**Preface**

The increasing concern regarding environmental issues has led to the adoption of stringent regulations worldwide to facilitate the urgent move towards green mobility and sustainable transportation. In this regard, electrified powertrains are bound to replace conventional thermal engines to reduce greenhouse gases and pollutant emissions.

Although full electric vehicles succeed in reducing onboard tank-to-wheel emissions, they are still affected by some major issues, such as range anxiety, battery lifetime, high cost, and limited recharging infrastructures. Moreover, the low specific energy of a pure electric powertrain prevents its final establishment beyond light- and medium-duty ground vehicles. On the contrary, the synergy between an internal combustion engine and an electric unit on hybrid electric vehicles (HEVs) overcomes the main battery limitations and fulfills even the most demanding energetic requirements, ranging from light- and heavy-duty vehicles to agricultural machinery, vessels, and aircraft.

As a result, HEVs significantly reduce fuel consumption and emissions while maintaining high vehicle performance and driving comfort, becoming the optimal sustainable solution in the short term. Nonetheless, the full potential of HEVs can only be exploited using a multidisciplinary approach to design the mechanical and electrical equipment and implement an optimal energy management strategy to pursue the best trade-off between fuel saving and performance.

The main goal of this Special Issue, "Frontiers in Hybrid Vehicles Powertrain", has been to gather original contributions from several scientific fields to provide a broad perspective on the current issues and research trends on the hybrid electric powertrain. In this respect, this book collects nine research articles covering efficiency-oriented control strategies and novel mathematical tools for power-split hybrid transmissions to reduce pollutant emissions, improve powertrain overall efficiency, and enhance battery life. Moreover, the adoption of the hybrid electric powertrain on agricultural machinery and air–ground vehicles is analyzed, along with the integration of the latest advancements towards zero-emission mobility regarding hydrogen-powered engines and renewable energy sources. Overall, the Special Issue outlines the major unsolved challenges for the final widespread establishment of HEVs.

Thanks are due to all the authors who contributed to this Special Issue, to the reviewers who improved the quality of the work, and to the editorial team of the *Applied Sciences* journal and MDPI Book staff.
