The turbocharger attains a high output by pumping compressed air into the cylinders. However engine parts will be unable to withstand the explosive force if the boost pressure rises too high. In this case, the waste gate valve is activated by the actuator, and controls the boost pressure so that the boost pressure does not rise above the specified value.
(1) Control when the boost pressure is below the specified value When the boost pressure is below the specified pressure, the actuator does not operate. All the exhaust gas is therefore routed into the turbine wheel because the waste gate valve remains closed.
(2) Control when the boost pressure is above the specified value When the engine speed rises and the boost pressure supplied by the turbocharger exceeds the specified value (intercept point), the actuator diaphragm is depressed. This causes the waste gate valve to open and part of the exhaust gas bypasses the turbine wheel. By allowing part of the exhaust gas to bypass the turbine wheel, the rotational speed of the turbine wheel is regulated so that the boost pressure becomes within the specified value.
With some gasoline engine, the boost pressure is also controlled in accordance with the octane rating of the fuel used (premium or regular gasoline).
Relationship between Boost Pressure and EngineSpeed
The relationship between the boost pressure and engine speed when the accelerator pedal is fully depressed, is shown on the left. The relationship between the boost pressure and engine speed varies according to the load placed on the engine.