Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 30

In the turbine section, the turbine housing receives the most attention due to its part in tuning the turbo into the engine application. A given model of turbo may have a turbine wheel design that can accommodate several different turbine housings. The turbine housing is typically where the application is tweaked once the correct model is found. It’s important to know that the turbo model selected for your application has more than one turbine housing. Although the wastegate makes up for some degree of turbine mismatch, it’s a distinct advantage to know you have other turbine housing choices within the turbo model family you have matched to your engine.

The external elements of the turbine housing that can be viewed include: (1) the turbine volute, (2) the turbine foot, and (3) the outlet connection (a slip ring type is shown). Sometimes the material type is shown, as in this case where it is made from ductile iron (note “DUC” on housing). (Courtesy Diesel Injection Service Company, Inc.)

Viewing the turbine housing from the bearing housing side we can more easily view the following: (1) the turbine housing contour, (2) bearing housing pilot diameter, (3) turbine throat area, and (4) bearing housing connection, which is typically a V-band or threaded holes for clamp tabs (V-band connection shown). (Courtesy Diesel Injection Service Company, Inc.)

While there are many types of turbine housing sizes and types, the two most commonly found are the tangential entry, open or divided type housings. Note the divider wall at the turbine gas entry point in the housing on the right. (Courtesy Diesel Injection Service Company, Inc.)

This turbine housing is cut in half perpendicular to the axis of turbine wheel rotation. The turbine wheel and shaft has been placed into the housing for perspective to help visualize the flow of exhaust gases as they enter tangent to the turbine wheel inducer tip. Note how the volute progressively has a smaller and smaller cross-sectional area. As the exhaust begins to exit the housing by way of the turbine wheel, exhaust energy is lost. The progressively smaller cross sectional area of the volute tends to create an even pressure all around the turbine wheel inducer tips to more effectively drive the turbine while stabilizing it in its dynamic path. The tongue of the turbine housing forces the remaining exhaust gas energy in the volute to exit the housing. (Courtesy Diesel Injection Service Company, Inc.)

While there are many specialty types of turbine housings, the two most common are divided and open housings. The divided housing uses a divider wall in the turbine volute to keep the exhaust path isolated from the exhaust port to the tip of the turbine wheel, or turbine inducer. This allows a maximum use of exhaust gas pulse energy, which sounds like a mouthful, but it’s actually very easy to understand. The engine is a reciprocating air pump. Consequently, the amount of energy during each exhaust gas pulse is greater than the amount of energy between each pulse. Similarly, if you placed your hand on the outlet of the shop air compressor you would feel rapid pulses. If you fill an air compressor’s tank and feel the air gun, you’ll feel a steady stream flow of air. A divided turbine housing allows each exhaust pulse to reach the turbine wheel tip and thereby transmit higher energy to the turbine.

By contrast, an open housing has a more efficient and less restrictive flow, but doesn’t transmit pulse energy as well. Therefore a divided turbine housing is best employed on low-speed and midrange turbo matches where engine RPM is not high, but peak torque is more the desired result, such as in a diesel engine moving large loads on or off-highway. The open housing is typically used more on high-speed, high RPM automotive applications where exhaust becomes more steady stream flow. In a high-RPM engine, a divider wall tends to be more restrictive due to the additional surface area that causes losses with turbine gas flow. This is another design tradeoff where the optimum type of housing for a particular application depends upon how the engine is to be used.

The turbine housing is sized to optimize the pressure and flow as in the garden hose analogy. Turbine housings are rated in A/R. Many call this an aspect ratio. I’ve never liked that term (it is incorrect) and development engineers don’t use it. It’s simply the “A/R ratio.” The ratio is where “A” is the area of the volute at the tongue of the turbine housing. The “R” is the radius from the center of the axis of rotation to the centroid of the volute. The easiest way to think of the A/R is the swallowing capacity of the turbine. The larger the A/R is, the larger the swallowing volume. Therefore the smaller the A/R of the housing, the higher the pressure becomes in the turbine stage. Refer to the section Understanding Turbines in Chapter 3 for a more complete explanation of A/R ratios and how they should affect what turbo you choose.

The A/R of a turbine housing may appear on the outside of the housing on the throat just beyond the turbine foot gas entry point as shown. (Courtesy Diesel Injection Service Company, Inc.)

This area of turbocharger design is one of the most frequently used, yet least understood relationships. In Chapter 3 I’ve tried to provide as complete of a definition as exists for better understanding of this portion of turbo design to help you more intelligently apply and communicate about turbos. The turbine A/R ratio is one of the most important variables and is a common necessary reference when it comes to tuning.

On some turbine housings the A/R will appear just inside the gas entry above the turbine foot as shown. (Courtesy Diesel Injection Service Company, Inc.)