An analytical investigation has provided a new and useful procedure for evaluating the peak power requirements of a vehicle, based upon the desired levels of accelerative performance and the vehicle usage, that is, city, suburban, or highway applications. These power requirements have been used to determine the required weight of the vehicle power source, based upon its effective power density and the propulsion system efficiency.
A computer solution to the one-dimensional equation of motion of a vehicle with a non-linear input was used to predict vehicle performance. Consideration has been given to various combinations of constant power-torque limited drives to determine the relationship between the torque-speed characteristics of the energy source and the vehicle performance. This information was used to find the locus of power which the energy source must deliver (as a function of vehicle speed) to provide the desired vehicle performance within specified limits.
To illustrate potential applications of this method, examples of a battery-electric drive and a hybrid (engine-battery) drive system were considered for a compact-size car with performance equivalent to today's compacts. Three types of batteries were used to demonstrate that the vehicle performance is severely degraded as the battery becomes discharged. Battery discharge derating factors, which were developed to characterize this behavior, can be applied to the rated power densities to determine the effective power density of a given battery at any predetermined maximum depth of discharge. It was found that the battery weight penalty was high when a vehicle was designed to provide minimum acceptable performance at a preselected state of battery discharge.
