How Automatic Transmissions Work: Torque Converters, Gears and Shifting Explained

Drivers who have never touched a clutch pedal still rely on a chain of mechanical and hydraulic parts that act every time the car sets off or changes speed. At the core of any automatic gearbox are two tasks: letting the engine idle without stalling the car, and changing the ratio between engine speed and wheel speed without the driver selecting a gear.

The torque converter does the job of the clutch

In the most common type of automatic, a mechanical clutch is replaced by a sealed, fluid-filled housing called a torque converter. According to the car-mechanics reference site How a Car Works, it contains three main parts: an impeller bolted to the engine's flywheel, a turbine connected to the gearbox input shaft, and a central reactor fitted with a one-way clutch.

As the impeller spins with the engine, it flings transmission fluid outward into the turbine, which turns the gearbox input shaft. At low engine speed the reactor stays still and redirects fluid back into the impeller, which amplifies torque and helps the car pull away from a standstill. Once engine speed rises, the reactor freewheels and the converter simply acts as a fluid flywheel, How a Car Works explains.

Australian outlet CarsGuide reports that this fluid link is why torque-converter automatics feel smooth and deliver strong pulling power at low revs. The trade-off, historically, has been lower efficiency than a direct mechanical link, which is why most modern torque-converter units add a 'lock-up' function once the car is up to speed, mechanically joining the impeller and turbine to cut power loss, according to CarsGuide.

Planetary gears change ratio without changing gears

Once power reaches the gearbox, it passes through one or more planetary (epicyclic) gear sets. How a Car Works describes these as a central sun gear, an outer ring gear with internal teeth, and a set of smaller planet gears that mesh with both. All three parts are always in contact, so the transmission never has to physically engage or disengage a pair of gears the way a manual box does.

Instead, hydraulically operated brake bands and multi-plate clutches hold different parts of the gear set still or let them spin freely. How a Car Works lists the basic combinations: locking the sun gear while the carrier drives the planets produces a speed increase; locking the ring gear and driving the sun gear produces a speed reduction; sending power to the sun gear while braking the planet carrier produces reverse; and locking the sun and ring gears together produces direct drive, a one-to-one ratio.

A single planetary set used this way can already provide several ratios. Pairing two epicyclic sets, the simplest classic layout, can yield three forward speeds, How a Car Works notes; manufacturers chain together more sets and clutches to reach the five-, six-, eight-, nine- and ten-speed transmissions common in today's cars.

Hydraulics and electronics decide when to shift

The locking sequence inside the gearbox is triggered by hydraulic pressure acting on the brake bands and clutches, routed through what How a Car Works calls a complex arrangement of hydraulic valves. In older designs those valves respond mechanically to engine load, road speed and throttle position; a 'kickdown' mechanism forces a downshift for extra acceleration when the accelerator is pressed to the floor. In modern cars, electronic sensors and a control computer perform the same job, opening and closing solenoid valves to time each shift.

Three different ways to build an automatic

Not every self-shifting gearbox works the same way. CarsGuide groups modern automatics into three main families, plus an older, largely discontinued fourth type.

• Torque-converter automatic — uses the fluid coupling described above; strong low-speed torque and smooth shifts, found in brands such as Mazda and Lexus, per CarsGuide.
• Dual-clutch transmission (DCT) — effectively two manual gearboxes sharing one casing, each with its own clutch; one gear is pre-selected while the other is engaged, so the system swaps clutches to change gear in milliseconds with minimal torque loss. CarsGuide cites the Volkswagen DSG, Nissan GT-R, Ferrari 488 GTB and McLaren 650S as examples.
• Continuously variable transmission (CVT) — has no fixed gears at all; a belt or chain runs between two pulleys whose effective diameter changes, giving, in CarsGuide's words, an action 'like a rubber band on a traffic cone' and a theoretically unlimited number of ratios for maximum fuel efficiency.
• Single-clutch automated manual — an older design in which a computer operates one clutch on a conventional manual gearbox; CarsGuide describes its shifts as jerky and delayed and says the type is now largely obsolete.

For comparison, CarsGuide notes that conventional manual gearboxes typically have six forward speeds, with some sports cars such as the Porsche 911 offering seven.

Why it matters for everyday driving

The type of automatic under the floor affects how a car feels rather than simply whether it has a clutch pedal. A torque-converter automatic's fluid coupling is what lets a car creep forward at idle without the engine stalling, a trait How a Car Works attributes directly to the torque converter's design. A DCT's two clutches are what let performance cars change gear without the momentary power gap felt in older automatics, according to CarsGuide. A CVT's absence of fixed ratios is what lets the engine stay near its most efficient speed regardless of road speed, which CarsGuide lists as its main advantage, offset by a constant, non-traditional engine drone some drivers notice.