# Influence of Drivetrain Hybridization on Transmission Lifetime
## 1 Introduction  
Hybrid and battery electric vehicles are paving the way to the reduction of the greenhouse gas and pollutant emissions in global individual transport. Purely battery electric vehicles (BEVs) still have the disadvantages of high battery costs and low range [1]. As a transitional technology, hybrid electric vehicles (HEVs) offer a compromise between low pollutant emissions from BEVs and the high range of conventional vehicles [2]. In principle, they have the potential to be operated more efficiently than purely combustion engine vehicles. In fact, HEVs dominate the European vehicle market, with a market share of 3.7% compared to BEVs with 1% [3]. In HEVs, one or more electrical motors (EMs) can be arranged in the drivetrain and thus define different drivetrain concepts. Parallel hybrid drivetrains form the group of concepts in which a mechanical coupling of the internal combustion engine (ICE) with the wheels is possible and all drives can simultaneously provide torque for propulsion [4]. Five basic parallel hybrid topologies (P0-P4) are distinguished depending on the arrangement of the EM in the power path from the ICE (internal combustion engine) to the wheels (Figure 1). By combining the EM arrangements of the basic topologies, new drivetrain variants can be created, such as the exemplary POP2 hybrid in Figure 1. Dual-clutch transmissions are particularly well suited for HEVs, as they combine high efficiency with ahigh level of driving comfort [5] and meet the additional functional requirements with few design modifications [6]. In the P2 arrangement, they find a particularly broad application in HEVs [7]. Among the development goals of the automotive industry, reliability is of central importance alongside efficiency, emissions and costs [8]. Automotive products are multidisciplinary and subject to increasing complexity with a high number of variants. The later correction of errors that occur during early development phases is particularly cost-intensive [9]. For this reason, the reliability of vehicle transmissions is continuously taken into account, from the early design phases to the realization of the physical product [10]. In early product creation phases, lifetime calculation is often used to assess reliability [11]. In the process, occurring loads for the investigated component are compared with their bearable loads and converted into component damage or lifetime. Occurring loads are, for instance, extracted from real driving tests, or are generated synthetically and condensed to load spectra by means of a suitable classification method. [PERSON] et al. [12] present a method to extract real load data for the durability design of transmissions considering numerous influences on the loads, and apply it to a conventional vehicle, a BEV and an HEV. Approaches to generating of synthetic load data can be found in [13; 14]. [PERSON] et al. [15] investigate the influence of dynamic overloads on the lifetime of gears in the transmission of an electric vehicle. For this purpose, the dynamic factor is determined analytically, using a rigid and a fully elastic simulation model of the gear pairs. A lifetime calculation of a six-speed manual transmission in a conventional vehicle drivetrain is performed by [PERSON] et al. in [16]. The development of a simulation model and a method of calculating the damage in gears in a conventional drivetrain is described by [PERSON] et al. in [17]. An approach to determining the remaining lifetime of transmission gears in real vehicle operation is presented by [PERSON] et al. in [18] and applied to a manual transmission in a conventional vehicle. Major influences on damage include the clutch actuation in gearshifts, the inclusion of the lower drivetrain natural frequencies in the model and special events such as the loss of tire adhesion. Negative torques from ICE coast operation are indicated as negligible [19]. A load-related operating strategy for the vehicle is presented and discussed in detail in [20], regarding damage to gears. [PERSON] et al.