Analysis of the Optimal Operating Strategy of a P24-Hybrid for Different Electric Power Distributions in Charge-Depleting and Charge-Sustaining Operation

In order to adhere with future automotive legislation and incentives, the electric range of plug-in hybrids has steadily increased. At the same time, the installed electric power has risen as well leading to future hybrid vehicles with an electric power share of more than half of overall system power and hybrid configurations with at least two electrical machines come into focus. The concept of adding a separate electrical axle to a P2-hybrid - a so called P24-hybrid, is of special interest. The system complexity of a such a system increases significantly as the number of possible system states increases. Thus, this paper analyzes the efficiencies and benefits of the different system states within the fuel-optimal operating strategy derived by global optimization. By varying the electrical power distribution between the two axles, the impact on fuel efficiency and the changes within the operating strategy are investigated. Due to the higher system complexity, the control and calibration effort for a P24-hybrid is also much higher. Thus, this paper evaluates the efficiency advantages of different system states in order to identify possible driving situations where a less complex calibration might be feasible without a large efficiency compromise. The analysis within this paper shows the following: The use of an additional all-electrical axle improves the overall system efficiency of the hybrid system. A parallel hybrid operation is more fuel-efficient compared to a series operation in almost all driving situations. And an electrical power distribution shifted towards the all-electrical axle is beneficial regarding fuel efficiency and stability of the operating strategy. Furthermore, by analyzing the fuel-optimal operating strategy within different driving profiles, the predominant distribution of load points of the internal combustion engine is identified from which one can conclude the most relevant areas for further engine improvements.