| 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:
- Pump/Impeller: attached to engine crankshaft (input); rotates at engine speed n_e; centrifugally accelerates ATF outward
- Turbine: attached to transmission input shaft (output); fluid impinges → rotates at n_t; transfers torque to drivetrain
- Stator/Reactor: between turbine exit and pump inlet; one-way clutch (sprag/roller); redirects return flow → torque multiplication
- 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).
