Reduction of nitrogen oxide emissions from marine diesel engines

Globally, nitrogen oxide (NOx) emissions are controlled through emission standards for marine diesel engines with a power output greater than 130 kilowatts (kW).

Since the 2000s, Nitrogen oxide (NOx) emissions from vessels have been regulated through the International Maritime Organization’s (IMO) International Convention for the Prevention of Pollution from Ships (MARPOL). 

NOx are air pollutants that harm the environment and human health. NOx damages the lungs and has been linked to heart disease, lower birth weight in newborns and increased risk of premature death. NOx can also lead to acid rain, which damages crops, vegetation and infrastructure, and contributes to the pollution of coastal waters and poor air quality in the form of smog.  

Emission requirements for marine diesel engines 

MARPOL Annex VI sets out tiered limits (Tiers I, II, and III) to progressively reduce NOx emissions globally from marine diesel engines with a power output above 130kW. The tier that applies is based on the ship’s construction date, date of any major conversions, and area of operation. For each tier, the NOx emission limit is based on the ship’s rated engine speed (crankshaft rpm). 

Tier I, II and III 

Tier I NOx emission limits apply to marine diesel engine installations in Australian vessels constructed until 1 January 2011 and to existing engines that underwent a major conversion prior to this date. 

Since 1 January 2011, new installations of marine diesel engines on vessels and existing engines that undergo a major conversion have been required to meet the less polluting Tier II limits. Replacement engines and additional engines installed on existing ships must meet Tier II standards unless they meet the defined criteria for an ‘identical replacement engine’. 

Tier III-compliant engines are only a requirement for vessels built after specific dates operating in IMO agreed NOx Emissions Control Areas (NECAs). There are currently two NOx ECAs - the North American area and the United States Caribbean Sea area. There are currently no plans to establish a NECA in Australian waters. 

These requirements are summarised in the following table. 

Tiers for NOx limits Effective date 
Tier I On or after January 1, 2000 
Tier II On or after January 1, 2011 
Tier III On or after January 1, 2016 when operating in North American ECA and United States Caribbean Sea Area ECA. 

Further details on the MARPOL Annex VI requirements relating to NOx can be found in MARPOL Annex VI and the NOx Technical Code 2008, as amended. 

Evidence that an engine meets the NOx emission requirements 

When purchasing a new marine diesel engine with a power output of more than 130 kW, shipowners and operators must ensure that the engine has been issued with an Engine International Air Pollution Prevention Certificate (EIAPP) and its supporting engine Technical File. These provide the evidence that an engine complies with NOx emission standards.  

From 1 January 2023, engine suppliers are required under Australian law to issue both the EIAPP certificate and Technical File at the point of sale

What is an identical replacement engine? 

Under MARPOL Annex VI, an engine is considered an identical replacement to the original engine if it is the same in respect to all of the following:  

  • design 
  • model 
  • rated power 
  • rated speed 
  • use 
  • number of cylinders 
  • fuel system type. 

If you are replacing an engine that is not an identical replacement, as defined by MARPOL Annex VI, for operation in Australian waters you must ensure the replacement engine is certified as compliant with the MARPOL Tier II NOx emission limits.  

Requirements for engines used for emergencies 

The NOx emission regulations do not apply to marine diesel engines: 

  • intended to be used solely for emergencies, or  
  • used to power any device or equipment intended to be used solely for emergencies on the vessel on which it is installed, or 
  • installed in lifeboats intended to be used solely for emergencies. 

Find out more 

Last updated: 13 November 2023