A Free-Piston Linear Generator Control Strategy for Improving Output Power

Abstract

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.

A Review of the Design and Control of Free-Piston Linear Generator

“…When the FPLG has two cylinders, the piston motion is mainly controlled by LEM during the starting process. Some studies focused on the starting process using position feedback or the constant-magnitude motoring force approach [37,[42][43][44][45]. In order to obtain a desired piston trajectory, the position error serves as a feedback variable to control the thrust force with appropriate value and direction [43].…”

“…More importantly, it will affect the compression process, scavenging process, combustion quality, and further produce cycle-to-cycle variations. In particular, under limiting operation conditions such as the dead positions, piston displacement and velocity, the state of reciprocation motion depends on the input and output energy conversion [37,38]. Where the input energy is from fuel burning and the output energy is produced by the resultant force in a cycle.…”

“…At the same time, the dead center positions of piston and bounce pressure were set at the supervisory control level, and they were controlled at piston motion level based on the signal from the low-level control. Due to the good logical structure of the multi-layer control system, this control framework is modified and implemented to control free piston motion by many works of research literature [7,37,49]. In the following study, the predictive piston motion method was proposed to improve dynamic performance on the top dead center and compression ratio, as shown in Figure 22 [50].…”

“…X. Gong et al studied nonlinear control scheme and proposed a model predictive control (MPC) strategy of dual-cylinder FPLG [49,[58][59][60][61][62]. The researchers built a nonlinear, discrete- In [37,38], considering the acceleration and deceleration characteristic of piston motion, the ladder-like reference electro-magnetic force control strategy was implemented to control piston dead centers, as shown in Figure 28. In a stroke, reference current is divided into two parts: small reference current in acceleration zone, and larger reference current in deceleration.…”

Optimal Design of Permanent Magnet Linear Generator and Its Application in a Wave Energy Conversion System

“…To achieve relatively high quality and efficiency for converting ocean wave energy into electrical energy, various ocean wave energy converters and corresponding electrical machines have been investigated. It was found that linear generators can be used in the direct drive wave energy conversion system, in which it can reduce the complexity of mechanical transmission and improve the reliability and efficiency of the system [6][7][8][9][10][11]. Several electrical generator topologies have been analyzed in the past years [12,13].…”