Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 26

There are many types of compressor wheel designs that have been used over the years. Each one has its own advantage and type of use. Care should be taken to understand the basics of each design to make sure that the turbo you intend to use for your application is of the type and variety that best suits your application.

Straight radial designs are not used much today. They develop high pressure but aren’t very efficient. When used in conjunction with a vaned-type diffuser the efficiency can be very high, but the tradeoff is a narrow flow range, which disqualifies it for use on an automotive application where the engine operates over a wide range of RPM. Historically they were used on diesel engines and generator applications that operate over a narrow RPM band.

A full-blade wheel is rarely seen and is typically on slower speed turbo applications. The full bladed wheel is not recommended for high-performance applications with high boost from high turbo speed. While the full bladed wheel is believed to produce a slightly higher pressure and has slightly higher efficiency, it tends to have trouble biting off enough air at higher speeds.

The straight radial wheel is easily identified by the blades that emanate in a straight line perpendicular to the axis of rotation. They develop high pressures but are not as efficient or have as broad of a flow range as today’s wheel designs. (Courtesy Diesel Injection Service Company, Inc.)

The splitter blade is the alternating shorter blade between each full blade. At higher speeds, the larger gap between the full blades is more capable of biting off more air. Once taken inside the compressor wheel, the splitter blade helps to efficiently manage and compress the air as it is accelerated and turned radially to the axis of rotation. The splitter blade is most commonly seen in automotive applications today.

The full-bladed compressor wheel means that all compressor wheel blades are full length rising from the max wheel diameter all the way to the inducer or inlet diameter of the wheel. (Courtesy Diesel Injection Service Company, Inc.)

The splitter blade wheel design is where every other blade is shorter than the full blade next to it. This allows for higher airflow at higher rotational speeds.

The backward curved impellor or BCI wheel design, is the most common type of highly efficient compressor used today. This design creates a wider compressor map that is compatible with automotive applications and raises compressor efficiency by beginning air diffusion within the wheel before it exits into the diffuser of the compressor.

The backwall of the compressor can be either a full backwall or a partial backwall. A full backwall is where the backwall is the same diameter as the overall diameter of the compressor wheel. The idea of the partial backwall is to reduce the overall wheel mass allowing the wheel to be more responsive. However, most backward curved impellors cannot be properly supported without the aid of a full backwall, thus there are design tradeoffs.

Backward curved impellor refers to that part of the blade element as it approaches the maximum diameter. Note how it curves backward relative to the direction of rotation, which is clockwise. This feature helps to begin air diffusion by slowing the air speed before it exits the wheel thereby making the airflow range broader in the higher efficiency regions of operation. (Courtesy Turbonetics)

A recent advancement patented by Honeywell Turbocharging Systems (Garrett brand) is the use of a boreless compressor wheel. The traditional compressor wheel has a shaft bore through its center that allows it to be fastened onto the turbine wheel and shaft assembly’s stub shaft.

These two wheels have nearly the same exducer diameter, but the wheel on the left uses a partial backwall while the wheel on the right uses a full backwall. You will almost never see a partial backwall turbo applied to a new application. (Courtesy Diesel Injection Service Company, Inc.)

Eliminating the thru bore high-stress core of the compressor wheel reduces the risk of hub fatigue in a highly cyclical application and increases the compressor wheel life. A boreless design can last five times as long as a thrubore wheel. (Courtesy Honeywell Turbo Technologies)

The extended tip is a special machining process that cuts the compressor wheel impellor back at an angle greater than 90 degrees relative to the backwall. This allows for the rotational diameter to extend beyond the backwall diameter and makes a little higher pressure on a large diameter wheel while minimizing the wheel mass in the outer diameter. The moment of inertia is very sensitive the farther away from the center of rotation. (See Chapter 6 for moment of inertia specifics relative to mass at a given radius.) (Courtesy Diesel Injection Service Company, Inc.)

The bore unavoidably passes directly through the area of greatest stress concentration in the wheel. This bore can cause a problem when the turbocharger is running at high rotational speeds, and of course, it goes through a high number of cycles, or accelerations. A single cycle can be a simple gear change where the turbo spools up then slows down for a gearshift, and then spools back up again. Aluminum alloy compressor wheels have a specific number of cycles they will last before fatigue sets in and the wheel bursts. For most automotive applications this does not represent a problem, but it does for commercial diesels.

The higher the amplitude of each cycle, the fewer cycles the compressor can withstand. For example, a compressor wheel that regularly cycles on a given application from 40,000 to 80,000 rpm would last longer in-terms of cycles than the same wheel if used on another application where the cycles ranged from 60,000 to 110,000 rpm. In the turbo world anything that fails in less than 100,000 cycles is deemed a low-cycle fatigue problem. This is why other materials, such as titanium, are necessary in certain applications.