How do engines with forced induction create vacuum?



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

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.

Usually, closing the throttle body to its idle position will allow the running engine to create vacuum in the intake manifold.

I think most replies here cover the essential point: under light load and light power requirements, the normal aspiration of a 4-stroke is sufficient to meet all the needs/economy for normal running. This, of course, means that you will only ever register vacuum on the manifold gauge. Atmospheric air pressure is constant at 1 bar (unless you have some wacky ram-air arrangement), so the gauge will only ever register a reading lower than that, by definition: a vacuum.

When you require more power, and therefore open the throttle up, normal aspiration is superseded with boosted air from the charger. This, by definition, is forced in – so air/manifold pressure rises to parity with atmosphere (1 bar) and then to 2 or 4 bar over-pressure or whatever is your system is limited to, or requires.

Point being: under normal aspiration, you can only ever see vacuum (negative pressure) on a gauge, even with the throttle wide open, as external air pressure never changes.

With a charger of some sort, you’re always into a pressure realm above 1 bar, if it is in use. Anything above normal air pressure is always into ‘boost’.

Presumably, you must have some kind of arrangement to be able to mix/route boosted air into the (normally aspirated) manifold stream, when needed. Otherwise, I would anticipate running issues at the transition point between the two states. This would easily achieved if the charger allows unimpeded air flow through it under non-boost conditions, but progressively boosts/mixes as required.