Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 34

Except for the carbon face positive oil seals used in the turbos applied to carbureted engines before the widespread use of fuel injection, turbos don’t really have oil seals. Those early applications used the positive oil seals because the turbo’s compressor was subjected to vacuum frequently due to being applied to a throttled engine in what was known as a pull-through turbo system. The pull-through system referred to pulling the air through the carburetor as opposed to blowing it through. These two systems were debated frequently as to which was best because they each had their merits. But fortunately both are now passé, along with the carburetor.

A close-up of a cutaway section of the turbine end of a bearing housing shows some of the design features of a typical bearing housing and how it manages to keep the high heat from the engine’s exhaust from migrating into the oil and harming the bearing system. The backplate (1) divides the turbine area of the bearing housing surface from the turbine gases by creating a dead air space (2). The seal ring bore area is made in a casting portion that is suspended and creates a long heat path for the heat to travel (3). This gives the oil flung off of the turbine-end flinger an opportunity to cool it as long as oil is being supplied. The bearing bore is further isolated in its own portion of the casting (4) and not directly connected to the high heat area. (Courtesy Diesel Injection Service Company, Inc.)

There is what’s called a piston ring on both the compressor and turbine ends. The name comes from the fact that they look just like very small piston rings. Like piston rings, their primary purpose isn’t oil control, it is to seal the boost pressure and exhaust gas pressure from entering the oil drain cavity and consequently pressurizing the engine’s crankcase, an undesirable condition indeed.

The piston rings in a turbocharger are primarily dynamic gas seals that limit pressure, from either the compressor or turbine end, from entering the engine’s crankcase by way of the turbo’s oil drain cavity. The smaller one is typically used on the compressor end, while the larger ring is used on the turbine side. (Courtesy Diesel Injection Service Company, Inc.)

Oil control is principally achieved by way of controlling the oil and deflection, which keeps the oil away from the areas of possible leakage. In a boost condition, oil cannot enter into the engine’s compressor or turbine due to the pressures in the respective housings. The oil that has drained away from the bearings is no longer under pressure and is subject to gravity to drain back into the oil pan through the oil drain cavity in the bearing housing. When a turbo is said to be leaking oil, it’s usually due to it idling too long, where there is no housing pressure to seal the oil away from the housings. That condition is called turbo slobber. Other conditions can exist that cause oil leakage and are covered in Chapter 10.

This chapter has discussed several different design variations of specific turbo parts such as turbine wheel material, different turbine housing availability, upgraded thrust bearings for high-load applications, etc. Trying to determine whether any of these options are necessary or even available for your model turbo can be difficult. One good way of obtaining this knowledge is from your closest turbo expert. There are many turbo experts who have years of experience and can help answer some of your questions. See the source guide at the back of this book for a list of contacts.

Once the oil leaves the thrust bearing area it is flung off at high speed. An oil deflector (1) shrouds the thrust bearing (2) and directs the oil to drain into the bearing housing drain cavity, thus keeping the area of potential leakage from being flooded with oil. (Courtesy Diesel Injection Service Company, Inc.)

The five major parts of a turbocharger include, the compressor cover (1), the bearing housing (2), the turbine housing (3), the compressor wheel (4), and the turbine wheel and shaft assembly (5). The rest of the parts, the bearings, seals, and oil control components are critical to the overall operation of the entire assembly. An understanding of these parts and their operation is helpful in the design and installation of a good turbo system, and is critical in turbo troubleshooting, maintenance, and rebuilding. (Courtesy Diesel Injection Service Company, Inc.)