BOOST SYSTEM [SKYACTIV-G 2.5T]

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Purpose, Outline

• he boost system rotates the turbine wheel of the dynamic pressure turbo using the pressure of the exhaust gas to rotate the compressor wheel on the intake air side on the same shaft, and compress the intake air which is then supplied to the engine.

• Continually obtains optimum boost pressure by changing the air flow according to the engine operation conditions.

As a result, vehicle acceleration performance, dynamic pressure turbo responsiveness (turbocharger lag) at low engine speeds, torque build-up at low speeds, and fuel economy have been improved.

Function

• The boost system operates based on the functions of mainly the following 5 parts. The boost system mainly controls the operation of the wastegate valve and performs optimum boost based on the functions of each part according to the driving conditions.

Dynamic pressure turbo

•  The dynamic pressure turbo rotates the turbine wheel of the turbocharger using the pressure of the exhaust gas to rotate the compressor wheel on the intake air side on the same shaft, and boosts the intake air which is then supplied to the engine.

Wastegate valve

Normal operation

•  The wastegate valve opening angle is adjusted according to the driving conditions to adjust the amount of exhaust gas entering the turbine and the targeted boost pressure value is achieved. Under the driving condition in which the engine load is low, the wastegate valve is opened to reduce pumping loss and improve fuel economy. In addition, when powerful acceleration is necessary, the wastegate valve is fully closed quickly to increase the boost pressure and exhibit excellent response performance.

Engine protection during overboost

•  If the boost pressure increases excessively, the opening angle of the wastegate valve is increased and the amount of exhaust gas entering the turbine is reduced to prevent engine damage.

Air bypass valve

•  The air pressure (positive pressure) from the dynamic pressure turbo to the throttle valve increases if the throttle valve is fully closed during dynamic pressure turbo operation. As a result, air hose breakage and damage to parts may occur, and surging *¹ or turbocharger lag occurs.
To reduce the possibility of the above from occurring, if the air pressure (vacuum) from the throttle valve to the intake manifold exceeds the specification, the air bypass valve opens and the boosted intake air pressure flows back to the upstream of the compressor wheel of the dynamic pressure turbo.

Exhaust manifold variable valve

•  The exhaust manifold variable valve closes when the engine speed is low. By closing the exhaust manifold variable valve, the flow speed of the exhaust gas flowing to the turbine is increased and the turbine wheel is turned efficiently.
When the engine speed increases, the exhaust manifold variable valve is opened so that the exhaust gas is sent to the dynamic pressure turbo without resistance.

Charge air cooler

•  To efficiently combust the intake air and fuel, the charge air cooler cools the intake air which is compressed and heated by the dynamic pressure turbo to increase the intake air density.

*1 :Surging is a phenomenon in which vibration and noise occur and pressure and flow rate fluctuate due to flow-back of the boosted pressure caused by having no exit.

Structure

System diagram

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• The boost system consists of the following parts:

Part name

Reference

Dynamic pressure turbo (See DYNAMIC PRESSURE TURBO [SKYACTIV-G 2.5T].)

Wastegate valve

(See WASTEGATE VALVE [SKYACTIV-G 2.5T].)

Wastegate valve actuator

Wastegate valve position sensor

(See WASTEGATE VALVE POSITION SENSOR [SKYACTIV-G 2.5T].)

Air bypass valve

(See AIR BYPASS VALVE [SKYACTIV-G 2.5T].)

Exhaust manifold variable valve

(See EXHAUST MANIFOLD [SKYACTIV-G 2.5T].)

Exhaust manifold variable valve actuator

Three way solenoid valve

(See THREE WAY SOLENOID VALVE [SKYACTIV-G 2.5T].)

Cylinder head (built-in vacuum chamber)

(See CYLINDER HEAD COVER [SKYACTIV-G 2.5T].)

Check valve (intake-air system)

(See CHECK VALVE (INTAKE-AIR SYSTEM) [SKYACTIV-G 2.5T].)

Charge air cooler

(See CHARGE AIR COOLER [SKYACTIV-G 2.5T].)

