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Volvo Car Corporation developing flywheel kinetic energy recovery system; considering broad application

Flywheel KERS component details. Click to enlarge.

Volvo Car Corporation (VCC) has received a grant of 6.57 million Swedish kronor (US$1.05 million) from the Swedish Energy Agency to develop and test a flywheel kinetic energy recovery system in a joint project together with Volvo Powertrain and SKF. SKF is also a partner in the Ricardo-led KinerStor project to demonstrate the viability of low-cost flywheel hybrid systems (earlier post).

The new system, known as Flywheel KERS (Kinetic Energy Recovery System), is fitted to the rear axle. During retardation, the braking energy causes the flywheel to spin at up to 60,000 revs per minute. When the car starts moving off again, the flywheel’s rotation is transferred to the rear wheels via a specially designed transmission.

Flywheel KERS system layout. Click to enlarge.

Flywheel KERS has the potential for reducing fuel consumption by up to 20% while giving a four-cylinder engine acceleration like a six-cylinder unit, says Derek Crabb, Vice President VCC Powertrain Engineering. Tests in a Volvo car will begin in the second half of 2011.

The combustion engine that drives the front wheels is switched off as soon as the braking begins. The energy in the flywheel can be used to accelerate the vehicle when it is time to move off once 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. However, this has a major impact on fuel consumption. Our calculations indicate that the combustion engine will be able to be turned off about half the time when driving according to the official New European Driving Cycle.

We are not the first manufacturer to test flywheel technology. But nobody else has applied it to the rear axle of a car fitted with a combustion engine driving the front wheels. If the tests and technical development go as planned, we expect cars with flywheel technology to reach the showrooms within a few years. The flywheel technology is relatively cheap. It can be used in a much larger volume of our cars than top-of-the-line technology such as the plug-in hybrid. This means that it has potential to play a major role in our CO2-cutting DRIVe Towards Zero strategy.

—Derek Crabb

[Porsche has applied flywheel KERS technology from Williams in two racing vehicles, the 911 GT3 R Hybrid (earlier post) and the 918 RSR (earlier post). Matthias Mueller, President and CEO of Porsche AG suggested that the flywheel technology might appear in “road-going cars as well.”]

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 stops and starts, Volvo says. In other words, the fuel savings will be greatest when driving in busy urban traffic as well as during active driving.

Flywheel module. Click to enlarge.

If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra boost of 80 horsepower; due to the swift torque build-up this translates into rapid acceleration, cutting 0 to 100 km/h figures significantly.

Volvo tested flywheel propulsion assistance in a Volvo 240 back in the 1980s. The flywheel that Volvo Car Corporation will use in its current test car is made of carbon fibre, weighs about 6 kg, and has a diameter of 20 centimeters. The carbon fibre wheel spins in a vacuum to minimize frictional losses.



Given the way things are going right now, I actually think the traditional battery / motor setup could end up cheaper. Certainly better packaging and no rotational inertia problems.


It is much harder to advance a mechanical system like a flywheel the way you can electronics and even batteries or supercapacitors.

Flywheels are already behind in energy density and the price for the others is dropping rapidly. This seems like an interesting attempt, but most likely a dead end competitively and price-wise.

We'll see.


The flywheel may store energy well. Perhaps a genset/motor would be cheaper and more flexible than the CVT.


In recovering energy during braking, mechanical systems have much higher efficiency than electrical systems. No energy conversion is needed. You go from kinetic energy in the car to – kinetic energy in the flywheel. Provided that an (efficient) infinitely variable mechanical transmission could be practically realized, nothing on earth can be more efficient. This is of course nothing new. An undergraduate student could make a simple calculation in a couple of minutes to show this. Still it is surprising to find that the faith in electric drive system is so great (religion?) that this fact is simply not accepted.

Does the poor efficiency of an electric drive system mean that it has greater development potential than a mechanical system? Probably, in relative terms and presumably, the gap could also be reduced but in the end, the efficiency will still be lower than for a well-developed mechanical system. Is this an indication that this is the beginning to the end of the (very short) HEV era and that HKVs will rule in the future? We do not know. So little funding is spent on HKVs that we have no idea if there are some practical limitations or any other kind of a “show stopper” that would prohibit the commercial introduction of HKVs. However, we cannot neglect this potential any more but simply have to find out if this is the case. Therefore, I am personally very happy for this project and I wish them good luck.

If >90% of the readers on this site find my discussion above provocative, I do not care. Instead, I hope that the rest of you might be spurred to participate in the fierce discussion that will come.



"In recovering energy during braking, mechanical systems have much higher efficiency than electrical systems."
You got a source for that, or are you just mouthing off?

