5.2 Automatic Transmissions: Torque Converters & Planetary Gearsets

Automatic Transmissions: Torque Converters & Planetary Gearsets

An automatic transmission utilizes a hydrodynamic fluid coupling (torque converter) and planetary gearsets controlled by clutches and bands to select gear ratios based on vehicle speed and engine load.

Torque Converter Operation and Fluid Dynamics

The torque converter transfers engine torque to the transmission input shaft, multiplies torque, and isolates engine vibrations. It is mounted directly to the engine flexplate and consists of three main components:

1. Impeller (Pump): Welded to the housing, it rotates at engine speed. Centrifugal force throws transmission fluid outward into the turbine.
2. Turbine: Positioned opposite the impeller and splined to the transmission input shaft. Fluid from the impeller strikes its blades, transferring rotational energy to drive the transmission.
3. Stator: Positioned between the impeller and turbine, it contains a one-way clutch (sprag or roller) that allows it to rotate in only one direction. The stator redirects fluid returning from the turbine back into the impeller, multiplying torque.
4. Torque Converter Clutch (TCC): A mechanical lock-up clutch controlled by a Pulse-Width Modulation (PWM) solenoid. It locks the turbine to the cover at cruising speeds, eliminating slippage and improving fuel efficiency.

Fluid Flow Phases

The fluid moves in two distinct flow patterns:

• Vortex Flow: The high-speed circular motion of fluid traveling from impeller to turbine and back through the stator. Vortex flow is greatest at stall speed (engine running, vehicle held stationary in gear, wide-open throttle) and decreases as turbine speed increases.
• Rotary Flow: The circular movement of fluid rotating with the converter housing. As turbine speed approaches impeller speed, vortex flow ceases and rotary flow dominates.

At the coupling point (turbine speed at ~90% of impeller speed), fluid strikes the back of the stator blades, causing the stator's one-way clutch to overrun (freewheel). Torque multiplication ceases, and the converter acts as a 1:1 fluid coupling.

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Planetary Gearset Calculations and Power Flows

Planetary gearsets achieve different gear ratios, reverse, and direct drive. A simple planetary gearset consists of:

• A central Sun Gear
• Multiple Planet Pinions rotating on axles attached to a Planet Carrier
• An outer Ring Gear (or annulus)

The number of teeth on the sun gear (S) and ring gear (R) determines the gear ratios. The effective number of teeth on the carrier (C) is represented by C = R + S.

Gear Ratio Rules

To determine the gear ratio, identify the driving member, the held (reaction) member, and the driven (output) member using the formula:

Gear Ratio = Teeth of Driven Member / Teeth of Driving Member

• Maximum Reduction (Underdrive): Driving = Sun Gear, Held = Ring Gear, Output = Planet Carrier. Gear Ratio = C / S = (R + S) / S.
• Moderate Reduction (Underdrive): Driving = Ring Gear, Held = Sun Gear, Output = Planet Carrier. Gear Ratio = C / R = (R + S) / R.
• Reverse Reduction (Underdrive): Driving = Sun Gear, Held = Planet Carrier, Output = Ring Gear. Gear Ratio = R / S.
• Maximum Overdrive: Driving = Planet Carrier, Held = Ring Gear, Output = Sun Gear. Gear Ratio = S / C = S / (R + S).
• Moderate Overdrive: Driving = Planet Carrier, Held = Sun Gear, Output = Ring Gear. Gear Ratio = R / C = R / (R + S).

Example Calculation: If S = 30 and R = 70, locking the ring gear and driving the sun gear yields: Carrier (Driven) = 70 + 30 = 100. Gear Ratio = 100 / 30 = 3.33:1 (Underdrive).

Locking any two members together produces a 1:1 direct drive ratio.

Modern transmissions combine multiple planetary gearsets. The Simpson gearset uses two planetary sets sharing a common sun gear. The Ravigneaux gearset uses a single carrier holding two sets of pinions, two sun gears, and one ring gear.

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Friction Elements and Hydraulic Controls

To direct power flow, the transmission control module operates clutches and bands:

• Multi-Disc Clutch Packs: Consist of alternating friction discs (splined to a shaft) and steel plates (splined to a housing). Hydraulic pressure behind a piston compresses the pack to lock the members together.
• Bands and Servos: A flexible steel band wraps around a clutch drum. A hydraulic piston inside a servo clamps the band around the drum, holding it stationary.
• One-Way Clutches (Sprags/Rollers): Lock in one direction to act as a reaction member and freewheel in the other.

The valve body is the hydraulic control center. A mechanical pump generates line pressure, regulated by a pressure regulator valve. Electronic solenoids direct fluid flow to shift valves, routing pressure to the appropriate clutch or band.

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Transmission Diagnostics and Testing

Diagnostics begin with checking fluid level and condition.

Hydraulic Pressure Testing

Connect pressure gauges to the test ports on the transmission case:

• Line Pressure Test: Check pressure in D and R at idle and stall. Low line pressure indicates a worn pump, clogged filter, stuck pressure regulator, or internal leaks. High pressure suggests a faulty control solenoid or regulator valve.

Stall Testing

A stall test checks engine performance, torque converter stator operation, and internal clutch/band holding capacity.

[!WARNING] Never hold a stall test for more than 5 seconds to prevent extreme fluid overheating.

1. Block wheels, apply parking and service brakes fully.
2. Shift to Drive (D), depress accelerator to wide-open throttle, and record engine RPM.
3. Repeat in Reverse (R).

• Results Analysis:
  • Low stall speed (e.g., 1,400 RPM instead of spec 2,200 RPM): Indicates low engine power or a slipping stator one-way clutch.
  • High stall speed (e.g., 3,000 RPM): Indicates slippage of the clutches or bands active in that gear range.