Study of Injection Method for Maximizing Oil-Cooling Performance of Electric Vehicle Motor with Hairpin Winding

energies

Article Study of Injection Method for Maximizing Oil-Cooling Performance of Electric Vehicle Motor with Hairpin Winding

Taewook Ha 1 and Dong Kyu Kim 1,2,*

1 School of Mechanical Engineering, Chung-Ang University, Seoul 06974, Korea; [email protected] 2 School of Computer Science and Engineering, Chung-Ang University, Seoul 06974, Korea * Correspondence: [email protected]; Tel.: +82-02-820-5192

Abstract: The oil injection method was studied to maximize the cooling performance of an electric vehicle motor with a hairpin winding. The cooling performance of the motor using the oil cooling method is proportional to the contact area of the oil and the coil. A numerical analysis was conducted to examine the effect of the spray nozzle type on the oil flow. The dripping nozzle forms the thickest oil film on the coil, making it the most effective for cooling of hairpin-type motors. Subsequently, an experimental study was conducted to optimize the nozzle diameter and number of nozzles. When the inlet diameter and number was 6.35 mm and 6, the oil film formation rate was 53%, yielding the most uniform oil film. Next, an experiment was performed to investigate the effects of the oil temperature and flow rate on the oil flow. The oil film formation rate was the highest (83%) when the oil temperature was 40 ◦C and the flow rate was 6 LPM.

Keywords: electric vehicle; hairpin winding; motor cooling; oil cooling; injection method

1. Introduction Citation: Ha, T.; Kim, D.K. Study of In recent years, interest in environmental issues has increased, accelerating the de- Injection Method for Maximizing velopment of electric vehicles and increasing their demand. In contrast to conventional Oil-Cooling Performance of Electric fuel vehicles, electric vehicles use only electricity and thus have no emissions [1]. The Vehicle Motor with Hairpin Winding. motor is the main power source of the electric vehicle and is the component that most Energies 2021, 14, 747. https:// signiﬁcantly affects the performance and life of the vehicle. Electric vehicle motors must doi.org/10.3390/en14030747 be high-power, and high efﬁciency for long and high-speed driving. In addition, because parts such as batteries and hydrogen tanks are located, they must be miniaturized. Thus, Academic Editor: Carlo Renno the heat-generation problem of motors is serious. The main heat losses of the motor are Received: 2 December 2020 joule losses, iron losses, stray load loss, and mechanical losses [2–4]. The power density Accepted: 26 January 2021 Published: 1 February 2021 should be increased to improve the motor performance. However, this increases the loss of the motor and the amount of heat generated. In particular, the coil generates a large

Publisher’s Note: MDPI stays neutral amount of heat owing to joule losses [5–7]. Therefore, effective cooling is important for with regard to jurisdictional claims in high-performance motors. The magnet of the rotor is demagnetized at high temperatures. published maps and institutional affil- In other words, when the temperature of the motor rises, the magnet is demagnetized, and iations. the drive stops and the performance decreases. Finally, the heat generated by the motor directly affects the performance and life of the electric vehicle. There are several methods to cool the motor such as air cooling, water cooling, and oil cooling method [8,9]. Thus far, motors have mainly used air and water cooling. In air cooling, a fan is employed to inject air into the motor and cool it through forced convec- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. tion [10]. Kim et al. [11] improved the cooling performance by optimizing the air-cooling This article is an open access article method of a brushless DC motor. By optimizing the inlet position and inner length of the distributed under the terms and rotor, the performance was improved by 24.3% compared with a reference brushless DC conditions of the Creative Commons motor. Kim et al. [12] analyzed the air-gap flow heating phenomena of a large-capacity Attribution (CC BY) license (https:// induction motor using an air-cooling system. In the large-capacity motor, the temperature creativecommons.org/licenses/by/ of the rotor was significantly higher than that of the stator. Hence, the air flowing between 4.0/). the rotor and the stator was heated by the rotor to a higher temperature than the stator.

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Thus, the stator was heated secondarily by air. The air gap of a motor exceeding 100 kW is in a superheating state; consequently, it is difficult to employ an air-cooling system. Electric vehicle motors that are at the same level as the performance of commonly used engine vehicles and capable of long-distance and high-speed driving use at least 100 kW of power [13], air cooling is not suitable. In water cooling, a coolant flows along a flow path in the housing and cools the motor through conduction with the housing [14]. Pechánek et al. [15] compared two types of water-cooling electric motors: axial water flow and tangential water flow. The tangential water flow had a lower water velocity, corresponding to a smaller heat-transfer coefficient between the frame and the water (1018.7 W/m2K). The axial water flow had a higher water velocity and a larger heat-transfer coefficient (1886.4 W/m2K). The higher water velocity resulted in better cooling performance. Rehman et al. [16] compared various coolant jackets with different numbers of flow passes. Considering the pump performance, a cooling jacket with six passes at a flow rate of 10 LPM had the best cooling performance. The maximum temperature was approximately 100 ◦C. For an electric-vehicle motor, a large amount of heat is generated at the coil. The water-cooling method is not appropriate for removing heat from the coil. This method has a low heat-transfer rate in the radial direction, because the motor is cooled indirectly through the housing. Direct cooling is more effective for removing the heat inside the motor [17]. Oil cooling involves spraying oil directly on the coil, which is a heat source inside the motor. This method is suitable for cooling high-performance motors. However, research on oil cooling for motors is insufficient. Huang et al. [18] compared the indirect cooling method with the direct cooling method. In the indirect cooling method, a water jacket was used, similar to the case of water cooling. In the direct cooling method, a groove was formed between the stator and the housing to directly cool the stator with cooling oil. Under similar cooling conditions, the motor with a direct cooling channel reduced the average temperature on the stator by a factor of approximately two, improving the heat-transfer performance. However, because there was insulating paper between the stator and the coil, the heat transfer in the radial direction was insufficient. This method is not suitable for the removal of heat from the coil. Ponomarev et al. [19] cooled the motor through a method in which the inside of the electric motor was completely immersed in cooling oil. The rotor and stator were cooled via direct contact with the coolant. Thus, the cooling performance of the motor was improved, and the maximum temperature of the coil was 124 ◦C. However, the performance of the motor was degraded by the friction loss between the cooling oil and the rotor, making this motor unsuitable for electric vehicles. Davin et al. [20] cooled the motor by spraying oil directly onto a coil. Different types of injectors were compared, including the full cone nozzle, flat jet nozzle, dripping, and multi-jets. When a dripping injector was used, the average temperature of the end winding was approximately 109 ◦C, and the best cooling performance was achieved. Additionally, a higher flow rate corresponded to better cooling performance at the end winding. However, previous studies on the motor using the oil-cooling method focused on general windings such as toroidal winding and multi-strand winding. These motors have limitations in miniaturization and power improvement compared to hairpin type motors. The motor using a hairpin winding was recently developed, which can improve the power density by reducing the amount of unnecessary space between coils [13,21]. As a result, the size of the motor can be reduced, and performance can be improved. The hairpin winding is suitable for miniaturized high-performance motors; however, because of the considerable heat generation, a new cooling method is needed. However, research on the cooling of motors using hairpin windings is lacking. In this study, the oil injection method was studied to maximize the cooling performance of an electric-vehicle motor with a hairpin winding. The Chevrolet Bolt EV motor, which uses the hairpin winding, was selected as the target motor. A parametric study is conducted to understand the injection conditions. It was selected as a variable of the factor that affects the behavior of oil, and the value of the variable to maximize the cooling Energies 2021, 14, x FOR PEER REVIEW 3 of 15

