Free-piston Engine Research Articles

Performance Characteristics of a Small Poppet Valve Crankshaft Engine in Free-Piston Engine Mode

The free-piston engine generator (FPEG) provides a novel method for electrical power generation in the range extender and hybrid electric vehicle application. In this paper, one-dimensional (1D) numerical simulations of a two-stroke poppet valve in crankshaft engine (CSE) and free-piston engine (FPE) modes are presented to illuminate the potential performance gain of a two-stroke poppet valve engine for free-piston engine generator application. The 1D numerical simulation for crankshaft and free-piston engine models focuses on the two-stroke engine performance response. Both models were subjected to variations of ignition and valve timings. The impact of lambda on engine performance was obtained. Finally, a single speed of 3000 rpm was chosen for detail performance behaviour of the free-piston engine model response. The results have shown that the CSE model has demonstrated traditional performance behaviour against ignition timing variation. In addition, FPE model performance is highly affected by both intake and exhaust valve timings as compared to the CSE model. Furthermore, CSE is superior to FPE across lambda variations for BSFC, brake thermal efficiency, brake power and bmep. These models have successfully portrayed realistic engine performance response as presented in the lambda variations simulation. When simulated at an intended operating speed of 50 Hz, the FPE model has shown poorer performance. The bmep and brake power of FPE model dropped by 3%, brake thermal efficiency dropped by 26%, and BSFC increased by 21%. This lower performance is attributed by 30% reduction in piston velocity suffers in FPE, which contributed to 13% reduction in peak cylinder pressure. Ignition delays promote better FPE performance which is able to match the CSE model. In conclusion, this paper has demonstrated the performance behaviour of a two-stroke free-piston engine model based on the baseline crankshaft engine model.

Free-Piston Stirling Engine Technologies and Models: A Review

The Stirling engine is an alternative solution to produce cleaner energy in order to achieve the reduction of the fossil fuel consumption and the CO2 emissions. It comprises an external combustion engine that can convert any external heat source into mechanical power, through cyclic expansion and compression of a working gas in a closed-regenerative cycle, with or without driving mechanisms. The free-piston Stirling Engine is significantly preferred because of the absence of any mechanical linkage resulting in longer operating life, lower noise pollution, maintenance and vibration free, self-starting and high thermal efficiency. The aim of this paper is to summarize the research works on the free-piston Stirling engine technologies and models. First, the working principles of the free-piston Stirling engine are described, identifying different configurations. Then, several applications are presented. Finally, a detailed review of the models available in literature is given, pointing out the main assumptions and equations.

Free-Piston Engine Design and Evaluation with Different Compression Ratios

Free-piston heat engines are being examined by a number of international research groups as an option for conservative technology, as they are not limited by the movement of the crankshaft as in conventional engines. The free-piston engines are employed in applications like power generation using conventional methods. The higher compression ratios provide higher cycle efficiencies and also boost in-cylinder temperatures increasing mechanical stress, pressures, and heat transfer losses. This paper presents different in-cylinder flows by computational fluid dynamics for various compression ratios ranges from 7 to 15 and estimates were made to find best compression ratio for the optimal engine operating performance and characteristic.

Generator Design Considering Mover Action to Improve Energy Conversion Efficiency in a Free-Piston Engine Generator

In a free-piston engine generator (FPEG), the power of the engine can be directly regenerated by linear generators without a crank. The mover motion of this system is interrelated with engine and power generation efficiencies due to the direct connection between the mover of the generator and the piston of the engine. The generator should be designed to improve the overall energy conversion efficiency. The dimensions and mass of the mover limit its operating stroke and drive frequency. Herein, we propose a method for designing linear generators and constructing FPEG systems, considering the mover operation to improve engine efficiency. We evaluated the effect of mover operation on the engine and generation efficiencies using thermal and electromagnetic field analysis software. The proposed design method improves the overall energy conversion efficiency compared with a generator that considers only the maximization of generation efficiency. Setting the mover operation for higher engine efficiency and designing a linear generator to realize the operation can effectively improve the energy conversion efficiency of FPEGs.

Research on Working Process and Stability of Hydraulic Free Piston Engine

This paper studies the working principle of the hydraulic free piston engine, and details the related contents of the working process of the hydraulic free piston engine. The related contents include the analysis of working state, the analysis of fuel injection characteristics, the analysis of the air exchange system, the analysis of the operating parameters, the working process in the cylinder, etc. The stability of the hydraulic free piston engine is studied by establishing a nonlinear model. Based on this, the stability control strategy is developed, which aims to accumulate reliable experimental data and lay a good application foundation for developing the dynamic control of hydraulic free piston engine in working process.

