Fei Zhao

Dynamic simulation of a novel free-piston linear generator

This paper proposes a new structure of free-piston linear generator (FPLG) which consists of double two-strokes combustion chambers with inner and outer piston-couples and a linear electric generator with multi-movers in order to improve the system efficiency. The one-dimensional thermodynamics model of the combustion chambers and the model of the linear electric generator have been respectively derived mathematically. And the hybrid simulation model of the whole FPLG has also been established using MATLAB/SIMULINK. The features and operation process with basic igniting and scavenging strategy have been analyzed. It indicates that the novel FPLG has comparative potential and capacity for producing higher electric power and reaching higher power density.

Vibration analysis of highl-speed ball-screw drive in machine tool feed system

This paper studies the dynamic response of a high-speed ball-screw drive in a machine tool feed system. The Timoshenko beam theory is employed to model the rotating screw, while the nut is modeled as a lumped moving mass subjected to an external force resolved into three mutually perpendicular components. The dynamic model of the ball-screw drive is formulated based on the assumed mode method. To study the deformation characteristics of the ball-screw drive with different constrains, the dynamic equations are solved by using the fourth order Runge-Kutta method. In this paper, the moving mass and force effect are taken into consideration synchronously, and the relative velocity relationship of screw and nut is studied as well. The results show that the pitch of the screw, which determines the relative speeds of the rotating beam with respect to the moving excitation, have strong influences on the lateral deformation. Major factors affecting the ball-screw drive deformation are identified to illustrate its vibration performance.

Fault recovery control strategy of a two-stroke free-piston linear generator

This paper proposes a recovery control strategy of a two-stroke free-piston linear generator. The recovery process is implemented through switching the linear electric machine into motoring mode once certain unstable operation like misfire occurs during stable cycles. Various operation states are illustrated. Both the steady operation strategy and the fault recovery strategy are analyzed and simulated. The analysis experiments results show that the proposed recovering strategy is feasible and easy to be implemented. The unstable situation can be effectively recovered during 20ms or less, and has no negative influence on the next stable operation.

DC bus voltage control of a Free-Piston Linear Generator

This paper presents a DC-bus voltage regulation strategy to rectify the fluctuated AC voltage into stable DC-bus voltage for the Free-Piston Linear Generator (FPLG) in stable operation state. Considering that the frequency and amplitude of the AC voltage change rapidly with the nonsinusoidal piston velocity, a three-phase PWM rectifier topology together with its controller are presented to stabilize the DC-bus voltage. The simulation is conducted with the overall coupled dynamic system in which the models of the internal combustion engine thermodynamics, linear electric machine and mechanical spring are all included. The simulation results show that the proposed method can stabilize the dc bus voltage around 400V and ensure stable operation of the FPLG.

Study on breakdown voltage and minimum ignition energy of sparking system for free-piston linear generator

This paper is aimed at studying the breakdown voltage and minimum ignition energy of sparking system for the free-piston linear generator (FPLG). Combing the ignition characteristics and demands specifically for the FPLG system, the breakdown voltage and conduction duration of the primary coil are determined theoretically with the Townsend and Paschen Laws. A computational fluid dynamics (CFD) mode is developed to investigate the in-cylinder pressure and temperature at various igniting moments. The theoretical minimum ignition energy is determined with the laminar combustion theory, which contributes to the design and optimization of the sparking system. The effects of the ignition points and initial in-cylinder temperature on the breakdown voltage and minimum ignition energy are analyzed. The results show that advanced ignition contributes to decreasing the breakdown voltage and the conduction duration of the primary coil, meanwhile has no negative influence on the minimum ignition energy. However, rising initial in-cylinder temperature lowers the breakdown voltage, shortens the conduction duration of the primary coil and reduces the minimum ignition energy.

Coupling thermodynamics modeling and analysis of a free piston linear generator range-extender for HEVs

This paper proposes a concept of free piston linear generator (FPLG) range-extender for the application of hybrid electric vehicles (HEVs). The coupling thermodynamics model of the FPLG range-extender is developed mathematically. Its stable operation characteristics are analyzed accordingly. In addition, the effects of the variations of the key operational parameters on the effective output power and overall efficiency are investigated. The simulation results verify that the designed FPLG range-extender can provide 15kW effective charging power with the overall efficiency of 38.2%, and the peak power can even reach as high as 47.5kW. It is a promising alternative range-extender of the HEVs that can dramatically enhance the battery life.

Effects of initial temperature on the performances of free piston linear generator

This paper investigates the influences of the initial temperature on the performance of free piston linear generator (FPLG). Based on multi-dimensional combustion model, the effects of initial temperature on in-cylinder pressure, in-cylinder temperature and NOx emissions are studied. The cause of NOx in internal combustion engine is introduced and the NOx formation mechanism of gasoline FPLG is explored. It is found that reducing initial temperature can rapidly increase in-cylinder pressure, including transient pressure, peak pressure and the maximum pressure rising rate. However, the in-cylinder temperature linearly rises up with the increasing initial temperature. And the NOx emissions increases accordingly. In order to improve output power and reduce emissions, it is feasible to decrease initial temperature properly.