The similarity ratio effects in design of scaled model experiments for marine diesel engines

Marine diesel engines usually span a large range of bore diameters or power. While optimization of spray combustion system based on scaled model experiments would be beneficial to reduce the cost, energy consumption and project cycle of marine engine development, the similarity ratio effects on the design target parameters such as in-cylinder maximum pressure, indicated thermal efficiency, and NOx and soot emissions are rarely investigated. In this study, firstly, the three-dimensional computational fluid dynamics models are calibrated against the experiment data such as in-cylinder pressure and heat release rate evolutions, indicated thermal efficiency and NOx emissions, and the boundaries of similarity ratio determined by the limits of piston speed, engine speed and fuel injection pressure are clarified. Then, the effects of similarity ratio ranging from 0.8 to around 3 are studied under the different engine loads, engine types and scaling laws. The results show that the prediction accuracy of the scaled model experiments decreases with the increase of the bore diameter difference, and the exponential function can well describe the relationship between the design target parameters and similarity ratio. The results also reveal that the scaling laws should be properly selected for different design target parameters. The scaling law based on constant injection pressure exhibits great potential for predicting indicated thermal efficiency and maximum in-cylinder pressure, as the maximum difference of these parameters between the base engine and targeted engine is less than 1.2% with the similarity ratio up to 3.00.