Can you really improve fuel economy and performance just by taking a production car with a turbocharged 5-cylinder engine and adding Formula 1 race technology? 

To find out, we headed to Volvo headquarters in Gothenburg, Sweden to test-drive a top-secret prototype Volvo has been testing for the past year.

It's an ordinary-looking 2013 Volvo S60 sedan with an 80-horsepower Kinetic Energy Recovery System (KERS) bolted to the rear axle. 

The Basics

Unlike the regenerative braking found on cars with start-stop, hybrid or all-electric drivetrains, KERS is an entirely mechanical system which bleeds off the car’s kinetic energy under braking, transferring it through a continuously variable transmission and storing it as mechanical energy in a vacuum-encased carbon-fiber flywheel.

As a consequence, a car with a KERS system does not require a heavy-duty alternator or motor, complex power electronics or additional batteries.

Instead, the flywheel stores the mechanical energy for up to 30 minutes, releasing it when required to aid in acceleration. 

Behind the wheel

Approaching Volvo’s one-of-a-kind prototype, there’s a noticeable whine from the S60’s trunk as its flywheel spins with energy from the previous test drive.

That’s due to a lack of sound insulation, says Volvo. Being an engineering prototype and proof-of-concept vehicle, it’s a little rough and ready around the edges. 

Inside the cockpit, too, some obvious giveaways remind us we’re sitting in an engineering mule. Alongside the standard S60 dash, there’s a tablet PC mounted to the dash, displaying KERS status information, and the obligatory red emergency disconnect switch required of all prototype vehicles. 

To start with, our chaperone disengages KERS completely, so as to remind us of the stock performance of Volvo’s usual 2.5-liter Turbocharged five-cylinder engine. Accelerating quickly onto the test track thanks to 254 horsepower and 266 foot-pounds of torque at the front wheels, there’s nothing strange or unusual about driving this car. 

Volvo S60 KERS Engineering Prototype

Volvo S60 KERS Engineering Prototype

Then KERS is switched on and placed into ‘hybrid’ mode, we enter head for a tight corner, and start slowing down. 

Somewhere behind the rear seat, we hear the sound of a jet engine crossed with a laser rifle and quickly realize KERS is spinning its flywheel up to an electronically-limited 60,000 rpm top speed. The sound is intoxicating, and a quick glance at the KERS system display tells us we’re recapturing somewhere around 30 kW of instantaneous power.

Exiting the curve and heading for a reasonably steep incline, we accelerate and notice the S60’s engine doesn’t roar as much as it did on entering the track. This time, KERS is doing some of the work and the gasoline engine is noticeably less strained. 

Cresting the hill and passing down the other side, we notice the T5 engine has completely switched off, coasting down toward the next corner. At low speeds, and when there’s enough stored energy, the prototype creeps along using just the flywheel-stored energy to move it along, starting the engine when more power is needed. 

Switch into ‘Sport’ mode, and KERS acts a little like a boost function, storing energy when the car is cruising as well as braking to release it under heavy acceleration. 

The result, Volvo says, is a 0-62mph time of 5.5 seconds, 1.5 seconds faster than the stock S60 T5. While Volvo’s tiny test track didn’t afford us much chance to test this, we can say the extra 80 horsepower boost from the KERS system gave the well-built Swede a sportier feel.

Meanwhile, the high-pitched whine as the KERS system charged up, combined with the way it pushed us into our seats every time it discharged encouraged us to squeeze the throttle to the floor at every apex, however short the straight.

Volvo S60 KERS Engineering Prototype

Volvo S60 KERS Engineering Prototype

It’s fun, but is it efficient?

Addictive as it is in sport mode, is KERS really a green solution? Volvo says yes. 

The prototype -- designed in conjunction with KERS specialist Flybrid Systems -- can store somewhere in the region of 150 watt-hours of energy. That's about one-third the energy in the very smallest hybrid-electric battery packs.

While that doesn’t sound much -- about the equivalent of 6 seconds of full-throttle boost -- it makes a massive difference to performance figures. 

As Volvo detailed earlier this year, the current KERS system has the potential to reduce fuel consumption by up to 25 percent, because it gives a turbocharged four-cylinder the same performance characteristics as a turbocharged six-cylinder engine. 

Less cylinders means less fuel and less pollution. 

Interestingly too, while great fun on the track, KERS is best suited to city driving, where lots of stop and go driving, combined with overall low speeds means the gasoline engine can turn off completely. 

Driven carefully, a fully charged KERS system of similar size to Volvo’s prototype could easily take you a few blocks in a busy city rush hour without burning a drop of gasoline.

For heavily congested cities with hour-long traffic jams like New York City however, KERS may not work so well. That’s because the system slowly loses energy over time, due to its mechanical nature, taking about 30 minutes to go from fully charged to empty even if no energy has been sent to the wheels. 

A promising future

Volvo S60 KERS Engineering Prototype

Volvo S60 KERS Engineering Prototype

Despite some of its more obvious drawbacks however, KERS seems to have a rosy future. While Volvo is careful to not comment on KERS’ place in future models, its interest in flywheel technology goes beyond a single-year test program. 

In fact, Volvo’s full-scale test was part of a $3 million research project, jointly funded by the Swedish government, Volvo Cars, SKF, Flybrid and Volvo Trucks. 

So far, the results are promising: while Volvo won’t talk price, it assured us that the KERS system was far cheaper than a comparable hybrid-electric system. 

It’s lighter too: at a little over 132 pounds for the entire system and 13 pounds for the 7.87 inch diameter wheel, Volvo’s KERS system could easily be integrated into its existing production line using the same mounting system used for its all-wheel drive and V60 Plug-in Hybrid cars. 

Combined with Volvo’s all-new Scalable Platform Architecture (SPA) -- which Volvo says could shed up to 330 pounds per car compared to previous models -- and Volvo’s commitment to lowering its fleet-wide average CO2 output to under 0.32 pounds per mile, we think KERS will be making an appearance in a production car some time soon. 

But what do you think? Let us know your thoughts in the Comments below.

Volvo provided airfare, lodging and refreshments to High Gear Media to enable us to bring you this first-person report.


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