Turbocharger and Charge Air Cooler (CAC) Systems
TURBOCHARGER AND CHARGE AIR COOLER (CAC) SYSTEMSTurbocharger System
A turbocharger is an exhaust gas driven device used to increase the power output of an engine by increasing the mass of air entering the engine. The increased mass of air is achieved by the compressor increasing the pressure of the air entering the engine. Compressing the air causes the temperature to increase. The temperature increase is counteracted using a charge air cooler to increase the density of the air prior to induction into the cylinder. The turbocharger uses exhaust gas energy to drive the radial compressor.
The 6 cylinder engine uses twin turbochargers in a parallel arrangement with one turbine connected to the exhaust of each cylinder bank. This configuration improves engine responsiveness due to the reduced inertia of 2 small rotating assemblies in the place of a single large one while pumping adequate air to achieve the rated power. Emission compliance is achieved by mounting the catalysts very close to the turbo outlet.
The engine control system manages bypass valves for both the compressor and turbine stages. Two bypass valves are used to bypass the compressors on heavy throttle releases to prevent an unwanted air rush noise from the turbocharger. The bypass valves provide a connection between the high pressure and low pressure sides of the compressor. The wastegate on the turbine side is opened to reduce exhaust gas flow through the turbine when boost is not needed. A pneumatic actuator changes the wastegate valve opening. The wastegate is controlled by the powertrain control module (PCM) through a pulse width modulated (PWM) turbocharger wastegate regulating valve solenoid. The turbocharger wastegate regulating valve solenoid then regulates the supplying pressure to the wastegate canister to open the poppet style wastegate valve. The wastegate may be pressure or vacuum actuated, and the poppet style wastegate valve could be normally opened or normally closed at idle or low engine airflow conditions depending on vehicle configuration and requirements such as fuel economy and boost performance. The wastegates are coupled by having one control output from the turbocharger wastegate regulating valve solenoid that drives both turbocharger wastegate actuators.
The 4 cylinder system is similar, but uses only one turbocharger, wastegate and bypass valve instead of two. The compressor bypass valve can be electric or pneumatic and it may be mounted in the air induction system or in the compressor cover.
Charge Air Cooler (CAC) System
The CAC system cools the intake air which has been heated by the turbocharger. The removal of heat from the pressurized air going into the CAC increases the air density which improves combustion efficiency, engine horsepower, and torque. The system consists of a CAC radiator in the grille and tubing to interconnect these components. The CAC is positioned after the turbocharger directly in the flow of the intake air. As the heated air flows through the CAC, it is cooled by the airflow through the grille. The PCM maintains a desirable intake air temperature by monitoring the TCBP/CACT (located at the throttle body) and the MAP/IAT2 (located at the intake manifold) sensors.
