Modeling the Effects of Transmission Gear Count, Ratio Progression, and Final Drive Ratio on Fuel Economy and Performance Using ALPHA

The Advanced Light-Duty Powertrain and Hybrid Analysis (ALPHA) tool was created by EPA to evaluate the Greenhouse Gas (GHG) emissions of Light-Duty (LD) vehicles [1]. ALPHA is a physics-based, forward-looking, full vehicle computer simulation capable of analyzing various vehicle types combined with different powertrain technologies. The software tool is a MATLAB/Simulink based desktop application. The ALPHA model has been updated from the previous version to include more realistic vehicle behavior and now includes internal auditing of all energy flows in the model [2]. As a result of the model refinements and in preparation for the mid-term evaluation (MTE) of the 2022-2025 LD GHG emissions standards, the model is being revalidated with newly acquired vehicle data.

This paper presents an analysis of the effects of varying the absolute and relative gear ratios of a given transmission on carbon emissions and performance. Energy-based methods of selecting absolute gear ratios are considered and the effects of alternative engine selections are also examined. An algorithm is presented for automatically determining ALPHAshift parameter sets based on the selected engine and transmission combination. It is observed that no single ratio progression optimizes fuel consumption for all applications, however, fuel consumption is also relatively insensitive to progression which implies a fixed set of ratios can still be used for a range of applications without necessarily compromising consumption. The energy-based ratio analysis may prove useful in determining the optimal overall top gear ratio for a given engine-vehicle combination and also helps to explain the relative insensitivity to ratio progression. Individual performance metrics can show high sensitivity to ratio progression, final drive ratio and shift calibration, in particular 30-50 and 50-70 MPH passing times.