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Torotrak and Xtrac Partner on F1 Energy Recovery System; Applicable for Road Vehicles

Torotrak and Xtrac variator in the KERS system. Click to enlarge.

Toroidal traction drive specialist Torotrak plc (earlier post) and vehicle transmission design and manufacturing company Xtrac Ltd have entered into a licence agreement that will enable Xtrac to develop highly efficient and compact continuously variable transmissions (CVTs) for use in the new kinetic energy recovery systems (KERS) proposed for Formula One (“F1”) motor racing.

In 2009, F1 is introducing new rules that will lower the environmental impact of the sport. (Earlier post.) Part of this is to recover deceleration energy that can be stored for acceleration. Xtrac will exploit Torotrak’s full-toroidal traction drive technology for use in kinetic energy recovery systems within the motorsport industry.

Some of the new KERS systems under development will be mechanically based and will utilize a flywheel to recuperate, store and subsequently discharge a moving vehicle’s kinetic energy, which is otherwise wasted when the vehicle is decelerated. The kinetic energy is stored during a braking maneuver and is then released back into the driveline as the vehicle accelerates.

The toroidal traction drive variator, being developed with Torotrak and using Torotrak’s patented technology, is a central element in these mechanical flywheel-variator KERS systems as it provides a continuously variable ratio connection between the flywheel and the vehicle driveline, via the vehicle’s gearbox.

The components within each variator include an input disc and an opposing output disc. Each disc is formed so that the gap created between the discs is ‘doughnut’ shaped; that is, the toroidal surfaces on each disc form the toroidal cavity.

Two or three rollers are located inside each toroidal cavity and are positioned so that the outer edge of each roller is in contact with the toroidal surfaces of the input disc and output disc.

As the input disc rotates, power is transferred via the rollers to the output disc, which rotates in the opposite direction to the input disc.

The angle of the roller determines the ratio of the variator and therefore a change in the angle of the roller results in a change in the ratio. With the roller at a small radius (near the center) on the input disc and at a large radius (near the edge) on the output disc the variator produces a “low” ratio. Moving the roller across the discs to a large radius at the input disc and corresponding low radius at the output produces the “high” ratio and provides the full ratio sweep in a smooth, continuous manner.

The transfer of power through the contacting surfaces of the discs and rollers takes place via a microscopic film of specially developed long-molecule traction fluid. This fluid separates the rolling surfaces of the discs and rollers at their contact points.

The input and output discs are clamped together within each variator unit. The traction fluid in the contact points between the discs and rollers become highly viscous under this clamping pressure, increasing its ‘stickiness’ and creating an efficient mechanism for transferring power between the rotating discs and rollers.

The combination of a Torotrak variator—providing mechanical efficiency that should be in excess of 90 per cent—with a flywheel of advanced construction results in a highly efficient and compact energy storage system.

Torotrak has granted a license to Xtrac to design, manufacture, assemble and distribute components or complete variator systems, which incorporate Torotrak’s technology, to its F1 customers.

While Xtrac will supply variator units to its customers, the flywheels for these energy recovery systems are being developed separately by the Formula 1 teams themselves and their specialist suppliers. Torotrak will provide the control system expertise.

Torotrak and Xtrac believe that the variator-flywheel solution provides a significantly more compact, efficient, lighter and environmentally-friendly solution than the traditional alternative of electrical-battery systems.

The variator weighs less than 5kg in these applications and provides a high level of mechanical efficiency, enabling the overall mass of the mechanical KERS systems to be minimized. This mechanical efficiency, combined with the variator’s ability to change ratio very rapidly, helps to optimize flywheel performance.

—Chris Greenwood, technology director at Torotrak

The two companies consider that the system is applicable to other motor sports and everyday vehicles and see the potential for wider applications—particularly on high-performance road cars—as an aid to performance and also as a means of developing future products with reduced CO2 emission levels.

The system supports the current trend in powertrain design for engine downsizing, by providing a means of boosting acceleration, overall performance and economy independently of the vehicle’s engine and without the need for complex electrical-battery hybrid architectures.

