Translator Dynamics and Performance Comparison on One and Two Cylinder Free Piston Engines

Abstract

Free Piston Linear Engines and 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, but inclusion of a stiff springs would serve to raise operating frequency. Higher spring constants drive higher frequencies and also reduce the variability of the FPLEA compression ratio. The major component choices include the use of one or two cylinders, a spring constant, a bore and a stroke, and volumetric heat release. For design, the alternator is a component with translating mass that depends by design on the frequency, stroke and electrical power. The alternator demand must be matched to the engine power or the operating condition will change for the next cycle. Though there are many different FPLEA configurations, the performance comparisons of several baseline configurations have not been completely explored. A MATLAB®/Simulink numerical model with translator rod dynamics and in-cylinder thermodynamics is employed to predict the overall performance and efficiency of diesel-fueled FPLEA. This allowed comparisons of different FPLEA configurations for a variety of design variables. First, a two-cylinder FPLEA design is considered where the spring constant is varied, changing the frequency of operation and the motion of the translator. The simulation results show that without springs the motion is far from sinusoidal, and low in frequency and power, whereas the presence of stiff springs in the system strongly dictates nearly sinusoidal motion and high power at high frequency. Further, Fourier coefficients are used to characterize the motion of springs for different configurations. Effects of other parameters such as stroke and bore are also examined. Comparison is also performed for competing designs with the same power, but with one or two cylinders. The results provide a basis for selecting major design parameters before proceeding with a detailed design.