Ship emissions and fuel economy under transient conditions: Revisiting the propeller law

In ports and coastal areas, ship engines frequently operate under aggressive, transient conditions such as manoeuvring, which contribute significantly to gaseous and particulate emissions. To address these dynamics, this study investigates diesel exhaust behaviour during acceleration using a six-cylinder testbed engine. A custom driving cycle was developed to emulate propeller law-based transient cycles across varying rates of rotational speed change and load conditions. Performance and emissions were assessed under diverse load-speed scenarios to evaluate the applicability of the propeller law during transient operations. The engine response lagged behind load demands: at 12.5-50 rpm/s, the engine met 92-100% of the requested load, but at rates up to 300 rpm/s, only 56% was achieved. Rapid acceleration increased brake-specific fuel consumption, reducing efficiency. Principal component analysis (PCA) identified NOₓ, CO, PM₁, and PN as strongly correlated with operating parameters, with the first two principal components explaining 71% of the variance. Slower acceleration ramps had elevated NOₓ and CO, while PM₁ and PN rose with faster rpm changes. Regression models based on the propeller law predicted fuel consumption, NOₓ, PM₁, and PN with R² > 0.63; CO predictions were less accurate (R² = 0.58), indicating additional influencing factors. These findings enhance understanding of transient engine behaviour and fuel economy, informing strategies to optimise ship propulsion and improve emission inventories in port environments. In line with MARPOL Annex VI, which mandates sulphur limits in marine fuels within environmental control areas (ECAs), this study employed ultra-low sulphur diesel (ULSD). While absolute emission values may differ, the observed trends are applicable to refined marine fuels such as marine gas oil (MGO) and ultra-low sulphur fuel oil (ULSFO).