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An alternative to the fixed geometry turbine is the variable geometry turbine. The benefits of variable geometry turbines over wastegated turbines include :
The idea of using a variable geometry turbine in a turbocharger dates back at least to the 1950s . Since that time, a number of different designs have appeared. Two of the more common ones are the pivoting vane and moving wall types, Figure 1 . Others include the variable area type, variable flow type and the sliding ring designs. These designs will be discussed in more detail in the following sections.
Figure 1. Pivoting Vane (left) and Moving Wall (right) Variable Geometry Turbochargers
1. Turbine housing; 2. Variable angle vanes; 3. Adjusting ring
There are a number of different acronyms that are commonly used when referring to turbochargers with variable geometry turbines. In most cases these are or have been trademarks that a particular manufacturer has used with reference to their product. In more common usage, a particular acronym can be used in a more general sense and not necessarily be a reference to a particular manufacturer’s product. Some of these acronyms include:
In many designs, a variable geometry turbine does not include a bypass so the turbine must be capable of handling all of the exhaust flow from the engine while avoiding overboost and overspeeding the turbocharger. For a given engine power rating, this would imply a larger turbine swallowing capacity than that required by a wastegated fixed geometry turbine and comparable with that used for a fixed geometry turbocharger with no bypass.
The fundamental difference between a fixed geometry turbine and a variable geometry turbine is illustrated in Figure 2 . Compared to a fixed geometry turbine, the variable geometry turbine allows significant flexibility over the pressure ratio/flow relationship across the turbine and by extension, the engine ΔP. This flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and in diesel engines, driving EGR flow.
Figure 2. Comparison of Fixed Geometry (BorgWarner KP39) and Variable Geometry (BorgWarner BV40) Mass Flow vs. Pressure Ratio
The peak efficiency of a variable geometry turbine occurs at about 60% nozzle opening. It is usually comparable to or a few percent lower than that for a fixed geometry turbine. However, efficiency drops off rather quickly as nozzle opening is reduced or increased from a mid-vane opening position, Figure 3 .
Figure 3. Effect of variable geometry turbine nozzle opening and blade speed ratio on turbine efficiency