Free Piston Expander-linear Generator Research Articles

Performance Prediction and Optimization of Single-Piston Free Piston Expander-Linear Generator Based on Machine Learning and Genetic Algorithm

This paper proposed a single-piston free piston expander-linear generator (SFPE-LG) prototype applied to organic Rankine cycle systems. Two valve timing control strategies, namely, time control strategy (TCS) and position control strategy (PCS), were developed. Based on the experimental data, a back propagation neural network (BPNN) prediction model was established. The effects of structural parameters such as neural network layers, transfer function, training function, hidden layer nodes, and learning rate on the prediction accuracy of this BPNN model were discussed. The training and prediction accuracy of the BPNN model was verified using 5-fold cross-validation and Wilcoxon signed-rank test. Moreover, the BPNN model was integrated with a genetic algorithm to predict and optimize the maximum output power of the SFPE-LG. The results showed that the BPNN model used to predict the motion characteristics and output performance of the SFPE-LG exhibits strong learning ability and high prediction accuracy. Notably, the prediction accuracy of the BPNN model is significantly higher under the PCS compared to TCS. The effect of hidden layer nodes on mean square error (MSE) and correlation coefficient (R) is greater than that of the learning rate. When the number of hidden layer nodes exceeds 30, the BPNN model consistently achieves low MSE and high R. The optimization results showed that the SFPE-LG can obtain a maximum output power of 141.69 W under the TCS, when the working parameters are inlet pressure of 0.7 MPa, intake duration of 35 ms, load resistance of 67 Ω, and expansion duration of 104 ms, respectively.

Research on Gas Recycling of Free-Piston Expander–Linear Generator for Organic Rankine Cycle of Vehicle

A (FPE-LG) is a new type of vehicle waste heat recovery device based on an organic Rankine cycle. It is expected to achieve the reuse of vehicle internal combustion engine waste heat and improve the comprehensive utilization rate of energy. To enable the FPE-LG to recover exhaust gas to a greater extent in practical applications, based on the FPE-LG coupling gas storage tank test platform, the gas is discharged from the expander cylinder. This paper analyzes the influence of differences to the tank volume, intake pressure, intake duration time, expansion duration time and exhaust duration time on piston motion characteristics and gas storage and release time during the cycle, and verifies the feasibility of gas working as a recycling medium. The results showed that the energy storage of lithium batteries increases with the increase of intake pressure, and the energy stored in lithium batteries during gas release is higher than that during gas storage; the intake duration time, expansion duration time and exhaust duration time have little effect on the storage of lithium battery energy during the cycle. When the intake pressure is 0.5 MPa, the volume of the gas tank is 30 L, and the intake duration time, expansion duration time and exhaust duration time are 50 ms, 80 ms and 30 ms, respectively, the maximum actual stroke of the piston can reach 89.592 mm.

Experimental study on charging performance of free piston expander-linear generator coupled lithium battery system

Free piston expander-linear generator (FPE-LG), as a new linear waste heat recovery device, has been investigated by many research groups. Particularly when the FPE-LG coupled with the lithium batteries, it demonstrates many potential advantages for vehicle power range extenders. Thus, this paper focuses on the output characteristics and the charging performances of FPE-LG couples tieh lithium batteries based on the experimental results. It is found that when the intake pressure increased, the output voltage and output power also increased. It increase firstly occurred as the intake duration time increased, and then it tended to stabilize.The maximum output power could reach 34.07 W when the intake pressure was 2.5 bar and the operating frequency was 5 Hz. The charging rate of the lithium battery increased firstly as the intake pressure and intake duration time increased, and then it tended to stabilize. And the lithium battery required charging duration time reduced from 6.94 h to 3.75 h when the intake pressure increased from 2 bar to 5 bar, and the required duration time for a full charge reduced from 4.65 h to 3.49 h when the intake duration time increased from 50 ms to 80 ms. Therefore, the reasonable operation parameters of FPE-LG can keep high efficiency when it couples with lithium batteries in future.

