A novel mode-switching hydraulic hybrid for an on-highway vehicle: A study of architecture and control
Increasing demand for fossil fuels, their limited reserves and the environmental effects resulting from the transportation sector has raised severe concerns to government agencies, transportation industry as well as the end-users. This has raised interests in improving the fuel economy of road vehicles. One of the promising technologies in this regard is hybridization of vehicle transmission. Hydraulic hybrids have progressively gained acceptance due to their high power density and low component costs relative to their electric counterpart. ^ Many different hydraulic hybrid architectures have been developed to achieve better power management and regenerative braking and have been tested for performance and efficiency on transmission test rigs and off-highway vehicles. 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, a blended hybrid architecture was recently developed. However, the complexity of the architecture results in diculties while developing control strategies and results in poor drivability while mode switching. Drivability is a major concern along with performance in an on-highway vehicle. This work focuses on the development of a new hydraulic hybrid architecture called the "Mode-Switching Hybrid". This novel architecture combines the merits of a hydrostatic transmission as well as a series hybrid and separates the power transmission and energy recovery function to achieve better drivability. The hydrostatic mode facilitates stiff response and hence, a good driving experience. On the other hand the energy recovered through regenerative braking can be used at a later time to boost the performance of the vehicle by operating it in secondary control mode. The aim of this work is to design the mode switching hybrid for an on-highway vehicle and implement it on a prototype and develop control strategies to improve its drivability. ^ For this work, a non linear system model was developed and the operating modes like acceleration, deceleration and braking along with energy recovery were simulated. The model was linearized and control strategies were developed to improve the drivability of the vehicle. A 1999 Range Rover 4.0 was selected as the prototype vehicle to test the new transmission. A packaging architecture was designed using 3D modeling and implemented on the prototype vehicle. A data acquisition system was designed to record different parameters while conducting the experiments. Different control strategies were implemented and the performance of these control strategies was demonstrated. ^
Monika Ivantysynova, Purdue University.