- A few months ago, we did an episode on why it's almost
impossible for car to go a thousand miles per hour.
Now I'm not gonna spoil it for you,
it's a good episode, but it turns out that top speed
of a car isn't limited by physics,
but rather by mother nature.
So I started to think about the limitations of engine speed.
And as it turns out
the fastest revving motor peaks at about 20,000 RPM.
(car engine revving)
So why is that?
Well today, we're going to figure
out what limits engine speeds,
how engines can be built to spin faster,
and why it's so hard to hit that 20,000 RPM ceiling.
And if you don't care
about any of those answers, don't worry.
I'm going to also light some fuel on fire.
Let's go.
(upbeat music)
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- So Jerry Berry,
what's so great about a high revving motor?
Well, other than the sound and the bragging rights,
it's all about horsepower.
Horsepower is torque times revolutions per minute
which means that while an engine is producing torque,
the faster it spins, the more horsepower it makes.
But all motors have some limit to how fast they
can spin typically because of friction heat and wear.
These problems affect everything
from jet turbines to electric motors.
But some of those can exceed 1 million RPM.
So why do piston engines struggle to reach 20,000 RPM?
Well, there are three main problems.
Uno, dos, tres.
Now whether it's a Camry or a Mercedes W10 F1 car,
all four stroke engines turn fuel into
horsepower using the same four steps.
Suck, squeeze, bang, blow.
Each step begins with the piston
at a dead stop at top dead center or at bottom dead center.
The piston accelerates from zero to its top speed as much
as 95 miles per hour or 42.8 meters per second.
And then it decelerates back to zero
so it can change direction in the next step.
This produces the first limiting factor
for how fast an engine can spin.
It's reciprocating masses
we need to overcome.
The moving object has an effective weight
equal to its mass times any G force exerted on it.
but a piston only weighs what, maybe 500 grams.
How bad could that be?
Well, take the Cosworth V8 Formula One engine for example,
that's one of the few engines to ever hit 20,000 RPM.
At that speed, the piston is accelerating,
decelerating and reversing over 600 times a second
and producing 9,500 G's.
That gives a 500 gram piston
an effective weight of a thousand pounds.
But it's not just the piston that has to
withstand those G-forces, oh no, it's the risk pins,
it's connecting rods, the crankshaft, the bearings,
they all got to keep that rapid acceleration
and deceleration of the piston in check.
So one reason why reciprocating engines have a limit
to how fast they can spin is because
beyond a certain speed,
the effective weight of a piston will break
ordinary bottom end parts.
But we can solve this problem
of reciprocating mass and that's by beefing things up.
You can strengthen the bottom end to deal
with the additional weight that comes
with higher engine speeds.
Using Ford's parts instead of cast for example,
but stronger often means heavier
and bottom end parts are also reciprocating or rotating.
So you generally have to deal
with the trade-off between strength and lightweight.
You can switch to materials that are stronger and lighter
but that's going to be more expensive.
(upbeat music)
The limits of bottom end parts
aren't the only reasons engines have rev limits.
The top end is vulnerable too,
and the second problem that high revs create is valve flow.
An engine valve is pushed open when the camshaft
or rockers press down on the valve shaft,
pushing it into the cylinder.
Once the cam lobe or rocker is no longer applying pressure,
the valve is closed by a spring.
So valve is also a reciprocating mass.
It travels in one direction stops then reverses,
which takes time.
In an overhead cam motor like the S2000, that's it.
But in the case of a push rod motor
like a Chevy small block, the tapits, push rods, and rockers
they are also reciprocating.
The valve shaft is supposed to remain
in contact with the cam lobe or lifter even when closed,
but springs, they take time to expand.
If the camshaft is moving too quickly
because the engine is spinning too fast
it remains open for too long protruding into the cylinder
when it shouldn't be or rapidly opening
and closing as the undammed valve spring oscillates.
Just sits there, just in a constant state
looking compressed.
The same thing happens in a push rod motor
except now all the other parts
between the valve and the can, can end up floating too.
Now when a valve floats the best case scenario is
the valve remains open a bit too long, reducing efficiency
because the cylinder doesn't seal properly
for the compression stroke,
but many engines have an interference design where
the valves and the pistons occupy the same space
just at different times.
