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Gen 2 low-cost flywheel-based Torotrak KERS for buses en route to production in mid-2016

Torotrak Group and Wrightbus have completed the in-service trial of a Wrightbus StreetLite midi bus fitted with Torotrak’s Flybrid mechanical kinetic energy recovery system (KERS). The trial was made possible by and conducted with Arriva, one of the largest bus operators in the UK.

The trial, conducted on a standard bus route in Gillingham, Kent from March 2015, successfully demonstrated the performance of the Flybrid KERS under real-world operating conditions including the capture, storage and release of energy from the brakes and the drivability of the system. The learnings from the trial have already proven invaluable in selecting the operating strategy for the volume production system, which will be a second generation design, the partners said.

The KERS captures the kinetic energy that is normally wasted when the bus is braking and stores it in a steel and carbon fibre flywheel weighing 8.5kg and spinning at up to 30,000 rpm. The energy stored in the flywheel is then delivered back to the wheels reducing the energy that is required from the engine to accelerate the bus and so reducing the fuel consumption of the vehicle. The process of capturing, storing and releasing energy back to the wheels is fully automated, giving a high quality driving experience. The Flybrid KERS technology was developed in Formula 1 and has been applied in a variety of vehicle applications including motorsport, passenger road cars and excavators.

Already undergoing detailed testing on rigs and in a new Euro VI StreetLite bus, the new KERS design is intended to further improve bus fuel efficiency and reduces the KERS weight and cost, helping to improve the bus operator payback. The new design has reduced parts count and system weight by 30% and 80kg respectively and incorporates a simpler 2x2x2 clutched flywheel transmission system reducing coast-down losses and improving efficiency during torque transfer.

In collaboration with its global Tier 1 manufacturing partner, Torotrak is on track to commence production of bus KERS systems from mid-2016 onwards. Tier 2 suppliers across the UK and other parts of the EU have been nominated and the manufacture of production tooling is currently underway.

The Flybrid KERS for buses is suited for the long service life and repeated stop-start duty cycle of public service buses. Using common materials and manufacturing methods, and removing the requirement for replacement batteries during service life, the flywheel-based technology can offer value and an opportunity for widespread hybridization.

After the invaluable in-service trials with Arriva, Wrights and Torotrak have confirmed that the next generation of flywheel is now under extensive durability testing, and after successful fuel trials, will be put into service trials from May 2016. Production vehicles will be able to be delivered into service late in 2016.

—Brian Maybin, Wrightbus Engineering Director

As one of the largest bus operators in the UK with over 5,900 buses, improving the efficiency of our bus fleet and reducing emissions is important to us and our customers. Having collaborated with Torotrak and Wrightbus on the flywheel project since 2012 we are looking forward to installing Flybrid KERS across our bus fleet from Q3 2016 onwards.

—Ian Tarran, Engineering Director at Arriva UK Bus



Don't see a reason to run the additional complication of flywheels in a vehicle unless they are more efficient and less costly than regeneration. The aim of the future is BEVs not a continuation of ICEs. This is especially true when discussing busses. The future is just waiting for the better battery and when it is available, all this other stuff become nonsense.


It's not just about efficiency with a bus. It's about duty cycle and sensitivity to weight. Buses don't care about added weight much, and they're going to cycle that flywheel WAY WAY WAY more than a privately owned compact car HEV/BEV with regen braking.


Lad: Flybrid KERS has nothing to do with ICE, it an alternative to batteries and supercapacitors.


Lad: “Don't see a reason to run the additional complication of flywheels in a vehicle unless they are more efficient and less costly than regeneration.” The round-trip efficiency of flywheel regeneration can be quite high, and they can “eat” an incoming high-power peak more readily than can most energy-dense batteries can. They also don’t care if they’re hot or cold. (This time of year it’s painful to watch regen power at about half the normal value when my Leaf battery is cold. BTW, before Henrik shows up and says “but TESLA…”: trust me, even the other-worldly Muskmobiles have the same problem when cold-soaked all day long.)

I am inclined to be more excited about ultra-cap technology, but flywheels have a place in vehicles like urban busses or delivery trucks.


With their inherent ultra quick charge, long lasting and very low maintenance nature, future ultra caps could become the ideal regen energy capture units.

However, their energy efficiency has to be tripled and cost reduced to better compete.

It took may decades for superior electronic watches to replace the mechanical units.


Is there a place for flywheels? Let's see what the research says. I'll for anything that will eliminate burning fossil fuels in the air.

Thomas Pedersen


Since the Flybrid delivers mechanical power (via clutch and CVT) rather than electrical, it appears to be particularly directed towards hybridization of ICE buses.


With all the stop and go with high miles the flywheel could be more cost effective.

Brian Petersen

A moving bus (or other large vehicle) possesses mechanical kinetic energy. A flywheel keeps that as kinetic energy. Then putting it back into the bus keeps it as kinetic energy. The power source now just has to make up losses (friction, aerodynamics). It doesn't matter what that power source is.

Keeping mechanical power mechanical is theoretically more efficient than converting it to electricity and then to chemical energy (battery) then back into electricity then back into mechanical energy. The regeneration process used in battery electric vehicles is not even remotely close to 100% efficient.

Obviously exchanging energy between the flywheel and the drive wheels requires a very good CVT (and this has its own set of difficulties), but it's quite conceivable that there are fewer losses that way than by doing regeneration via the batteries.


E-motors/generators and ultra caps are getting very efficient and may not wear as fast as flywheels?

Brian Petersen

That's what they pay the engineers for ... running the simulations and crunching the numbers to see whether this sort of system is worthwhile for each application. It might be ... it might not be ... depends how each application is used. Electric motor/generators have a relationship between the voltage and the frequency (the "V/Hz ratio") and there are limits to how far the motor drive can go "off design" and still function efficiently. If it has to be capable of 120 km/h then the efficiency of regeneration at 20 km/h is going to be lousy. Flywheels and mechanical CVTs don't have that limitation.

Henry Gibson

A high pressure nitrogen hydraulic fluid bladder tank is adequate for regeneration for stops and starts in a series hydraulic hybrid system. Double the distance for fuel use or more has be proved by Artemis Digital Displacement. Parry People Movers have demonstrated very high efficiency and regeneration and low fuel use with its long in service and much longer demonstrated flywheel hydraulic systems. ..HG..

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