A design methodology which uses the regenerative brake model is introduced to determine the major system parameters of a parallel electric hybrid bus drive train. Hybrid system parameters mainly include the power rating of internal combustion engine (ICE), gear ratios of transmission, power rating, and maximal torque of motor, power, and capacity of battery. The regenerative model is built in the vehicle model to estimate the regenerative energy in the real road conditions. The design target is to ensure that the vehicle meets the specified vehicle performance, such as speed and acceleration, and at the same time, operates the ICE within an expected speed range. Several pairs of parameters are selected from the result analysis, and the fuel saving result in the road test shows that a 25% reduction is achieved in fuel consumption. 1. Introduction Facing more and more stringent emissions regulations and the world ever-increasing petrol price, the automotive industry is looking for cleaner and more efficient powertrain for the next generation of vehicles [1]. Hybrid electric vehicle (HEV) which combines the traditional internal combustion engine with the electric motor and batteries is the prime choice for automakers as the next generation of alternative powertrain because of its good fuel economy and better operating range compared to the traditional vehicle and acceptable price and cost. Many Chinese bus manufacturers and powertrain producers are engaged in projects to develop clearer and more economical hybrid buses to meet the requirements [2]. City buses often have low average speed and rapidly accelerated movement when they move in city routes. In order to transfer more and more heavier guests, most cities of China adopt the 12？m long buses. The average speed is less than 35？km/h, and the distances between two stops often are less than 500 meters, even if there exit many traffic lights between them. In most situations, the maximal velocity of city buses is less than 50？km/h. Naturally the hybrid system should be able to emulate the engine driving in low speed, to recover the brake energy, and to stop the engine when the bus is in stops or waiting for the traffic lights [3, 4].(i)The EV function uses the motor to drive the bus from zero to a certain speed, so the start process of low efficiency clutch-slipping, high fuel consumption, and high emissions in traditional engine buses is avoided.(ii)The larger the brake regeneration is, the better the fuel economy is. To increase the brake regeneration, the engine must be disengaged from the powertrain to
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