As for your critique of electric systems...are you referring to the the 99% ROUND TRIP efficiency of the supercapacitors they are getting now? Yeah, that's just horrible LOL

And I'm sure that Formula 1 teams who spend as much as $400Million a year developing their cars ALL use battery systems instead of flywheels for their KERS because of...religious issues ROFLMAO.

Whatever dude. Show some actual facts or just admit you're the religious nutjob.


You forgot that the supercapacitor is not the only component of an electric drive system.

Flywheel-based KERS were initially developed just for Formula 1. However, the current regulation does not give any advantage for a system that has higher efficiency than electric KERS.


I think flywheel KERS makes sense on paper, but a car is going to have electrical systems anyway, so it makes sense to have electrified drive systems.

Also the CVT will have to deal with converting 2k rpm to 60k rpm and back again sounds like its waiting for problems!


"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 stops and starts..."

My concern is about for how long can the energy can be stored in a flywheel KER? Is it practical only in lab test conditions/race tracks, or in real-life say, traffic light stops?


I don't buy it. You're saying that Formula 1, where they'd pay you $30million dollars to find an advantage of a few tenths of a second, is not using the most efficient, power dense and energy dense system they can get their hands on? I simply don't buy it. If flywheels were better, they would be using it.

And no, I did not forget that the supercaps or batteries are not the only part of the system. But you're not cutting out much, if anything of all the other components by going to a fly wheel. It still has to go through a "specially designed transmission".

The newest generation of electronics are coming out on
gallium nitride for the IGBTs, etc are over 99% efficient and the electric motor can directly drive the axles or at least go through a single gear reduction.

In other words, the same mechanical losses as the electric KERS systems encounter would hit the flywheel systems.

As far as advances...try coming up with a way to double or quadruple the energy density of the flywheel system the way they're doing with graphene and other breakthroughs on the supercaps or the coated nanowires on the battery front.

No, flywheels are most likely going to be interesting footnotes in history for autos.


With the current rules of F1 you do not get a benefit of using kinetic KERS (as I already clearly stated). That was the reason why Williams stopped the development. Current rules also limit the budget in F1. Why spend millions of dollars to develop something new when you can buy the electric KERS (cheaper, but still expensive, of course) from someone else.

Poor efficiency causes losses. Eventually those losses are converted to heat. An electric drive system must have cooling. Kinetic KERS does not need cooling. I hope this clarifies the issue.


All the F1 teams (although Williams in particular) investigated flywheel KERS rigorously and found it to be too large, too heavy, and too prone to gyroscopic effects upsetting the car's balance during direction changes. It's not a regulation issue, it's a speed issue.

With some lithium chemistries demonstrating >95% efficiency in terms of energy out vs energy in, and motors / generators also at >95% efficiency, who really cares about an extra 5-10% here or there?


Well, I have seen an interview with Williams where the regulation was mentioned as the main issue. I hope they did not lie then. The weight was lower than for electric KERS so this is actually an advantage. Is 6 kg for the Volvo flywheel too heavy? The other issues in racing could be overcome with development. Note that electric KERS in F1 has been prone to technical failure. Most teams did not use KERS the first year due to frequent failures. In the last F1 race, Sebastian Vettel had do switch off his KERS. Why? Because the batteries overheated! Poor efficiency creates heat that is difficult to dissipate. An extra 10% in efficiency could be the difference between first and second place in F1. Thus, I hope that the regulation will change and become fair for all options in the future.

By the way, tell me what you think the average “roundtrip” efficiency of recovering and using the kinetic energy of the car is?


Actually, only Red Bull are having KERS issues this year, and this is because Adrian Newey refuses to compromise on aerodynamics to allow sufficient cooling for their system. He thinks that having downforce is better than having KERS, so the car was not designed to accommodate it. That's why their KERS is failing, every other team is doing fine, including MacLaren who are using 20 kW/kg A123 cells.

As for the Volvo unit, 6 kg is only for the carbon fibre flywheel, the rest would be very much heavier. Roundtrip efficiency for the Prius system is reported to be low, at around 50%, because it uses a lossy NiMH battery, but with decent lithium-ion and efficient power electronics that could easily be increased to 80%.

Another issue is packaging. For the flywheel systems, they have to be mounted in a particular place and orientation, which limits design flexibility for the rest of the car (a huge issue in both F1 and passenger cars). Motors are small and batteries can be placed anywhere. Check out the size of the Porsche system to see just how bulky these things really are, what's shown in the picture is just the accumulator!


As you indicate, very good cooling is needed when you have high losses. Kinetic KERS does not need cooling. Vettel could have been calm with kinetic KERS under the hood.

