Variable Geometry Turbochargers...

Introduction

       An alternative to the fixed geometry turbine is the variable geometry turbine. The benefits of variable geometry turbines over wastegated turbines include:

• no throttling loss of the wastegate valve;
• higher air–fuel ratio and higher peak torque at low engine speeds;
• improved vehicle accelerations without the need to resort to turbines with high pumping loss at high engine speeds;
• potential for lower engine ΔP (the difference between exhaust manifold and intake manifold pressures);
• control over engine ΔP that can be used to drive EGR flow in diesel engines with High Pressure Loop (HPL) EGR systems;
• a better ability to cover a wider region of low BSFC in the engine speed–load domain;
• ability to provide engine braking;
• ability to raise exhaust temperature for aftertreatment system management.

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 section.

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