Simulation-based methods for dynamic analyses of electrified powertrains

Abstract

Electrification of powertrains is an important step in reducing the emissions and fuel consumption of off-road machinery. Computer simulations of powertrains and vehicle dynamics have become an integral part of the design process. This study aims to improve modeling techniques to predict the dynamic behavior of electrified powertrains by adequately describing details at the system and component levels. Traditionally used powertrain models use simplified methods of simulating loads that are not sufficient to describe off-road machinery. Therefore, a co-simulation platform is presented for comparing the energy efficiency of electrified powertrains with dynamically simulated off-road machinery loads. At the component level, the ball bearing is a source of uncertainty in predicting the dynamic behavior of powertrains due to simplifications in conventional bearing modeling techniques. Therefore, a rotor-bearing modeling procedure is presented that takes into account the full bearing stiffness matrix, including cross-coupling terms and a three-dimensional finite element rotor model describing rotors of complex geometries with flexible components. This modeling technique considers realistic bearing mounting conditions like ball bearing arrangement and tilting of the loosely fitted bearing outer race.

System-level simulations of hybrid electric powertrain configurations with various types of load coupling and transmission designs are investigated with a multibody tractor model to demonstrate the capabilities of the developed co-simulation platform. The standardized plowing work cycle of the Deutsche Landwirtschafts-Gesellschaft PowerMix and a human-operator-driven soil digging and dumping work cycle are used to simulate the dynamic traction and hydraulic implement loads of the tractor. The series-parallel topology of coupling the traction loads through a double planetary gearset transmission shows the lowest fuel and total energy consumption. The coupling of hydraulic implements with the battery saves fuel with a rapid depletion of the battery charge. Typically, the ball bearing supporting the electric machine in a powertrain is loosely fitted to the housing, causing a tilt of the bearing outer race in the gap. The results show reduced bearing stiffness due to such tilting misalignment. The critical speeds of the system decrease significantly, depending on bearing arrangement and misalignment. The results presented illustrate the advantages of eliminating simplifications in existing powertrain modeling practices.