Free-piston Engine Linear Generator Research Articles

Research on Piston Dynamics and Engine Performances of a Free-Piston Engine Linear Generator Coupling with Various Rebound Devices

Free-piston engine linear generators (FPELGs) are an innovative linear power device that exhibits the distinctive dynamics of pistons and performance of free-piston engines. Furthermore, the single-cylinder/single-piston FPELG structure type has more advantages than other FPELG structure types, including a straightforward structure and ease of control. However, when coupled with various rebound devices, the operational characteristics and piston and engine performance of single-cylinder/single-piston FPELGs are quite different. Therefore, this paper aims to quantitatively compare the dynamics of the piston and engine performance of single-cylinder/single-piston FPELGs coupled with various types of rebound device. The results indicate that when the equivalent stiffness of the gas spring is greater than that of the mechanical spring, the operating frequency of the piston of the FPELG coupled with a gas spring will be higher than that when coupled with a mechanical spring. During the compression stroke, the piston velocity of a FPELG coupled with a mechanical spring changes linearly, while the piston velocity of a FPELG coupled with a gas spring changes nonlinearly. FPELGs coupled with gas springs have shorter compression and expansion durations compared to those coupled with mechanical springs. In addition, the indicated powers of FPELGs coupled with ideal gas springs and mechanical springs are 1.5 kW and 1.3 kW, respectively. However, due to leakage, the thermal efficiency of a FPELG coupled with an actual gas spring is reduced by approximately 2.5% compared with the FPELG coupled with the ideal gas spring. Furthermore, the operation frequency of the piston is positively correlated with the stiffness of the mechanical spring. In addition, as the stiffness of the mechanical spring increases, the combustion process of the engine becomes close to an isovolumetric process. The changes in piston dynamics and engine performance when increasing the initial gas pressure of the gas spring are similar to those when increasing the stiffness of the mechanical spring.

Combustion Characteristics of a Free Piston Engine Linear Generator using Various Fuel Injection Durations

Combustion Characteristics of a Free Piston Engine Linear Generator using Various Fuel Injection Durations

Development of a coupling model and parametric analysis of an opposed free-piston engine linear generator

The opposed free-piston engine linear generator shows unique dynamic characteristics due to the elimination of the complex crankshaft and connecting rod mechanism. This paper investigated the behaviors of the design parameters and operating conditions on engine performance based on the simulation and experimental results. Considering the unique dynamic characteristics and the coupling of free-piston engine and linear generator, a numerical model was established and validated with prediction errors of 2 %. According to the experimental results, an optimal ignition position of 27 mm was obtained, and the both of indicated power and indicated work achieved their maximum value of 1.85 kW and 87.37 J improved by 17.1 % and 20.9 %, respectively. Further parametric analysis about the design parameters was carried out based on the coupling model. Simulation results showed that the brake thermal efficiency could be improved significantly from 31.95 % to 36.06 % considering the coupling effects of the piston assembly mass and the stroke length. Additionally, higher indicated power could be obtained with a shorter stroke length and lower piston assembly mass.

Operation Range of Intermittent Velocity Control for Improving Top-Dead-Center Accuracy in Dual-Sided Free-Piston Engine Linear Generator

Operation Range of Intermittent Velocity Control for Improving Top-Dead-Center Accuracy in Dual-Sided Free-Piston Engine Linear Generator

Optimisation of operating parameters on the performance characteristics of a free piston engine linear generator fuelled by CNG–H2 blends using the response surface methodology (RSM)

The free piston engine linear generator (FPELG) is a simple engine structure with few components, making it a promising power generation system. However, because the engine works without a crankshaft, the handling of the piston motion control (PMC) is the main challenge influencing the stability and performance of FPELGs. In this article, the optimal operating parameters of FPELG for maximising engine performance and reducing exhaust gas emissions were studied. Moreover, the influence of adding hydrogen (H2) to compressed natural gas (CNG) fuel on FPELG performance was investigated. The influence of operating parameters on in-cylinder pressure was also analysed. The single-piston FPELG fuelled by CNG blended with H2 was used to run the experiments. The response surface methodology (RSM), including the central composite design (CCD), was used. Then, adequacy models were developed and verified by ANOVA. Three independent factors on seven responses were utilised for optimisation. Results showed that the optimal operating conditions of lambda, ignition velocity, and injection position were 0.96, 0.53 m/s, and −14.9 mm, respectively. The best-predicted values were as follows: indicated mean effective pressure (IMEP) of 7.6 bar, in-cylinder pressure of 27.87 bar, combustion efficiency of 39.64%, CO of 9531.41 ppm, CO2 of 2.4%, HC of 551.75 ppm, and NOX of 113.737 ppm. Furthermore, results showed that the experimental data could be fitted well with the predicted quadratic model.

