INTRODUCTION An intracity bus follows the same route each time that the energy usage is easily expected. Thus, the hybridization of the intracity bus is the urgent matter of concern [ 1, 2, 3]. Hybrid vehicle research generally focuses on increasing fuel economy or reducing emissions. Many strategies have been investigated to accomplish these two goals. The energy distribution control strategy is one of the most important issues to address because hybrid vehicles strongly depend on the supervisory control strategy. It influences each component of the vehicle. Even though the engine, the generator, the battery and the motor are very powerful, without an appropriate control strategy, the output performance may be no greater than that of an internal combustion engine vehicle. There are several types of energy distribution control strategies-e.g., DP (Dynamic Programming), ECMS (Equivalent Consumption Minimization Strategy), and PMP (Pontryagin Minimum Principal) [ 4, 5, 6]. Energy distributionresearch generally focuses on the energy flow from the engine/generator unit to the traction motor in Figure 1. However, if the energy distribution at the traction motor is not efficient, even if a very effective or powerful methodology is used to supply the optimal energy from the engine/generator unit to the traction motors, the most efficient energy flow to the wheels cannot be achieved in the system. This research focuses on the energy flow at the traction motors. Unlike the parallel-type HEV bus, in the series-type vehicle, dual traction motors are often used instead of just a single motor because the required traction motor capacity is much higher than that of the parallel-type vehicle. For example, the existing series-type HEV intracity bus is equipped with a 240 kW single traction motor or 120 kW dual motors [ 7, 8]. To raise the fuel economy of such series- type hybrid buses, using more efficient traction motors might be a solution. However, because a motor generally has a uniformly good efficiency and already has an efficiency of up 2012-01-1035 Published 04/16/2012 Copyright © 2012 SAE International doi:10.4271/2012-01-1035 saecomveh.saejournals.org Fuel Economy Research on Series-Type HEV Intracity Buses with Different Traction Motor Capacity Combinations Minjae Kim, Daebong Jung and Kyoungdoug Min Seoul National Univ ABSTRACT Research on HEV (hybrid electric vehicle) intracity buses has become a topic of interest because the well-known service routes of intracity buses and the frequent stop/go pattern make the energy management of the vehicle straightforward. Thus, the energy flow and the energy management of the intracity bus have been studied extensively in order to improve fuel economy. However, the HEV buses that have been studied previously were equipped with a single traction motor or with dual motors with the same capacity for the convenience of the equipment without considering the motoring or generating efficiency of the traction motor. Therefore, the energy flow from the engine/generator unit to the traction motor that has been optimized by many kinds of energy distribution strategies could not be transferred to the wheels in the most efficient manner. This paper investigates this aspect of the energy flow. It studies the traction motor energy distribution by investigating combinations of the different traction motors with the different motoring/generating energy distribution methodology. The research suggests that instead of using a single traction motor or dual motors with the same capacity, dual motors with the different capacity combinations should be used to access the best efficiency distribution map. The different capacity combinations here not only refer to the physical capacity but also to the ratio of electrical energy distributed to the traction motors. Thus, the proposed approach distributes the required motoring/ generating energy to the dual motors with the better efficiency. Therefore the system effici