Turbocharger bearings systems are an often overlooked but critical component of the turbocharger. A properly designed bearing system can spell the difference between a turbocharger design that operates efficiently and effectively over the life of the engine and one that is plagued by durability problems. Turbocharger bearing systems are also evolving in the face of increased pressure to reduce engine fuel consumption and emissions. Newer engines often demand higher turbocharger efficiencies that in in many cases, can be partially achieved by reducing losses due to the bearing system.
The turbocharger bearing system must be tolerant of
- High thrust loading. High boost pressure acting on the compressor wheel can create significant thrust loads. In variable geometry turbines, the thrust loading can be even higher due to the ability of a VGT to drive the compressor to higher boost pressures at low flows. A low flow in the VGT usually means a small nozzle setting and low static pressure acting on the turbine wheel that is unable to significantly offset the corresponding compressor thrust.
- Oil contaminants. Longer engine oil change intervals and higher soot loading due to engine emission controls such as EGR can lead to oil contamination that can corrode bearing surfaces.
- Oil supply delay. Low ambient temperature and long oil feed pipes can increase the time it takes for lubricating oil to reach the turbocharger upon engine start-up. Even at low idle the turbocharger speed can be relatively high shortly after start-up and lead to potential problems with bearing system wear.
- Hot shutdown. Continuous operation at high exhaust temperature followed immediately by an engine shutdown without idle can cause localized overheating and coking of the oil in the bearing housing and subsequent damage to bearing surfaces.
In an era of increased pressure to reduce emissions and fuel consumption, reducing bearing friction can play a critical role in improving cold start emissions and fuel economy. Also, as engine oil viscosities become lower, either through the use of low viscosity oils or dilution by fuel during post injections for DPF regeneration, turbocharger bearing systems must adapt to maintain rotor-dynamic stability and to avoid increased wear
Modern commercial turbocharger bearing systems can be split into two principal types: hydrodynamic journal bearing systems and ball bearing systems. Hybrid systems are also possible that combine journal and ball bearings.
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