Automatic Transmissions — Core Gear Mechanism for Ratio Changes Within the gear unit of a conventional hydraulic automatic transmission, which mechanism provides the different forward and reverse gear ratios?

Introduction / Context:
Automatic transmissions achieve multiple ratios and smooth shifts without manual synchronization. Recognizing the internal mechanism that enables compact, multiple gear ratios is essential for automotive engineering basics.

Given Data / Assumptions:

• Hydraulic automatic transmission (not DCT or CVT).
• Focus is on the gearset that creates ratios; apply bands and clutches as needed.
• Modern units may have several planetary sets in compound arrangements.

Concept / Approach:
Planetary (epicyclic) gearsets provide multiple ratios by holding, driving, or outputting different members (sun, ring, carrier). Clutches and bands select which elements are locked or free, producing various forward speeds and reverse. Synchronizers are manual gearbox components. Multi-plate clutches are actuators that engage elements but do not themselves form the ratios.

Step-by-Step Solution:
Identify ratio-forming hardware: planetary gearset.Recognize role of clutches/bands: select and hold elements.Select “planetary gear” as the core mechanism.

Verification / Alternative check:
Exploded views of ATs show one or more planetary sets (Simpson, Ravigneaux, Lepelletier) used for ratio spread.

Why Other Options Are Wrong:
Synchronizer: used in manual transmissions.Magnetic clutch: not typical in AT gear units.Hydraulic multi-plate clutch: engages/locks elements but does not create gear ratios alone.Lay shaft with sliding dogs: manual gearbox architecture.

Common Pitfalls:
Confusing actuators (clutches/bands) with the ratio mechanism (planetary set).

Final Answer:
planetary gear