Performance Investigation of Single–Piston Free Piston Expander–Linear Generator with Multi–Parameter Based on Simulation Model

The structural design and operating strategy of a free piston expander–linear generator (FPE–LG) has a major impact on performance. In this paper, the simulation model of single–piston FPE–LG was built and verified by combining the structural parameters of the existing test rig with a set of kinetic and thermodynamic equations. On this basis, the influence of the design and operating parameters of the device on the performance was studied, while keeping other parameters fixed. Then, a sensitivity analysis of power output and operating frequency was carried out. The results show that within a certain range of external load and intake beginning position, increasing the diameter of the intake and exhaust pipes, or reducing the piston rod diameter can improve the power output. Within a certain range of frictional coefficient and intake time, increasing the cylinder diameter and intake pressure, or reducing the piston assembly mass and back electromotive force (EMF) constant can increase the operating frequency. Both the power output and the operating frequency are most sensitive to the cylinder diameter among the design parameters. Among the operating parameters, power output is the most sensitive to intake pressure, and operating frequency is the most sensitive to intake beginning position. The optimization of structural design and operation strategy in expander provides important guiding significance for ORC waste heat recovery system.

Force and energy analysis of single-piston free-piston expander—linear generator

The movement of piston in free-piston expander-linear generator (FPE-LG) mainly depends on the dynamic force balance among four kinds of forces (air impetus, air resistance, electromagnetic resistance and frictional resistance). In this paper, the Newton's Second Law and the first law of thermodynamics are used to analyze the single-piston FPE-LG, the change of four kinds of forces and work produced by the four kinds of forces are analyzed. Nominal work-electricity conversion loss coefficient is proposed for the firs time to represent the energy loss. The experiment results show that the work produced by four forces from top dead center (TDC) to bottom dead center (BDC) are higher than those from BDC to TDC. The minimum proportion of air resistance energy consumption is also more than 50%, the proportion of electromagnetic resistance energy consumption is less than 20%, and the proportion of frictional resistance energy consumption is more than 20%. Therefore, the total nominal work-electricity conversion loss can be effectively reduced by increasing intake time or intake pressure.

Experimental research and artificial neural network prediction of free piston expander-linear generator

For the purpose of better matching the performance of the organic Rankine cycle (ORC) system concerning the vehicle engine waste heat recovery, this paper studies the output performance of free piston expander-linear generator (FPE-LG). A test bench of FPE-LG is established for small scale ORC system, and timing and displacement control strategy is proposed. Furthermore, the impact of the intake pressure and the torque on motion characteristics and output performance of FPE-LG are analyzed. According to evaluating different learning rates, number of hidden artificial neural networks and training functions, a prediction model of FPE-LG based on artificial neural network is established. Genetic algorithm is used to optimize the key operating parameters, to maximize the power output of FPE-LG. In consideration of the mean square error and determination coefficient, the artificial neural network model is verified and tested by experimental data. Finally, combining genetic algorithm with artificial neural network model, the maximum power output of FPE-LG is optimized and its performance is predicted. The results show that the maximum value of electric current, voltage and power output are 2.8 A, 14.75 V and 28.5 W, respectively. Based on artificial neural network, this method can provide useful guidance for performance prediction and coordinated optimization, with advantages of minimum deviation and high precision.

Experimental research and performance analysis of a free piston expander–linear generator coupled with a driving motor

This paper establishes a test bench for a free piston expander–linear generator (FPE–LG) system coupled with the drive motor (DM) to promote the application of FPE–LG on vehicle. Firstly, it discusses the operating characteristics and output performance of the FPE–LG under the simulated operation conditions of vehicle. Meanwhile, it investigates the operating characteristics of FPE–LG under different torques and expansion times. Finally, this paper analyzes the influence of different torques and expansion times on the operating frequency of FPE–LG. The experimental results show that the operating frequency of FPE–LG is related to the expansion time and torque. The power output of the DM decreases as the torque increases, the power output of the DM increases first and then decreases as the expansion time increases. Therefore, the selection of suitable expansion time under different intake pressure plays an important role in improving the power output of the DM. The maximum of the peak power output of DM is about 25.8 W, when the intake pressure is 4 bar, the expansion time is 30 ms and the torque is 0.00571 N m.

