AMT

Automated manual transmissions add electro-hydraulic or electric actuators to a manual gearbox. Learn actuation of clutch and gear selection, shift sequence, torque interruption, shift time and its effect on acceleration, cost/efficiency benefits and applications in small cars and trucks.

An AMT keeps the efficient, cheap manual gearbox but replaces the driver's clutch pedal and gear lever with actuators controlled by a TCU. It offers two-pedal driving at low cost — popular in budget cars and heavy trucks.

The trade-off is a pause in drive (torque interruption) during each shift, felt as a 'nod'.

An analogy

An AMT is a robot driving a manual gearbox: it presses the clutch, moves the lever and releases the clutch — faster and more consistent than most people, but still one step at a time.

How it works

Sensors (gear position, shaft speeds, pedal) feed a TCU. Actuators (electro-hydraulic or electric motors) open the clutch, select and engage the gear, and re-close the clutch while the ECU reduces and restores engine torque.

Torque interruption

While the clutch is open, the car decelerates under road resistance — noticeable as a lurch during hard acceleration. Shift times of 0.3–1.0 s are typical; clever calibration (lifting throttle smoothly, shifting earlier at light load) improves comfort.

Pros and cons

Pros: low cost, low weight, manual-level efficiency, easy to add to existing gearboxes, good for fuel economy and commercial vehicles. Cons: shift-shock/interruption, creeping and hill starts need careful clutch control, less smooth than AT/DCT.

Commercial-vehicle AMTs

12–16-speed truck AMTs with predictive shifting using GPS topography, eco-roll and skip-shifts have largely replaced manual trucks, improving fuel economy and driver fatigue.

Formulas

• Speed loss during torque interruption: Δv = (F_res / m) · t_shift
• Shift sequence time: t = t_torque-down + t_clutch-open + t_select + t_sync + t_clutch-close
• Efficiency: η ≈ 95–97 % (same gear set as manual)

Worked example: Speed lost during a shift

Given: Road resistance 600 N, mass 1500 kg; Shift time 0.4 s

Find: Speed loss.

• Deceleration = 600/1500 = 0.4 m/s²
• Δv = 0.4 × 0.4 = 0.16 m/s (0.58 km/h)

Answer: ≈ 0.16 m/s

Worked example: Shift time budget

Given: Torque ramp-down 0.08 s, clutch open 0.06 s; Select/engage 0.15 s, clutch close + torque ramp-up 0.12 s

Find: Total shift time.

• 0.08 + 0.06 + 0.15 + 0.12 = 0.41 s

Answer: ≈ 0.4 s

Common mistakes

• Wrong: Expecting AT-like smoothness from an AMT. — Right: Interruption is inherent; lift throttle slightly when it shifts.
• Wrong: Holding on hills using the clutch. — Right: Use brakes/hill-hold; clutch slipping overheats it.

Applications

• Budget cars: Affordable automatics in small hatchbacks.
• Trucks & buses: Predictive AMTs cut fuel and driver workload.
• Motorsport: Sequential gearboxes with automated flat-shift.

Key takeaways

• Manual gearbox + actuators + TCU.
• Efficient and cheap, but torque interruption.
• Δv = (F_res/m)·t_shift.
• Dominant in heavy trucks; common in budget cars.

Going further

• Predictive shifting: GPS topography and cruise control choose gears ahead of hills.
• Torque-fill AMT: An electric motor fills torque during shifts in hybrid AMTs.
• Clutch temperature models: Protect the clutch during creeping and hill starts.