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Refer to Regulation 13 of Annex VI MARPOL, all marine diesel engines with a power output of 130 kW and more that are installed or replaced on a vessel (except for exactly the same or identical diesel engines), the requirements related to the emission of NOX are mandatory.

This article provides a brief explanation of the different tiers of NOX emission. Accordingly, for ships constructed on or after January 1, 2000, their NOx emissions must fall into one of the following tiers:

1. Tier I: Ships constructed on or after January 1, 2000 and before January 1, 2011, their NOx emissions based on engine RPM shall be less than the following values:
   1. 0 g/kWh when n is less than 130 rpm;
   2. 45 · n^(–0.2) g/kWh when n is 130 or more but less than 2,000 rpm;
   3. 8 g/kWh when n is 2,000 rpm or more.
2. Tier II: Ships constructed on or after 1 January 2011, their NOx emissions based on engine RPM shall be less than the following values:
   1. 4 g/kWh when n is less than 130 rpm;
   2. 44 · n^(–0.2) g/kWh when n is 130 or more but less than 2,000 rpm;
   3. 7 g/kWh when n is 2,000 rpm or more.
3. Tier III: Ships engaging in NOX emissions control area (Tier III), the emission limit based on engine RPM must be as follows:
   1. 4 g/kWh when n is less than 130 rpm;
   2. 9 · n^(–0.2) g/kWh when n is 130 or more but less than 2,000 rpm;
   3. 0 g/kWh when n is 2,000 rpm or more;

According construction year, NOX tier III emission control area is defined as follows:

1. Ships constructed on or after 1 January 2016, sailing in North American and Caribbean seas’ ECA.
2. Ships constructed on or after 1 January 2021, sailing in Baltic Sea and North Sea’s ECA.

It should be noted that for ships built from 1 January 1999 up to 1 January 2000 have one of the following conditions, shall comply with the emission limits Tier I.

1. Output power of 5000 kW and more
2. Cylinder displacement at or above 90 L

To review all the details of this topic, you can refer to the sixth appendix of the MARPOL Convention, Regulation 13.

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