torque-converter

name	torque-converter
description	Torque converter design and analysis — three-element fluid coupling (pump/impeller, turbine, stator/reactor), torque ratio K, speed ratio SR, efficiency η, capacity factor K_c, torque multiplication at stall, coupling point, lockup clutch, stall torque calculation, pump and turbine characteristic curves, torus geometry, ATF viscosity effects, and integration with automatic transmission.
metadata	{"priority":7,"promptSignals":{"phrases":["torque converter","fluid coupling","torque converter stall","torque ratio converter","lockup clutch","automatic transmission converter"],"minScore":3}}

Torque Converter — Complete Skill

Operating Principles

Three-Element Construction

Elements:

1. Pump/Impeller: attached to engine crankshaft (input); rotates at engine speed n_e; centrifugally accelerates ATF outward
2. Turbine: attached to transmission input shaft (output); fluid impinges → rotates at n_t; transfers torque to drivetrain
3. Stator/Reactor: between turbine exit and pump inlet; one-way clutch (sprag/roller); redirects return flow → torque multiplication
4. Lock-up clutch: mechanical clutch in turbine assembly; locks turbine to pump at high SR → eliminates slip losses

ATF circuit: pump → turbine → stator → pump (torus circuit) Stator one-way clutch: engages (freewheels) when turbine slower than pump → redirects → K > 1 Stator freewheels above coupling point when turbine exits tangentially → K = 1 (fluid coupling mode)

Torque Converter Characteristics

Speed Ratio and Torque Ratio

Speed ratio: SR = n_t / n_e [turbine speed / pump speed; SR = 0 at stall; SR → 1 at coupling point]

Torque ratio (multiplication factor): K = T_t / T_e [turbine torque / engine torque (pump torque)]

At stall (SR = 0): K_stall = 1.8–2.5 (typical automotive converters) Higher K_stall: better for vehicle launch; costs: larger size, more heat generation T_turbine_stall = K_stall × T_engine

At coupling point: SR_coupling ≈ 0.85–0.95; K = 1.0; stator unloads (freewheels) Above coupling point: torque converter acts as fluid coupling; η ≈ SR (no multiplication)

Efficiency

Efficiency: η = (T_t × n_t) / (T_e × n_e) = K × SR [power out / power in]

At stall: η = 0 (all input power dissipated as heat in ATF); T_output high but no rotation At coupling: η ≈ SR × 1.0 ≈ 0.85–0.95 Peak efficiency: typically 85–90% at SR near coupling point

Efficiency curve: η = 0 at SR = 0 (stall) η increases linearly ≈ K × SR up to coupling point η = SR above coupling (K = 1) Peak η at SR ≈ 0.80–0.90 depending on design

Capacity Factor (K-Factor)

Capacity factor (input capacity): K_c = n_e / √T_e [RPM / √(N·m) or RPM / √(ft·lb); engine operating point where engine torque equals converter input torque]

At given SR: the converter will absorb specific torque for given speed; K_c characterizes this Pump curve: T_e = (n_e / K_c)² [parabolic; torque proportional to n²]

Matching converter to engine: Engine torque curve must intersect converter pump curve at desired operating RPM At stall: engine must not exceed stall torque absorption capacity → K_c_stall = n_e_stall / √T_e_stall Typical K_c at stall: 150–300 RPM/√N·m for automotive converters

Stall speed calculation: T_engine(n_stall) = T_converter_input(n_stall) → intersection on speed-torque curve Example: engine T_max = 350 N·m at 3,500 RPM; converter K_c = 200 RPM/√N·m n_stall = K_c × √T_stall = 200 × √350 = 3,742 RPM → converter stall ≈ 3,700 RPM (check must be near T_max RPM)

Fluid Mechanics of Torque Converter

Euler Turbomachinery Equation

Torque from angular momentum change (pump): T_pump = ρ × Q × (r₂ × c_u2 − r₁ × c_u1) [c_u = tangential (whirl) velocity; r = radius; Q = flow rate]

Pump head rise: ΔH = (u₂ × c_u2 − u₁ × c_u1) / g [u = blade tip speed = ω × r; Euler head]

Turbine torque: T_turbine = ρ × Q × (r₃ × c_u3 − r₄ × c_u4) [stations 3, 4 = turbine inlet, exit]

Stator effect: Without stator: turbine exit flow has backward tangential component → pump inlet penalty → K = 1 only With stator: stator blades reverse tangential component → pump inlet benefits → K > 1 (torque multiplication) Stator torque: T_stator = T_turbine − T_pump [reaction torque; equals multiplication above 1]