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that affects the behavior of oil, and the value of the variable to maximize the cooling effect was found. First, the effect of oil injection type on the oil film was compared through nu- mericaleffect analysis. was found. The First,types theof spray effect ofnozzles oil injection included type a ondripping the oil nozzle film was and compared 45° and through60° ◦ full-numericalcone nozzle. analysis. The effects The of types the nozzle of spray diameter nozzles and included the number a dripping of nozzles nozzle on and the 45for-and ◦ mation60 offull-cone the oil film nozzle. were The experimentally effects of the examined. nozzle diameter Subsequently, and the numberthe effects of of nozzles the flow on the rate formationand temperature of the oilof the film oil were on the experimentally formation of examined.the oil film Subsequently,were investigated. the effects Finally, of the we proposedflow rate andan optimal temperature oil injection of the oilmethod on the for formation an oil-cooling of the motor oil film using were a investigated. hairpin winding.Finally, Using we this proposed method, an a optimal uniform oil oil injection film can methodbe obtained for anon oil-coolingthe coil. The motor results using of a this hairpinstudy provide winding. important Using this information method, aregarding uniform oilthe film flow can characteristics be obtained and on the oil coil.film The formationresults of of oil this-cooling study providemotors importantwith hairpin information windings. regarding This provides the flow important characteristics infor- and mationoil filmfor conducting formation ofrelevant oil-cooling resea motorsrch, and with they hairpin can contribute windings. to the This development provides important of a highinformation-performance for motor conducting for electric relevant vehicles research, by providing and they can an contributeoptimal injection to the development method for improvingof a high-performance the oil-cooling motor performance. for electric vehicles by providing an optimal injection method for improving the oil-cooling performance. 2. Materials and Methods 2. Materials and Methods 2.1. Motor2.1. Motor Description Description FigureFigure 1 illustrates1 illustrates the thestator stator and and coil coil of the of the target target motor. motor. The The Chevrolet Chevrolet Bolt Bolt EV EV mo- motor tor fromfrom General General Motors Motors was selected asas thethe targettarget motor. motor. Figure Figure1a 1a shows shows the the motor motor geometry ge- ometryused used for thefor the numerical numerical analysis. analysis. The The geometry geometry of the of the motor motor was was simplified simplified to reduce to re- the duceanalysis the analysis time. time. The outer The outer coil had coil a had diagonal a diagonal shape similarshape similar to that to in that the actualin the motor.actual The motor.oil The flow oil was flow affected was affected by the coilby the shape. coil shape. The inside The ofinside the coilof the was coil simplified was simplified as a cylinder as a cylinderthat was that blocked was blocked between between the coils. the coils. Figure Figure1b shows 1b shows the motor the motor used inused the in experiment, the experiment,which which was a permanent-magnetwas a permanent-magnet synchronous synchronous motor withmotor a hairpinwith a hairpin winding. winding. The motor The hadmotor a peak had torquea peak oftorque 360 Nm of 360 and Nm a peak and power a peak of power 150 kW. of The150 kW. stack The length stack was length 125 mm, was and125 themm, outer and diameterthe outer was diameter 204 mm. was The 204 length mm. The of the length stator of used the instator the experimentused in the was experiment180 mm. was There 180 weremm. sixThere coil were conductors six coil perconductors slot of the per stator. slot of The the endstator. winding The end on the windingright on side the of right the motor side of was the calledmotor the was crown called part, the crown and the part, end and winding the end on thewinding left side on was the leftcalled side the was welded called part. the Therewelded was part. white There insulating was white paper insulating between paper the protruding between the coils of protrudingthe welded coils part, of the in welded contrast part, to the in crowncontrast part. to the crown part.

(a) (b)

FigureFigure 1. Target 1. Target motor: motor: (a) CAD (a) CAD geometry; geometry; and ( andb) motor (b) motor to be to tested. be tested. 2.2. Numerical Analysis 2.2. Numerical Analysis A numerical analysis was performed to compare the various types of spray nozzles.A numerical To analyze analysis the oilwas ﬂow, performed we used to thecompare Moving the Particlevarious Simulationtypes of spray (MPS) nozzles. method. To analyzeParticleWorks the oil® flow,was usedwe used for thethe analysis.Moving Particle The continuum Simulation equation (MPS) (massmethod. conservation Parti- ® cleWorkslaw) and was Navier–Stokes used for the equationsanalysis. The (momentum continuum conservation equation (mass law) were conservation used as the law) funda- and mentalNavier– governingStokes equations equations (momentum of the MPS conservation method: law) were used as the fundamen- tal governing equations of the MPS method: Dρ/Dt = 0 (1)

Du/Dt = −∇P/ρ + ν∇2u + g + ∇φ (2)

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Dρ/Dt = 0 (1) where Dρ/Dt represents a Lagrangian derivation, ρ represents the density, u represents the Du/Dt = −∇P/ρ + ν∇2u + g + ∇ϕ (2) velocity, P represents the pressure, ν is the kinematic viscosity coefficient, and g represents thewhere gravitational Dρ/Dt represents acceleration. a Lagrangian In the fluid derivation, analysis, theρ represents surface tension the density, potential u represents model (∇theφ) wasvelocity, used. P represents the pressure, ν is the kinematic viscosity coefficient, and g rep- resentsAs the the cooling gravitational oil, ATF acceleration. SP-IV—an automatic-transmissionIn the fluid analysis, the oil—was surface tension used. Figure potential2 presents the changes in the oil properties according to the temperature. Figure2a shows model (∇ϕ) was used. the oil density with respect to the temperature, and Figure2b shows the oil viscosity with As the cooling oil, ATF SP-IV—an automatic-transmission oil—was used. Figure 2 respect to the temperature. These oil properties are the important factors for analyzing presents the changes in the oil properties according to the temperature. Figure 2a shows the oil flow because they vary significantly with respect to the temperature. The density the oil density with respect to the temperature, and Figure 2b shows the oil viscosity with varies linearly with respect to the temperature; a higher temperature corresponds to a lowerrespect density. to the Thetemperature. viscosity These of the oil oil properties decreased are rapidly the important until the factors temperature for analyzing reached the 50oil◦C, flow after because which they it became vary significantly gentle. with respect to the temperature. The density varies linearly with respect to the temperature; a higher temperature corresponds to a lower density. The viscosity of the oilρ (T)decreased = −0.5988 rapidlyT + 1042 until the temperature reached 50 (3) °C , after which it became gentle. µ(T) = 2 × 10−13T6 − 4 × 10−10T5 + 4 × 10−7T4 − 0.0002T3 + 0.54T2 − 8.2514T + 522.74 (4)

