Abstract
The miniaturization of electrical equipment and popularization of portable devices is an appealing motivation for the development of small-scale heat engines. However, the in-cylinder charge leaks severely as the engine dimension shrinks. The free-piston engine on a small scale provides better sealing than other miniature heat engines. Therefore, a miniature free-piston generator (MFPG) with a single-piston internal combustion engine (ICE) and a voice coil motor (VCM) was proposed in this work. A dynamic model with special attention on the heat transfer and leakage was established accordingly, upon which parametric studies of leakage and its effects on the performance of the MFPG system were performed. Four key parameters, including scavenging pressure, ignition position, combustion duration and piston mass, were considered in the model. The results showed that the mass leakage during the compression decreases with the rise of the motoring current. The indicated thermal efficiency can be improved by boosting scavenging pressure and increase motoring current. The critical ignition position is 2 mm before the top dead center. When ignition occurs later than that, the MFPG system is incapable of outputting power. The chemical to electric energy conversion efficiency is about 5.13%, with an output power of 10~13 W and power density around 4.7~5.7 W/cc.
Highlights
The scavenging pressure psccentre influences scavenging efficiency φsinand mass in. This results in a gentle increase in the effective compression ratio ε ec, and of gas sealed in the cylinder at the exhaust port closing, affecting the mass of fuel a relatively higher combustion pressure to be compensated by the piston movesupplied per cycle mfuel
The operating frequency of the prototype is around 20~24 Hz, which is equivalent to 1200~1440 rpm, the miniature free-piston generator (MFPG) prototype outputs a power of 10~13 W, with a power density around 4.7~5.7 W/cc
This work centers on the influences of the system parameters on characteristics of the in-cylinder mass leakage and the corresponding system performance
Summary
With the reduction in the scale, the heat loss caused by the resulting enormous area-to-volume ratio and the leakage caused by the sealing problem between the piston and cylinder both increase severely, which limits the improvement of engine efficiency. Lungu et al [20] proposed a small-scale high-aspect-ratio free-piston engine Various solutions are proposed for different prototypes, including the usage of sealing parts and lubricant [41], adopting more advanced bonding technology for the sandwich structure [36], applying liquid piston structure [42] and improving the fabrication technology to reduce the gap width [43] Heat loss is another major factor leading to inefficiency or flame extinction of microengines [44,45,46].
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