Study on the Control Strategy of Free Piston Expander-Linear Generator Used for Organic Rankine Cycle Waste Heat Recovery

This study presents a free piston expander-linear generator (FPE-LG) for engine waste heat recovery. The control strategies based on displacement (control strategy A) and valve timings (control strategy B) are proposed. The operation and output characteristics with different control strategies are conducted. The results show that control strategy has significant effect on the FPE-LG performance. When FPE-LG operates with control strategy A, the operation frequency is determined by the intake pressure, advanced exhaust duration, fixed position point and external load resistance, whereas it is only related with the intake/exhaust time and advanced exhaust duration (ta) for control strategy B. The external load resistance has slight influence on the stroke length for these two control strategies. For control strategy A, when advanced exhaust duration is more than 40 ms, the stroke length rises with the intake pressure, whereas it first rises and then reduces when advanced exhaust duration is less than 40 ms. There exist the optimal external load resistance and advanced exhaust duration to achieve the maximum power output with control strategy A, whereas the average power output reduces with the increasing advanced exhaust duration when using control strategy B. Stable operation and high power output can be obtained by using different control strategies and adjusting relevant operation parameters.

Experimental study on the performance of single-piston free-piston expander—linear generator

In this study, a single-piston free-piston expander—linear generator test system for the power-electricity conversion is presented using compressed air as the working fluid. This study investigated the effect of intake time and intake pressure on the performance of the test system. Examples include the following: in-cylinder pressure, bias displacement, one-way time, indicated work, output electric energy, and work-electricity conversion efficiency in a single cycle using the position-time control method with the intake position of 21and 45 mm. Result shows that: in the operational process of the test system, all values obtained when the piston runs from top dead center to bottom dead center are all different compared with those obtained when the piston runs from bottom dead center to top dead center. When the test system operates with low intake pressure and short intake time, the working chamber without a piston rod will over-expand. With the increase of intake time or intake pressure, the bias displacement, indicated work, and one-way output electric energy show an increasing trend. Moreover, the one-way time shows a decreasing trend and eventually tends to a fixed value, and the one-way work-electricity conversion efficiency initially increases and then decreases.

Analysis on operation characteristic and output performance of free piston expander-linear generator based on displacement control strategy

In this paper, a free piston expander-linear generator (FPE-LG) test rig is established and the control strategy based on displacement is applied. The operation characteristic, output performance, and energy conversion efficiency of the FPE-LG are analyzed. The coefficient of equilibrium oscillation (C) is identified to evaluate the stability of the free piston assembly (FPA). Experimental results show that the FPA has best stability when fixed position point (FPP) is between 20 - 75 mm. The actual stroke and operation frequency increase with the intake pressure and load resistance. The variations of voltage and power output are in accordance with velocity. The average power output increases with intake pressure and FPP, but reduces with the advanced exhaust duration. The maximum average power output can be achieved when the load resistance is 20 Ω. The energy conversion efficiency first increases with intake pressure and then remains constant. The energy conversion efficiency reaches up to 17.5 % when intake pressure is 0.9 MPa, FPP is 40 - 55 mm, advanced exhaust duration is 0 ms, and load resistance is 20 Ω. The lower the coefficient of equilibrium oscillation is, the higher the average power output is.

