I have a question about the turbocharger.

I understand how a turbocharger works: there are two impellers on the same axle. The turbine impeller spins due to the exhaust gases, while spinning the compressor impeller, which in turn pumps air into the engine intake. But why without load on the engine, there is almost no boost pressure at high RPM? If I understand correctly, at this moment the wastegate opens, but why?


Example:

 

you are rev limiting the car, for it to not overrev the engine disables the throttle imput and the wastegate opens when throttle is stopped

 

Turbo behavior on a diesel engine significantly differs from turbo behavior on a petrol engine:

Diesel engines are quality controlled, they always induct all the air available to them by suction/forced induction.

Petrol engines are quantity controlled, they induct only as much air as needed for current load conditions and are throttle limited.

So when you are flooring an turbocharged engine and then release the accelerator pedal, this is what happens:

Diesel Engine: "Ok, you want less power, so I will just stop injecting fuel for now". No need to do anything about the charged air still rushing into the engine. The engine will just cycle it without burning. Turbo can keep spinning and delivering charged air to the engine without issues.

--> Boost pressure drops slowly and recovers instantly if you acclerate again.

Petrol Engine: Oh crap, the throttle valve has just mostly closed and we now have an tidal wave of charged air that has nowhere to go because the mostly closed throttle valve wont allow most of it into the intake manifold. Quick, open the blowoff valve so that it does not run backwards to the turbo again, causing a compressor surge. This also allows the turbo to keep spinning and creating charge air, however most of that air never reaches the intake manifold as it is vented before the throttle valve.

--> Boost pressure drops instantly and it takes a moment to recover it even with the turbo still spinning at nearly maximum RPM because not only does the Blowoff valve have to close first, the now mostly vacuum space of the intake manifold has to be filled with air again before full boost pressure is restored to the inlets of the cylinders.


High engine RPM may not necessary involve creating boost pressure, a petrol engine will have its throttle mostly closed due to low load and as such there will be little in terms of exhaust spinning the turbo. A diesel may have much more air going through the engine but at low load, it also creates not much in terms of exhaust and so boost pressure may drop too.

BeamNG may have a somewhat higher treshold for turbo spinning than in real life, too. (Meaning that while a turbo in real life would spin and create some boost pressure in real life, it may not spin and create boost pressure in BeamNG. This mostly involves low to middle load conditions.