It’s been said that the current hybrid power
units (and in fact all electric or hybrid
engines) are more ‘torquey’ than previous
engines that did not have an electric motor
as part of their power delivery.
In this video we’ll look at what torque
actually is, and then see how torque is delivered
to the rear wheels and - crucially - what
it means to say an engine is more ‘torquey’
or that it ‘delivers torque instantly’.
Right - what’s torque then? Torque is a
turning force, so we use it for the most part
when we referring to forcing something to
rotate. So it could be the force of turning
a wheel around on an axle, pulling open a
door on its hinge or undoing a nut from a
screw.
In fact, let’s imagine this nut for a second
to better understand torque. Let’s imagine
the nut is on pretty tightly and we’re trying
to undo it with a spanner. Let’s use a really
long spanner for reasons we’ll see in a
second.
The amount of torque applied is measured by
taking the distance from the pivot point at
which you’re applying the force, and multiplying
it by how much force you’re putting in.
So let’s think about this in action. I could
push down 15 cm from the pivot point on this
spanner with 150 Newtons of force (a Newton
is just a measure of force). Outside of America
we tend to use Newton-metres as units of torque
so we just multiply distance by force here
see we’re putting 22.5 Nm of torque into
the nut, trying to get it to turn.
Now, the nut is really tight, so we decide
to double the torque. Now we could do that
by pushing down twice as hard, but let’s
say that 150 N is the hardest we can push.
Instead, we can just double the distance,
because that will double the torque. So we
push with 150 N of force 30 cm from the pivot
and that generates 45 Nm of torque, causing
the nut to come loose.
So anyway - we double the distance, double
the torque. We didn’t have to put more energy
in to get more turning force out, which is
impressive and interesting, right?
This is also how the jacks work in pit stops.
One person can lift the entire back of an
F1 car because they push the jack REALLY far
from the pivot, which generates a lot of torque
in the system. The lifting pad of the jack
is really close to the pivot, though so the
force pushing the car up is magnified as (for
constant torque), shrinking the distance multiplies
the resultant force.
Now let’s apply this to an engine.
An engine has a piston, which is forced down
by a little explosion in the cylinder. So
it’s applying a force downwards, which is
carried through the rod, and this turns the
crankshaft which is just a cylindrical axle
beneath the engine.
We see that the piston applies a force to
the crankshaft, causing it to spin so it and
the other 5 pistons are collectively generating
some torque to spin this crankshaft at incredible
speeds.
If we stick a gear cog on the end of the crankshaft
we can measure the force the gear pushes with
at its circumference to work out how much
torque the engine is delivering to the crankshaft
at any given time.
Now this torque - the turning force measured
at the crankshaft - is the engine torque we
talk about when describing the torque of the
engine.
But this is not the torque delivered to the
wheels that makes them spin.
To get there, we need to go on a little journey.
And I’m going to describe it in broad terms
and illustrate is using models, not the exact
systems you’ll find in an F1 car.
So, the crankshaft connects to the transmission,
which drives a drive shaft, which connects
to an axel gear which drives the wheels.
The transmission has gears of different sizes
that will connect to the crankshaft depending
on which gear the driver selects. The different
gears are described by their ratios, which
just describes how big the gear is compared
to the gear it connects with at the crankshaft.
First gear has the biggest ratio, so it might
be 10 times the size of the crankshaft gear.
These ratios get smaller and smaller, until
8th gear which might have a ratio of 0.75:1
with the crankshaft gear.
Why are gear ratios important? Well because
the ratio in size of the two gears directly
influences the change in torque between the
crankshaft and the driveshaft.
You should have all the information to figure
out why, so let’s step through it.
Let’s keep it super simply and say we’re
in first gear and first gear has a gear ratio
of ten, so it’s ten times bigger than the
gear at the crankshaft.
Now, when gears connect there is a driving
gear (the one with all the power going through
it) and a driven gear (the gear that gains
power from the driving gear). So in this case,
the driving gear is the one at the crankshaft
and it’s driving our first gear, here.
The driving gear carries the torque of the
crankshaft. So if the engine is producing
200 Nm of torque, the driving gear of the
crankshaft has 200Nm of torque. The gears
connecting means the teeth of the driving
gear are pushing with a force on the teeth
of our first gear. So the force where two
gears meet is the same, obviously ‘cause
they are connected via this force.
But because our first gear is ten times bigger
in diameter, the torque delivered to it is
ten times bigger than it is at the crankshaft,
because - as we saw - if you apply a force
further away from the pivot, it multiplies
the torque you generate.
This goes straight to the driveshaft so we
can say the driveshaft has ten times as much
torque as the engine is pulling at the crankshaft.
The gear that converts this turning force
to the axle remains constant as is somewhere
around 3:1, so we then multiply the torque
by 3 to give a final torque at the wheels
of 6000 Nm. This is a good 30 times greater
than the torque the engine delivers directly
to the crankshaft.
This is what a gearbox does: takes the torque
of the engine and multiplies it up in different
amounts depending on how much torque the wheels
need.
To accelerate from standstill, the wheels
need a lot of torque to get going as it’s
a lot harder to get the system moving. That’s
why the ratio of first gear is the biggest,
as it has the biggest multiplying effect.
Once everything’s moving though, you need
less and less torque to accelerate and maintain
the wheel rotation speed
You’ll also notice when you’re driving
rotation to a larger gear, the gear rotates
more slowly. If the gear ratio is 10:1, as
it if with our pretend first gear, you need
to rotate the driving gear 10 times to get
the driven gear to turn once. So you’re
getting a lot of torque out of this gearing
but not actually getting it to rotate very
fast.
