Free Piston Linear Engine Research Articles

The effects of design parameters on the dynamic performance, in-cylinder pressure and electrical power generation of a free piston linear engine

Operation of a free piston linear engine is modeled based on the combination of three mathematical models, including a piston dynamic model, a linear alternator model and a thermodynamic model. The simulated in-cylinder pressure, piston velocity, and electric power output are compared with the corresponding experimental results to validate the models mentioned above. The influences of the design parameters, including cylinder dimension ( Le), number of coil turns ( N), and air gap ( g) between the translator and stator, on the dynamic performance, in-cylinder pressure and electric power generation of the free piston linear engine are studied. The study results show that the reduction of Le has a benefit for improving the piston dynamic performance and output electric power, however it also reduced the cylinder pressure. The increase of number of coil turns N results in the reduction of the peak piston velocity, displacement, acceleration, and pressure in the cylinder, however, it increases the output electric power of the free piston linear engine. The peak piston velocity, displacement, acceleration, and pressure in the cylinder are considerably decreased when g is reduced. However, the reduction of g has a benefit to improve the output electric power of the free piston linear engine. The energy conversion efficiency can be maximized when g and Le are reduced, and N is increased.

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.

A Preliminary Study of Spark-Ignition System for Free-Piston Linear Engine

In recent years, the free-piston linear engine (FPLE) has attracted increasing interest from researchers worldwide. With the development of computer control technology, internal combustion engine technology, and material science, there have been significant advances in the aspects of FPLE simulation, experimental techniques, as well as systems FPLE control. This paper presents a model of a two-stroke, spark-ignition free-piston engine converted from two small two-stroke engines. It has a bore size of 34 mm and a maximum stroke of 28 mm. The linear engine uses gasoline fuel that is available on the market. The purposes of the research show a model of the starting system, investigation the effects of spark timing, and effective pressure. A premixed air-fuel charge is conducted by a carburetor then it is directly supplied into each cylinder. The spark timing was varied at 1 mm and 2 mm away from the maximum top dead center with the speed starting at 10 Hz. The model successfully ignited with a maximum pressure of 7.5 kg/cm2 at the ignition of 1 mm before top dead center (TDC) and 9.5 kg/cm2 at the ignition of 2 mm before TDC. The research has completed the initial intentions such as a theoretical investigation, calculation, determination of the engine model, and starting mechanism. Preliminary results have confirmed the feasibility of proposing the FPLE model.

Numerical simulation of the effect of spring dynamics on the combustion of free piston linear engine

Free piston linear engine (FPLE) is a new potential alternative to conventional engine, and the mechanical spring of the engine is a key energy storage device to maintain reciprocating operation and then affect combustion and thermodynamic. This article presents a study to reveal the effect of spring dynamics on the combustion performances of a linear diesel engine. A system model which couples spring dynamic model and multi-dimensional combustion model is proposed to discuss the variable spring stiffness effect on the combustion and heat release characteristics of the FPLE. The results indicate that the spring stiffness gives a positive excitation on the reciprocating frequency and engine compression ratio, although it brings about low fuel-air mixture uniformity for combustion reaction. Moreover, the great spring stiffness leads to high combustion pressure and fast combustion process due to its large compression ratio. The results suggest that properly increasing the spring stiffness is beneficial to increase the thermal efficiency of the engine, but too high spring stiffness will also result in higher NO emission and soot generation.

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.

Control of Magnetoelectric Load to Maintain Stable Compression Ratio for Free Piston Linear Engine Systems

The free piston linear engine system (FPLE) is considered as a promising powerplant, which has the advantages such as compact structure, short transfer path and variable compression ratio (CR) because the crank connecting rod is removed. However, the absence of crank-connecting rod inevitably produces uncertainty to the stable operation of the FPLE. A control system of the piston motion regulating for the FPLE is necessary. In this paper, the nonlinear dynamic model simulating the piston motion in a dual-piston FPLE is derived based on energy and force balance. The feasibility of the dynamic model is verified by experiment and simulation results. Based on instability mechanism analysis, a magnetoelectric load controller with motion stroke feedback is designed to maintain the piston position in a predefined CR by regulating the magnetoelectric force. The proposed magnetoelectric load controller is shown to have good control performance for the FPLE. The piston is always stabilized at the predefined position after a short adjustment time. The time of eliminating disturbance for the operation process is less than the start process. Furthermore, the increase in disturbance will result in the increase of time for adjustment.

