A Linear Magnetic-geared Free-piston Generator for Range-extended Electric Vehicles

Abstract

This paper proposes a novel linear magnetic-geared machine for the free-piston generator in range-extended electric vehicles. The key is to integrate a linear magnetic gear into a linear permanent magnet synchronous machine to form a single machine, which can offer high efficiency and high power density for electricity generation. The low-speed mover of the proposed machine is directly coupled with the pistons of the internal combustion engine and reciprocates with the pistons. Due to the magnetic gear effect, the high-speed mover which is also the translator of the synchronous machine can travel at high speeds, hence offering high power density design and generating electricity with a high voltage. By using finite element analysis, the machine performance is analyzed and verified. Keywords linear magnetic gear, linear synchronous machine, free-piston generator, range-extended electric vehicle, integrated machine 1. INTRODUCTION With ever-increasing concerns on global environmental deterioration and fossil energy shortage, there is a fast growing interest in electric vehicles (EVs) for road transportation. EVs can be classified into three main kinds: battery EVs, fuel cell EVs and hybrid EVs (HEVs). Due to the consideration on driving range and initial cost, the HEV which incorporates an internal combustion engine (ICE) and an electric motor has been successfully commercialized by automakers [Chan and Chau, 2001; Chau et al., 1999; Chau et al., 2007; Zhang et al., 2008 ]. According to the relationship between the ICE, electric motor and transmission system, the HEVs can be classified as the series hybrid, parallel hybrid, series-parallel hybrid and complex hybrid. The series hybrid is the simplest kind of HEVs. Its engine output is first converted into electricity using a generator. The converted electricity either charges a battery set or propels the wheels via an electric motor and mechanical transmission system [Chau and Wong, 2001]. The range-extended electric vehicle (REV) is a new concept of series HEVs. It is designed that the electric motor propels the vehicle whenever possible. When the battery voltage falls below a certain level, it can be plugged into the grid for charging or charged by an onboard generator. In this REV, a rotational generator is adopted to convert the engine power into electricity.