Free-piston Engine Research Articles

Quantitative comparison of the operating characteristics and engine performances of typical configurations of free-piston engine linear generator

Due to their unique operating characteristics and high thermal efficiencies, free-piston engine linear generator (FPELG) has attracted research interest in recent years, thanks to the crankshaft absence. The FPELG has three typical structure configurations: single-cylinder single-piston, dual-cylinder dual-piston, and opposed-piston. These structure configurations have different benefits owing to their distinctive piston operating characteristics and engine performances. This paper analysed the piston operating characteristics and engine performances of the three typical FPELG structure configurations using a quantitative comparison method. The results revealed that the output power of the dual-cylinder dual-piston FPELG was the highest, at 3.7 kW. The opposed-piston FPELG exhibited the highest indicated thermal efficiency of 34%. In addition, the indicated power of FPELG was negatively correlated with the heat transfer area. The friction loss of each FPELG was < 5% of the overall energy consumption. Our study comprehensively analysed the various FPELG structure configurations and highlighted their advantages.

Influence analysis of position error to the operating characteristics of opposed-piston free-piston engine generator with position observation optimization

Stable operation is a crucial challenge for opposed-piston free-piston engine generator (FPEG). Due to the absence of mechanical constraints, variations position-based operation parameters would have a significantly impact on the operation performance of the system. This paper conducted a comprehensive investigation into the effects of position fluctuations on the system's dynamic characteristics and thermodynamic performance by leveraging a detail system model with stable operation. Additionally, a novel real-time position observation approach for the mover was proposed, leveraging observation coils. The findings indicate that the opposed-piston FPEG system can quickly recover to stable operational state within a few cycles for a one-time position error, if there is no misfire in the power cylinder. The acceptable range for stable operation of the opposed-piston FPEG is between +35 % and −48 % for the one-time position error. With the continuous position error varying from +20 % to −30 %, there is a noticeable decrease in the compression ratio, operating frequency, and overall system performance. A significant linear relationship has been found between the flux linkage value of the observation coil and the position of the mover. The maximum difference between the observed position calculated by the proposed approach and the actual position was below 4 %.

Experimental study on the power generation characteristics of a free piston engine generator prototype with dual linear generator modules

The dual-piston dual-cylinder free piston engine generator (DD-FPEG) has garnered significant attention for its high thermal efficiency, power density, and suitability for various fuels. To improve performance and stability, this paper proposes a DD-FPEG prototype integrating dual linear generator (LG) modules. The structure and working process of the prototype is introduced, followed by testing its operating characteristics and power generation performance from the cold start to power generation process. When LG1 operates in motor mode, and LG2 operates in generation mode with no load, the motor thrust must exceed 126 N to ensure piston movement. As the motor thrust decreases from 221 N to 0 N and then switches to generation mode, the peak cylinder pressure, operating frequency, output voltage and power show a decreasing trend. The coefficients of variation for peak cylinder pressure position, top and bottom dead center positions are lower than that of peak cylinder pressure, and their values are below 3 %. When the load is 50 Ω, the prototype operates at about 31.2 Hz, with an average indicated power of 1422.8 W and a peak output power of about 1002.5 W. These results demonstrate that the proposed control strategy enables the designed prototype to achieve stable operation and smooth switching.

Investigation on the oscillation characteristics and performance prediction of a linear range extender: An analytical and numerical combined method

Linear range extender (LRE) system is directly coupled with free piston engine and linear motor and has great application potential in extended-range electric vehicles. In this paper, an analytical and numerical combined method was adopted to investigate the effect of key design parameters on the design and performance prediction of an LRE system. The results showed that the energy input and consumption reach a balance state, the LRE system maintains a stable steady vibration. Increasing the system load will reduce the piston operating stroke. The peak operating velocity of the piston, the average operating velocity of the piston gradually decrease, the operating frequency and the output power decreases as the load increases. When S/B is 0.8, the system output power reaches 4.5 kW, and when S/B is 1.2, the output power is only 2.2 kW. When the mover mass is 3.0 kg, the system output power reaches 3.8 kW, and when the mover mass is 8.0 kg, the output power is only 2.4 kW. When the excess air ratio increases from 0.8 to 1.2, the peak piston speed was reduced from 6.79 m/s to 6.03 m/s, the average piston operating speed was reduced from 5.30 m/s to 4.96 m/s, and the system operating frequency was reduced from 51.2 Hz to 47.96 Hz. If the LRE system wants to have stronger resistance to load fluctuations, it should choose a larger S/B ratio and a relatively larger mover mass during design. In order to pursue higher power output, the S/B and mover mass must be relatively small.