Comparative analysis of different valve timing control methods for single-piston free piston expander-linear generator via an orthogonal experimental design

This paper presents a single-piston free piston expander-linear generator for small-scale organic Rankine cycle. In order to achieve stable operation, two types of valve timing control methods are developed: time control method (TCM) and position control method (PCM). The orthogonal experimental design is adopted to evaluate the influence degree and significance level of different factors on the prototype performance. Results show that the symmetry motion characteristics are worse when the TCM is used than those of the PCM. Furthermore, such characteristics could be improved by appropriately increasing expansion duration and intake duration tBT. For the PCM, operating frequency gradually increases as external resistance, intake pressure, and intake duration but significantly decreases as theoretical stroke. For the TCM, operating frequency is only related with intake and expansion durations. Intake pressure has the greatest influence on the peak power output, peak velocity, and actual stroke length, followed by intake duration. Theoretical stroke is the dominant factor affecting operating frequency, followed by intake pressure, and then by intake duration and external resistance. Thermal-work, work-electric, and thermal-electric conversion efficiencies can achieve a maximum of 5.4%, 28.8%, 1.4%, respectively.

Modelling and validation of a free piston expander-linear generator for waste heat recovery system

In this study, an air driven free piston expander-linear generator (FPE-LG) test rig and the FPE-LG simulation model based on GT-Suite/Simulink have been established. The simulation model is validated by experimental results. Afterwards, the performance of FPE-LG under different conditions and the influence of intake temperature on FPE-LG are conducted. Since the simulation results are in good agreement with the experiment results, this proves the rationality of the simulation model. When intake pressure and temperature rise, the in-cylinder volume increases. Furthermore, the displacement, velocity and acceleration increase with intake temperature. The peak voltage output of FPE-LG linearly increases when increasing the intake temperature and pressure. The peak voltage output is more sensitive to intake pressure than temperature. The higher the intake pressure is, the more obvious the intake temperature effects the peak voltage output. The maximum voltage output can reach up to 23.6 V. The peak power output demonstrates an initial increasing and then reducing trend with the external load resistance. And the maximum peak power output can reach up to 22.23 W when the external load resistance is 9 Ω and intake temperature is 373 K.

Experimental study and artificial neural network based prediction of a free piston expander-linear generator for small scale organic Rankine cycle

In this paper, a novel free piston expander-linear generator (FPE-LG) prototype has been developed for small scale organic Rankine cycle system. The effects of three key operating parameters, including intake pressure, operation frequency and external load resistance on piston dynamics, output characteristics of the linear generator, and system energy conversion efficiency are investigated. An artificial neural network (ANN) based prediction model is established after evaluating different learning rates, hidden layer neural numbers and train functions. The ANN model is also validated and tested using the experimental data with consideration of mean squared error and correlation coefficient. Finally, combined the genetic algorithm with the ANN model, a parametric optimization and performance prediction for maximum power output of the linear generator is conducted. The results show that the free piston assembly operates stably with good consistency. Higher intake pressure and external load resistance are beneficial for improving the piston dynamics and output characteristics of the linear generator while the optimal operation frequency corresponding to the maximum peak power output is more dependent on the coordinated variation of the operating parameters. The maximum system energy conversion efficiency can reach up to 28.81% with the intake pressure of 0.2 MPa, operation frequency of 1.5 Hz and external load resistance of 5 Ω. The proposed ANN model shows a strong learning ability and generalization performance. The correlation coefficients between the ANN predictions and experimental data obtained from the validation and test processes are all close to 1. The optimized peak power output can reach up to 100.47 W based on the proposed ANN model. The ANN based method can provide a useful guidance for the performance prediction and coordinated optimization with the least deviation and high accuracy.

