By recuperating a moving car's kinetic energy as it slows down and storing it as electrical energy, it can then be used to help the car speed up again, reducing fuel use.
But is there a better way of storing that energy? Volvo's flywheel-based Kinetic Energy Recovery System, or KERS, is one alternative to storing energy in a battery.
The Swedish marque has been testing kinetic systems for a few years now, and according to Autocar, says the technology could improve the fuel efficiency of a regular vehicle by 25 percent.
Not only that, it does away with the need for heavy batteries, and can store energy three times quicker than more conventional recuperation systems like you might find on a Toyota Prius.
While it can't store as much energy as the battery in a conventional hybrid, the speed at which it "recharges" means total energy storage isn't as important.
When running at its maximum 60,000 rpm, the flywheel can supply up to 80 horsepower for ten seconds of full-power assistance..
But because so much energy is expended when braking, Volvo says just eight seconds of gentle braking is enough to fully recharge the flywheel. It can be charged by the engine too, meaning storage is possible even at a constant highway cruise.
In fact, Volvo suggests it's as useful here as it is in city driving. Constant highway cruising uses very little power--Volvo reckons less than 30 horsepower on its S60 T5 test vehicle--so the flywheel could propel the vehicle for up to half a mile at a time.
And when it runs down, it'd only take another half a mile of combustion power to charge the flywheel again--at which point the cycle repeats.
Essentially, your engine may only run for half the time on a regular highway cruise, traffic and topography permitting.
Performance is good too--that 80-horsepower boost cuts the 254 bhp T5's 0-62 mph sprint by 1.5 seconds.
The flywheel itself is made of steel and wrapped in a 10 mm layer of carbon fiber. It runs in a vacuum for frictionless spinning. Power is sent through a Tototrack CVT, which in turn runs a series of gears to send its power to the rear wheels.
This rear-wheel drive layout may not be used on production vehicles--though it's worth noting the current Volvo V60 Plug-In Hybrid sends its electric power to the rear wheels--but the low weight should: The total system weighs just 132 lbs.
Production reality is still a few years away yet--Volvo suggests 2020 and after--but further refinement could improve those fuel and performance benefits further.
And unless there's some big battery breakthrough in the meantime--and reduced costs--the flywheel system is also cheaper than conventional hybrid vehicles.