The 'wastegate valve' deserves its own section in the in-depth discussion of other system components.

The wastegate is a overpressure valveused for monitoring the operating conditions of the turbine.

Management consists of ensuring that the exhaust gases bypass the turbine should a 'positive feedback' situation arise: loop effect caused by exceeding the turbine speed limit and consequent increase in pressure (overcoming turbine speed = turbocharger speed increase = turbine speed overshoot) detrimental to both the turbo-compressor and (above all) the engine.

In practical terms: the faster the turbine spins, the less effort it makes to pump air; the flow rate of air processed increases with the square of the rotation speed, so that while a turbine at 80,000 rpm pumps about 0.4 cubic metres of air per second, at 160,000 it does not pump twice as much, but four times as much, and at the same time the overpressure increases exponentially from 0.2 bar to 1.6 bar, an increase of eight times.
This explains the two main features of the system:

1. Initial response slowness (so-called turbo-lag)

2. Subsequent exceptional thrust with uncontrolled power escalation.

In phase (1), at low rotational speeds, the thrust of the exhaust gases is not sufficient to turn the turbine blades quickly and the engine supply pressure does not exceed the atmospheric; in phase (2), on the other hand, the flow and temperature of the gases produced by combustion increase and as soon as these are sufficient to provide overpressure, a chain reaction is triggered that leads to a real escalation of power, which can only be stopped by opening the wastegate valve, on pain of damaging the engine.

With this strategy, part of the exhaust gases are channelled outwards, causing the turbine to decelerate together with the compressor, thereby reducing the boost pressure.

Wastegate valve management

The wastegate can be controlled in two different ways: pneumatic drive (see figure opposite). The valve is controlled by a tube connected to the compressor side. When the attention limit is exceeded, the internal pressure of the compressor overcomes the resistance of a spring inside the valve, causing it to open.

Electronic driveIn the new generation of cars, the wastegate valve is electronically controlled by the Engine Control Module (CCM), which optimises its opening thanks to the parameters provided by the sensors.

When the specific turbo pressure sensor detects a pressure above the threshold value, the CCM activates the solenoid valve which releases the high pressure to the wastegate actuator, allowing it to open. Once the regulation phase is over, power is cut to the solenoid valve and the high pressure is discharged upstream of the turbocharger.

We'll stop here: see you in 'part three' with more on the other main components of the system. Bye 

• wastagateMito2