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ICCT analysis finds newer Tier II marine engines have significantly higher NOx emissions than older Tier I engines

A new working paper from the International Council on Clean Transportation (ICCT) finds that newer Tier II engines have significantly higher NOx emission rates than older Tier I engines. Moreover, there was no statistical difference in NOx emission rates between unregulated Tier 0 engines and Tier II engines.

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Boxplot showing distribution of observed NOx emission rates by engine tier. White dots show mean; vertical bars show median; error bars show interquartile range; diamonds show outliers. Source: ICCT


The researchers analyzed 615 samples of real-world NOx emissions from 545 ships operating in Danish waters between the North Sea and the Baltic Sea in 2019. Measurements were obtained using sniffers attached to helicopters flown into exhaust plumes. The data includes measurements from ships covering all engine age categories (tiers).

The greatest mean NOx emission rates were found at main engine loads below 25%, with emissions averaging 12 g/kWh across all vessel types and engine tiers. Emission rates decrease as main engine loads increase, with mean emission rates of 8.1 g/kWh at loads greater than 75%.

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Boxplot showing the distribution of NOX emission rates by engine load and IMO tier. Source: ICCT


Existing NOx test cycles assume that marine engines most often operate at higher engine loads; however, this study finds that engines typically operate at lower engine loads.

The ICCT suggests that NOx regulations could be revised to make them more effective at reducing air pollution. Rather than relying on weighted emissions limits, the International Maritime Organization could consider implementing not-to-exceed (NTE) standards for new and existing ships, particularly focusing on operations at low loads, and including a test point below 25% load.

This would result in more complete emissions profiles for ships, especially during low load operations, the ICCT said.

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