Experimental and numerical study on the performances of a free-piston engine generator using ammonia and methane fuel mixtures

The opposed-piston free-piston engine generator (FPEG) has high energy conversion efficiency, but its thermodynamic performance is significantly affected by various system parameters owing to the absence of mechanical constraints. This paper introduces a novel approach to predicting the performance of opposed-piston FPEG by leveraging a comprehensive thermodynamic model with a stable operating process and a quadratic regression model based on response surface methods incorporating both design and operating parameters. The established models were verified using variance analysis, enabling the evaluation of the statistical significance of various independent parameters. Furthermore, key factors were identified, and their coupling effects on crucial indicators like indicated work, indicated thermal efficiency, and power output were thoroughly analyzed. A Multi-Objective Evolutionary Algorithm based on Decomposition was proposed to maximize the thermodynamic performance, and the optimal parameter set was obtained from the Pareto solution by means of three decision-making methods. The findings indicate that a higher power cylinder scavenging pressure and rebound cylinder base pressure, lower ignition position and rebound cylinder diameter, along with an appropriate optimal value of power cylinder diameter, effectively enhanced the overall system performance of the opposed-piston FPEG. The optimized maximum indicated work, indicated thermal efficiency, and power output were found to be 395.7 J, 64.74%, and 5.10 kW, respectively. In conclusion, the integration of response surface methodology, multi-objective optimal algorithms, and decision-making methods was demonstrated to be an effective strategy for designing the comprehensive parameters of the opposed-piston FPEG system, leading to a significant enhancement in overall system performance. 

Performance prediction and parameters optimization of an opposed-piston free piston engine generator using response surface methodology

Linear engines, or widely called free piston engines, have their advantages compared to conventional engines with crankshafts, for example, lower friction loss, compact structure, fuel adaptability, flexible compression ratio, etc. Among the linear engine conceptual designs and prototypes, Linear Engine Generator (LEG) incorporating a linear engine (internal or external combustion) and a linear alternator provides an alternative and attractive solution for hybrid electrical powertrains in transport applications. Due to the advantages of linear engines, hydrogen, synthetic hydrogen carrier fuels and biofuels are easier to adopt on LEG, making it an appealing technology for decarbonisation. This paper explicitly reviews LEG research and development on state-of-art solutions and recent progress in linear engine improvement, linear alternator design optimisation, and integrated LEG system control. Key performance indicators of the linear engine generator, like power output, thermal and electrical efficiency, etc., are summarised to present a general picture of up-to-date LEG development. Recommendations for further LEG development are provided at the end of the paper regarding the review outcomes.

Recent progress on performance and control of linear engine generator

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.

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

The unique motion profile of Free Piston Engine Generators (FPEG) results in a brief dwell period at the top dead centre (TDC). Effectively achieving swift combustion within the cylinder during this limited interval is an urgent challenge that needs to be solved. In this study, a three-dimensional simulation model of FPEG was established and the model underwent calibration utilizing experiment data. Subsequently investigating the impacts of varying ignition positions, distinct spark plug spacing, and distribution on the combustion of the FPEG when subjected to the dual spark plug ignition configuration. The research results show that with the advancement of the ignition position, the maximum combustion duration of the dual spark plug FPEG was shortened by 34 %. With a reasonable spark plug spacing, the dual spark plug FPEG could shorten the flame propagation distance, which is conducive to the realization of constant volume combustion. When the dual spark plugs were arranged on both sides of the auxiliary intake port, the peak pressure in the cylinder could reach up to 49.5 bar. When the dual spark plugs were arranged in the main intake port and the auxiliary intake port, the indicated thermal efficiency reached 34.7 %. 

Numerical investigation of the impact of dual spark plug coordinated ignition strategy on the combustion process in a free piston engine generator

High-efficiency and low-emission combustion in methanol-fueled linear range extender: experimental investigation of injection and ignition timing effects

Effect of key parameter on the energy consumption and start-up time in the cold start-up process of an opposed-piston free-piston linear generator

Generation characteristics and power quality control of a free piston linear generator considering system uncertainty

Enhancing energy efficiency of free-piston engine generator through the controllable cycle method: Analysis of energy gain and consumption

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