superchargers @ high rpms

Question

why are superchargers not good at producing boost at high rpms like turbos?

In: Engineering

Answer 1

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.