Simulation Research On Combustion Process Of Highly Intensified And Low Heat Dissipation Diesel Engine

Under the background of "carbon peak" and "carbon neutrality",how to continuously reduce the carbon emissions of power system is an important issue that needs to be solved urgently.This puts forward higher requirements for the improvement of engine thermal efficiency.Improving system energy utilization by reducing engine cooling loss is an important way to improve thermal efficiency.The cooling loss of diesel engine can be effectively reduced by adding thermal insulation coating in combustion chamber space.This is the low heat dissipation diesel engine.However,the thermal insulation coating will inevitably cause the wall temperature of the combustion chamber to rise.At the same time,the increase in ambient temperature in the cylinder causes intake efficiency to decrease,resulting in a low air-fuel ratio.The highly intensified diesel engine has the characteristics of large fuel injection and high heat load.This aggravates the high wall temperature combustion caused by heat insulation,which leads to the deterioration of combustion condition and is not conducive to the improvement of thermal efficiency.It has important practical application value to reveal basic problems of high wall temperature combustion through research and analysis,master technical methods of improving high wall temperature combustion,and optimize combustion process of low heat dissipation and high power density diesel engine.In this paper,the combustion deterioration problem of high power density and low heat dissipation diesel engine was studied by numerical simulation methods.Through modeling and verification of 3D CFD model,the calculation model of high wall temperature combustion based on constant volume combustion bomb was built,and the influence of the high wall temperature environment on the combustion process was simulated and analyzed.The phenomenon of early combustion caused by high wall temperature combustion was revealed,and the improvement effects of increasing fuel injection pressure and changing spray impingement distance on high wall temperature combustion were studied.Aiming at the target diesel engine with high power density and low heat dissipation,the technical methods of increasing injection pressure and optimizing combustion chamber structure to achieve efficient and fast combustion were obtained through research.The influence of high wall temperature environment on combustion process and combustion improvement measures were studied by numerical simulation.Firstly,the three-dimensional modeling of constant volume combustion bomb was carried out.The model was calibrated according to the experiment of fuel spray impingement combustion at high wall temperature.The influence of high wall temperature on spray combustion process was studied and wall temperature was set at 600℃,700℃,800℃ and 900℃ respectively.Secondly,the influence of high injection pressure(180MPa～250MPa)on spray combustion at high wall temperature was investigated.Finally,the influence of spray impingement distance(25mm～45mm)on high wall temperature combustion was studied.The calculation results showed that high wall temperature environment under the target operating conditions shortened ignition delay period of fuel spray,that is,early combustion,increased combustion duration,and cause serious after-combustion.By increasing fuel injection pressure to 250 MPa,the combustion duration at high wall temperature can be shortened by 7.1%.By increasing spray impingement distance to 45 mm,the combustion duration at high wall temperature was reduced by 16.8%.Therefore,the method of increasing fuel injection pressure and spray impingement distance can accelerate spray combustion speed at high wall temperature to a certain extent.Based on the basic simulation study of constant volume combustion bomb,the parametric modeling method was used to optimize combustion chamber of high power density diesel engine with CAESES coupled with CONVERGE as an automatic optimization platform.The original ω-type combustion chamber,the optimized I-type combustion chamber and a double-layer diverging combustion chamber(II-type combustion chamber)were compared and analyzed.The results showed that the combustion effect of type I combustion chamber was better and can improve the growth of combustion duration in high wall temperature environment.