How do engines with forced induction create vacuum?

Question

I’m specifically focused on how engines with centrifugal supercharger create vacuum at the manifold when the engine in high RPM with low load.
Let’s say engine is at 4K RPM with 20% throttle, how can the manifold have a vacuum?

In: Engineering

Answer 1

Centrifugal superchargers create pressure or boost by spinning an impeller at very high speeds, relative to the RPMs of the engine driving it (often they spin upwards of 8:1) They increase the air density in this way. Since they are inline with the intake of air, they can only really add to the air dynamics of the engine. The vacuum effect is just the displacement caused by varying degrees of air pressure. Obviously in a low load situation the impeller spins slower and creates little, if any, boost. But it wouldn’t harm the natural flow of air the engine creates by running.