Numerical comparison of HCCI combustion mode between a free piston linear engine and traditional crankshaft engine

The free piston linear engine (FPLE) eliminates the crankshaft components and supports variable compression ratio operation. This characteristic makes it as one of the most appropriate choices to adopt HCCI combustion mode. This work aims to reveal the preliminary characteristics of HCCI combustion in FPLE and compare it with traditional crankshaft engines (TCE). The HCCI combustion model of TCE and FPLE was established by using a chemical kinetic reaction mechanism, and it then was numerically calculated to obtain the combustion and emission performance. The results indicate that the typical two-stage reaction characteristic of HCCI combustion also occur in the FPLE. Although the final cumulative heat release of the two types of engines is approximately the same, the low-temperature reaction stage of FPLE comes later, which also leads to the lag of its high-temperature reaction stage. In addition, the TCE features with earlier heat release rate, higher peak pressure and temperature, and longer duration of high temperature, which are beneficial to promote the reaction of soot, thereby slightly reducing soot emissions, but also increasing NO emissions.

Experimental investigation of operating characteristics and thermal balance of a miniature free-piston linear engine

A miniature free-piston linear engine fueled by butane is designed using glow plug ignition. This study investigates the engine operation characteristics and thermal balance. Experimental results show that the engine can be manually started successfully and achieve self-sustainable operating without the energy support from the glow plug. Thermodynamic analysis of the engine reveals that the energy loss resulting from scavenging process contributes the largest part to the total energy loss. The running frequency and indicated power increase with the mixture flow owing to the increased charging efficiency, and the best performance is achieved when the engine operates at a slightly fuel-rich condition. The energy loss of heat transfer and incomplete combustion are important contributors to energy loss due to the increased surface-to-volume ratio. Leakage and friction loss account for small contributor (less than 3%). The order of importance of engine loss components is different between the miniature FPLE and conventional engine. For the miniature free-piston linear engine, the energy loss components arranged by order of importance are scavenging, heat transfer, incomplete combustion, exhaust, leakage and friction. For conventional-scale spark ignition engine, the energy loss components arranged by order of importance are exhaust, heat transfer, aerodynamic, friction and incomplete combustion.

Effect of hydrogen addition on the operating characteristics of a free piston linear engine

A free piston linear engine (FPLE) with glow plug ignition was designed and manufactured. This paper investigated effects of hydrogen addition on the FPLE performance and emission characteristics. Experimental results showed that the FPLE can be started manually and then steady FPLE operating state is achieved. Compared with the crankshaft model engine, the FPLE has a greater speed around the top dead centre. The peak acceleration of the FPLE (4100 m/s2) is more than three times greater than that of the crankshaft model engine. Results also showed that the FPLE performance including the indicated power, indicated thermal efficiency and indicated mean effective pressure improves remarkably with hydrogen addition. The optimal FPLE performance is achieved when the hydrogen volume fraction is 4%. However, the engine performance cannot be further improved when the hydrogen volume fraction continues to increase due to the occurrence of over-advanced combustion phasing. Moreover, the FPLE operating range is significantly broaden and the operating stability is improved with hydrogen addition since the fast burning speed and wide flammability of hydrogen. Besides, the heat release rate, in-cylinder peak pressure and pressure rise rate are also obviously enhanced. The high-speed flame images showed the flame color becomes lighter and the yellow area is smaller compared with that of pure methane condition, indicating that the hydrogen addition to the fuel contributes to burn completely and reduce the soot formation.

