Experimental Research And Simulation Of Opposed-piston Hydraulic Free Piston Engine

Hydraulic free piston engine(HFPE),a new form of power,has many advantages,but in terms of reliability,stability and control,some problems need to be solved.This paper is based on the study of the first generation of(HFPF)in the earlier stage.HCCI combustion experiments on the first generation of HFPE are made.In light of the problems that high Solenoid valves are required and the prevailing problems of stability of the bottom dead center position existing on free-piston engine,the solutions that using small volume of the compression piston accumulator and energy recovery piston are proposed.Moreover,MATLAB / Simulink software is used to build widely-used hydraulic free piston engine model.With this model,the key structure of the second generation HFPE can be optimized and its property can be compared with that of the first-generation HFPE and the conventional hydraulic free piston engines.HCCI combustion experiments on the first generation of HFPE are made,which mainly focus on the features and influencing factors of HCCI heat release rate and indicated thermal efficiency.The solutions that using small volume of the compression piston accumulator and energy recovery piston are proposed.Besides,MATLAB / Simulink software is used to build widely-used HFPE simulation model and the structure of the second generation of HFPE is designed.Structure parameter optimization to the compression accumulator of the second-generation HFPE shows that using a smaller volume of compression accumulator can increase the operating frequency of the engine and reduce engine compression ratio and sensitivity of bottom dead center position to the working oil pressure,allowing the engine to have a greater equivalence ratio change interval.When plunger diameter increases,single cycle time of the engine is shortened,the maximum operating frequency of the engine increases,and the concentration and range of the allowable burning mixture by the engine increase.Increasing the area of energy recovery piston can reduce the fluctuation range influenced by the bottom dead center position of the piston under the impact of equivalent ratio and increase the allowable mixture concentration of the engine,reduce engine operating cycles and increase the maximum operating frequency.At the ending stage of the expansion stroke,when the plunger piston and energy recovery piston impact,the oil valve begins to flow,and the maximum flow increases with the increasing of energy recovery piston area.The conclusion drawn from the comparative study with the second generation of HFPE shows that when the working oil pressure is 25.2 MPa,the area of plunger piston is 314.16 mm2 and the cross-sectional area of energy recovery piston becomes tripled,the single-cycle time of the second generation HFPE increases 0.28 ms over the original prototype and decreases 6.28 ms over the Two-cavity-type HFPE.Its maximum working frequency is same with that of the first generation HFPE and 8 Hz bigger than the two-cavity-type HFPE.The second-generation HFPE has a good compression ratio consistency.When its initial position of compression stroke down until departs from the positive position of the bottom dead center position,the compression ratio will become bigger,which is benefit for the engine to restore working stability.When the initial position of the compression is between-5 mm and 5 mm,its bottom dead center position is between-0.19 mm and 0.67 mm and it has a good compression ratio consistency.When it is in a stable-working condition,after its fuel equivalence ratio and working oil pressure changing,the engine will make adjustments to its compression ratio and the bottom dead center position automatically in the following cycles.In the next few cycles,the compression ratio and the bottom dead center position fluctuate in a convergence trend which eventually make the engine reach a steady operating state.