Free-piston Engine Research Articles

Analysis of the Scavenging Process of a Two-Stroke Free-Piston Engine Based on the Selection of Scavenging Ports or Valves

The free-piston engine generator (FPEG) is a linear energy conversion device with the objective of utilisation within a hybrid-electric automotive vehicle power system. In this research, the piston dynamic characteristics of an FPEG is compared with that of a conventional engine (CE) of the same size, and the difference in the valve timing is compared for both port scavenging type and valve scavenging type, with the exhaust valve closing timing is selected as the parameter. A zero-dimensional simulation model is developed in Ricardo WAVE software (2016.1), with the piston dynamics obtained from the simulation model in Matlab/SIMULINK (R2017a). For the CE and FEPG using scavenging ports, in order to improve its power output to the same level as that of a CE, the inlet gas pressure is suggested to be improved to above 1.2 bar, approximately 0.2 bar higher than that used for a CE. If a CE cylinder with exhaust valves is adopted or referred to during the development of an FPEG prototype, the exhaust valve is suggested to be closed earlier to improve its power output, and a higher intake pressure is also suggested if its output power is expected to be the same or higher than that of a CE.

Effectiveness and Setting Operation Range of Resonant Output Distribution Control in Consideration of Spring Characteristics on a Free-Piston Engine Linear Generator System

Effectiveness and Setting Operation Range of Resonant Output Distribution Control in Consideration of Spring Characteristics on a Free-Piston Engine Linear Generator System

Stirling Generators: Challenges and Opportunities

The current state of the market of Stirling generators shows that high-technology engines that have been tuned in the process of cooperation or competition between powerful corporations are winning. The main advantages of Stirling engines are the ability to use various heat sources and combustion chambers meeting environmental requirements, a low level of noise and vibration; favorable characteristics for both vehicles and stationary electric generators, and good consistency with a linear electric machine. High cost and weight—size parameters, as well as limited aggregate capacity and useful life, are constraints on the development of Stirling generators. It is a feature of a Stirling engine that the pressure-circuit, generator, and combustion-chamber parameters need to be regulated. The control system should be intelligent to provide starting conditions. Power control of a free-piston Stirling engine is usually accomplished by changing the piston stroke, which is realized a linear generator electronic control unit. At the moment, due to the development of alternative energy and distributed power supply, a potential niche for the use of Stirling generators is growing.

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.

Study on adaptive behavior and mechanism of compression ratio (or piston motion profile) for combustion parameters in hydraulic free piston engine

The Hydraulic Free Piston Engine (HFPE) piston motion profile is controlled by the composite force of the in-cylinder gas and hydraulic fluid. Based on the analysis of HFPE piston motion by dynamic calculations, it is found that the piston motion profile and compression ratio (CR) are self-adaptive for the variation of combustion parameters. As the combustion timing is advanced, the piston commutation time is advanced, and the CR is decreased. Further, the existence of this self-adaptive features are demonstrated by a prototype test of HFPE, and it also proves that HFPE has self-adaptive features for the variation of mixture concentration. The self-adaptive features can make HFPE effectively avoid the phenomenon of knocking or post-combustion, make it stable of the maximum in-cylinder gas pressure and maximum value of heat release rate, reduce the loss of indicated work, and keep the output power relatively stable.

Experimental investigation and performance analysis of a direct-driven linear generator

Electricity generation of renewable sources has got great development to reduce the usage of fossil fuels. Linear generator is a newly-developing orientation to utilize the power with reciprocating movement. In the paper, the structure and principle of the direct-driven linear generator (DLG) are introduced. The induction electromotive force is given in detailed based field & circuit method. Meanwhile, the paper introduces the design, fabrication and testing of a DLG prototype, theoretical model of the free-piston engine is presented, and the DLG operation driven by Simulated Power Source (SPS) is discussed. Furthermore, the performance of the system is investigated in detail under different conditions. The results show that, there is an optimal capacitance to get the highest coefficient, which is 25μF here. The frequency has obvious effect on the output at lower load resistance, but it has little effect on the efficiency, the efficiency depends more on the ratio of the load resistance and generator internal resistance. Lastly, according to the analysis results with the phasor algorithm analysis, the proposed model and analytical method are correct and effective. The DLG has a good application prospect in the energy conversion field.

