Evaluation of split-path extended range continuously-variable transmissions for automotive applications

Abstract

One potentially important application of continuously-variable transmissions (CVT's) is the improvement in fuel economy of internal combustion engine vehicles. However, many otherwise attractive CVT designs have a ratio coverage less than ideal for such applications. Split-path configurations can be used to extend the ratio coverage, providing even a geared neutral and reverse capability with some combinations of parameters. The increased coverage is in general achieved, however, only at the expense of lower overall CVT efficiency and a larger required variable unit capacity. All possible single-regime designs which use a single differential to extend the ratio coverage of the basic variable unit are considered, as well as two 2- regime designs. Data for the CVT efficiency characteristics are developed for both otherwise conventional vehicles and those utilizing an energy-storage flywheel, with steady speeds and three driving cycles used for the evaluation. It is found that the efficiency penalty for extending the ratio coverage can be quite severe for some combinations of system parameters and operating conditions, so that a complete analysis of this aspect is important when considering the concept for any specific application.