Scaled Model Experiments for Marine Low-Speed Diesel Engines

Diesel engines have been widely used as marine propulsion, with a wide range of bore diameters. Since some similar characteristics of spray combustion exist in different size diesel engines, the ability to accurately reproduce engine performance by existing engines is therefore beneficial for reducing time, cost and energy consumption in new engine development. However, so far knowledge on scaling diesel engines is far from adequate, particularly for large marine low-speed diesel engines. The aim of this study is to explore the potential of scaled model experiments for marine low-speed diesel engines with different bore diameters. After calibration of the computational fluid dynamics (CFD) simulation model using the experimental data of the in-cylinder pressure trace and the apparent heat release rate (AHRR) from a marine two-stroke diesel engine with 340 mm bore diameter, the similarity of spray mixture formation and combustion processes as well as pollutant emissions is numerically studied for three similarity rules between this 340 mm engine and an up-scaled 520 mm engine. The results reveal that the in-cylinder pressure, HRR and in-cylinder temperature can be well scaled between the large and small engines, while the three similarity rules exhibit different degrees of similarity in terms of NOx (nitrogen oxides) and soot emissions. With the sophisticated control of the cylinder wall temperature of the small engine, the discrepancy in NOx and soot emissions between the large and small engines is reduced. These results are valuable for evaluating the similarity rules for guiding new diesel engine development.