Answer
Engine derating means reducing the approved MCR of an engine to a lower power and/or RPM. It is done only within the maker’s approved layout diagram. Practically it is done by limiting maximum fuel index through governor or electronic control system, reducing maximum RPM if required, matching turbocharger and propeller, and adjusting injection/control parameters. For permanent derating, maker, class and flag approval are required, and engine technical file, NOx technical file, EIAPP certificate and EEXI documents may need amendment. After derating, performance trials are carried out and fuel limiter or EPL/ShaPoLi arrangement may be sealed by class. The main purpose is fuel saving, emission reduction, EEXI/CII compliance, propeller matching and reduction of engine stress.
Why engine is derated?
Engine may be derated for:
1. Fuel economy
Main engine may not need full design power in actual service. By derating, the engine can operate closer to optimum specific fuel oil consumption.
2. Slow steaming
Many ships operate at lower speeds to save fuel and reduce emissions. Derating makes the engine suitable for continuous low-load operation.
3. CII / EEXI compliance
To reduce attained EEXI or improve operational carbon intensity, engine power may be limited by Engine Power Limitation, EPL or Shaft Power Limitation, ShaPoLi.
4. Propeller matching
If propeller is changed, damaged, fouled, or if hull resistance changes, engine may be derated to match the propeller load curve.
5. Emission control
Lowering fuel consumption reduces CO₂ emission. In some cases derating also helps keep NOx emission within approved technical file conditions.
6. Reliability
Reducing peak pressure, thermal loading, exhaust temperature, and mechanical stress improves engine life.