We know that the rear brakes on an F1 car are smaller than they used to be because the
MGU-K helps to slow the car down by stealing energy from the rear wheels to charge the
battery.
The obvious next question is: “how does the MGU-K do this?”.
What is the mechanism that allows the MGU-K to take that energy from the wheels such that
it dramatically slows the car down?
Well first let’s clarify a common misconception – that the MGU-K captures heat energy from
the brakes.
This is not true and is a bit of miscommunication I’ve heard repeated.
Now, in order to slow a car down, you have to convert its kinetic energy (its “movement
energy”) into another form of energy.
With standard brakes, we convert kinetic energy into heat energy; the car slows down and the
brakes get hot.
Heat energy conversion like this is often considered ‘wasted energy’ because it
tends to just dissipate into the air and isn’t reused for anything.
But the MGU-K converts the kinetic energy of the wheels in a different way, instead
of heat energy.
It doesn’t use the wasted heat energy of the brakes, it instead means we waste less
energy by using the brakes less.
Instead we convert it to electrical energy.
OK, we’re going to get a little bit science now.
Some people might remember this from school, others might be yet to see this in school,
depending on your age or forgetfulness.
The MGU-K is a Motor/Generator.
The “motor” function means it’s taking electrical energy and delivering kinetic energy
to the rear wheels, i.e. delivering power to accelerate the car.
The “generator” function means it’s taking the kinetic energy from the rear wheels
and turning it into electrical energy – i.e. charging the battery.
As we’ll see, the Motor and Generator are actually the exact same system working in
almost exactly the same way, using principles of electromagnetism.
I’m going to start by showing how the motor part works and then we’ll see how the generator
part works.
OK, so – electromagnetism explores the deep interconnection between electric charge and
magnetic fields.
As you probably know, magnets have a north and south pole.
And when you’ve got a north and south pole, a magnetic field exists between them, flowing
from north to south.
You don’t really have to know much more than this right now, just know that magnetic
fields exist and flow from the north pole to the south pole of magnets.
Now electrical current – what’s generally called “electricity” is just electrons
flowing in a wire.
If you imagine a simple circle with a battery powering it, the current will always flow
from positive to negative Magnetic fields flow from north to south.
Electric current flows from positive to negative.
That’s what you need to know for everything else to make slow into place.
OK, so here’s the fun part.
Magnetic fields interact with electric currents.
If you run a live wire through a magnetic field, the wire will move, due to the interaction
between the two.
In fact, the wire will move in a direction at right angles to both the direction of current
and direction of the magnetic field.
The magnetic field is going this way, the current is going this way – so the wire
will move either up or down.
Which is it?
Here we have to whip out an old technique called Fleming’s Left-Hand Rule.
If you imagine your left hand like this (assuming you’ve got all the necessary fingers), when
you put an electric current in a magnetic field, this finger represents the direction
of the Field, this finger represents the direction of the current and the thumb will then tell
you which way the wire will move.
So, if we apply that to our wire between two magnetic poles, we see the wire will move…
up.
All good so far?
These are just fundamental rules about how electricity and magnetism interact, but you
can see we’ve already created motion out of electricity.
That is, we’ve turned electrical energy into kinetic energy.
By putting an electrical current in a magnetic field, the wire has shot upwards without anything
else provoking it.
So that’s a start.
But what we really want is a rotational force as, you know, that’s what wheels do.
Picture this then: imagine looping the wire back on itself like this.
So, we’ve got current going this way up one side of the loop and the current coming
back down this side of the loop.
Doing a quick check with the left-hand rule, we see the left side of the loop is driven
up...
And the right side is driven down and the whole loop starts to rotate.
Excellent.
But oh, wait, once the loop rotates past ninety degrees, the directions of current have switched
to opposite sides of the loop.
So now the right side of the loop is driven up and the left side down.
Uh oh.
You can see the problem here.
There are a number of ways to get around this problem but F1 uses an alternating current.
This means that the current in the wire is made to keep switching direction intermittently.
So as the loop of wire flips over, the current switches direction … and the forces on the
loop keep acting in the same direction.
A constant rotation, over and over and over.
This is a motor.
We’ve used electrical energy and turned it into constant kinetic energy – i.e. we’ve
made this loop of wire rotate and rotate and rotate.
You can imagine sticking a gear, a fan or a wheel onto this rotating device and boom
– you can start to see how this idea can drive machines.
The first thing to note here, before we move on, is: obviously this is a simplified version
of what’s actually going on in an F1 car.
