Experimental study on the performance of single-piston free-piston expander—linear generator

Highlights: • The in-cylinder pressure and gas force are analyzed. • The concept of offset displacement is proposed. • One-way time is introduced and the effect of the intake time is analyzed. • Indicated work, electric energy and conversion efficiency in single cycle are analyzed. In this study, a single-piston free-piston expander—linear generator test system for the power-electricity conversion is presented using compressed air as the working fluid. This study investigated the effect of intake time and intake pressure on the performance of the test system. Examples include the following: in-cylinder pressure, bias displacement, one-way time, indicated work, output electric energy, and work-electricity conversion efficiency in a single cycle using the position-time control method with the intake position of 21and 45 mm. Result shows that: in the operational process of the test system, all values obtained when the piston runs from top dead center to bottom dead center are all different compared with those obtained when the piston runs from bottom dead center to top dead center. When the test system operates with low intake pressure and short intake time, the working chamber without a piston rod will over-expand. With the increase of intake time or intake pressure, the bias displacement, indicated work, and one-way output electric energy show an increasing trend. Moreover, the one-way time shows a decreasing trend and eventually tends to a fixed value, and the one-way work-electricity conversion efficiency initially increases and then decreases.