Active SCR catalytic converter: BLUETEC with additive
and selective catalytic reduction

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The SCR (selective catalytic reduction) process involves adding an aqueous reducing agent to the exhaust flow. This process cuts levels of nitrogen oxides by up to 80 percent, and is thus the most effective exhaust treatment method for diesel drives today. The person responsible for the idea of applying SCR technology to passenger cars was Frank Duvenage, who was head of Diesel Drive Advanced Development at a time when SCR was already being used successfully in commercial vehicles.

As is the case with the BLUETEC variant of NSK Advanced, the exhaust gas is initially sent through an oxidizing catalytic converter, which reduces the levels of hydrocarbons (HC) and carbon monoxide (CO). The downstream particulate filter then captures diesel particulates. After that, the nitrogen oxides are broken down by injecting the reducing agent AdBlue into the pretreated exhaust flow via a dosage valve.

AdBlue is an aqueous carbamide solution that is transformed into ammonia (NH3) in the exhaust pipe in a two-stage process. First, the heat in the exhaust pipe vaporizes the water in the additive, and the resulting thermal decomposition leads to the creation of the first of two ammonia molecules from one molecule of carbamide. Isocyanic acid, which is also formed by this process, binds with a water molecule, leading to the creation of the second ammonia molecule. This ammonia then breaks down the nitrogen oxide (NOX) into harmless nitrogen (N2) and water in the downstream SCR catalytic converter. Exact dosage of AdBlue in accordance with the engine’s operating state is crucial for the process’ high efficiency, and the dosage is regulated by a nitrogen oxide sensor at the end of the exhaust line. The AdBlue is stored in a special tank.

AdBlue consumption in a passenger car is around one or two percent of diesel consumption, which means a car can go a very long way on just one tank of the solution. Daimler developers designed the tank in such a manner that it only has to be refilled by service personnel during regular service checkups. This also makes things very easy for customers because it allows AdBlue to be refilled during regular oil changes. An electric heating unit for the AdBlue tank and feed lines ensures that the system continues to work reliably even at freezing temperatures.