Experimental and Computational Investigation of a Quarter-Wave Resonator on a Large-Bore Marine Dual-Fuel Engine 2017-24-0017

Large pressure pulsations and a non-uniform distribution of charge air temperature along the intake manifold were detected on a large-bore marine Dual-Fuel engine. These two phenomena were found to impact negatively on the knock resistance of individual cylinders, when the engine is operated in gas-mode. As it happens with marine gas engines, the cylinder most prone to knocking drives the engine tuning for all the others, thus reducing the overall fuel conversion efficiency. In order to effectively tackle this issue, a comprehensive study was carried out, which included both experimental testing and fluid-dynamics simulation. A detailed GT-POWER 1D engine model was built, representing the laboratory 8L (i.e. inline eight-cylinder) engine configuration. The model was extensively correlated against measurements at different speeds and loads and it proved capable of closely reproducing both the pressure fluctuations and the temperature gradient along the intake manifold. Models for the other cylinder configurations (6L, 7L, 9L, 12V, 14V and 16V) were built scaling the validated 8L one. Linear acoustic analyses were then used to investigate the eigenvalues (natural frequencies) of the intake air path, for each engine variant. It was possible to infer that the second natural frequency of the intake volumes is to be considered as the main cause of the pressure fluctuations. The temperature gradient was on the other hand identified as a side-effect of the large pressure oscillations, as they increase locally the heat transfer coefficients between the charge air and the warm manifold walls. A quarter-wave resonator was designed, which was tuned on the resonance frequency to mitigate the pressure pulsations. For packaging reasons, it was decided to place the resonator inside the air receiver. As the resonator length was very close to the manifold’s one in some cylinder configurations, a bent resonator pipe was proposed. Its shape, cross-section and dimensions were investigated in detail and optimized by means of 1D and 3D CFD analyses. Eventually, tests on the 8L laboratory engine, fitted with the quarter-wave resonator of choice, supported the simulation results: pressure pulsations decreased by 40% and, after engine re-tuning, brake efficiency increased by 0.3% (absolute) at the same knock-margin.