The Compressor Wheel

The compressor wheel is perhaps the most commonly discussed component inside of a turbo. This may be because it’s the most easily understood, and because it’s the focus of the turbo being an air pump to begin with. The turbocharger’s compressor wheel is called a radial compressor because it takes in fresh air and accelerates it radially, or turns it 90 degrees, unlike axial compressors used in jet engines that accelerate air in the same direction it’s already going.

The compressor wheel has a number of critical areas, many of which are changeable within turbocharger model families, resulting in the various trims available to adjust flow parameters and correctly match the compressor to the engine. These changes must not be done by the turbo owner and must be done by the manufacturer at the time the turbo is manufactured. Changing these shapes by the turbo owner will destroy the design relationship between the compressor wheel and the compressor cover contour.

Most compressor wheels have been made from various aluminum alloys. However, there are more and more applications that push the limits of what the current aluminum alloys will stand. The higher the rotational speed of the aluminum, the shorter the life cycle. This is typically not a problem in most performance automotive applications due to the relatively low hours of operation. However, in some extreme high-boost applications, such as tractor pulling, this can pose a problem with wheel burst.

The compressor wheel has several important areas of design consideration: (1) inducer diameter, (2) tip height or tip width, (3) wheel contour, (4) splitter blade, (5) full blade, (6) backwall, (7) wheel diameter, tip diameter, or exducer diameter, (8) tip, impellor, or exducer, and (9) nose. (Courtesy Diesel Injection Service Company, Inc.)

While most compressor wheels are a casting, this compressor wheel has been machined from billet. Careful examination of the wheel floor reveals the machining marks left after the formation of the wheel’s hub as the blades were made. This process is done by a five-axis milling machine and is used in special low-volume, high-durability, or high-performance applications where no production wheel will fit the intended need. These types of wheels are tremendously expensive but are justifiable in many instances. (Courtesy Diesel Injection Service Company, Inc.)

This compressor wheel has been cut in half. The shaft bore (1) typically runs all the way through the wheel, the compressor wheel hub (2) supports the blades and its shape forms the wheel floor (3) that turns the inlet airflow 90 degrees to make it a radial-type compressor. Some wheels will have an extended backwall (4), which strengthens the wheel at its highest point of stress for improved durability. The root of the blades (5) will have a small fillet to support the stresses of compression.

Compressor wheels are balanced on two planes: the nose and backwall. Note the balance stock removal off the nose and off the backwall in a process called scallop balancing. (Courtesy Diesel Injection Service Company, Inc.)

Special note: Any compressor or turbine wheel burst is an extremely dangerous situation and can be lethal. It is most advisable that a burst shield be employed in these extreme applications.

Some commercial diesel and performance racing applications are beginning to use titanium wheels machined on five-axis mills. While these wheels are extremely expensive, they do tend to solve some of the premature failures seen on highboost applications. In most cases however the cast aluminum alloys are more than adequate for most street and strip applications.

The compressor wheel is balanced on two planes: the nose and the back face. Because the wheels in the turbocharger rotate at such high speeds balance is critical for correct operation and life. The compressor wheel is set up on specially designed balance machines and the two planes of balance are defined. A balance spec is set by the manufacturer based upon turbo size and intended operational speed. Typically the balance spec is held to within hundredths of an ounce-inch. On extremely high-speed applications the rotor group can be dynamically balanced by stacking up all of the rotating components and indexing them, then balancing them all as a group. This is explained further later in this chapter.