## 1 Introduction  
The electric vehicle (EV) has become the main solution in the struggle to minimise transportation-related emissions, which represent 14% of the total weight of greenhouse gasses that are emitted, according to the Intergovernmental Panel on Climate Change [1]. EVs have an average efficiency of 77% compared to 21% for vehicles driven by an internal combustion engine (ICE) [2]. The greater efficiency of EVs results in a large reduction in CO\({}_{2}\) emissions [3], especially when using electricity generated from renewable sources [4]. There are different classes of EVs, including battery electric vehicles (BEV), extended range electric vehicles (EREV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), mild hybrid electric vehicles (MHEV), and fuel cell electric vehicles (FCEV). Regardless of the EV type, the EM can be placed in different positions [5]. The position of the EM with respect to the ICE has an important role in the design and operation of hybrid vehicles. Although there is no industry agreement, the position of the EM is usually referred to as P0, P1, P2, P3, or P4 [6]. P0 and P1 are used when the EM is located before or after the ICE, respectively, and outside the transmission. In these cases, little change is expected for the ATFs because they do not interact with the EM. P2 and P3 refer to the case of an EM inside the transmission housing, either immediately before or immediately after the transmission, so the EM is in contact with the ATF. P4 describes the arrangement in which the EM drives the rear axle [6], and hence is physically separated from the transmission and ICE. When the EM is located within the transmission of the hybrid vehicle, the automatic transmission fluids (ATFs) come into contact with the EM, so fluid formulators must bear the following several requirements in mind: thermal compatibility, magnetic compatibility, corrosion protection, the problem of aeration and foaming at high speeds, electrical compatibility, and material compatibility [7; 8; 9]. To date, EVs are still using conventional ATFs to lubricate electrified drivetrains, and they have not been optimized for this application [10; 11]. A variety of polymers are used in the electric motors, mainly for structural purposes, such as, for example, plastic bearing housings, and the ATFs that are employed must be compatible with these polymeric components. Among these polymers can be found polyether ether ketone, polyamide (commonly known as nylon), polyethylene, and polytetrafluoroethylene, but new materials that can withstand higher loads and operation temperatures will begin to be used [6; 8]. [PERSON] et al. [12] studied the compatibility of some structural polymers (polyether ether ketone-PEEK, polytetrafluoroethylene-PTFE, and ultrahigh-molecular-weight polyethylene-UHMW PE) with different bio-oils. They found that PEEK and PTFE showed negligible degradation after ageing. [PERSON] et al. [13] and [PERSON] et al. [14] studied the compatibility of PA66 with ATFs. [PERSON] et al. [13] pointed out the tendency of PA66 to crystallize when aged in ATFs, although they concluded that it was the high temperature that caused this effect. They also reported a drop in tensile strength. [PERSON] et al. [14] confirmed the crystallization of PA66 in ATF after 2000 h at 130 \({}^{\circ}\)C, although they specified that the degradation was even greater in air at the same temperature, due to thermo-oxidation. Nonetheless, EV manufacturers recommend the use of conventional ATFs (originally designed for vehicles driven by ICEs) in EVs, without any reference to their overall compatibility when the EM is located inside the transmission. The objective of this research is to study the compatibility of four conventional ATFs with three structural polymers and the feasibility to using them in EVs with the EM located inside the transmission.