The Simulation Study And Optimal Research Of The Combustion Progress In An Spark Ignition Free Piston Engine

Spark ignition engines have the advantages of direct combustion phase control,no soot emission generation during combustion process,convenient disposal of harmful emissions,and low vibration noise.However,due to the resist of knocking phenomenon,the compression ratio and applicable bore of engines are difficult to improve,and improving the thermal efficiency of the SI engine is difficult in the internal combustion engine field.In this paper,three-dimensional simulation software is used to study the combustion process and knocking phenomenon in the spark ignition hydraulic free piston engine,willing to explore a new way to improve the compression ratio and the conversion efficiency of SI engines.A three-dimensional simulation model suited for combustion and knocking process of ignited free piston engine was established.The influence of different piston motion laws on the combustion process and the knocking process of the engine was studied.The results show that when the compression ratio is too high,Spontaneous combustion occurs in the mixed gas region near the cylinder wall,with the appearing of knock.Using the fast piston movement,the knocking tendency and intensity of the ignited hydraulic free piston engine can be reduced,but the maximum indicated thermal efficiency of the engine would reduce.The influence of multiple spark plug ignition on the combustion process of the engine is simulated.The results show that multi-spark plug ignition can accelerate the heat release rate of the fuel,increasing the combustion capacity,and reduce the heat loss of the engine,finally improved the thermal efficiency of the engine.The single spark plug ignition has the same effect as the multipoint spark plug when the knocking tendency is measured by the CA50 of knocking ignition phase.The effect of different combustion chamber structures on the combustion process of the engine was studied.The results show that the peak value of heat release rate increases with the increase of the first contact time of the flame.With the new chamber,the surface-to-volume ratio became smaller,and high temperature zone contact with the piston wall prematurely is avoid r.Both of them conduct to reducing the heat loss during combustion.Finally,the optimized hemispherical combustion chamber enhanced the thermal efficiency of the engine.Based on the research results of this paper,a new way to improve the conversion efficiency of SI free piston engine is proposed: The fast piston motion law is adopted to increase the limit compression ratio,the multi-spark plug ignition is used to increase the heat release rate,and the hemispherical combustion chamber is used to shorten combustion duration and reduce heat loss.