(a) (b)

Figure 2. Cooling-oilCooling-oil properties: (a) density; and (b) viscosity. Equations (3) and (4) give the oil density and viscosity, respectively, as polynomial functions of the temperature T. The analysisρ(T) = − was0.5988 conductedT + 1042 with consideration of the oil(3) properties that vary with respect to the temperature. μ(T) = 2 × 10−13T6 – 4 × 10−10T5 + 4 × 10−7T4 − 0.0002T3 + 0.54T2 − 8.2514T + 522.74 (4) 2.3. Experimental Setup Equations (3) and (4) give the oil density and viscosity, respectively, as polynomial functionsThe experimental of the temperature equipment T. The for analysis the flow-visualization was conducted experiment with consideration of the motor of the is oil shownproperties in Figure that3 vary. The with motor respect used to in the temperature. experiment is shown in Figure1b. The cooling oil was transferred to the motor using an oil pump (TOP-210 HBE, DH PUMP Engineering Co.,2.3. Gimhae-si, Experimental Korea). Setup To control the temperature of the oil supplied, heat supplied by a boiler (Kiturami Electric Boiler, Kiturami, Cheongdo County, Korea) was transferred to The experimental equipment for the flow-visualization experiment of the motor is the oil through a heat exchanger. In an electric vehicle, as the motor is heated, the motor shown in Figure 3. The motor used in the experiment is shown in Figure 1b. The cooling is cooled with the oil temperature of 80 ◦C. The experiment in this paper is a motor flow oil was transferred to the motor using an oil pump (TOP-210 HBE, DH PUMP Engineering visualization experiment, and since no heat is generated in the motor, the oil temperature Co., Korea). To control the temperature of the oil supplied, heat supplied by a boiler (Kitu- does not rise. Therefore, we used a boiler to control the oil temperature. The temperature rami Electric Boiler, Kiturami) was transferred to the oil through a heat exchanger. In an was precisely controlled using a line heater. The oil flow rate was measured using a electric vehicle, as the motor is heated, the motor is cooled with the oil temperature of 80 °C. flowmeter (KTR-550-MF-T-Ex, Korea Flowmeter Ind. Co., Gunpo-shi, Korea), and the oil The experiment in this paper is a motor flow visualization experiment, and since no heat pump was controlled by an inverter (SV022iG5A-2, LS Electric Co., Anyang-si, Korea) to

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Energies 2021, 14, 747 is generated in the motor, the oil temperature does not rise. Therefore, we used a boiler5 of to 15 control the oil temperature. The temperature was precisely controlled using a line heater. The oil flow rate was measured using a flowmeter (KTR-550-MF-T-Ex, Korea Flowmeter Ind. Co.), and the oil pump was controlled by an inverter (SV022iG5A-2, LS Electric Co.) toset set the the oil oil ﬂow flow rate. rate. The The transferred transferred oil oil was was sprayed sprayed by by a nozzlea nozzle onto onto the the end end winding winding of ofthe the coil coil and and was was sprayed sprayed directly directly onto onto the the crown crown and and welded welded parts. parts.

(a) (b)

Figure 3. Experimental equipment: ( a) schematic; and ( b) photograph.

3.3. Results Results and and Discussion Discussion 3.1.3.1. Effect Effect of of Spray Spray Nozzle Nozzle on on Flow Flow Field Field BecauseBecause of of the the insulating insulating paper between the end windings of the welded part, the oil diddid not pass between the coils and the oil film film was evenly formed. In contrast, the crown part had an empty space between the coils; thus, oil passed through it. The oil flow flow in the crowncrown part was moremore difficultdifficult to to analyze analyze than than that that in in the the welded welded part. part. Therefore, Therefore, we we studied studiedthe the formation formation of theof the oil oil film film by by analyzing analyzing the the oil oil flow flow in in the the crown crown part. part. AA numerical analysisanalysis was was conducted conducted to to investigate investigate the the effect effect of of the the spray spray-nozzle-nozzle type type on on the the flow flow field. field. ◦ ◦ The spray nozzles nozzles examined included a dripping nozzle an andd 45° 45 andand 60° 60 fullfull-cone-cone nozzles. nozzles. The full full-cone-cone nozzles had significant significant oil splashing; thus, it it was was difficult difficult to to observe observe directly directly inin the the experiment. experiment. To To compare compare the the exact exact oil oil flow flow field field and and the the amount amount of oil of splashing, oil splashing, we usedwe used the analytical the analytical method. method. Figure Figure 4 shows4 shows the theflow flow field field and andoil velocity oil velocity in the in front the front and crossand cross sections sections of the of coil. the In coil. front In of front the coil, of the the coil, flow the fields flow of fields the three of the nozzles three nozzleswere similar. were similar. The oil flowed in the oblique direction of the coil owing to the coil shape. Figure4a The oil flowed in the oblique direction of the coil owing to the coil shape. Figure 4a shows shows the oil flow with the dripping nozzle. The oil flowed well along the outer surface of the oil flow with the dripping nozzle. The oil flowed well along the outer surface of the the coil, and the oil did not splash after hitting the coil. Additionally, the oil film formed coil, and the oil did not splash after hitting the coil. Additionally, the oil film formed on on the coil was thicker than those for the other nozzles. Figure4b,c shows the oil flow the coil was thicker than those for the other nozzles. Figure 4b,c shows the oil flow for the for the 45◦ and 60◦ full-cone nozzles. A larger spray angle corresponded to a wider area 45° and 60° full-cone nozzles. A larger spray angle corresponded to a wider area over over which the oil was sprayed onto the coil. However, in contrast to the dripping nozzle, which the oil was sprayed onto the coil. However, in contrast to the dripping nozzle, a a large amount of oil splashed after hitting the coil. This is because the oil inlet velocity large amount of oil splashed after hitting the coil. This is because the oil inlet velocity was was high. To accurately compare the amounts of oil splashing and the flow through the high. To accurately compare the amounts of oil splashing and the flow through the coil, coil, the results were quantified according to the width and thickness of the oil film. The the results were quantified according to the width and thickness of the oil film. The oil oil film formation rate was used to quantify the oil film formed on the coil. It was defined filmas the formation area where rate the was oil used film to was quantify formed the to oil the film total formed surface on area the of coil. the It coil. was The defined oil film as the area where the oil film was formed to the total surface area of the coil. The oil film formation rate Roilfilm is given as follows: formation rate Roilfilm is given as follows: R = A /A (5) oilfilmRoilfilm = oilAoil/Acoilcoil (5)

where Acoil representsrepresents the the total total surface surface area area of of the coil, and Aoil representsrepresents the the contact contact area betweenbetween the the coil and the oil. Q = k · A · ∆T (6) Q = C · m · ∆T (7)

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Energies 2021, 14, 747 6 of 15 Q = k ∙ A ∙ ∆T (6)

Q = C ∙ m ∙ ∆T (7)

(a) (b) (c)

Figure 4. CrossFigure section 4. Cross of coil: section (a) drippingof coil: (a) nozzle; dripping (b )nozzle; full cone (b) nozzlefull cone (spray nozzle angle: (spray 45 angle:◦); (c) 45°); full cone(c) full nozzle (spray angle: 60◦). cone nozzle (spray angle: 60°).

