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Volvo Cars’ tests of flywheel technology confirm fuel savings of up to 25%

Volvo Car’s flywheel KERS system. Click to enlarge.

Results of Volvo Car Group’s testing of kinetic flywheel technology on public roads during 2012 show that the flywheel technology combined with a four-cylinder turbo engine has the potential to reduce fuel consumption by up to 25% compared with a six-cylinder turbo engine at a comparable performance level, according to Derek Crabb, Vice President Powertrain Engineering at Volvo Car Group. (Earlier post.)

If the energy in the flywheel is combined with the combustion engine’s full capacity, it will give the car an extra 80 horsepower. Combined with the swift torque build-up, this translates into rapid acceleration, cutting 0 to 100 km/h figures by seconds. The experimental car, a Volvo S60, accelerates from 0 to 100 km/h in 5.5 seconds.

The experimental 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 component details. Click to enlarge.

The combustion engine that drives the front wheels is switched off as soon as braking begins. 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 stops and starts. In other words, the fuel savings will be greatest when driving in busy urban traffic and during active driving.

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 flywheel that Volvo Cars used in the experimental system is made of carbon fiber. It weighs about six kilograms (13 lbs) and has a diameter of 20 centimeters (7.9 inches). The carbon fiber wheel spins in a vacuum to minimize frictional losses.

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



I have said it couple of times on this forum but I want to state it once more: We have to take flywheel technology (KHV, KERS) very seriously as a potential contender to all other hybrid systems, including HEVs! Theoretically the roundtrip from kinetic energy in the car via stored kinetic energy in the flywheel and back to the wheels and the car again, could be the highest of all hybrid options. Progress in the development of the IVT transmission has increased efficiency substantially, which could facilitate this. Low cost and weight are also important advantages. In addition, it is easy to envision that a downsized 4-cylinder engine with a slightly bigger KERS could replace a V8. With an acceleration of 5.5 seconds from 0 to 100, Volvo is already pretty close. Similarly, a 6-cylinder engine could be replaced with a small 3-cylinder engine.

For those who absolutely want an all-electric range, it can also be combined with the necessary components and a battery size chosen according to the desired EV range. However, a large battery would make the concept look like a big dinosaur in most applications.


@Peter XX, this sounds impressive. 80 hp in little more volume than a 12V car battery.

I think there was a prior article with KERS at $1500, but the full power worked out to ~10 seconds worth.

How does "..possible to turn off the combustion engine about half the time when driving.." happen?


Marketing should tell us if the buyers want 20% better mileage for $2000 more or 40% better for $10,000 more. Do they want PHEV with 20 miles range in EV for $20,000 more, better performance, better mileage or both?

It is a trade off, but buyers want all the above. They want better mileage and better performance at NO additional cost. The first company that can come closest to that just might get some sales.


I've got lots of arguments about why a flywheel is not going to be more efficient than a battery system, and vice versa.
But I've stopped worrying about that because it comes down to a simple question: Which one gives you more bang for the buck?
Both will give you almost the same energy savings from a system view. Which one is cheaper to build, deploy and maintain?

Unless some type of silly legislation (ALWAYS prompted by scummy lobbyists buying off even scummier politicians) affects the outcome, manufacturers will use which ever one is cheapest and gets the job done.

Frankly, I can't believe they even bother with a study at this point. ANY type of KERS system has been proven to work and save why don't they just focus on getting the bloody things to market cheaply???

Roger Pham

>>>>How does "..possible to turn off the combustion engine about half the time when driving.." happen?

Easy! Use the engine to accelerate the flywheel, then turn off the engine and power the car using the flywheel until the flywheel unit runs out of energy, then restart the engine. This power-and-glide method can easily double the mpg, even for freeway cruise, when your engine does not operate at its peak efficiency point in the map. You don't want to do this too much in a HEV because the battery will wear out. Flywheel energy storage is far more durable with respect to cycling, even deep cycling, when carbon fiber is used, so has advantage over that of battery in HEV.

Quite an impressive achievement here. If there is enough room in the front, perhaps some complexity can be reduced if the KERS can be incorporated into the front drive, when the engine is downsized from 4 cylinders to 3 cylinders, or from 6 to 4. However, the rear wheel drive KERS can give the advantage of 4-WD when extra traction is needed. Perhaps a small electric motor can be used to accelerate the flywheel to prepare for launching the car, then after a longer stop, the motor can recuperate the energy from the flywheel to charge the battery. Perhaps permanent magnets can be incorporated into the flywheel to turn it into an electric motor as wheel. So, the car can be bi-hybrid, both KERS and electric hybrid and enjoy advantages of both.


