Simulation Study Of The Gas Exchange Process Of Two-stroke Hydraulic Free Piston Engine

The gas exchange quality of a two-stroke hydraulic free-piston engine is critical to the power and economy of the engine.Based on the prototype of the free piston engine of the research group,this thesis uses CONVERGE to simulate the influence of the structural parameters of the gas exchange system on the gas exchange quality.Based on the prototype,a hydraulically driven variable valve mechanism for the free piston engine is developed.Simulink simulation optimizes its structural parameters;finally,the hydraulic valve mechanism and the direct injection in the cylinder are combined to further improve the engine’s gas exchange effect and fuel economy.In this thesis,the three-dimensional simulation study of the free piston engine gas exchange process is carried out.The results show that the scavenging efficiency is mainly affected by the specific time-area value of the scavenging when the exhaust gas backflow phenomenon does not occur,and increases with the increase of the specific time-area value of scavenging.The capture ratio is mainly affected by the specific timearea value of exhaust,and decreases with the increase of the specific time-area value of exhaust.When the exhaust gas backflow occurs,the scavenging efficiency and the capture ratio decrease with the increase of the backflow intensity.Increasing the scavenging case compression ratio is beneficial to the scavenging efficiency,and the capture ratio is gradually reduced.The scavenging case compression ratio is increased from 1.2 to 2.1,and the scavenging efficiency is increased by 9.8%,but the scavenging case compression ratio is also increased.Increase the negative work done by the scavenging case on the piston;properly increase the piston angle,which is conducive to the improvement of scavenging efficiency and capture ratio;reducing the diameter of the piston rod is conducive to the improvement of scavenging efficiency,but at the same time,the capture ratio is reduced.In order to achieve a better gas exchange effect,and achieve the variable timing of the valve.A scheme of hydraulically driving variable valve is proposed,and a Simulink simulation model of hydraulic variable valve mechanism is established.The influence of structural parameters of hydraulic valve on valve lift is studied.Finally,the diameter of the plunger head is determined to be 10 mm,and the gap of the plunger coupler is 12 μm,the orifice diameter and the buffer distance are both 1mm,the seating speed at this time is 0.475 m/s,and the seating time is 5.26 ms.Based on the structure of the free piston engine,the uniflow scavenging cylinder direct injection system is designed.It is found through simulation that the scavenging efficiency of uniflow scavenging is more than 9% compared with cross-flow scavenging.The main factor affecting combustion efficiency is the mixing time of oil and gas.At a fuel injection time of 1 ms,the combustion efficiency was 97.4%.