Today's video has been
brought to you by Squarespace but more on that, later in the video. Let's imagine a scenario. You're idling at, say 600 RPM. And then you
decide to put the vehicle in gear, and floor it. The butterfly valves under throttle body
take a fraction of a second to open fully and allow maximum air into the engine. It takes that air even less time to actually
get into the engine, into the combustion chamber And then takes the injectors another absolutely minuscule amount of time to deliver
the fuel needed to match this air. So within a fraction of a second, we're giving the
engine everything it needs to build maximum power. We're allowing maximum air into the engine
and we can deliver maximum fuel instantly. So then why can't the engine deliver maximum
power and maximum torque instantly? Why does it need to rev to here and here to build peak torque and peak power? Why can't it build peak torque and peak power right after idle if we're giving
it everything it needs to do so? Now, I know we're accustomed to seeing power
and torque as curves. But have you ever wondered Why are they actually curved? Now, I'll let you think about it one for just
a moment, as I tell you a bit about Squarespace. So, what is Squarespace? Well, Squarespace is everything you need it
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of a domain for your website. And now back to engines. So do you know the answer? Why is power and torque a
curve, and not a flat line? Well the answer might be both
surprising and kind of obvious. And the answer is piston speed. Why piston speed? Because a fully open throttle valve, may allow
a lot of air to 'potentially' get into the engine. But how much air, actually gets into the
engine, is determined by the piston. Now, I know a lot of you right now are going Huh? What is this lunatic talking about? Piston speed? No it's the intake valves. The size of the intake valves, and
how much they get open to their lift. And the duration of how long they stay open That's what determines how
much air gets into the engine. Well yes, this is technically true. But again, intake valves are
just like a throttle body. A large intake valve gets opened a lot, has a
lot of lift, and stays open for a long time, only creates potential for a lot
of air to get into the engine. Again, how much air actually gets into the
engine, is determined by the speed of the piston. So, how does this work? Well, it's actually pretty simple. When the piston moves down the bore, it
creates a void, an absence of air, a vacuum. When a vacuum appears, air of
course moves to fill that vacuum. And this vacuum which is constantly being
created by the piston as the engine is running, is the true source of the engine appetite for air. Now, the higher the engine RPM,
the faster the crankshaft rotates, and the faster the piston travels down the bore. The faster the piston travels down the bore,
the more vacuum it creates at a more rapid rate. The more vacuum it creates, the
more air rushes into the engine. The more air is being pulled into the engine. And this is the reason why
power and torque are curves At 700 RPM, there simply isn't enough piston
speed to pull in a lot of air into the engine. You may open the throttle fully and
create a for a lot of air to get in, but a lot of air won't get in, because
there isn't enough piston speed. And this is why when you floor it
from idle, nothing really happens. There's no drama until RPMs
increase at least a bit. By the time the engine builds
up, let's say 5000 RPM. The piston's travelling so fast, that
it can ingest the maximum possible air that the throttle body
and intake valves will allow. You can match this with maximum fuel and create
the maximum combustion power you can generate, which generates maximum combustion pressure
and pushes the piston down with maximum power. And then using the connecting rod
and crankshaft pin as leverage, the piston can act on the crankshaft, and
the crankshaft spins with maximum torque Now, I know that again some of you are going Ah! who cares?
This only applies to naturally aspirated engines. Because when it comes to forced induction,
we can use a turbo or supercharger to stuff in more air into the engine
than a silly little vacuum could hoped for. Well, yes, a supercharger and a turbo can
increase the peak power output of an engine Definitely. But, the power and torque curves of
forced induction engines are still curves. No conventional mass produced forced
induction device can create flat power and torque curves, nor can it generate
instant power and torque right off idle. And this is of course because no turbo
or supercharger can create boost at idle. A turbo needs heat and exhaust
gases to be driven, to create boost. A supercharger is directly connected to
the crankshaft pulley usually via a belt, and needs engine RPM to spin
fast enough to create boost. At idle neither is creating boost. And this means that again, the initial
combustion, that creates exhaust gases, and increases RPM, and then
drives the turbo or supercharger, is again dependent on the vacuum
generated by the piston speed. Which draws in the air, creates the combustion,
and then drives the turbo or supercharger Piston speed and the vacuum which is generated
by the downward piston movement, is at the core of the internal combustion engine, and as
such it shapes the power and torque curves. Now, it's time for the Level 2 question. Why is peak torque always generated
at a lower RPM, than peak horsepower? Aren't torque and horsepower linked together? Isn't horsepower essentially torque x RPM? So if they're linked together, why
don't the curves follow each other, why don't they look similar or the same? Well the answer to that question
is in the question itself. The curves can't be the same,
because horsepower is torque x RPM. If you're using the horsepower formula, then
horsepower equals torque in feet pounds x RPM. Divided by 5252 If you're using kilowatts, then the
formula is torque in Newton meters x RPM. Divided by 9549. But it doesn't matter which formula your
are using, you can see that at the essence, at the core of the formula,
torque is multiplied by RPM. So what do these two formulas tell us? Well they tell us that it's simply
impossible to have similar or identical horsepower and torque curves, on the
same graph and at the same scale. Why? Because both torque and RPM are
a multiplier for horsepower. Let's use a dyno chart of an electric
