Study On The Improvement Of Thermal Efficiency Of Marine Medium-Speed Diesel Engine

In the face of the international trend of "energy saving and emission reduction",achieving higher thermal efficiency has become the main challenge for diesel engines.It has been well known and trusted that the thermal efficiency of Otto cycle is higher than other cycles under the same conditions.But in practice,the higher degree of constant-volume combustion is,the higher the in-cylinder pressure and temperature will rise substantially,which will lead to the increase of mechanical loss and heat transfer loss and affect the improvement of brake thermal efficiency,so it is difficult to improve the brake thermal efficiency of the diesel engine under the condition of limited temperature and pressure.Therefore,in order to achieve the maximum improvement of brake thermal efficiency,it is necessary to reasonably match the in-cylinder combustion process of the constant-volume combustion and constant-pressure combustion.In this paper,a zero-dimensional calculation model of medium-speed diesel engine is established based on the working process principle of medium-speed diesel engine,and in the model the combustion model is substituted by constant-volume combustion and constantpressure combustion.The calculation program is written by Fortran programming language,and the model is calibrated and verified based on a medium-speed diesel engine.By setting the degree of constant-volume combustion,the brake thermal efficiency,heat transfer loss and mechanical loss are obtained with the different degree of constant-volume combustion.The trend of brake thermal efficiency,mechanical loss and heat transfer loss with the proportion of constant-volume and constant-pressure combustion was analyzed,by which the exothermic characteristics and matching law of constant-volume combustion and constant-pressure combustion were analyzed,so as to match the constant-volume and constant-pressure combustion process of medium-speed diesel engine,and the optimal exothermic rate curve of effective thermal efficiency was obtained.GT-Power model was established for medium-speed diesel engine and the model calibration was completed.Through the substitution of virtual heat release rate profile for combustion process,the trend of brake thermal efficiency under different pressure rise ratio and constant volume combustion duration was analyzed.Under the condition that the pressure rise ratio is limited,the heat release rate profile that maximizes the effective thermal efficiency is obtained.GT-Power software was used to establish the performance simulation model of mediumspeed diesel engine with three injectors,and the optimization of the oil distribution,injection strategy,injection hole diameter and injection pressure of the three injectors was carried out to realize the constant-volume and constant-pressure combustion process of the diesel engine.The change law of cylinder pressure and heat release rate under different pressure ratios is analyzed,and the technical scheme to improve the effective thermal efficiency is put forward.The results show that under the condition that the maximum pressure is not higher than 25 MPa and NOx emission limit,the maximum pressure value is 24.21 MPa and the effective thermal efficiency is 44.04% when the constant volume combustion ratio is 0.25,which is higher than the calibration effective thermal efficiency of 42.21%.