Valve lift profile design of marine natural gas engines by using multi-objective optimization and impact-free cam methods

The valve lift profile plays a crucial role in the gas exchange process of natural gas engines, as well as in the operational stability of the valve mechanism. Through optimization and design of the valve lift profile, it is possible to effectively reduce engine gas exchange loss, enhance engine power output, and improve the operational stability of the valve mechanism. This study establishes a simulation model for a natural gas engine, which is validated against experimental data. The impact of various parameters on engine performance is explored using a parametric method and experimental design. The paper focuses on optimizing the valve lift profile at a typical operating point, with targets of minimizing brake-specific fuel consumption and NOx emissions. The non-dominated sorting genetic algorithm was employed to optimize the valve lift profile. Given that the optimized lift curve alters the original kinematic characteristics, an impact-free function cam design method was adopted to perform the forward design of the optimized curve, followed by a comprehensive valve motion verification. The results show that the fullness coefficient of the optimized exhaust valve lift profile increased from 0.609 to 0.637, and the fullness coefficient of the intake valve lift profile improved from 0.578 to 0.608. The larger fullness coefficient indicates improved engine gas exchange performance. At 90.9% load, the brake-specific fuel consumption of the natural gas engine decreased by 2.76%, and NOx emissions were reduced by 22.93%. The valve lift profiles designed using the cam method with the impact-free function achieved third-order continuity in lift, velocity, and acceleration. This ensures smoother operation of the engine valve mechanism and prevents abnormal vibration.