Thermodynamic performance of lunar surface nuclear power system with heat sink temperature change in a rotational period

The thermodynamic performance of the lunar surface nuclear power system with Free-Piston Stirling Engines (FPSE) was analyzed based on the energy conservation of the system. The heat sink temperature was assumed to follow the sine function law. The cold side temperature of the FPSE was changing with time in a rotational period and would be increased with the increase of the heat sink temperature. The thermodynamic performance of the power system was changing with lower thermal efficiency and high amount of exhaust heat rejection, during the day time with heat sink temperature higher than 200 K. During the dark time, the power system could be kept as a steady state with higher thermal efficiency and less amount of exhaust heat rejection. The energy storage option could be required, if the electrical power output was expected to meet the grid. The highest thermal efficiency could be increased from 0.21 to 0.235, if the area of the heat rejection system was increased from 120 m2 to 180 m2, since the cold side temperature of the PFSE could be decreased. Larger area of the heat rejection system could increase the specific area but has the advantage of lightweight for the power system.

Numerical evaluation of pilot-ignition technology used for a hydrogen fuelled free piston engine generator

The spark-ignited free-piston hydrogen engine is characterized with relatively slow combustion and low indicated thermal efficiency. This study explored the pilot-ignition technology to the free-piston hydrogen engine and presented a research to evaluate its applicability. An advanced full-cycle operation model which couples with dynamic, combustion and gas exchange was established to iteratively simulate the effect of pilot-ignition technology on a free piston hydrogen engine. The simulated performances were compared with a corresponding spark-ignition situation. Results indicate that compared with the spark-ignition, the pilot-ignition technology can speed the hydrogen fuel combustion in the free-piston engine and enhance the level of isochoric heat release around top dead center, resulting in more indicated work and higher indicated thermal efficiency. The hydrogen engine adopting the pilot-ignition technology features with faster cycle, larger compression ratio, higher generating power, and slightly more NO emission than the spark-ignition situation. Moreover, the pilot-ignition technology also brings about longer ignition delay and shorter rapid combustion period for the hydrogen engine. However, the released heat in rapid combustion period is significantly more than the spark-ignition situation, which makes that the hydrogen engine operates with larger peak combustion pressure and temperature, and the peak values appear earlier and last shorter.

Cogging force investigation of a free piston permanent magnet linear generator

Better performance and higher efficiency of the vehicles can be achieved by using free piston engine, in which the piston is connected directly to the linear generator and waiving of any mechanical means. The free piston engine has the ability to overcome or reduce many of the challenges, such as the carbon dioxide (CO2) emission and fossil fuel consumption. The cogging force produces undesired vibration and acoustic noise in the generator. However, the cogging force must be minimized as much as possible, in order to have a high performance. This paper studies the effects of ferromagnetic materials on the cogging force of the permanent magnet linear generator (PMLG) to be used in a free piston engine using nonlinear finite-element analysis (FEA) under ANSYS Maxwell. The comparisons have been established for the cogging force of the PMLG under various translator velocities and three different ferromagnetic materials for the stator core, namely, Silicon Steel laminations, Mild Steel and Somaloy. It has been shown that the PMLG with a stator core made of Somaloy has a lower cogging force among them. Furthermore, the induced voltage of the PMLG at different accelerations has been studied. It is found that the PMLG with Mild Steel and Somaloy, respectively give larger induced voltage. Moreover, as the translator speed increase the induced voltage increased.

The numerical assessment of motion strategies for integrated linear motor during starting of a free-piston engine generator

Free-piston engine generator (FPEG) provides a novel method for electrical power generation in hybrid electric vehicle applications with scarcely reported prototype development and testing. This paper is looking into the motion control strategy for motoring the FPEG during starting. There are two motion profiles investigated namely, trapezoidal velocity and Scurve velocity. Both motion profiles were investigated numerically and the results have shown that the S-curve motion can only achieve 80% of the stroke when operated at the proposed motoring speed of 10Hz.

An optimization of free-piston engine generator combustion using variable piston motion

The free-piston engine generator has been a research focus lately due to its crankless structure and unconstrained piston motion, which brings about some weaknesses for the combustion process. This...

