Numerical Simulation Research On Expansion Of Operation Range In Micro HCCI Free-piston Power Devices

Micro free-piston power devices have the advantages of simple structure,high energy conversion rate and high output power,so it stands out in many micro power devices.According to the characteristics of micro scale compression ignition,micro-scale HCCI combustion can alleviate the problems of insufficient combustion and prone to misfire and quenching.But even so,the development and application of micro free-piston power devices is still facing with various challenges.Firstly,under the high operating frequency,it has a short air exchange time,and the intake and exhaust are difficult to get enough,which leads to low combustion efficiency and unstable operation;secondly,the large surface-to-volume ratio of the micro combustor makes the wall heat loss serious,and the high heat loss will reduce the reaction temperature in the combustion chamber,which will reduce the ignition performance,resulting in high compression ratio and the rarefied combustion limit.However,high compression ratio and high temperature combustion under the rarefied combustion limit will inevitably put forward more stringent requirements for the strength performance and thermal stress of micro combustor wall materials.Therefore,from the perspective of expanding the operation range of micro free-piston power devices,this paper studied the expanding effect of various parameters on its compression ignition limit and rarefied combustion limit by numerical simulation,which provides some theoretical basis for the research and development of micro free-piston power devices.In order to improve the problem of insufficient air change of micro free-piston power devices at high frequency,two kinds of air change structures and methods of micro free-piston engines were designed in this paper.In the design,the movement of free piston is coupled with the air change process of micro engine,and the piston directly drives the intake and exhaust process in the movement process.The design has the advantages of simple structure,high feasibility,and can guarantee the filling of air inlet and the sufficiency of exhaust.According to the characteristics of single compression combustion process of micro free-piston power devices,and considering the influence of mixture leakage and wall heat transfer factors,a three-dimensional numerical model of micro HCCI free-piston powerdevice was established,and the accuracy of the model was verified.Based on the model,the combustion performance of methane and dimethyl ether(DME)in micro free-power devices was simulated.The results showed that the ignition limit of DME is wider than that of methane,so it is more suitable for micro free-piston power devices.Then,based on DME fuel,this paper explored the influence of mixture equivalence ratio and intake initial temperature on combustion performance and compression ignition limit of micro HCCI free-piston power devices.The results showed that reducing the equivalence ratio of the mixture in a certain range can achieve a good low-temperature combustion effect,and help to improve the combustion efficiency.In addition,it can also help to improve the ignition performance of the device,so as to broaden the ignition limit of the micro free-piston power devices.Increasing the initial temperature of the mixture can advance the ignition time of the free-piston power devices,reduce the critical ignition compression ratio,and thus broaden the compression ignition limit of the device.In addition,the effects of hydrogen addition to methane and preheating of intake charge on the expansion of the rarefied combustion limit in the device were studied.After the micro free-piston power devices adopts the method of methane mixed with hydrogen,the ignition delay period of the fuel mixture was shortened,the combustion rate was accelerated,and the ignition performance was improved,so the rarefied combustion limit was also expanded;Preheating the intake change of the micro free-piston power devices can broaden its flammable limit equivalence ratio,and with the increase of the preheating temperature,the rarefied combustion limit of the micro free piston power devices was expanded,the lower the fuel equivalent ratio.The purpose of this paper is to expand the compression ignition limit and the rarefied combustion limit of micro free-piston power devices,and to provide theoretical guidance for solving the problem of narrow operation range of micro free-piston power devices.