Figure 5a showsFigure the 5oila showsfilm formation the oil film rate formation and Figure rate 5b and shows Figure the5 oilb shows film thickness. the oil film thickness. The dripping Thenozzle dripping and 45° nozzle and 60° and full 45-cone◦ and nozzles 60◦ full-cone had oil film nozzles formation had oil rates film of formation56%, rates of 58% and 60%,56%, respectively. 58% and 60%, The respectively. nozzles had The similar nozzles oil film had similar formation oil film rates formation and flow rates and flow fields. This is fields.because This the isoil because flow was the affected oil flow by was the affectedcoil shape. by The the coiloil film shape. thickness The oil of film thickness the full-cone nozzlesof the full-conewas approximately nozzles was 1 mm. approximately The oil film 1thickness mm. The of oil the film dripping thickness nozzle of the dripping was 2.5 mm (approximatelynozzle was 2.5 2.5 mm times (approximately larger than that 2.5 timesof the largerfull-cone than nozzles). that of The the drip- full-cone nozzles). ping nozzle hadThe the dripping thickest nozzle oil film. had The the full thickest-cone oil nozzles film. Thesprayed full-cone the oil nozzles more sprayedwidely the oil more than the drippingwidely nozzles, than the increasing dripping the nozzles, oil film increasing formation the rate. oil However, film formation they did rate. not However, they form a thick oildid film not because form a thickof the oil large film amount because of of oil the that large splashed amount after of oilhitting that splashedthe coil. after hitting In contrast, thethe dripping coil. In contrast,nozzle had the little dripping oil splashing nozzle had and little formed oil splashing the thickest and oil formed film. the thickest According to oilEquation film. According (6), the cooling to Equation performance (6), the cooling is proportional performance to the is proportionalcontact area to the contact between the oilarea and between the coil. the According oil and the to coil.Equation According (7), the to cooling Equation performance (7), the cooling is pro- performance is Energies 2021, 14, x FOR PEER REVIEWproportional to the contact capacity between the oil and the coil. The area of the oil film7 of 15 portional to the contact capacity between the oil and the coil. The area of the oil film was proportional towas the proportional amount of heat to the transferred, amount of and heat the transferred, thickness of and the the oil thickness film was ofpro- the oil film was portional to theproportional heat capacity to the. Thus, heat the capacity. use of the Thus, dripping the use nozzle of the increase drippings nozzlethe heat increases ca- the heat pacity, and thiscapacity, nozzle andis optimal this nozzle for motor is optimal cooling. for motor cooling.

(a) (b)

FigureFigure 5. Oil 5. Oil film film for for different different spray-nozzle spray-nozzle types: types: (a) ( oila) filmoil film formation formation rate; rate; (b) ( oilb) filmoil film thickness. thickness.

3.2. Effect of Inlet Diameter and Number of Inlets on Flow Field The foregoing analysis indicated that among the nozzle types considered, the dripping nozzle is most effective for oil injection. An experimental study was conducted to optimize the diameter and number of inlets. The oil temperature was 60 °C , and the flow rate was 1 LPM. The oil was sprayed through the dripping nozzle. Figure 6 shows the experimental results for the oil film formation rate. We waited until the flow field of oil reached dynamic equilibrium and took photographs. Then, we calculated the exact area of each area using the photograph. Through the comparison with numerical analysis, the results of oil film formation rate obtained by calculation using photographs and the numerical results are 3% of error. First, for the same diameter, the effect of the number of inlets on the oil film formation rate was examined. When the inlet diameter was 3.175 mm, the oil film formation rate increased with the number of inlets. The oil inlet velocity varied with respect to the number of inlets. When the number of inlets was 8, 6 and 4, the velocity was 0.527, 0.351 and 0.263 m/s, respectively. A smaller number of inlets corresponded to a higher oil inlet velocity. Consequently, the oil film did not form well. For the inlet diameter of 6.35 mm, the most uniform oil film was formed when the number of inlets was 6. In contrast to the other conditions, the inlets were located at the left, right and middle; thus, the oil was sprayed evenly. The oil flow rate was low (0.088 m/s); thus, there was no oil splashing. Consequently, the oil film was formed evenly throughout the coil. When the inlet diameter was 9.525 mm, the oil film formation rate decreased as the number of inlets increased. The oil inlet rate was the lowest (0.029–0.059 m/s) for this diameter, with no oil splashing. However, owing to the wide inlets, the amounts of oil injected were not identical among the inlets. As the number of inlets increased, the amounts of oil injected at the inlets became more unbalanced, making it difficult to form the oil film. Figure 7 shows experimental photographs taken when the oil film was well formed for each inlet diameter. Figure 7a presents the experimental results for the inlet diameter of 3.175 mm and 8 inlets. Oil splashing occurred on the wall because the oil velocity was 4–9 times faster than those for the other inlet diameters. Figure 7b presents the experimental results for the inlet diameter of 6.35 mm and 6 inlets. As shown, the oil was sprayed evenly at each inlet. Addi- tionally, in contrast to the other inlet-number conditions, the inlets were located at the left, right and middle; thus, the oil was evenly sprayed throughout the coil. Figure 7c shows the experimental results for the inlet diameter of 9.525 mm and 4 inlets. The oil inlet velocity was the lowest for this condition; thus, there was no oil splashing. However, the