Investigation of performance of free-piston engine generator with variable-scavenging-timing technology under unsteady operation condition

A substantial number of free-piston engine generators still use cross scavenging, which limits scavenging quality and performance under unsteady condition. Variable-scavenging-timing technology is a novel method to change the fixed scavenging process so that the scavenging process is coordinated with piston movement. This paper investigated effects of the variable-scavenging-timing technology implemented on the free-piston engine generator. In designed case that the free-piston engine generator has different compression ratios, cyclic variation and frequency, the variable-scavenging-timing technology improved the scavenging quality and performance. The delivery ratio of the free-piston engine generators applied with the variable-scavenging-timing technology decreased about 0.3 at different compression ratios, 0.18 at different cyclic variations and 0.2 at different frequencies on average. The trapping efficiency of FPEG rose over 70% and the scavenging efficiency was maintained at least 85%. As for different compression ratios, the power increased by an average of 0.18 kW. While the power was improved 0.2 kW on average under the other condition. The increase of indicated efficiency had an increase varying from 1% to 2% compared with before. The changes of the performance about free-piston engine generators under different unsteady condition also were discussed.

Comparative analysis on friction characteristics between free-piston engine generator and traditional crankshaft engine

Compared with traditional crankshaft engines (TCEs), free-piston engine generators (FPEGs) present many potential advantages due to the abandonment of crankshaft mechanism. The friction loss in FPEGs is considered to be lower than that in TCE. However, few studies have calculated and analyzed the friction differences between them in any great detail. This paper presents an equivalent design of two-stroke TCE based on an existing FPEG prototype size, and then determines the main friction components of their prototypes. The comprehensive simulation models of FPEG and TCE are established in MATLAB/Simulink. Following this, the friction models and lubrication characteristics of each part in FPEG and TCE are described and compared based on the fundamental lubrication theory. The simulation results show good agreement with the experimental data for both the cold start and combustion power generation. And the overall error between them is less than 5%, indicating that the simulation model is reliable and can predict the prototype performance. Moreover, the effects of lubricating oil grade, temperature and operating frequency on the friction performance of FPEG are also investigated. Results show that the piston assembly friction power is less powerful in FPEG (118.3 W) than that in TCE (142.1 W), while the friction power of piston rings is slightly more powerful in FPEG than TCE. The total mean friction power is significantly lower in FPEG than that in TCE, with the proportion of total mean friction power to the indicated power being 11% in TCE and 6.7% in FPEG. This distribution of friction power indicates that when concerning friction loss, FPEG has an advantage over TCE. The piston ring friction power increases with an increase in lubricating oil temperature and operating frequency, and decreases with an increase in lubricating oil viscosity. The proportion of mean friction power to the indicated power shows a decreasing trend as the viscosity and operating frequency increase, and an increasing trend as the lubricating oil temperature increases.

Parametric Simulations on Leakage and Performance of a Miniature Free-Piston Generator (MFPG)

The miniaturization of electrical equipment and popularization of portable devices is an appealing motivation for the development of small-scale heat engines. However, the in-cylinder charge leaks severely as the engine dimension shrinks. The free-piston engine on a small scale provides better sealing than other miniature heat engines. Therefore, a miniature free-piston generator (MFPG) with a single-piston internal combustion engine (ICE) and a voice coil motor (VCM) was proposed in this work. A dynamic model with special attention on the heat transfer and leakage was established accordingly, upon which parametric studies of leakage and its effects on the performance of the MFPG system were performed. Four key parameters, including scavenging pressure, ignition position, combustion duration and piston mass, were considered in the model. The results showed that the mass leakage during the compression decreases with the rise of the motoring current. The indicated thermal efficiency can be improved by boosting scavenging pressure and increase motoring current. The critical ignition position is 2 mm before the top dead center. When ignition occurs later than that, the MFPG system is incapable of outputting power. The chemical to electric energy conversion efficiency is about 5.13%, with an output power of 10~13 W and power density around 4.7~5.7 W/cc.

