Research On Multiple-Injection Combustion Process Of Marine Medium Speed Diesel Engine

At present,our country has achieved independent research and development of medium speed marine diesel engines,but there is still a big gap with advanced foreign countries.High power density,high thermal efficiency,high reliability and low emissions have become the direction of development of medium-speed marine diesel engines in our country.The shortcomings of large-bore marine diesel engines have seriously slowed down the realization of our country’s goal from a"large shipbuilding country"to a"strong shipbuilding country".Since the marine medium-speed diesel engine has already achieved high power density,but the mechanism and evolution of spray and combustion is not so clear,which makes it difficult to continue to improve the effective thermal efficiency of the high-power density marine medium-speed diesel engine.Multiple-injection technology in automotive diesel engines has been proven to be efficient to improve the effective thermal efficiency of diesel engines,and is regarded to be promising in the combustion organization of marine diesel engines with larger cylinder diameters.First,this paper established a simulation model of a constant volume bomb and a marine medium-speed diesel engine based on the Computational Fluid Dynamics(CFD)software CONVERGE.A spray model was calibrated in the constant volume bomb,then the spray characteristics and combustion characteristics of double injections and single injection were compared respectively by the calibrated model.The results showed that the double injections could effectively improve the characteristics of hollow flame and combustion accumulation near the chamber under a high injection pressure and a large injection amount.Considering that the constant volume bomb could not reflect the airflow motion induced by the in-cylinder piston movement and fuel propagation,a CFD simulation for the multiple-injection combustion process of a marine medium-speed diesel engine was carried out.A simulation model of the marine medium-speed diesel engine was established,with the combustion model calibrated.Under the common operating loads of the original diesel engine,the injection strategies of pre-main injection and main-post injection with different proportions and intervals are designed to enhance the effective thermal efficiency and meet the emission standards.Then the advanced multiple-injection strategies were proposed according to the obtained effective thermal efficiency of the marine medium-speed diesel engine.Finally,in order to cope with the excessively high NO_x production accompanied with the improved effective thermal efficiency,the study of matching strategy between the multiple-injection and the combustion chamber shape was carried out.Based on the results of effective thermal efficiency under different advanced multiple-injection strategies,the effects of combustion chamber bore and pit depth as well as pit radius on the effective thermal efficiency and emissions were analyzed.Then several advanced matching strategies between the multiple-injection and the combustion chamber shape were proposed in 75%load:Under the condition that the NO_x emission standard of Tier 2 is 9.8g/k W·h,when the pre-injection ratio is 20%,and the pre-main injection interval is 10°CA,the NO_x emission is 9.48g/k W·h,and the effective thermal efficiency is increased to 45.53%from 43.44%,soot emissions is decreased by 32.56%.When the pre-injection ratio is 30%and the pre-main injection interval is 5°CA,the NO_x emission is 9.29g/k W·h,the effective thermal efficiency is increased to46.28%from 43.44%,soot emissions is decreased by 51.16%.