For those a bit vague about electric motors, you might want to read this. The gist is that feed electricity in and, using magnets, the electric motor turns electric energy into rotational force. Very relevantly, a generator (or dynamo) is the same technology in reverse. Turn an electric motor axle by force and electricity will come out of it where normally it goes in. Unsurprisingly, F1 teams that employed KERS used the same unit as both motor or generator as required.
Kinetic energy is the energy of an object in motion. It is proportional to the mass of the object. If you were hit in the face by a ping-pong ball travelling at 15 mph, it would hurt much less than if you were hit by a cricket ball at the same velocity. Kinetic energy is proportional to the square of the object’s velocity. Bullets are quite light and would do little damage if one was tossed at you, but if fired from a gun would blow a hole through you because of the very high kinetic energy from the high velocity, despite the low mass.
KERS is short for kinetic energy recovery system. A Formula One car is wasteful in that it expends huge amounts of energy provided by the fuel-burning engine to accelerate the car up to high speeds on the straights, and then uses the friction of the brakes to convert that kinetic energy into wasted heat to slow down the car for the corners. The concept of F1 KERS is some of that kinetic energy can be converted instead into stored energy. Subsequently, that collected energy can be used to add extra acceleration to the car in addition to that provided by the conventional engine. Teams employing KERS in 2009 stored that energy using batteries and this article will cover the flywheel option later.
The KERS allowed in 2009 was very limited because F1 cars are wasteful of energy in other ways, such as the huge heat generated by the engines and energy contained in the exhaust emissions. Even with braking, the regulations only allow the collecting of energy from the rear wheels whereas most the braking retardation is from the front wheels (since under braking the mass of the car leans forward).
The basic principle of battery KERS is that under braking, a generator is connected to the back wheels. The rotational force required to turn the generator both generates electricity and slows the back wheels, instead of, or in addition to, conventional friction braking. This generated electricity is used to charge up batteries. The charged batteries can be used to power the electric motor (as the generator now becomes) to add to conventional engine power when the driver presses the boost button providing extra acceleration and speed.
In 2009, generally the motor/generator was placed in front of the engine connected (when required) to the engine drive-shaft, and thus to the back wheels via the transmission. (For the uninitiated, the power from the engine goes through the clutch and gearbox, into the differential, that splits the power between the two ‘half-shafts’ which turn each of the back wheels. This arrangement for passing the engine-power to the driving wheels is known collectively as the transmission.) The motor/generator could be connected more directly to the wheels behind the engine and gearbox but designers like to keep weight as much towards the centre of the car as possible.
It is quite odd to think that in charging mode, the power is going through the transmission in the ‘wrong’ direction. Of course, this happens with ‘engine-braking’. If a driver in a car lifts off the throttle without using the brakes, the driving wheels will rotate the engine faster than the rpm that the reduced fuel supply on its own would sustain. The friction of the engine and the power needed for the compression strokes of the engine will thus slow down the vehicle. (In the past, racing drivers have struggled on without brakes by using engine-braking, changing the gears down as the car slows, for in the next gear down the driving wheels will turn the engine faster again.) So the generator adds to engine-braking.
The flywheel option is to store the energy not in batteries but as kinetic energy. Flywheels are traditionally heavy spinning disks connected to engines or mechanisms to maintain smooth rotation. Of course, F1 designers do not want big, heavy disks so they prefer to have a much smaller, lighter disk spinning at horribly high speeds, possibly peaking at 40,000 to 80,000 rpm. To eradicate air-friction, which at that speed would be substantial, the disk would be housed in a vacuum. This would require very strong housing, not only to contain the vacuum but for safety. A disk spinning at 40,000 rpm that flew out of the car after a big crash would be hugely dangerous.
The intended Williams flywheel system (which they have yet to use in anger) used electricity to pass the power to and from the flywheel. Under braking, the unit connected to the back wheels generates electricity that powers an electric motor built into the flywheel itself. When the driver presses the boost button, the motor in the flywheel switches to generating electricity (which slows the flywheel), which powers the unit, now in motor-mode, connected to the back wheels.
Flybrid flywheel unit
The Flybrid system designed for Honda before their withdrawal and yesterday reported as potentially the standard KERS system for Formula One next year is entirely mechanical (I am fascinated to know how Flybrid get a rotating shaft into a vacuum chamber without breaking the seal). A continuously variable transmission (CVT) will connect mechanically, as needed, the back wheels to the flywheel to move rotational kinetic energy from one to the other.
(With normal engine gears, a manual transmission has a gearbox offering a set of gear ratios. The driver changes up to a higher gear when engine rpm is getting too high, and changes down when rpm is getting too low. CVT, the engine operates at a constant rpm and the gear ratio is continuously variable to match the speed needed. See this for how. Williams developed such a system for F1 which FIA banned before they could use it.)
For a fully mechanical flywheel KERS system, CVT is essential as during charging, the back wheels slow down rapidly as the flywheel speeds up rapidly, with the opposite problem as the power goes the other way. An engine can raise or lower rpm to suit the wheel-speed and selected ratio but with KERS the ratio has to match the flywheel rpm. (Although illegal to use CVT to replace the engine gearbox, it would be permissable for connecting the flywheel to the transmission.)
The problem with KERS was it was introduced for the wrong reasons in the wrong way. My next entry will touch more on this in the context of contemporary and future engine technology now I have explained KERS basics. In short, it was introduced to look green but very limited in what was allowed so it would not add a big boost to engine power not that long after FIA reduced engine size to reduce power.
Teams could charge up their system in a few seconds of braking (Mercedes said half-a-second was enough) with a small limit on how much energy they could collect and only use it a few seconds a lap (actually they could collect as much as they liked but only use 400kj a lap at 60kW (60 kj/sec) which gave an extra 80 hp for 6·6 seconds). They still had to lug the KERS equipment around the rest of the lap with weight distribution and packaging compromises off-setting at best much of the advantage KERS provided. Also, the lightest batteries were those that lost some of their capacity to hold electricity with repeated recharges, so were un-evironmentally replaced as frequently as possible.
For 2010, using the KERS boost above 300 kph was banned. Had the teams not decided to mutually drop the technology anyway, it would have become more effective at preventing overtaking by using it out of corners than enabling overtaking with use towards the end of straights ruled out. It now seems the main reason to re-introduce it next season is to look environmental and fit in with the manufacturers’ hybrid cars marketing. If you ask me, three manufacturers in F1 may be three too many.