Simulation on stirling reverse osmosis desalination power system

In this article, a new power system based on the Stirling engine and hydraulic free piston engine is proposed to provide power for reverse osmosis (RO) desalination. First, the design of the Stirling free piston RO desalination power system was carried out, and the size parameters of the main structures such as the regenerator, heater, cooler, power piston, and valve distribution piston were obtained. A physical model was established to simulate the process of the Stirling engine driving the RO desalination power system based on finite element CFD. The flow field distribution cloud map and output power of the Stirling engine were analyzed. The results indicate that the gas motion of the Stirling engine is stable, and the entire power system can operate normally.

Application of semi-direct fuel injection system to free piston engine generator for better performance: Simulation approach with validation results

This paper explores methods to improve the fuel economy of a two-stroke free piston engine generator (FPEG) with no additional cost. In order to solve the problem of low fuel capture and poor fuel economy of two-stroke FPEG using port fuel injection (PFI), the fuel injection method is optimised in this paper. This fuel injection system is semi-direct injection (SDI). In contrast to PFI, SDI also uses a low-pressure injector, which introduces fuel directly into the ports of the engine. This study explores the feasibility of SDI on a free-piston engine generator through numerical simulation. The results show that SDI can effectively avoid short-circuit losses and improve the fuel capture rate by more than 60 % relative to PFI. Meanwhile, the ISFC is reduced by 42.3 %, and the lowest fuel consumption rate is 259.7 g/(kW·h). The combustion efficiency of SDI is maintained at over 95 % under warm-up conditions. Compared with PFI, the stagnation period is shortened by up to 10.9 % and the centre of gravity of combustion is advanced by up to 16.7 %. SDI effectively reduces HC emissions and has a lower NOx emission level than PFI. At the same time, SDI produces more CO due to stratified combustion.

A new one-dimensional model of free-piston Stirling engine based on quasi-steady flow approximation and simi-implicit numerical method

Free-piston Stirling engine is a widely used device for thermal-to-electric conversion especially in aerospace applications. A dynamic model for free-piston Stirling engine is established in this paper for the purpose of modeling the free-piston Stirling engine more accurately and understanding its operation characteristics. This paper proposes a new one-dimension numerical model with quasi-steady flow approximation for free-piston Stirling engine with linear alternator, which can be categorized as third-order theoretical model for Stirling engine. The corresponding simi-implicit discretizationmethod is presented. In order to accurately obtain the thermal efficiency of the Stirling machine, various energy loss mechanisms are taken into consideration. A beta type free-piston Stirling engine, Component Test Power Convertor, is studied to demonstrate the correctness and accuracy of the numerical model established in this paper. The dynamic steady-state anticipated operation condition is simulated first. Then, the sensitivity analysis is carried out in which the effect of load resistance, heater temperature and cooler temperature, and charged pressure on the system operation parameter is studied. The predicted system parameters including frequency, displacer and piston amplitude, output power, and alternator output voltage and current agrees well with the designed value. Under the dynamic steady operation condition, the simulated indicated power is 15.51 kW and the electric power is 13.12 kW. The relative deviation between the predicted electric power and the measured output electric power of 12.78 kW is 2.66 %. The corresponding indicated efficiency and thermal-to-electric efficiency is 25.06 % and 21.20 % respectively. The amplitude of displacer and piston is 13.96 mm and 13.69 mm for this condition. Compared with the experimental data at different piston amplitude, the derivation of predicted engine parameters is within 10 %. The comparison with experimental results proves the accuracy of the proposed model in this paper. Compared with the previous model based on the adiabatic thermodynamic model, the third-order model presented in this paper shows higher accuracy on system parameter prediction for free-piston Stirling engine.

Development of a hydrogen free-piston engine generator for range extenders

Free-piston engine generator (FPEG), as an important branch of range extender, is a very competitive development direction. In this paper, a hydrogen-fueled opposed-piston FPEG is proposed, which adopts a mechanical-pressurized structure and a two-layer control strategy based on the inner and outer dead center. The prototype well meets the requirements of stable power generation and vehicle loading. The experimental tests have been conducted, which show that the pure hydrogen is successfully ignited under the cold start condition, the combustion efficiency and generation efficiency reach 30 % and 20 % respectively, and the peak pressure is close to 40 bar. However, the prototype was unable to enter the stable operation stage due to frequent misfire phenomena. Based on the above, the causes of abnormal combustion were discussed and the energy loss of the prototype was calculated. The prototype was simulated to run in the stable operation stage finally, and the future optimization scheme based on control strategy and structure was concluded.