Gas exchange processes of a small HCCI free piston engine – A computational study

Free Piston Linear Alternator engines (FPLAs) can be used for efficient electric power generation because they can provide high mechanical efficiencies due to low friction. Combining the free piston architecture with Homogeneous Charge Compression Ignition (HCCI) can enable combustion of dilute fuel–air mixtures with high thermal efficiency. However, trapping losses can be a major source of unburned hydrocarbon emissions and combustion inefficiency in two-stroke engines. Therefore, controlling the gas exchange process of an HCCI FPLA is critical for delivering fresh charge in the combustion chamber, retaining high levels of residual gas, and minimizing trapping losses. In the current study, Computational Fluid Dynamics (CFD) are utilized to model the gas exchange process and investigate the underlying phenomena behind trapping losses in a loop-scavenged FPLA. The effects of engine design parameters such as air box compression ratio, port target point, location, and inclination angle on scavenging and trapping efficiency were investigated. The results indicated that a moderate air box compression ratio (1.2:1) combined with high inclination angles of the scavenge ports can result in acceptable scavenging efficiency for HCCI combustion (∼35%) and very high trapping efficiency (>98%). The ability to predict the gas exchange process in consecutive cycles was also investigated. A repeatable gas exchange process over 10 cycles was simulated with the coefficients of variance (COVs) of trapped mass and effective equivalence ratio (φ′) being lower than 3%. Gross indicated efficiency of 38.1% was calculated with acceptable pressure rise rate.

Modeling the dynamic and thermodynamic operation of Stirling engines by means of an equivalent electrical circuit

The Stirling engines are inherently efficient; their thermodynamic cycles reach the Carnot efficiency. These technologies are suitable to operate under any low temperature difference between the hot and the cold sources. For these reasons, these engines can be considered as reliable power conversion systems to promote the conversion of low-grade waste heat generated by industrial plants. The need of a model to predict the behavior of these engines is of primary importance. Nevertheless, a great difficulty is encountered in developing such a model since it is not simple to take into account coupled thermodynamic and dynamic effects. This is the main reason why several models make use of electrical analogies to describe Stirling engines (in particular, free-piston machines): by assuming the pressure equivalent to a voltage and the flow rate to an electrical current, a coupled dynamic-thermodynamic analysis of the engine can be performed. In this paper, an electrical circuit whose behavior is equivalent to that of the engine is derived from the electrical analogy theory. To this aim, we propose an electrical analogy model based on the three conservation laws (mass, momentum and energy). Since limited experimental information is available in the open literature, the results obtained with the proposed model are compared with the experimental data collected at the NASA Lewis Research center for a free-piston Stirling engine i.e., the RE-1000 engine.

Comparison of the gas exchange of a loop scavenged free-piston engine alternator and the conventional engine

The free-piston engine alternator (FPEA) is a new energy conversion engine system, which moves without mechanical restriction. This paper aims to compare the gas-exchange performance of a two-stroke, loop scavenged, diesel FPEA and a corresponding conventional reciprocating engine through numerical modeling. A gas exchange model which coupled with piston dynamic and combustion was developed, and an iterative calculation method was used to simulate the model. The different effects of the piston motion and combustion on the gas exchange in the FPEA and the conventional engine were discussed. The results make it clear that the FPEA has shorter gas exchange duration and more residual burned products than the conventional engine because its piston moves faster in the gas exchange process, which causes the scavenging efficiency of FPEA to be more inferior. While the FPEA has more in-cylinder burned gas through the exhaust port flowing to outside in free blow-down stage, less fresh air through the scavenging port flowing into the cylinder in forced scavenging stage, and more in-cylinder fresh through the exhaust port flowing to the outside in additional exhaust stage.

A Model-Based Control Design Approach for Linear Free-Piston Engines

A general design approach is presented for model-based control of piston position in a free-piston engine (FPE). The proposed approach controls either “bottom-dead-center” (BDC) or “top-dead-center” (TDC) position. The key advantage of the approach is that it facilitates controller parameter selection, by the way of deriving parameter combinations that yield both stable BDC and stable TDC. Driving the piston motion toward a target compression ratio is, therefore, achieved with sound engineering insight, consequently allowing repeatable engine cycles for steady power output. The adopted control design approach is based on linear control-oriented models derived from exploitation of energy conservation principles in a two-stroke engine cycle. Two controllers are developed: A proportional integral (PI) controller with an associated stability condition expressed in terms of controller parameters, and a linear quadratic regulator (LQR) to demonstrate a framework for advanced control design where needed. A detailed analysis is undertaken on two FPE case studies differing only by rebound device type, reporting simulation results for both PI and LQR control. The applicability of the proposed methodology to other common FPE configurations is examined to demonstrate its generality.