Design Optimization of Flexure Springs for Free-Piston Stirling Engines and Experimental Evaluations with Fatigue Testing

The free-piston Stirling engine is a closed-cycle regenerative heat engine that converts heat energy into mechanical work, and requires a spring element for vibratory operations of the displacer and power pistons. In this study, the geometry of the flexural spring design was optimized through structural finite element analyses and fatigue test evaluations. First, we constructed a target design space considering the required natural frequency of the displacer spring assembly under the geometric constraints of total mass and module height. The design of experiments was employed to construct simulation cases for design factors such as the outer diameter, thickness, and number of spirals in the spring sheet. As a result, the optimized design values were obtained to satisfy the design requirements. We also fabricated a test spring specimen and conducted fatigue tests using a linear actuator system developed to have the same motion as the engine. The test results indicated that the optimized spiral spring had no fracture under operating conditions with the design piston amplitude, revealing the effectiveness of the design method.

Research on starting process and control strategy of opposed-piston free-piston engine generator _ Simulation and test results

The operation process of free-piston engine generator (FPEG) can be divided into starting process and power generation process, and each of them faces severe challenges especially for the opposed-piston FPEG. As the first part of the investigation, this paper focuses the starting process of opposed piston FPEG. Two novel general methods to start the opposed-piston FPEG are investigated by the linear electric machine operating as linear motor. Both simulation and test results are presented, and the numerical model is successfully validated. The test results show that the maximum in-cylinder pressure of the engine cylinder is 11.85bar, indicating that the engine is ready for ignition, and the synchronization error of opposed-piston fluctuates within ± 2 mm with the limitation of the synchronization mechanism Further investigations on the opposed-piston scheme performance with the displacement master–slave control strategy and current master–slave control strategy are carried out. The results demonstrate that both control strategies are feasible to start the opposed-piston FPEG. More importantly, the current master–slave control strategy has faster response to external interference and therefore the piston synchronization error is smaller. And the smaller motor force of the current master–slave control strategy makes the peak in-cylinder pressure reach the target value. Further research shows that the mass of moving parts, constant motor force and the synchronization mechanism are crucial factors for such schematic prototype during starting process.

Optimized design of a novel yokeless mover permanent magnet linear generator for free‐piston stirling engines applied to solar cogeneration systems

Considering the growing environmental issues, the use of renewable energy converters such as solar cogeneration systems has received more attention during recent years. Solar cogeneration systems convert the thermal energy of solar radiation into mechanical energy (by means of a free-piston Stirling engine) and then into electrical energy (by means of a linear generator). This paper introduces the novel yokeless mover permanent magnet linear generator (YMPMLG) to reduce mover mass, increase power density, increase thrust and decrease the normal force on the mover, which can make this generator a more suitable structure for operating in solar cogeneration systems. This paper is initiated with a comparative study of a YMPMLG with two reported prototypes in this field. Three generators with similar characteristics are simulated by the finite element method (FEM). The magnetic field flux density, power density, thrust coefficient, normal force, and efficiency parameters are analysed and compared in three generators. Following that, some dimensional parameters of the YMPMLG, including generator diameter and length and permanent magnet dimensions are optimized to achieve the highest possible power density and thrust coefficient using the ‘hybrid genetic and pattern search’ algorithm. The highest power density and efficiency are 0.58 kW/kg and 90.3%, respectively. In the end, the YMPMLG prototype has been tested to validate the results of the FEM method.

Control of linear generator based on hysteresis‐SVPWM current rectification and bidirectional buck/boost converter used for energy storage

The free-piston Stirling linear generation system has tremendous applications in space power supply, biomass generation, solar thermal generation, factory waste heat generation, and other industrial fields. However, the free-piston Stirling linear generation system suffered from the disadvantages of fluctuation of output electric energy and self-starting. To solve the aforesaid problems, a two-level electric energy conversion control mode which combines three-phase AC/DC and bidirectional buck/boost DC to DC converter is proposed in this paper to make the generation system with stable output DC power. Then, both the linear and non-linear mathematical model of the free-piston Stirling engine are established. Additionally, to improve the performance of frequency and displacement tracking, the traditional proportional-integral (PI) in the position loop is replaced by Proportional Resonance (PR) controller. Moreover, the Hysteresis-SVPWM current control strategy with high robustness and dynamic response is applied to the rectifier to realize the unit power factor control of the AC side of the generator and the stability and controllability of the DC side voltage. Finally, the three-phase voltage Source Rectifier (VSR) and Bi-directional DC/DC with an appropriate inductance and capacitance for energy storage are modelled. The effectiveness of the proposed system is validated using theoretical analysis and simulations in Matlab.

