Free-piston Engine Research Articles

Mathematical simulation and parameter determination of regulation of a linear electrical reciprocating machine

A linear switched-reluctance electrical machine (LEM) intended to be used as an electric-power generator together with a free-piston internal-combustion engine (ICE) has been analyzed. Taking into account the hard service conditions of an electrical machine (EM) in close proximity to a cylinder ICE, hottemperature zones and heavy mechanical loads of the EM of the vent-inductor type have been recommended to be used. Development of a control system for this type of machine is one of the most complicated problems when developing it. It has been suggested to determine the reasonable principles and regulating algorithms of the machine that is being considered, as well as its regulation system and energy-data evaluation, on the basis of a model of the electromagnetic process. Taking into account the characteristic properties of the linear switched-reluctance machine as a modeling object, a mathematical model has been developed to solve the problems mentioned above. As stator phases have separate coil-flux guides and are not connected magnetically, the electromagnetic processes in each phase are analyzed independently. For experimental corroboration of ideas for design calculation, an experimental model of the machine has been constructed. Parameters required for modeling the characteristics’ correspondences of flux linkage and phase current’s generating force at various positions of a moving element have been determined for this machine experimentally. Performance of the linear switched reluctance electrical reciprocating machine has been analyzed with the mathematical model with moving-element movement frequencies unfeasible for study on a proving stand. Reasonable parameters of the current pulses’ phases of the machine have been determined, and its energy data have been evaluated. The obtained results have confirmed the correctness of the design approaches, choice of regulation principles, and adjustment of a linear vent-inductor electrical machine of forward and backward action and the feasibility of using it as an electric-power generator together with a free-piston ICE.

Design Rules of Hybrid Stepping Machine for Free Piston Engine

Design Rules of Hybrid Stepping Machine for Free Piston Engine

Experimental study of the operation characteristics of an air-driven free-piston linear expander

Free-piston engine is a kind of linear internal combustion engine, and shows advantages on simple mechanical structure, low frictional losses, high thermal efficiency and operational flexibility. In this research, an experimental test rig of a dual-piston air-driven free-piston linear expander (FPLE) is established using the FPE concept. A linear generator is used to convert the mechanical work of the pistons into electricity during the expansion process. The piston dynamics, the output voltage of the generator, and the expander operation frequency, as well as the system energy conversion efficiency are identified. It is observed that the piston displacement profile is similar with a sinusoidal wave. The piston is found to run at relative high speed during the middle stroke, and peak velocity is usually achieved when the piston approaches the middle stroke. The output voltage of the generator is sensitive with the piston velocity. With higher driven pressure, the expander frequency is higher. The energy conversion efficiency increases with higher driven pressure and can reach up to 55% with a driven pressure of 3.75bar. This research presents a fundamental analysis of a FPLE prototype, which can be used as a guidance for the future design of this FPLE type.

A Decoupled Design Parameter Analysis for Free-Piston Engine Generators

The free-piston engine generator (FPEG) is a novel power generation device with an estimated brake efficiency (energy contained in the fuel that is transformed into useful work) of up to 46%, compared to the 25–35% reported in conventional reciprocating engines. This paper seeks to address a major challenge in the development of new and complex technologies—how do we effectively communicate and understand the influence of key design parameters on its operating performance? In this paper, the FPEG is described using a simple numerical model, a model which is reduced to a forced mass-spring vibration system under external excitation, enabling all the major input parameters to be decoupled. It proved that the engine piston position as a function of time and output power could be predicted directly from the input parameters with acceptable accuracy. The influence of the key FPEG design parameters on the piston oscillation characteristics and electric power output can be characterised with respect to one another and summarised. Key design parameters include piston mass, compression stroke length, piston cross sectional area, and electric load. Compared with previous and more complex numerical models, the presented methods can be used to simply describe the sensitivity of key design parameters on the FPEG performance. It will provide useful general guidance for the FPEG hardware design process.

