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For city buses, especially hybrid electric buses, the requirements for the fuel economy and low noises are stricter, comparing with the momentum quality. Since hybrid electric buses sometimes run without the engine, the noises that the transmission makes become the major type. To get better fuel economy and lower noises, this paper focuses on optimizing the characteristics of the automatic mechanical transmission (AMT) in a hybrid electric city bus, and the studies are done as follows. Firstly, in order to reduce the fuel consumption, the transmission ratios are optimized by the co-simulation and optimization in CRUISE and MATLAB, with the limitation of the quality of driving momentum. Secondly, for the purpose of lightweight and lower transmission noise, multi-objective optimization based on reliability is applied in transmission geometric optimization design, the objective function are the smallest volume and the biggest transmission gear contact ratio of the transmission.

The paper describes an algorithm, which estimates the mass of large buses and axle load distribution using pedal position, wheel speed and the wheel cylinder pressure sensors. This algorithm is allowed to achieve the purpose without additional sensors by using the rotational speed sensors from ABS system and air pressure sensors in brake cylinders form ESP system. The axle load distribution algorithm mainly consists of three steps. Firstly, deceleration of the bus is estimated and then the mass of the bus is estimated. After that, the position of the mass centre is estimated. Taking account of the tire nonlinear characteristics under longitudinal forces and vertical forces, mass estimation, deceleration and the position of the mass centre of buses is corrected by the coefficient, which is determined by the wheel cylinder pressure, the wheel speed and mass estimation.