Automatic

Conventional automatics use a torque converter and planetary gear sets shifted by clutches and brakes. Learn fluid coupling and torque multiplication, stall speed and K-factor, planetary gear ratios (Willis equation), clutch-to-clutch shifting, lock-up clutches and 8–10-speed designs.

The torque-converter automatic (AT) is the classic 'automatic' — smooth, durable and able to handle big torque. Modern 8–10-speed ATs rival manuals for efficiency thanks to lock-up clutches and wide ratio spreads.

Planetary gear sets give compact, coaxial ratios; the controller shifts by releasing one clutch while applying another.

An analogy

A torque converter is like two fans facing each other in a sealed box of oil: one blows oil that spins the other — with a stator redirecting the flow to multiply torque at low speed.

Torque converter

Impeller (driven by engine), turbine (drives gearbox) and stator (on a one-way clutch). At low speed ratio the stator redirects oil and multiplies torque up to ~2×; near coupling point (SR ≈ 0.85) it freewheels and the converter acts as a fluid coupling. A lock-up clutch eliminates slip at cruise.

Planetary gear sets

Sun, planets on a carrier, and ring gear. Holding one member and driving another gives reduction, overdrive, direct drive or reverse. Several sets linked with clutches and brakes create 6–10 ratios (e.g. Lepelletier, Ravigneaux arrangements).

Shifting

The TCU controls hydraulic pressure to each clutch: during an upshift, the off-going clutch releases as the on-coming clutch fills (torque phase), then engine speed drops to the new ratio (inertia phase). Shift quality comes from precise pressure control and engine torque reduction.

Efficiency and trends

Losses: converter slip, pump, drag of open clutches. Countermeasures: early lock-up, variable-displacement pumps, more ratios with wider spread (≈ 7–10). Hybrid ATs replace the converter with an electric motor and clutch.

Formulas

• Converter efficiency: η = TR × SR
• Capacity factor: K = N / √T → stall speed N = K √T_engine
• Willis equation: ω_s z_s + ω_r z_r = ω_c (z_s + z_r)
• Sun in, ring fixed, carrier out: i = 1 + z_r / z_s

Worked example: Planetary reduction

Given: Sun 30 teeth, ring 72 teeth; Sun input, ring held, carrier output

Find: Ratio.

• i = 1 + 72/30 = 3.4

Answer: 3.4 : 1 reduction

Worked example: Converter operating point

Given: TR = 1.5 at SR = 0.6

Find: Efficiency.

• η = 1.5 × 0.6 = 0.90

Answer: 90 % (torque multiplied 1.5×)

Common mistakes

• Wrong: Assuming automatics are always inefficient. — Right: Modern ATs with lock-up and many ratios are near manual efficiency.
• Wrong: Choosing planet teeth arbitrarily. — Right: z_r = z_s + 2z_p and equal spacing: (z_s + z_r)/n_planets must be an integer.

Applications

• Luxury & trucks: 8–10-speed ATs for smoothness and high torque.
• SUVs & towing: Converter torque multiplication for launch with heavy loads.
• Hybrids: Motor-integrated ATs in parallel hybrids.

Key takeaways

• Torque converter: multiplication at low SR, coupling at high SR, lock-up at cruise.
• Planetary: Willis equation; sun-in/ring-fixed i = 1 + z_r/z_s.
• Clutch-to-clutch shifts via hydraulic pressure control.
• 8–10 speeds + lock-up = good efficiency.

Going further

• Adaptive shift control: TCUs learn clutch fill times and pressures as parts wear.
• Lepelletier gear train: Simple + Ravigneaux sets give 6 ratios with 5 shift elements.
• Converter damping: Pendulum absorbers in lock-up clutches allow early lock-up without vibration.