You almost answered the question yourself. If the fuel savings are similar and KERS is cheaper, it gives you more bang for the buck.


A fraction of a minute of 80 hp(total energy storage) could not run a car half the time - esp. up a long incline.

'Restarting the engine' won't put 'free energy' in the flywheel. The only 'free energy' comes from capturing wasted braking momentum. Flywheels have been in ICE since pistons.

In any case, market KERS and we will see.


I have heard about stationary flywheel projects where they bury the flywheels in the ground so when they have the occasional "mechanical failure" no one gets impaled with shrapnel. Now, I am not suggesting they are dangerous I am just asking if anyone knows if this is an issue for above ground flywheel systems for cars. Perhaps they are small enough to not have that issue of challenging the capabilities of the construction materials, I just don't know.

I say all this, because this seems a no brainer if it is just that simple.


I really don't think it's answered or probably even very close yet. I haven't seen any pricing for this type of system yet but would love to see it.

Of course, both would have their advantages and disadvantages:
- The flywheel would probably last longer as Roger points out
- But you have to factor in maintenance for the CVT and the output gears and how long they last before they need work or replacing.

- The HEV batteries would probably not last as long (unless you use supercaps and then you have more cost up front)
- But it can be feeding an electric motor that is tied directly to the rear axle with no gears or transmission at all.
- Of course, then you have to throw in the cost of the electric motor and its maintenance vs the transmission and gears of the flywheel system.

As far as the life, there are plenty of HEVs running around today that have been on the road more than 10 years and the batteries haven't been replaced.

And regardless of which system you use, it won't take much energy storage because it only takes ~220Wh to accelerate a 1,600kg vehicle from 0 - 113kph (about 70mph). Considering that this will most likely be put on the rear wheel and using regen power from the last stop, it will never get more than about 30-35% of the kinetic energy recaptured anyway. So you're really only talking about storing maybe ~70Wh of energy.

2kg of the Toshiba SCiB batteries would quadruple this and you'd never have to do deep charges/discharges.

Pricing the two systems out, both upfront cost as well as maintenance over a 15 year life span? Hell, I have no clue. Haven't been looking at market prices for any of this in a couple of years and NO clue on the flywheel stuff.

Roger Pham

The flywheel tip is traveling at 628 m/s at 60,000 rpm, and assuming 5kg weight is at the tip. So the kinetic energy stored is: .5 x 5kg x 628^2= 986,960 J = 274Wh. Let's say that a car can travel 4 miles/ kWh. So, the energy stored can propel a car for ~1 mile. Then, the engine is restarted again.

The carbon fiber material does not fail catatrospically. It will cause the wheel to sag, and brush against the casing, and the whole thing will slow down.

HEV is for those who can drive gently. Flywheel KERS is for those leadfooted drivers who like to floor on the gas pedal and slam hard on the brakes. Different strokes for different folks. Neither cost a whole lot, and the owner will recoup far more in saving in fuel cost. Adding some permanent magnets on the flywheel and put somes stator coils on the flywheel housing and you will have yourself a bi-hybrid and everyone will be happy! :)


LOL @Roger, that depends on which batteries you choose. You can put your foot down and slam it with more power than your wheels could handle easily. There are batteries like the version A123 does for Formula 1 that will literally give you 20kW/kg!

And there were some vanadium batteries on here last week where they were claiming something like 190C discharge rates. Again, a few kg of these bad boys will give you any acceleration you want.

Again, in all these what cost? What about the flywheel? I don't have a clue on any of them, and can't wait to see anyone actually price out a system for ANY of this. :-)


I meant to comment on your discussion about the "bi-hybrid". I've always wondered if anyone was going to do that but haven't seen anything. Surely someone is working on it or am I missing something?

I mean, an electric motor with permanent magnets on the rotor essentially IS a flywheel. They make the rotor light for stopping and starting it easier for wear, tear and performance, but if you made it heavier and treated it like a flywheel storing kinetic energy and used electronics to handle the stator coils....seems like it would work?


@RP, I'm for what's most efficient and economic.

Yet, imagine gearing energy spinning from 60,000 rpm to ?10,000 rpm? to 60,000 rpm over and over for a ~0-1,000 rpm axle reliably, cheaply, and with little loss for over a 100,000 driven miles.

If 274 Wh is sufficient, why would a light Prius use a 1,300 Wh battery?


Those are the next customer questions, can the flywheel come apart, do the hydraulics leak, will the batteries catch fire?

People may want 20 mpg to be 40 mpg for $10,000 more, but if it is dangerous or unreliable, no thanks.


