Boost Regulation

PURPOSE
To control turbocharger boost pressure.

LOCATION
^ Wastegate valve:
In turbocharger exhaust turbine housing.
^ Wastegate controller:
On outside of turbocharger.

OPERATION
Basic function of wastegate valve
As the engine speed increases with the opening of the throttle valve, the amount of exhaust gas increases. This leads to increase in the rotational speed of the turbine (approx. 20,000 to 120,000 rpm), the boost pressure and engine output. However, excessive boost pressure may cause engine ping and excessive combustion chamber temperatures leading to piston damage. To prevent this, the system is equipped with a wastegate and controller. By sensing boost pressure, the wastegate bypasses some of the exhaust gas, therby controlling turbine rpm.

Fig. 1 Turbocharger Full Boost:

When boost pressure remains below a predetermined level, the wastegate valve remains closed so that all exhaust gas passes through the turbine, creating maximum rpm. See Fig. 1

Fig. 2 Turbocharger Full Boost (Close-up):

Since the wastegate controller diaphragm is supplied boost pressure through a rubber hose, it actuates the rod to the wastegate valve when pressure reaches the predetermined level. As the wastegate valve opens some of the exhaust gas bypasses the turbine, slowing the turbine. See Fig. 2

Fig. 3 Turbocharger Partial Boost:

This decreases the turbine rotating energy to keep the boost pressure constant.
It means P2 - P1 = constant.
P1 = Atmospheric pressure
P2 = Boost pressure
See Fig. 3