Automatic Transmissions

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1 Automatic Transmissions A Study of Internal Heat Generation and its Effects on ATF By MolaKule 1

2 Agenda General Review of AT Components Heat Transfer Studies ATF Formulations and Heat Effects 2

3 Mola s Early Years 3

4 Mola s Early Years 4

5 Then I got Hitched! 5

6 Then I did some Electronics Work! 6

7 Later Worked In Aerospace and Defense 7

8 Now for the Good Stuff! 8

9 Automatic Transmissions - Main Components A very complex (Electro) Mechanical Device Transfers Engine Mechanical Energy (Torque) to wheels using Step-Shifting or CVT Components: Flex Plate, Case, Torque Converter, Oil Pump(s), Gearing, Clutch Packs, Special Hydraulic Fluid (ATF), Actuators (Pistons), Valves, Sensors for feedback. 9

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12 Automatic Transmissions Torque Converter Most Torque Converters have an integrated torque converter clutch or TCC. Some TCCs are continuous-slip type. Torque Converter : Impeller - Connected to Engine, Turbine - Connected to Transmission, Stator to increase torque (freewheeling). Lock-Up of TCC Increases fuel economy in cruise. Continuous Slip Torque Converter (CSTCC) smooths engagement, reduces shock loads. Increases heat load for ATF and Heat Exchanger 12

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14 Automatic Transmissions ATF A Special Hydraulic Fluid Mechanical Energy (Torque) Transfer via Torque Converter assembly Lubrication Fluid Film and Wear Protection Cooling - dissipation of Heat Load Friction modification for Clutch Packs and TCC Pressurization and Control of Actuators 14

15 SUN Gear without Planet Carrier 15

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17 Automatic Transmissions Planetary Gears In order to lock the gears to perform gear ratio change, a band (linear clutch) is used. The band is connected to an actuator piston by way of a lever link. The piston pushes the lever link to force the band to lock the gear. By using more planetary gear sets and clutch packs, more gear ratios can be accommodated. 17

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21 Automatic Transmissions Wet Clutch Description Contains a number of alternating outer-splined clutch plates and inner-splined clutch discs which are stacked alternately in the annulus of a hub and a housing. Clutch plates are made of steel. The inner-splined clutch discs are attached to the hub and the outer-splined clutch plates are attached to the housing. The clutch discs are made of porous friction materials which are bonded to steel core plates. Friction materials may include cellulose, carbon fiber, Aramids, fiberglass, or sintered metals (HD), or a combination of materials (composites). Today, composites rule. 21

22 Automatic Transmissions Wet Clutch Friction Material Basic Friction Coefficients Must exhibit: μo, low speed dynamic coefficient; affected by friction material ingredients and ATF additives adsorption Must exhibit: μi, initial dynamic coefficient at high speed; affected by hydrodynamic effects/porosity/compression/roughness 22

23 Automatic Transmissions Wet Clutch Operation Obviously before engagement, the friction clutch discs and clutch plates are rotating at different RPMs. Oil flow is supplied to the clearance between the clutch discs and clutch plates through radial pathways on the hub for cooling and lubrication purposes. During a clutch engagement, a piston compresses the plates. The oil between the plates is squeezed out of the interface and permeates into and or out of the pores in the friction material. Engagement is completed when no speed differential exists between clutch discs and clutch plates. 23

24 Automatic Transmissions Wet Clutch Packs and TCC Heating Any time the TC is operating, heat is being dissipated because energy transfer is NOT 100% efficient. A clutch pack engagement/disengagement, or TC engagement/disengagement creates heat. Internal Heat transfer is the main topic of this presentation. 24

25 Automatic Transmissions Clutch Pack Temperatures Earliest study was by M. Haviland ( 61) of GM who placed temperature sensors (thermocouples) on clutch plates. Power: 394 Cu. In. engine TC coupled to a three-speed AT. Clutch plate surface temps (CPST) were measured for various Sump temperatures. Sump = 100F; CPST = 275F at 0.6 sec. after engagement. Sump = 150F; CPST = 325F at 0.5 sec. after engagement. Sump = 200F; CPST = 360F at 0.45 sec. after engagement. Sump = 250F; CPST = 410F at 0.4 sec. after engagement. Sump = 300F; CPST = 625F at 2.55 sec. after engagement. 25

26 Automatic Transmissions Clutch Pack Temperatures Questions. What did you notice about clutch plate surface temperatures in general? Trends? What did you notice about clutch plate surfaces temperatures verses ATF Flash Point temperatures? (FP is about 425F for a synthetic ATF). Why didn t ATF combust? Flash Point refers to the minimum temperature at which the vapor-air mixture near the surface of a liquid can be ignited. 26

