# Cooling Performance of Fresh and Aged Automatic Transmission Fluids for Hybrid Electric Vehicles
###### Abstract  
The cooling performance of automatic transmission fluids (ATFs) plays an important role in hybrid electrical vehicles, in which the electric motor (EM) is placed inside the transmission housing due to their mission of cooling the EM. The cooling performance of the ATFs depends on their thermophysical properties, but these properties change with the oxidation of the ATFs. This work studies the influence of the oxidation of three ATFs (A, B, C) on their thermophysical properties, as well as on some figures-of-merit (FOMs) which are relevant for evaluating the cooling performance. The results indicated that the influence of the molecular structure on thermal conductivity and heat capacity is stronger than on density and viscosity, whereas the molecular structure hardly affects the FOMs of the fresh ATFs; ATFs B and C, formulated with base oils from API Group III, indicated better cooling performance than ATF A which was formulated with base oils from API Group I; the sensitivity to temperature of the variation with oxidation of the studied properties, including the FOMs, was almost null, except for ATF A; therefore, FOMs should be used to compare the cooling performance of ATFs for electric drivetrains instead of a single property, such as thermal conductivity. automatic transmission fluids; hybrid electric vehicles; thermal conductivity; heat capacity; viscosity; density +
Footnote †: journal: Applied Sciences  
There are different methods for cooling EMs: air cooling, water cooling and oil cooling, but the miniaturization and required high performance for EVs led to the use of the oil-cooling method as the best option [7]. Heat conduction enhancement and hybrid thermal management have also been studied as options for cooling EMs [8]. [PERSON] et al. [9,10] compared indirect and direct oil cooling methods, direct ones being superior. They reduced the average temperature rise of the coolant at the inlet by a factor of between 1.5 and 3 and improved the heat transfer performance. The best cooling results were obtained with a direct oil method, reducing the maximum temperature to 403.6 K (130.45 \({}^{\circ}\)C). [PERSON] et al. [11] reported maximum temperatures of 420.15 K (147 \({}^{\circ}\)C) and 377.15 K (104 \({}^{\circ}\)C) for the rotor and the stator, respectively, when general oil cooling was used. On the other hand, [PERSON] et al. [12] reported temperatures of 380.15 K (107 \({}^{\circ}\)C) and 362.95 K (89.8 \({}^{\circ}\)C) for those components, respectively, when lubricating the coil and housing with the oil spray cooling method. Lower temperatures, 350.15 K (77 \({}^{\circ}\)C) for the rotor and 333.15 K (60 \({}^{\circ}\)C) for the stator, were reached using a hybrid cooling method (hollow shaft oil cooling + housing cooling). Automatic transmission fluids (ATFs) should ideally maintain the materials, magnetic, and electrical compatibilities, the copper corrosion protection, and avoid foaming and aeration over time, and thus achieve the desired \"fill-for-life\" status. For this reason, studying how the heat transfer characteristics change due to the aging of the ATFs and the influence of their formulation on these characteristics is very important. Today EV and HEV designs are also using conventional ATFs for ICEV (internal combustion engine vehicles) with acceptable performance, but this solution is not optimized for the performance of the EV and the HEV [3]. Effective cooling of the EM allows for a higher vehicle efficiency and therefore the heat dissipation requirement of the lubricant is increased. This demands higher thermal stability and improved heat transfer performances of the ATFs used in HEVs and EVs than of their conventional counterparts. Conversely, the use of low-viscosity lubricants in engines and transmissions is a current trend, intended to reduce churning and drag losses. However, these low-viscosity lubricants must also protect the hardware and  
Figure 1: Toyota Multistage Hybrid System [6].