Free-piston Engine Research Articles

A Plate Moving-Magnet Linear Generator Designed for Free-Piston Engines

Free-piston engine coupled with linear generator is a research hotspot in the field of novel vehicle mounted power generation devices. A plate moving-magnet linear generator (PMMLG) is proposed and designed for free-piston engines in this paper. Magnetic circuit of the novel linear generator is analyzed and modeled in detail. The dimensions of a 2 kW prototype with the max stroke of 40 mm and the max electromagnetic force of 800 N is designed by using the derived magnetic circuit model. Static and transient electromagnetic field finite element models with motion solvers are created. Magnetic field distribution, electromagnetic force characteristics, EMF characteristics, coil inductance, generating power and losses of the linear generator are analyzed and compared with two reported prototypes. Computational fluid dynamics model of the PMMLG is established, and the temperature field is analyzed to verify that the air-cooling conditions can ensure long-term stable running of the linear generator without demagnetization. The research results show that the linear generator has the characteristics of low moving mass, low cogging force, high generating efficiency and high response speed.

A moving-coil linear generator with dual axial-magnetized stator for free-piston engines

Free-piston engine coupled with linear generator is a new type of thermoelectric energy converter, which has the advantages of high efficiency and simple structure. In this paper, a novel moving-coil linear generator with dual axial-magnetized stator is proposed for this kind of short-stroke high frequency oscillation generating applications. Finite-element (FE) model of the generator is created, the static and transient characteristics are analyzed, and the losses in different components are compared. A full-bridge power converter is designed to meet the control requirements of reversible operation. A 540 W experimental prototype is developed and tested to validate the FE analysis. According to the tested and simulated results, the generator has advantages of high efficiency and fast response. The generating efficiency of 92% can be obtained at the rated generating power. The average inductance of the proposed generator is about 0.7 mH, which benefits from the low armature effect and ensures a very quick electromagnetic response.

ILQ Optimal Voltage Control for Biomass Free-Piston Stirling Engine Generator System

In this paper, we study the control of output voltage of Free-Piston Stirling Engine Generator (SEG) System. The Stirling is normally used by connected to the grid. The grid is very robust because grid can be considered as infinite bus. When the SEG is applied to the based power-station in the remote rural or disaster areas, the SEG must be operated as a standalone system or off grid power system, this situation may lead to unstable generation. To solve the problem, the ILQ (Inverse Linear Quadratic) can be used to realize robust voltage controller, in this paper we propose a new power generation system based on renewable energy sources specifically, wood pellet as the heat source. The experimental results show that up to 1.6kW can be generated from the generator system, and possible production of around 200L of hot water (45°C). In order to rescue us from serious conditions.

Compression Ratio Control of an Opposed-Piston Free-Piston Engine Generator Based on Artificial Neural Networks

Opposed-piston free-piston engine generator (FPEG), as an energy conversion device, has attracted the attention of researchers with its advantages of variable compression ratio (CR) and good dynamic balance performance. At the same time, the variable compression ratio poses a challenge to the stable operation of the engine. The controller needs to overcome the interference caused by the combustion variations to the piston movement, so that the compression ratio of the engine remains stable. This paper proposes an opposed-piston synchronous motion control strategy based on master-slave position following and a compression ratio control strategy based on artificial neural networks. A test prototype and simulation model were established, and the model was verified by the prototype. The performance of the control strategy was studied through simulation analysis. The results showed that the engine achieved stable operation and the compression ratio was well controlled. Compared with the reported control strategies in the literature, the artificial neural network algorithm applied in free-piston engine generator system shows better control accuracy and good response characteristics.

Study on HCCI combustion improvement by using dual assisted compression ignition (DACI) on a hydraulic free piston engine fueled with methanol fuel

A dual assisted compression ignition combustion mode is researched on a hydraulic free piston engine. The dual assisted compression ignition combustion mode is realized by combining a glow plug with a spark plug in the ignition system to assist the homogeneous charge compression ignition combustion. The effects of control parameters and operating parameters on dual assisted compression ignition combustion mode are revealed by experiments. It is found that as the compression ratio increases, the combustion characteristics of dual assisted compression ignition change from spark ignition to two-stage heat release. The dual assisted compression ignition combustion has more controllability on the combustion phase and the heat release rate than glow plug assisted compression ignition due to the direct and stable spark ignition. When the temperature of the glow plug is lower, the heat release phase is dominated by the spark plug. With the temperature further increases, the heat release process is mainly controlled by the glow plug. The dual assisted compression ignition combustion has a wider compression ratio range than spark assisted compression ignition to guarantee the high indicated thermal efficiency. The suitable methanol/air equivalence ratio range for dual assisted compression ignition combustion mode is between 0.6 and 0.8.