Performance analysis of a single-piston free piston expander-linear generator with intake timing control strategy based on piston displacement

A single-piston free piston expander-linear generator (FPE-LG) prototype for a small-scale Organic Rankine Cycle system is presented. The steady-state operation of the prototype is successfully achieved, which indicates the feasibility of single-piston FPE-LG with intake timing control strategy based on piston displacement. The effects of intake duration, intake pressure, theoretical stroke, and external load resistance (R) on operational performance and energy conversion efficiency are investigated. Results show that the piston is running at a relatively high velocity when it approaches the predetermined special displacement point (X1 or X2). The peak voltage, current, and power output increase linearly as the intake pressure and duration increase. The theoretical stroke exerts a minimal effect on peak velocity and peak voltage output but considerably affects the operating frequency. The peak power output and work-electric conversion efficiency (ηw-e) increase first and then decrease with increasing external load resistance. The peak output power and ηw-e reach the maximum value of 32.5 W and 30.8%, respectively, with an optimal R = 80 Ω. As the intake duration increases, the indicated efficiency increases gradually, whereas ηw-e shows an increasing and then decreasing trend. The optimal intake duration for reaching the maximum ηw-e is variable under different operation conditions.

A comparison study and performance analysis of free piston expander-linear generator for organic Rankine cycle system

In this paper, a free piston expander-linear generator (FPE-LG) experiment rig equipped with different linear generator (LG) is established. A comparison study on the performance of FPE-LG with different LGs is conducted. Then the operation characteristics and output characteristics of FPE-LG are investigated. The results show that the selection and design of LG has significant influence on FPE-LG performance. The displacement and velocity with LG-1 are higher than that LG-2’s, but the operation stability and output performance with LG-2’s are better. Considering the high pressure and temperature operation conditions in ORC system, LG-2 is suitable for this experiment rig. Both actual stroke and velocity increase with intake pressure and external load resistance. The variations of actual stroke, velocity, indicated efficiency, voltage and power output are more sensitive to intake pressure than external load resistance. The maximum power output can reach up to 111 W. The voltage output is in agreement with that of velocity. The peak voltage output is usually obtained when the velocity reaches peak value. The energy conversion efficiency shows first increases and then decreases with external load resistance and intake pressure. The maximum energy conversion efficiency of 74% can be obtained.

Performance investigation of a free piston expander-linear generator for small scale organic Rankine cycle

This paper presents a free piston expander-linear generator (FPE-LG) for small scale organic Rankine cycle (ORC). Based on FPE-LG experiment configuration, performance investigation and efficiency analysis on FPE-LG are conducted. Experiment results show that intake pressure and external load resistance have great influence on actual stroke, velocity and exhaust temperature. The exhaust pressure is hardly relevant to intake pressure and operation frequency. When FPE-LG operates with low operation frequency, the entropy generation decreases with intake pressure, whereas it increases with intake pressure when the operation frequency is high. The indicated work increases when increasing the intake pressure and external load resistance, whereas it decreases when increasing operation frequency. The indicated efficiency decreases with the intake pressure and external load resistance, and the maximum indicated efficiency can reach up to 81%. The voltage and power output increase when the intake pressure and external load resistance increase, and the maximum voltage and power output can reach 44.4 V and 111.6 W, respectively. High energy conversion efficiency can be achieved in the area of high external load resistance and intake pressure. Generally, the higher power output is, the higher energy conversion efficiency is. The maximum energy conversion efficiency of 73.4% is achieved.

Development and validation of a single-piston free piston expander-linear generator for a small-scale organic Rankine cycle

To improve the energy efficiency of internal combustion engines, this study develops a single-piston free piston expander-linear generator (FPE-LG) for a small scale organic Rankine cycle (ORC) in a waste heat recovery system. The prototype can operate continuously and stably, thereby indicating that the single-piston FPE-LG has a feasible working principle. The operation characteristics of FPE-LG are revealed, and the effects of the intake valve opening time (IVOT) and the exhaust valve opening time (EVOT) on the output performance are investigated. The experimental results reveal that the piston can run at a relatively high velocity near the dead centers, and peak acceleration is typically achieved when the piston arrives at the operation left/right dead centers (OLDC/ORDC). The piston stroke length is decreased significantly as the operating frequency is increased. The peak power output, peak in-cylinder pressure, peak velocity of the free piston, and utilization ratio of the piston stroke (η) are increased as the external load resistance is increased. In addition, η increases with longer IVOT and can reach up to 67.4% when the IVOT is 40 ms. The exhaust pressure decreases as the EVOT is increased, and the power output can be improved by regulating the IVOT rather than the EVOT.