Design and Analysis of a Linear Electric Generator for Harvesting Vibration Energy

This paper provides a proof of concept for a linear electric generator that can be used to harvest energy from various sources of linear motion, such as vibrations, free-piston engines and wave energy. The generator can be used to power small electronic devices, such as sensors, or charge household batteries. The literature was reviewed to develop an understanding about the applications, control methods, excitation methods and mechanics of rotating and linear electric machines. A bidirectional, two-sided linear machine was designed with two stator cores and a single mover core. The stator windings and mover winding can be independently excited, allowing for three modes: no mover excitation, a DC excited mover, and an AC excited mover. Simulation results showed that the magnetic flux generated by DC excited stator cores were concentrated in the centre of the mover core. The use of two stator cores eliminates lateral flux in the mover core when it is not excited, minimising attraction and repulsion forces. Parametric analysis showed that flux cutting occurred in all operation modes, verifying that the generator will produce power when operating. Hardware tests produced an output current when the machine was electrically and mechanically excited, verifying the proposed design.

Performance investigation of an active free-piston Stirling engine using artificial neural network and firefly optimization algorithm

The aim of this study is to explore the characteristics of an active Free-Piston Stirling Engine (AFPSE) through the use of machine learning methods. Due to the time-intensive nature of extracting simulation results from complex thermal equations, an Artificial Neural Network (ANN) is utilized to expedite the process. To construct a nonlinear model, 5000 samples are extracted from simulation results. Input parameters included in the model are the hot and cold source temperatures, the voltage given to the DC motor, spring stiffness, and the mass of the power piston, while output parameters are the amplitude and frequency of power piston displacement. The proposed ANN model structure comprises two hidden layer with 10 and 20 neurons, respectively, indicating the applicability of the ANN model in estimating significant parameters of AFPSE in a shorter amount of time. The firefly optimization algorithm is utilized to determine the unknown input parameters of ANN and maximize the output power. Results indicate that a maximum output power of 23.07 W can be attained by applying 8.5 V voltage on the DC motor. This study highlights the potential of machine learning techniques to explore the primary features of AFPSE.

Study of hydrogen injection strategy on fuel mixing characteristics of a free-piston engine

The formation of the gas mixture in the cylinder has a significant effect on the combustion and emission of the engine. The main difference between a conventional engine and a free piston engine is that the piston movement mode is different. Therefore, a new coupled iterative method is adopted in this paper to study the injection position and duration of hydrogen fuel in a free piston engine where diesel fuels hydrogen, and reveal its influence on gas mixing and combustion in the cylinder of a free piston engine. The results show that it is suitable to spray hydrogen 55 mm from TDC. At this time, the mass fraction of H2 in the cylinder is relatively uniform, the cumulative heat release is about 620 J, and the pressure and temperature are relatively low, which are 7.2 MPa and 1529 K respectively. The duration of hydrogen injection is 0.4 ms, at this time, the mixing degree in the cylinder and the gas flow speed are better, although there is a little shortage of unburned equivalent ratio, but this has little impact on the overall, and the final cumulative heat release reaches 655 J.

Design and Analysis of a Free-Piston Stirling Engine for Microreactor Applications

Abstract With the development of microreactors, a free-piston Stirling engine (FPSE) is an excellent candidate to support the development of microreactors. Based on the advantages of microreactors such as the compact design, long-lasting, highly-efficient, and remote-control operation, an FPSE can provide almost the same requirements. In this paper, a 20 kWel FPSE is proposed to support the development of microreactors. The calculation method was done through matlab to analyze the design with all the significant losses in the engine. Through various designs and operating conditions for the engine, the proposed design has 21.7% efficiency with a total output power of 20.7 kWel. With the testing through different parameters in the engine, the current design is well optimized to balance all the constraints which offers a highly-efficient, compact design, and reliability. However, there is room for improvement during the design process, such as using the heat flux instead of a heat exchanger, robust foil for the regenerator, and simulation through 3D modeling to maximize the potential of the design. This study provides theoretical support for the design and analysis of the FPSE for microreactor applications.

