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Volvo Car and Flybrid vehicle testing showing flywheel KERS can deliver fuel savings up to 25%, with significant performance boost

Rear-axle KERS unit. Click to enlarge.

Volvo Car Group and engineering company Flybrid Automotive, part of the Torotrak Group, have been conducting UK tests of lightweight Flybrid flywheel KERS (Kinetic Energy Recovery System) technology. (Earlier post.) The test car applies flywheel technology to the rear axle, while the combustion engine drives the front wheels.

The four-year partnership, using real-world driving data from tests on public roads and test tracks in both Sweden and the UK, has shown that the flywheel-based hybrid technology can deliver an 80 hp (60 kW) performance boost, together with fuel savings of up to 25%. (Earlier post.) The research forms part of Volvo’s continued Drive-E Powertrain research and development program; the Flybrid KERS tests show that it is a lightweight, financially viable and efficient solution for efficiency and performance, the partners said.

The system is the first full-scale trial of a rear-axle mounted flywheel system in a front-wheel-drive passenger car and is the result of a partnership between Flybrid, Volvo and the Swedish government.

We are the first manufacturer that has applied flywheel technology to the rear axle of a car fitted with a combustion engine driving the front wheels. The next step after completing these successful tests is to evaluate how the technology can be implemented in our upcoming car models.

—Derek Crabb, Vice President Powertrain Engineering at Volvo Car Group

The Flybrid KERS is fitted to the rear axle of an S60 powered by a 254 hp (189 kW), 5-cylinder T5 gasoline engine. Under braking, kinetic energy which would otherwise be lost as heat is transferred from the wheels to the KERS, and is used to spin a 6 kg carbon fiber flywheel at up to 60,000 revs per minute. The combustion engine that drives the front wheels is switched off as soon as braking begins.

When the car starts moving off again, energy stored in the spinning flywheel is transferred back to the rear wheels via a specially designed transmission, and can either boost power or reduce load on the engine. The energy in the flywheel can then be used to accelerate the vehicle when it is time to move off again or to power the vehicle once it reaches cruising speed.

The flywheel’s stored energy is sufficient to power the car for short periods. This has a major impact on fuel consumption. Our calculations indicate that it will be possible to turn off the combustion engine about half the time when driving according to the official New European Driving Cycle.

—Derek Crabb

Since the flywheel is activated by braking, and the duration of the energy storage—i.e., the length of time the flywheel spins—is limited, the technology is at its most effective during driving featuring repeated deceleration and acceleration cycles. In other words, the fuel savings will be greatest when driving in busy urban traffic and during active driving.

If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra 80 horsepower (60 kW); due to the swift torque build-up, this translates into rapid acceleration, cutting 0 to 62 mph figures by seconds.

The experimental car, a Volvo S60 T5, accelerates from 0 to 62 mph around 1.5 seconds quicker than the standard vehicle. The KERS drive to the rear wheels also offers the experimental car part-time four wheel drive to add extra traction and stability under acceleration.

Flywheel propulsion assistance was tested in a Volvo 260 back in the 1980s, and flywheels made of steel have been evaluated by various manufacturers in recent times. However, since a unit made of steel is large and heavy and has rather limited rotational capacity, this is not a viable option.

The Flybrid flywheel that Volvo Cars used in the experimental system is made with the combination of a steel hub and carbon fiber outer. The ~6 kg flywheel has a diameter of 20 centimeters. The carbon fiber wheel spins in a vacuum to minimize frictional losses.



6 kg and dia of 20 centimeters. Why do some people think that we need >100 kg of batteries in a car?


Flywheels have been around for many decades. Why wasn't a flywheel Hybrid mass produced years ago?

Since the flywheel is rather light, why not couple it with a battery (to extend range without the ICE running) and make it an extendable range AWD plug-in hybrid.


What PeterXX said. Why does this method lag so far behind electric hybridization, when it sure seems like it might be nearly as effective, far less expensive, require less vehicle weight and complex battery manufacture?


If you click back further into the archived GCC posts on this topic you'll see that Volvo has been working on the KERS-based hybrid since 2009. Moreover, that's from public releases -- likely they've been tinkering in labs well before that.

The electrical solutions have gotten quite a bit more emphasis for a number of good reasons: power electronics have made huge leaps outside of automotive applications, electric energy storage supports auxiliaries (e.g., HVAC) more readily for start/stop environments, etc.

