BorgWarner has developed its first turbocharger roller bearing system to series maturity. These new bearings considerably increase the efficiency of turbocharging systems, which in turn improves the response and fuel economy of the engine quite significantly. What‘s more, the innovative system also sets new standards for roller bearings with regard to NVH behavior.
M odern combustion engines have already reached a high level of efficiency. The challenge therefore becomes even greater when attempting to further optimize these units in terms of fuel economy, power output and reduced emissions. The developers at BorgWarner tackle this task by continuously increasing the efficiency of the turbocharging systems. The best example of this is the company‘s new roller bearings, which significantly reduce frictional losses over plain bearings of the same size. Further key criteria for the development included noise characteristics at plain bearing level, stable start-stop behavior - in particular in hybrid drive systems - and high resistance to solid and liquid impurities in the oil, also when used with biogenic fuels.
Roller bearings offer benefits over plain bearings, particularly at low speeds. To control the rotor dynamics and noise characteristics, however, it is necessary for the outer ring of the ball bearing to be mounted as a „floating“ unit. The developers therefore equipped the system with cartridge-type angular-contact ball bearings. Balls made from silicon nitride, coupled with high-performance materials for the bearing rings, secure excellent system performance and robustness, as they also offer sufficient hardness reserves for the high limit temperatures encountered. Many other design optimizations served to improve the NVH behavior (noise, vibration, harshness), as this has become a key criteria for the success of roller bearing technology in turbocharger applications.
Comprehensive tests performed by BorgWarner clearly highlight the benefits of the new ball bearings over plain bearings. By way of direct comparison, friction is reduced by up to 50% when using cold oil at low speeds and still maintains a reduction of up to 40% at operating temperature. The friction of the ball bearings also increases more slowly than with plain bearings as the axial thrust and stage pressure ratio rise.
The complex design of the new roller bearing system increases turbocharger efficiency, which in turn has a positive effect on engine response. As boost pressure is built up more quickly at low engine speeds, this significantly improves dynamic driving performance. The increased efficiency also improves fuel efficiency, while reducing CO2 emissions in the consumption-critical partial-throttle range by around 2%.
The outer ball bearing ring is floating, so that it can handle rotor dynamics, while maintaining excellent noise characteristics.
The innovative ball bearing system from BorgWarner
significantly improves turbocharger efficiency.
Photos: © BorgWarner
In comparison with a turbocharger fitted with plain bearings, the new ball bearings deliver increased torque from very low revs.
Robust and low-noise
High speeds, dynamic load changes, fast temperature changes, external vibrations, as well as liquid and solid impurities in lubricants are the main causes of wear to bearings in turbochargers. The developers from BorgWarner performed tough tests to assess the robustness of the new roller bearing system. During these tests, turbochargers were subjected to high loads and operated with 200-times the typical amount of filterable, hard impurities. The new ball bearings remained operable, while the thrust bearing of a reference turbocharger with plain bearings was no longer functional following the test. An accelerated endurance test developed in-house that uses the hot chamber and engine test bays, as well as a vibration endurance test also confirmed the robustness of the new system. Since one of the objectives when developing the new ball bearing technology was to deliver excellent NVH behavior, a whole series of vehicle and endurance tests was then performed. During this testing, it became clear that the new system offers a very low noise level, which was also subsequently confirmed by customers.
Ready for series production start
With the new roller bearings, BorgWarner is offering a high-performance, robust and low-noise solution which is more than capable of meeting the strict requirements of both current and future applications. The advanced technology increases the efficiency of the turbocharger by 4% and thereby contributes to a significantly improved response behavior. At the same time, the system reduces CO2 emissions by up to 2%. The innovative bearings will soon be available for use in all passenger vehicle sizes.