Under MARPOL Annex VI and the NOx Technical Code 2008, engines above 130 kW installed on ships built from 2000 (Tier I), 2011 (Tier II), and those operating in ECAs/certain zones from 2016 (Tier III) must comply with a NOx emission limit curve based on engine rated speed. Procedures/equipment:

1. Engine certification: the engine is a "certified engine" tested on the test bed (engine pre-certification) and issued a NOx Technical File (NTF) containing the EIAPP certificate and the engine's parameter values (engine's adjustable parameters, inc. fuel injection timing, compression ratio, cylinder oil feed), with the approved NOx emission value.
2. On-board NOx verification: procedures for verifying compliance on board per the Technical Code include either the parameter-check method (verifying engine is as per the NTF) or the simplified/full measurement method using a PEMS (Portable Emissions Measurement System) analyser measuring NOx in the exhaust at test points.
3. On-board monitoring/control: fitting of NOx abatement equipment (e.g. SCR) and its control to hold NOx below the limit; where applicable the EGC (exhaust gas cleaning) NOx control and monitoring; data logging.
4. The on-board NOx verification procedure/per second stage onboard check using PEMS by a surveyor, and the EIAPP certificate is renewed at the first and subsequent in-service surveys.
5. Keeping records and the ship operating within the approved parameters, with in-use engine adjustments logged, and the NOx Technical File available on board.

1. Internal engine measures (Tier II, don't need add-on):

• Injection timing retard: retarding the start of injection reduces peak combustion temperature (pressure) by spreading combustion, lowering thermal NOx formation, at a cost of some fuel economy and smoke (part profile).
• Improved combustion chamber/fuel injection (raising injection pressure, better atomization, shaping) reduces local hot zones and NOx.
• Miller cycle / advanced (early) inlet valve closing: reduces the effective compression ratio and hence charge-air temperature, lowering peak combustion temperature and NOx.
• Variable injection timing (VIT) and load-dependent tuning.
• Compression ratio/boost, cooled air. These limit the peak flame temperature where NOx forms (Zeldovich).

1. Exhaust after-treatment (Tier III):

• SCR (selective catalytic reduction): urea injected into the exhaust, ammonia reduces NOx on the catalyst to N2 and H2O - achieves up to 80-90% removal.
• EGR (exhaust gas recirculation, some medium-speed/low-speed): part of the exhaust is recirculated into the intake, reducing the oxygen partial pressure and lowering combustion temperature, cutting NOx.
• Water/liquid fuel emulsion injection or water injection into the cylinder (some engines) reduces flame temperature and NOx.
• Select adaptive operating parameters.

Each method acts to lower the temperature, or reduce the oxygen availability, in the combustion zone, reducing the formation of thermal NOx (or removing it by the catalyst); the chosen method is matched to the emission requirement and load range.