Why does combustion engine power taper off at a certain point while the crankshaft continues to speed up?


Why wouldn’t a faster engine speed give more power?

In: 19

Power = torque x RPM, so if RPM is going up but power is dropping you know that torque must be dropping (more than RPM increasing).

So your question boils down to “why does torque drop off at higher RPM?”.

And that’s because two things are working against you as RPM gets high…time available to burn fuel (to make pressure to make torque) is dropping, and loses (energy you spend to make the engine work) are going up.

Even if the engine was taking in the same charge of air & fuel per stroke (it’s not), the faster it goes the less time there is to burn the fuel. As you get really fast, you don’t get peak burn until relatively late in the power stroke and so your maximum pressure and maximum torque falls off.

And as you go faster, you need to move air through the system faster. You get more pressure drop in the intake, so less air & fuel enter the cylinder per stroke, and you do more work to push the exhaust out, which steals torque from the crankshaft. And the friction in the bearings and valve train and everything else is continuously going up with RPM and starts to eat more an more into your total available power.

An engine is designed to work within a certain RPM range. At the top or bottom of the range, it makes less power. Outside the range, it doesn’t work at all. The range can only be so wide, given the nature of the system.

On the high RPM end, it’s usually because the valves don’t slam shut quickly enough to trap in the explosion. This is known as valve float. If the valves are open even slightly, it means that the explosive pressure escapes the cylinder instead of being used to push the piston down. It gets worse with higher RPM.

You can try to raise the limit, but not indefinitely. 10,000 RPM is considered the realistic upper limit for an extensively modified 4 cylinder engine (in a street car), usually bigger engines rev considerably less. 8500 rpm is considered pretty good for a V8, for example. However, at the top of the RPM range, it will always run into this problem.

You can put a stronger spring in there to get the valve to shut faster (allowing you to rev higher), but this makes the car lose low RPM power, and can even raise the minimum idle RPM needed to keep the car from stalling out. Different cams are also used to change the timing of the valves (try to shut them a little earlier) and also how far they open up. Cams that favor high RPM make the idle really erratic as well.

They can make engines run up to around 20,000 RPM, but they are for the most extreme race cars or motorcycles only (formula 1). They have to idle at really high RPMs (3-5000+ RPM) just not to stall out. It would be very inconvenient to drive something like this on the street, and it would be loud as hell too.

This doesn’t even consider the reliability concerns associated with high RPM performance, of which there are many.

There’s a few specific design features. Really you’re right. The faster the better. Old formula one engines used to run up to 24,000rpm, where your old VW air cooled engine might run out of revs at around 4000 or less.

You could write a phd on this, and I’m sure some have. The eli5 would be that you only get so many points to build an engine with. Like assigning your skills at the start of a video game.

If you want to make it cheap, you can’t use fancy materials. A mass production car engine running 20,000 rpm will just explode into pieces.

Then you have to pick how you’d like the engine to feel. If you want it to run the most power, then has to rev to faster speeds, but then an engine that runs fast speeds also by nature will run horrible at low speeds.

Then you have economy, if you’re putting this in a commuter car, those people care more about mileage than power, so you need to set the points toward mileage.

Then you have lifespan, if your engine needs to last years rather than just one race, then the parts can’t be stressed to near failure.

Those settings are decided by how wide the piston is, how far it travels up and down, and when the valves open and close and how closely you can control the fuel input. All of that is decided by how by how you want the explosion to behave when you set fire to the fuel in the cylinder.

Engines are designed to work best at certain RPM ranges. The main factor that affects this is the profile of the cams and the design of the valves. In a perfect world every exhaust stroke would expel 100% of the exhaust gasses inside each cylinder; if this were the case, the amount of power the engine produced would be directly proportional to RPM, i.e. if an engine made 300HP at 6000 RPMs, it would make 150HP at 3000 RPMs; it doesn’t work that way. When gasses leave the cylinders they leave in 3 separate pulses.

1. When the exhaust valve opens the higher cylinder pressure forces the gas to rush out into the exhaust manifold
2. The majority of the exhaust gasses leave as the cylinder rises during the exhaust stroke
3. The intake valve opens up slightly before the exhaust stroke finishes and the exhaust valve remains open during part of the intake stroke. The inertia from the gasses inside the exhaust pipes draws the remaining gasses inside the cylinder and fresh air fuel mix is drawn in. This is very bad for low end performance because the gasses aren’t flowing fast enough for this to have a positive effect. That’s why muscle cars have that classic rumble at low RPMs.

Modern cars have a trick called variable valve timing that allows them to adjust when the intake and exhaust valves open in relation to each other depending on engine speed and load. At low RPMs there is very little overlap and at high RPM and high engine load the intake and exhaust valves have much more overlap.