27 Automatic Transmissions Clutch Pack Temperatures ; Later Studies In the 70 s, K. Tataiah of Shell Oil Company did some experimental and analytical studies in order to develop mathematical models for clutch plates. For a passenger car transmission of 4 plates with an energy input of 18,000 ft.lbs., an applied plate pressure of 100 psi, he found temperature rises of as much as 225 F above non-engagement (150 F ambient) temps. This means for this test, the friction plate temperatures reached almost 400F for a brief period. For a test rig with an energy input of 30,000 ft.lbs., an SAE No. 2 test rig with one friction plate, an applied plate pressure of 63 psi, he found temperature rises of as much as 640 F above non-engagement (150 F ambient) temps. This means for this test, the friction plate temperature reached almost 800F for a brief period. 27

28 Automatic Transmissions Clutch Pack Temperatures ; Later Studies Later, Cameron of Kettering and Tersigni of Afton (2005) co-developed more extensive mathematical models that predicted clutch surface temperatures. From these Basic Thermodynamic and Friction Equations: Q + W = m*c*deltat; (Eq. 1) DeltaT is temperature rise, c is specific heat of material, m is mass of material, W is work done on or to system, Q is heat added to system. Friction Heating: Wfr = mu*a*p*y(t); (Eq. 2). y(t) is sliding distance as a function of time, P is contact pressure, A is effective area of sliding surface, mu is coefficient of friction, Wfr is work due to friction. 28

29 More Math and Aliens 29

30 Automatic Transmissions Clutch Pack Temperatures Temperature Rise By combining a number of additional equations and using some DiffyQ gyrations, simpler(?) equations result: For a clutch plate system with poor thermal conduction and small thermal mass, the temperature is given by: Tf = Ns*mu*v*P*t/(rho*c*L); (Eq. 3) For the denominator, L is plate thickness, c is specific heat of material, rho is density of material. For the numerator, mu is friction coefficient, v is relative sliding velocity, P is pressure, t is engagement time, Ns is number of friction surfaces generating heat on a plate of thickness L. Note: N = Number of active surfaces so Ns = N/2 because of symmetry. In the case of Mercon and Dexron tests, Ns = 2. For Temperature rise in TCC s see (Eqs. 4) 30

31 Automatic Transmissions Clutch Pack Temperatures ; Temperature Rise Predictions (Cameron and Tersigni) Mercon (E = 15,526 ft.lbf, Ns = 2, A = 9.66 sq. in., L = 0.14 in., ts = 0.9), TB = 275F, Tfrise = 176F, Total Temperature at clutch = 419F Dexron (E = 11,753 ft.lbf, Ns = 2, A = 6.9 sq. in., L = 0.18 in., ts = 0.6), TB = 284F, Tfrise = 219F, Total Temperature at clutch = 471F JASO (E = 18,239 ft.lbf, Ns = 2, A = 6.87 sq. in., L = in., ts = 0.9), TB = 189F, Tfrise = 190F, Total Temperature at clutch = 347F A is effective area of each clutch, L is thickness of plate, Ns is number of active friction clutches. These temperature predictions most likely under estimate actual temperatures by 15-30%. 31

32 Automatic Transmissions Clutch Pack Temperatures ; Heat Dissipation Processes Heat in clutches can be dissipated by several methods: 1. Conduction into solids that are in contact with the clutch materials, 2. Convection into liquids that are in contact with the clutch materials, 3. Vaporization of heated liquids that are in contact with the clutch materials. Some evaporation does occur but is negligible (Closed System). 32

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34 Automatic Transmissions Clutch Surface Temperatures ; Results of Experiments and Simulation The Temperature Rise of the fluid can be determined from an equation of the general form: DeltaT = Q/(F*rho*Cp); where DeltaT is temp rise, Q is heat removal rate (J/s), F is ATF flow rate across all clutch surfaces (approx. 1.25L/min.), rho is ATF density (8 gm/cm^2), and Cp is the ATF specific heat capacity (2160 J/kg/K). The temperature charts show that clutch pack surface temperatures peak immediately after engagement and the temperature then drops off to initial temperature after 2.5 to 10 seconds - dependent upon the amount of torque applied and shift transition time. For a bulk ATF fluid temp initially at 180F, the temp of the fluid flowing out of the clutches may be briefly as high as 479F! This means the temperature of the ATF across the clutch surfaces has risen approx. 300F. 34

