# Special Issue on Frontiers in Hybrid Vehicles Powertrain
###### Abstract  
The present review of the recent advances in the field of active  
consumption. Preserving the battery life was also pursued by [PERSON] et al. [5], who proposed an EMS for a series hybrid electric air-ground vehicles that can both run on land and fly. Series hybrid includes an electric generator directly linked to the ICE, which is mechanically uncoupled from the wheels, and another electric machine providing the power coming from the generator or the batteries for the propulsion or performing regenerative braking tasks. The EMS presented by the authors involves [PERSON]'s minimum principle-based model predictive control framework where speed information from intelligent network technology predicted the reference trajectory for the battery state of charge. Compared with more traditional EMS, fuel efficiency was improved and battery temperature was reduced. As the latter contribution suggested, a hybrid powertrain can be effectively adopted in those applications where the complete disposal of the ICE in favor of a pure electric powertrain appears less practicable. In this regard, [PERSON] and [PERSON] [6] developed a power-split hybrid electric layout for an orchard tractor. The power-split hybrid architecture merges the advantages of both series and parallel hybrid and overcomes their drawbacks. Indeed, the ICE is kinematically decoupled from the wheels, such as in the series hybrid. However, some of its power can still be delivered for vehicle propulsion without any intermediate electric conversion, such as in the parallel hybrid. Consequently, the ICE could always operate within the most efficient operation and a significant downsizing of both the engine and the electric unit can be realized. The authors presented two control strategies: a charge depleting mode to use full power for the most power-intensive scenarios and a charge sustaining mode to optimize efficiency and battery use throughout the whole working day. Promising results in terms of peak power capabilities and fuel savings were obtained. In the power-split hybrid architecture, the ICE, two electric machines and the output shaft are coupled by a power-split unit consisting of one or more planetary gear sets, as well as ordinary gear sets. Furthermore, embedding a clutch system enables multimode operations to increase powertrain adaptability in various road conditions. Nonetheless, the high constructive complexity of power-split transmissions requires proper mathematical tools for the analysis and the design. [PERSON] and [PERSON] [7], taking the Voltec multimode power-split transmission as a case study, described a procedure to evaluate a global efficiency map in various battery scenarios that can be exploited to extract data to implement a real-time EMS. The contribution is based on a universal parametric approach that can be applied to any power-split transmission. Besides the ICE and electric machines efficiency, it also considers the transmission meshing losses, which are rather challenging to evaluate, especially for multimode architecture with multiple planetary gearing, and thus often neglected. In the same regard, [PERSON] et al. [8] proposed general and reliable formulas to compute the mechanical efficiency of a planetary gear set. The new formulation exploits the concept of potential or virtual power and highlights the dependency between kinematics and efficiency. [PERSON] et al. [9] evaluated the power flows and the efficiency of a power-split transmission with two planetary gear sets, showing that adopting this solution would be more advisable if included in a multimode framework. The study aimed to provide designers with the advantages and disadvantages of implementing this topology within a complex compound powertrain, paving the path for developing an optimization tool that identifies the most efficient configuration for each working condition. The last two contributions foresee the embedding on HEVs of some of the most cutting-edge advancements towards zero-emission mobility.