WHY are HORSEPOWER and TORQUE CURVED?

Let's say you're idling at 600 rpm. You put the car in gear and you floor it, you open the throttle completely. It takes a fraction of a second for the butterfly valve in your throttle body to open fully and allow large amounts of air into the engine. The air takes even less time to actually get into the engine and it takes the injectors another absolute miniscule amount of time to deliver the fuel needed to match this air.
So everything the engine needs to be build maximum power and torque is delieved in a split second. Maximum air is allowed into the engine and we can deliver maximum fuel pretty much instantly. So why doesn't the engine deliver maximum power and torque instantly? Why does it need to rev higher to make maximum power? Why can't it deliver that same power right after idle if we're giving it everything it needs to do so? Why can't internal combustion engines generate instant torque like electric vehicles such as a tesla can?

Why is power and torque a curve and not just a flat line?
Well the answer is piston speed! Why piston speed? Because the speed of the piston determines how much air can actually get into the engine. A fully open throttle body may ALLOW a lot of air to potentially get into the combustion chamber. But how much of that air actually gets in is determined by the piston.

But aren't intake valves what determines how much air gets into the chamber. Zero air gets into the chamber when the intake valve is closed. the timing of the intake valve opening and the duration of how long the intake valve stays open actually determines how much air gets into the chamber. Well yes, technically this is correct. But the valves too are just like the throttle body. A fully open intake valve creates potential for maximum air to enter into the engine, but whether maximum air actually gets into the chamber is determined by the piston. How does the piston do this?

Well it's actually pretty simple. When the piston moves down the bore it creates a void, or vacuum, an senescence of air. When this absence appears air of course moves to fill it. This vacuum which is constantly being created by the piston is the true source of the engine's appettite for air.
Now the higher the engine rpm the faster the crankshaft spins and the faster the piston travels. Now the faster the piston moves down the bore the faster it creates more vacuum and the faster the air rushes into the engine. And this is why power and torque are curves. At 700 rpm the piston simply doesn't travel fast enough to create enough vaccum to ingest maximum air.
But when the engine builds up 5000 rpm the piston travels fast enough to ingest the maximum possible air and then you match that with fuel and you get the maximum possible combustion intensity which generates the maximum possible combustion pressure which pushes the piston down with maximum force then using the connecting rod and crankshaft pin as leverage the piston causes the crankshaft to rotate at maximum torque.

But forced induction engines don't care about the vacuum generated by the piston because they can use a turbo or supercharger to stuff in more air than a silly little vaccum could ever hope to create? True, forced induction increases power but again no amount of forced induction can create a flat power and torque curve. A turbo needs a sufficient amount of exhaust energy to be driven at sufficient speed to generate maximum boost, and the engine can only generate this maximum exhaust energy at certain rpm. Same goes for the supercharger which is driven by the crankshaft usually via a belt so it's rotation speed is actually synced to the rpm of the engine. And to achieve maximum boost the supercharger also needs to achieve a certain rpm. And although some very modern turbocharged engines can generate maximum torque starting from as little as 1500rpm and keep it flat for most of the rpm range thanks to modern ultra low resistance and ultra smart aerodynamics turbos and continuously variable valve timing and valve lift.....maximum power is still always generated at a much higher rpm.

So here's the next level question for you: How can maximum torque be generated at much lower rpm than horsepower. Aren't the two linked together because horsepower is essentially torque x rpm. So why doesn't the horsepower curve simply follow the torque curve? Why don't they look the same?
The reason behind this is that horsepower is essentially torque x rpm.

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#d4a #horsepower #torque

00:00 Why are they not flat
02:29 Gates and piston speed
05:10 Forced induction and vacuum
06:41 Why peak torque before peak power
09:04 Why do they fall off

[MrAndrius]

Another thing to take into consideration is thermal efficiency - the faster the piston speed the less time the heat generated from the fuel has to leave through the cylinder walls.

[kylesebring]

another great video, im always suprised by how i will think i know a solid 80% of what there is to know about a topic, then I watch your video and become humbled

[roflchopter]

The main reason for the reduction in BMEP (and thus torque) at low speeds is the heat lost to the cylinder (due to the long time between ignition and BDC), and the tradeoff between combustion chamber geometry during the timing of the combustion event (delaying combustion so the high pressure coincides with a favorable rod-crank angle will decrease the effective compression ratio).

