Study On Broadening The Combustion Boundaries Of Micro HDDI Free-piston Engine

As variety of miniature devices appeared in some fields,the micro-power system based on hydrocarbon fuel has gradually become a key research point for scientific research institutions and scholars.The micro HCCI(Homogeneous Charge Compression Ignition)free-piston engine has the following advantages: light weight,small size,high energy density fuel,and uninterrupted energy supply for a long time.However,due to the small size of the device,the ratio of the surface area to the volume of the micro-power device increases sharply,the heat loss is severe.It's difficult to obtain sustained and stable combustion under a relatively wide range of conditions.In view of the problem of combustion instability in the micro free-piston power plant,the visualization test and numerical calculation are combined.In order to expand the stable combustion boundaries of HCCI free-piston engines,the combustion properties of methane with preheating,hydrogen addition,hightemperature EGR and catalysis were investigated.The visualized experimental platform of the micro free-piston power plant with hot-water preheating system was established.A high-speed digital camera was used to take dynamic images of the HCCI combustion process in the micro-chamber.High-frequency pressure sensors and temperature sensors were used to capture the pressure and temperature trends of the combustion.Based on the experimental platform,the images of combustion process under three typical operating conditions were obtained.In addition,it has been confirmed that the preheating of the combustion process can effectively expand the stable combustion boundaries of the micro-chamber.However,the power performance of the micro power plant will be reduced if the preheating temperature is too high.At the same time,the chemical reaction mechanism of the space gas phase of methane combustion was confirmed,a three-dimensional modes was established for the free-piston power plant,the combustion process with preheating,hydrogen addition,high-temperature EGR and catalysis in the micro combustor was numerically simulated using Fluent software based on experiments.The effects of preheating,hydrogen addition,high-temperature EGR and catalysis on the ignition limit,ignition time,ignition process and critical burning energy of the critical combustion were studied,and the combustion properties of fuel mixtures were analyzed.The research results show that,the ignition limit of compression ignition gas can be widened by above four measures,the reliability of ignition and combustion is improved and the ignition time is advanced.But these measures will decrease the work capacity of the micro free-piston plant.Therefore,the specific situation should be specifically analyzed,the preheating temperature,hydrogen addition ratio,EGR rate,catalysis type and coverage area must be appropriately chosen.Both widen stable combustion boundary and high dynamic output performance should be assured.The research results provide a theoretical basis for improvement of the combustion property of HCCI free-piston power plant,and establish foundation for the development of the prototype of the micro free-piston engine.