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.
Highlights
A free-piston linear generator is a novel energy converter which normally consists of an internal combustion engine (ICE), a linear electric machine (LEM) and a rebounding device [1,2]
Where, Fp is the pressure force of the combustion chamber, Fatm is caused by the atmospheric pressure, G is the gravity acting on the piston assembly, Fsp is the force generated by mechanical spring, Ff is the frictional force between the piston components and the linear guide, and Fmag is the electromagnetic force, M is the mass of the piston assembly, and the displacement between the piston and cylinder head is defined as x for the purpose of describing the piston motion
Where, Ecom is the in-cylinder internal energy generated by the compression of the air pressure when the piston is from bottom dead center (BDC) to top dead center (TDC), xivo is the switch position of the intake valve, x0 is the initial position of piston, P0 is the intake pressure, and V 0 is the initial volume at the end of the scavenging process
Summary
A free-piston linear generator is a novel energy converter which normally consists of an internal combustion engine (ICE), a linear electric machine (LEM) and a rebounding device [1,2]. Their research proposed a control strategy which used a constant electromagnetic force profile generated by the reference current during one stroke. Gong described a nonlinear linear quadratic regulator (LQR) controller to adjust the input energy for reaching the fast response in an opposed-piston FPLG [22,23] In these strategies, though the electromagnetic force was adjusted by the load coefficient, it was defined as a constant under a specified working condition. These control strategies didn’t take into account the effects between the power demand and the load requirement They rarely consider the relationship between the output power and electromagnetic force profile, only by regulating the injected fuel mass and load factor could not be a suitable method to improve output power. The simulation results are compared with different control strategies
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