Using the same, proven and patented, high-speed flywheel technology as other Flybrid® products the CFT KERS uses an all-new transmission arrangement for connection between the flywheel and the car.
The CFT transmission uses a number of discrete gears and special Flybrid-developed high-speed clutches that perform a controlled slip to transmit the drive.
When connected to an engine speed shaft within the vehicle transmission the three gears in the CFT KERS are multiplied by the number of gears in the main vehicle transmission to provide a large number of available overall ratios between flywheel and wheels. The efficiency of a slipping clutch depends upon the speed across it and with so many gears to choose from a high efficiency option is always available.
In a Formula One application the 21 speeds available (3 CFT KERS x 7 car gearbox speeds) mean that during a typical racing lap the efficiency will be around 64% round trip. In typical road car use the overall efficiency can be even higher so that fuel savings on the NEDC drive cycle are comparable with CVT based flywheel hybrid systems.
When in use a computer controller selects the most appropriate gear by partially engaging the high-speed clutch associated with that gear. The control system uses hydraulic pressure to close the normally open clutches and transmit the drive, seamlessly changing from one gear to another with no torque interruption as the speed across the engaged clutch reduces to near zero.
The hydraulic system is fully sealed so in F1 applications it is possible to use the normal car hydraulic system. For other applications Flybrid make and supply their own self contained hydraulic system including a very small Flybrid developed hydraulic pump and control valve block.
The CFT KERS uses conventional lubricants running at normal engine temperatures and the average heat rejection is low (just 3.0 kW average for a F1 lap) so it is easy to dissipate the heat generated by the slipping clutches. By splitting the heat to be dissipated across a number of clutches the heat management task is made much easier and the life of the wet multiplate clutches is calculated to be many thousand full power cycles.
Flybrid has granted patents for several features of the CFT KERS including the general arrangement as well as details of the cooling and lubrication system required for the high-speed clutches.