Development of an Automatic Transmission Model

Photo of JamesLee Fabian, Monique Messman, and Ryan Phillips Students: JamesLee Fabian, Monique Messman, and Ryan Phillips

Sponsor: The University of Texas Society of Automotive Engineers

Date: Fall 2009

Requirements:
The requirements for this project are to create a MATLAB program that will calculate the efficiency of an automatic transmission, given specific operating conditions, within 10% accuracy. Constraints placed on the transmission are simply to have the transmission operating at steady-state, normal conditions, ignoring outside factors like ground temperature and weather. Additional constraints include the complexity of the transmission itself and the fact that people drive differently so shift maps could vary.

Problem:
Develop a mathematical model that calculates the efficiency of an automatic transmission. An updated model is being used for manual transmissions, however, the current model for automatic transmissions is about half a century old. Therefore, it is essential to develop a new model since the percentage of automatic transmissions in vehicles in the US is about 87% and rising due to an increase in demand from young truck drivers who are more comfortable with automatics.

Solution:
To begin the program, the user enters transmission specifications. Once the transmission information is known, the program will reads an Excel file that contains all the ranges of engine speed, engine torque, speed ratio, and vehicle gear that the user will have to input. The program then calculates power losses from the torque converter, pump, and gearbox, and component efficiencies. The program calculates the overall transmission efficiency, output speed, and output torque. Once the range of outputs is known, program presents graphs for the transmission's efficiency versus the engine rpm, efficiency versus the engine torque, efficiency versus the torque converter speed ratio, and finally the transmission output torque versus output rpm. The design team created an input excel file that contained all torque and converter speed ratios for three different RPMs obtained from an SAE document provided by Dr. Matthews. This file was then run in the model to generate a graph (in first gear) of the transmission efficiency vs. converter speed ratio. Figure 2 shows the graph generated from this model compared to the actual experimental data from the SAE document.

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