Miller Cycle with Early Intake Valve Closing in Marine Medium-Speed Diesel Engines

In this study, a one-dimensional simulation was performed to evaluate the performance of in-cylinder combustion to control NOx emissions on a four-stroke, six-cylinder marine medium-speed diesel engine. Reducing the combustion temperature is an important in-cylinder measure to decrease NOx emissions of marine diesel engines. The Miller cycle is an effective method used to reduce the maximum combustion temperature in a cylinder and accordingly decrease NOx emissions. Therefore, the authors of this study designed seven different early intake valve closing (EIVC) Miller cycles for the original engine, and analyzed the cycle effects on combustions and emissions in high-load conditions. The results indicate that the temperature in the cylinder was significantly reduced, whereas fuel consumption was almost unchanged. When the IVC was properly advanced, the ignition delay period increased and the premixed combustion accelerated, but the in-cylinder average pressure, temperature and NOx emissions in the cylinder were lower than the original engine. However, closing the intake valve too early led to high fuel consumption. In addition, the NOx emissions, in-cylinder temperature, and heat release rate remarkably increased. Therefore, the optimal timing of the EIVC varied with different loads. The higher the load was, the earlier the best advance angle appeared. Therefore, the Miller cycle is an effective method for in-engine NOx purification and does not entail significant cost.