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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