Motors can be made even more potent by adding more loops of wire, by increasing the strength
of the magnets or current.
We can also make the whole process a lot smoother by adding two more loops spread evenly at
120 degrees to each other to create what’s known as a three-phase motor generator.
And thirdly, note that it doesn’t actually matter if the loop spins and the magnets stay
still or if the loop stays still and the magnets rotate around them.
You can even have the magnets spinning on the inside and the electric loops stationary
around the outside.
In any case, the stationary part of the motor/generator is called the stator and the rotating part
is called the rotor, simply enough.
Now we’re not privy to all the inner workings but, to the best of my knowledge, the F1 MGU-K
uses a three-phase system with the electrical part as the external stator and the magnets
rotating inside them (which is an inside-out version of what I’ve illustrated leading
up to this).
The principles are all exactly the same though, and I’m going to keep using the version
I’ve drawn as it’s both easier to understand visually and (ahem) easier to animate.
But just know there are several ways of arranging this system, with different benefits.
The motor as part of the wider system is set up thus: The battery feeds electrical energy
to the MGU-K which converts it to rotating kinetic energy, which adds power to the crankshaft,
which feeds that energy through the transmission to drive the back wheels.
Now to use this system as a generator, we just have to imagine the energy flowing in
reverse.
The driver comes off the accelerator.
Now the back wheels are spinning with lots of rotational kinetic energy which will spin
the rotor part of the MGU-K.
So now the MGU-K rotor is being spun by the kinetic energy of the car.
But the relationship between electric current and magnetic fields works both ways.
Putting a current through the wire in a magnetic field will make it move, we just saw that.
But also moving a wire circuit within a magnetic field will generate a current in the wire.
The rotor spins in exactly the same direction as it did when acting as a motor; it doesn’t
have to stop and reverse when switching between motor-mode and generator-mode, as you might
think.
The only thing you have to flip-reverse in your mind is that when in generator mode,
you have to use the right-hand rule to figure out what was going on.
When we looked at the motor, we used the left-hand rule to work out the directions of current,
magnetic field and motion.
We use our right hand to do the same thing when it’s behaving like a generator.
So the wire loop is being rotated clockwise: the magnetic field goes left-to-right, the
left side of the loop is going up, so this creates a current going up the left hand side
and down the right hand side, flowing through the wire like this.
And naturally, the current flips direction every time the loop of wire flips past the
half-way point, creating an alternating current.
You notice though: as we’re using this right-hand rule instead of our left-hand rule, the current
is flowing the opposite direction than it was when it was in motor-mode.
Why is this important?
Well, because as soon as you induce a current in the wire it’s naturally inclined to rotate
the other way.
As we remember from motor-mode, putting a current through the wire loop induces a rotational
movement in the wire.
The motor-effect, using the left hand rule, says the current generates an anti-clockwise
force which is the opposite direction to way the wheels are driving the motor.
By spinning the rotor, the wheels are generating a current that increasingly resists the direction
of spin.
The magnetic field and current really want the rotor to go the other way.
It’s in overcoming this resistance that the system generates electrical energy and
steals energy from the rear wheels.
It takes a lot of work to rotate the rotor against its wishes and that work is being
done by the spinning back wheels.
Putting that work into getting the MGU-K to spin saps kinetic energy from the rear wheels,
fast.
That is, it slows the rear wheels down.
The spin of the rear wheels is being resisted by the electromagnetic forces inside the generator.
So as the kinetic energy of the wheels goes down, the electrical energy of the MGU-K goes
up.
Straight energy conversion.
OK so we’ve just done a lot of sciencing and not everyone is used to that so if you’re
thinking ‘that’s a lot of information to absorb in a short amount of time,’ let’s
just shake it off and remember these basic points:
If you put an electric wire in a magnetic field, the magnetic field, electric current
and movement of the wire are all linked.
If you change one, it affects the others.
Running current through a wire in a magnetic field will cause the wire to move.
This is the premise of an electric motor – turning electricity into motion.
Moving a wire through a magnetic field will generate electric current in the wire.
This is the premise of a generator – turning motion into electricity.
Shaping the wire in a loop allows the system to rotate, so you’ve got a motor-generator
that works with rotational forces like how wheels work.
When you use the rotation of the wheels to generate energy, the rotational energy of
the wheels is reduced and the wheels slow down, causing a kind of braking force on the
car.
There – simple enough right?
You now have a basic knowledge of motor-generators and “regenerative braking”; you can understand
how the MGU-K contributes to the braking effect of the car.