Novel Mode-Switching Hydraulic Hybrid - A Study of the Architecture and Control

With the need for improvement in the fuel economy along with reduction in emissions due to stringent regulations, powertrain hybridization has become the focal point of research for the automotive sector. Hydraulic hybrids have progressively gained acceptance due to their high power density and low component costs relative to their electric counterpart and many different architectures have been proposed and implemented on both on and off-highway applications. The most commonly used architecture is the series hybrid which offers great flexibility for implementation of power management strategies. But the direct connection of the high pressure accumulator to the system often results in operation of the hydraulic units in high pressure and low displacement mode. However, in this operating mode the hydraulic units are highly inefficient. Also, the accumulator renders the system highly compliant and makes the response of the transmission sluggish. In contrast, a hydrostatic transmission has a very stiff response which ensures a good drivability. However, it lacks energy storage. Keeping these in mind, the blended hybrid architecture was recently developed [1]. First realization of the hybrid architecture in Maha’s SUV showed that the complexity of the architecture results in difficulties while developing control strategies and results in poor drivability while transitioning between modes. This paper focuses on the development of a new mode-switching hybrid which is a novel combination of a hydrostatic transmission and a series hybrid. This architecture initially operates in hydrostatic mode offering a stiff response and later when the system demands higher pressure, the mode of operation can be switched to secondary control like in series hybrid using an on-off valve. In the secondary control mode, the accumulator energy is used as a power boost. This architecture helps to achieve better drivability along with improved efficiency. The development of control strategies and implementation of the architecture for an on-highway vehicle are also discussed.