Effect Of Combustion Chamber Shape On Pilot Diesel Auto-ignition And Flow Field In Cylinder

Energy crisis,environmental pollution and more strict emission regulations promote the research of natural gas engine.Diesel/natural gas dual fuel engine as an important form of natural gas engine has been concerned widely.Diesel/natural gas dual fuel engine are usually modified from diesel engine.However,there are many problems in diesel/natural gas dual fuel engine.This is due to the ? combustion chamber is not the most suitable for diesel/natural gas dual fuel engine.A large number of studies show that the combustion chamber shape has influence on the combustion process in the cylinder.Auto-ignition of pilot diesel as ignition source of dual fuel engine has an important influence on the subsequent combustion of natural gas.However the detailed study about the influence of combustion chamber shape of diesel/natural gas dual fuel engine on auto-ignition and combustion process of diesel is rare currently.In order to put forward some design views and explore the suitable combustion chamber shape for diesel/natural gas dual fuel engine,in this study,four groups of combustion chamber shapes were designed.Four combustion chambers were obtained by changing the combustion chamber throat diameter in each groups,therefore,there were 16 kinds of combustion chambers.Based on the simulation of the ? combustion chamber and 16 different combustion chamber shapes,the influence of combustion chamber shape on atomization and evaporation,auto-ignition and combustion process of pilot diesel,and ignition and combustion process of natural gas influenced by pilot diesel were studied.The influence of combustion chamber shape on the flow field in the cylinder was also studied.The results show that: in the ? combustion chamber,the evaporation rate of pilot diesel is fast in the early stage of diesel injection,diesel burns and releases heat quickly after auto-ignition,and the first heat release rate is high.At the same time,the distribution of turbulence is narrow and the turbulent kinetic energy is weak,and there is only weak turbulence distribution in the area of diesel injection and near the cylinder wall.In addition,a large amount of natural gas is left in the central region of cylinder and the bottom of the combustion chamber.The study of different combustion chamber shapes findings: with the decreasing of the throat diameter of the combustion chamber,the evaporation rate of diesel in the combustion chamber is increased,the ignition timing of diesel is delayed,and the combustion speed of diesel is slowed down;at the end of the numerical calculation,the percentage of residual natural gas decreases first and then increases slightly,the average pressure peak increases first and then decreases;When the throat diameter is small,the pressure peak in the cylinder is advanced.The diesel distribution and combustion are similar in the ? combustion chamber and the larger throat diameter combustion chambers in the six groups.Through the study of many combustion chambers,in order to promote the fuels burns completely,it is necessary to expand the distribution of diesel,reduce the combustion speed of diesel.In addition,the turbulence intensity and the turbulence distribution should be enhanced in order to promote the distribution of diesel widely and the combustion of natural gas rapidly.Among them,reducing the throat diameter of combustion chamber is good to form strong turbulence intensity and the turbulence distribution.The design of concave and convex on the side wall of the combustion chamber can promote the combustion of the natural gas near the side wall.However,while reducing the throat diameter of the combustion chamber,it is necessary to avoid the large distribution of diesel on the side wall of the combustion chamber.A small amount of diesel vapor attached to the side wall can reduce the combustion speed of diesel,but a large number of diesel vapor attached to the side wall is not conducive to the wide distribution of diesel.And when the throat diameter is small,two counter rotating vortices are formed on the side wall of combustion chamber.Therefore when diesel is sprayed into the regions of two counter rotating vortices,it is good to promote the wide distribution of diesel,and promote diesel into the bottom of the combustion chamber by matching the injection timing.However,the depth of combustion chamber should not be designed too deep,otherwise it is not conducive to the combustion of natural gas in the bottom of combustion chamber.The boss designed at the bottom of the combustion chamber can guide the flow in the cylinder to promote the ignition resource into the bottom of the combustion chamber,and reduce the residual natural gas in the central region of combustion chamber.