why are superchargers not good at producing boost at high rpms like turbos?
In: Engineering
A supercharger kind of works the same at any engine speed. It takes a certain amount of energy to rotate it once and it will deliver a certain amount of compressed air to the cylinders. At higher engine speeds the supercharger takes more power to turn but provide more air which matches what the engine needs. The difference is more on the turbo side. A turbo does not get power from the engine crank but rather use waste energy from the exhaust. Technically it does take some energy from the engine output as it does increase exhaust manifold pressure, but this is typically very minimal. The issue is that a turbo spins best as the speed it is designed for. At low engine speeds there is not enough exhaust gasses to make the turbo spin properly. So you first need to get the engine speed up to provide enough exhaust gasses to spin the turbo, then wait for the turbo to spin up before you eventually get enough air for the engine.
The difference at high engine speeds are that superchargers still require power from the engine to spin it around, while the turbo does not. A supercharger can maybe use 20% of the engine power. That is power that could have gone to the wheels. And the turbo is able to provide just as much compressed air without this power draw. But at lower engine speeds the turbo will not spin as much and just let the engine exhaust go past the turbine blades without contributing much power. So you starve the engine of air lowering its power output. So the 20% power draw that a supercharger takes is less then what you get in return for enough air to the engine.
A common solution is to use a smaller turbo. Smaller turbos takes less time to spin up and require less exhaust gasses to spin so they work at lower engine speeds. But they do not provide as much air to the engine as a larger turbo. So these work better at lower engine speeds but not as good at higher engine speeds. Some engine builds have two different sized turbos, one for low speed boost and one for high speed boost. This is to compensate for issues compared with a supercharger.
a supercharger is tied to the rotation of the engine so when the engine RPM rises the supercharger RPM also rises.
At the same time the engines need for air is also rising so overall a supercharger gives a relatively consistent boost over the RPM range.
A turbocharger is driven by the exhaust gasses of the engine which is why at low RPM a big turbo wont be spinning fast enough to produce any boost but at high RPM the amount of exhaust gasses rises exponentially which in turn spins the turbo faster which results in more boost which means you can inject more fuel which again results in more exhaust gasses and the cycle continues until you either hit the RPM limit of the engine or theres a point where you cant inject more fuel or the friction losses become greater than the extra boost you would get from the extra exhaust gasses.
oh and of course most high power turbo cars will have a valve to limit the boost pressure so this cycle doesnt literally blow up the engine.
Simply put, a turbo can spin faster at higher RPMs than a supercharger can. A supercharger is connected to the engine, but the turbo is spun by exhaust gasses so as more air is passed through the motor at higher RPM the turbo spins faster and faster. Depending on the geometry of the turbo’s blades, this can result in a turbo going wicked fast. A supercharger is more ‘linear’, giving you a proportionate amount of boost with the spinning of the engine. You can change the pully length of a supercharger to overdrive it even more, but it remains tied to the motor’s RPM.
It is misleading to say a supercharger is not good at producing boost at high RPM and more correct to say that turbos are *more efficient* at creating boost at high RPMs than a supercharger and wildly less efficient at producing boost at low RPMs. My wife’s car has a two stage boost, an electronically driven supercharger at low RPMs and a nice big turbo for higher RPM.
Superchargers and turbos are, for all intents and purposes, the same device. The primary difference is that one is driven by a belt, the other is driven by exhaust.
Any improperly sized device is going to run out of steam, so it’s important that the pulley/exhaust turbine be properly matched to the expected operating range of the impeller. So if you put a larger impeller (it’s not that simple, but for the sale of conversation let’s pretend) on a supercharger that used to run out of boost at 6k rpm, you could make more boost at 6k and still have room to go up making boost at 7k. You’re going to loose some response at lower rpm though, so that needs to be considered..
It’s a matter of pairing the engine efficiency with the supercharger/turbo efficiency.
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