Formula for success

Remarkable developments are now hitting the road when it comes to regenerating energy to meet the demands of both new F1 regulations and future car technology. Tom Shelley reports

New F1 regulations come into force with the 2009 season – and the front-runner to meet these stringent demands is a flywheel kinetic energy storage system, both powered and delivering power through a Torotrak continuously variable transmission,

According to Tony Purnell, technical consultant to the FIA and the man entrusted with the job of making the sport more road relevant: “Car manufacturers are already working flat out to develop optimal low emission engines and there is little that engine development programmes in F1 will add to this effort. However, in five years or so, their attention will turn more and more to subsidiary devices incorporating energy recovery. By opening up this area now, Formula One can make a real different to this important facet of future car technology.”

The new F1 regulations require that the system transfers power in and out at a maximum rate of 60kW. Braking requires dissipation of up to 750kW and amounts of energy per lap can be as much as 8000kJ, so rear wheel brakes will still be very much required. Maximum energy release per lap is 400kJ and there is to be no release of energy below 100km/h. Energy recovery is only allowed when the throttle pedal is below a threshold, the brake pressure is above a threshold and longitudinal acceleration is below a threshold. Release of the power must remain under the complete control of the driver, and the power demand must be scalable between 0% and 100% of the maximum driver demand. The system must connect to the rear wheel drive train, or 4-wheel drive for 2011. And while a CVT (continuously variable transmission) can be used for the energy transfer, it cannot be employed for the main transmission, which must have between four and seven gear ratios. It can be controlled by its own electronic control unit, but must accept signals from the standard FIA ECU. The system can be used for anti-stalling and engine starting, but must disconnect if the car comes to rest with the engine stopped.

These regulations mean that, while the rate of energy storage and release is governed, the maximum amount of energy that can be stored is not. However, the system can only be charged from braking energy and not from ‘spare’ engine capacity. The trigger to release the power must be driver controlled, but the energy release and energy storage control can be automated.

There are three possible methods that have been identified that will meet these requirements.

The most promising one, and the only one described to Eureka to date, is to use a flywheel to store the energy, which requires some means of spinning it up and recovering the energy generated. When starting to brake, the flywheel will initially be rotating slowly while the car is moving quickly; and when starting to accelerate, the flywheel will be rotating at maximum speed, while the car is moving relatively slowly. Due to the need to match speed ratios, there would either have to be a CVT or some kind of motor generator combination able to produce a speed ratio of up to 6:1.

Alternatively, the system could be entirely electrical, using hybrid car technology, with a motor/generator charging either a capacitor bank or batteries and then drawing power from them, when required.

Thirdly, the system could be hydraulic, with energy stored in - and released from - accumulators, an idea already being researched and developed, primarily for heavy goods vehicles, as featured in our October 2006 issue.

The only purely mechanical solution on offer so far has been developed by Xtrac and Flybrid Systems, based on Torotrak’s toroidal traction drive variator technology, which uses rollers running in toroidal cavities between two pairs of discs. The rollers are positioned by hydraulic cylinders in such a way that they can run on, and be driven by, paths of different circumferences inside each pair of discs.

Speaking at the Autosport Engineering show in January 2008, Cliff Hawkins, development director of Xtrac, commented: “We have taken the design and engineered it [the CVT for the overall system, which is put together by Flybrid] in a manner suitable for Formula 1. There’s a lot of our technology we have put into it. It’s been load tested on rigs. We expect client rig testing to start in March and one to be running in a car this summer. It’s all on schedule for 2009.”

Nobody would be drawn as to which team’s car this was. However, Xtrac’s commercial manager Paul Leeming observed: “Most of the teams are pursuing multiple solutions. Most of the other options are electric hybrids, but there are issues with batteries. Our system will stand typical engine temperatures, while batteries will need cooling. Our system will give the mandated performances across the entire speed range.”

On the Flybrid Systems stand, Eureka found one of the completed transmission units, and encountered design and development engineer Tobias Knichel, who told us that the whole package weighed less than 25kg, yet would store 400J and deliver the maximum allowed 80HP (60kW) 6.66s. As regards getting this technology into more cars, he said: “They all have to do it, but they have been sitting on the fence a bit. We have been talking to all the Formula 1 teams. With an electrical system, you have a lot of losses when the energy is transferred. Also, our system is made of conventional materials. With electrical batteries, there are always some rare and expensive materials that are likely to give trouble at end of life. We are very pleased with the Torotrak solution, not least because it is torque controlled.”

Xtrac claims its system has a round trip efficiency of more than 70%. According to Jon Hilton, managing partner of Flybrid Systems, electrical hybrids have a round trip efficiency of around 40% overall, because of the losses associated with multiple changes of energy state – “mechanical to AC electrical, AC to DC, DC to chemical in the battery, and then all the way back again”.

Mechanical efficiency of the CVT has been found to be 78% at low loads and is said to improve at high loads. Weight is about 6kg. Maximum torque at input is 90Nm. Input speed is 12,000 rpm and output speed is 16,000 rpm. Ratio variability is from 0.41:1 to 2.45:1. Control pressure is about 50 bar pre-load. Design fatigue life for F1 is 20 hours.

The flywheel is made of carbon fibre and runs at up to 60,000 rpm. Overall, the system is said to offer low weight, low heat rejection, long-lifed components, low operating costs and is scaleable for other applications.


The mechanical flywheel system for gathering and extracting vehicle kinetic energy power via a Torotrak CVT seems to work and is, so far, the only viable system that has been described to us

Electrical hybrid systems may be very satisfactory for road cars, but suffer from problems associated with battery weight, the need to keep them cool and low round trip efficiencies

The only other alternative is hydraulic, which has been researched for other applications, but nobody seems to be adopting this route for F1

Tom Shelley

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