More KERS(13 seconds):


The 1,300Wh battery in the Prius makes sense. They don't want to do deep charges/discharges on the batteries so only using about 20% of it's available energy keeps it in the "sweet spot" and really extends the life.

Roger Pham

Electromotorization of the flywheel is not a new idea. It is done on flywheel electricity storage for the grid. Electricity is used to spin the wheel, then, current is released when the wheel is slowed down.

@kelly and SJC,
Good point. The long-term reliability of the flywheel KERS is still remained to be proven, while HEV's components are well-proven after 15 years of commercial release and mass production. However, with continual development, testing and refinement, it should be at least as reliable as any other mechanical components of the vehicle. A NuVincy-type of CVT should be more durable than gear-type of transmission and can absorb more torsional vibration that can cause gears to break or wear out rapidly. I think that Volvo is using hydraulic CVT transmission? This is also durable and can absorb torsional vibration well, and is low cost to replace. The flywheel itself is very simple and low-cost to replace.

Those drivers who like to floor the gas pedal and slam the brakes are not likely to ask for the most long-lasting of vehicles, in comparison to more gentle HEV owners.

The 1300Wh battery in the Prius is charged only to 70% and discharged only to 30%, so only 40% of the battery capacity, or 520 Wh, is usable, and only 27 kW of power is available from the battery. The flywheel KERS here can utilize most if not all of its 270 Wh of capacity, and can deliver up to 60 kW of power. 270 Wh is more than adequate for launching the vehicle from rest to 75 mph from the KERS itself without engine power, or to recuperate the energy from this speed to a standstill.

Adding permanent magnet and stator coils to the flywheel and a 600 Wh battery (250 Wh usable) in the vehicle and you can match the usable energy storage capacity of the Prius for summer A/C use and prolonged stops without using engine power. Since the flywheel will handle most heavy loads, the battery will be lightly used and will last a long time, and can be made light and compact and no larger than the existing lead-acid batteries of most cars. I see this development as a very promising alternative to existing HEV's.


So, you do not have a clue about the costs? Is this a joke? Just have a look at the components for this KERS and a conventional HEV. If you cannot figure out which one is cheaper you must be blind!


'How does "..possible to turn off the combustion engine about half the time when driving.." happen? '

The explanation is at the end of the quote: "when driving according to the official New European Driving Cycle."

Logically, that specific cycle includes a substantial amount of "stop and go." In a perfect world, this driving pattern would be a fairly good match for typical European big-city driving.

Flywheel technology won't be a boon in all cases (driving up the Alps, or down the Autobahn), but the stop/go case is common enough that many drivers could benefit.


In the mid and long terms, electrical deceleration energy recovery/use, will be more compatible with future electrified vehicles.

Mechanical flywheels, (like ICEs) will be progressively phased out. They were phased out as No-Break Power units many years ago and replaced by batteries.


If you have pricing for this flywheel system, CVT, the output gear train, an 80HP electric motor, and the price of batteries needed (in volumes of 100's of thousands of units) then please share.

If you can even give us a link to the price of this flywheel alone, much less volume shipments, then please share.



Tone it down, we don't want personal insults, we have had WAY too many of those on here.


Thanks SJC.
@Peter, I really wasn't trying to be offensive and was just enjoying an interesting topic.
If I said something to irritate you, i didn't mean to. Sorry.


About KERS cost
I think we have to wait a couple of years before we know the exact cost for KERS in mass production. The development status is at the state it was for HEV systems some 20 years ago, i.e. a couple of years before Prius I was introduced. I have made my own cost estimates using analogies with other IVT/gearboxes and the flywheel and anyone could do the same. In any article about kinetic KERS where cost is mentioned, it is stated that the cost will be lower than for an electric hybrid. Already here you have an indication. Could anyone find any information that it should be more expensive? No! Does it contain any rare/precious metals? No! Does it contain any other expensive materials? Well, carbon fibre. How much? Six kg (=almost nothing)! Is the manufacturing process complicated and/or expensive? No! How could it be expensive?

Why not increase the size of the KERS system and get more power and lower fuel consumption? It will come, I assure you.


"that the flywheel technology combined with a four-cylinder turbo engine has the potential to reduce fuel consumption by up to 25% compared with a six-cylinder turbo engine "

There are a lot of assumptions here. Nevertheless, the flywheel only works when breaking. So on long trips there will be no reduction in fuel consumption.

To Roger Pham - your power-and-glide idea sounds like free energy. These tiny flywheels store very little energy, only enough for 80 hp for 6 seconds. You can't get a constant 80 hp output anyway, because the flywheels start slowing down the instant a load is put on them. Shutting down and restarting the motor every 6 seconds is absurd.

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