As the emissions regulations have become more stringent, reducing NOX emissions is of great importance to the shipping industry. Due to the price and emissions advantages of natural gas, the diesel-natural gas engines have become an attractive solution for engine manufacturers. Firstly, in this paper, the NOX emissions prediction model of a large marine four-stroke dual-fuel engine is built by using AVL-BOOST. In addition, the model is further calibrated to calculate the performance and emissions of the engine. Then, the influences of boost pressure, compression ratio, and the timing of intake valve closing on engine performance and emissions are analyzed. Finally, the response surface methodology is used to optimize the emissions and performance to obtain the optimal setting parameters of the engine. The results indicate that the response surface method is a highly desirable optimization method, which can save a lot of repeated research. Compared with the results from manufactured data, the power is increased by 0.55% and the BSFC, the NOX emissions, and the peak combustion pressure are decreased by 0.60%, 13.21%, and 1.51%, respectively, at low load.
This study aims at the parametric investigation of a large marine four-stroke dual-fuel engine in order to identify the pre-injection effects on the engine combustion, knocking and emissions parameters.A model was employed that was developed by integrating a 1-D engine model in AVL-BOOST and a 3-D CFD model in CONVERGE. The MAN 51/60DF marine engine is modelled and the simulation results were validated against experimental data. Subsequently, parametric runs for various pre-injection timings and mass ratios are performed and the simulation results are analysed and discussed. The derived incylinder pressure oscillations at determined points are employed to calculate the knock index (KI), which was used as an evaluation indicator for the knocking intensity. A number of pre-injection strategies with varying timing and fuel mass ratios are studied. This study results reveal that a lower knock trend and NO X emissions can be achieved by early pre-injection timing and increasing pre-injection fuel mass ratio.In addition, the medium pre-injection interval increases the engine IMEP while reducing the NO X and total hydrocarbon emissions. Larger pre-injection mass ratio reduce the KI and NO X emissions, but reduces IMEP and causes the wetted-wall phenomenon. Besides, the excessive pre-injection intervals and pre-injection mass ratio result in a change in combustion mode from the conventional diesel compression ignition mode to a two-stage auto-ignition mode. This study provides a better understanding of the underlying interactions of involved parameters and proposes pre-injection solutions to improve the engine performance, emissions and knocking behaviour.
Compared to the standard cycle, the Miller cycle decreases the cylinder maximum combustion temperature which can effectively reduce NOx emissions. In this paper, a 0-D two-zone combustion model is used to establish the simulation model of a marine dual-fuel engine, which is calibrated according to the test report under different loads. Due to the high emissions under part load, the Miller cycle (early intake valve closing method) is used for optimization. By analyzing the cylinder pressure, temperature, heat release rate, and NOx emissions under different cases, it can be found that the effective working volume and thermal efficiency decrease with the advance of intake valve closing and improve with the increase of the geometric compression ratio. In all optimization cases, the NOx emissions and fuel consumption are reduced by 72% and 0.1%, respectively, by increasing the geometric compression ratio to 14 and the intake valve closing timing to 510 °CA (the reference top dead center is 360 °CA). The simulation results show that the early intake valve closing Miller cycle can effectively reduce the NOx emissions and cylinder peak pressure.
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