With smaller gears, you’re putting as much
power in, but need to rotate the wheels much
faster to achieve those high speeds, and you
can use these small gears at high speeds cause
you don’t need lots of torque to keep the
rotational momentum up.
So, if we ignore normal energy losses for
the moment, just note that the power of the
system doesn’t change between the engine
and the wheels. Power is just the rate at
which the engine delivers power to the wheels.
If the engine is putting out 500 horsepower
of power at the crankshaft, it’s delivering
500 horsepower to the wheels. Again, ignoring
losses due to friction.
The power will go up and down depending on
the speed of the engine, i.e. its RPM, but
not with how you gear up the car. All that
changes is the torque and the rotational speeds.
If you up the torque, you sacrifice turning
speed at the wheels.
So what’s interesting is that whatever the
power at the engine you can deliver as much
or as little torque as you like to the wheels
if you’ve play around with the gear ratios
enough. If won’t give you any more power
though. So what are we talking about when
we say ‘ooh Lord, these electric engines
have a lot of torque!’ ?
Well, firstly we don’t mean “they have
a lot of torque”, that’s a bit of a misunderstanding.
But we can talk about torque delivery or - in
a colloquial way - a car being ‘torquey’.
Now we have to look at torque curves. This
means graphs, but I’m gonna make it super
simple. So let’s draw up some axes. Along
the horizontal axis here, we’re going to
represent RPM, revs per minute. So as we move
along this axis, the engine does this:
[SOUND EFFECT OF ENGINE REVS INCREASING]
[VROOM VROOM VROOM]
Up this axis, we’re going to measure torque,
so the higher up the chart we go, the more
turning force the engine is delivering to
the crankshaft. Not the wheels, the crankshaft.
So we’re talking about pure torque delivery
before we’ve used gears to play about with
it.
We’re going to look at the petrol engine
part on its own here, just the combustion
engine, not the electric motor. Now with this
engine we don’t get maximum torque straight
away. At low revs, as the engine is building
up speed, it’s taking its time to reach
maximum torque. There’s a few reasons for
this: partly, as there are a lot of moving
parts right through the drive train, the engine
has to put some of its energy into overcoming
the friction between these parts; partly as
the combustion parts of the engine work on
sucking fuel and air into the cylinder, it
takes a certain amount of engine speed to
bring the optimal amount of fuel and air into
play.
So the torque curve ends up looking like this,
with torque getting higher and higher as the
engine gets faster and faster. It peaks around
12,000 RPM then starts to drop.
It drops because the mechanical resistance
of the engine starts to dominate and the cylinder
can’t pull in any more air and fuel fast
enough to create more power. So this is the
peak torque of the engine, this sort of area
around 10-12,000 RPM.
You can confirm this because you’ll notice
after the initial acceleration phase, the
gears are specifically set up such that changing
up through the gears keeps the revs in this
peak torque range right through to maximum
speeds. Keep your eye on the telemetry graphics,
like the ones here:
[PAUSE]
So, looking at this curve we can see that
the torque has a sort of lag, not reaching
its maximum until the engine is basically
at its fastest speed. From in the car, the
driver will feel an initial pull of acceleration
but this will grow and grow up to this point.
Not very “torquey”. Slow to reach its
full torque.
Now an electric motor has a very different
torque curve. Unlike the combustion engine,
it has very few mechanical parts and it doesn’t
rely on the flow of fuel and air. In fact,
we know from our previous video on the MGUK
that it’s just a rotor spinning in a magnetic
field. Very simple.
So there’s next to no resistance on this
motor so the instant you put your foot down,
you’ll pretty much jump straight to maximum
torque.
The torque curve looks like this, showing
that max torque is delivered from the very
beginning.
That’s why electric cars and hybrid cars
feel ‘torquey’ - ‘cos they’re delivering
all this torque right up front, which puts
a lot of the engines potential power straight
to the wheels. In hybrid systems like the
ones in F1 cars, the electric motor ‘fills
in’ some of the torque lag at low engine
speeds, giving more instant high torque to
the wheels that was missing before.
You’ll see that after a point, the torque
drops off dramatically with an electric motor.
And the thing with electric motors is that
when putting a rotational spin into the electric
motor, we generate some electromotive force
in resistance to the direction of the current.
After a certain RPM it dominates and overpowers
the current in the motor, forcing the drop
off of torque delivery.
Now just to reiterate, an engine feeling very
torquey doesn’t mean it’s more powerful
(a Formula E car feels more torquey than an
F1 car, for example). But the power delivered
by the engine to the wheels ramps up faster
the more torque it being delivered.
We can show, with a bit of mathematical manipulation
that Power is just Torque times RPM, so if
there was a constant torque then the Power
would just ramp up directly with RPM. If there’s
more torque, the power ramps up more quickly.
If there’s less torque, the power ramps
up more slowly.
So a high torque engine isn’t more powerful,
but it does help deliver that power to the
engine more quickly.
So hopefully that’s given you a better idea
of what torque is. Engine torque is simply
the turning force of the engine that’s then
translated down the drive train, manipulated
by the gear box to multiply it up and provide
Drive torque to the rear wheels. You need
maximum torque at low speeds to get the car
accelerating and lower torque at high speeds
to keep the wheel spinning faster, which is
why higher gears are smaller and do not crank
the torque up as much as low gears.
And Torque does not affect the amount of power
in the system, which is just the rate at which
the engine delivers energy.