Further, the long goal of zero propulsion system emissions frankly gets more near-term attention than perhaps it should (the perfect as enemy of the good). This causes external funding and subsidy sources such as the US ATVM program and ZEV credits to be focused around electric (or H2 FCV) and practical, effective measures like flywheel KERS are orphans. (I would add that there is a tremendous "emotional" appeal to driving electric. Although my Leaf is a sensible car only because of a confluence of personal circumstances that fit VERY few people's lives, even doubters are stunned at the smooth, comfortable, quiet and seamless rush of power this odd little car provides. You can fall in love with the EV ride pretty quickly. That's why people are able to convince themselves that 600kg of battery in the Tesla Model S is "disruptive" and "visionary" when it is in fact mostly just a cool and lovely engineering stunt.)

Finally, you can't ignore the industrialization factor. High-speed carbon fiber flywheels and hermetic vacuum chambers do not have the giant industrial base of Lithium chemistry batteries, high-power IGCTs, etc., so the parallel activities of cost optimization present additional challenges. The target production cost for this addition to the car is USD1600 (2009 economics), or somewhere less than USD2000 today. Considering the low weight of the module (flywheel + coupling + gear reduction under 75kg) and the relatively minor packaging challenge to fit it to the rear axle, it is a very interesting solution. When you further consider there are no cold-weather operability or other thermal management hurdles, no public infrastructure heroism needed, a very benign failure mode, and relative transparency to the operator, your next question really has to be: "when can I get one?"


Why haven't flywheels already reached mass production is a simple question with a simple answer.
Materials science had to develop to a stage where they would be both economic and safe.
That has happened about now.


Thanks guys. I agree. This really seems like the sub-$1000, sub 100kg (with rear drive assembly), zero rare earths, solution to mass hybridization.


Herman, I could have been persuaded but a closer look at the KERS unit convinced me that their unit cannot possibly be manufactured for anything like $2000. But even if it was, there are still some serious doubts in my mind now that I think about it.

A 60mph to rest energy capture is only about 600MJ in a Prius sized vehicle and that's if you can capture energy at an initial peak rate of 240Kw. And it pre-supposes you are going to be able to re-use that energy before the flywheel spins down. On the race track that is not a problem but in real life... ?

Then there are these considerations at play. First, only the rear axle where the KERS mounts can actually harvest kinetic energy. Second, wheel skid would definitely occur if the system attempted to recuperate half of the KE (300MJ) in a reasonable short space of time. Third, it is only the front axle that has the ability to do any wild braking bearing in mind that the vehicle will attempt to stand on its nose during a severe emergency braking manouvre thus removing the downward pressure at the rear and therefore the wheel friction that the rear tires could exert.

It also begs the question. How simple and reliable can a transmission be, that is able to slow down one mechanical system while speeding up another and shortly after have the capability to do the opposite ?

Obviously this is race car technology that is looking for application in the mass automobile market. But where ?

With an EV handling large amounts of regenerated power suddenly is never a problem. Whereas with a hybrid, braking power of only 10Kw seems to have significantly reduced brake wear, even though owners say it probably satisfies only 95% of braking opportunities.


Flywheels compete with supercapacitors (or small, specialized packs of batteries that have characteristics that match up with hybrid usage).

You're also confusing issues for electric cars with things flywheels will deal with. There is no point in talking about the weight of the Tesla pack or your Leaf. The comparison here is whether 20kg of ultracaps (or 10kg of A123 cells like they were using in F1 from 2010-2013) plus something like a 18kg Yasa-250 motor is better from a total life cycle perspective compared to the flywheel and its components.

Toyota didn't put batteries in the MILLIONS of hybrids because they like spending extra money. If flywheels were better, then they would have used them. If something has changed then it may be worth revisiting. But I doubt Toyota spends extra $$$ building each car because they've got some secret love affair with batteries.

If Volvo has found something cheaper and better in some way, then I'm sure the world will beat a path to their door.


DaveD I think I understand your point but just to be clear, nobody is using ultracaps in hybrids right now, correct? How far away from general usage is the ultracap technology?

The 3600 lb curb weight of the Ford C Max Hybrid is due to batteries, IIRC.


Nobody is using flywheels in production yet either. Some F1 teams were using supercaps in their hybrid last year (notably Red Bull) and Toyota was using them in their Hybrid in LMP1 while Audi was using a flywheel in their LMP1 car.

So I think my statements are correct because nobody is using either supercaps or flywheels in production yet but both are being used in experimental cars and racing.

Production hybrids are still using batteries.