A CVT-controlled flywheel is particularly suited to stop-start driving situations when real-world fuel economy is often at its worst. In these conditions, the variator-flywheel system can assist the launch of a vehicle which has slowed down or come to a standstill, by utilizing the kinetic energy stored in the flywheel. In heavily congested traffic, where a car is frequently stopped and restarted, the system can help alleviate the heavy fuel consumption and emissions of greenhouse gases normally associated with these conditions.

For the F1 applications, the stored kinetic energy can be applied by the driver on demand whenever required—at a rate and for a time period set by the regulations—to boost performance for rapid acceleration. The device is particularly beneficial when exiting corners or for tricky overtaking maneuvers.

The mechanical efficiency, compactness and mass of the variator system is critical since it directly influences the size and the ability to package such as system into an F1 car, or into a road vehicle. The size, torque capacity and response of the unit is critical to take the full advantage of having a flywheel KERS system.

—Adrian Moore, technical director at Xtrac


Rafael Seidl

F1 cars are light but a flywheel will still need to be made of carbon fiber in a high vacuum and rotate really fast (50,000RPM+) in a high vacuum to provide enough capacity. That means using magnetic bearings, which cannot sustain the gyroscopic forces involved so the whole thing has to be mounted in a gimball. That in turn means energy tranfer into and out of the device has to be electromagnetic. Rosen Motors did pioneering work on this in the 1990s:


They ultimately folded, mostly because you also need a containment shell in case the flywheel ever disintegrates for any reason. Advanced composite materials such as GLARE that did not exist at the time (see p40 for a blast-proof air cargo container developed after Lockerbie):


Containment is essential because at nominal RPM the energy stored in a superflywheel is equivalent to that needed to lift the whole vehicle some 80m into the sky. In other words, the whole thing is a very expensive IED.

For F1, the energy needs to be stored in e.g. ultracaps. The motor/generators may be connected to the front wheels. Note that F1 cars feature a very expensive slab of super-dense tungsten (~40kg) that can be repositioned to tune weight distribution for each track. Ultracaps are bulkier but at least the weight would be doing something useful.



The illustration does not appear to show the type of setup you are describing. Could it be they are not going to store as much energy as you think? I do wonder about the effect of the gyroscopic forces on the handling of the vehicle, however.


How rapidly does the flywheel lose energy? If it is high (~1% per minute) it wouldn't be the best fit for most passenger cars, which often sit for several hours. However, high initial energy capture might make this system good for delivery vehicles, taxis, and urban buses even if the energy loss is fairly rapid.

Rafael Seidl

@Nick -

F1 cars weigh 550kg and need to slow down from well over over 300kph to perhaps 150kph in tight corners. That's 1.86MJ or 0.6kWh. Such an extreme braking process only lasts ~3 seconds, so the system has to dissipate ~600kW.

A hybrid system would most likely only be laid out to recuperate a fraction of that, e.g. 150kW. Even so, the flywheel has to be spun up and down very rapidly, while the vehicle is in a tight bend. Unless the flywheel is isolated via a gimball cage, the gyroscopic forces will produce very high mechanical and thermal stresses on the bearings, plus steering will be affected. Thermal load on the bearings is also the primary reason why you don't want a high rate of energy loss in a flywheel, even if it is discharged again immediately after a recuperative braking event.

So yes, the system I described earlier is very different from the one ToroTrak is proposing. I just don't think their idea for recuperation is going to work, though there may be value in switching from a sequential step-by-step gear box to a mechanical CVT, provided it can be cooled. Toroidal designs scale to higher torque levels than the belt or cone-ring types.

I've seen examples of flywheel energy storage on large commercial vehicles (e.g. the Frauenhofer Autotram) but in those applications, space and weight are not quite as critical as in F1. Nevertheless, by the time you add up all the high-tech engineering and manufacturing costs, I doubt you'll save a whole lot relative to ultracaps.


What happened to the hydraulic accumulator idea? Have the F1 teams given up on this route?


The amount of energy releasable and the rate at which it can be released is stipulated in the proposed FIA regulations for KERS devices for 2009.

If you have a quick look, you will find that the maximum released energy is limited to 400kJ in any one lap and the power of the device is limited to 60kW, so nothing like as high as the numbers mentioned above.

Have a look here for a good discussion of the limitations of the FIA proposals:


richard schumacher

Fearless/rash prediction: nothing like this system will ever be successful in a production vehicle.

