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SCR and EGR

Today, heavy-duty diesel engine trucks are equipped with the latest EPA compliant equipment. The device comes in the form of two different methods for the aftertreatment of exhaust emissions. These methods are Selective Catalytic Reduction (SCR) and increased Exhaust Gas Recirculation (EGR). Either (or both) is used to reduce NOx, which are basically excess unburned oxygen from the combustion process. In the early days, most trucks had only the EGR system, with the diesel particulate filter (DPF) inline with the exhaust system, so the problem of SCR and EGR was related. Modern diesel engines, however, now make use of both technologies in harmony. There still seems to be some confusion between what each system does and how it works, so hopefully this article clarifies those issues.

Both technologies use exhaust gas recirculation to send a certain amount of engine exhaust back into the combustion chamber, and both use diesel particulate filters (DPF). This is how particulate matter is removed from exhaust.

Selective Catalytic Reduction (SCR)

Let’s start by explaining what SCR is. This solution allows the engine to operate at optimal combustion temperatures, but in turn produces unacceptable levels of NOx. It is used in combination with EGR, as it does recirculate some exhaust gas through the engine, but only in a marginal amount. However, the main process of disposing of waste gas is done through chemical reactions. The exhaust gas passes through a system called a decomposition reactor, which reacts nitrogen oxides (which come from the exhaust gas) with diesel exhaust fluid (DEF), which is usually a combination of deionized water and urea. The reaction effectively produces environmentally sound water and nitrogen. It is worth noting that there are two versions of SCR technology, one using copper zeolite catalysts and the other using iron zeolite catalysts. Because copper zeolite is more effective at reducing nitrogen oxide emissions, it can improve fuel economy by 2%. Cummins and Paccar have both implemented this version.

Exhaust Gas Recirculation (EGR)

Now you may be wondering what the main differences are. The enhanced (or increased) EGR method reduces NOx by recirculating more exhaust gas through the engine’s combustion chamber without the use of DEF. These exhaust gases are cooled on its way back to the engine and mixed with oxygen-rich air from the intake. Once this happens, we find that the number of oxygen molecules in the combustible air is reduced because there is less oxygen-rich exhaust gas, the combustion temperature is lowered, and the NOx gas is reduced. The downside is that we now have a less efficient combustion process that reduces fuel mileage due to lower combustion temperatures.

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