Multi-objective optimization of a free-piston Vuilleumier heat pump using a genetic algorithm

A free-piston Vuilleumier heat pump (FVHP) has unique advantages for residential/commercial heating and cooling applications. This work focuses on optimizing the adjustable machine parameters of a FVHP to maximize its thermal performance for both cold climates, in which the FVHP primarily operates in heating mode, and warm climates, where cooling mode operation dominates. Nine machine parameters are optimized using a thermodynamic model combined with a non-dominated Sorting Genetic Algorithm. The typical optimization time on a desktop PC is several hours. The machine performance is evaluated using a thermal model for a FVHP for the most favorable designs, with the optimized solutions found to provide a 20–30% improvement in output, COP and parasitic energy use compared to the parameters from a current prototype that serves as the baseline. The approach developed should be applicable to most FVHP designs and related devices, such as free-piston Stirling engines.

Feasibility of Multiple Piston Motion Control Approaches in a Free Piston Engine Generator

<div class="section abstract"><div class="htmlview paragraph">The control and design optimization of a Free Piston Engine Generator (FPEG) has been found to be difficult as each independent variable changes the piston dynamics with respect to time. These dynamics, in turn, alter the generator and engine response to other governing variables. As a result, the FPEG system requires an energy balance control algorithm such that the cumulative energy delivered by the engine is equal to the cumulative energy taken by the generator for stable operation. The main objective of this control algorithm is to match the power generated by the engine to the power demanded by the generator. In a conventional crankshaft engine, this energy balance control is similar to the use of a governor and a flywheel to control the rotational speed. In general, if the generator consumes more energy in a cycle than the engine provides, the system moves towards a stall. If the generator consumes less energy, then the effective stroke, compression ratio and maximum translator velocity must rise steadily from cycle-to-cycle until the heat transfer losses stop the increase. Moreover, when stiff springs are added to the FPEG system, the dynamics becomes more sinusoidal and more consistent with increasing spring stiffness. To understand the behavior of proposed control and cycle-to-cycle variations, a comprehensive FPEG numerical model with a 1 kW target electric power was developed in MATLAB<sup>®</sup>/Simulink. An FPEG system corresponding to that numerical model has been operated in the laboratory. This MATLAB<sup>®</sup>/Simulink numerical model has been used to examine the sensitivity of FPEG dynamics and performance parameters to the changes in design and operating inputs. A difficulty during the modeling is associated with the cycle-to-cycle energy balance, and this difficulty is also reflected in the real-world FPEG control. Therefore, the authors have devised a control strategy similar to the real world intended control methodology. In this numerical model, two different feedback control methodologies were implemented and investigated. These control methodologies were applied to regulate the generator load with selected control or input variables, namely peak pressure, mid-stroke piston velocity, trapped compression ratio and dead center set points. The controllers with optimized coefficients demonstrated the feasibility of energy balance management during the transient operation. Based on the simulation results, the controllers with compression ratio, peak pressure and dead center clearance set points as control variables demonstrated stable FPEG operation whereas the mid-stroke velocity failed to achieve the steady-state operation due to deviation in the piston dynamics. The simulation results from this study will be used as the pathway for improving and optimizing the experimental FPEG design.</div></div>

The design and testing of a kW-class free-piston Stirling engine for micro-combined heat and power applications

This research paper describes the design methodology of the free-piston Stirling engine (FPSE) and its demonstration. The FPSE is equipped with a dual-opposed linear alternator. The design and experiment are mainly focused on understanding how the FPSE dynamically behaves. Based on the data obtained from experiments, the operational issues regarding the output and dynamic characteristics are discussed in detail. The FPSE can accept approximately 4.2 kW at the engine head with a temperature of 525 °C. As a heat source, city-supplied natural gas is used. The linear alternator stably produces 961 We-electricity at a resonance frequency of 60 Hz. The deduced efficiency of the linear alternator is 70–75% of the thermodynamic work generated by the engine. The integrated system, FPSE, has an overall efficiency of 23.0% when comparing the heat supplied to the electrical output. The FPSE was also proven with a direct connection to the grid, so it can generate and sell electricity back to the grid. The equipped resonant capacitor enables this by compensating for the power factor. The experiment results show that the design methodology is able to adequately explain the behavior of the FPSE.