Other factors include reduced inlet air velocity for mixing and complete combustion, more time for leakage per engine cycle, and being outside the range of speeds the intake and exhaust have their resonances tuned for.

The "pistons not moving fast enough to suck air in" fails the sniff test. The pistons moving slowly gives more time for the cylinder to fill. At very low speeds, the cylinder pressure will be equal to the manifold pressure at the end of the intake stroke.

[Gnerko]

Maximum torque is when cylinder fill is best, i.e. highest cylinder pressure when the intake valve closes. Higher rpm tends to decrease this, as flow resistance from the intake tract becomes higher. Due to some nonlinear effects, torque isn't maximized at minimum rpm. Piston speed, however, is not the chief reason for this.

At 4:17, the video notes that at 700 rpm, there is not enough piston speed to pull a lot of air into the engine. This is true, that is why power is low. But it is not the reason that torque is low. Torque is not about how much air the engine can suck in per unit time, but about how much air it can suck in per crankshaft rotation. At higher rpm, pumping losses from the intake tract increase, but nonlinear effects such as scavenging can still cause an optimum, leading to a peak in the torque curve.

[NickShelden]

There are many other variables, though I think he touched upon the primary ones. Parasitic losses from bearings, thermal efficiencies, variable valves, etc all factor in. The biggest factor that comes to mind for me, especially for why torque drops off in higher rpm, even if the valves and intake are not a restriction, is the flame front and the time it takes for the pressure to build after ignition. At higher rpm the fuel air mix has less time to burn, thus providing less pressure, thus less torque. At extremely high piston speeds the piston may even outpace the flame front, at which point the engine rpm is usually self-limiting. This situation is not normally possible on most engines due to other factors and is obviously avoided by engineering design (piston speeds in this range also risk con rod or wrist pin failure, or simply overheating)

[ThisRandomUsername]

The torque curve is almost entirely down to how much air gets into the engine per cycle, it's got very little to do with how many cycles happen per second. How much air gets in is a result of tuning. If you close the intake valves at bdc you get almost 100% of the air into the engine at low revs, but less in at high revs. If you close them later you get better higher end torque because of the momentum of the intake charge (and wavefront if your port is properly tuned) forces more air in even when the piston is coming back up.

That's why a lot of modern engines have such flat torque curves: variable valve timing.

As others have said, at low revs you're limited by the amount of heat rejected into the cylinder walls, while at higher revs you're limited by the breathing capability of the intake and exhaust. (edit: exactly as you said in the latter part of the video)

[gabeshaw]

Another way to think about this is to break it down into two different ideas: an engine spends half its time basically pumping air, and the other half extracting power from the air. Engines work best when the intake air is turbulent and mixed well, and at low RPM the only way to generate that turbulence is by sacrificing airflow, but sometimes that isn’t possible (DOHC).

[QDWhite]

For the calculus nerds, power is the integral of torque WRT rpm, or conversely, torque is the derivative of power.

[aaronaaronsen]

Thanks for this video ! Now I understand why some electric vehicles still do have some sort of gearbox. The power band is very large compared to a ICE but it still has a limit.
And I didn't knew the reason behind the falling off of the torque curve, which in turn causes the power curve to fall. But when you know it is limited by the air intake, it all makes sense !

[jimstepan]

@ 4:12, I heard the "nails on the chalk board" words that paint the picture of air being "pulled" into the engine.. This is IMPOSSIBLE ‼️ Air is a gas. Gases, and liquids, can not be pulled !!

Every time the piston in a naturally aspirated, reciprocating, internal combustion engine travels through its 'intake stroke', air, and possibly fuel, are PUSHED in to the vacated space by ATMOSPHERIC PRESSURE ‼️

Even at higher piston speeds, where a 'ram effect' takes place, the air is being pushed from the higher pressure areas to the lower pressure areas during each and every intake stroke ‼️

I am fascinated by the range of subjects you cover !! Keep it up !! 😉 🙋🏻‍♂️

[thearth]

Back in the 60s, a company called Turbonique sold superchargers that were independently powered by a gas turbine with its own fuel supply (of literal rocket fuel!). Since they operated completely independently of the engine itself, they could provide full boost at any RPM and had no parasitic drag.