Development and Performance Measurements of a Beta-Type Free-Piston Stirling Engine Along With Dynamic Model Predictions

This paper presents the development and performance measurements of a beta-type free-piston Stirling engine (FPSE) along with dynamic model predictions. The FPSE is modeled as a two degrees-of-freedom (2DOF) vibration system with the equations of motion for displacer and piston masses, which are connected to the spring and damping elements and coupled by working pressure. A test FPSE is designed from root locus analyses and developed with flexure springs and a dashpot load. The stiffness of the test springs and the damping characteristics of the dashpot are identified through experiments. An experimental test rig is developed with an electric heater and a water cooler, operating under the atmospheric air. The piston dynamic behaviors, including the operating frequency, piston stroke, and phase angle, and engine output performance are measured at various heater temperatures and external loads. The experimental results are compared to dynamic model predictions. The test FPSE is also compared to a conventional kinematic engine in terms of engine output performance and dynamic adaptation to environments. Incidentally, nonlinear dynamic behaviors are observed during the experiments and discussed in detail.

Stability, Control, and Constraint Enforcement of Piston Motion in a Hydraulic Free-Piston Engine

A free-piston engine (FPE) requires stabilizing feedback control to manage piston motion and ensure safe operation. A high-fidelity model, which simulates the FPE behavior by combining dynamic, thermodynamic, and hydraulic submodels, is exploited as an engine surrogate for control development. A control-oriented model implicitly describes the FPE piston turnaround position using energy balance principles for an idealized Otto cycle. Local eigenvalues of the control-oriented model suggest that the FPE system is open-loop unstable. The control system relies on dynamic inversion and state feedback to stabilize the plant and track a target piston turnaround position. A robust reference governor is introduced to manage load changes, such that the piston position does not violate constraints, even in the presence of parameter uncertainty. In high-fidelity simulations, the reference governor constrains piston turnaround position to ±0.5 mm of the target during a load change.

In-cylinder heat transfer and gas motion of a free-piston diesel engine generator

The free-piston engine generator is an attractive alternative to the conventional reciprocating engine due to the feature that it moves without crankshaft system. This paper presented a simulation for the investigation on the characteristic of in-cylinder gas motion and heat transfer in a compression ignited free-piston engine generator. An operation experiment was performed to obtain the precise piston motion for the modeling of heat transfer and gas flow. The development of the multi-dimensional model was described, and simulation results were presented and showed good similarity with the experimental data. Then, the heat transfer and gas motion in the free-piston engine generator were discussed, on which the influences of piston motion were also investigated compared with a corresponding conventional reciprocating engine. The results indicated that compared with the conventional reciprocating engine, a higher level of squish and reverse squish effect was found for the free-piston engine generator due to its faster motion around top dead center, while its slower piston motion led to weaker gas turbulence in the compression process. Moreover, the free-piston engine generator and conventional reciprocating engine did not show a significant difference in heat transfer during the compression process, however, an obvious advantage of heat transfer was indicated for the free-piston engine generator in combustion and expansion processes due to its lower combustion temperature and the reduced time that is available for heat transfer caused by its faster expansion. The mechanism for such differences is that the free-piston engine generator moves with uneven equivalent speed.

Parametric study on the starting of a free-piston engine alternator

The free-piston engine alternator is a new crankless engine. The traditional starting method cannot be used to conveniently initialize the new engine. This article introduced a resonance starting m...

Sensorless control of a linear reciprocating switched-reluctance electric machine

It is proposed to use a simple switched-reluctance electric machine that does not have windings or permanent magnets on a movable part to develop a linear reciprocating electric machine intended for use as an electric generator together with a free-piston internal-combustion engine. It is noted that one of the most difficult problems in the development of such a linear electric machine is the development of a control system. A sensorless control algorithm is considered. Information on the stator-phase inductance of the electric machine and its variation is used to determine the position of the movable part. For its estimation, probing voltage pulses of known duration are applied to the phase. The amplitude of the current pulses caused by them will be proportional to the phase inductance. A mathematical model of electromagnetic processes is used to test the efficiency of the proposed control algorithm and determine appropriate control parameters. Since stator phases have separate magnetic circuits and are not magnetically connected, the electromagnetic processes in each phase are considered independently. Model parameters are determined experimentally using an experimental prototype of the considered electric machine, for which the dependences of the phase flux linkage and the generated force on the phase current for different positions of the movable part are obtained. The results of the investigation of processes in the considered electric machine at different frequencies of the movable part are given. It is found that, at frequencies of the movable part close to the nominal frequency, the phase should be connected to the power supply even before the movable part reaches the extreme position. Control using probing pulses applied to the operating phase is impossible, because the level of these pulses does not reach the maximum value until the phase is switched on. In this case, it is necessary to use the probing pulses of another, nonoperating phase to determine the position of the movable part. Such an algorithm makes it possible to control a linear reciprocating switched-reluctance electric machine at both low and high frequencies of the movable part. The obtained results confirmed the correctness of the adopted approaches to the development of a sensorless control algorithm.