Turbochargers are composed of an exhaust gas-driven turbine and a radial air compressor mounted at opposite ends of a common shaft and enclosed in cast housings. (see Figure 1) The shaft itself is enclosed and supported by a center housing, to which the compressor and turbine housings are attached. The typical turbocharger can rotate at speeds of 100,000 RPM and up.

The turbine section is composed of a cast turbine wheel, a wheel heat shroud and a turbine housing, with the inlet at the outer diameter of the turbine housing.

It is a centripetal, radial-or mixed-inflow device in that exhaust gas flows inward, past the wheel blades, and exits at the center of the housing�s diameter.

The compressor section is composed of a cast compressor wheel, a backplate, and a compressor housing, with the inlet at the center of the compressor housing diameter. It is a centrifugal, or radial-outflow device in that the air flows outward, past the wheel blades, and exits at the outer diameter of the housing.

A turbocharger without the compressor and turbine housing attached is called a Center Housing and Rotating Assembly (CHRA). The center housing (bearing housing) supports the compressor and turbine wheel shaft in a carefully designed bearing system. The bearing system, designed for high speed, does not see heavy loading as with crankshaft bearings, but rather must delicately position the wheels as closely as possible to the contour of the end housings. Key to this positioning is the oil filling the clearances between the center housing bore, bearings, and shaft. This oil filled clearance is vital to a turbocharger�s operation and longevity.

The Oil Flow Schematic in (Figure 2) describes how oil delivered from the engine�s lubrication system is directed by ports and grooves to the journal bearings. Oil flows through the holes in the bearings to lubricate and cool the bearings, bores and shaft journals. Oil is also directed from the inlet port to the thrust bearing or through a passage in the backplate to the thrust bearing. Oil drains from the center housing by gravity.

Seal systems separate the center housing from both the turbine and compressor stage. The seals restrict the oil from entering the compressor and turbine areas and reduce the flow of gases from those areas into the center housing. These systems may include piston ring seals, carbon and O-ring seals, oil slingers and labyrinth (threaded) seals to accomplish the task. The oil seals are activated when the shaft rotates and housing pressure develops as pictured in (Figure 3, Figure 4 and Figure 5).

Various retaining rings and bolts secure or hold in place the rotating components. Thrust components maintain axial integrity. A shaft nut or threaded wheel is used to join the compressor wheel and the turbine wheel. The turbine and compressor housing are attached to the center housing and rotating assembly by bolts, V-band clamps and/or clamp plates, lockplates, and bolts.

Wastegates swing valves, and poppet valves are types of exhaust control devices. They help control turbine speed, (see Figure 6 and Figure 7) which in turn helps to control boost. They are either integral to the turbine housing or are remotely� mounted. They are activated either by diaphragms or cylinder and pistons filled with air or oil. When opened, excess exhaust pressure is released from the turbine housing, directed to the exhaust system and expelled into the atmosphere.