Torus Geometry

Torus diameter D: primary sizing parameter Torque capacity: T ~ D⁵ × ρ × ω² [scaling law; doubling D increases torque 32×] D = 200–300 mm for passenger car; 350–500 mm for trucks

Blade angles: Pump exit angle β_2: typically 30–50° from radial; controls flow direction and K_stall Turbine inlet angle β_3: matches pump exit for design SR Stator blade angle: optimized for maximum efficiency at design SR

Lock-Up Clutch

Function and Operation

Lock-up clutch: friction clutch between pump housing and turbine; engaged at highway speeds When locked: SR = 1.0; η = 1.0 (no slip losses); reduces fuel consumption 10–15%

Lock-up strategy: ECU controls apply pressure; engages above SR = 0.95 and v > 40–60 km/h Slip control (soft lock-up): partially applied to provide damping + slip compensation during transients; modern 8/9-speed autos use this extensively

Torsional damper: spring-mass damper in lock-up clutch assembly absorbs engine torque fluctuations (combustion frequency) Natural frequency of damper must be below idle RPM to avoid resonance Spring stiffness: k_damper = T_fluctuation / θ_damper; target f_n < 5–10 Hz

Thermal Analysis

Heat generation in fluid coupling: P_heat = T_e × n_e × (1 − η) = T_e × ω_e × (1 − K × SR) At stall: P_heat = T_e × ω_e (all engine power → heat) Example: Engine 200 kW stall condition → ATF absorbs 200 kW → must circulate and cool

ATF cooling requirement: Q̇_cooler = P_heat (steady-state; transient allows some thermal mass) ATF temperature limit: 130–150°C (conventional); 160–170°C (full-synthetic) Cooler: external (radiator-mounted); additional if trailer towing

ATF viscosity effect on K_stall: Lower viscosity (hot ATF): K_stall decreases slightly; pump efficiency increases slightly Viscosity grade: DEXRON VI (GM); MERCON V (Ford); ZF Lifeguard 6/8/9; Aisin ATF WS

Integration with Automatic Transmission

Combined efficiency: η_total = η_TC × η_transmission [TC efficiency × gear/clutch efficiency] At lock-up: η_TC = 1.0; η_total = η_transmission ≈ 0.95–0.97

Launch feel: determined by K_stall and coupling point; K_stall = 2.0+ for performance; 1.8 for economy Fuel consumption impact: larger converter D → higher K_stall → better launch but worse cruise efficiency Lock-up aggressive engagement: reduces loss but can cause shudder; damper tuning critical

Continuously variable transmission (CVT): uses torque converter or start clutch; lighter/smaller; belt/chain CVT has own efficiency Dual-clutch transmission (DCT): wet or dry clutch; no torque converter; lower launch smoothness; better efficiency

Standards and References

Standard	Scope
SAE J643	Performance of automatic transmission torque converters
SAE J651	Torque converter lockup clutch definitions
GM DEXRON VI specification	ATF requirements
Bosch Automotive Handbook	Powertrain reference
Jandasek (SAE)	Classic torque converter design reference

Output

Provide: application (engine: rated torque [N·m] at [RPM]; rated power [kW]; transmission: automatic/CVT; stall RPM desired; launch feel: smooth/sporty), torus sizing (D [mm] from torque capacity scaling; torus cross-section width W_t [mm]; aspect ratio), characteristic curve (K_stall [value]; SR_coupling [value]; η_max [%] at SR = [value]; K_c [RPM/√N·m]; pump curve T_e = (n_e/K_c)² overlay), stall speed (n_stall from engine T curve ∩ converter pump curve [RPM]; verify within 200 RPM of engine torque peak), torque multiplication (T_output at stall = K_stall × T_engine [N·m]; vehicle force at launch = T_stall × drive ratio / r_wheel [N]), efficiency at operating points (highway: SR ≈ 0.90 → η [%]; lock-up engaged: η = 100%; fuel economy benefit of lock-up [%]), lock-up clutch (engagement SR: [value]; engagement speed [km/h]; damper natural frequency f_n [Hz] vs. engine idle [Hz]; soft-lock slip strategy), thermal (P_heat at stall [kW]; ATF cooler capacity required [kW]; ATF temperature rise ΔT [°C] for given stall duration [s]), ATF selection (viscosity grade; ATF specification: DEXRON/MERCON/OEM; service interval), and applicable standard (SAE J643 for TC performance; Jandasek reference for blade design; OEM ATF spec).