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3.2. Effect of Inlet Diameter and Number of Inlets on Flow Field The foregoing analysis indicated that among the nozzle types considered, the dripping nozzle is most effective for oil injection. An experimental study was conducted to optimize the diameter and number of inlets. The oil temperature was 60 ◦C, and the flow rate was 1 LPM. The oil was sprayed through the dripping nozzle. Figure6 shows the experimental results for the oil film formation rate. We waited until the flow field of oil reached dynamic equilibrium and took photographs. Then, we calculated the exact area of each area using the photograph. Through the comparison with numerical analysis, the results of oil film formation rate obtained by calculation using photographs and the numerical results are 3% of error. First, for the same diameter, the effect of the number of inlets on the oil film formation rate was examined. When the inlet diameter was 3.175 mm, the oil film formation rate increased with the number of inlets. The oil inlet velocity varied with respect to the number of inlets. When the number of inlets was 8, 6 and 4, the velocity was 0.527, 0.351 and 0.263 m/s, respectively. A smaller number of inlets corresponded to a higher oil inlet velocity. Consequently, the oil film did not form well. For the inlet diameter of 6.35 mm, the most uniform oil film was formed when the number of inlets was 6. In contrast to the other conditions, the inlets were located at the left, right and middle; thus, the oil was sprayed evenly. The oil flow rate was low (0.088 m/s); thus, there was no oil splashing. Consequently, the oil film was formed evenly throughout the coil. When the inlet diameter was 9.525 mm, the oil film formation rate decreased as the number of inlets increased. The oil inlet rate was the lowest (0.029–0.059 m/s) for this diameter, with no oil splashing. However, owing to the wide inlets, the amounts of oil injected were not identical among the inlets. As the number of inlets increased, the amounts of oil injected at the inlets became more unbalanced, making it difficult to form the oil film. Figure7 shows experimental photographs taken when the oil film was well formed for each inlet diameter. Figure7a presents the experimental results for the inlet diameter of 3.175 mm and 8 inlets. Oil splashing occurred on the wall because the oil velocity was 4–9 times faster than those for the other inlet diameters. Figure7b presents the experimental results for the inlet diameter of 6.35 mm and 6 inlets. As shown, the oil was sprayed evenly at each inlet. Additionally, in contrast to the other inlet-number conditions, the inlets were located at Energies 2021, 14, x FOR PEER REVIEW 8 of 15 the left, right and middle; thus, the oil was evenly sprayed throughout the coil. Figure7c shows the experimental results for the inlet diameter of 9.525 mm and 4 inlets. The oil inlet velocity was the lowest for this condition; thus, there was no oil splashing. However, the amountamount of of oil oil injected injected was was not not constant, constant, owing owing to to the the large large inlet inlet diameter. diameter. Thus, Thus, when when the the inletinlet diameter diameter was was 6.35 6.35 mm mm and and the the number number of of inlets inlets was was 6, 6, the the oil oil film film formation formation rate rate was was 53%,53%, and and the the oil oil film film formation formation was was optimal. optimal.

FigureFigure 6. 6. OilOil film ﬁlm formation formation rate rate according according to to the the number number and and diameter diameter of inlets.

(a)

(b)

(c)

Figure 7. Oil flow according to the number and diameter of inlets: (a) 3.175 mm and 8 inlets; (b) 6.35 mm and 6 inlets; (c) 9.525 mm and 4 inlets.

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amount of oil injected was not constant, owing to the large inlet diameter. Thus, when the inlet diameter was 6.35 mm and the number of inlets was 6, the oil film formation rate was 53%, and the oil film formation was optimal.

Energies 2021, 14, 747 8 of 15 Figure 6. Oil film formation rate according to the number and diameter of inlets.

(a)

(b)

(c)

FigureFigure 7. 7. OilOil flow ﬂow according according to the to thenumber number and anddiameter diameter of inlets: of inlets: (a) 3.175 (a) mm 3.175 and mm 8 inlets; and 8 ( inlets;b) 6.35(b) 6.35mm mmand and6 inlets; 6 inlets; (c) 9.525 (c) 9.525 mm mm and and4 inlets. 4 inlets. 3.3. The Effects of Oil Flow Rate on Flow Field

The use of six dripping nozzles with a diameter of 6.35 mm at the inlet yielded the optimal formation of the oil film. According to the given inlet geometry, the effect of the oil flow rate on the flow field was analyzed experimentally. The temperature of the oil was ◦ 60 C. Figure8 shows the experimental results for the oil film formation rate. The error of the oil film formation rate was approximately 4.6%. The oil film formation rate was the lowest (63%) when the oil flow rate was 2 LPM. When the oil flow rate was 4 LPM, the oil film formation rate was 70% (7% higher than that at 2 LPM). When the oil flow rate was 6 LPM, the oil film formation rate was 75% (approximately 5% higher than that at 4 LPM), which was the highest rate observed in the experiment. As the oil flow rate increased, the oil film formation rate increased. However, the increase in the oil film formation rate gradually decreased as the flow rate increased. Figure9 shows the flow-visualization experiment results for each oil flow rate. Figure9a shows the oil flow when the oil flow rate was 2 LPM. The oil inlet velocity was 0.176 m/s, which was lower than those of the other flow-rate conditions, because the flow rate was low. Little oil splashing occurred after the oil was sprayed onto the coil, and the oil flowed well along the coil. Figure9b shows the oil flow when the oil flow rate was 4 LPM. Even at 4 LPM, the oil inlet velocity was low (0.351 m/s); thus, there was almost no oil splashing. However, because the amount of oil increased, the oil flowed downward between the coils. This oil did not flow along the coils but was sprayed onto the lower coil to form the oil film. Compared with the case of 2 LPM, the formation of the oil film was better, because the flow rate was higher. Figure9c shows the oil flow when the flow rate was 6 LPM. The oil inlet velocity at 6 LPM was 0.527 m/s, which was three times higher than that at 2 LPM. Energies 2021, 14, x FOR PEER REVIEW 9 of 15

3.3. The Effects of Oil Flow Rate on Flow Field The use of six dripping nozzles with a diameter of 6.35 mm at the inlet yielded the optimal formation of the oil film. According to the given inlet geometry, the effect of the oil flow rate on the flow field was analyzed experimentally. The temperature of the oil Energies 2021, 14, 747 was 60 °C . Figure 8 shows the experimental results for the oil film formation rate.9 The of 15 error of the oil film formation rate was approximately 4.6%. The oil film formation rate was the lowest (63%) when the oil flow rate was 2 LPM. When the oil flow rate was 4 LPM, theAs theoil film oil flow formation rate increased, rate was oil70% splashing (7% higher occurred than that onthe at 2 wall. LPM). Additionally, When the oil the flow amount rate wasof oil 6 thatLPM, flowed the oil downwardfilm formation between rate was the coils75% (approxim increased.ately This 5% oil formedhigher than a film that on at the 4 LPM),lower coil.which As was the flowthe highest rate increased, rate observed the amount in the of experiment. oil splashing As increased; the oil flow however, rate thein- creased,amount the of oil oil forming film formation the oil film rate also increased. increased. However, Thus, thethe oilincrease film formation in the oil improved film for- mationwith the rate increase gradually in the decreased oil flow rateas the and flow was rate optimized increased. at6 LPM.

