Enhancing energy efficiency of free-piston engine generator through the controllable cycle method: Analysis of energy gain and consumption

The free-piston engine generator is a promising alternative to the traditional crankshaft engine, particularly for hybrid electric vehicles. By eliminating the traditional crankshaft mechanism, it allows the piston to move freely with a variable trajectory from top dead center to bottom dead center. Leveraging this unique feature, this paper proposes the controllable cycle method to improve energy efficiency. The core principle of this method is modulating the engine's thermal cycle to achieve higher indicated work by controlling the piston's dynamic cycle using a linear motor. To evaluate the efficiency improvement contributed by the controllable cycle method, a new metric, net energy gain is introduced, which considers the improvement of indicated work (termed as energy gain) and the electrical energy used for piston motion control (termed as energy consumption). To investigate the feasibility and effectiveness of the controllable cycle method, a dual-motor free-piston engine generator prototype is established. This prototype features a modular arrangement with two linear motors to physically decouple the functions of power generation and piston motion control. Additionally, the reference dynamic cycle function is designed to reflect various piston motion patterns, with the trajectory active control strategy employed to track it. The prototype is tested with various dynamic cycles, and their effects on energy gain and consumption are further quantitatively analyzed. The test results indicate that higher indicated work and energy gain can be achieved by controlling the dynamic cycle with higher frequency, faster compression process, and slower expansion process. However, it is observed that energy consumption increases with higher frequencies, thereby reducing the energy efficiency. Ultimately, through the net energy gain anlysis considering the trade-off between the energy gain and consumption, the optimal dynamic cycle is identified, achieving a net energy gain of 0.98 J and an efficiency improvement of 2.60 %. Overall, the test results demonstrate that the controllable cycle method effectively enhances the energy efficiency of the free-piston engine generator. Furthermore, this method has potential for application in other hybrid power systems that integrate engines and motors.