Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 32

While journal bearings are the most widely used in today’s turbos, ball bearings are coming onto the scene. Ball bearings that will withstand the hostile environment of a turbocharger are extremely expensive, but there are definite advantages. Ball bearings can entirely eliminate the thrust bearing, which accounts for approximately 40 percent of the bearing system drag on the turbo’s rotor assembly. Also, ball bearings reduce the viscous drag of conventional journal bearings and allow the turbo to spool-up approximately 15 percent faster than journal bearings. In any competitive application where it’s turbo car versus turbo car, ball bearings must be considered or you’ll be giving up too much to the competitor who is using them.

The graph on the next page shows a comparison between identical turbos on the same engine when accelerated from a 2,000-rpm start point on a 2.0-liter spark-ignition engine. If we analyze the graph closely we’ll find that the greatest separation between the ball-bearing and journal bearing performance comparisons is at about 0.4 second. That’s characteristic of the transient response challenge of turbos to come on boost quickly with rapid changes in throttle position. The journal bearing turbo jumps to about 42,000 rpm (700 Hz) while in the same 0.4 seconds, the ball-bearing jumps to 72,000 rpm (1,200 Hz). As the time passes, the differences in rotor speed tend to diminish as the journal bearing catches up. For this reason, all of the benefits from a ball-bearing system cannot be seen adequately on the engine dynamometer which takes data points at a steady state where speeds and load are stabilized. However, there are still benefits, they’re just more difficult to measure depending upon test accuracy. Championship race teams, for which Garrett originally designed the ball-bearing system, claim that the reduction in power lost to the bearing system is up to 50 percent.

The Garrett ball-bearing system is contained in a cartridge assembly. The angular contact ball bearings totally eliminate the need for a separate thrust bearing, which reduces drag in the system. (Courtesy Honeywell Turbo Technologies)

The load capacity of a traditional thrust bearing is about 20Hg (inches of mercury). In high-boost, high-demand conditions this pressure differential can be exceeded and damage the bearing, causing turbo failure. Another plus for the ball bearing is its load carrying capacity, which is approximately 10 times that of the traditional journal and thrust bearing design. Certain conditions cause surges, and the ball bearing will better withstand the poundings than a traditional thrust bearing.

Turbine acceleration rate between journal and ball bearings is shown in Hz, or cycles per second, multiply values on the X-axis times 60 to convert to RPM. (Courtesy Honeywell Turbo Technologies)

Ball bearing equipped turbos do not require as much oil volume as traditional bearings and will commonly come with an oil restrictor in the bearing housing oil inlet or contain a reference to a specific size orifice as a restrictor. Too much oil present will tend to negate some of the ball bearing advantages.

There is also what’s known as a hybrid bearing system, which uses one ball bearing and one journal bearing. Since there is a journal bearing, there is also a traditional thrust bearing. Consequently, the hybrid design carries most of the disadvantages of the journal bearing design.

However, it should be noted that unless you are competing in a race class where ball bearings dominate, then traditional journal bearings operate just fine. Some enthusiasts just want the best, and crisp response is desired along with the horsepower that turbos help develop. Turbos have been blamed for being slow to react and the term “turbo” lag is well known. Turbo lag is usually a combination of factors that are caused by several things such as a poor match, bad manifolding, and spool-up time. Ball bearings will positively contribute to the elimination of turbo lag in a well-designed system.