Torotrak Lover

richard schumacher.

completely ridiculous statement with no means to backup your arguament. i suggest you do some research into Torotrak and their IVT technology and you will see that it could be immediately successful in production vehicles if given the chance.
Torotrak IVT would allow drivers to immediately tap the full power of any engine by allowing acceleration of a vehicle at its maximum engine torque, thus providing sustained and uninterrupted acceleration.
alternatively from a fuel efficiency and environmental point of view, it would allow for a vehicle to accelerate steadily at a low and constant engine revolution thereby saving fuel. it could also act as a cruise control. any engine characteristic such as speed, torque, acceleration, consumption would be completely controllable and any one of these characteristics could be maintained at a constant level.

when they finally do get this into production vehicles, it will exceed all expectations; trust me!!


It will be interesting to see if Nissan's experience in CVTs helps Renault, or if Toyota's CVT experience helps them.

Rafael Seidl

@Torotrak Lover -

acceleration requires power, a transmission can only optimize the match between the speed the engine delivers it at and the speed the differential requires. Maximum engine torque is not relevant for acceleration, though keeping the engine speed low for a given level of power demand will keep down the noise and fuel consumption.

The reason Torotrak hasn't been very successful is that their toroidal CVTs are very expensive. For moderate engine torque ratings, the simpler cone-ring type is a lot cheaper.


Nice animation of how it works (140MB):



I also think it is difficult for this system to work in F1 cars (for energy recuperation)
Regarding switching from sequential gearbox to CVT,
for now, CVT type transmissions are not allowed in F1 cars according to the FIA regulations.


Chrysler built a flywheel hybrid race car in the early 90s which was set to contest Le Mans before the project was cancelled. They claimed that the gyroscopic effects of the flywheel and the potential for catastrophic failure caused them to abandon the flywheel design. I wonder how this new system will overcome these problems.

See this article from 1994:

Blair Anderson

GHG emmissions from F1 are diddly squat compared to GHG from the worlds stop/start refuse collection and other similar utility vehicles. It's not as sexy as F1 but to city dwellers the pay off in reducing kerbside noise and smelly emmissions, let alone the fuel savings/carbon compliance involved would make adoption of KERS a 'best practice' public good. When can we have it!

Andres Beltran

Bueno es cuestión de tecnologías y no de cómo se escucha bonito.
La formula uno es eso y no se pueden estancar en boberías, en este momento necesitamos desarrollar la tecnología mas eficiente, en CVT tenemos una carrera por la mas eficiente y NO se puede quedar fuera la F1.
Actualmente la CVT es cara delicada débil inestable complicada y lo peor de todo para POCO PESO.
Para mi es un gran gusto poder escribir la presente, esperando que en un momento de tranquilidad puedan leer estas líneas y cuantificar el contenido, y de esa manera darme un poco de su Atención que para mi es de ALTA IMPORTANCIA.
Por esta y otras muchas razones me dirijo con respeto.
Quiero DESARROLLAR MI INNOVACION Ustedes son La Clave, por los conocimientos y contactos que tiene.
Uno de mis proyectos es interesante por el resultado de sus funciones.
Ahorros Aproximados:
a- 25% En todo tipo de Combustibles.
b- 40% En todo tipo de Motores., (controlado régimen de RPM)
c- 30% Emisión de Contaminantes.
d- 35% Tiempo aceleración (Velocidad de Crucero)
e- 30% Caída de velocidad en Pendientes.
f- 70% Crucetas (No hay jaloneo)
Es difícil creer en alguien o en algo, hasta que se puede comprender, y para esto tengo que demostrar el funcionamiento del EQUIPO EN CUESTION.
Muy bajo CAPITAL:
1- PROTOTIPO: Para demostrar su funcionamiento y enumerar cada una de sus cualidades, como primer paso y estar seguros antes de patentar.
2- PATENTAR: De esta manera ya convencidos del buen funcionamiento del EQUIPO y con Gusto se conformaría la SOCIEDAD, y seguir con los gastos de Patentes.
MI CVT ES (TODO VEHICULO Y TODO PESO) bici coche tráiler locomotora etc..
ATTE: Andrés Beltrán T. [email protected] [email protected]

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