Optimal configuration design for hydraulic split hybrid vehicles

Hydraulic hybrid vehicles are more suitable for heavy-duty applications in urban driving than hybrid electric vehicles because of the high power density and low cost of hydraulic devices. However, the low rotational speeds of hydraulic pump/motor and the low energy density of the accumulator impose severe constraints on the design and control for these vehicles. The split configuration is an efficient and more flexible transmission configuration and is the focus of this study. A three-step design methodology is developed to systematically and exhaustively explore all possible split configurations using two planetary gears: Step 1 checks mechanical feasibility of configurations; Step 2 develops optimal power management inspired by Pontryagin´s Minimum Principle; and Step 3 finds optimal component sizes. After the screening, we identify three new configurations that achieve best fuel economy for the hydraulic vehicle/drive cycle studied, together with their best design and control executions.