Compressor wheels in turbochargers are often categorized into two main design types based on the shape of their exducer (the outer diameter of the wheel where the blades meet the housing):
Flatback: The exducer of the wheel is essentially flat, meaning the back surface of the wheel (opposite the inducer, where air enters) is a straight plane perpendicular to the shaft.
Superback: This design features a curved or "cupped" exducer. The back surface of the wheel curves outward and then back inward towards the shaft, creating a more complex, contoured shape.
The key difference and the reason for these designs lies in how they manage stress and improve the structural integrity, and boost capabilities.
Here's a breakdown:
- Stress Distribution: Superback wheels are generally better at distributing stress, particularly centrifugal stress, that arises from the extremely high rotational speeds (often exceeding 100,000 RPM) that turbocharger wheels experience. The curved back helps to spread the load more evenly, reducing the likelihood of stress concentrations that could lead to cracks or failure. Flatback are more simple to produce.
- Boost Potential and Efficiency: The superback design can, in some cases, contribute to a slightly higher boost potential and improved efficiency. The curved exducer can influence the airflow as it exits the compressor wheel, potentially improving pressure recovery and reducing backflow. However, this effect is highly dependent on the overall turbocharger design (housing, blade profile, etc.) and isn't always a guaranteed advantage.
- Manufacturing Complexity: Superback wheels are more complex to manufacture than flatback wheels. The intricate curvature requires more sophisticated machining processes, which can increase production costs. Flatback are typically simpler.
Application Flatback are most common type. Superback is more usual to be found in high performance applications.
In summary, while both designs serve the same fundamental purpose (compressing air), the superback design is generally favored in high-performance applications where durability, high boost pressures, and potentially marginal efficiency gains are prioritized. Flatback designs are more common in applications where cost-effectiveness and simpler manufacturing are key considerations, and where the extreme stress demands of high-performance applications aren't as critical. The choice of using flatback or superback compressor is a balance of multiple factors.
