Abstract
•Starting strategies of free-piston engine generator by linear electric machine is investigated.•A novel method to start the engine by mechanical resonance is proposed.•A detailed comparison of the implementation of open-loop and close-loop strategies.•A detailed discussion of how the motor force can influence the starting process and the following combustion process.
Two general methods to start the engine are investigated by the linear electric machine operating as a linear motor and identified one for a specific FPEG prototype configuration. Based on that, a novel method to start the engine by mechanical resonance is proposed. Both simulation and test results are presented, and the numerical model is successfully validated. The results show that with a fixed motor force of 110N, within 0.5s, the maximum in-cylinder pressure could reach 13bar, indicating that the engine is ready for ignition. Further investigation on the engine performance with the closed-loop control strategy is carried out. The results demonstrate that it is feasible to start the FPEG with mechanical resonance with a closed-loop controlled electric linear machine that applies a proper constant motor force in the direction of the natural bouncing motion. With different starting motor force, the top dead centre (TDC) value for both cylinder is different during the first few running cycles, but the difference reduces and tends to be zero during the stable resonance state. There is not any significant difference observed on the engine frequency and piston profile during combustion process.