| Marine low-speed engines have become the prime movers for large ocean-going ships because of its high power,good economy and stable operation.Different from the traditional four-stroke diesel engines,the air intake valves are replaced by scavenging ports in marine lowspeed engines.For example,the ultra-long stroke low-speed engine 7G80ME-C achieves high output power with a stroke of 3.72 m,which increases the gas flow stroke during the air ventilation process as well as the instability.Complex gas motion would cast influence on the combustion process and the performance of low-speed engines because it affects the ventilation process.As there is a lack of the relevant theoretical and experimental research,it is of great significance to study the ventilation process of marine ultra-long stroke low-speed engines.Based on the proceeding of the Intelligent Ship 1.0 project,the in-cylinder flow of the whole ventilation process by using CFD software.It is found that the exhaust gas residue in the center of the cylinder will have a great influence on the ventilation process.Different models are developed and calculated by changing the number and structural size of the scavenging ports.Besides,the ventilation process under design condition and off-design condition are studied by changing the the engine rotation speed.By analyzing the simulated 3D results and test results,the effects of different factors on the ventilation process are summarized.It is found that the rotation speed of the ultra-long stroke low speed engine has a significant impact on the ventilation process.The scavenging efficiency will be reduced when the speed increases.The structural parameters of the exhaust ports also have a significant impact on the ventilation process.The increasing elevation angle of the exhaust port weakens the scavenging effect at the cylinder bottom,whilst it improves the ventilation efficiency of the entire ventilation process.When the tilt angle of the exhaust port is small,the gas flow path would get close to the central axis of the cylinder,and the cleaning effect of the central axis position can be improved.The number of scavenging ports will affect the flow area of the ventilation process and determine the intake effect together with the circulation speed.In the case of 7G80ME-C,the air sweep efficiency can be improved significantly when the number of flushes is set to be 30.Compared to the commonly used zero-dimensional model,the CFD simulation modelling method used in this study can accurately simulate the flow situation and the air exchanging effect of the ultra-long stroke low-speed engine.The details of the in-cylinder flow process and the phenomena caused by the gas flow are further understood.This study is of high academic value as it is capable of providing certain theoretical support for the in-cylinder flow study of marine ultra-long stroke low speed engines. |