Lumped circuit‐based sizing of quasi‐ Halbach PM excited T‐LSMs: application to free piston engines

The study is aimed at a lumped circuit-based sizing of tubular-linear synchronous machines (T-LSMs) equipped by quasi-Halbach magnetised PMs in the mover. The proposed approach is initiated by the synthesis the T-LSM lumped circuit, taking into account: (i) the saturation of the magnetic circuit, (ii) the leakage fluxes, (iii) the armature magnetic reaction, and (iv) the variation of the mover position. Then, formulations of different no-load and load operation features are derived in terms of selected variables yielded by the proposed lumped circuit. Among these features, the main value of the developed force is selected as a sizing criterion whose variation with respect to different influent design parameters is investigated along with the armature current density. The lumped circuit-based sizing approach enables the selection of a set of appropriate design parameters for which the formulated no-load and load operation features are predicted. A dual validation of the obtained results by an analytical model based on the air gap flux density formulation on one hand, and a 2D finite element analysis on the other hand, proves the effectiveness of the proposed sizing approach.

Tribological characteristics of piston rings in a single-piston hydraulic free-piston engine

PurposeA free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).Design/methodology/approachA time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.FindingsThe study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.Research limitations/implicationsIn this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.Practical implicationsThis paper contributes to the tribological design method of HFPE.Social implicationsNo social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.Originality/valueThe main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

Design and analysis of linear oscillatory single-phase permanent magnet generator for free-piston stirling engine systems

This study focuses on the design and analysis of a linear oscillatory single-phase permanent magnet generator for free-piston stirling engine (FPSE) systems. In order to implement the design of linear oscillatory generator (LOG) for suitable FPSEs, we conducted electromagnetic analysis of LOGs with varying design parameters. Then, detent force analysis was conducted using assisted PM. Using the assisted PM gave us the advantage of using mechanical strength by detent force. To improve the efficiency, we conducted characteristic analysis of eddy-current loss with respect to the PM segment. Finally, the experimental result was analyzed to confirm the prediction of the FEA.

Hybrid Stirling engine-adsorption chiller for truck auxiliary power unit applications

This paper presents preliminary experimental test results for a novel truck auxiliary power unit (APU) design consisting of a 1 kWe free-piston Stirling engine and 2 kWt zeolite-water adsorption chiller that is powered via waste heat from the engine's cooling jacket. A prototype system was built and tested to study the interaction dynamics between the Stirling engine and adsorption chiller and to determine the performance of the chiller in extreme ambient temperature conditions. The results show that pulsed thermal loading from the chiller on the engine results in engine power spikes to 110–115% of rated power. An average COP of 0.42 ± 0.06 was achieved for the adsorption chiller. An investigation into mitigating the negative effects of low buffering was also conducted and estimates that a buffer volume of 50 litres is sufficient to minimise the effects. This volume can potentially be achieved through integration with the main truck engine eliminating the need for a supplementary tank. The proposed system has many benefits over existing technologies such as low noise, high reliability and clean emissions without any need for additional exhaust treatment.

A single-phase axially-magnetized permanent-magnet oscillating machine for miniature aerospace power sources

A single-phase axially-magnetized permanent-magnet (PM) oscillating machine which can be integrated with a free-piston Stirling engine to generate electric power, is investigated for miniature aerospace power sources. Machine structure, operating principle and detent force characteristic are elaborately studied. With the sinusoidal speed characteristic of the mover considered, the proposed machine is designed by 2D finite-element analysis (FEA), and some main structural parameters such as air gap diameter, dimensions of PMs, pole pitches of both stator and mover, and the pole-pitch combinations, etc., are optimized to improve both the power density and force capability. Compared with the three-phase PM linear machines, the proposed single-phase machine features less PM use, simple control and low controller cost. The power density of the proposed machine is higher than that of the three-phase radially-magnetized PM linear machine, but lower than the three-phase axially-magnetized PM linear machine.

Transient motion control for a free-piston engine

The free-piston engine has a unique internal-combustion engine architecture which features unconstrained piston movement because the crankshaft has been removed. The engine design is thus more compact, modular and efficient. One of the technical challenges that hinder the development of free-piston engine technology is the lack of precise control of the piston motion. Previously, a robust repetitive controller was designed and implemented to form a virtual crankshaft to provide stable and precise control of the piston motion. The virtual crankshaft mechanism ensures stable and repeatable free-piston engine operation and opens up an opportunity for controlling the in-cylinder gas pressure–temperature trajectories by altering the piston trajectory. The experimental data from engine motoring tests with the virtual crankshaft demonstrate the effectiveness of the controller. However, the presence of a transient period after combustion prevents the engine from continuous firing. Thus, a transient control scheme is presented in this paper, which consists of a detection and shifting algorithm to remove the transient period, so that continuous engine firing operation can be achieved.

