Figure 9-6 illustrates an air-cooled Ishikawajima-Harima turbocharger of the type found on engines in the 100–150-hp range. The inset shows the water-cooled version of the same turbocharger, plumbed into the engine cooling system. All modern small-engine turbochargers follow these general patterns.
Floating bushings, located in the central bearing chamber, support the shaft. These bushings, like the connecting rod bearings specified for the original Ford V-8, float on the ID and OD. Thus, bushing speed is half that of shaft speed, which is to say that the bushings can reach speeds of 60,000 or 70,000 rpm. A floating thrust bushing contains axial motion.
Also note the way the impeller wheels cantilever from the bushings, so that the masses of the rotating assembly are concentrated near the ends of the shaft. This “dumb-bell” configuration requires precise wheel balance and extremely accurate shaft alignment.
The bearing section is lubricated and cooled by engine oil, generally routed through external pipes or hoses. Shaft seals keep oil from entering the turbine and compressor sections.
The turbocharger is usually the last component to receive oil pressure and might continue to rotate after the engine stops. To ensure an adequate oil supply to the bearings, operators should idle the engine for at least 30 seconds upon starting and for the same period before shutdown.