A turbocharger consists of a compressor wheel and an exhaust turbine wheel connected together by a solid shaft to increase the intake pressure of an internal combustion engine. Exhaust gas Turbines extract energy from exhaust gas and use it to drive compressors and overcome friction. In most automotive applications, compressors and turbines are radial flow types. Some applications, such as low - and medium-speed diesel engines, can use axial flow turbines instead of radial flow turbines. The flow of gas through a typical turbocharger with a radial flow compressor and turbine wheels
Center Housing: The compressor co-shaft is supported by a bearing system in a central Housing (bearing Housing) located between the compressor and turbine. SWA refers to a shaft with a compressor and turbine attached, i.e. rotating assembly. A center housing rotating assembly (CHRA) is a SWA mounted in the center housing but without the compressor and turbine housing. The center housing is usually cast from gray cast iron, but aluminum can also be used in some applications. Seals help prevent oil from passing through compressors and turbines. Turbochargers for high exhaust gas temperature applications, such as spark-ignition engines, can also incorporate cooling channels in the central housing.
Bearings. The turbocharger bearing system appears simple in design but it plays a key role in a number of critical functions. Some of the more important ones include: the control of radial and axial motion of the shaft and wheels and the minimization of friction losses in the bearing system. Bearing systems have received considerable attention because of their influence on turbocharger friction and its impact on engine fuel efficiency.
With the exception of some large turbochargers for low-speed engines, the bearings that support the shaft are usually located between the wheels in an overhung position. This flexible rotor design ensures that the turbocharger will operate above its first, and possibly second, critical speeds and can therefore be subject to rotor dynamic conditions such as whirl and synchronous vibration.
Seals. Seals are located at both ends of the bearing housing. These seals represent a difficult design problem due to the need to keep frictional losses low, the relatively large movements of the shaft due to bearing clearance and adverse pressure gradients under some conditions.
These seals primarily serve to keep intake air and exhaust gas out of the center housing. The pressures in the intake and exhaust systems are normally higher than in the turbocharger’s center housing which is typically at the pressure of the engine crankcase. As such, they would primarily be designed to seal the center housing when the pressure in the center housing is lower than in the intake and exhaust systems. These seals are not intended to be the primary means of preventing oil from escaping from the center housing into the exhaust and air systems. Oil is usually prevented from contacting these seals by other means such as oil deflectors and rotating flingers.
Turbocharger seals are different from the soft lip seals normally found in rotating equipment operating at much lower speeds and temperatures. The piston ring type seal is one type that is often used. It consists of a metal ring, similar in appearance to a piston ring. The seal remains stationary when the shaft rotates. Labyrinth-type seals are another type sometimes used. Generally turbocharger shaft seals will not prevent oil leakage if the pressure differential reverses such that the pressure in the center housing is higher than in the intake or exhaust systems.