Energies 2021, 14, x FOR PEER REVIEW 10 of 15

◦ FigureFigure 8. 8. OilOil film ﬁlm formation formation rate rate according according to to the the mass mass flow ﬂow rate rate at at 60 60 °C.C.

Figure 9 shows the flow-visualization experiment results for each oil flow rate. Figure 9a shows the oil flow when the oil flow rate was 2 LPM. The oil inlet velocity was 0.176 m/s, which was lower than those of the other flow-rate conditions, because the flow rate was low. Little oil splashing occurred after the oil was sprayed onto the coil, and the oil flowed well along the coil. Figure 9b shows the oil flow when the oil flow rate was 4 LPM. Even at 4 LPM, the oil inlet velocity was low (0.351 m/s); thus, there was almost no oil splashing.

However, because the amount of oil increased, the oil flowed downward between the coils. This oil did not flow along(a the) coils but was sprayed onto the lower coil to form the oil film. Compared with the case of 2 LPM, the formation of the oil film was better, because the flow rate was higher. Figure 9c shows the oil flow when the flow rate was 6 LPM. The oil inlet velocity at 6 LPM was 0.527 m/s, which was three times higher than that at 2 LPM. As the oil flow rate increased, oil splashing occurred on the wall. Additionally, the amount of oil that flowed downward between the coils increased. This oil formed a film on the lower coil. As the flow rate increased, the amount of oil splashing increased; however, the amount of oil forming the oil film also increased. Thus, the oil film formation improved with the increase in the oil flow rate and was optimized at 6 LPM.

(b)

(c)

◦ FigureFigure 9. 9. OilOil flow ﬂow according according to to the the mass mass flow ﬂow rate rate at at 60 60 °C:C: ( (aa)) 2 2 LPM; LPM; ( (bb)) 4 4 LPM; LPM; ( (cc)) 6 6 LPM. LPM.

3.4. Effect of Oil Temperature on Flow Field The viscosity of oil changes rapidly with changes in the temperature; thus, it was important to investigate the effect of the oil temperature on the oil flow. An experiment was conducted to examine the effect of the oil temperature on the flow field. Figure 10 shows the experimental results for the oil film formation rate at a flow rate of 6 LPM. The error of the oil film formation rate was approximately 4%. When the oil temperature was 20 °C , the oil film formation rate was 80%. The oil film formation rate was the highest (83%) at 40 °C . The oil film formation rate at 60 °C was 75%, which was 8% lower than that at 40 °C. The oil film formation rate was the lowest (70%) at 80 °C . The oil film formation rate depended on the temperature. When the temperature increased beyond 40 °C , the oil film formation rate decreased, and the reduction rate was the highest between 40 and 60 °C .

Energies 2021, 14, 747 10 of 15

3.4. Effect of Oil Temperature on Flow Field The viscosity of oil changes rapidly with changes in the temperature; thus, it was important to investigate the effect of the oil temperature on the oil flow. An experiment was conducted to examine the effect of the oil temperature on the flow field. Figure 10 shows the experimental results for the oil film formation rate at a flow rate of 6 LPM. The error of the oil film formation rate was approximately 4%. When the oil temperature was 20 ◦C, the oil film formation rate was 80%. The oil film formation rate was the highest (83%) at 40 ◦C. The oil film formation rate at 60 ◦C was 75%, which was 8% lower than that at 40 ◦C. The oil film formation rate was the lowest (70%) at 80 ◦C. The oil film formation rate Energies 2021, 14, x FOR PEER REVIEW ◦ 11 of 15 depended on the temperature. When the temperature increased beyond 40 C, the oil film formation rate decreased, and the reduction rate was the highest between 40 and 60 ◦C.

FigureFigure 10. 10. OilOil film ﬁlm formation formation rate rate according according to to the the temperature temperature at at 6 6 LPM. LPM.

◦ ◦ FigureFigure 11a11a shows shows the the results results for for an an oil oil temperature temperature of 20 °CC.. At At 20 20 °CC,, the the oil oil viscosity viscosity waswas 0.0547 kg/m··s.s. Owing Owing to thethe highhigh viscosity,viscosity, almost almost no no oil oil splashed splashed after after hitting hitting the coil.the coil.Additionally, Additionally, the oil the flowed oil flowed well alongwell along the coil. the The coil. oil The film oil was film formed was formed on the outeron the surface outer surfaceof the coil; of the however, coil; however, the oil filmthe oil was film not was well not formed well formed between between the coils. theFigure coils. Figure11b shows 11b the results for an oil temperature of 40 ◦C. At 40 ◦C, the oil viscosity was 0.0222 kg/m·s shows the results for an oil temperature of 40 °C .◦ At 40 °C , the oil viscosity was 0.0222 kg/m·s (approximately(approximately 2.5 2.5 times times lower lower than than that that at at 20 20 °CC).). However, However, the the oil oil was was still still very very viscous; viscous; thus, little oil splashed, and the oil flowed well along the coil. Compared with the case thus, little◦ oil splashed, and the oil flowed well along the coil. Compared with the case of 20of °C 20, theC, the oil oilfilm film was was thinner thinner but but more more uniform. uniform. FigFigureure 11 11c,dc,d showshow the the results results for for oil oil temperatures of 60 and 80 ◦C, respectively. The oil viscosities at these temperatures were temperatures of 60 and 80 °C , respectively. The oil viscosities at these temperatures were 0.0113 and 0.0067 kg/m·s, respectively. As the temperature increased, the viscosity of the 0.0113 and 0.0067 kg/m·s, respectively. As the temperature increased, the viscosity of the oil decreased, and the amount of oil that splashed after hitting the coil increased. At 80 ◦C, oil decreased, and the amount of oil that splashed after hitting the coil increased. At 80 °C , more oil splashed compared with 60 ◦C, and a large amount of oil was lost. Thus, when more oil splashed compared with 60 °C , and a large amount of oil was lost. Thus, when the oil flow rate was 6 LPM, the oil film was optimal at the oil temperature of 40 ◦C. the oil flow rate was 6 LPM, the oil film was optimal at the oil temperature of 40 °C . 3.5. Studied Oil Injection Method The correlation between the oil flow rate and the temperature on the oil film formation rate was analyzed. Figure 12 shows the oil film formation rate under each oil condition. When the oil temperature increased at the same flow rate, the oil film formation rate was significantly affected by the viscosity of the oil. When the oil flow rate was 2 LPM, the oil inlet velocity was low; thus, little oil splashed after hitting the coil. There was no oil splashing even when the viscosity was reduced; thus, a higher temperature corresponded to a higher oil film formation rate. In contrast, when the oil flow rate was >4 LPM, the ◦ ◦ formation of the(a) oil film was optimal at 40 C.( Whenb) the oil temperature was 20 C, the viscosity was high, and the oil film was thick. When the oil temperature was 40 ◦C, the oil film was thinner but more uniform than that at 20 ◦C. When the oil temperature was >60 ◦C, the oil viscosity decreased significantly; thus, the amount of oil that splashed after

Energies 2021, 14, x FOR PEER REVIEW 11 of 15

Figure 10. Oil film formation rate according to the temperature at 6 LPM.