35 Automatic Transmissions Clutch Surface Temperatures ; Results of Experiments and Simulation (Cont.) If you are using an OBDII scanner in real time, this is why you will see a spike in the bulk oil temperature for about 15 seconds after a shift, especially on a hill climb! Of course, on the down slope the bulk oil temperature will start to decline. Example: 2003 Nissan PathFinder. Engine - VQ35DE, Transmission 4 Speed Automatic, ATF Dexron III eq. A spirited down shift on a hill at 55 mph, with no towing load, the ATF bulk temp went from 185F to 260F in 4.0 seconds and then went back down to about 190F in 12 seconds later, a total time of 16 seconds. 35

36 Automatic Transmissions Clutch Surface Temperatures ; Results of Experiments and Simulation (Cont.) Why didn t the bulk oil temperature go 400F? Continual cooling of clutch packs by fluid, and continual cooling of fluid by heat exchanger. Mass of ATF (11.5 Liters) Short engagement time of about 0.6 seconds. Had clutch pack or TCC been slipping for a longer time, fluid temp would have risen sharply to critical values. If you plot oxidation percentages verses temperatures, then a sump temp of 300F will certainly degrade fluid faster than having an average sump temperature of 185F! So fluid oxidation and degradationis highly dependent on how long (the dwell time) the fluid has been at that elevated temperature. 36

37 Automatic Transmissions Clutch Pack Temperatures; Effects on ATF and Materials High fluid temperatures oxidizes fluid. Oxidation alters the Friction Modifier, causes sludging, degrades Anti-Wear (AW) agent, and degrades seals. High fluid temperatures varnishes clutch discs, plates, bands, valves and actuators. Varnishing of clutch discs severely modifies clutch disc material surface friction coefficients ui and u0. High fluid temperatures can cause clutch disc material breakup. High fluid temperatures can cause clutch plate warping, distortion, wavyness, high spots, low spots. Lower bulk fluid temperature TB, means lower surface temps, longer transmission life. 37

38 Automatic Transmissions External Heat Sources High engine temps. Timing advance too low, restriction in exhaust system such as blocked CAT(s), muffler. Radiator fins blocked (bugs?), internal radiator orifices blocked, air bubbles in cooling system, bad head gasket. Too low fuel octane. Low fluid level. Sensor malfunctions. Increased Loads. Too high a pulling load in Mountainous or hilly terrain raises engine temps, which raises coolant temps, which raises ATF heat exchanger fluid temps. May need an external cooler (ahead of radiator) to reduce ATF temperatures. 38

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40 Automatic Transmission Fluid Formulations Mixtures of Base Oils (Group II-V) of various viscosities (two or more viscosities) depending on Specification Performance Improver (additive pack) Viscosity Index Improver (VII) Dye Red, to differentiate type of fluid and for leak detection 40

41 Automatic Transmission Fluid Formulations Performance Improver PI (Additive) Package A PI package includes these agents: Anti-wear, rust and corrosion inhibitors, detergents, dispersants and surfactants, seal conditioners, anti-oxidants, anti-foam, friction modifiers for bands, clutch packs, and Torque Converter Clutch. 41

42 Automatic Transmission Fluid Majority Group IV - based Formulation (Cont.) PI 7.2% VII 3.8% Ester 2.6 cst 11.6% PAO 2 cst 9.4% PAO 6 cst 68.0% Dye Red 150 to 250ppm Viscosity 6.02 cst Note: Ratios may vary. 42

43 Automatic Transmission Fluid (Cont.) Majority Group III based Formulation PI 8.27% VII 3.7% Ester 7% Group III 3 cst 39.8% Group III 6 cst 41.2% Dye Red 150 to 250ppm Viscosity 6.02 cst Note: Ratios may vary. 43

44 Automatic Transmissions TCC Temperatures ; Temperature Rise Predictions (Cameron and Tersigni) By combining the above equations and using some DiffyQ gyrations, simpler equations(?) result: For a TCC with good thermal conduction and large thermal mass: Tf = F*y*Wf/SQRT(rho*c*Lambda*t); (Eq. 4). t is sliding time, Lambda is thermal conductivity, c is specific heat of material, rho is density of material, y is heat partition factor = 1, Wf is the Work done due to friction, F is friction heating distribution factor =

45 Useless (tongue-in-cheek) Errata: Potential Origin of Dexron name: Dex is a contraction for decimal exponent or 10^x, implying orders of magnitude (better?). ron is a currency of Romania based on the decimal system. ron literally means lion. With this information, Dexron may potentially mean, A Bigger or Better Lion? 45

46 Thank Goodness It s Over! 46

47 Automatic Transmissions Thanks for watching. Questions? 47

Module 5: Cooling Fundamentals

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