Analysis, functional design, and energy management of power-split hybrid electric vehicles

To mitigate the ongoing escalation of global warming and air pollution, new regulations have been established in the transport sector in order to minimise greenhouse gases and toxic emissions. To this purpose, a transition from conventional vehicles powered by an internal combustion engine towards more sustainable solutions has been fostered by governments and regulatory bodies worldwide. In this regard, the hybrid electric powertrain appears as an effective alternative to be widely adopted in the short term.The powertrain of a hybrid electric vehicle (HEV) includes an internal combustion engine and an electric unit. The synergy between these two power sources leads to a significant reduction of both fuel consumption and emissions, avoiding the most critical issues that affect pure electric vehicles, i.e., the low range and the need for significant enhancements to the electric infrastructure.Among the available hybrid technologies, the power-split powertrain is the most versatile solution. The speed, torque and power ratios between the engine, the electric machines, and the wheels are established by the power-split continuously variable transmission (PS-CVT), consisting of a power-split unit (PSU) that includes one or more planetary gear trains (PGs) and, optionally, ordinary gear trains. The PSU enables two kinematic degrees of freedom, making the engine kinematically decoupled from the wheels, thus being able to always operate close to the best efficiency. The simplest power-split layout includes a single PG, but some solutions deploy two or more PGs. Moreover, a system of brakes and clutches can be embedded in a PSU to realise multi-mode PS-CVTs, which makes available multiple power-split layouts to select according to the current driving condition so as to pursue high-efficiency performance.However, any HEV can achieve an actual reduction in fuel consumption and emissions in comparison with a conventional vehicle only if an effective energy management strategy (EMS) is implemented onboard. Hence, the demanded power should be instantaneously split between the engine and the battery so as to keep the ICE operating as efficiently as possible, minimise the powertrain power losses, and maintain the battery state of charge (SOC) around a desired value.Due to the different nature of the main components of a hybrid electric powertrain, its design and analysis often require expertise in several fields, e.g., mechanics, electrics, and control systems. Thus, owing to the two kinematic degrees of freedom of PS-CVTs, the high constructive complexity, the wide variety of the feasible solutions, and the possibility of switching the operating mode, the power-split powertrain requires dedicated mathematical tools that must be accessible to scholars and engineers from different scientific areas.The most common approaches of the relevant literature use an equivalent representation of the PSU based on the lever analogy or the graph theory. The lever analogy is mainly adopted for analysis purposes, but it is not suitable to address more complex PS-CVTs with ordinary gearing and multiple modes. On the other hand, the graph theory is mainly adopted in the design stage, but the enabled design procedure relies on a merely explorative approach achievable only by the aid of extensive computation, which hinders the designer's awareness towards the optimal solution. Moreover, both approaches are not suitable for a rapid assessment of the PSU power losses, which, thus, are often neglected.On the contrary, this dissertation aims to study and extend a unified parametric model for PS-CVTs that enables a universal formulation suitable for both analysis and design purposes. The mathematical treatment relies on physically-consistent functional parameters that univocally characterise any PSU. The resulting equations of speed, torque, and power ratios do not depend on the PSU constructive arrangement, which, instead, only affects the numerical value of the functional parameters. Moreover, a modular, hierarchical design procedure is enabled, as well as a rapid assessment of the PSU meshing losses. As a result, the model provides all the crucial features that a mathematical tool for PS-CVTs requires within a comprehensive formulation.The main advancements of this research are the extension of three previous contributions already available in the literature, where the fundamentals of the modular parametric design and the analysis of single-mode PS-CVTs with up to two PGs were addressed, as well as the PSU meshing losses. Firstly, the analysis procedure has been extended also to multi-mode PS-CVTs with any number of PGs, not only in power-split operation, but also in the full-electric mode. This enabled a comprehensive assessment of the powertrain response, considering also the PSU meshing losses. Moreover, the modular design procedure has been used for the global design of a power-split powertrain. Thanks to the utmost generality of the approach, it has been applied to propose the first power-split hybridisation of an oil drilling rig to recover braking energy during the gravity-driven work phases. Nonetheless, the analysed case study has revealed that the integration of an energy management strategy is essential to pursue the optimal sizing of the thermal and electric unit.Therefore, the research has been focused also on the implementation of effective EMSs relying on the parametric model to assess the optimal operations of power-split hybrid electric powertrains. In this regard, two different approaches have been developed to optimise the operation of the power-split hybrid electric powertrain. The first method deals with the offline assessment of the optimal operating maps resulting in the maximisation of the powertrain global efficiency. The second contribution integrates the unified parametric model within an EMS based on the model predictive control. The universal mathematical formulation of the parametric model and the possibility of a rapid evaluation of the PSU meshing losses have allowed proposing a universal model predictive controller with integrated mode switch and assessing how the consideration of PSU meshing losses and electric machines efficiency affects the controller performance, by comparing internal models with different complexity.