This paper presents a control strategy to improve the output power for a single-cylinder two-stroke free-piston linear generator (FPLG). The comprehensive simulation model of this FPLG is established and the operation principle is introduced. The factors that affect the output power are analyzed theoretically. The characteristics of the piston motion are studied. Considering the different features of the piston motion respectively in acceleration and deceleration phases, a ladder-like electromagnetic force control strategy is proposed. According to the status of the linear electric machine, the reference profile of the electromagnetic force is divided into four ladder-like stages during one motion cycle. The piston motions, especially the dead center errors, are controlled by regulating the profile of the electromagnetic force. The feasibility and advantage of the proposed control strategy are verified through comparison analyses with two conventional control strategies via MatLab/Simulink. The results state that the proposed control strategy can improve the output power by around 7–10% with the same fuel cycle mass.

In an electromagnetic valve actuator (EMVA), precise motion control is desired to achieve fully variable valve actuation, so that the fuel economy, the emissions, and the torque output performance of the engine will be improved. This paper presents an inverse system method (ISM) to achieve precise valve motion control for a moving-coil EMVA. The dynamic model of the EMVA is established, and the inverse system of the EMVA is obtained. By linearization of the non-linear EMVA system, state feedback control is used to control the valve to follow the desired valve motion. A state variable observer is designed to estimate the unmeasurable state variables. Simulations and an experimental set-up are used to demonstrate the effectiveness of the proposed control method. The performance of proportional–integral–derivative control is compared with ISM control in the simulations, and ISM control is robust to large parameter variations of 50 per cent. The experimental results show close agreement with the simulation results. The proposed method proved effective by achieving a transition time of 3.8 ms and a seating velocity of 0.05 m/s. Fully variable valve actuation has been achieved and the valve motion profile is given.

Free-piston engine coupled with linear generator is a new kind of thermoelectric energy converter. It has been highly concerned due to its high efficiency and low emission in recent years. In this paper, a tubular moving-coil linear generator is developed for the application of free-piston engines. The two-dimensional finite-element model of the linear generator is established, and its electromagnetic characteristics are analyzed. A power converter is designed to control the operating state and the electric current of the linear generator. A prototype is developed and used in a free-piston engine with a bore of 102mm, and the main performance parameters are tested. According to the simulated and tested results, the linear generator can realize the stable running of free-piston engines. The generating efficiency at peak power of the linear generator is about 95.2%, the averaged generating efficiency during a working cycle is about 87.5%, the peak power of the linear generator is about 16.77kW, and the averaged generating power of 3.37kW can be achieved at the working frequency of 10Hz.

A free-piston engine is a novel internal combustion engine which has the advantages of a variable compression ratio and multi-fuel adaptability. This paper focuses on numerical simulation for combustion process and spray angle optimization of an opposed-piston free-piston engine. The working principle and spray-guided central combustor structure of the engine are discussed. A three-dimensional computational fluid dynamic model with moving mesh is presented based on the tested piston motion of the prototype. Calculation conditions, spray models, and combustion models were set-up according to the same prototype. The effects of spray angle on fuel evaporation rate, mixture distribution, heat release rate, in-cylinder pressure, in-cylinder temperature, and emissions were simulated and analyzed in detail. The research results indicate that the performance of the engine was very sensitive to the spray angle. The combustion efficiency and the indicated thermal efficiencies of 97.5% and 39.7% were obtained as the spray angle reached 40°.