Experimental Study And Performance Analysis Of Single-piston Free Piston Expander-linear Generator

Organic Rankine cycle（ORC）system has been highly concerned by domestic and foreign scholars in the field of vehicle engines waste heat recovery due to its advantages.Researchers have carried out a lot of research work on different types of expanders in ORC systems of vehicle engines.However,there are no commercial or large-scale production expanders as the best choice.For this reason,our group has developed a single-piston free piston expander-linear generator（FPE-LG）prototype,which is used in ORC system to realize thermoelectric conversion.Based on the single-piston FPE-LG test rig and Lab VIEW software,two kinds of piston motion control methods are developed:time control method（TCM）and position control method（PCM）,which can realize the stable operation of the prototype.A large number of experiments were carried out with high-pressure air as working fluid.The influences of intake pressure,intake duration,load resistance,expansion duration and theoretical stroke on the operating performance of single-piston FPE-LG are discussed.The results show that the peak cylinder pressure of chamber B（with piston rod side）is usually higher than that in chamber A.The curves of displacement and velocity of the piston mover assembly（PMA）and power output with time are approximately sine wave,and the velocity of the PMA varies with displacement is similar to an ellipse.The piston is run at relatively high velocity near the operation top dead center（OTDC）or operation bottom dead center（OBDC）,and peak acceleration is achieved when the piston arrived at OTDC or OBDC.The voltage output is directly related to the velocity of the PMA,and the trend of their changes with time is similar and has a good consistency.With the increase of intake pressure or intake duration,the displacement and velocity of the PMA,peak cylinder pressure,peak voltage,current and power output increase gradually.As the load resistance increases,the displacement and velocity of the PMA,peak cylinder pressure and peak voltage output increase gradually,while the peak current output decreases.The peak power output increases first and then decreases with the increase in load resistance.As the expansion duration or theoretical stroke increases,the displacement of the PMA increases gradually,while the peak cylinder pressure shows a slight downward trend,and the peak velocity and peak power output change slightly.The comparative analysis shows that the symmetrical motion characteristics of the PMA are better than that of the TCM when the PCM is used.However,the symmetrical motion characteristics can be effectively improved by increasing the intake duration t _Btor expansion duration,thereby improving the stroke utilization rate.When the PCM is used,the operating frequency increases gradually as the intake pressure,load resistance,and intake duration increase,and increases significantly with the decrease in theoretical stroke.When the TCM is used,the operating frequency is only related to the intake and expansion durations.The orthogonal experiment design is adopted to evaluate the influence degree and significance level of different operating parameters on the performance indices.The results show that the influences of intake pressure and intake duration on the actual stroke length,velocity and power output are the largest and significant,followed by load resistance.The expansion duration and theoretical stroke have similar effects on the peak velocity and power output,which are the smallest.The theoretical stroke is the primary factor affecting the operating frequency,followed by intake pressure,intake duration and load resistance.Combined with the uncertainty analysis,it can be seen that the experimental error and uncertainty are small and can be ignored.Based on the experimental data,the energy conversion efficiencies of the prototype are analyzed.The results show that the single-piston FPE-LG has a thermal-work conversion efficiency of 1.7%to 5.4%,a work-electric conversion efficiency of 22.9%to 28.8%.The thermal-electric conversion efficiency（total efficiency）is relatively low,and its maximum value is only 1.4%.Based on the experimental data,a back propagation（BP）neural network model of single-piston FPE-LG is established.The effects of transfer function,training function,neural network layers,hidden layer neuron number and learning rate on the prediction accuracy are discussed.The results show that the BP neural network model has high prediction accuracy and can be used to predict and optimize the operating performance of single-piston FPE-LG.The optimization results of BP neural network model and genetic algorithm show that the maximum power output can reach 141.69 W under the TCM and 138.03 W under the PCM,respectively.