added SCR to their exhaust systems such as; Volvo, Mack, Daimler, and Hino to name a few. Navistar is the only OEMs to use a standalone EGR system.
While stand alone EGR systems help to reduce NOx, there are some disadvantages:
- EGR lowers power density
- EGR increases heat rejection
- EGR reduces fuel economy
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