the trick is to keep the slip
percentage in our clutches small
Like other Flybrid Systems offerings, the CFT KERS uses a fl ywheel as the storage medium. However, it uses a slipping clutch mechanism to transmit drive to the rear wheels
at the losses in the clutch it is very straightforward – the torque across both faces of the clutch is the same and the difference in speed represents the losses. What you get is this:
Power in = torque in x rotational speed in (Nm x rad/s = Kw)
Power out = torque out x rotational speed out
But, torque in = torque out, so effi ciency = rotational speed out / rotational speed in
‘In our CFT, the trick is to
keep the slip percentage in our clutches small and then it’s really quite effi cient. If you can slip where one side is only going 10 per cent faster, or slower, than the other side then the loss is only 10 per cent. That’s more effi cient than CVT [continuously variable transmission].’
EXTRA RATIOS With three clutches controlling the drive, the transmission essentially gains an extra three ratios and that creates an effect Hilton compares to another type of vehicle entirely: ‘We have three gears to choose from but, because we are connected [to the] gearbox input shaft side
rather than the wheel side, we multiply our three ratios by the six in the gearbox already to give us 18 speeds effectively to choose from. It’s like a mountain bike with three gears on the front and six on the back – some of those ratios overlap each other, but we have a wide range to play with, and this means there is always a reasonable effi cient gear available. We then choose to close the right one by computer to minimise the slip across it. It’s hydraulically actuated, but is controlled electronically, and we write all the software to get the right clutch instantaneously, to choose the one with the least slip. Then, before it grips solid, you change to the next closest and it automatically does that. ‘If you can arrange your ratios
properly and set it up in the car so the slip across the clutches is relatively small, it’s actually a pretty effi cient method of doing the power transfer.’ A Flybrid-developed computer
controller selects the most appropriate gear by partially engaging the high-speed clutch associated with that gear. The control system then uses hydraulic
Le Mans •
www.racecar-engineering.com
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