Xona Turbocharger X2C XR7864S
Xona Rotor's second-generation turbine wheel has arrived. Known as UHF (Ultra High Flow), its mission was to maximize flow capacity and raise efficiency.UHF is Xona Rotor's most advanced wheel yet. Among other details, UHF's patent pending splitter-blade design allowed the blade loading to be fine-tuned in order to achieve its performance objectives.
Turbine design is a multi-variable juggling act. One feature that heavily influences a turbine's mass flow "swallowing capacity" is the throat region located near the exit of the wheel. An effective method to increase the throat area is to decrease the number of blades. While reducing blade count tends to increase the turbine's flow capacity, efficiency typically suffers.
Introducing splitter blades provides an elegant solution to the trade-off between flow and efficiency. The splitter blades alleviate the blockage in the throat area while preserving the fluid momentum at the wheel inlet.
The result is that UHF has the swallowing capacity of a low blade-count turbine with the efficiency of a high blade-count configuration. Its improvements were observed analytically, validated experimentally on a gas stand and then confirmed in on-vehicle testing.
Why Flow Capacity Matters
So why all this effort to maximize the turbine's flow capacity? For a given engine operating point - say, full boost near redline - a higher-flowing turbine operates at reduced expansion ratio than a lower-flowing turbine. The lower expansion ratio translates directly to reduced exhaust manifold pressure ("EMAP").
Lower EMAP means more engine power - the engine's pumping work is reduced and volumetric efficiency improves. If the engine's valve events (i.e., overlap) and ignition timing are modified to take advantage of the reduction in EMAP, further gains can be reaped. In the bargain, with reduced EMAP, the engine becomes less knock-sensitive and less prone to overheating.
Engineered for Life
Xona Rotor's ball bearing system is unique. We spared no expense in its specification, starting with a high-precision ABEC 7 angular contact dual-row ball bearing cartridge. Its M62 tool steel races and silicon nitride ceramic balls deliver the highest possible load capacity, and its metallic retainers won't melt.
We didn't stop there. By integrating a proprietary variable preload feature (US patent no. US9062595B2) to our ball bearing cartridge, we've achieved a best-of-all-worlds scenario.
In a turbocharger, the correct bearing preload is a moving target. The pressures acting on the turbine and compressor wheels are often unbalanced, which generates a variable thrust (axial) load through the turbine shaft. This thrust load is exerted on one row of balls in the bearing.
To maintain contact of the unloaded row of balls to the races, a certain amount of preload is required. Insufficient preload can cause the unloaded row of balls to slide and skid on their respective races, generating fretting wear on the races that results in reduced life. Excessive preload also accelerates wear and generates heat. Compounding this situation is that the correct amount of preload is a moving target.
Xona Rotor's variable-preload system extends bearing life by seamlessly introducing additional preload when needed and less when it isn't.
Conventional Preload Techniques Have Limitations
Typical methods to develop preload in a bearing are via a press fit or a spring force. The press fit method requires very tight axial tolerances to be held between the bearing's inner races and opposing outer races. To maintain consistent preload, a press fit preload system relies on materials of equal coefficient of thermal expansion and uniform operating temperatures across the entire bearing system.
However, the extreme temperature excursions inherent to a turbocharger present a challenging environment for a press fit-preloaded ball bearing. The inner bearing race is typically pressed to the turbine shaft and runs hotter than the outer race, which is thermally decoupled and more directly bathed in cooling oil. Even in a water-cooled bearing housing, the temperature gradient between the outer and inner bearing races can be hundreds of degrees Fahrenheit.
Due to the difference in operating temperature between the turbine and compressor, turbocharger applications also present a significant axial temperature gradient along the bearing's length. These temperature differentials will not allow optimum preload to be maintained for all operating conditions. Compared to a press-fit preload system, a spring-based system can maintain a more consistent preload across a wider range of temperatures, but provides lower system stiffness.
These conventional approaches to applying preload can only be optimized for a single design point. In all other cases they are either employing insufficient or excessive preload.
How Xona Rotor's Bearing Is Different
Xona Rotor's variable-preload system addresses the limitations of conventional bearing systems. It harnesses the oil pressure delivered to the bearing housing to selectively apply preload to the rotating group as needed. It compensates for the variations in preload induced by thermal expansion and can vary preload as a function of rotational speed. This patented system is exclusive to Xona Rotor.
Unlike some ball bearing turbochargers, Xona Rotor turbochargers also use the oil to hydraulically damp radial shaft motions that would otherwise compromise bearing life. The oil also damps axial motions, a feature exclusive to Xona Rotor.
Power Application in HP: 410-820
Compressor Diameter Inlet/Outlet in mm: 62/86
Mass Flow in LB/min: 78
Turbine wheel diameter inlet/outlet in mm: 72/64
compressor housing: X2C
