Numerical Investigation On The Efficient And Clean Combustion Technology For Low-speed Two-stroke Marine Diesel Engines

Low-speed two-stroke diesel engines are dominant in marine propulsion installations due to their high power output and reliability.The IMO Tier ? standard issued by the International Maritime Organization(IMO)requires a NOx reduction limit of approximately 80%compared to the Tier I standard,which poses a serious challenge to marine engine combustion technology.At present,technologies used to optimize emissions and performance of marine diesel engines include in-cylinder efficient and clean combustion technologies and after-treatment technologies,in which the former is the core process of engine development.In this paper,a three-dimensional numerical simulation of a typical low-speed two-stroke marine diesel engine is carried out.The influence of different in-cylinder decontamination technologies on the combustion process is studied and an effective technical scheme for realizing Tier ? is discussed.This study is of theoretical guidance for the development of a new generation of autonomous low-speed diesel engines.Firstly,a two-stroke marine diesel simulation model was built by using the Converge software(three-dimensional numerical simulation software).The KH-RT spray model and the Shell-CTC combustion model were used,with their tunable parameters calibrated with the engine test results.On this basis,the effects of EGR,injection strategy(including injection timing,injection pressure,multiple injection,injection angle),on the combustion process and emissions of marine diesel engines were studied,and the possible solution on meeting the Tier?standard was discussed.The results show that EGR is most effective in reducing NO_x emissions.Using EGR alone can achieve Tier ? emission standard,but it will also cause deterioration of BSFC.Under EGR conditions,fuel injection timing and injection pressure can be adjusted simultaneously to control the combustion heat efficiency and combustion phase,optimizing the relationship between NO_x and BSFC.From the perspective of energy conservation,it is effective to reduce the BSFC by maximizing the injection pressure and match the appropriate injection timing.From the perspective of emission reduction,a modest increase in fuel injection pressure with postponed fuel injection timing can reduce NO_x emissions and alleviate the increase in of BSFC.Pilot injection can the main injection combustion temperature,with the potential of optimizing both BSFC and NO_x.The combined application of EGR,increasing injection pressure,adjusting fuel injection timing and pre-injection technology can significantly improve fuel economy on the basis of meeting the Tier ? emission.The results of the fuel injection-air motion matching study show that the BSFC can be modestly optimized by adjusting the horizontal inclination of the scavenging ports,however the potential is limited.The combined adjustment of the injection angle in the swirl section and the longitudinal section can optimize the mixture formation process and therefore improve the Trade-off relationship between NO_x and BSFC.