自由活塞内燃发电机低速不稳定性影响因素分析

单缸低速试验表明,自由活塞内燃发电机低速模式缸内燃烧并不稳定,呈间隔循环着火。为探索其原因,构建了系统CFD(计算流体力学)模型,对比分析了自由活塞内燃发电机换气过程与传统二冲程内燃机换气过程之间的差异,通过计算解释了试验中着火不稳定的现象,并得到了换气过程中影响换气效果的关键因素是扫气箱容积变化率的结论。随后探寻改善自由活塞内燃发电机低速不稳定性的设计策略,在计算模型中进一步研究了动子(磁棒)直径、扫气箱容积、进气增压压力对换气过程的影响。研究发现:减小动子直径与缸径比、对进气增压能显著改善自由活塞内燃发电机的换气效果,而在保证提供充分混合气量基础上扫气箱容积的改变对换气效果影响不大。

Analysis on influence factors of low-speed instability in free-piston engine-generator

Abstract: Free-piston engine generator (FPEG) contains two gasoline engines and a linear generator, which has attracted considerable research interest for its potential application to electricity generation. The device has low mechanical loss and variable compression ratio which means energy conservation and multiple fuel adaptability. Previous studies have found that the scavenging process was vital to the stable operation of the FPEG system. In order to know the features, an experiment was carried out to analysis the scavenging process of FPEG. In the experiment, both the pressure in cylinder and in scavenge box were tested. Experiment results showed that: the combustion process of FPEG variated cycle to cycle at low speed operation condition for the instability of scavenging process, while in traditional mode the engine performed very well. Computational fluid dynamics (CFD) model designed according to prototype was introduced to analysis the difference between FPEG and traditional two-stroke engine in scavenging process. Calculation results showed that: the relative low scavenging box volume variation rate of FPEG was the effective factor on the scavenging efficiency, which led to low-speed instability. And the difference of scavenging structure between FPEG and traditional engine was responsible for it. In order to optimize the scavenging system, operation parameters including diameter of piston assembly, volume of scavenging box and charging pressure were investigated. Results showed that: the low proportion of piston assembly diameter on bore and high inlet pressure effectively improved the scavenging efficiency. Especially, when the proportion of piston assembly diameter on bore was less than 0.4, the difference of scavenging efficiency of FPEG and traditional engine was less than 6%.