Design and Function of a Turbocharger: Bearing System
basic functions have not fundamentally changed since the times of Alfred Büchi.
A turbocharger consists of a compressor and a turbine connected by a common shaft.
The exhaust-gas-driven turbine supplies the drive energy for the compressor.
The turbocharger shaft and turbine wheel assembly rotates at speeds up to 300,000
rpm. Turbocharger life should correspond to that of the engine, which could be 1,000,000
km for a commercial vehicle. Only sleeve bearings specially designed for turbochargers
can meet these high requirements at a reasonable cost.
Turbocharger bearing system (cut-away model)
Radial bearing system
With a sleeve bearing, the shaft turns without friction on an oil film in the sleeve
bearing bushing. For the turbocharger, the oil supply comes from the engine oil
circuit. The bearing system is designed such that brass floating bushings, rotating
at about half shaft speed, are situated between the stationary centre housing and
the rotating shaft. This allows these high speed bearings to be adapted such that
there is no metal contact between shaft and bearings at any of the operating points.
Besides the lubricating function, the oil film in the bearing clearances also has
a damping function, which contributes to the stability of the shaft and turbine
wheel assembly. The hydrodynamic load-carrying capacity and the bearing damping
characteristics are optimised by the clearances. The lubricating oil thickness for
the inner clearances is therefore selected with respect to the bearing strength,
whereas the outer clearances are designed with regard to the bearing damping. The
bearing clearances are only a few hundredths of a millimetre.
The one-piece bearing system is a special form of a sleeve bearing system. The shaft
turns within a stationary bushing, which is oil scavenged from the outside. The
outer bearing clearance can be designed specifically for the bearing damping, as
no rotation takes place.
Axial-thrust bearing system
Neither the fully floating bushing bearings nor the single-piece fixed floating
bushing bearing system support forces in axial direction. As the gas forces acting
on the compressor and turbine wheels in axial direction are of differing strengths,
the shaft and turbine wheel assembly is displaced in an axial direction. The axial
bearing, a sliding surface bearing with tapered lands, absorbs these forces. Two
small discs fixed on the shaft serve as contact surfaces. The axial bearing is fixed
in the centre housing. An oil-deflecting plate prevents the oil from entering the
shaft sealing area.
The lubricating oil flows into the turbocharger at a pressure of approximately 4
bar. As the oil drains off at low pressure, the oil drain pipe diameter must be
much larger than the oil inlet pipe. The oil flow through the bearing should, whenever
possible, be vertical from top to bottom. The oil drain pipe should be returned
into the crankcase above the engine oil level. Any obstruction in the oil drain
pipe will result in back pressure in the bearing system. The oil then passes through
the sealing rings into the compressor and the turbine.
The centre housing must be sealed against the hot turbine exhaust gas and against
oil loss from the centre housing. A piston ring is installed in a groove on the
rotor shaft on both the turbine and compressor side. These rings do not rotate,
but are firmly clamped in the centre housing. This contactless type of sealing,
a form of labyrinth seal, makes oil leakage more difficult due to multiple flow
reversals, and ensures that only small quantities of exhaust gas escape into the
Turbocharger for passenger car gasoline applications with water-cooled bearing housing
Petrol engines, where the exhaust gas temperatures are 200 to 300 °C higher
than in diesel engines, are generally equipped with water-cooled centre housings.
During operation of the engine, the centre housing is integrated into the cooling
circuit of the engine. After the engine's shutdown, the residual heat is carried
away by means of a small cooling circuit, which is driven by a thermostatically
controlled electric water pump.