Torque Converter

The three-element machine

Strip a torque converter open and you find three bladed wheels sealed inside a fluid-filled steel doughnut, all sharing a common axis:

• Pump (impeller): welded to the converter housing, which is bolted to the engine flexplate. It always turns at engine speed and acts as a centrifugal pump, throwing oil radially outward.
• Turbine: splined to the transmission input shaft. Oil thrown by the pump strikes its curved vanes and drives it. The turbine is the output.
• Stator (reactor): sits in the middle, mounted on a one-way clutch around a fixed stationary shaft. It is the element that turns a fluid coupling into a torque converter.

Power crosses each gap not through gear teeth or a friction plate but through automatic transmission fluid (ATF) circulating in a closed toroidal loop: outward through the pump, across to the turbine, inward and back through the stator, then outward through the pump again. Because nothing solid connects input to output, the engine can keep idling while the turbine — and the car — sits perfectly still.

Where the extra torque comes from

The defining trick is torque multiplication, and it is entirely the stator's doing. Oil leaving the turbine is still carrying angular momentum, but it is now swirling in a direction that would slam back into the rear of the pump vanes and try to slow the engine down. The stator's aggressively curved vanes intercept that returning flow and bend it through a large angle so that it re-enters the pump in the same direction the pump is already spinning. Instead of fighting the pump, the redirected oil gives it a push.

The bookkeeping is a torque balance on the whole device. Torque in and torque out are linked through the reaction torque the stator absorbs from the fluid:

T_turbine = T_pump + T_stator        (torque balance)

where:
  T_pump    = torque from the engine (input)
  T_turbine = torque to the gearbox  (output)
  T_stator  = reaction torque held by the one-way clutch

Torque ratio:   TR = T_turbine / T_pump = 1 + (T_stator / T_pump)

At stall (turbine held still):  TR ≈ 2.0 – 2.5
At the coupling point:          TR → 1.0   (stator freewheels)

As long as the stator is held stationary by its one-way clutch, it carries a reaction torque, and the output torque is the input torque plus that reaction. There is no free lunch — the extra torque comes with proportionally lower output speed, because power (torque × speed) can only fall through the device, never rise. The converter trades speed for torque exactly when a launching car needs it most.

Three operating phases

	Stall	Acceleration	Coupling point	Lockup
Speed ratio (turbine / pump)	0	0 → ~0.85	~0.85–0.9	1.0
Torque ratio	2.0–2.5	falling toward 1	1.0	1.0
Stator	Locked (reacting)	Locked (reacting)	Begins to freewheel	Freewheeling
Slip	100 %	large → small	10–15 %	0 %
Efficiency	0 %	rising	~85–92 %	~98–100 %
Where it happens	Brake-torque launch	Pulling away	Light cruise	Steady highway

The most important takeaway from the table: torque multiplication and high efficiency never happen at the same time. Multiplication is a stall and low-speed-ratio phenomenon; efficiency is a high-speed-ratio phenomenon. The job of the converter design — and of the lockup clutch — is to spend as little time as possible in the lossy middle.

Stall speed and the K-factor

A converter's character is summarized by its stall speed: the highest engine RPM reachable with the turbine held stationary (brakes on, throttle floored). Stall speed scales with the engine torque the converter must absorb and with the converter's capacity, captured by the K-factor:

Capacity factor:   K = N_pump / sqrt(T_pump)

Stall speed:       N_stall ≈ K · sqrt(T_engine_at_stall)

  N = speed (RPM)
  T = torque (N·m)

Lower K  → "tight" converter → low stall speed → efficient, good for towing
Higher K → "loose" converter → high stall speed → engine reaches power band first

K depends mostly on converter diameter and vane angles. Shrinking the diameter or steepening the vanes raises the stall speed. A factory passenger car runs a tight converter stalling around 1,400 RPM for smoothness and economy; a drag car might fit a small-diameter 3,000+ RPM converter so the engine is already making power the instant the brakes release.

Worked example: launch torque

A 2.0-litre engine makes 250 N·m at its stall RPM. The converter has a stall torque ratio of 2.2. What torque reaches the transmission input the instant the car begins to move?