Coupled effect of injection position variation on gas exchange stability of a free-piston engine

Gas exchange fluctuations may lead to operation instability of the free-piston engine (FPE), because the engine has a coupling effect between the gas exchange and the piston motion, which is different from a conventional engine. It is, therefore, important to study the gas exchange instability to ensure the stable operation of the engine. This study focuses the effect of variable piston position when injecting on the gas exchange stability of a looped scavenging FPE, and a systematic full-cycle gas exchange model is presented by developing a dynamic model, a combustion model, and a multidimensional scavenging model. Meanwhile, an iterative method between motion and scavenging is used to solve the model numerically for gas exchange simulation, and the effects of variable piston position when injecting on piston motion, thermodynamic in cylinder, and gas exchange process are studied. The results indicate that retarding injection induces instability of gas exchange duration which first increases and then decreases, and there is an injection position which is most conducive to the increase of capture efficiency of gas exchange, although the scavenging efficiency is lowest under this condition. This result also suggests that more complete exhaust may be achieved by a different injection position.

Performance Characteristic of Permanent Magnet Linear Generator for Energy Conversion with Renewable Fuels on Free-Piston Engines

Renewable fuels attract more attention owing to reduction of both air pollution and greenhouse gas emissions compared to fossil fuels. The free-piston engine generator is a new energy conversion with compact structure, multifuel combustion mode possibility, and less friction loss, and it can be applied to a variety of renewable fuels. This study investigated the effect of operating parameters on power generation and motion characteristics for free-piston engine generator. A test bench and a detailed and validated numerical model of the free-piston engine generator were presented. Results show that with the increase of the average velocity, the output power root mean square (RMS) and efficiency of the generator gradually increase. The output power RMS grows rapidly, whereas the generator efficiency gradually stabilizes. Under the same average velocity but different strokes and frequencies, the generator power and efficiency are the same. This result means that the multiple-point operating condition can achieve the same output power and generator efficiency goals. Moreover, the optimal operating conditions can be selected comprehensively, considering factors such as fuel properties, combustion efficiency, and performance. The influence of the generator on motion characteristics of the system under pure resistive load and high-frequency operation should be noted.

Optimizing design of a free piston Stirling engine using response surface methodology and grey relation analysis

The performance analysis and optimization of a γ-type free-piston Stirling engine (FPSE) are conducted using Response Surface Methodology (RSM) and Grey Relation Analysis (GRA). The input-output parameters table is determined by the RSM. A regression model is presented to investigate the influence of structural parameters of FPSE on its performance. The relation coefficients for the four output parameters obtained from the RSM are determined to be 0.9980, 0.9983, 0.9999, and 0.9995, respectively, thus unequivocally demonstrating the exceptional precision of the RSM model. Also, the relationship between displacer and piston amplitudes, operating frequency, and output power and these parameters of Stirling engine is presented via 3D surface plots. The performance of the FPSE is optimized using the desirability approach and GRA, respectively, followed by a comparison of four groups of optimization results to determine the most suitable model. The maximum output power achieved is 78.02 W, with corresponding operating frequency, displacer amplitude, and piston amplitude values of 60.24 Hz, 2.55 mm, and 7.27 mm. Finally, by comparing pre- and post-optimization, the output power is increased by 36.85%. The GRA exhibits superior optimization compared to the RSM, thus providing valuable guidance for enhancing the optimization of FPSE in this study.

Deterministic Simulation of Surface Textures for the Piston Ring/Cylinder Liner System in a Free Piston Linear Engine

The use of synthesis gas (SYNGAS) from waste gasification has been pointed out as a key strategy to help the energy transition. However, SYNGAS’ low calorific power is considered a difficult obstacle to its technological use in internal combustion engines. To overcome this, a novel free-piston linear motor has been proposed to pave the way for the use of SYNGAS in the mobility sector. Surface texturing has vast potential to reduce friction losses in this system. This study utilizes a deterministic numerical model to investigate the mixed lubrication performance of a textured piston ring/cylinder liner conjunction in a free piston engine. The model considers the simultaneous solution of the lubrication and asperity contact problems at the roughness scale, including texturing features on the cylinder surface. The numerical model employs the Reynolds equation with mass-conserving cavitation to calculate the inter-asperity fluid pressure. The rough contact model utilizes the Hertz theory for elastic contact to calculate the contact pressure at each asperity between the piston liner surface and the admitted smooth and rigid ring surface. Surface texturing demonstrated remarkable effectiveness, particularly in the hydrodynamic lubrication regime, with a maximum friction reduction of 38.5% observed for an area coverage of 50%. This was accompanied by a notable shift in the transition from the boundary to the mixed lubrication regime. The textured surfaces exhibited consistent efficiency in reducing fluid pressure and shear stress as the coverage of the textured areas increased. The incorporation of dimples on these surfaces played a crucial role by augmenting the lubricant storage capacity while concurrently reducing the real shear and contact areas. This study offers valuable insights into the nuanced friction-reducing mechanisms of surface textures, illuminating their influence on the coefficient of friction and the formation of lubricant films across various lubrication regimes.

