The Influence of Intake Pressure on the Performance of a Free-piston Hydrogen Engine

Abstract

Background: Free-piston engines have attracted significant research interest as a promising alternative to conventional internal combustion engines due to their simplified mechanical structure and potential for improved efficiency and lower emissions. Early studies primarily focused on scavenging and gas exchange processes, highlighting the strong dynamic coupling between piston motion, in-cylinder flow characteristics, and scavenging performance resulting from the absence of a crank–connecting rod mechanism. Aims: This paper selects the free-piston hydrogen engine as its research subject, concentrating on investigating the coupled impacts of intake pressure on in-cylinder combustion, airflow dynamics, heat transfer, and emissions, with the aim of identifying the optimal operating conditions. Study Design: A simulation model was developed by employing a coupled dynamics and thermodynamics methodology, and iterative computations were utilized to dynamically update the piston motion trajectory and combustion model. Subsequently, a three-dimensional model was constructed, incorporating the ECFM 3Z combustion model and the Han Reitz heat transfer model, while the extended Zeldovich mechanism was adopted to delineate NO formation. Ultimately, the reliability of the simulation model was validated through a comparative analysis of in-cylinder pressure and heat release rate, thereby laying a groundwork for subsequent parametric investigations. Methodology: This article conducts research on parameters such as combustion, heat transfer, and emissions under different intake pressures, aiming to obtain the optimal operating conditions. Results: When the intake pressure increased from 0.10 MPa to 0.18 MPa, the total heat transfer increased from 35.72 J to 38.83 J. The multi-objective evaluation results in terms of power performance, fuel economy, and emissions indicate that both the indicated mean effective pressure (IMEP) and indicated thermal efficiency exhibited increasing trends. Specifically, the IMEP increased from 0.5866 MPa to 0.6446 MPa, while the indicated thermal efficiency increased from 41.999% to 45.329%. In addition, with increasing intake pressure, the NO mass fraction increased significantly, whereas the soot mass fraction continuously decreased. Conclusion: This article employs the linear weighted sum method to conduct multi-objective optimization analysis for various operating conditions. The optimization results indicate that within the studied operating range, the comprehensive evaluation result is optimal when the intake pressure is 0.16 MPa. However, due to certain limitations in both the research scope and computational workload of this article, further detailed analysis of the aforementioned range was not conducted.