Performance Comparison of a Single Cylinder and a Dual Cylinder Free Piston Engine

Abstract

Free piston linear engine alternators (FPLEA) may be designed following several different baseline configurations. Common designs include a translator that carries permanent magnets, with either one piston attached to one end of the translator, or a piston at each end of the translator. The single cylinder engine requires a reversing force from a spring so that it can operate, whereas, the dual cylinder version can operate without a spring. However, inclusion of stiff springs would serve to raise the operating frequency and reduces the cycle-to-cycle variations in a dual cylinder version. A matlab/simulink numerical model with translator rod dynamics and in-cylinder thermodynamics was employed to predict the overall performance and efficiency of a FPLEA. This numerical model allowed comparison of different FPLEA configurations for a variety of design variables. First, a dual cylinder FPLEA design was considered where the spring constant was varied, changing the frequency of operation and the motion of the translator. The simulation results showed that without springs the motion was far from sinusoidal, and low in frequency and power, whereas the presence of stiff springs in the system strongly dictated nearly sinusoidal motion and high power at high frequency. Effects of other parameters such as stroke and bore were also examined. Finally, the same tests were carried out for a single cylinder FPLEA design. The simulation results showed that the dual cylinder engine produced twice the electrical power output with higher efficiency than the single cylinder engine for the same cylinder geometry.