Comparative Analysis of Performance and Emissions of a Two-Stroke Marine Diesel Engine According to CPP Modes

This study experimentally investigates the performance and exhaust emission characteristics of a low-speed two-stroke marine diesel engine operated with different controllable pitch propeller (CPP) modes during actual sea operation. Full-scale measurements were conducted on the training vessel T/S Baek-Kyung, equipped with a MAN B&W 5S35ME-B9.5 engine, operating under IMO Tier II fallback (FB) conditions. Two CPP control strategies were compared: a constant-speed mode, in which engine speed was maintained at approximately 162 rpm and load was controlled by propeller pitch, and a combinator mode, in which engine speed and pitch were jointly controlled. In the combinator mode, the propeller pitch reached saturation (100%) at approximately 25% load, and further load variation was governed primarily by engine speed. The analysis focused on an engine-load range of approximately 25–75% SMCR and evaluated propulsion performance, including specific fuel oil consumption (SFOC) and shaft torque, together with estimated brake-specific exhaust emissions expressed in g/kWh. The combinator mode achieved superior fuel efficiency under partial-load conditions, reducing SFOC by up to 10.5 g/kWh (5.4%) at 25% load, while increasing shaft torque by up to 47%, indicating improved engine–propeller matching. However, this benefit was accompanied by higher estimated emissions at low load, with BSNOx increasing from 13.61 to 16.95 g/kWh. As engine load increased, differences in both performance and emissions between the two modes diminished. These results reveal a clear load-dependent trade-off between fuel efficiency and exhaust emissions in CPP operation and emphasize the importance of load-based switching or optimal joint control strategies under off-design conditions.