Scaling spray combustion processes in marine low-speed diesel engines

Scaled model experiments would be very effective in reducing cost, time and energy consumption in marine diesel engine development, however, relevant researches are rarely reported. The aim of this paper is to explore the potential of scaled model experiments for marine low-speed diesel engines, and provide references for researchers who attempt to conduct scaled model experiments in marine diesel engine development. At first, the computational fluid dynamics simulation model is established and calibrated against the experimental data from a marine low-speed diesel engine with 340 mm bore diameter. Then, three scaling laws for spray mixture formation and combustion processes as well as pollutant emissions are numerically studied with the baseline 340 mm-bore engine and an up-scaled 520 mm-bore engine at various injection timings. The results reveal that the spray mixture formation processes, in-cylinder temperature and pressure traces, and indicated thermal efficiency can be well scaled between the 340 and 520 mm-bore engines, while the three scaling laws show different degrees of similarity in soot and nitrogen oxides emissions. The increased heat transfer losses of the small engine are identified as a major obstacle to achieve the overall similarity, and the wall temperature setting methods to scale the heat transfer loss are theoretically derived and numerically verified. With the sophisticated control of the cylinder wall temperatures of the small engine, the discrepancy in soot and nitrogen oxides emissions between the large and small engines is reduced.