INTRODUCTION With growing concerns about global warming, depleting fossil fuels, and air pollution, the importance of eco-friendly vehicles with low CO 2 emissions and exhaust free of environment pollutants is increasing. A hybrid electric vehicle (HEV) is one of the most practical options for eco-friendly vehicles. In order to satisfy the demands of automotive consumers, further improvement in the fuel economy of HEVs is required. Hence, the improvement of system efficiency for hybrid powertrains is still a main area of development. Electric motor driving during low road-load driving conditions and vehicle energy recuperation using the electric motor during braking conditions are major gain factors for fuel economy improvement in hybrid powertrains. In addition to these factors, an improvement in transmission efficiency also contributes to improved fuel economy . In order to improve the transmission efficiency, the losses in the transmission system need to be reduced. Depending on the hybrid powertrain configuration, the total amount of loss as well as its generation mechanism differs. Hence, it is important to investigate the optimal configuration considering the new drivetrain components that improve the system efficiency of the hybrid system. Hybrid powertrain configurations can be categorized into three types: parallel hybrid systems, series hybrid systems, and series-parallel hybrid systems. In a parallel hybrid system, an electric motor is attached to the conventional mechanical transmission, enabling electric motor driving and regenerative braking [ 1,2]. One of the advantages of the parallel hybrid systems is good transmission efficiency at high speeds, especially for large vehicles. On the other hand, in order to complete other functions such as vehicle moving-off and engine starting, conventional devices such as torque converters or frictional clutches and engine starters are individually required, respectively. These devices are inevitable for conventional vehicles. However, for HEVs in which at least one electric motor is installed, these conventional devices are not necessarily efficient for improved fuel economy. In both series hybrids [3,4] and series-parallel hybrids [ 5], which are called two-motor hybrid systems, two independent electric motors allow the realization of multiple functions such as vehicle moving-off, engine starting, engine driving, electric motor driving, and regenerative braking. Due to these multiple functionalities, no other mechanism is required for the vehicle. On the other hand, electric motors are large, and the transmission loss increases as the transmitting power increases in series hybrid systems. In series-parallel hybrid systems, the improvement in transmission efficiency at a high gear ratio is an essential research topic. In this paper, we present a new hybrid system that takes the advantages of both the parallel hybrid systems and the two-motor hybrid systems. In order to downsize the system with two motors High Efficiency Electromagnetic Torque Converter for Hybrid Electric Vehicles Takao Watanabe, Eiji Tsuchiya, Masaki Ebina, and Yasumitsu Osada Toyota Central R&D Labs Inc Tomoyuki Toyama Aisin Seiki Co Ltd Akira Murakami Toyota Motor Corporation ABSTRACT A new concept of an electromagnetic torque converter for hybrid electric vehicles is proposed. The electromagnetic torque converter, which is an electric system comprised of a set of double rotors and a stator , works as a high-efficiency transmission in the driving conditions of low gear ratio including a vehicle moving-off and as a starting device for an internal combustion engine. Moreover, it can be used for an electric vehicle driving as well as for a regenerative braking. In this concept, a high-efficiency drivetrain system for hybrid electric vehicles is constructed by replacing a fluid-type torque converter with the electromagnetic torque converter in the automatic transmission of a conventional vehic