Abstract Hybrid Electric Vehicles with a power split system provide a variety of possibilities to promote the fuel economy of vehicles and better adapt to various driving conditions. In this paper, a new power split system of a hybrid electric bus which consists of double planetary gear sets and a clutch is introduced. The system is able to decouple both the torque and speed of the engine from the road load, which makes it possible for the engine to operate on its optimal operation line (OOL). Considering the features of the system configuration and bus driving cycle, the driving mode of the bus is divided into Electric Vehicle (EV) mode, Electric Variable Transmission (EVT) mode and Parallel mode. By controlling the engagement of the clutch at high vehicle speed (after the mechanical point), the system operates in the parallel mode rather than EVT mode. This avoids the problem that the system efficiency sharply declines in high speed region which EVT configurations are generally faced with. Accordingly, a control strategy based on the engine OOL is proposed and a corresponding co-simulation model on the platform of Matlab/Simulink and A VL/ Cruise is established. The simulation results verify that the bus is able to cover the dynamic performance requirements under UDDS cycle and reaches a fuel consumption as low as 17.84L/100km. I. Introduction With the booming development of vehicle industry, the environment and energy problems that come along is becoming increasingly severe[1 ]. This makes it necessary to develop alternative propulsion and energy solution regarding personal and commercial transportation. The pure electric vehicle is not an efficient solution to this problem because it is limited by the current battery techniques[ 2]. Hybrid electric vehicle (HEV) which combines the advantages of conventional vehicle and pure electric vehicle is an effective way to solve the problem of energy shortage and environmental pollution. Hybrid electric vehicle can be divided into series HEV , parallel HEV and series-parallel HEV according to its power assembly configuration[ 3]. Series-parallel HEV combines the advantages of series and parallel HEV and has comprehensively optimum performance. The HEVs manufactured by Toyota Motor Corporation and General Motor Corporation are the most typical HEVs in current auto market. The Prius developed by Toyota in 1997 which used the Toyota Hybrid System (THS) as its power split device was the first mass- produced hybrid electric vehicle in the world[ 4]. Toyota also developed the new generation THS II and applied it in the SUV , realizing excellent power performance and fuel economy, while meeting stringent emission standard [ 5, 6].GM developed its hybrid configurations which consists of two or three planetary gear sets and realized dual power split mode [ 7 8]. Generally, Toyota Hybrid System is an input-split configuration and GM hybrid systems are able to realize two operation modes: the input-split mode and compound-split mode. THS is feasible as a full-range single mode hybrid system but requires high engine power and motor torque and speed. GM hybrid system is able to provide full ratio coverage with moderate component speeds and torques, and less electrical path power as it combines input-split and compound-split mode[9]. However, the GM hybrid system is hard to manufacture and requires more complicated control strategy because of its relatively complex structure. Accordingly, this study proposes a hybrid configuration comprised of two planetary gear sets and a clutch. The front gear adopts input-split configuration and the rear is used as a fixed speed ratio with its ring gear fixed. The clutch is connected to the sun gear of the front gear set and will be engaged at the mechanical point in order to realize parallel mode. This configuration has relatively simple structure and achieves favorable dynamic performance and fuel economy as it is able