Hybrid powertrains, which recover and reuse kinetic energy traditionally wasted via braking with the target of reducing fuel consumption and emissions, are widely viewed as an essential solution for the road transport sector.  They are also potentially applicable to other vehicles, particularly trains.  The majority of hybrid systems both in series production and under development are based upon electrical architectures with a variety of designs and storage media.  However, converting mechanical energy to electrical energy and then to chemical energy and vice versa produces rather poor system efficiencies.  In addition the storage media, power electronics and motor/generators produce a complex powertrain with corresponding impacts on system cost, weight and package,
An alternative to the electric hybrid powertrain is a mechanical hybrid system.  A mechanical hybrid powertrain utilises a rotating flywheel as the energy storage device and a variator to transfer the energy to and from the vehicle driveline. 
Flywheel mechanical hybrid systems offer advantages of higher efficiencies due to the removal of the energy conversions together with a significantly less complex system providing considerable weight, package and cost benefits over electrical systems.  Flywheel mechanical hybrids are not new and have been previously developed by a number of companies.  Interest in them has been renewed following the introduction of regulations in Formula 1 motor racing which will require racing cars to be capable o I kinetic energy recovery and re-use. 
A problem can arise in using a conventional variator in a KERS system, in that the range of ratios provided by a typical variator can be smaller than the expected spread of speed ratios between the engine and the flywheel.