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The free piston linear engine (FPLE) eliminates the crankshaft components and supports variable compression ratio operation. This characteristic makes it as one of the most appropriate choices to adopt HCCI combustion mode. This work aims to reveal the preliminary characteristics of HCCI combustion in FPLE and compare it with traditional crankshaft engines (TCE). The HCCI combustion model of TCE and FPLE was established by using a chemical kinetic reaction mechanism, and it then was numerically calculated to obtain the combustion and emission performance. The results indicate that the typical two-stage reaction characteristic of HCCI combustion also occur in the FPLE. Although the final cumulative heat release of the two types of engines is approximately the same, the low-temperature reaction stage of FPLE comes later, which also leads to the lag of its high-temperature reaction stage. In addition, the TCE features with earlier heat release rate, higher peak pressure and temperature, and longer duration of high temperature, which are beneficial to promote the reaction of soot, thereby slightly reducing soot emissions, but also increasing NO emissions. [ABSTRACT FROM AUTHOR]

Variable stroke (or compression ratio) has been expected as a potential technology for optimizing engine combustion all the time. The free piston motion of linear engines introduced by its unconstrained dynamics is well suited for this expectation. This paper introduces a numerical analysis to explore the effects of variable stroke operation on the combustion and heat release of a linear hydrogen engine (LHE). A system model which couples zero-dimensional dynamics, multi-dimensional combustion, and one-dimensional gas exchange is established and verified experimentally to predict the combustion of the LHE, and then a series of simulations are performed over a range of motion stroke from 62 mm to 72 mm in 2 mm interval to evaluate its effect on LHE combustion. Results indicates that short stroke operation of the LHE shows obvious advantages in thermal efficiency and high peak combustion pressure, although the completion level of hydrogen combustion is slightly poor. Fast combustion, large heat release, and low level of post-combustion effect can be obtained by neither lengthening nor shortening from the certain stroke length of 68 mm, while serious NO emission is indicated. Long-stroke operation makes the LHE clean, although it induces slow engine speed, low thermal efficiency and output power. • The variable stroke effect on hydrogen combusting in LHE was revealed. • The feasibility of using variable stroke technology in LHE was estimated. • The LHE behaves high energy efficiency in short stroke conditions. • Long-stroke operation makes clean combustion for the LHE. [ABSTRACT FROM AUTHOR]

Free-piston engine generator (FPEG) is an unconventional engine; which is characterized by freely moving pistons without crankshaft system. This paper focused on an experimental analysis for the combustion and heat release characteristics of a free-piston diesel engine generator. An experimental prototype was established to test the operation performances of the new engine under different injection conditions. Experimental results show that the actual compression ratio of the FPEG is influenced by the injection advance position (IAP), which decreases with injection position advancing; but the peak in-cylinder gas pressure varies in positive correlation with the IAP. Moreover, the heat release characteristics of the unconventional engine were also analyzed using the experimental data. The results indicate that at the optimum injection position, the ignition delay of the FPEG prototype is 0.86 ms and generally shorter than a corresponding conventional crank engine due to its faster piston motion around the top dead center; and the heat release process has more significant pre-mixed combustion, which differs from the conventional engine situation. The heat release center of the FPEG at all injection position lags a stable time after injection; and the departure time of heat release center (DTHC) is increased significantly with the retarded injection. Meanwhile, a particular feature was found that the optimum efficiency of the FPEG does not occur in the lowest DTHC condition due to its variable compression ratio. [ABSTRACT FROM AUTHOR]

The hydrogen vibration combustion alternator (HVCA), utilizing diesel pilot ignition, represents a novel power system capable of reducing emissions while enhancing combustion efficiency. This study investigates the effects of varying spring stiffness on the dynamics and thermodynamics of the HVCA system. A system-level model was developed and validated with experimental data to explore the influence of spring stiffness changes on piston motion, mixing characteristics, and combustion performance. The results indicate that increasing spring stiffness from 70 kN/m to 78 kN/m led to an increase in maximum piston velocity from 12.4 m/s to 12.9 m/s. However, higher spring stiffness did not significantly improve the mixing of hydrogen and air, with the Sauter Mean Diameter (SMD) increasing from 4.2 m to 6.3 m, resulting in a 2.76% decrease in thermal efficiency. Additionally, NO emissions increased significantly under higher stiffness, while soot emissions decreased. The study also found that lower spring stiffness helped enhance cumulative heat release, reaching 573.6 J at 70 kN/m. This research reveals the critical impact of nonlinear dynamics and thermodynamic coupling on the diesel pilot ignition of hydrogen combustion, providing important insights for further optimizing combustion efficiency and emissions reduction. • Nonlinear asymmetric motion's impact on combustion in a vibration combustor is studied. • The coupling between nonlinear dynamics and combustion characteristics is revealed. • The maximum heat release rate does not appear under the highest spring stiffness. • Appropriate spring stiffness improves the performance of vibration combustors. [ABSTRACT FROM AUTHOR]