A tubular hybrid Halbach/axially-magnetized permanent-magnet linear machine

A single-phase tubular permanent-magnet linear machine (PMLM) with hybrid Halbach/axially-magnetized PM arrays is proposed for free-piston Stirling power generation system. Machine topology and operating principle are elaborately illustrated. With the sinusoidal speed characteristic of the free-piston Stirling engine considered, the proposed machine is designed and calculated by finite-element analysis (FEA). The main structural parameters, such as outer radius of the mover, radial length of both the axially-magnetized PMs and ferromagnetic poles, axial length of both the middle and end radially-magnetized PMs, etc., are optimized to improve both the force capability and power density. Compared with the conventional PMLMs, the proposed machine features high mass and volume power density, and has the advantages of simple control and low converter cost. The proposed machine topology is applicable to tubular PMLMs with any phases.

Performance and power quality assessment of a linear electric generator focused on microgeneration applications

Summary The use of new generation devices based on renewable energy is a subject that increasingly becoming more important for the development of new distributed generation systems; one device developed for this purpose is the permanent magnet linear generator (PMLG). This work deals with the performance and power quality assessment of the PMLG under 2 different generation scenarios of microgeneration: the wave energy converter and the free piston engine linear generator. The performance of the PMLG is evaluated using the finite element method and experimental results under steady state condition of a 100-W prototype are presented to test and validate the proposed process design. The actuator displacement inside the PMLG was obtained from the equation that governs the movement of the 2 systems under study; this displacement was the excitation function used to conduct the simulation and the laboratory experiments. This work contributes to the design and development of power components devoted to microgeneration and creates the need for the modelling and development of power electronic converters for helping in control and power quality.

Computational Investigation on the Effect of Compression Ratios on Performance Characteristics of Two Stroke Free Piston Spark Ignition Engine

This research work depicts the effect of compression ratios on performance characteristics of Free Piston Engine (FPE) by using computational fluid dynamics (CFD) to find out the optimum compression ratio for the best operating characteristics and performance of FPE. CFD analysis showed that the highest peak pressure was seen in high compression ratio for which the scavenging efficiency was around 87% with the marginal increase in piston velocity. The high compression ratio increased the indicated power efficiently. Further, the peak temperature was greatly lesser than the traditional engine in the similar circumstances. It's possibly decreased the temperature contingent emissions from the engines.

Wear resistance of composite plasma coatings with graphite

The wear-resistant composite solid lubricant coatings formed by atmospherec plasma spraying method of cladding Ni[Cg] powder in relation to the extreme working conditions of friction units of aircraft engines and power plants have been investigated. Examples of studies of technological processes of the plasma spraying of protective coatings on elements of liquid-propellant rocket and gas turbine engines have been given, and the possibilities of sealing technologies to increase their service life have been shown. The evaluation of efficiency of coatings obtained by plasma spraying from mechanical mixtures of powders based on Ni[Cg] in relation to power plants of vehicles based on freepiston engines.

Optimizing Exhaust Port in 2-stroke Free Piston Engine for Improving Scavenging Efficiency using CFD

Optimizing Exhaust Port in 2-stroke Free Piston Engine for Improving Scavenging Efficiency using CFD

Friction characteristics of piston rings in a free-piston engine generator

Free-piston engine generator is a new alternative to traditional reciprocating engine, which moves without mechanical restriction of crankshaft system. This article investigated numerically the friction characteristics of piston rings in a free-piston diesel engine generator by adopting coupled models of dynamic and friction. The development of the dynamic model and friction model was described, and an iterative calculation method was presented, giving insight into the coupled parameters of these two models. The detailed effects of the dynamic on friction and lubrication were investigated compared with a corresponding traditional crank engine. The friction characteristics of the free-piston engine generator were found to differ clearly from those of the traditional engine due to its special piston motion. Compared with the traditional engine, the minimum lubricant film thickness of piston rings in the free-piston engine generator is thicker and lasts shorter at the dead center regions, but it is generally thinner at other positions. The average friction force, friction power, and friction work of the piston rings in the free-piston engine generator are less than the traditional engine due to the better lubrication in endpoints region. Meanwhile, the friction power of the free-piston engine generator increases with the increase in fuel mass or decrease in load. The friction efficiency varies in correlation with the generator load; the optimum friction efficiency can be obtained by either increasing or decreasing from a certain generator load.

