Optimization Of Combustion Chamber Structure And Matching Research Of Intake System Of Marine Diesel Engine

With the rapid development of the shipping industry,the exhaust pollution of ships has been paid more and more attention.IMO has developed increasingly stringent regulations to control the emissions of marine diesel engines,especially NO_x emissions.The combustion process of diesel engine directly determines the amount of NO_x generated,so optimizing the combustion is the basic method for NO_x purification.Optimization research of combustion chamber geometry and intake system was carried out in this paper by using simulation method.The temperature reduction effect of miller cycle coupled with intake turbocharging system and EGR as well as the better fuel-air mixing effect of combustion chamber modification and swirl ratio can decrease emissions effectively.Based on a six-cylinder turbocharged and inter-cooled marine diesel engine,the simulation models were built by using Boost and Fire software code respectively.The effects of combustion chamber structure,swirl ratio,Miller cycle and EGR on combustion,performance and emissions of marine diesel engine were studied.The scheme of matching the swirl ratio on the different chamber designs and the small EGR rate with medium degree Miller cycle were studied,and their applicability to large bore marine diesel engine was investigated.Results showed that chamber diameter and bowl depth have a great influence on in-cylinder flow.The deeper depth of the re-entrant bowl can result in enhanced swirl motions and improve the combustion,while a deficient combustion occurs at the lower depth open chamber.Further,high turbulent kinetic energy with a large swirl is important to improve the quality of combustion.It's also found that re-entrant chamber makes a higher power output at the cost of a smaller increase of NO_x emissions.The straight chamber has lower emissions at the same output power level with the prototype.And open chamber is found to have lower engine-out power.Optimization of the combustion chamber structure and matching of reasonable swirl ratio can improve the in-cylinder flow,further reduce emissions,and increase the indication power.With the advance of miller timing,the temperature in cylinder decreases,and the NO_x emissions decrease as well,while the too early intake valve close timing increases NO_x emissions and makes heat release rate increase sharply.EGR strategy can evidently reduce the high temperature distribution and NO_x emissions,while soot emission increases.When the intake valve closing advance angle is small,EGR plays a major role in reducing the in-cylinder oxygen concentration,while the increase of the stagnation period plays an important role in the improvement of fuel and gas mixture as the intake valve closing advance angle increases.The optimization of miller cycle,combustion chamber,initial swirl ratio and EGR can reduce NO_x emissions by 90.6%,and increase the output power slightly.