Coordination of forces of mechanical and electric subsystems of power plant with free piston engine and electric generator of reciprocating type

Autonomous power supply system of modern mobile special-purpose equipment requires the development of electromechanical energy converters with high energy and minimum weight- and size indicators. In industrialized countries, the system “free piston engine – reciprocating electric generator of transverse type” is considered as a promising power plant. The main feature of this kind of power plant is the lack of crank mechanism in the engine design. This allows: increasing the efficiency of the engine up to 50–60 % and overall power by 2.5–3 times while reducing the specific gravity and metal consumption compared to traditional engines; reducing the specific fuel consumption of the engine up to 30 %; increasing the resource to overhaul by 30–50 thousand hours; implementing a modular structure. The main drawbacks of this kind of power plant are high probability of failure when passing the ignition of the working mixture and instability of work with significant load fluctuations. The noted drawbacks are due to the inconsistency of the forces of the electrical and mechanical subsystems of the power plant throughout the operating cycle. The solution for the problem of matching the forces of the electrical and mechanical subsystems of the power plant in the extreme positions of the piston group of the free piston engine is of particular complexity. In this regard, a method for solving the problem of matching the forces of the mechanical and electrical subsystems of the power plant with a free-piston engine throughout the operating cycle was developed, characterized by the use of an electromechanical reciprocating energy converter with transverse- and longitudinal nonlinear changes in the magnetic flux in the electrical subsystem. Coordination of the forces of mechanical and electrical subsystems of the power plant on the entire operating cycle makes it possible to fulfill the conditions of continuous electromechanical energy conversion at all work cycle and to reduce the specific gravity of the electrical subsystems of the plant while improving efficiency.

An averaging-based Lyapunov technique to design thermal oscillators: A case study on free piston Stirling engine

This article introduces a novel averaging-based Lyapunov approach to design complex oscillators such as free-piston Stirling engines (FPSEs). First, a class of thermal oscillators namely the FPSE is introduced. Then, the existence of limit cycle in dynamic response of the oscillator, which is a required criterion for steady operation, is investigated via the proposed technique. Accordingly, a systematic way containing two conditions is presented to justify the performance of the oscillator system. As the preliminary condition, instability of the equilibrium state of the oscillator is studied through the proposed averaging-based Lyapunov theorem (necessary condition). Subsequently, as the second condition, the existence of stable limit cycle in the oscillator response is investigated through the mentioned method (sufficient condition). Next, some useful equations for effective prediction of the oscillator performance are presented using the proposed averaging-based Lyapunov method. Finally, the simulation outcomes, as well as the experimental data, clearly reveal the effectiveness of the proposed methodology in designing the free piston Stirling oscillator, which can be further extended to other oscillators with periodic response such as engines, energy harvesters, vibration/acoustic isolators, and resonant absorbers as well.

A Tubular PM Linear Generator With a Coreless Moving-Coil for Free-Piston Engines

Free-piston engine coupled with linear generator is a new kind of thermoelectric energy converter. It has been highly concerned due to its high efficiency and low emission in recent years. In this paper, a tubular moving-coil linear generator is developed for the application of free-piston engines. The two-dimensional finite-element model of the linear generator is established, and its electromagnetic characteristics are analyzed. A power converter is designed to control the operating state and the electric current of the linear generator. A prototype is developed and used in a free-piston engine with a bore of 102 mm, and the main performance parameters are tested. According to the simulated and tested results, the linear generator can realize the stable running of free-piston engines. The peak power of the linear generator in a cycle is about 16.77 kW, and the averaged generating power of a cycle is 3.37 kW at the working frequency of 10 Hz. The generating efficiency at peak power of a cycle is about 95.2%, the averaged generating efficiency of a cycle is about 87.5%.