Figure 11a shows the results for an oil temperature of 20 °C . At 20 °C , the oil viscosity Energies 2021, 14, 747 was 0.0547 kg/m·s. Owing to the high viscosity, almost no oil splashed after hitting11 ofthe 15 coil. Additionally, the oil flowed well along the coil. The oil film was formed on the outer surface of the coil; however, the oil film was not well formed between the coils. Figure 11b shows the results for an oil temperature of 40 °C . At 40 °C , the oil viscosity was 0.0222 kg/m·s (approximatelyhitting the coil increased,2.5 times lower reducing than thethat oil at film20 °C formation). However, rate. the The oil oilwas film still formation very viscous; rate thus,increased little withoil splashed, the flow and rate, the regardless oil flowed of thewell oil along temperature. the coil. Compared The viscosity with of thethe oilcase at of 20 ◦ 20and °C 40, theC oil was film high; was thus, thinner even but when more the uniform. flow rate Fig increased,ure 11c,d there show was the no results oil splashing, for oil ◦ temperaturesand the oil film of 60 was and well 80 formed.°C , respectively In contrast,. The theoil viscosities viscosity at at 80 theseC wastemperatures very low; were thus, Energies 2021, 14, x FOR PEER REVIEW0.0113as the and flow 0.0067 rate increased, kg/m·s, respectively. the oil inlet velocityAs the temperature increased, and increased, the amount the viscosity of splashing 12of ofthe oil15

oilincreased. decreased, Therefore, and the evenamount when of oil the that flow splashed rate increased, after hitting the oil the film coil formation increased. rate At did 80 °C not, moreincrease oil splashed significantly. com Accordingly,pared with 60 the °C formation, and a large of theamount oil film of wasoil was optimal lost. whenThus, thewhen oil temperature was 40 ◦C and the flow rate was 6 LPM. the3.5. oilStudied flow Oilrate Injection was 6 LPM, Method the oil film was optimal at the oil temperature of 40 °C . The correlation between the oil flow rate and the temperature on the oil film formation rate was analyzed. Figure 12 shows the oil film formation rate under each oil condition. When the oil temperature increased at the same flow rate, the oil film formation rate was significantly affected by the viscosity of the oil. When the oil flow rate was 2 LPM, the oil inlet velocity was low; thus, little oil splashed after hitting the coil. There was no oil splashing even when the viscosity was reduced; thus, a higher temperature corresponded to a higher oil film formation rate. In contrast, when the oil flow rate was >4 LPM, the formation of the oil film was optimal at 40 °C . When the oil temperature was 20 °C , the viscosity was high, and the oil(a) film was thick. When the oil( btemperature) was 40 °C , the oil film was thinner but more uniform than that at 20 °C . When the oil temperature was >60 °C , the oil viscosity decreased significantly; thus, the amount of oil that splashed after hitting the coil increased, reducing the oil film formation rate. The oil film formation rate increased with the flow rate, regardless of the oil temperature. The viscosity of the oil at 20 and 40 °C was high; thus, even when the flow rate increased, there was no oil splashing, and the oil film was well formed. In contrast, the viscosity at 80 °C was very low; thus, as the flow rate increased, the oil inlet velocity increased, and the amount of splashing oil increased. Therefore, even when the flow rate increased, the oil film formation rate did not increase significantly. Accordingly,(c) the formation of the (oild) film was optimal when the oil temperature was 40 °C and the flow rate was 6 LPM. FigureFigure 11. 11. OilOil flow flow according according to to the the temperature temperature at at 6 6 LPM: LPM: ( (aa)) 20 20 °C;◦C; ( (bb)) 40 40 °C;◦C; ( (cc)) 60 60 °C;◦C; (d (d) )80 80 °C.◦C.

FigureFigure 12. 12. OilOil film ﬁlm formation formation rate rate according according to to the the temperature temperature at at each each mass mass flow ﬂow rate.

FigFiguresures 13 13 and and 14 show14 show the results the results of applying of applying the oil-cooling the oil-cooling method with method the optimized with the oiloptimized injection oil conditions. injection conditions.A dripping A nozzle dripping was nozzle used as was the used spray as thenozzle, spray and nozzle, the inlet and diameterthe inlet diameterand number and of number inlets were of inlets 6.35 were mm 6.35and mm6, respectively. and 6, respectively. The nozzles The were nozzles located were atlocated the top at of the the top end of thewinding end winding of the coil, of the and coil, the and crown the crownand welded and welded parts each parts had each three had nozzles.three nozzles. At each At part, each the part, nozzles the nozzles were placed were placedat the left, at the right, left, and right, middle and middle sides of sides the of the coil. The inlet oil temperature and flow rate were 40 ◦C and 6 LPM, respectively. coil. The inlet oil temperature and flow rate were 40 °C and 6 LPM, respectively. Figure 13a Figure 13a shows the oil flow in the crown part. No oil splashed after hitting the coil, and shows the oil flow in the crown part. No oil splashed after hitting the coil, and the oil the oil flowed well along the coil. The oil flow was affected by the diagonal direction of the flowed well along the coil. The oil flow was affected by the diagonal direction of the coil. coil. The oil film formation rate was 83%. Figure 13b shows the oil flow in the welded part. The oil film formation rate was 83%. Figure 13b shows the oil flow in the welded part. Here, the oil flowed well, without splashing. In contrast to the crown part, the welded part had insulating paper between the coils. Because of the insulating paper, the oil film was formed more evenly than that in the crown part. The oil film formation rate was 90% for the welded part (7% higher than that for the crown part). Figure 14 shows the results of the numerical analysis of the crown part, which support the results in Figure 13a and provide details regarding the oil flow over the coil. The oil flow was similar between the two cases; however, there was a slight difference due to the simplification of the geometry.

