Research On Operation Stability Of Free Piston Engine Generator

Free piston engine generator (FPEG) is a new type of energy conversion device whichcombined with a free-piston engine and linear generator, due to its advantages on dynamicand economic performance, it has being investigated by a number of research groupsworldwide. Due to the cancellation of the mechanical constraints and storage devices, themovement of piston assembly is controlled by combustion stability in cylinder, so operationstability become one of the difficulties to constraints its further development.In order to investigate the combustion characteristic, a prototype of FPEG wasdeveloped. For its wide operation range, a set of segment control strategy was introducedfor the piston assembly. According to the operating principle, a double Hall sensor positiondetected method was adopted for the piston assembly. A mathematical model of theprototype was built for analysis the force on piston assembly and energy conversion processat the starting and operation condition. In order to avoid pistons hit on cylinder head, asingle-end constrained FPEG experiment was taken on; effect of operation parameters oncombustion stability was researched.The main contents are summarized as follows:(1) Prototype designFirstly, a prototype of FPEG was designed for experiment. The prototype structure wastwo cylinders opposed, two-stroke oprating cycle, and the scavening box was desigened byour group for gas exchange process. In the starting process, the motor works as startermotor, the switching circuit was designed for a steady state switching process. In order toachieve precise control over a wide operating range and higher power conversion efficiency,the throttle opening degree and operation frequence were adopted as stardards for dividinginto segments, and strategies were design for different control target at each operatingcondition.(2) Displacement detection method of piston assemblyPiston assembly is an important component of the combustion chamber, which worksat high temperature and pressure. So, the commonly used sensors like electrical parameterstype, photo-electric type and magnetic type can not be adopted to detect the displacement.Based on the sinusoidal magnetic field of linear motor mover, a pair of Hall sensor wasintroduced to detect the displacement of piston assembly. The Hall sensor is a quarter of thepole pitch away from the other, so the outputs of the sensors are a set of quadrature signals, from which the displacement and movement direction of the piston assembly can be got. Inorder to suppress common mode interference, differential transmission was inoduced. Adifferential amplifier circuit and an adder circuit were adopted for the differential signal,through which the signals canbe input to the microcontroller.(3) Simulation and analysis on the starting and operation process of FPEGFor mathematical model of the prototype was desigend, the simplified expression ofdriving force startup process was derived, and the effect of structural parameters on drivingforce was discussed. After starting process, the prototype transferred into operationcondition. A zero-dimensional model of prototype was used for thermodynamic and kineticmode of operation condition. The modules were coupled with input and output parametersin Matlab/Simulink environment, and thedisplacement, velocity and acceleration of pistonassembly canbe calculated after the model is validated. For the prototype of FPEGdiscussed in this paper, the minimum driving force of the linear motor during startingprocess canbe derived. In the operation conditon; the effect of paramenters on outputcharateristic is analyzed.(4) Resesrch on combustion stability of FPEGIn order to avoid pistons hit on cylinder head, a single-end constrained FPEGexperiment was taken on; effect of operation parameters on combustion characteristic wasresearched. Misfire percent and heat-release cycle variation was adopted as standards toevaluate the low-speed instability of FPEG at different operating parameters.Three-dimensional simulation results show that: the scavenging efficiency is the key factorthat influencing the stablility of combustion process in cylinder, and the mechanicalstructure of scavenging box design led to inadequate ventilation. In order to improve thecombustion process in the cylinder, a two-stage piston scavenging box design wasintroduced.