 |
|
Turbocharger bearing system (cut-away
model) |
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.
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 center 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
optimized 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 millimeter.
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 center housing. An oil-deflecting
plate prevents the oil from entering the shaft sealing area.
Oil drain
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.
Sealing
The center housing must be sealed against the hot turbine exhaust gas and
against oil loss from the center 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 center 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 crankcase.
Water-cooling
 |
|
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 center housings. During operation of the engine, the center 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.