Part name	Reference
Dynamic pressure turbo (See DYNAMIC PRESSURE TURBO [SKYACTIV-G 2.5T].)	Wastegate valve	(See WASTEGATE VALVE [SKYACTIV-G 2.5T].)
Wastegate valve actuator
Wastegate valve position sensor	(See WASTEGATE VALVE POSITION SENSOR [SKYACTIV-G 2.5T].)
Air bypass valve	(See AIR BYPASS VALVE [SKYACTIV-G 2.5T].)
Exhaust manifold variable valve	(See EXHAUST MANIFOLD [SKYACTIV-G 2.5T].)
Exhaust manifold variable valve actuator
Three way solenoid valve	(See THREE WAY SOLENOID VALVE [SKYACTIV-G 2.5T].)
Cylinder head (built-in vacuum chamber)	(See CYLINDER HEAD COVER [SKYACTIV-G 2.5T].)
Check valve (intake-air system)	(See CHECK VALVE (INTAKE-AIR SYSTEM) [SKYACTIV-G 2.5T].)
Charge air cooler	(See CHARGE AIR COOLER [SKYACTIV-G 2.5T].)

Operation

• The boost system operates the wastegate valve, air bypass valve, and exhaust manifold variable valve according to the engine operation conditions.

• Four patterns of engine operation conditions are described below.

― Ignition is switched off

― Boost pressure is insufficient relative to control target value during low engine loads (when increasing rotation speed of dynamic pressure turbo)

― Boost pressure is high relative to control target value (when flowing amount of exhaust gas is large)

― Boost pressure is high relative to control target value (when amount of intake air is large)

Ignition is switched off

• The open/closed condition of each valve when the engine is stopped is as follows.

― Wastegate valve: Fully open

― Air bypass valve: Fully closed

― Exhaust manifold variable valve: Fully open

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Boost pressure is insufficient relative to control target value during low engine loads (when increasing rotation speed of dynamic pressure turbo)

• When the boost pressure is insufficient relative to the control target value, the wastegate valve is closed to enhance boosting, and the amount of exhaust gas flowing into the turbine of the dynamic pressure turbo is increased.

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Boost pressure is high relative to control target value (when flowing amount of exhaust gas is large)

• The exhaust gas flow amount applied to the turbine wheel of the dynamic pressure turbo is reduced by opening the wastegate valve and bypassing the exhaust gas pathway.

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Boost pressure is high relative to control target value (when amount of intake air is large)

• When the boost pressure is high relative to the control target value (when amount of intake air is large) or the throttle valve is suddenly closed based on the driver's request, the air bypass valve is fully opened to make the intake air after passing through the compressor wheel of the dynamic pressure turbo to flow back to the upstream of the compressor wheel.

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• For boost control details, refer to the CONTROL SYSTEM. (See BOOST SYSTEM [SKYACTIV-G 2.5T].)

Dynamic pressure turbo	• The dynamic pressure turbo rotates the turbine wheel of the turbocharger using the pressure of the exhaust gas to rotate the compressor wheel on the intake air side on the same shaft, and boosts the intake air which is then supplied to the engine.
Wastegate valve	Normal operation • The wastegate valve opening angle is adjusted according to the driving conditions to adjust the amount of exhaust gas entering the turbine and the targeted boost pressure value is achieved. Under the driving condition in which the engine load is low, the wastegate valve is opened to reduce pumping loss and improve fuel economy. In addition, when powerful acceleration is necessary, the wastegate valve is fully closed quickly to increase the boost pressure and exhibit excellent response performance. Engine protection during overboost • If the boost pressure increases excessively, the opening angle of the wastegate valve is increased and the amount of exhaust gas entering the turbine is reduced to prevent engine damage.
Air bypass valve	• The air pressure (positive pressure) from the dynamic pressure turbo to the throttle valve increases if the throttle valve is fully closed during dynamic pressure turbo operation. As a result, air hose breakage and damage to parts may occur, and surging *¹ or turbocharger lag occurs. To reduce the possibility of the above from occurring, if the air pressure (vacuum) from the throttle valve to the intake manifold exceeds the specification, the air bypass valve opens and the boosted intake air pressure flows back to the upstream of the compressor wheel of the dynamic pressure turbo.
Exhaust manifold variable valve	• The exhaust manifold variable valve closes when the engine speed is low. By closing the exhaust manifold variable valve, the flow speed of the exhaust gas flowing to the turbine is increased and the turbine wheel is turned efficiently. When the engine speed increases, the exhaust manifold variable valve is opened so that the exhaust gas is sent to the dynamic pressure turbo without resistance.
Charge air cooler	• To efficiently combust the intake air and fuel, the charge air cooler cools the intake air which is compressed and heated by the dynamic pressure turbo to increase the intake air density.