The Future of Mild Hybrid Systems in the Transmission

An innovative mild hybrid transmission concept is presented and the opportunities described that arise from the addition of an e-machine to the transmission system and the introduction of a freewheel mechanism. Design studies and simulation have shown that hybridising elements of the transmission system can extend the efficiency gains of 12V engine-based mild hybrid systems, specifically in small passenger cars. In addition, combined with the freewheel mechanism, this concept adds desirable features and functionality that justify its cost. The paper will demonstrate how some of this functionality will enable hybridisation of a far broader range of vehicles, helping manufacturers to significantly reduce their fleet CO2 emissions. While the energy storage requirements over a standard drive cycle for such a system have been shown in simulation to be well within the capability of a 12V battery pack, the peak power limitations of a 12V machine, if not addressed correctly, can lead to issues around driving refinement and safety. As well as reviewing the impact on transmission design and control, the paper also examines how the electrical system can be designed to overcome these limitations.