Coupled dynamic effect of combustion variation on gas exchange stability of a free piston linear engine

The fluctuation of gas exchange is easy to induce the free piston linear engine (FPLE) to run unsteadily, because the gas exchange, piston motion and combustion of FPLE are intimately coupled with each other. This work presents a simulation to investigate the effect of combustion variation on the gas exchange stability of a FPLE. A full-cycle zero-dimensional dynamic model and a geometric model for FPLE are built separately, while a coupled three-dimensional combustion and gas exchange is carried out based on the piston motion profile and the geometric model. Subsequently, the effect of combustion variation (including ignition position and combustion duration) on FPLE gas exchange stability is analyzed and discussed. It is clarified that combustion variation has a significant effect on the piston motion and initial conditions of gas exchange, resulting in a variation of gas exchange stability. Specifically, the retarded ignition and extended combustion duration allow FPLE to a longer piston travel distance, providing gas exchange process with sufficient intake duration, thereby resulting in the increase of intake flow mass and fresh charge in cylinder, which improves the scavenging efficiency to 96.82% at the highest, and promotes the reduction of the residual exhaust gas in cylinder, but also inevitably causes the trapping efficiency to drop to 44.06%.

Effect of injection position on fuel spray and mixture preparation of a free-piston linear engine generator

Fuel spray and mixing in linear engines is coupled by dynamics, combustion, and gas exchange, which differs from that in conventional engines. This work presents a system simulation to reveal the multi-process coupling effect of injection position on the fuel spray and mixing of a free piston linear diesel engine (FPLE). A full-cycle fuel spray model which couples with dynamic, combustion, and gas exchange is established to predict the coupled effect on mixture formation. The results indicate that the variable injection position changes the FPLE motion through multi-process coupling effect, resulting in different boundary conditions for fuel spray and mixing. Relatively large injection advance position leads to more residual gas, fast speed, intense turbulence, low gas pressure, and temperature at the moment of injection for mixture formation. The earlier fuel injection generally makes the longer spray penetration, smaller Sauter mean diameter of droplets, less fuel impingement, faster fuel evaporation rate, and more evaporated fuel mass. However, too early injection does not support the above results. Suggesting that in order to achieve homogeneous combustion mode, the large injection advance position injection schedule operation is a good choice for the FPLE due to its long ignition delay duration for fuel atomization, evaporation, and mixing.

Experimental study on the operating characteristics of a reciprocating free-piston linear engine

A miniature reciprocating free-piston linear engine (FPLE) with glow plug ignition (GPI2) was designed and manufactured. The steady-state operation of the prototype is successfully achieved with and without load. The operating characteristics of the FPLE were investigated using a premixed methane/air mixture. The experimental results showed that the in-cylinder mixture was successfully ignited by the glow plug within five cycles and the FPLE could be successfully started under the single-cylinder self-starting strategy. Based on the comparison of the FPLE operating characteristics in the GPI and spark-ignition (SI3), it is found that the performance and operating stability of the FPLE were significantly improved in the GPI due to the flexible ignition timing. Besides, the effects of key parameters such as glow plug heating power (GPHP4), piston assembly mass and fuel/air flow rate on the performance of the FPLE were studied. The electric power and work conversion efficiency increased first and then decreased with increasing external load, and both of them reached maximum values of 31.6 W and 42.8%, respectively, with an external resistance of 6.3 Ω.

Numerical evaluation of pollutant emissions of a two-stroke linear engine.

Combustion engines bring a lot of pollution products to human living environment. This study introduced a green engine called free-piston linear engine (FPLE), and performed an evaluation on the pollutant emissions of a diesel FPLE by comparison with a corresponding conventional crankshaft engine (CCE). The combustion reaction and pollutant formation were described by a comprehensive model which considers the special operation mechanism of the FPLE. The effect of dynamic and fuel quantity on the unclean combustion products of the FPLE and the CCE was compared. Results indicate that in the case of the fuel quantity of 7.1mg condition, the pollutant forming in the FPLE is generally later than that in CCE because of the higher speed of FPLE around the reverse point of stroke, and a significant advantage in nitric oxide (NO) emission is found for the FPLE, although it emits more carbon monoxide (CO) and soot. Moreover, compared with the CCE, the FPLE is cleaner in lean fuel conditions because the NO production of the FPLE is only half of that of the CCE, and that of other pollutants is almost at the same level. However, in heavy fuel conditions, the FPLE shows a disadvantage in CO and soot emission due to the serious afterburning effect and poor oxygen environment in some local areas, although its NO production is still lower than that of the CCE.

