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This paper investigates a novel tubular transverse-flux permanent-magnet linear machine (TFPLM) for a free-piston energy converter, which has advantages in efficiency, emissions and reliability for serial hybrid electric vehicles. Most importantly, the novel structure solves the problems of low power factor and complex process which exist in conventional transverse-flux permanent-magnet machines (TFPMs). In this paper, a staggered structure is applied for the adjacent stator teeth of a transverse-flux machine. This kind of structure has two topologies: three phases in the axial direction and three phases in the circumference direction. To minimize flux leakage, stator and permanent magnet dimensions are optimized by three-dimensional finite-element method (3-D FEM). As the key technology of TFPMLM, the method to increase power factor and force density has been investigated. And low thrust fluctuation, high power factor and force density are achieved by further optimization. The simulation results prove that the machine has both higher power factor and high efficiency.