# Cooling Performance of Fresh and Aged Automatic Transmission Fluids for Hybrid Electric Vehicles
## 4 Conclusions  
The ATFs used in EVs, where the EM is located inside the transmission housing, have the role not only of forming a friction- and wear-reduction tribofilm but also protecting against corrosion and removing heat. The lubricants' properties, such as density, viscosity, thermal conductivity, and heat capacity, among others, are important for the heat-transfer rates in many applications, including electric drivetrains. These properties can change with the oxidation of the lubricant that takes place during the use of the ATF and the cooling performance can be seen to be affected. The cooling performance of three conventional ATFs when fresh and aged were studied and the following conclusions could be drawn:  
1. The influence of the molecular structure on thermal conductivity and heat capacity is stronger than on density and viscosity, but the differences in these properties of the three conventional ATFs in fresh conditions did not differentiate their cooling performance as expressed through some figures-of-merit (FOMs).
2. The oxidation of the ATFs changed thermal conductivity and heat capacity in a different manner depending on the API Group of the base oils. ATFs B and C, formulated with base oils from API Group III, demonstrated better cooling performance than ATF A, which was formulated with base oils from API Group I
3. The sensitivity to temperature of the variation with oxidation of the studied properties, including the FOMs, was almost null, except for ATF A.
4. The FOMs should be used to compare cooling performance of ATFs for electric drivetrains instead of a single property such as thermal conductivity.
5. Testing of the cooling performance of ATFs at higher oxidation levels should be addressed in future research.  
Figure 8: Sensitivity to temperature of the variation of figures-of-merit with oxidation.  
**Author Contributions:** Conceptualization, J.L.V. and A.H.B.; methodology, N.R., A.G. and L.L.; investigation, N.R., J.L.V. and J.I.P.; writing--original draft, A.H.B., N.R. and L.L.; supervision, J.L.V.; funding acquisition, A.H.B. and J.L.V.; project administration, A.H.B.; writing--review and editing, A.H.B., J.I.P., L.L. and A.G. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Ministry of Science, Innovation and Universities (Spain) and by the Government of the Principality of Asturias (Spain), grant numbers PID2019-109367 RB-I00 and PA-21-AYUD/2021/50987, respectively.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data is contained within the article.  
**Acknowledgments:** The Scientific-Technical Services of the University of Oviedo are also acknowledged.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclature**  
\begin{tabular}{l l} \(C_{p}\) & heat capacity \\ FOM & figure of merit \\ \(k\) & thermal conductivity \\ Mo & Mouromtseff number \\ Greek symbols & \\ \(\mu\) & dynamic viscosity \\ \(\rho\) & density \\ \end{tabular}