An important marine pollution issue identified by the International Maritime Organization (IMO) is NOx emissions; however, the stipulated method for determining the NOx certification value does not reflect the actual high emission factors of slow-speed two-stroke diesel engines over long-term slow steaming. In this study, an accurate method is presented for calculating the NOx emission factors and total amount of NOx emissions by using the actual power probabilities of the diesel engines in four types of bulk carriers. The proposed method is suitable for all types and purposes of diesel engines, is not restricted to any operating modes, and is highly accurate. Moreover, it is recommended that the IMO-stipulated certification value calculation method be modified accordingly to genuinely reduce the amount of NOx emissions. The successful achievement of this level of reduction will help improve the air quality, especially in coastal and port areas, and the health of local residents.
Implications: As per the IMO, the NOx emission certification value of marine diesel engines having a rated power over 130 kW must be obtained using specified weighting factor (WF)-based calculation. However, this calculation fails to represent the current actual situation. Effective emission reductions of 6.91% (at sea) and 31.9% (in ports) were achieved using a mathematical model of power probability functions. Thus, we strongly recommend amending the certification value of NOx Technical Code 2008 (NTC 2008) by removing the WF constraints, such that the NOx emissions of diesel engines is lower than the Tier-limits at any load level to obtain genuine NOx emission reductions.