# Influence of Drivetrain Hybridization on Transmission Lifetime
## 4 Summary, Conclusions and Outlook  
In this paper, the bearing lifetime of a transmission in a passenger car application was investigated and the influence of regenerative braking on this lifetime was quantified. Here, both a conventional and a parallel hybrid drivetrain were investigated. The bearing lifetimes were calculated using ISO 281 as part of a method for estimating the lifetime of technical systems. The loads on the rolling bearings in the transmission were determined with the help of a detailed drivetrain model consisting of ICE, EM, mechanical drivetrain, vehicle, driver and HCU. In the conventional drivetrain, the EM model was omitted. To take into account the additional loads from the eigenbehavior of the drivetrain, the DMF, side shafts and tires were considered to be elastic. The WLTC was chosen as the representative driving cycle. Additional loads from the actuation of the clutches in gear shifts were taken into account by the implemented power shifts. A simple approach was considered as the operating strategy, which splits the driver's braking torque demand between the drag torque of the ICE, the EM and the MB. The results show that in the conventional drivetrain, the bearing lifetime of the locating bearings of the intermediate shafts dominates the failure distribution. The lifetime of the bearings as a group was evaluated as plausible on the basis of literature references. In the hybrid drivetrain without torque limitation, a 36.9% reduction in bearing lifetime was determined. The lifetime in the hybrid drivetrain is shorter, because high negative input torques occur more frequently during regenerative braking. These torques cause additional bearing loads, depending on the gear engaged. A reduction of the regenerative torque, as can be justified by the numerous boundaries of regenerative braking, was investigated in a sensitivity analysis. The bearing lifetime decreases with falling gradient up to the maximum regenerative torque of the EM. The findings reported here demonstrate that the additional loads occurring in HEV must be taken into account in transmission development and operation. Design possibilities are a stronger dimensioning of the components, a design adaptation of the transmission layout or the use of a hybrid topology with a different position of the EM. If the regeneration can be realized without additional power flow through the transmission (e.g., topologies P3 or P4), this influence on the lifetime remains negligible. However, these topologies require a higher speed spread of the EM. In addition, the loads can be taken into account on the operating strategy side. During development, the operating strategy can be adapted to the load capacity of the transmission by limiting the maximum regenerative torque or by a load-related gear shifting strategy that takes into account uniform loads on the bearings. During operation, the regenerative torque can be adjusted online to the damage observed in the components. The simultaneous consideration of operating strategy and transmission lifetime  
Figure 13: Results of the sensitivity analysis. Bearing lifetime over maximum regenerative torque in the WLTC. leads to a multiobjective optimization problem, the solution of which can yield a suitable tradeoff between energy saving and transmission lifetime. Although the results are considered plausible, and a transmission model validated with regard to losses was used, the calculation approaches should nevertheless be validated in future work. This includes the physical behavior of the torsional vibration model, as well as the bearing force calculation. The torsional vibration model can be validated by means of a modal analysis. A validation of the bearing force calculation is possible by measuring the bearing forces. However, the necessary application of strain gauges in the proximity of the bearing is costly. In addition to the physics, the results of the lifetime calculation should be validated.