Thermal characteristics and effects of oil spray cooling on in-wheel motors in electric vehicles

Highlights: • The effect of the in-wheel motor oil spray cooling with a hollow shaft cooing channel is studied. • A hollow shaft cooling channel suitable for in-wheel motor cooling was designed. • Oil spray cooling method shows a maximum of 16% decrease in absolute temperature at coil. • As the motor speed increases, a thick and uniform oil film is formed on the coil surface and the cooling effect is increased. -- Abstract: The output, efficiency, and durability of in-wheel motors in electric vehicles (EVs) depend on heat dissipation performance; therefore, effective active cooling is essential. However, investigating flows under high-speed conditions and predicting component temperatures that cannot be measured by direct contact requires a reliable numerical analysis model. In this study, the oil spray cooling of an in-wheel motor was compared with an in-wheel motor with only simple channel cooling, and to motors with conventional stagnant and circulating which is adapted conventional motors, and at base speed (4400 RPM) the coil absolute temperature induced by spraying was 25.0%, 11.6%, 15.8% lower than conventional methods, respectively. At base speed (4400 RPM) and maximum speed (11,000 RPM), the average heat transfer coefficients of the direct spray cooling area were 5270 Wm⁻² K⁻¹ and 10,849 Wm⁻² K⁻¹, respectively; the heat transfer coefficients of the secondary flow cooling area were 2372 Wm⁻² K⁻¹ and 8913 Wm⁻² K⁻¹. The oil film's cooling effect increased with increasing motor speed, and the ratio of the oil film's conduction heat transfer to the oil spray's convection heat transfer was 9.8 times higher at maximum speed that at base speed.