Experimental Analysis for the Influence of Ignition Time on Combustion Characteristics of a Free Piston Engine Linear Generator

Free Piston Engine Linear Generator (FPELG) is a modern engine and promising power generation engine. It has many advantages compared to conventional engines such as less friction, few numbers of parts, and high thermal efficiency. The cycle-to-cycle variation one of the big challenges of the FPELG because it is influence on the stability and output power of the engine. Therefore, in this study, the effect of ignition time on combustion characteristics is investigated. The single-cylinder FPELG with spark ignition (SI) combustion type by using compressed natural gas (CNG) fuel type was set to run. LabVIEW is used to run the engine and control of input parameters. All experimental data have been collected and processed based on LabVIEW and Macro tool. The analysis results of the experimental based on ignition time show that the in-cylinder pressure with double peak shape was produced when the ignition delay after Top Dead Centre (TDC). On the other hand, the in-cylinder pressure and output power with ignition before TDC is higher in approximately (33.8%, and 17.8% respectively) compared to the in-cylinder pressure with ignition after TDC. Though investigate the influence of ignition time on the combustion characteristics but with further study and investigation for such as the combustion during expansion, exhaust gas temperature, and emission under various ignition time could be achieved in our future work.

Numerical Investigation of a Free-Piston Hydrogen-Gasoline Engine Linear Generator

The free-piston engine linear generator (FPELG) is being investigated by many researchers because of its high thermal efficiency and its variable compression ratio. However, all researchers focused on the FPELG characteristics with mono-fuel. Therefore, in this paper, the performance of the FPELG that has adopted gasoline with hydrogen as fuel is investigated. The method of coupling the zero-dimensional dynamics model with the multi-dimensional CFD (Computational Fluid Dynamics) combustion model was applied during the simulation process. According to the results, the piston TDC (Top Dead Center), the piston peak piston velocity, and the system operation frequency show a negative correlation with the increase of hydrogen fractions. However, the peak in-cylinder pressure was increased with the hydrogen volume fraction increase, due to the fast flame speed and short combustion duration characteristics of hydrogen. Meanwhile, the indicated efficiency of the free-piston engine was increased from 32.3% to 35.3% with the hydrogen volume fraction change from 0% to 4.5%, when the free-piston engine operates at stoichiometric conditions with fixed ignition timing. In addition, with the ignition timing advance increase, the piston TDC was decreased. The peak piston velocity and the peak in-cylinder pressure were in negative correlation with the ignition timing advance. While the engine indicated that the efficiency was increased with the equivalent degree of ignition timing from 20° to 16°. Therefore, the ignition timing of the FPELG under the spark-ignition combustion mode is supposed to be an effective and practical control variable.

Development of free piston engine linear generator system and a resonant pendulum type control method

A free piston engine linear generator, which has the potential of the compact physique, high brake efficiency and high flexibility for fuel, has been developed. The developed free piston engine linear generator consists of a two-stroke combustion unit, an air-bounce chamber and a linear generator. The key technologies to realize the continuous operation are the control method and lubricating and cooling strategies. The proposed structure is featured as a piston shape with two different diameters coaxially, called a “W-shape” piston, which has an empty space inside for the oil cooling path. The performance of the structure is evaluated by a one-dimensional cycle simulation. The result indicates the possible output power of 10 kW and thermal efficiency of 42% using premixed charge compression ignition combustion strategy. The control method is another challenge of the free piston engine linear generator. This work proposes two strategies of the position feedback control method and the resonant pendulum type control method. The first method has the function of the feedback loops for the piston position and velocity so that the piston motion follows the reference profile calculated in advance. The experimental results show the limited range of operation because the fixed profile does not absorb the deviation of the piston motion due to the combustion deviation. The second method is based on the speed control without a fixed reference profile of the piston motion. The experimental result shows the robustness to the change in operating parameters such as ignition position, amount of fuel and desired power output.

