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What is SCR System Selective Catalytic Reduction Explained

In this article, we will explore what is SCR system is, how it works, and its benefits. This system is integral to meeting stringent environmental regulations and improving air quality. The Selective Catalytic Reduction SCR system is an advanced emission control technology used in diesel engines to reduce harmful nitrogen oxide (NOx) emissions.

What is the SCR System?

The SCR system is an after-treatment technology designed to convert nitrogen oxides (NOx), which are harmful pollutants, into harmless nitrogen (N2) and water (H2O). This process occurs through a chemical reaction facilitated by a catalyst in the presence of a reducing agent, commonly known as Diesel Exhaust Fluid (DEF) or AdBlue.

Key Components of the SCR System:
  1. Catalyst: A device coated with precious metals that facilitates the chemical reactions necessary to reduce NOx emissions.
  2. DEF (Diesel Exhaust Fluid) Injector: Injects a precise amount of DEF into the exhaust stream.
  3. DEF Tank: Stores the DEF, which is a mixture of urea and deionized water.
  4. Control System: Monitors and adjusts the injection of DEF to ensure optimal performance.

How Does the SCR System Work?

The SCR system operates in several steps to reduce NOx emissions:

  1. Exhaust Gas Enters the system: Exhaust gases containing NOx are directed into the SCR catalyst.
  2. Injection of DEF: DEF is injected into the exhaust stream. The heat from the exhaust converts DEF into ammonia (NH3).
  3. Chemical Reaction: Ammonia reacts with NOx in the presence of the catalyst. This reaction breaks down NOx into nitrogen and water vapor.
  4. Emission Release: The cleaned exhaust gases, now containing harmless nitrogen and water, are released into the atmosphere.

Detailed Process Breakdown:
  • Step 1: Diesel exhaust gases are directed towards the SCR system after passing through the diesel oxidation catalyst (DOC) and particulate filter (DPF) to remove other pollutants.
  • Step 2: The DEF injector sprays a controlled amount of DEF into the hot exhaust gases.
  • Step 3: The catalyst facilitates the reaction between ammonia (from DEF) and NOx, converting them into nitrogen and water.
  • Step 4: The final, cleaner exhaust gases exit the tailpipe.

Benefits of the SCR System

Several key benefits:

  • Significant NOx Reduction: Reduces NOx emissions by up to 90%, making diesel engines much cleaner.
  • Regulatory Compliance: Helps vehicles comply with stringent emission standards such as Euro 6 and EPA Tier 4.
  • Fuel Efficiency: Allows for optimized engine performance and fuel efficiency by enabling engines to operate at higher temperatures without producing excessive NOx.
  • Environmental Protection: Contributes to improved air quality and reduced environmental impact.

Proper maintenance is crucial for its efficiency and longevity:

  • Regular DEF Refills: Keep the DEF tank filled to ensure continuous operation of the SCR system. Vehicles typically have indicators to alert when DEF levels are low.
  • Quality DEF: Use high-quality DEF to prevent contamination and ensure the effectiveness of the SCR system.
  • System Checks: Regularly inspect the SCR components for any signs of wear or malfunction.

Common Issues and Troubleshooting

System are generally reliable, some issues can arise:

  • DEF Crystallization: DEF can crystallize if exposed to air, leading to blockages. Ensure the DEF tank is properly sealed.
  • Freezing: DEF can freeze at temperatures below -11°C. Most systems have heaters to prevent this, but it’s important to ensure these are functioning correctly.
  • Sensor Malfunctions: Regular diagnostics can help detect and address sensor issues that may affect the SCR system’s performance.


The SCR system is a vital technology for reducing NOx emissions from diesel engines, contributing to cleaner air and compliance with environmental regulations. Understanding how this system works and maintaining it properly can ensure optimal performance and longevity. Regular maintenance, using quality DEF, and staying informed about the system’s operation are key to reaping the full benefits of SCR technology.

For more insights on diesel engine technologies and maintenance tips, visit our blog and stay updated with the latest information.

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