Under real driving conditions, the efficiency of Prius KERS is below 50%. Losses are not only in the batteries. The problem is that you have so many components in the system and so many energy conversions that even if losses in each component are relatively low, they add up. You could probably reach 80% as top efficiency but not as average under real driving conditions. Probably 60% is a more realistic number for a future system. Kinetic KERS has no energy conversion whatsoever.

Very much heavier??? Flywheel speed is high but mass is low. In F1, the maximum torque experienced for supporting the unit was 130 Nm. A small kid with a torque wrench can do much more than that. Of course it is possible to make a support rigid enough to mount this in a F1 car; or any other car. Weight of the whole F1 system was 18 kg and it fits on an A4 piece of paper. I am sure it would be possible to cut weight even further. Referring to your figures, batteries alone weigh more the whole kinetic KERS system. Weight is an a-d-v-a-n-t-a-g-e for kinetic KERS; not a disadvantage. The mentioned size of an A4 paper should answer your question about packaging issues.

Furthermore, why should kinetic KERS not be able to benefit from the use of new materials and improved technology? Did flywheel development stop with the iron flywheel of the Ford model T and bearing technology with the invention of the spherical roller bearing by SKF about 100 years ago in both cases? You are shooting at a moving target!


The overall discussion was about different issues - Peter XX proposing existing IEC car paradigm development when others propose electric paradigm. But this is about fuel not the dynamics of the car or even efficiency. If sticking to the traditional petrol may be "gyroscope" systems would be preferential. The flywheel always been part of the engine and this just question of how we in more advanced ways employ freewheeling energy and get some braking energy back. If not think about perspective of EREV - Prius would be nonsense. Better go for efficiency improvements or simply go for diesel car. Flywheel systems could interesting be for locomotives or trucks.


You could use kinetic hybrid drive on diesel cars as well as on gasoline cars. Diesel engines are more expensive to produce than gasoline engines. Hybrid drive systems are prohibitively expensive when combined with diesel engines. Kinetic hybrids will be far less expensive than electric hybrids, as clearly pointed out by Volvo. Thus, the ideal combination might be a diesel engine and a kinetic hybrid system. The cost will be in the same range as a gasoline hybrid but fuel consumption and performance will be much better.

william g irwin

Hey guys, I remember that counter rotating flywheels mostly resolve the inertial dynamics (precession?) issues, but 60k rpm presents its own challenges - mostly the CVT beast of a system, and don't forget the bearings. Thats a huge rpm/torque exchange - ~1000:1.
If Volvo et/all gets to put as much effort into solving the KERS system as the world has put into batteries, super caps, electronics, and ECVTs, then they may be able to solve the unique KERS problems.
There are respectable system advantages too. Good luck!

Big Al

You notice some interesting features about this system

1) The system is for the back axle on a front wheel drive car. As a result the flywheel has it's own clutch and transmission.
2) The control on this system will be independent of the main engine. You could even charge up this system while the main engine is powering you down the road.
3) You could put any kind of engine you want as the main engine, the only connection would be through a computer that would have input and output for both the main engine and the KERS system.


You have listed a couple of interesting features, which would enable a fairly straightforward installation in contemporary cars without significant modifications (as in the case for full-hybrid HEVs for example). The system is so compact that it might be fitted to many front-wheel drive vehicles without major redesign and with minimal impact on fuel tank and/or cargo capacity. Other options would be to integrate this KERS in the gearbox. The combination with kinetic KERS and a 7 or 8-speed double-clutch gearbox would be very interesting.

In the longer run, it is plausible to increase power from 80 hp in this case to a significantly higher level (e.g. doubling the power). Although some who have commented on this site consider a flywheel at 6 kg heavy and bulky - yet might accept 150 kg of batteries in a PHEV/EREV - doubling this weight would not pose any significant installation problems. Increasing power would enable significant downsizing of the ICE with further improvements of fuel consumption and increased drivability as results.

With the potential of reaching up to 90% roundtrip efficiency for brake energy recovery (compared to <50 % for current electric KERS), the potential for reducing fuel consumption is impressive.

A great potential is not sufficient for a commercial success. Volvo Car and the Swedish government have funded this project with ~1 M USD each. However, an increase in funding by at least a factor of 10 would be necessary to fully develop “new” technology, as in this case. If the results from this project are promising, we will probably also see increased funding in the future.



You don't know what you're talking about. You're taking a few articles and statements completely out of context. Of course Williams is going to make some claims about their flywheel systems because they're trying to sell it to others and recover some of their costs.

I worked with a team to develop a HEV system and we were able to get over 70% recovery on individual braking instances, with no effort and no tweaking to the system AND we limited the power back into the system to 70kW to protect the batteries until we had a chance to study the effects on battery life cycle.

You're talking out of your A$$ with nothing to go by other than some "gut feeling" that flywheels are better.

You need to talk with someone who has done both and see which one they ended up choosing.