Numerical investigation into the fuel evaporation and mixture formation characteristics of a free-piston diesel engine

The free-piston engine generator becomes a new-type potential substitute for the conventional crankshaft combustion engine. This article presents a simulation to study the fuel spray and mixing characteristics of a diesel free-piston engine generator by comparing a corresponding crankshaft combustion engine. A full-cycle model which couples with piston dynamics, combustion, and gas exchange is developed to simulate the fuel spray, atomization, and mixing in the free-piston engine generator. The result indicates that compared with the crankshaft combustion engine, the free-piston engine generator provides a higher temperature and pressure for fuel spray and mixing during the ignition delay, but its ignition delay lasts shorter. The free-piston engine generator shows a shorter spray penetration and more fuel impingement due to its smaller combustion chamber volume during the injection process. The free-piston engine generator exhibits a lower level of air utilization and worse uniformity of fuel–air mixture in combustion chamber. In addition, the shorter ignition delay of free-piston engine generator makes the time of atomization, evaporation, and mixing of fuel shorter, and the mixing effect of free-piston engine generator is worse, resulting in less combustible mixture formed during the ignition delay. In addition, some guiding suggestions have been proposed to improve the fuel spray and fuel–air mixing characteristics of free-piston engine generator.

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.

Performance Analysis of a Flexi-Fuel Turbine-Combined Free-Piston Engine Generator

The turbine-combined free-piston engine generator (TCFPEG) is a hybrid machine, generating both mechanical work from the gas turbine and electricity from the linear electric generator for battery charging. In the present study, the system performance of the designed TCFPEG system is predicted using a validated numerical model. A parametric analysis is undertaken based on the influence of the engine load, valve timing, the number of linear generators adopted, and different fuels on the system performance. It is found that when linear electric generators are connected with the free-piston gas turbine, the bottom dead centre, the peak piston velocity, and engine operation frequency are all reduced. Very minimal difference on the in-cylinder pressure and the compressor pressure is observed, while the peak pressure in the bounce chamber is reduced. When coupled with a linear electric generator, the system efficiency can be improved to nearly 50% by optimising engine load and the number of the linear generators adopted in the TCFPEG system. The system is able to be operated with different fuels as the piston is not limited by a mechanical system; the output power and system efficiency are highest when hydrogen is used as the fuel.

Performance characteristics of compressed air-driven free-piston linear generator (FPLG) system – A simulation study

The integrated model of the two-stroke free-piston engine system driven by compressed air is established using MATLAB/Simulink software and the relationship and the influence of the reciprocating motion, friction force, cylinder process and electromagnetic characteristics of permanent magnet linear generator are fully investigated. The piston displacement, velocity, acceleration, thrust, air pressure in cylinder and generator voltage and their relationship are simulated and analyzed, and the motion performance, the characteristics and interaction relationship of the system are obtained. The experimental platform of air driven free-piston linear expander system is tested. Simulation results are compared with experimental data and the whole model is validated. The developed model is then used to study the influence of piston assembly mass, the timing of air intake and exhaust on piston motion characteristics. The peak voltage output, voltage harmonic distortion and efficiency of permanent magnet linear generator under different air pressure and the output power and efficiency with pressure of FPLG under different loads are obtained. The results show change rules of piston displacement, velocity and frequency changes with air pressure, piston assembly mass. Finally from the simulation result, the air pressure and air intake position of highest efficiency of the linear generator are found.

Multi Cycle Modeling, Simulating and Controlling of a Free Piston Engine with Electrical Generator under HCCI Combustion Conditions