Energies 2021, 14, 747 12 of 15

Here, the oil flowed well, without splashing. In contrast to the crown part, the welded part had insulating paper between the coils. Because of the insulating paper, the oil film was formed more evenly than that in the crown part. The oil film formation rate was 90% Energies 2021, 14, x FOR PEER REVIEWfor the welded part (7% higher than that for the crown part). Figure 14 shows the results13 of 15 Energies 2021, 14, x FOR PEER REVIEW 13 of 15 of the numerical analysis of the crown part, which support the results in Figure 13a and provide details regarding the oil flow over the coil. The oil flow was similar between the twoFig cases;ure however,14a,c show theres the was oil aflow slight at differencethe right and due left to thenozzles, simplification respectively. of the The geometry. oil flowed FigFigureurewell 14a 14 alonga,c,c showshows thes obliquethe the oil oil flow flowdirection at at the the of right right the andcoil, and left leftwithout nozzles, nozzles, splashing. respectively. respectively. As the The Theoil oilflowed oil flowed flowed down- wellwell ward,along along it the was obliqueoblique evenly directiondirection distributed of of the the on coil, coil,the without coil.without Figure splashing. splashing. 14b shows As As the the oil flowedoil flowedflow downward, at down-the middle ward,it wasnozzle. it evenly was The evenly distributed oil flowed distributed onalong the onthe coil. the coil Figure coil. without Figure 14b splashing. shows 14b shows the However, oil the flow oil at inflow the contrast middleat the tomiddle nozzle. the actual nozzle.Thesituation, oil The flowed oil there flowed along was the along no coil empty the without coil space without splashing. between splashing. However, the coils; However, inthus, contrast no in oil contrast to passed the actual through.to the situation, actual situation,there was there no empty was no space empty between space between the coils; the thus, coils; no thus, oil passed no oil through.passed through.

(a) (a)

(b) (b)

Figure 13. Overall oil flow at◦ 40 °C , 6 LPM: (a) crown part; and (b) welded part. FigureFigure 13. 13. OverallOverall oil oil flow ﬂow at at 40 40 °CC,, 66 LPM: LPM: (a (a) )crown crown part; part; and and (b (b) )welded welded part. part.

(a) (b) (c) (a) (b) (c) Figure 14. Oil ﬂow at each nozzle: (a) right; (b) middle; (c) left. Figure 14. Oil flow at each nozzle: (a) right; (b) middle; (c) left. Figure 14. Oil flow at each nozzle: (a) right; (b) middle; (c) left. 4. Conclusions 4. Conclusions The oil injection method was studied to maximize the cooling performance of an elec- tricThe-vehicle oil injection motor method using awas hairpin studied winding. to maximize The Chevrolet the cooling Bolt performance EV motor (Chevrolet, of an elec- US), tric-vehiclewhich usesmotor a hairpinusing a winding,hairpin winding. was selected The Chevrolet as the target Bolt motor. EV motor To maximize (Chevrolet, the U coolingS), whichperformance uses a hairpin of the winding, motor, wasthe oil selected must beas evenlythe target distributed, motor. To because maximize the theamount cooling of heat performancetransfer is of proportional the motor, the to theoil mustcontact be area evenly between distributed, the oil becauseand the coil.the amountTherefore, of heatanalyses transferand is experiments proportional were to the performed contact area to betweeninvestigate the the oil andeffects the of coil. various Therefore, conditions analyses on the and formationexperiments of werethe oil performed film. Finally, to investigate the geometry the andeffects conditions of various of theconditions inlet were on studiedthe formationfor maximizing of the oil thefilm. width Finally, of the the oil geometry film. and conditions of the inlet were studied for maximizing the width of the oil film.

Energies 2021, 14, 747 13 of 15

4. Conclusions The oil injection method was studied to maximize the cooling performance of an electric-vehicle motor using a hairpin winding. The Chevrolet Bolt EV motor (Chevrolet, Detroit, MI, USA), which uses a hairpin winding, was selected as the target motor. To maximize the cooling performance of the motor, the oil must be evenly distributed, because the amount of heat transfer is proportional to the contact area between the oil and the coil. Therefore, analyses and experiments were performed to investigate the effects of various conditions on the formation of the oil film. Finally, the geometry and conditions of the inlet were studied for maximizing the width of the oil film. First, a numerical analysis was conducted to compare different types of spray nozzles. When 45◦ and 60◦ full-cone nozzles were used, the oil was sprayed widely and formed a wide film. However, the oil film was as thin as 1 mm, because a large amount of oil splashed after hitting the coil. A dripping nozzle formed an oil film similar to that for the full-cone nozzles. However, the oil film had a thickness of 2.5 mm. Thus, the use of the dripping nozzle increased the heat capacity, and this nozzle is the best type for cooling. Subsequently, an experiment was conducted to analyze the effects of the number of dripping nozzles and the nozzle diameter on the oil flow. A smaller inlet diameter corresponded to a higher oil inlet velocity and a larger amount of splashed oil. With an increase in the inlet diameter, the oil flow rates at the different inlets differed, resulting in a non-uniform oil film. When the inlet diameter was 3.175 mm, the oil film formation rate decreased as the number of inlets decreased. This is because the oil inlet velocity increased when the number of inlets decreased. Consequently, the amount of oil that splashed after hitting the coil increased. When the inlet diameter was 9.525 mm, the oil film formation rate decreased as the number of inlets increased. This is because the inlets were wide, and the oil flow rates differed among the inlets. Lastly, in the case of the 6.35 mm inlet diameter, the formation of the oil film was optimal (oil film formation rate of 53%) when the number of inlets was six. Next, an experiment was conducted to analyze the effects of the oil temperature and flow rate on the oil flow for the studied inlet geometry. As the flow rate increased, the oil inlet velocity increased. Consequently, the amounts of oil that splashed and flowed downward through the coils increased. The oil that flowed downward between the coils was sprayed onto the lower coil to form the oil film. Thus, the formation of the oil film was improved when the oil flow rate increased. The viscosity of the oil changed rapidly as the temperature increased. When the oil temperature was 20 ◦C, the viscosity was high, and the oil film was thick. When the oil temperature was 40 ◦C, the oil film was thinner but more uniform than that at 20 ◦C. When the oil temperature reached >60 ◦C, the viscosity of the oil decreased significantly, and the amount of oil that splashed after hitting the coil increased. Thus, the formation of the oil film was optimal (oil film formation rate of 83%) when the oil temperature was 40 ◦C and the flow rate was 6 LPM. To form the widest oil film, the dripping nozzle was used as the spray nozzle, and the inlet diameter and number of inlets were 6.35 mm and 6, respectively. The inlet oil temperature and flow rate were 40 ◦C and 6 LPM, respectively. In an experiment, the optimized oil injection method was applied, and the flow field and formation of the oil film in the crown and welded parts were analyzed. In the crown part, no oil splashed after hitting the coil, and the oil flowed well along the coil. The oil film formation rate was 83%. The oil film was formed more evenly in the welded part than in the crown part owing to the insulating paper between the coils. The oil film formation rate was 90% in the welded part. Thus, we studied the oil injection method for cooling high-performance electric-vehicle motors with hairpin windings. The results of this paper provide important information for the development and research of high-performance electric vehicle motors. Furthermore, we will do more research to apply the oil cooling method to a hairpin type motor based on the results of this paper. Energies 2021, 14, 747 14 of 15

Author Contributions: Conceptualization, T.H.; methodology, T.H.; validation, T.H.; formal analysis, T.H.; investigation, T.H.; writing—original draft preparation, T.H.; writing—review and editing, T.H. and D.K.K.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 20206810100030), (No.20008021). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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