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The objective of this paper is to demonstrate a possibility of optimizing the fixed gear ratios of an automatic transmission by using a special modeling and simulation technique. It is a problem to design the set of ratios in general, and especially for transient driving situations. Sometimes there is a lack of systematic methods to optimize dynamic systems, and particularly vehicle powertrain systems, which is the focus of this paper. The techniques used involve a dynamic engine model and a refined gearbox model. To give each set of ratios a fair judgement, the gearbox control system is calibrated for each new set. The influence of a set of ratios on the vehicle acceleration performance and fuel economy in a given driving cycle is evaluated against an optimization criterion. Calibration of the gearbox control system is performed with stationary analysis. This is the most suitable way with respect to the input signals of gearbox control systems of today.

An engine model developed earlier is validated through comparison between measurements and simulations of transient situations. In particular boost pressure build up is studied. The engine model is intended for use in computer simulation of transient behavior in drivetrain and vehicle systems. The model studied is semi-empiric. Some of the model properties are empiric: engine data in steady state operation and mappings of compressor and turbine performance. Others are physical properties, e.g. displacement and compression radio. The measurements have been carried out in a transient engine test bed. In the measurements the delivered fuel amount is measured, as opposed to pedal position, and thus the engine control system and fuel injection system are not included in the comparisons. The time constants of the measured and the simulated boost pressure buildups are compared for eleven different step changes in pedal position. All step changes go from lower to higher pedal position.

An engine model for use in computer simulation of transient behavior in drivetrain and vehicle systems is presented. Two elements, important for deviation (e.g. turbo-lag) from steady state characteristics, are the inertia of the supercharging unit (turbo shaft) and the fuel injection control system. No extensive combustion calculations are carried out within the model. Instead it uses condensed results from existing combustion models and measurements. The model is semi-empirical. Some of the engine specific properties needed for simulation are (e.g. for a turbocharged diesel): engine data in steady state operation, mappings of compressor and turbine performance, inertia of the engine components condensed to the crankshaft, turbo shaft inertia, displacement, compression ratio and the essentials of the fuel injection control strategy. Input parameters to the computer program based on the model are accelerator pedal position and external torque acting on the flywheel.