As for power density of flywheels vs batteries: the F1 teams are using less than 3kg of batteries compared to the 6 kg for the flywheel and they don't have to deal with the gyroscopic affect.

You are correct about the heat problems and that has been the main issue for Red Bull among others.

I just got back from London last week and did a tour of the McLaren facility because the guy who designed their ECU is a friend. I'm talking directly with the guys who do this stuff, what is your source for all of your claims? Something you read from Williams as they try to peddle their flywheel system???


@DaveD, I had a tour of the MacLaren facilities too - amazing setup! Interestingly they are very strict about the no jeans on site rule, all the way from the top I'm told....

@Peter, DaveD is right that they are using about 3 kg of batteries to provide 60 kW of KERS. The electric motor is about the size of a toilet roll so very good for packaging issues too. As for the use of flywheel KERS in F1, remember the Williams system has to convert kinetic energy to electrical, which then powers a motor to spin up the flywheel, and then it has to convert that kinetic energy back to electrical with a generator which then powers the motor attached to the gearbox. With all of these energy conversions you can see why F1 prefers batteries!

Back to passenger cars, another point that manufacturers might consider is that it's a lot easier to add plug-in functionality at a later date if you've gone the electric motor route rather than the kinetic flywheel route.


It is more than a gut feeling. I have done computer simulations about efficiencies for electric KERS with validated electrical components. Current Prius-type systems are usually below 50% in average efficiency in real-life driving cycles. Yes, it is very likely that the batteries used in F1 give higher efficiencies. You can probably get 70 % efficiency under ideal conditions but it is still very difficult to achieve that as an average. If you do not have energy conversions, as with a mechanical system, the efficiency potential is higher. That is, of course, on the condition that the whole system works as it should and this is the scope of the project.

Anyhow, I finally understand that you have gained an enormous knowledge and experience in this field. Perhaps you should share this knowledge with those who are less fortunate. I suppose that you could convince Volvo Car and the Swedish Energy Agency that this is just going to be a footnote and that they should terminate all efforts in this area immediately. Thus, Volvo and the Swedish Government could both save ~1 M$ of their own and the tax payer’s money. I suggest that you make direct contact with the following persons in leading positions at their company/organization respectively:

Derek Crabb, Vice President of VCC Powertrain Engineering:

Tomas Kåberger, Director General of the Swedish Energy Agency:

The whole idea with a mechanical system is to avoid energy conversions. The potential to use plug-in functionality also with flywheels has already been shown. As I already mentioned, the rules of F1 are not in favor of kinetic KERS.



Peter, At least apply some logic to the situation. Some company getting their bought and paid for politician to give them money does not validate a technology.

Ok, let's follow your logic here: Everyone, please call George W. Bush because he was President and he spent HUNDREDS OF BILLIONS OF DOLLARS on hydrogen fuel cells! So they must be the greatest technology in the history of man!!! Quick, everyone run out and buy one for their car!!!

As for your statements about F1, you keep saying the same thing with no proof whatsoever. What is this rule you keep referring to that puts flywheels at a disadvantage or makes it illegal?

The only rules regarding KERS are that it can only have 400kJ, can only use it for 6.67s per lap and no more than 60kW.

So could you kindly post us any rules that prohibit flywheels? You're still speculating as to why all the Formula 1 teams use batteries rather than flywheels with absolutely no facts to back it up.

You want to compare credentials of people making these decisions: Adrian Newey is probably going to go down as the greatest engineer in the history of Formula 1, and he didn't choose the wrong technology. His designs at Williams, McLaren and Red Bull have dominated Formula 1 for years...I'll take his judgement over yours or any other person you can look up and post on here.

You want to compare car companies: Have your buddy from Volvo call the guys at Toyota and explain to them that the millions of Prius cars they sold should have been flywheels and how he knows better.

Whatever. No wonder people always attack you on here. It's ok to state an opinion, but you started this conversation asserting that flywheels were better with no proof.

Admit that it's just your opinion and state why you think it's better and stop asserting these things are "facts".



Yes,the McLaren facilities are amazing! Our corporate HQ is about 10 miles from McLaren in Surrey so a lot of the guys on my team have worked at McLaren at one time or another. Another good friend and partner in the HEV effort used to head up engineering at Cosworth so between them, it's impossible for me not to be a huge F1 fan.

I think I'm going back over to "work with the guys" July 8th-10th and I may get lost and find myself at Silverstone! :-)


At the fast rate that super caps are evolving, they will become the favored technology, for most vehicles with with variable speed or stop and go traffic pattern unless future batteries can do even better. The future is towards energy storage units without moving parts and capable of almost unlimited charge/discharge quick cycles. That sounds a lot like super caps.

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