Operation of a turbine-compound free-piston linear alternator

A free-piston linear-alternator combined with combustion chambers has been examined in many studies. However, only simplified thermodynamic and mechanical models were developed to mimic the actual behavior of the free-piston engine. The purpose of this study is to establish a fully dynamic model that can calculate the energy transformation under the operation of the free-piston engine. The Matlab/Simulink® model uses non-constant-volume combustion event, the piston transient dynamics, flow, heat losses, and thermodynamics as bridges to connect control volumes. The model successfully captured the behavior and measurements of a GS-34 free-piston engine, based on a thermodynamic calculation calibrated with experimental data. The resulting model is used for a series of parametric studies to understand the very complex system behavior, including low load operation. Operation parameters (injection timing and bounce chamber mass) are optimized to generate the engine map for different alternator sizes. At the end, the advantages of the opposed free-piston engine with a linear alternator are presented through the energy analysis.

Theoretical Modeling and Energy Conversion Process of Free Piston Internal Combustion Power System

A numerical model for energy flow calculation of the free piston internal combustion power system (FPICPS) was established, which was compared with the traditional internal combustion engine (IC engine) with the same structure and input conditions. The piston motion characteristics and energy transfer process of FPICPS are studied by numerical simulation method. The results show that the changes of cylinder pressure, temperature and piston motion characteristics of free piston engine (FP engine) lag behind those of IC engine, and the piston peak acceleration of the FP engine is approximately 2.8 times that of IC engine. The proportion of compression stroke and expansion stroke of IC engine is basically the same, while the expansion stroke of FP engine only accounts for 41.6% of the whole operating cycle. FP engine have obvious advantages in heat transfer and air leakage loss, their percentages are 11.82% and 0.83%, respectively. The percentage of indicated power in FP engine and IC engine are 27.44% and 25.98%, respectively. And 79.64% of FP engine indicated power is converted into the motor electromagnetic energy.

Investigations for a Trajectory Variation to Improve the Energy Conversion for a Four-Stroke Free-Piston Engine

Internal combustion engines with a crankshaft have been successfully developed for many years. They are lacking in the fact that the piston trajectory, i.e., position as a function of time, is limited by the crankshaft motion law. Position-controlled electric linear machines directly coupled to the piston allow to realize free-piston engines. Unlike the crankshaft-based engines, they allow for a higher degree of freedom in shaping the piston trajectory, including adaptive compression ratios, which enables optimal operation with alternative fuels. The possibility of adapting the stroke course results in new degrees of freedom with which the combustion process can be optimized. In this work, four-stroke trajectories with different amplitudes and piston dynamics have been proposed and analyzed regarding efficiency. A simulation model was created based on experimental measurements for testing the proposed trajectories. It could be proved that the variation of the trajectory resulted in an improvement of the overall efficiency. The trajectories were described analytically so that they can be used for a prototype in a future work.

Numerical Analysis and Parametric Study of a 7 kW Tubular Permanent Magnet Linear Alternator

Free-Piston Stirling Engines (FPSEs) are known for their easy maintenance, longer lifetimes, high reliability, quiet operation due to no crankshafts, and having fewer seals compared to the traditional Stirling engine. Free-piston systems are popular in the conversion of thermal energy into electrical energy and are compatible with many types of heat sources. This research paper concentrates on the development of a Permanent Magnet Linear Alternator (PMLA) and parametrically analyzing it to predict its limitations and performance over variable operable conditions and material choices. Operable conditions including stroke length and frequency of the translator, and material choice for the stator and magnets, are varied in this study to analyze the machine and put it to test for its extreme limitations. Spacing between slots is introduced to reduce the overall mass of the stator and increase the power density. The load test is carried out with varied parameters. It induces a load EMF of 2.4 kV, yields a power of 7 kW, and has a power density of 314 W/kg by FEM analysis in peak variations. This study enumerates the performance variation of a PMLA over these varied conditions and illustrates the limitations of such power-dense machines.

Research on the Operating Characteristics of Hydraulic Free-Piston Engines: A Systematic Review and Meta-Analysis

The hydraulic free-piston engine (HFPE) is a kind of hybrid-powered machine which combines the reciprocating piston-type internal combustion engine and the plunger pump as a whole. In recent years, the HFPE has been investigated by a number of research groups worldwide due to its potential advantages of high efficiency, energy savings, reduced emissions and multi-fuel operation. Therefore, our study aimed to assess the operating characteristics, core questions and research progress of HFPEs via a systematic review and meta-analysis. We included operational control, starting characteristics, misfire characteristics, in-cylinder working processes and operating stability. We conducted the literature search using electronic databases. The research on HFPEs has mainly concentrated on four kinds of free-piston engine, according to piston arrangement form: single piston, dual pistons, opposed pistons and four-cylinder complex configuration. HFPE research in China is mainly conducted in Zhejiang University, Tianjin University, Jilin University and the Beijing Institute of Technology. In addition, in China, research has mainly focused on the in-cylinder combustion process while a piston is free by considering in-cylinder combustion machinery and piston dynamics. Regarding future research, it is very important that we solve the instabilities brought about by chance fluctuations in the combustion process, which will involve the hydraulic system’s efficiency, the cyclical variation, the method of predicting instability and the recovery after instability.