ABSTRACT For distributed electrical power generation, linear engine-alternator systems have the potential to operate at high efficiency. They avoid the frictional losses present in rotating generators and enable the use of efficient combustion modes. However, their design and control optimization are matters of novelty. In this research, a single cylinder, two-stroke, free piston engine model has been built and simulated. The model describes the coupling between the piston motion dynamics and the chemical kinetics of the in-cylinder charge mixture undergoing homogeneous charge compression ignition (HCCI) combustion. The chemical kinetics reactions have been employed to find the net heat release rate and the start of combustion. Piston motion depends on the cylinder pressure as well as the dynamics of the mass-spring-damper system translator. For advanced investigations, the model results are represented by a multi-cycle simulation where 100 engine cycles were simulated with an average frequency of 125 Hz. At near-steady state conditions, the electrical generator converts 47% of the total fuel energy to electrical work at stoichiometric combustion conditions. It is shown that HCCI ignition timing strongly affects the performance of the engine. Combustion timing just before piston top dead center position encourages better engine stability and performance for the given model. The stability of the engine strongly depends on the rate of work done on the translator by the generator, putting the generator thrust force at the core of the engine control scheme. Cycle to cycle stability, with HCCI combustion occurring for each compression stroke, is achieved with the use of a modified proportional-derivative controller which alters the generator thrust force based on the cyclic change in the piston bottom dead center position. The research goal is to develop an optimized design with stable controls that yields a high efficiency conversion of chemical to electrical energy.

Independent Pressure and Flow Rate Control Enabled by Hydraulic Free Piston Engine

Fluid power consumes a significant portion of energy annually with relatively low efficiency, especially for mobile applications. This is mainly due to the current architecture where an internal combustion engine (ICE) driven hydraulic pump is used to supply fluid to various driving and working circuits. This architecture as well as the response time limitation of the ICE and the hydraulic pump cause significant fluid throttling loss and low-efficiency engine operation at part load conditions. As a novel alternative, the hydraulic free piston engine (HFPE) eliminates the mechanical crankshaft and integrates the ICE and the hydraulic pump into one unit. Due to its ultimate freedom on the piston motion and reduced inertia, the HFPE has the potential to produce variable fluid power in an efficient manner with fast response time. In this paper, such potential is investigated. The working principle of a prototype HFPE is described and a comprehensive HFPE model is developed. A novel control method, namely the independent pressure and flow rate control (IPFC), is proposed to regulate the HFPE's output flow rate at any given load pressure and realize throttleless fluid control. The effectiveness of the method is demonstrated through simulations, which show the capability of the control method in providing different output flow rates at variable load pressures within tens of milliseconds. Additionally, a detailed analysis of the HFPE efficiency, in terms of thermal efficiency of the engine and mechanical efficiency of the pump, has been conducted under different working conditions. The results show that with the IPFC, the HFPE is an efficient and flexible mobile fluid power source.

Analysis of ways of increasing the energy efficiency of traction electric transmission of shunting diesel locomotive

The article is devoted to the problems of increasing the energy efficiency of traction electric transmission of a shunting diesel locomotive. In the context of rising fuel costs, to maintain the competitiveness of rail transport, the issue of energy efficiency of the traction rolling stock, which accounts for about 90% of the total consumption of hydrocarbons by rail, is quite acute. At present, a number of measures have been investigated to improve the efficiency of diesel engines of locomotives, which allows getting an increase in the efficiency at the level of 1-5%. The use of new efficient types of electromechanical transducers with static converters, in turn, also enables us to reduce fuel and lubricant consumption by a diesel locomotive. However, solving the problem of energy efficiency in traction transmission of a locomotive requires a comprehensive approach. One of such approaches is the application of new types of diesel generator sets for locomotives – free-piston combustion engines with linear generators. The application of new designs of thermal engines makes it possible to significantly extend the possible limits of increasing the efficiency of diesel traction.

Performance of a tubular machine driven by an external‐combustion free‐piston engine

Free-piston engines have the potential to offer a compact machine capable of delivering electrical power. In this work, the design and modelling of a linear tubular generator are presented and used to investigate the effect of the electrical machine design on engine performance. A cogging-force-reduction technique is presented and used as a case study to demonstrate the importance of the integrated design.

Experimental comparison of two linear machines developed for the free piston engine

In this study, a linear synchronous machine is compared with a linear transverse flux machine. Both machines have been designed and built with the intention of being used as the power take off in a free piston engine. As both topologies are cylindrical, it is not possible to construct either using just flat laminations and so alternative methods are described and demonstrated. Despite the difference in topology and specification, the machines are compared on a common base in terms of rated force and suitability for use as a generator. Experience gained during the manufacture of two prototypes is described.