Performance Comparison of a Single Cylinder and a Dual Cylinder Free Piston Engine

Free piston linear engine alternators (FPLEA) may be designed following several different baseline configurations. Common designs include a translator that carries permanent magnets, with either one piston attached to one end of the translator, or a piston at each end of the translator. The single cylinder engine requires a reversing force from a spring so that it can operate, whereas, the dual cylinder version can operate without a spring. However, inclusion of stiff springs would serve to raise the operating frequency and reduces the cycle-to-cycle variations in a dual cylinder version. A matlab/simulink numerical model with translator rod dynamics and in-cylinder thermodynamics was employed to predict the overall performance and efficiency of a FPLEA. This numerical model allowed comparison of different FPLEA configurations for a variety of design variables. First, a dual cylinder FPLEA design was considered where the spring constant was varied, changing the frequency of operation and the motion of the translator. The simulation results showed that without springs the motion was far from sinusoidal, and low in frequency and power, whereas the presence of stiff springs in the system strongly dictated nearly sinusoidal motion and high power at high frequency. Effects of other parameters such as stroke and bore were also examined. Finally, the same tests were carried out for a single cylinder FPLEA design. The simulation results showed that the dual cylinder engine produced twice the electrical power output with higher efficiency than the single cylinder engine for the same cylinder geometry.

An optimization method for flexural bearing design for high-stroke high-frequency applications

A method for optimization of a high-stroke, high-frequency flexural bearing was completed. The main goal was to design a flexural bearing system as the energy restoration component of a one kW free piston linear engine alternator (LEA) operating at 90 Hz and maximum targeted displacement of 22 mm. Defined electricity output of 1 kW, with more than 30% efficiency, required engine operation under high speed and high stroke conditions. Under such challenging stroke and frequency requirements, a method of design was required to select between a wide range of parameters in flexural bearing design. A parametric CAD model was prepared and used in the optimization package of a finite element analysis (FEA) software. Four main parameters of flexural bearing geometry were identified and included the outer diameter, thickness, number of arms, and sweep angle of spiral cuts. Other design parameters including spiral gap width, shape factor and spiral start points angle were shown to be less important compared to these four main parameters in primary design and were examined separately. A neural network algorithm was used to investigate the interactions and effects of design parameters. The sensitivity of each parameter on output criteria such as maximum stress and natural frequency was examined and design charts developed. Methods for further modification of the spiral curve at end points were introduced to reduce the maximum stress level by up to 20%. Several steel alloys and a titanium-based alloy were identified as proper material candidates for high cycle applications and the final designed spring was manufactured and tested to validate the FEA results of axial stiffness and strain level throughout the flexure arms. The percent of the flexure’s moving mass was estimated to be 29% with FEA analysis and compared well with the experimental value of 31%. The stress/strain results of the FEA analysis matched with the experimental results – within ±5%.

A Study of the Scavenging Process in a Two-stroke Free Piston Linear Engine using CFD

Unlike the conventional two-stroke engine, a two-stroke free piston linear engine can change compression ratio due to free motion of a piston in a cylinder. The free motion of the piston directly affects the gas exchange process or scavenging process of the free piston engine which is key to improving the engine performance. In this study, a numerical simulation using computational fluid dynamics (CFD) is conducted to examine the scavenging process taking place in a two-stroke free piston linear engine. The motion of the free piston is built first based on a dynamic model to define the motion profiles of the piston, and afterward, the piston’s motion profiles are imported into a commercial CFD software (Ansys Fluent v.14) to simulate the scavenging process. The simulation results in piston motion and in-cylinder pressure are compared with experimental results to validate the presented simulation. To provide information for this study, some key parameters such as operating frequency of the piston, exhaust port distance, and inlet pressure are changed to find out their effects on the trapping efficiency. The simulation results show that the trapping efficiency has a maximum value of 83% at optimal operating frequency f=33Hz. Besides, by increasing exhaust port distance and reducing inlet pressure, the trapping efficiency is increased.

