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In order to extend the usable range of automobile transmission parameters by satisfying the performance requirements of the vehicle, a new optimum model is presented. In this model, design variables become interval. The aim of optimization is to find maximum feasible interval of design variables. It can successfully solve the question about the mating gears in optimum design of transmission parameters. A typical interval optimum example of the transmission parameters of an urban bus is given in this paper. The optimum results indicate that the fuel economy of the bus is improved obviously,and the dynamic performance is not notably changed in contrast with results of the original vehicle. The method proposed in this paper is not only adapted to optimal design of automobile transmission parameters, but also in other engineering design fields.

An Analytical system of the automobile powertrain matching is introduced in this paper. The following mathematical models are included in the system: 1. a predictable model of engine performance; 2. a predictable model of automobile performance; 3. an analysis model of the relation between transmission parameters and automobile performance; 4. an analysis model of the relation between engine performance and automobile performance; 5. a diagnosis and appraisal model of powertrain matching; 6. an optimum model of power transmission parameters. When certain engine or transmission is chosen, the system can not only select the best transmission or engine from given set, but also give the allowable range of recommended transmission parameters or engine indices to allow the user to select or adjust.

In general, the automobile transmission parameters are optimized, based on the simulation method of automobile performance and fuel economy, by using performance requirements as restraint conditions and the multi mode fuel economy as objective functions. In this paper, a new optimization method for the automobile transmission parameters is set up. The restraint conditions are described with fuzzy mathematics and the degree of emphasis on performance requirement is considered, and a wider range of accepted design variables is arrived. The transmission parameters of an urban bus are optimized. The result shows a marked improvement in the fuel ceonomy of the urban bus.