DaveD, you're the confused one here. I don't see how you don't get it -- my point is what attracts funding, both politically (the notion of "ZEV" and associated gov't R&D spend/tax credit) and in terms of industrial maturity of the targeted technologies. I'm not comparing life cycle costs or mission (would that politicians and legislative staffs actually did). You should really read what other posters are saying before you go off.

And this reflects your hair-trigger reflexiveness:
"But I doubt Toyota spends extra $$$ building each car because they've got some secret love affair with batteries."

Just to match your tone: are you ENTIRELY clueless about Project G21, and Yaegashi-san's very specific mission to bridge the gap between pure battery EV and gasoline vehicles, to in fact accomplish the G21 goals with an electric hybrid? Yes, they were completely committed to this technology, since you incorrectly (and snidely) propose otherwise.


Thanks, T2, for a rational view. I'll have to think about much of what you said -- good points. I would openly disagree with one: "With an EV handling large amounts of regenerated power suddenly is never a problem". It's "never a problem" because when the battery can't accept the power, there's no regeneration. When the battery is fully charged, too cold, or too hot, it doesn't accept the regen and the brakes do the work. I spent plenty of time with only one or two battery bars this winter and frequently saw no regen lights until things warmed up. Simply rejecting the regeneration from the flywheel when, for example, the system senses skidding doesn't seem too difficult.


Looking back at my post, I can see how you would read it that way. I was actually in a good mood and *trying* to make more of a light-hearted tone and just executed poorly. Wasn't intended to be snide. Sorry.


I just found a reference that Mazda is shipping some supercaps starting this July. It's not using the kinetic side for propulsion, but rather excess energy from braking for running other electric components in the car. They are claiming a 10% improvement from this system and that it's total weight is just 9.3kg. I assume it can be that light as it's not including any kind of motor or transmission.



Ditto an apology re: my grumpiness. I have enjoyed the relative civility of GCC and do not want to be the guy who ruins it.

BTW, I did see articles on GreenCarReports and C&D coinciding with this presser. GCR links to test drive report from 2013 here:

Not much different from here except a weight number of 60kg, and a statement that Volvo would expect to see it in service "around 2020". So we have plenty of time to speculate.


One more slightly OT note about efforts to achieve efficiency without significant electrical energy storage/conversion in a SAE article...

I did not see this speech referenced in February's GCC archives, so forgive me if I'm way late to the table. CARB member Dr. Daniel Sperling spoke to the 2014 SAE Hybrid & EV Symposium at the February meeting in La Jolla. In my mind the key message in his speech was this:

"[W]ith the possible exception of full-size trucks, meeting the 54.5-mpg fleet target will not be an overly difficult task, most experts believe. Because of the comparatively high cost of batteries, traction motors, power control, and charging systems, it would take an estimated outlay of $150 billion to achieve rough cost parity with conventional ICE vehicles by 2025, according to a University of California-Davis analysis cited by Dr. Sperling. The cost of both types won’t likely converge until 2050, he said, when production scale and systems innovations for plug-ins are realized."

This isn't some Exxon guy speaking -- he is a card-carrying CARB exec and ZEV supporter. Clearly, the industry will seek non-electric solutions if they are reasonably effective, and some of them seem to be promising.


I have just quickly skimmed through your comments. I can agree on much, e.g. lack of industrialization, focus on electrification instead, etc. but I think you have missed the crucial point. This is the CVT needed in the system. It has to be very efficient and of course also small, light, durable etc. Not until Torotrak made substantial improvements in this area, KERS has become a viable option. Still, I think there are more improvements in the pipeline; not only by Torotrak but also other development companies. The flywheel itself could have been made many years ago, as many of you already noted.


Thanks Herman. I shouldn't be making comments when I'm running between meetings at work and not taking the time to read through what people are really saying and making sure my responses are thought out.
I like GCC because it's usually a civil place to ask questions or throw out some ideas. I avoid sites where everyone is spewing vile so I don't want to make it bad here.
In response to your original post, you're absolutely right that the money/investment in the tech follows political goals very often rather than the best potential technology.


I think we should be glad they there are several approaches to hybridisation being taken: electric, kinetic and hydraulic.

Electric seems effective, and is a step towards a fully electric vehicle, but it is expensive.

If kinetic (or hydraulic) was nearly as good, but considerably less expensive, it would make a big difference, + as people start to really use it, they will find ways of improving it further (after say 5-10 years) and we will see greater savings at lower cost.

For instance, a diesel kinetic hybrid might be very economic and still affordable.


Toyota and BMW will probably use new ultra caps instead. Are those two making a mistake?

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