Design and analysis of a novel yokeless mover permanent magnet linear generator for free piston engine converter

Free piston engine generator (FPEG) is one of the recent solutions by researchers used in hybrid vehicles, which eventually can contribute to the reduction of environmental pollutants from public transports. The purpose of the this work is to introduce a novel dual-stator permanent magnet linear generator with yokeless mover (DSPMLG) to improve the key parameters of the linear generator used in FPEG systems. In this work, an analytical model of the DSPMLG magnetic field is developed to investigate magnetic field flux density distribution and for back-EMF prediction. Then, a two-dimensional (2D) model of the DSPMLG is put forth using the finite element method (FEM) to validate the analytical results. In the next section, 1D modelling and simulation of a two-stroke free-piston engine (FPE) with dual-piston structure is done using Ricardo’s WAVE software. The obtained results are used in the FEM simulation and prototype development in order to accurately evaluate the performance of the introduced generator in the FPEG system. Since the FPE velocity profile distorts the back-EMF, the effect of the velocity profile on back-EMF distortion is investigated using fast Fourier transform. Also, the DSPMLG is optimised for minimum back-EMF distortion, considering the maximum output power. To validate the simulation results, the DSPMLG is developed and the obtained experimental results have proper conformity with the simulation results. Finally, in order to weigh up the effectiveness of the proposed generator, its structure is compared with other reported structures in previous research studies based on the key indicators required for the linear generator used in FPEGs.

Linear Dynamic Analysis of Free-Piston Stirling Engines on Operable Charge Pressure and Working Frequency along with Experimental Verifications

This paper presents a linear dynamic analysis on operable charge pressure and working frequency of free-piston Stirling engines (FPSE) along with experimental verifications. The equations of motion of the FPSE are formulated as a 2-degree-of-freedom (DOF) vibration system of the power piston (PP) and displacer piston (DP), based on the state equation of ideal gas and the isothermal Stirling cycle model. The dynamic models of FPSE we considered are the 1-DOF simple vibration model of each piston and the 2-DOF root locus model of coupled pistons. We developed a test FPSE for verification of the dynamic models and conducted a series of experiments to measure the dynamic behaviors of PP and DP under varying charge pressures for various masses and stiffnesses of the PP. As a result, both prediction models showed good agreements with experimental results. The 1-DOF vibration model was found to be simple and effective for predicting the operating frequency and charge pressure of FPSE. The root locus method showed reasonable predictions with an operation criterion of the PP–DP phase angle of 90°. In addition, the FPSE was confirmed to operate in resonant oscillations when the DP–PP phase angle is 90°, based on analysis of the force vector diagram of the two pistons.

Effects of hydrogen addition on combustion characteristics of a free-piston linear engine with glow-assisted ignition

In this paper, the combustion characteristics of a methane-fueled free-piston linear engine (FPLE) with glow-assisted ignition (GAI) under different hydrogen fractions were studied by numerical simulations. The results showed that the chain-branching reactions start to process after the ignition source appears. The fuel-air mixture can be successfully ignited under the compression of the piston. Hydrogen addition to the fuel accelerates the elementary reaction and helps enhance the ignition and combustion processes. Compared with that of pure methane, the maximum HRR increases by 9.4%, 49.4%, 75.6%, 124.7%, and 241.4% with hydrogen fractions of 1.0%, 3.0%, 5.0%, 7.0%, and 9.0%, respectively. High hydrogen concentration (greater than 7.0%) can bring the occurring of over-advanced combustion, resulting in higher compression negative work. With the hydrogen fractions changing from 0% to 9.0%, the CA0–10 first decreases and then increases, with its minimum value atαH2=7.0%, and the CA10–90 is gradually decreased. The hydrogen addition raises the combustion temperature, which in turn, results in higher NOx emission. High combustion temperature helps convert carbon monoxide into carbon dioxide, decreasing the emission amount of CO. Hydrogen addition to methane has competition effect on soot emission. On one hand, it reduces the soot emission by soot oxidation. Another hand it causes chemical effect leading to higher soot nucleation and surface growth rates. In this paper, the result of competition suppresses soot formation.