T_turbine = TR × T_pump
          = 2.2 × 250 N·m
          = 550 N·m  at the transmission input

Multiply by first-gear ratio (say 3.6) and final drive (3.9):
  Wheel-end torque ≈ 550 × 3.6 × 3.9 ≈ 7,720 N·m (minus slip/efficiency)

That 2.2× boost is why an automatic can out-launch a same-engine manual from a standstill: the manual's clutch can only ever pass 1× engine torque, while the converter momentarily delivers more than double. The catch is that all of the input power not appearing at the output is dumped into the oil as heat.

Heat, cooling and the loss budget

Whenever there is slip, the difference between input and output power becomes heat in the ATF. At stall the turbine produces zero output power, so every watt the engine pours in becomes heat. The numbers are brutal:

Heat at stall:  Q = T_pump × ω_pump   (all input power, 0 output)

Example: 250 N·m at 2,000 RPM (209 rad/s)
  Q = 250 × 209 ≈ 52 kW dumped into the oil

A few seconds of brake-torque stall can boil ATF past 150 °C.

This is why every automatic routes converter fluid through a cooler (an oil-to-water loop in the radiator tank, plus an auxiliary cooler on towing vehicles), and why prolonged brake-torque stalls are a fast way to cook a transmission. It is also the single strongest argument for the lockup clutch.

The lockup clutch

Even at the coupling point a fluid coupling slips a few percent, and at highway cruise that slip is pure waste — typically 3 to 6 percent of engine power turned into heat and burned fuel. The fix is a lockup clutch: a friction plate inside the converter that, above a programmed speed, hydraulically clamps the turbine directly to the converter cover. The result is a rigid 1:1 mechanical link, zero slip, and efficiency near 100 percent.

• Full lockup gives the best economy but transmits engine vibration and torsional pulses straight to the driveline.
• Controlled-slip (partial) lockup holds a small, regulated slip of 20–60 RPM so the fluid still damps vibration while recovering most of the efficiency. Modern transmissions apply this aggressively, sometimes from second gear onward.
• A torsional damper with springs in the lockup plate absorbs the shock of engagement and filters combustion pulses.

Torque converter vs. the alternatives

	Torque converter	Fluid coupling	Manual clutch	Dual-clutch (DCT)
Elements	Pump + turbine + stator	Pump + turbine	Friction disc + pressure plate	Two dry/wet friction clutches
Torque ratio	2–2.5× at stall	Always 1:1	1:1	1:1
Coupling medium	Oil (hydrodynamic)	Oil (hydrodynamic)	Dry friction	Friction
Launch smoothness	Excellent (self-modulating)	Good	Driver-dependent	Good, software-tuned
Steady-cruise loss	0 % locked, 3–6 % open	2–4 %	0 %	~0 % engaged
Wear part	Lockup plate only	None (sealed)	Clutch disc (consumable)	Clutch packs
Typical use	Automatics, towing, off-road	Older buses, industrial drives	Manuals, motorsport	Performance autos

Failure modes and trade-offs

• Stator one-way clutch failure. If the sprag seizes locked, the stator can never freewheel: the car launches fine but cannot reach speed because the locked stator chokes flow at high speed ratio — poor top-end and overheating. If it seizes free (always slipping), there is no torque multiplication: weak, sluggish launches with normal cruise. These two opposite symptoms are a classic diagnostic.
• Overheated, oxidized ATF. Repeated stall or towing without an adequate cooler degrades the fluid, varnishes the lockup plate, and erodes the vanes. Burnt-smelling dark ATF is an early warning.
• Lockup-clutch shudder. Glazed friction material or contaminated fluid makes the lockup engage with a chatter felt as a shudder at light throttle around 60–80 km/h.
• Ballooning. Extreme line pressure or RPM can swell the steel housing, letting internal clearances open up and the elements rub — a high-horsepower failure mode addressed with a furnace-brazed or strengthened converter.
• Cavitation. Insufficient charge pressure or very high stall lets the ATF flash to vapor on the low-pressure side of the pump vanes; collapsing bubbles pit the blades, much as in a centrifugal pump.
• The fundamental trade-off: a loose, high-stall converter launches hard and lets a peaky engine reach its power band, but bleeds efficiency and heat; a tight, low-stall converter is efficient and cool but feels soft off the line. Designers thread this needle with diameter, vane angle, and ever-earlier lockup.