Clutches & Torque Converters

The two devices that connect and disconnect engine and gearbox — one a friction plate, the other a fluid coupling that also multiplies torque.

Automotive EngineeringPowertrainFree preview

⏱️ About 14 min

Clutches & Torque Converters — illustration

Decorative illustration.

An engine can't start a stationary car while bolted rigidly to the wheels — something must slip to bridge the gap between spinning engine and stopped gearbox. That something is the clutch, or its automatic twin, the torque converter.

💡

The big idea: A clutch (friction plate) or torque converter (fluid coupling) couples engine to gearbox while allowing controlled slip at launch; the torque converter additionally multiplies torque at low speed, while a lock-up clutch removes its slip losses at cruise.

🎯 By the end, you'll be able to

Explain why a coupling device that can slip is essential for launch
Describe friction-clutch operation and the friction-torque relation
Explain the torque converter's three elements and torque multiplication
Describe the lock-up clutch and why it exists

📎 Helpful to know first

Torque & Power Curves

The coupling that must slip

An idling engine turns at ~800 rpm; a stationary car's gearbox input shaft is at zero. Bolt them together rigidly and either the engine stalls or the car lurches. A launch device bridges that gap by allowing controlled slip — transmitting torque while the two sides rotate at different speeds, smoothing the difference away. Two technologies dominate. The friction clutch (manual and dual-clutch gearboxes) squeezes a friction disc between a pressure plate and the engine flywheel; clamp force × friction × mean radius sets the torque it can transmit, and the driver (or actuator) modulates that clamp force to control slip during launch. The torque converter (classic automatics) is a fluid coupling — three bladed wheels (impeller, turbine, stator) full of transmission fluid — where the engine spins the impeller, the fluid drives the turbine, and a one-way-clutch stator redirects flow to multiply torque at low output speed.

\[ T_{\text{clutch}}=\mu\,F_{\text{clamp}}\,R_{\text{mean}}\,n_{\text{surfaces}},\qquad T_{\text{converter}}=T_{\text{impeller}}\times(\text{torque ratio, }\downarrow\text{ with output speed}) \]

Clutch torque = friction coefficient × clamp force × mean radius × number of friction surfaces. The torque converter multiplies input torque by a ratio that is highest at stall (zero output speed) and falls to 1:1 as output catches up.

Friction clutch (left): pressure plate squeezes the friction disc to the flywheel; clamp force sets slip. Torque converter (right): engine-driven impeller pumps fluid to the turbine; the stator redirects flow to multiply torque at low speed.

✨ Why the lock-up clutch exists

A pure fluid coupling always slips a little — the turbine never quite reaches impeller speed — and that slip wastes fuel as fluid heat. At cruise, where slip isn't needed, modern torque converters engage a lock-up clutch that mechanically bolts the impeller and turbine together, eliminating slip and giving near-100% efficiency. So a modern automatic's converter behaves like a fluid coupling at launch (for smoothness and torque multiplication) and like a rigid clutch at cruise (for efficiency). The lock-up clutch is the reason modern automatics approach manual gearbox efficiency on the highway.

📝 Worked example: A clutch friction disc has friction coefficient mu = 0.3, is clamped with F = 4000 N at a mean radius of 0.12 m, with 2 friction surfaces. What torque can it transmit?

T = mu × F × R_mean × n = 0.3 × 4000 × 0.12 × 2 = 288 N·m

✓ 288 N·m

✏️ Practice: A torque converter has a stall torque ratio of 2.2:1. If the engine produces 300 N·m at the impeller at stall, what torque reaches the turbine (gearbox input)?

N*m

Solution

T_turbine = T_impeller × torque ratio = 300 × 2.2 = 660 N·m
This launch torque multiplication is why old automatics feel strong off the line.

Check your understanding

1. A friction clutch's transmittable torque is increased by:

T = mu·F·R·n, so more clamp force, grippier friction material, or a larger disc radius all raise clutch capacity.

2. The lock-up clutch in a torque converter exists to:

At cruise the converter doesn't need to slip; the lock-up clutch mechanically couples impeller and turbine, removing the slip loss that would otherwise waste fuel.

✅ Key takeaways

A launch device (clutch or converter) couples engine to gearbox while allowing controlled slip
Friction clutch torque = mu·F·R·n; the driver/actuator modulates clamp force to launch smoothly
The torque converter is a fluid coupling that multiplies torque at low output speed via the stator
A lock-up clutch removes converter slip at cruise for near-manual efficiency

➡️ Launch handled, the next lesson surveys the gearbox families that take that engine (or converter) output and multiply torque across a range of speeds.

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