Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 15

Let’s begin with the turbine. A turbocharger obtains its energy in the form of heat and pressure from the engine’s exhaust. Any given turbine has a designed set of parameters that dictate what amount of exhaust flow it is compatible with. A properly applied turbine will actually raise the backpressure in an engine’s exhaust system, which will, in turn, raise the pressure to drive the turbine and increase the heat in the exhaust system.

The turbine has two principal components; the turbine wheel and the turbine housing. The exhaust from the engine is routed to the turbine through the exhaust manifold and enters the turbine inlet. The exhaust gas fills the turbine housing and creates a static pressure by way of a volute shaped turbine housing that feeds the turbine wheel at its tip or the inducer area of the turbine wheel. Since high pressure always seeks low pressure, the exhaust gas flows through the turbine wheel and the high-pressure gas expands as it travels through the turbine wheel. This exhaust gas expansion is what drives the turbine wheel in rotation inside the turbine housing.

The turbine is the machine that extracts the energy present in the form of heat and pressure from the exhaust and converts it to mechanical energy via the turbine wheel’s rotation. Therefore, turbine design and efficiency is critical to overall turbocharger effectiveness as it relates to engine performance. Even more important than subtle design enhancements within the turbine is a properly sized turbine to correctly “see” the engine’s exhaust.

This is a cutaway photo of a turbocharger showing the major sections of turbo design. (Courtesy Honeywell Turbo Technologies)

The turbine section of a turbocharger consists primarily of the turbine wheel and turbine housing. This 120-degree cutaway section illustrates the turbine wheel position relative to the turbine housing. (Courtesy Diesel Injection Service Company, Inc.)

The household garden hose is a simple analogy to help understand optimized flow matching. If you let the hose run with the end unrestricted it flows out in high volume and will typically run out 3 to 6 feet from the hose end. Now begin to restrict the hose end a little at a time and you find the pressure rising and the water sprays out farther and farther. It may spray as far as 30 to 40 feet. But if you continue to cover more and more of the hose, at some point you see a point of diminishing return. The water spray distance begins to drop off because you have restricted flow too much and all you have done is caused excessive backpressure beyond what was optimized for maximum flow pressure and volume.

A turbine is sized for a given engine’s exhaust gas flow in much the same way. But since an engine operates at a variety of engine speeds, the challenges are more complex than the constant flow through the hose. For example, a given engine that is used in a street driven vehicle and reaches engine speeds of only 5,000 to 6,500 rpm may require one size turbine. The same engine designed to perform in a racing environment and operate at engine speeds of 7,000 to 9,000 rpm will need a completely different sized turbine.

It’s extremely important to understand the turbine-to-engine-size relationship because all too often turbos are mismatched to engines based upon the premise that smaller turbines raise boost pressure. While that may be true, the point of diminishing return is often met or exceeded and a turbine housing that is too small is now robbing horsepower due to turbine choke and what is known as pumping losses (excessive backpressure). This will be discussed in greater detail in Chapter 3. But for now it is only important to understand this flow relationship and how a turbine can be sized too small as well as too large.