The turbine wheel plays a crucial role within the turbocharger. It provides the drive pressure needed to spin the compressor wheel for it to create boost pressure.
In order for a turbocharger to perform efficiently, the compressor and turbine wheels must roughly match their respective flow rates. An ideal scenario would be a slightly higher flowing turbine wheel in comparison to the compressor to compensate for the added fuel in order to allow for effective exhaust flow as well as keeping turbine drive pressure similar to intake manifold pressure created by the compressor wheel.
What affects spool up?
In order for the turbine wheel to respond quickly, gases must be directed at the tip of the inducer. This produces the maximum amount of leverage due to the distance away from the centre of the shaft.
The turbine wheels blade count has an effect on the spool. This is determined by the turbine wheels inducer, with more blades providing more drive to the turbine wheel in comparison to a turbine wheel with less blades.
Weight also plays a crucial role for response and spool up. Design characteristics such as a small hub diameter can be key in saving weight and reducing rotating mass.
The turbine wheels inducer design can also play a crucial role in spool up. An example of this would be a mixed flow turbine wheel. They respond quickly due to having less weight while having a longer & pointy inducer tip. This provides the additional benefit of increasing leverage on the turbine wheel.
What determines turbine flow?
Turbine flow is mainly determined by the exducer diameter. This is the overall flow limit through the 'restriction' in the exhaust. However, the volume the turbine wheel takes up within the bore also reduces the overall flow of exhaust gases.
Turbine wheel volume is added up from the thickness of the blades, centre hub size and quantity of blades.
This is why lower blade count turbine wheels provide more overall flow than higher blade count wheels - because they take up less space in the exhaust flow. However, too low of a blade count causes a lack of turbine drive pressure, which causes a lack of flow.
This is where the staggered turbine wheel comes in. Originally brought to the market by Xona Rotor, the design is similar to a staggered compressor wheel design, using primary (full size) and secondary (half size) blades. This provides an incredible benefit used in a turbine wheel application as it maximizes the amount of inducer blades, still providing full drive pressure. While also creating more free space at the exducer of the turbine wheel, allowing for more overall turbine flow. Another additional benefit is also weight saving as all of the blades are not primary blades.