The Effect of Spring System Design on Scavenging of Two Stroke Single Cylinder Spark Ignition Free Piston Linear Engine

To increasing the efficiency research a single cylinder linear engine with spring system has been done. Issues raised in the study was the analysis of air flow into the combustion chamber in a single cylinder linear engine, with a spring system for its return cycle. Solidworks has been used to design and simulation as well. This study compared the fluid flow in the combustion chamber between the conventional engine and linear combustion engine. The parameters used are the engine speeds 1000 rpm, 4000 rpm and 4500 rpm. The speeds is chosen because has been predicted that the inlet port on the linear engine can not work properly. Results of analysis that has been done appears average pressure of conventional has a higher value than the linear combustion engine. The same thing happened in velocity, in the average value of the conventional engine is higher than the linear combustion engine.

A two stroke free piston engine’s performance and exhaust emission using artificial neural networks

The performance and exhaust emissions of a Free piston linear engine (FPLE) were ascertained for various equivalence ratios (0.7, 0.8, 0.9, 1.0, 1.1 and 1.2). After that, the usability of Artificial neural networks (ANNs) in case of FPLE has been tested. Actually, the aim was to examine the best suited operational condition of, and show the possibility of using ANN, for this kind of engine technology. We first interrogated the thermal efficiency, generated power, total heat release rate, indicated mean effective pressure, exhaust gas temperature, and exhaust emissions such as CO, CO2, NOx and O2 for the chosen range of equivalence ratios and then gathered experimental data in order to train and test an artificial neural network model for prediction. The experimental results showed that, running the engine on the slightly lean side stoichiometrically, can fulfil the goals of higher engine performance and lower emissions. We used the back propagation learning algorithm for ANN and observed that the correlation coefficients (R) vary between 0.990-0.999, Mean absolute percentage error (MAPE) vary between 0.885-5.9%, and coefficient of determination (R2) vary between 0.937-0.999; which showed a welldefined relationship between the predicted and experimental values.

A study of operating parameters on the linear spark ignition engine

In this paper, we present our experiment and simulation study of a free piston linear engine based on operating conditions and structure of the linear engine for generating electric power. The free piston linear engine includes a two-stroke free piston engine, linear generators, and compressors. In the experimental study, the effects of key parameters such as input caloric value, equivalence ratio, spark timing delay, electrical resistance, and air gap length on the piston dynamics and electric power output are investigated. Propane is used as a fuel in the free piston linear engine, and it is premixed with the air to make a homogeneous charge before go into the cylinder. The air and fuel mass flow rate are varied by a mass flow controller. The experimental results show that the maximum generating power is found with the value of 111W at the input caloric value of 5.88kJ/s, spark timing delay of 1.5ms, equivalence ratio of 1.0, electric resistance of 30Ω, and air gap length of 1.0mm. In order to check the durability of the linear engine, a durable test is conducted during 100h. The experimental results show that there are no problems for the linear engine after about one hundred hours of the durable test. Beside experimental study, a simulation study is conducted to predict operating behavior of the linear engine. In the simulation study, the two-stroke free piston linear engine is modeled and simulated through a combination of three mathematical models including a dynamic model, a linear alternator model and a thermodynamic model. These mathematical models are combined and solved by a program written in Fortran. Besides, the effects of key parameters such as reciprocating mass, spark timing and spring stiffness on the piston dynamics and electric power output of the linear engine are also investigated. The simulation results show that the simulation and experimental data are nearly similar at the same initial conditions. In addition, a highest generating power of the linear engine can be easily found by optimizing the key parameters.

인공 신경망을 이용한 프리피스톤 리니어 엔진의 연구

Free piston linear engine (FPLE) is a promising concept being explored in the mid-20th century. On the other hand, Arficial neural networks (ANNs) are non-linear computer algorithms and can model the behavior of complicated non-linear processes. Some researchers already studied this method to predict internal combustion engine characteristics. However, no investigation to predict the performance of a FPLE using ANN approach appears to have been published in the literature to date. In this study, the ability of an artificial neural network model, using a back propagation learning algorithm has been used to predict the in-cylinder pressure, frequency, maximum stroke length of a free piston linear engine. It is advised that, well-trained neural network models can provide fast and consistent results, making it an easy-to-use tool in preliminary studies for such thermal engineering problems.