Experimental and simulation study of a free piston expander–linear generator for small-scale organic Rankine cycle

This paper presents a new free piston expander–linear generator (FPE–LG) for small-scale organic Rankine cycle (ORC) waste heat recovery systems. An FPE–LG experimental test rig is developed, and a simulation model is established with Matlab/Simulink. Motion characteristics and output performance are discussed based on experimental results, and the simulation model is validated. Further investigation of the motion characteristics and output performance of FPE–LG is conducted at a relatively high intake pressure and operating frequency. Results show that intake pressure and operating frequency significantly affect motion characteristics and output performance. The velocity and displacement of the free piston assembly increase with increasing intake pressure, and the peak power output initially increases then decreases with increasing operating frequency. The peak power output is 676 W when the intake pressure, external load resistance, and operating frequency are 10 bar, 9 Ω, and 15 Hz, respectively. Using the simulation model to study FPE–LG operating parameters is convenient, and the model can serve as a reference for the future design and optimization of FPE–LG.

Experimental investigation of a free piston expander-linear generator with different valve timings

In this paper, a new free piston expander-linear generator (FPE-LG) prototype for small-scale organic Rankine cycle (ORC) waste heat recovery system has been developed. Firstly, the in-cylinder pressure, motion characteristics and output performance of the FPE-LG with different valve timings are investigated. Then, the effect of external load resistance on motion characteristics and output performance with cam plate A is analyzed. Finally, the effect of external load resistance on the indicated efficiency with cam plate A is studied. The results show that the intake duration and exhaust duration have significant influence on the motion characteristics and output performance of FPE-LG. Compared with other cam plates, cam plate A (the intake exhaust duration is 60°, and the exhaust duration is 180°) can be considered as the best choice for FPE-LG. The external load resistance not only influences the motion characteristics but also effects the output performance. The displacement and velocity increase with increasing the external load resistance. The peak power output with cam plate A is up to 18.6 W. The indicated efficiency shows a downward tendency when the external load resistance increases, and the maximum indicated efficiency of 66.2% is achieved.

External load resistance effect on the free piston expander-linear generator for organic Rankine cycle waste heat recovery system

For this study, a test rig of free piston expander-linear generator (FPE-LG) for organic Rankine cycle (ORC) waste heat recovery system is built. Based on the free piston expander-linear generator test rig, an experiment of external load resistance effect on the motion characteristics, indicated efficiency, output performance and energy conversion efficiency of the free piston expander-linear generator is conducted. The displacement and velocity of the free piston assembly increase with the external load resistance. The indicated efficiency of the free piston expander (left cylinder) decreases with the external load resistance. As the external load resistance and intake pressure increase, the peak voltage output of the free piston expander-linear generator increases. The maximum peak voltage output is 44.4 V when the intake pressure is 3.0 bar, the operating frequency is 1.5 Hz and the external load resistance is 40 Ω. The peak power output of the free piston expander-linear generator initially shows a non-linear relation increase with external load resistance and tends to stabilize in the end. When the intake pressure is 3.0 bar, the operating frequency is 1.5 Hz and the external load resistance is 30 Ω, the maximum peak power output of 110 W can be achieved. The energy conversion efficiency of the free piston expander-linear generator increases gradually with growing of the intake pressure and external load resistance until the energy conversion efficiency reaches the maximal value, and then the energy conversion efficiency decreases with the intake pressure and external load resistance. The maximum energy conversion efficiency can reach up to 73.33%.