Parametric analysis of a dual-piston type free-piston gasoline engine linear generator

Abstract The free-piston gasoline engine linear generator (FPELG) is linear type power device, which couples a linear internal combustion engine and a linear electrical generator together. Free-piston gasoline engine is commonly modeled using simplified zero-dimensional models. In this paper, parametric analysis of a dual-piston type FPELG is presented, aiming to find the piston operation characteristics and the performances of the FPELG during the generating process. Model validation was undertaken with the test data from a running prototype, which showed good agreement. The results show that with fixed stroke, lower moving mass would lead to higher indicated power. With fixed moving mass, shorter stroke would make higher indicated power. The FPELG was observed to be operated at high load, and it was prone to operate at relative low speed. The influence of combustion parameters, i.e. the ignition timing and the combustion efficiency to the FPELG engine performance were also investigated respectively.

Effect of Copper Loss Reduction with Output Dispersion in a Free-Piston Engine Linear Generator System

Effect of Copper Loss Reduction with Output Dispersion in a Free-Piston Engine Linear Generator System

Operation Range of Generation Braking Force to Achieve High Efficiency Considering Combustion in a Free-Piston Engine Linear Generator System

Operation Range of Generation Braking Force to Achieve High Efficiency Considering Combustion in a Free-Piston Engine Linear Generator System

Design of a Linear Synchronous Generator and Examination of the Driving Range for a Free-Piston Engine Linear Generator System

Design of a Linear Synchronous Generator and Examination of the Driving Range for a Free-Piston Engine Linear Generator System

Research on the engine combustion characteristics of a free-piston diesel engine linear generator

The free-piston diesel engine linear generator (FPDLG) shows a great deal of merits due to its unique structure and operation characteristics. For this reason, most researchers mainly investigated on the FPDLG characteristics. However, there has not been any report on the engine operation characteristics. So this paper focuses on the engine operation characteristics of the FPDLG during the generating process. According to the experimental results, it is found that the output power of the designed prototype can reach 5.16 kW with the thermal efficiency of up to 38.5%. Experimental test results are obtained and processed both in time and crank-angle coordinates. Compared to the conventional diesel engines, the ignition delay of the FPDLG is shorter and the premix combustion quality is better. By analyzing the operating cyclical variations of the prototype, the coefficient of variation (COV) of the compression ratio is found to be higher than 27%. While the peak in-cylinder gas pressure and pressure rising ratio cyclical variation can maintain a stable operation (similar to the conventional diesel engines), higher COVs of the compression ratio can always complicates the engine combustion. Therefore proper control strategies are necessary for the FPDLG.

Effect of fuel injection characteristics on the performance of a free-piston diesel engine linear generator: CFD simulation and experimental results

The free-piston diesel engine linear generator (FPDLG), as a novel energy conversion device, is investigated by many researcher groups. The injection characteristics are important for the FPDLG operation performance. Therefore, the effects of fuel injection characteristics on the FPDLG are researched by CDF simulation and experimental results in this paper. According to the results, it is found that the trends of the compression ratio and the peak in-cylinder pressure vs the injection timing are the identical with convex function curves. And the IMEP of the FPDLG also show the same trend. Although the heat-release rises in the rapid combustion period, the IMEP vs the injection timing various law present convex function curve on account of reduced heat-release in the normal combustion period. Four injection rate-profiles, i.e. rectangle, wedge, trapezium and triangle are simulated and compared. It is observed that the combustion process with the trapezium rate-profile curve is smoother. The peak in-cylinder pressure and the thermal efficiency can be kept high value while the center of heat-release curve near the TDC. Therefore the trapezium rate-profile is suggested is appropriate and can keep high efficiency of the FPDLG in order to get high engine efficiency.

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

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.

