Turbocharger bearing development
While journal bearings currently reign supreme for applications such as crankshafts, other areas of the powertrain use different types. There is a host of different applications, but those intended for use in turbochargers are some of the most interesting.
Providing a low-friction bearing capable of operating at the very high speeds a turbocharger sees – more than 150,000 rpm – and which can also survive the high temperatures experienced within a turbine housing is no easy task. For many years, the most common solution was a simple bronze-bushed affair, with an oil supply and in some cases a water cooling jacket. In high-performance applications, this approach has given way to more efficient ball bearing systems, which generate less friction and therefore help to increase turbine efficiency, allowing for faster spooling and thus reduced turbo lag.
Ball bearing designs also reduce the amount of oil required to provide adequate lubrication compared to journal bearings, and this lower oil volume reduces the risk of seal leakage. Ball bearings are also more tolerant of marginal lubrication conditions, reducing the possibility of failure.
While such systems are undoubtedly more efficient than a simple bush, there is still a lot of scope for improvement. The answer to achieving greater durability and lower friction has been the introduction of ceramic-based bearing elements, using silicon nitride instead of steel to provide a number of advantages over regular steel bearings and races. Ceramic bearings are generally available in one of two types – pure ceramic, where all the elements of the bearing are made from ceramic material; and hybrid, where ceramic balls or rollers run in steel rings.
The advantages of ceramic bearings are considerable. Ceramic bearing balls can weigh up to 40% less than steel ones, depending on their size and the materials used, and this can reduce centrifugal loading and skidding, allowing hybrid ceramic bearings to operate 20-40% faster than conventional steel bearings. As a result, ceramic and hybrid ceramic bearings have less inertia, and thus in turbo applications can increase response rate over steel bearings. Ceramics also need far less lubrication than steel, further reducing the oil supply requirements over both ball bearing and journal bearing systems.
The thermal characteristics of ceramic balls are also very attractive, as they experience minimal expansion even at very high temperatures. To give an example that quantifies the performance possible with such bearings, one turbo manufacturer worked with its bearings supplier on producing a hybrid ceramic ball bearing turbo unit that saw peak speeds of more than 220,000 rpm, with a shaft temperature in excess of 300 C – conditions far beyond those in which a traditional steel bearing could survive.
From this it is clear that ceramic technology provides the best current solution for high-efficiency turbocharger bearings. There are other potential development avenues when it comes to further reducing friction in turbo applications, but currently they exist only in theory or in very large machinery applications. For example, the use of foil-air or magnetic bearings offers the prospect of almost completely eliminating friction, but a practical application of these technologies in an automotive turbocharger has yet to be realised.
Written by Lawrence Butcher