Xtrac and Flybrid are both licensees of Torotrak’s technologies, which employ a continuously variable transmission (CVT). The CPC-KERS differs as it incorporates the flywheel entirely within the vehicle’s hub. In the CPC-KERS, a differential transfers torque between the flywheel, drive wheel and road wheel.
The Flybrid system weighs 24 kg and has an energy capacity of 400 kJ, providing a maximum power boost of 60 kW for 6.67 seconds. Its flywheel, with a diameter of 240 mm, weighs 5.0 kg and revolves up to 64,500 rpm.
Formula One regulations allowed the use of 60 kW KERS units in the 2009 season. Ferrari, Renault, BMW and McLaren all used KERS at some point during that season, though Renault and BMW later discontinued its use. Nick Heidfeld achieved the first podium finish with a KERS-equipped car at the 2009 Malaysian Grand Prix. Lewis Hamilton, driving for McLaren Mercedes, secured the first Formula One Grand Prix win with a KERS-equipped car at the 2009 Hungarian Grand Prix, with his teammate finishing fifth. Hamilton also took the first pole position with a KERS car at the same event, with Kovalainen qualifying second. Kimi Räikkönen won the 2009 Belgian Grand Prix with a KERS-equipped Ferrari, with Giancarlo Fisichella noting that Räikkönen’s victory was aided by the system: “Actually, I was quicker than Kimi. He only took me because of KERS at the beginning”.
Although legal in 2010, all Formula One teams agreed not to use KERS that season. The 2011 season saw a return to KERS, with new regulations raising the minimum car and driver weight to 640 kg. Three teams initially chose not to use the system. WilliamsF1 developed a flywheel-based KERS but opted for an electrical system due to packaging issues, later establishing Williams Hybrid Power to sell their developments. Audi’s Le Mans R18 hybrid cars used Williams Hybrid Power systems in 2012.
Since 2014, KERS unit power capacity increased to 120 kilowatts, coinciding with the transition to 1.6 litre V6 turbo engines.
Beyond Formula One, Peugeot unveiled the Peugeot 908 HY, a hybrid variant with KERS, at the 2008 1000 km of Silverstone. Toyota used a supercapacitor-based regeneration system in its Supra HV-R, which won the 2007 Tokachi 24-Hour endurance race. Porsche tested a flywheel-based KERS system in its 918 concept car, though it presented imbalance issues. Mazda introduced i-ELOOP in 2011, a system using a variable-voltage alternator to store energy in a double-layer capacitor. FIAT launched a KERS-equipped Panda mild-hybrid in 2020, and Roewe also utilizes the technology.
KERS has also been applied in motorsport beyond cars. KTM raced with a KERS system on a motorcycle during the 2008 Valencian Community motorcycle Grand Prix, but it was later ruled illegal. The system has even been demonstrated on bicycles, with the EPA and University of Michigan developing a hydraulic Regenerative Brake Launch Assist (RBLA).
The Automobile Club de l'Ouest has promoted KERS use in the LMP1 class at the 24 Hours of Le Mans since the late 2000s. Hope Racing entered a Flybrid Systems mechanical KERS car in the 2011 24 Hours of Le Mans, the first car to compete at the event with a hybrid system. Audi and Toyota both developed LMP1 cars with KERS for the 2012 and 2013 races, while Porsche used a battery system in its 2014 Porsche 919 Hybrid.
KERS technology has also found application in public transport, with a carbon fibre flywheel system retrofitted to London double-decker buses from 2014 to 2016, aiming for a 20% fuel efficiency improvement. The development team received the Dewar Trophy in 2015. The Parry People Mover railcars also use a KERS system with a flywheel and regenerative braking.