Slippage Equalization for Energy Loss Minimization in a Four-Wheel Drive Vehicle

Abstract

Tire slippage will inevitably occur during vehicle acceleration and deceleration, with the amount of slippage depending on parameters such as tire force, vertical load, and road surface conditions. In a decelerating electric vehicle, tire slippage can result in considerable energy loss to a regenerative brake system. The purpose of this study is to derive the optimal drive/brake force distribution for minimizing slippage loss; doing this will help to determine an optimal torque distribution strategy for electric vehicles driven by individual motor/generators that are connected directly to the wheels. In this study the wheel slippage parameters of a four-wheel drive vehicle running straight on a uniform road lane are estimated. The tires are modeled using the brush model, and vertical load shifting is estimated as a function of longitudinal acceleration. By using conventional brush model assumptions, a cubic equation in terms of the slip ratio can be derived. The analytical solution to this equation can then be used to estimate energy loss during acceleration or deceleration; from this, a figure for energy efficiency in terms of what percentage of the energy consumed in driving is converted into kinetic energy of vehicular motion can be calculated. A similar index showing the percentage of the kinetic energy used to drive the motor/generators in the wheels during deceleration can then be defined. From these relationships, theoretical efficiencies can be precisely estimated, and it is found that energy loss can be closely approximated as a sum of quadratic functions of the slip ratios of the front and rear wheels. This result can be used to develop a torque distribution method that equalizes the slip ratios of all four wheels (which equates to synchronizing the revolution of all of the wheels) in order to minimize total energy loss. The results of this study can therefore serve as a useful guide for implementing an efficient drive system controller. Although the quantitative estimation of relevant factors is driven by the brush model, slippage equalization is not essentially dependent on this estimation. This study provides a more firmly analytical description of an improved torque distribution method for minimizing slippage loss than has been previously developed; however, further study will still be needed to implement an actual drive system controller.

F2012-G02-021