The Investigation Of Controllable Combustion Technique For High Thermal Efficiency Marine Diesel Engine

Improving the effective thermal efficiency of low-speed two-stroke marine diesel engine is of great significance for energy saving,CO2 emission reduction,shipping cost reduction and sustainable development.Since the performance and emission of the diesel engine largely depend on the combustion process,the reasonable choice of combustion mode is essential to achieve efficient and clean combustion.It has been proved that constant volume and pressure combustion mode,by controlling the proportion of premixed combustion and diffusion combustion,can reduce heat loss and mechanical loss as well as the maximum detonation pressure and temperature in the cylinder,and thus improving the effective thermal efficiency and reducing NOx emission.Therefore,in this paper,the combustion and emission characteristics of the low-speed Marine engine in the constant volume and pressure combustion process is studied by means of simulation.This paper developed a zero-dimensional model based on the SABAT cycle combining with the heat loss and mechanical loss formula,after calibration using experimental cylinder pressure curve and effective power,the influence law of constant volume combustion ratio on the cylinder pressure and thermal efficiency is investigated.So that the constant volume combustion ratio corresponding to the 340 diesel engine maximum limit pressure is determined to provide a reference for subsequent three-dimensional simulation parameter Settings.Secondly,the CFD model was established and calibrated,and then,according to the current researches that use three injectors to achieve the constant pressure constant volume combustion process,the cylinder head is redesigned in this paper.First of all,the exhaust duct of the original engine is simplified,and the influence of duct structure on the combustion process is simulated to guarantee the combustion consistency of the original engine model and the simplified model under the same conditions;After that,the cylinder head is designed as a four-gas valve with a central injector on the top.In order to verify the feasibility of the established three-dimensional model,the scavenging process of the four-air valve model is calibrated by the results of simplified exhaust duct model,and the combustion process is calibrated by the experimental cylinder pressure curve.Finally,according to the calculation results of the zero-dimensional model,the new CFD model is used to achieve the constant volume and pressure combustion process with different constant volume combustion ratios.Besides,the influence law of pressure ratio,injection pressure and nozzle diameter on the combustion and emission characteristics is studied.The results showed that,with the increase of pressure ratio,the flame distribution area and the high-temperature concentration area increased in the initial and middle period of combustion,the effective thermal efficiency and NOx emission also increased.With the increase of injection pressure,the flame distribution and the high temperature concentration area is more extensive,the thermal efficiency and NOx emission increased throughout the entire process.With the increase of nozzle diameter,the flame distribution in the cylinder and the thermal efficiency was basically unchanged,but the area of high temperature concentration decreased,NOx emission significantly decreased under high pressure ratio,and increased under low pressure ratio.The effective thermal efficiency reaches its maximum when the injection pressure is 120 MPa and the nozzle diameter is 0.8mm.Under the conditions of maximum pressure limitation and Tier II,the maximum effective thermal efficiency is 49.6% and 48.6% respectively,3.3% and 2.3% higher than the experimental effective thermal efficiency.