Gareth Allan

Probably obvious but..

When the car is in neutral and I blip the throttle to 4000rpm, the boost goes from maybe -0.5 to 0 bar.

When driving down the road, the boost goes upto one bar.

Now, it only boosts when the engine is under load, but how does the engine know when it is under load? Or is it boosting because of some other factor?

Gareth

SecretAgentMan

During load you're putting much more air and fuel through the engine, which creates more exhausts, which in turn spins the little fan more and more...and it starts to create positive preassure in the intake = boost.
The increased mass of air caused by the positive preassure from the turbo creates even more gases for the turbo, and so on...

The turbo itself needs a certain amount of gases before it starts to produce boost, and this depends on the size, and the design of the impellers.

Boost is controlled by opening and closing the wastegate (in our Mitsu/IHI VF turbos an internal bypass "valve"), that is controlled by a solenoid....that's controlled by the ECU.



(I'm certain a better description can be had by a more technically minded scoobnut.)

/J