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Selective Catalytic Reduction and Clean Diesels

Managing Diesel NOx Emissions


SCR catalytic converter system

Clean diesel exhaust treatment system using SCR

© Robert Bosch GmbH

What is Selective Catalytic Reduction?

Selective Catalytic Reduction (SCR) is a method of converting harmful diesel oxides of nitrogen (NOx) emissions, by catalytic reaction, into benign nitrogen gas and water. SCR can deliver near-zero emissions of NOx, an acid rain and smog-causing pollutant and greenhouse gas, in modern highway clean diesel engines.

The SCR system does not alter the design of the modern Common Rail Diesel (CRD) engine, therefore it can continue to deliver excellent fuel economy and durability. Rather, SCR provides emissions after-treatment well into the exhaust stack, in a way similar to the soot containment achieved by the Diesel Particulate Filter (DPF).

SCR works by injecting Diesel Exhaust Fluid (DEF) , such as AdBlue, into the hot exhaust stack. DEF works in conjunction with the hot exhaust gases and catalyst to break NOx into two components of our normal atmosphere—water vapor and nitrogen.

The Process - How SCR Works

Engine: The NOx reduction process starts with an efficient CRD engine design that burns clean Ultra Low Sulfur Diesel (ULSD) and produces inherently lower exhaust emissions—exhaust that is already much cleaner due to leaner and more complete combustion.

Diesel Exhaust Fluid (DEF) tank and pump: Under the direction of the vehicle’s onboard computer, DEF is delivered in precisely metered spray patterns into the exhaust stream just ahead of the SCR converter.

SCR Catalytic Converter: This is where the conversion happens. Exhaust gases and an atomized mist of DEF enter the converter simultaneously. Together with the catalyst inside the converter, the mixture undergoes a chemical reaction that produces nitrogen gas and water vapor.

Control device: Exhaust gases are monitored via a sensor as they leave the SCR catalyst. Feedback is supplied to the main computer to alter the DEF flow if NOx levels fluctuate beyond acceptable parameters.

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