Core powertrain efficiency improvement is currently the most effective way of improving fleet CO2 emission but is limited in its maximum impact. A novel Hybrid Drive System is described. An advanced composite flywheel is the core element of this system, which does not rely on power transmission through electricity. Key components of the system are identified and their roles discussed. Different options for the architecture of the system are described, referring also to potential integration with other typical elements of hybrid drive systems. It can be shown that electric hybridisation has greater impact on CO2 emissions but at a higher cost and kinetic hybridization offers electrical hybrid CO2 impact at core powertrain cost increment. Operational properties of the system are discussed. Innovative aspects and options of drivetrain integration are described. The complete system can be summed up: The central component is a flywheel. It retrieves energy from wheels under braking. The energy is stored in the rotating flywheel It returns energy to the wheels when boost is required, e.g. during acceleration. It is a mechanical-only hybrid system , no energy transformation required. The characteristics of the flywheel, e.g. energy density and power density are derived from basic design considerations and dimensioning constraints like UTS etc. Delivery of innovative solutions through adapting magnetic technology is enabling delivery of kinetic hybridization in a robust and cost effective manner. The benefit of the mechanical hybrid drive system for different applications is identified and some examples from passenger cars to industrial equipment and railcars are explained. The further development potential is estimated and future embodiments and applications are hinted at