Parry People Movers (PPM) is a British company manufacturing lightweight railbuses that use Flywheel energy storage (FES) to store energy for traction.
PPMs utilise rotating flywheels as a store of kinetic energy which is then used to power the vehicle. A typical PPM flywheel is made from steel laminates, 3 feet in diameter and 1120 lbs in mass, rotating at a maximum speed of 2,500rpm. The flywheel is mounted horizontally at the centre of the unit, beneath the seating area.
The flywheel allows the direct capture of brake energy (when slowing down or descending gradients) and its re-use for acceleration. Since the short-term power demand for acceleration is provided by the energy stored in the flywheel, there is no need for a large engine. A variety of small engine types can be used including LPG, diesel or electric.
On a route with stations a short distance apart it is theoretically possible to use the unit as a streetcar without any engine or overhead electrification at all. Instead the flywheel could be reenergised at each station storing enough power to carry it on to the next one.