vehicle to demonstrate this nicely. In this case, it's a Tesla Model 3. And as you can see, because it has an
electric motor, it can generate instant torque An electric motor obviously doesn't care
about piston speed and vacuum and what not. The battery supplied energy, the
electric motor starts spinning. And voila! Instant torque. And as you can see, the
electric motor can also keep a flat torque for a pretty large junk of its RPM band. But okay the horsepower curve, although the torque
curve is flat the horsepower curve is increasing. It's rising. Again, because RPM is a multiplier for horsepower Here we're multiplying torque by 1000. Here we're multiplying torque by 2000. Here we're multiplying torque by 3000 And so on and so forth. Even if torque is flat, RPM is always
increasing and it's a multiplier for horsepower. And thus horsepower is going to be increasing. The same goes if torque starts falling off. As you can see on this dino chart, torque is
falling off but horsepower is still increasing. And this is going to happen as long as
torque doesn't fall off too sharply. Because again, although torque is
falling off, the RPMs are increasing, and we're multiplying the torque by the
RPMs thus increasing the horsepower value. And now the final question. Level 3
The boss question. No not really, it's not hard, just
something you might be curious about. And the question is,
why do torque and horsepower eventually start falling off? Why do they reach a peak ,and then fall off. If the business moving faster
and faster as the RPMs increase, shouldn't torque just keep
increasing, until the RPM limit? if we're getting more and
more air into the engine? Well, the answer behind this is that it will be stupid and useless
to have peak torque, occur near the RPM limit. You may indeed see that some racing engines do
have peak torque pretty close to the RPM limit. But a racing engine spends most of its very short life being revved all the time to the RPM limit and driven all out. But other engines that see a very wide variety of uses, so you need to join on the highway, stay on the highway You drive to curvy back roads, drive to city traffic. In this case, you really don't want to be forced to rev to the red line all the
time to get the vehicle moving. You need torque to get the vehicle moving. And you want the torque to be somewhere reasonable, so you can access
this peak torque relatively easily. This means that the location of peak torque
is calibrated for the engine's intended use. And it's calibrated predominantly by the size
of the intake valves and the throttle body. As we said, the intake valves
in the throttle body determine the potential air, the maximum potential
air, that can get into the engine. And the piston speed, the
vacuum generated by the pistons, determines how much of that air actually gets in. But how much air (Within the
maximum potential determined by the intake valves in the throttle body) So the size of your inlet devices, is a
limiting factor to how much air can get in Because at a certain point, the speed of the
piston is going to try to ingest more air, than can actually come through the
intake valves and the throttle body. At this point when the piston becomes too
fast, and tries to suck in too much air, we have reached the maximum possible
flow of the size diameter of our orifice. Which is a throttle body. And the intake valves. If you install a very large throttle body and
very large intake valves onto your engine, you are creating potential for a
lot of air to come into your engine. But to realize this potential, you
need a lot of piston speed to actually ingest the maximum amount of air that the large
throttle body and intake valves can provide. This means that you need to rev
high to realize this potential. So, your ultimate power/your maximum
power, will increase with a larger throttle body and larger intake valves, but
your peak torque will be moved higher up. And you will need to rev
higher to actually generate this power that has been allowed by the
larger throttle body and intake valves. But even with the larger intake valve
and throttle bodies, at some point your piston speed can become too high, and try
to ingest more air than our orifices allow. At this point we have reached the maximum
mass flow rate through our orifices. And now they're becoming a
restriction, choking the engine. Revving higher, and increasing piston
speeds higher beyond the maximum amount of air that can come into the engine, will
obviously not result in a torque increase. Instead, the torque is going to start falling off. An analogy will be letting a
child breathe through a straw. A child might be able to breathe through a straw, because the maximum amount of air that he or she
can draw in might be sufficient for the straw. But if you give the same straw to an
adult, an adult will start choking and will be unable to breathe through the
straw, because the orifice is too small for the amount of air that the adult is
trying to draw into his or her lungs. What about electric cars? Why does their torque start falling off? If they have an electric motor and
a constant supply of energy from the batteries, why isn't the torque curve just flat? All the time, throughout the entire RPM range? Well, the answer is back EMF,
or a Back Electromotive Force. Basically, it's a force that opposes
the changing current which induced it. In other words, it's a voltage fighting another voltage. The faster the motor spins, the higher
the back EMF which counters the motor, and reduces the torque output. Now, when it comes to the instant
torque of electric vehicles, this is often presented as an advantage
over internal combustion vehicles. And although it definitely is in situations like city driving, on a different type of road, it really isn't an advantage. Because having to build power, to reach peak power
and peak torque, isn't necessarily a bad thing For many, it leads to a more rewarding, more
involved, more focused driving experience. And actually very few things are
absolute advantageous or disadvantageous. And many have merits depending
on different conditions. And there you have it. That's pretty much it, when it comes to today's video. A bunch of obvious questions,
but maybe not so obvious answers, in case you didn't have a firm grasp on all these things already. As always, thanks all for watching. And I'll be seeing you soon,
with more fun and useful stuff... ...on the D4A channel.
