The fuel economy benefits of any given hybrid drive train depend greatly on the vehicle type, size of hybrid components and control strategy. The main goal of this work is to simulate and control the power train of a hydraulic hybrid vehicle. Hydraulic hybrid system has high power density and the ability to accept charge and discharge at high rates/high frequencies. Therefore, hydraulic hybrid drive train is well suited for off-road vehicles such as heavy-duty trucks and buses. The objective of this simulation and control is to reduce fuel consumption and emissions. This process which also includes modeling, optimization of design and power management of hydraulic hybrid system is applied for an O457 city bus. A parallel hydraulic hybrid structure is used. Also, the designed hybrid system is able to regenerate the braking energy and to control the engine operation in its high efficiency zone. To perform this research, modeling of hydraulic components such as hydraulic accumulator and hydraulic pump/motor are done. The hydraulic accumulator modeling is based on the thermodynamic equation with a real gas state equation and the elastomeric foam that used to increase the charge/discharge efficiency. Wilson's pump/motor theory provides the basis for modeling of variable displacement piston pumps. The developed models are then implemented in Advisor software to complete dive train modeling and simulation. The simulations are preformed over UK Bus, European city and W. Virginia city standard drive cycles. A fuzzy torque control strategy based on the vehicle load changes is developed to real-time control the energy distribution for the proposed PHHV. Simulation results demonstrate that the proposed PHHV drive train with torque control strategy takes advantage of the high power density and efficiency characteristics of the hydraulic hybrid system, minimizes the disadvantages of low energy density, and effectively improves the fuel economy of PHHV. Average of 31% fuel economy improvement and 30% emission reduction are achieved for aggressive city-cycles. Key words: Hybrid vehicle, Hydraulic system, Accumulator, Pump/motor, Fuzzy logic control
