ICE-24 — Selective Catalytic Reduction (SCR)

ASSUMPTION — Dosing ratios and conversion efficiencies vary by system; a qualitative framework is given.

In this lesson you will learn:

• The SCR principle
• AdBlue/DEF dosing
• EGR + SCR combination

1. Why Is It Needed?

A diesel runs lean, so a three-way catalyst cannot reduce NOx. SCR fills that gap.

FACT — SCR selectively reduces NOx to nitrogen (N2) and water (H2O). The reducing agent is ammonia, produced from a urea solution (AdBlue/DEF).

2. AdBlue/DEF and Dosing

FACT — AdBlue (aqueous urea solution, DEF) decomposes in hot exhaust to form ammonia, which reacts with NOx over the SCR catalyst. Dosing is set by NOx sensors and a dosing control strategy.

3. EGR + SCR Together

INTERPRETATION — EGR lowers in-cylinder NOx (engine side); SCR reduces remaining NOx in the exhaust (aftertreatment side). Both are optimized together for high NOx conversion (ICE-41).

4. Trade-offs

FACT — SCR adds a tank, dosing system, sensors and catalyst — cost and complexity — but is among the most effective ways to meet NOx limits. AdBlue consumption is also an operating cost.

5. Summary

• SCR reduces NOx to N2 and H2O.
• Ammonia source is AdBlue/DEF.
• EGR + SCR reduce NOx on both engine and exhaust sides.

6. Sources

• Bosch, Emissions Control Technology
• Bosch, Diesel-Engine Management

Next Lesson

• ICE-25 — NOx: nitrogen oxide formation and control.