PORSCHE Turbocharging System: Operation
The turbocharger is driven by exhaust gases being expelled from the cylinder combustion chambers. At idle speeds there is no pressurization of incoming air, and the engine operates like a normally aspirated engine.
At partial load, the throttle valve is open and more fuel-air mixture is drawn into the combustion chamber. Speed and volume of exhaust gases increase, in turn. The increasing volume of exhaust gases causes the turbocharger turbine to turn with greater speed. See Fig 1. The compressor, mounted on the same shaft turns at the same speed as the turbine, producing boost pressure for incoming air. Opening the throttle valve reduces manifold vacuum, closing the pop-off valve located in the compressor housing between the inlet and outlet.
At full load, a large volume of exhaust gases are fed to the turbine, increasing speed of both the turbine and compressor wheels. Boost pressure will then reach the maximum. When it increases to 7.5 psi (.52 kg/cm2 ), the wastegate (boost pressure control valve) opens. Part of the exhaust gases are now routed directly into the main exhaust pipe, bypassing the turbine wheel. The turbocharger speed and boost pressure remain almost constant at 7.5 psi (.52 kg/cm2 ).
Should the wastegate fail to function properly, an electric boost pressure safety switch located in the compressor discharge pressure duct turns off the electric fuel pumps. Fuel shut-off occurs as boost pressure reaches 16-20 psi (1.1-1.4 kg/cm2 ). This prevents engine damage from excessive pressures.
During deceleration, the throttle valve is closed. The turbocharger now operates against a closed throttle valve, which could damage the turbocharger. To prevent this from occurring, the pop-off valve located between the pressure and intake duct opens due to intake manifold vacuum. This sets up a bypass circuit between pressure and intake ducts. Since the wastegate valve is shut, all exhaust gases are routed to the turbocharger turbine upon acceleration.