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The Differences Between In-Cylinder EGR and SCR

Many OEMs today use Selective Catalytic Reduction (SCR) technology, sometimes even in conjunction with Exhaust Gas Recirculation (EGR) to minimize diesel emissions. Both of these technologies bring several advantages to reducing the amount of nitrogen oxides (NOx) that are typically released into the atmosphere. These technologies are used for the diesel-powered vehicles and equipment in the transportation, agriculture, and mining industries. These emission-reduction technologies help multiple industries to meet the standards of NOx emissions as set by the Environmental Protection Agency (EPA). Read on to discover the main differences between EGR technology and SCR technology as well as why SCR has become more popular.

VW_TDI 265-6

Exhaust Gas Recirculation focuses on reducing emissions within the actual combustion chamber. EGR lowers the exhaust gas temperature within the combustion chamber. Essentially, EGR recirculates exhaust gases back into the cylinder and burns them again. In-cylinder EGR first cools exhaust gases through a heat exchanger. Afterword, the exhaust gases are blended with fresh air and are returned to the cylinder. They have less oxygen and more CO2 than the rest of the air, which lowers the combustion temperature. Thus, EGR attempts to reduce the temperature without having an excessive impact on combustion efficiency.  EGR has a less efficient combustion process that requires lower temperatures, the engine produces more heat. Additionally, EGR results in higher injection pressures. In EGR, the customer doesn’t benefit very much since there is decreased fuel efficiency and lower engine durability. More fuel is burned in order for the technology to work and since the technology is in the combustion chamber, it requires increased complexity in most engine parts.


Selective Catalytic Reduction focuses on reducing emissions through the use of a catalyst and diesel exhaust fluid (DEF) after combustion. The DEF is sprayed into the hot exhaust gas. Since DEF is made up of urea and deionized water, the mixture passes through the catalytic chamber and combines with the hot exhaust. The exhaust reacts with the DEF to break down into harmless nitrogen and water vapor. The catalyst and DEF react with the nitrogen oxides to release only nitrogen, water, and small amounts of carbon dioxide. This lowers the need for EGR since the emissions can be dealt with outside of the combustion chamber, meaning that the engine can operate under ideal conditions to create as much horsepower and torque as possible. Engine performance is not hindered or diminished by SCR. Additionally, it’s been proven that SCR results in cleaner available tailpipe emissions and has been used across the globe. So for the customer, this means better fuel efficiency and fewer regenerations than in-cylinder EGR. SCR also results in improved durability which means that the overall cost will be lower since it requires less maintenance as well as less fuel.

SCR has become the main technology to use in order to comply with EPA standards. If you have any questions about either EGR or SCR, please contact Victory Blue. We are happy to answer any questions you might have!