What is EGR and SCR on a ship? Troubleshooting and Fault Finding

Modern marine diesel engines operate under strict environmental regulations established by the International Maritime Organization (IMO), particularly Tier II and Tier III NOx limits. To comply with these standards, ship engines are equipped with exhaust gas emission control systems.

The two main technologies used onboard are:

• EGR (Exhaust Gas Recirculation)
• SCR (Selective Catalytic Reduction)

Both systems are designed to reduce nitrogen oxides (NOx), but they work in fundamentally different ways.

As an example, the article considers the MITSUI-MAN B&W 6S50ME-C9.5-EGRBP main engine with an EGR system and auxiliary Daihatsu 6DE-18 Tier III 630kW engines with an SCR system.