Research on the Combustion Characteristics of a Free-Piston Gasoline Engine Linear Generator during the Stable Generating Process

The free-piston gasoline engine linear generator (FPGLG) is a new kind of power plant consisting of free-piston gasoline engines and a linear generator. Due to the elimination of the crankshaft mechanism, the piston motion process and the combustion heat release process affect each other significantly. In this paper, the combustion characteristics during the stable generating process of a FPGLG were presented using a numerical iteration method, which coupled a zero-dimensional piston dynamic model and a three-dimensional scavenging model with the combustion process simulation. The results indicated that, compared to the conventional engine (CE), the heat release process of the FPGLG lasted longer with a lower peak heat release rate. The indicated thermal efficiency of the engine was lower because less heat was released around the piston top dead centre (TDC). Very minimal difference was observed on the ignition delay duration between the FPGLG and the CE, while the post-combustion period of the FPGLG was significantly longer than that of the CE. Meanwhile, the FPGLG was found to operate more moderately due to lower peak in-cylinder gas pressure and a lower pressure rising rate. The potential advantage of the FPGLG in lower NOx emission was also proven with the simulation results presented in this paper.

Study of the Injection Control Strategies of a Compression Ignition Free Piston Engine Linear Generator in a One-Stroke Starting Process

For a compression ignition (CI) free piston engine linear generator (FPLG), injection timing is one of the most important parameters that affect its performance, especially for the one-stroke starting operation mode. In this paper, two injection control strategies are proposed using piston position and velocity signals. It was found experimentally that the injection timing’s influence on the compression ratio, the peak in-cylinder gas pressure and the indicated work (IW) is different from that of traditional reciprocating CI engines. The maximum IW of the ignition starting cylinder, say left cylinder (LC) and the right cylinder (RC) are 132.7 J and 138.1 J, respectively. The thermal-dynamic model for simulating the working processes of the FPLG are built and verified by experimental results. The numerical simulation results show that the running instability and imbalance between LC and RC are the obvious characters when adopting the injection strategy of the velocity feedback. These could be solved by setting different triggering velocity thresholds for the two cylinders. The IW output from the FPLG under this strategy is higher than that of adopting the position feedback strategy, and the maximum IW of the RC could reach 162.3 J. Under this strategy, the prototype is able to achieve better starting conditions and could operate continuously for dozens of cycles.

State Estimation Algorithm Based on a Disturbance Observer for a Free-Piston Engine Linear Generator

In order to realize a thin, compact, efficient, and fuel-flexible power-generation system, we have been developing and investigating a free-piston engine linear generator (FPEG), which consists of a two-stroke combustion chamber and an adjustable gas spring chamber. This paper proposes a novel observer algorithm that has two advantages. First, the piston position can be interpolated at high accuracy beyond the resolution of the piston position sensor, which improves the accuracy and stability in controlling the combustion parameters, such as ignition timing and compression ratio. Second, the pressure in the gas spring chamber and the coefficient of damping (lubrication condition) are estimated simultaneously and independently. In this paper, the principle of the developed observer algorithm is described and tested through simulation and experimentation using the developed FPEG system. Based on the simulation, the output of the position sensor (resolution: 0.55 mm) was interpolated with an accuracy of ±0.1 mm. Based on the experimental results for the estimation of pressure in the gas spring chamber and the coefficient of damping, the feasibility of sensorless monitoring of the pressure in the gas spring chamber and the coefficient of damping has been verified.

Stable Operation and Electricity Generating Characteristics of a Single-Cylinder Free Piston Engine Linear Generator: Simulation and Experiments

We present a novel design of a single-cylinder free piston engine linear generator (FPELG) incorporating a linear motor as a rebound device. A systematic simulation model of this FPELG system was built containing a kinematic and dynamic model of the piston and mover, a magneto-electric model of the linear generator, a thermodynamic model of the single-cylinder engine, and a friction model between the piston ring and cylinder liner. Simulations were performed to understand the relationships between pre-set motor parameters and the running performance of the FPELG. From the simulation results, it was found that a motor rebound force with a parabolic profile had clear advantages over a force with a triangular profile, such as a higher running frequency and peak cylinder pressure, faster piston motion, etc. The rebound position and the amplitude of rebound force were also determined by simulations. The energy conversion characteristics of the generator were obtained from our FPELG test rig. The parameters of intake pressure, motor frequency, and load resistance were varied over certain ranges, and relationships among these three parameters were obtained. The electricity-generating characteristic parameters include output power and system efficiency, which can measure the quality of matching the controllable parameters. The output power can reach 25.9 W and the system efficiency can reach 13.7%. The results in terms of matching parameters and electricity-generating characteristics should be useful to future research in adapting these engines to various operating modes.