Realization And Research Of Two-stage Assisted Combustion Ignition On Hydraulic Free Piston Engine

Homogeneous charge compression ignition(HCCI)characterized with homogeneity,lean,low temperature and compression ignition,which has shown the potential to deliver high thermal efficiency and low emissions,is a hot technology in the research area of internal combustion engine.However,HCCI combustion process control has always been an obstacle to the further practical application of HCCI.The difficulty of HCCI combustion process control is mainly reflected in ignition timing control,combustion heat release rate control and expansion of working conditions.In this study,a two-stage assisted compression ignition(TACI)combustion mode is realized by combining a spark plug and a glow plug.The mixture around the glow plug is heated by hot surface and it can be easily ignited by spark.Then the heat released from the combustion of the local mixture can ignite the cold mixture in the peripheral space.When the temperature and pressure of the unburned mixture reach the autoignition,the HCCI is induced.This TACI combustion mode can directly control the combustion phase through spark ignition,while using the two-stage heat release process to effectively control the heat release rate and expand the operating range under the premise of efficient combustion.In this study,TACI combustion mode was achieved on a rapid compression and expansion machine.The effects of control parameters(ignition phase,glow plug power)and operating parameters(in-cylinder initial temperature,compression ratio and equivalence ratio)on TACI combustion process were investigated.Results revel that TACI combustion mode of methanol exhibits typical spark ignition(SI)combustion characteristics at low compression ratio,and exhibits two-stage heat release characteristics of SI-HCCI at higher compression ratio.Compared with spark assisted compression ignition,TACI allows a wider working range and a lower compression ratio.Compared with glow-plug assisted compression ignition,TACI can directly and stably control the combustion phase by spark ignition and the electric power of the glow plug can be lower.Under the experimental conditions(?=0.7,CR=16)of this study,with the ignition phase advancing and the glow plug power increasing,the indicated thermal efficiency of TACI combustion mode increases,and further advancing the ignition phase or increasing the glow plug power will lead to the knocking,reducing the indicated thermal efficiency.The concentration range of the methanol mixture suitable for the TACI combustion mode is from 0.6 to 0.8.When the mixture is rich(?>0.8),the method of SI can be used.When the mixture is very lean(?<0.6),the GACI method can be adopted.In order to further reveal the mechanism of TACI combustion process,the threedimensional simulation model of TACI was constructed by CONVERGE software.The in-cylinder flame expansion processes of SI,TACI and GACI combustion mode were further compared.The simulation results show that with the thermal effect of the glow plug,the early flame expansion process accelerates,and then the heat released from flame propagation will promote the auto-ignition of surrounding mixture.This study proposes a new method to optimize the control of HCCI combustion process.It has certain theoretical and engineering application value in the development of clean and high efficiency hydraulic free piston engine.