No-load Features of T-LSMs With Quasi-Halbach Magnets: Application to Free Piston Engines

The paper is devoted to the prediction followed by the enhancement of the no-load features of tubular linear synchronous machines (T-LSMs) with quasi- Halbach magnetized PMs in the mover. The study is initiated by the prediction of the spatial repartition of the no-load air gap flux density. Then, a formulation of the back-EMF and the cogging force, based on the predicted spatial repartition of the no-load air gap flux density, is developed, considering 1) the case of an infinite length machine and 2) the case of a finite length one. A case study, corresponding to an initial concept, is treated with the prediction of its no-load features. The study is extended to the enhancement of these features with emphasis on the effects of two influent sizing parameters, assuming an infinite length machine. This enables the identification of a preoptimized concept. The back-EMF and the cogging force of this latter are then predicted in the case of a finite length machine. A further enhancement of these features is gained, thanks to a quasicancellation of the end effect. The prediction of the back-EMF and the cogging force of the optimized T-LSM with quasi- Halbach magnetized PMs has clearly demonstrated the effectiveness of the proposed approach.

Trajectory-based combustion control for renewable fuels in free piston engines

Previously, the authors have developed an advanced combustion control, namely the trajectory-based combustion control, to further leverage the flexibility of free piston engine (FPE). With the assistance of this control method, the FPE enables optimization of both engine efficiency and emissions by implementing optimal piston trajectories. Extensive simulations have been conducted to prove the effectiveness of this combustion control on fossil fuels. In this paper, the investigation is extended to renewable fuels. Seven renewable fuels are considered herein including hydrogen, biogas, syngas, ethanol, dimethyl ether (DME), biodiesel, and Fischer-Tropsch fuel. The influences of both compression ratio (CR) and piston motion pattern between the two dead centers on the combustion process are considered in the study, which demonstrates the ultimate fuel flexibility and large tolerance of fuel impurity possessed by the FPE. In addition, the simulation results show that at a fixed CR, the thermal efficiency of the FPE can still be enhanced (5% in DME case) by varying the piston motion patterns alone. Furthermore, specific asymmetric piston trajectories are synthesized to further improve the engine thermal efficiency (8% in hydrogen case) and reduce the NOx emission simultaneously (around 70% reduction in hydrogen case). In other words, due to its ultimate fuel flexibility, large tolerance of fuel impurity, and controllable piston trajectory, the FPE, with the trajectory-based combustion control, enables a co-optimization of renewable fuels and engine operation.

Cold starting characteristics analysis of hydraulic free piston engine

The cold start characteristic of hydraulic free piston diesel engine may affect its stable operation. Therefore the specific cold start characteristics, such as BDC or TDC positions, pressure in-cylinder, heat release rate, should be investigated in detail. These parameters fluctuate in some regularity in the cod start process. With the development of the free piston engine prototype and the establishment of test bench, the results are obtained. For the dynamic results, the fluctuation range of TDC and BDC positions is 8 mm and decreases with time. The thermodynamic results show that the combustion process is not stable and the pressure in-cylinder fluctuates largely in the cold start process. In addition, the combustion is rapid and knock happens inevitably. In order to investigate the reasons, a CFD model is established for temperature analysis in-cylinder and heat transfer conditions. It is found that higher start wall temperature will lead to more uniform temperature distribution. The delay period may decreases and heat release will move forward. This reason is analyzed by thermodynamic derivation based on the first law of thermodynamics. Finally, the improvement suggestions of cold start strategy are proposed.

Disturbance analysis of a free-piston engine generator using a validated fast-response numerical model

In this paper, a fast-response numerical model was used to investigate potential disturbances to a free-piston engine generator (FPEG), i.e. engine cycle-to-cycle variations, misfire and immediate electric load change. During the engine operation, there could be one disturbance taking place or several disturbances take place simultaneously. By identifying different types of system disturbance with specific occurring times, the influence on the system was characterised. It was found that a step change of electric load would induce a corresponding top dead centre (TDC) step change. Low variations on piston TDC are observed when cycle-to-cycle variations take place. When unsuccessful ignition occurs, the engine will stop after one oscillation cycle. Reducing the electric load after misfire would cause more oscillation cycles and require a restart of the engine. Technically feasible control variables were identified and coupled with a PI feedback controller design to minimise the impact of each kind of disturbance, a design which could be used in future FPE control system designs. The controller performance was seen to be satisfactory for the electric load step change, and the piston TDC was controlled to back to the set point in 0.5s.