Advanced Torque Control Strategy for the Maha Hydraulic Hybrid Passenger Vehicle

An increase in the number of vehicles per capita coupled with stricter emission regulations have made the development of newer and better hybrid vehicle architectures indispensable. Although electric hybrids have more visibility and are now commercially available, hydraulic hybrids, with their higher power densities and cheaper components have been rigorously explored as the alternative. The most commonly used architecture is the series hybrid, which requires a power conversion from the primary source (engine) to the secondary domain. A positive displacement machine (pump) converts the rotational power of the engine into hydraulic power and a second positive displacement machine (motor) converts the hydraulic power back into rotational power to drive the axle or wheel. Having at least one variable displacement unit enables the system of the pump and the motor to form a continuously variable transmission. A series hybrid also includes a secondary power storage device, which in most cases is a high-pressure hydro-pneumatic accumulator. During braking, power flows from the wheels, which drive the second positive displacement machine into the high-pressure accumulator and during acceleration, the power flow is reversed, i.e. power from the high-pressure accumulator is used as an input for the second positive displacement machine which will run in motoring mode and drive the axle or wheel. A mode-switching hydraulic hybrid, which is a combination of a hydrostatic transmission and a series hybrid was recently developed at the Maha Fluid Power Research Center. This thesis focuses on the development of a new torque-based controller for the mode-switching hydraulic hybrid prototype. The aim of this work is to use a uniform control strategy across all vehicle modes instead of multiple controllers for multiple modes. With that in mind, an entirely new system model is developed. This torque-based control strategy, along-with a supervisory controller decides on the usage of the high-pressure accumulator, thereby switching the vehicle from non-hybrid to hybrid mode. A separate engine speed controller is designed to control the engine throttle based on the measured engine speed and a piecewise constant reference engine speed. The model is simulated using standard drive cycles demonstrating the different vehicle modes of operation and the controller action. The architecture of the existing prototype vehicle is modified to implement the new controller and also to prevent leakages when the vehicle is not in use. The data acquisition system is modified to incorporate new installed components. Lastly, baseline measurements taken with the prototype vehicle are compared with the simulations. This improved control strategy allows the vehicle to operate in higher powertrain efficiencies and the uniform nature of the controller results in a better driver-feel