Experimental research of an integrated scavenging pump coupled with a buffer tank for a free-piston engine

Experimental research of an integrated scavenging pump coupled with a buffer tank for a free-piston engine

Optimal power distribution control in modular power architecture using hydraulic free piston engines

Vehicle modularization has become an emerging trend in the automotive industry, leading to research on modular configuration, composition, and related control strategies. In this paper, we propose a modular power system with a hydraulic free piston engine (HFPE) as the power unit and develop a power distribution control strategy to enhance the overall efficiency of the system. Firstly, we determine the configuration scheme of the modular power system and establish a simulation model of the HFPE using MATLAB/Simulink. We conduct principle verification of the simulation model. Secondly, based on the simulation model of HFPE, we research the power unit control strategy using the machine learning regression prediction algorithm, enabling dynamic working condition switching of the power unit. Next, we propose a power distribution optimization algorithm which is named as the Rule Based Double Iterative Optimization Algorithm (RBDI) and compare it with several mature optimization algorithms under the framework of model predictive control, considering related constraints. Finally, we validate the performance of the proposed power distribution control strategy using a hardware-in-loop system. The results demonstrate that the output power of the modular power system can be effectively ensured. Compared with the average distribution algorithm (AVE), the genetic algorithm (GA), and the ameliorated particle swarm optimization algorithm (APSO), the overall working efficiency of the modular power system using the proposed control strategy is increased by 6.57%, 6.13%, and 5.59%, respectively, under the three test driving cycles.

Numerical investigation of the impact of dual spark plug coordinated ignition strategy on the combustion process in a free piston engine generator

The unique motion profile of Free Piston Engine Generators (FPEG) results in a brief dwell period at the top dead centre (TDC). Effectively achieving swift combustion within the cylinder during this limited interval is an urgent challenge that needs to be solved. In this study, a three-dimensional simulation model of FPEG was established and the model underwent calibration utilizing experiment data. Subsequently investigating the impacts of varying ignition positions, distinct spark plug spacing, and distribution on the combustion of the FPEG when subjected to the dual spark plug ignition configuration. The research results show that with the advancement of the ignition position, the maximum combustion duration of the dual spark plug FPEG was shortened by 34 %. With a reasonable spark plug spacing, the dual spark plug FPEG could shorten the flame propagation distance, which is conducive to the realization of constant volume combustion. When the dual spark plugs were arranged on both sides of the auxiliary intake port, the peak pressure in the cylinder could reach up to 49.5 bar. When the dual spark plugs were arranged in the main intake port and the auxiliary intake port, the indicated thermal efficiency reached 34.7 %.

Operating characteristics and design parameter optimization of permanent magnet linear generator applied to free-piston energy converter

A tubular permanent magnet linear generator (PMLG) with axial magnetized is proposed for the free-piston engine generator (FPEG). The structures and test rigs of the PMLG and FPEG prototypes are introduced. The working characteristics of the PMLG in FPEG system are analyzed under ignition conditions. A 2D finite element model of the PMLG is designed and verified based on initial design parameters and test results. The influences of key operating parameters on the output performance of PMLG are studied. On these basis, when considering the actual motion characteristics of the PMLG in FPEG system, the effects of different structural parameters on the power generation performance and efficiency (h) are investigated. Results show that when the working frequency of the FPEG is relatively small, the displacement profile resembles a sinusoidal curve. As the working frequency and stroke length increase, the power generation increases rapidly. As the air gap length and back iron shaft diameter decrease, and the stator slot number increases, the output voltage (U), current (I), power (Pout), and total loss power (Ploss) show an approximately linear increasing trend. As the pole arc coefficient and stator tooth width increase, the U, I, Pout, and Ploss first increase and then decrease. As the stator yoke thickness increases, the U, I, and Pout increase rapidly and then tend to stabilize. Besides, these structural parameters have little impact on power generation efficiency of this PMLG.