[humbertobm]

Thank you very much for making the description very complete and detailed, as I don't understand English, I can translate and understand the video and its animations clearly.

[jumboegg]

Piston speed and rpm are directly related to speed of the rotatin crankshaft/flywheel/drivetrain, and it is this rotating assembly that stores the energy we have created, in the form of momentum. A big heavy flywheel takes a lot of energy to get going, but once its spinning at the designed rpm, it takes relatively little energy to keep its momentum going.

[keithjurena]

Positive displacement superchargers generate flow. The engine's momentary valve opening restricts this flow, causing a rise of pressure. This pressure rise is nearly constant until dynamics of intake valve opening and mass-spring relationship of the intake charge limit flow.
Unfortunately, most positive displacement superchargers have leakage at apex seals and this is why idle speed boost is limited. Most positive displacement superchargers have boost bypass for all but wide open throttle position

[danamoroso-xjq]

My understanding has always been that RPMs determine power (and total torque value too to some extent) simply because of the number of explosions for "unit time". At 1000 rpm you have 1000xCilinders amount of explosions pushing you forwards in a minute, while at 6000rpm you have 6 times as many.

The curves normally drop off at the top RPM because at that point the pistons are going so fast that the explosions have little to push off against. The theoretical limit of this is when a piston is going as fast as the expanding explosion, where the explosion is not accelerating the piston any further.

[isotropicantenna]

Electric motor torque doesn't fall due to back EMF when the motor is ran with a motor controller. Controller can always step up the voltage to counter the back emf.

Torque is allowed to fall intentionally to prevent overheating of the motor. once the motor reaches peak power it can handle, trying to maintain the maintain the Torque means motor is being overloaded and will overheat.

[INeatFreak]

Thanks, this was very helpful. I was confused on why the torque and power curves are different when they're related with each other.

[errolfeistl]

Respectfully I would like to raise the question of combustion speed in relation to the powerband at higher RPM... EG it takes an amount of time to combust the fuel/air mixture and as you increase RPM you need to advance the timing (Eg setting off the mixture earlier and earlier in relation to piston TDC giving enough time for peak cylinder pressure to occur shortly after TDC resulting in more power). When you have larger displacement cylinders if the piston is moving at really high speed/rpm the combustion literally doesn't have time to fully occur as the flame front isn't instantons resulting in less peak pressure on the piston/less power output as RPM increases. Hence smaller displacement cylinders (or rather, shorter crank distances) allow for much higher RPM as the physical distance for the combustion to occur is shorter allowing for higher and higher RPM.

The old F1 V8 engines had really short stroke lengths to allow for higher and higher RPM and conversely when you "stroke" a V8 engine with longer stroke the RPM limit tends to be lowered (although you make more torque/power overall due to increased displacement). This is why "Stroker" V8s are great to drive on the road (Lots of power down low) but aren't suited for racing.

Also the other thing I thought was worth mentioning is the "Flat" torque curves you see in modern engines are solely a result of tuning and NOT the natural torque curve of the engine. This is because gearbox/driveline components are rated on their torque limit (Remember torque breaks things, NOT horsepower), so engine manufactures tune their engines to not exceed this limit but stay as close to it for as long as possible (thus giving maximum effective horsepower over a wider RPM range without breaking parts).

Anyway as always loving the video (And hope my comment came across as constructive and not argumentative).

Cheers

[alejandro]

Another consideration is the exhaust pulse timing and overlap of the valves, David Vizard has attributed more of the low pressure on the chamber being due to the exhaust than the piston.

[johannriedlberger]

In ship diesels is a way to come around the limitations of high torque at low rpm. When abusing the compressed air from the starting system during run they fill the cylinders instantly. During the next exhaust stroke the turbo wakes up quickly so the compressed air reservoir is not the limitation. The use of nitro oxide will also give instant torque. But if the system starts at idle the engine will throw its rods.