Here's a concise yet comprehensive English description of the SCR system:

SCR (Selective Catalytic Reduction) System‌

Purpose:‌
Reduces harmful ‌nitrogen oxides (NOx)‌ emissions from diesel engines (trucks, buses, ships, industrial equipment) to meet strict environmental regulations (e.g., Euro VI, EPA Tier 4).

Core Principle:‌
Converts NOx into harmless ‌nitrogen (N₂) and water (H₂O)‌ via a chemical reaction using a ‌reductant‌ and a ‌catalyst‌.

Key Components:‌

Reductant Tank:‌ Stores ‌aqueous urea solution‌ (e.g., AdBlue, DEF - Diesel Exhaust Fluid).
Dosing System:‌ Precisely injects urea into the exhaust stream.
Decomposition Tube:‌ Hot exhaust gases vaporize urea into ‌ammonia (NH₃)‌ and carbon dioxide.
SCR Catalyst:‌ Typically a honeycomb structure coated with catalytic metals (e.g., vanadium, zeolites).
Facilitates the reaction: ‌NH₃ + NOx → N₂ + H₂O‌.
Control Unit (DCU):‌ Uses sensors (NOx, temperature) to optimize urea injection based on engine load and emissions.

Chemical Reactions (Simplified):‌

Urea Decomposition:‌
(NH₂)₂CO + H₂O → 2NH₃ + CO₂
NOx Reduction (primary):‌
4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O
2NO₂ + 4NH₃ → 3N₂ + 6H₂O

Advantages:‌

High Efficiency:‌ Reduces NOx by ‌80-95%‌.
Fuel Economy:‌ Often better than alternative NOx reduction methods (e.g., EGR alone).
Proven Technology:‌ Widely adopted globally for heavy-duty diesel applications.

Challenges:‌

Reductant Consumption:‌ Requires periodic refilling of urea/DEF (~3-5% of diesel consumption).
Cold Weather:‌ Urea solution freezes below -11°C (12°F), requiring tank heaters.
Cost & Complexity:‌ Higher initial cost and maintenance than non-SCR systems.
Potential Clogging/Crystallization:‌ If dosing is incorrect or exhaust temperatures are low.

Why "Selective"?‌
The catalyst promotes the reaction between ‌ammonia (NH₃)‌ and ‌NOx‌ specifically, minimizing unwanted reactions with other exhaust components like oxygen (O₂).

Critical Role:‌
Enables diesel engines to achieve ultra-low emissions targets while maintaining performance and efficiency, playing a vital role in reducing air pollution.

Note:‌ SCR is often combined with other technologies like Diesel Particulate Filters (DPF) and Exhaust Gas Recirculation (EGR) for comprehensive emission control.