Comparative Analysis of Single and Combined Hybrid Electrically Variable Transmission Operating Modes

Electrically variable transmissions divide power between the electrical and mechanical paths using input, output, or compound split schemes. When combined with an electrical energy storage element such as a battery, these systems allow numerous fuel saving and performance benefits. This paper examines the design tradeoffs in each of the three topologies in order to balance fuel economy, system performance against requirements, and electrical component size. A general EVT analysis method is presented and used to study the fuel economy and performance sensitivity of the three configurations to motor, inverter, and battery constraints, and planetary gear ratios. To evaluate fuel economy, the three systems are assessed for each of the primary fuel economy mechanisms enabled by hybridization. To evaluate performance tradeoffs, system performance against typical vehicle performance design points is compared. The effects of combining two modes that are optimized for individual speed ranges vs. a single mode covering all ranges are also discussed. The use of 2 modes provides significant advantages over a single mode design including reduced motor power for a given vehicle performance.