Fuel-optimal control of CVT powertrains

Continuously Variable Transmissions (CVTs) offer the potential to improve the part-load fuel efficiency of Spark-Ignited (SI) engines by shifting the operating points towards higher loads and lower speeds. The control of CVTs has traditionally been designed using (quasi) static arguments, i.e., by identifying the best efficiency points in the engine map for each constant power requirement and by following that curve using some heuristics as much as possible also in transients. In this paper a complete analysis and solution of the fuel-optimal control problem for transient conditions is presented. The powertrain equations include a detailed engine and CVT model. Further a physically meaningful optimization criterion is defined. The optimal solution is found using the numerical optimization package DIRCOL. Based on this optimal solution a simplified control strategy is proposed which offers almost the same fuel economy benefits, Connections of this approach to well-known CVT control strategies are shown. Copyright (C) 2001 IFAC.