You can, you can probably see where the problem is.
If an interference engine is spinning too fast
and a valve stays open for too long
the piston can hit that valve,
called fricking crap day.
It's your crappy day, okay.
It crushes the valve and you got
a ton of repair work to do so.
How do you make valves that can cope
with increased engine speeds and avoid flow?
Well, you can use stiffer springs
so if the valve snap shut faster,
of course that means more force is required to
open those valves as they open more slowly.
The solution in some racing engines is to switch
to a pneumatic spring,
a nitrogen filled piston retracts the valve more quickly
and doesn't oscillate like a mechanical spring.
The downside, that's a much more complex
and expensive system than the traditional
overhead valve design or overhead cam designs.
There's also some electronic valves
which is pretty interesting or Koenigsegg,
they have their free valve technology again, very cool,
but very expensive.
So you might have noticed a trend so far,
the factors that limit engine speed
all can relate to carefully timed movements of its parts.
But it's not just the movement
of engine parts that have to be carefully timed.
Third problem for revving high
is that combustion itself takes time.
(upbeat music)
Combustion starts at an ignition point.
The spark plug in an engine and moves outward
into a process called flame propagation.
The moving line of combustion is the flame front
and how fast it moves is the flame speed.
You can see the flame propagate downward
from the ignition point at the top of this bottle,
that's like the flame front inside
an engine cylinders moving down from the spark plug.
Every type of fuel has an intrinsic burning velocity.
It's flame speed under a set of ideal conditions.
So we have two fuels that we tested.
We used a standard ethanol based fuel.
Super cas.
(fuel exploding)
And we also use isopropyl, alcohol, 91% isopropyl alcohol.
(fuel exploding)
Pretty potent stuff.
You're not going to find this high quality isopropyl alcohol
at your local CVS, actually yeah we did,
we bought it at CVS.
If we compare the speed of the flame propagation
between these two, we can see
that this fuel burns faster than this fuel.
Different fuels, different burn rates.
But burning velocity is just the beginning.
Actual flame speed is affected by temperature,
pressure, turbulence in the fuel
and the distribution of oxygen.
When I used compressed air
I was creating turbulence, which slows flame speed
but I was also vaporizing the fuel,
kind of like a fuel injector.
So what does all this mean for rev limits?
At 1500 RPM and at a 12 to one compression ratio
the flame speed in a gasoline engine is around 16
and a half meters per second.
But with changes in temperature,
turbulence, and fuel air mixtures
flame speeds can vary between 10 and 80 meters per second.
Piston speeds can reach 40 or 50 meters per second
meaning that under the right conditions at higher revs
the piston will outrun the flame propagation.
If that happens, the combustion isn't exerting any force
on the piston and the engine simply can't spin any faster.
It's sort of a built-in rev limiter
that particularly affects engines that have a long stroke
because of their higher piston speeds.
In fact, one of the primary reasons
why F1 engines can rev so high
is because they have incredibly short stroke,
just a bit over 53 millimeters, that's two inches.
A short stroke keeps piston speeds low, prevents outrunning
the flame front, and keeps effective masses manageable
without heavy bottom end parts
as you approach the 20,000 RPM ceiling.
It also increases the space for valves
meaning you can feed more air into the cylinders.
Large boards, small stroke is part
of how Bruno's 2006 F1 engines
capable of revving to a record 20,500 RPM.
But there's always a couple of downsides
and one of them is with a short stroke
very little power is made at lower RPM.
That's why an F1 engine has to idle
at 5,000 RPM just to stay running.
And one reason why we don't have 20,000 RPM engines
in our road cars.
So now we know that if we want to rev high
we've got three problems to overcome.
Damage to the bottom end
from rapid acceleration and deceleration of the piston.
We got damage to the top end
from valves that aren't retracting quickly enough.
And we got pistons that outrun combustion.
The reason why every car doesn't red line
at 20,000 RPM is because the solutions
to these problems all come with trade-offs.
You're a trade-off, okay.
Your parents traded off having a fun life for,
thank you guys so much for watching this episode of B2B.
We're getting up towards 5 million subscribers.
Thank you guys so much
for being part of the Donut family.
We love you.
Until next week, bye for now.