Contents — find the section you need
Background and reading guide
Background Basic force, motion and energy terminology; use the linked background explanations.
Fundamentals · Explanations of principles, structure and trade-offs.
0. What this article explains
- Why a wheel cannot simply be coupled to an engine, and why a ratio is needed.
- The mechanical elements, efficiency, heat, and control differences among MT, torque-converter AT, CVT, and DCT.
- Examples including ZF 8HP, Toyota's planetary THS power split, and DCTs, plus the reason EVs often converge on a reduction gear.
1. Bottom line: what is a transmission?
A transmission converts prime-mover speed and torque to match vehicle speed, grade, and load. Here define the reduction ratio i as input over output. For input torque T_i, output torque T_o, input speed \omega_i, and output speed \omega_o,
The middle relation is the lossless ideal case; the right-hand relation includes gear, bearing, and oil losses in efficiency \eta_g. A larger reduction ratio i increases wheel torque but lowers wheel speed.
2. Why a ratio is needed
An internal-combustion engine is efficient and torquey only over a limited speed range and cannot sustain rotation at zero speed. Wheels must cover standstill to highway speed, so launch uses a large reduction and cruising uses a smaller one. Electric motors offer high low-speed torque and a broad constant-power region, so they can omit multiple gears, although heavy vehicles, high speed, and towing can justify two or more ratios.
A clutch absorbs speed difference and disconnects the source; a differential permits left-right wheel speed difference. Transmission efficiency therefore includes shift interruption, hydraulic-pump loss, cooling, and control software—not only gear mesh loss.
3. Basic architecture
Figure 1 — The ratio is selected, the clutch absorbs speed difference, and the differential distributes torque to the wheels.
4. MT: the most direct shift
A manual transmission (MT) asks the driver to disengage the clutch, synchronize shaft speed, and engage a gear. Constant-mesh gears are common: even unused gears rotate, while synchronizer rings and dog clutches connect the selected one. The design is mechanically transparent with little hydraulic control, but launch, hill starts, and shifting depend on the driver.
5. AT: torque converter and planetary gears
A conventional multi-speed automatic combines a fluid torque converter, planetary gearsets, hydraulic clutches, and an electronic control unit. Pump, turbine, and stator speed difference can multiply launch torque; a lock-up clutch reduces slip in cruise. A planetary set changes ratio by selecting which of its sun, planet carrier, and ring is held, driven, or output.
ZF publishes an 8HP design that selects four gearsets with five shift elements, illustrating that more ratios are not simply more gears. Current 8HP variants are modular for combustion, mild-hybrid, and plug-in-hybrid vehicles.
6. CVT: a continuously variable ratio
A belt CVT varies the effective radii of two sheaves with hydraulic pressure. Its ratio is
With no slip, ωin rdrive = ωout rdriven. A 50 mm drive radius and 100 mm driven radius give i=2, so output speed is half the input speed.
It can hold an engine near an efficient speed, but belt clamping force, pulley friction, and the hydraulic pump consume energy. A launch torque converter or planetary stage may be added, and a hybrid power-split system may route an electrical path; two systems called “CVT” can therefore have very different power flows.
7. DCT: two clutches in alternation
A dual-clutch transmission assigns odd and even gears to separate clutches and preselects the next gear. A shift opens one clutch while closing the other, minimizing interruption. A dry clutch has low oil-churning loss but faces launch heat and wear; a wet clutch cools better but needs pump power.
8. Comparing the approaches
| Type | Mechanical idea | Efficiency tendency | Difficult condition |
|---|---|---|---|
| MT | Fixed gears and one clutch | High, with little control loss | Traffic, launch, driver variation |
| Multi-speed AT | Torque converter, planetary sets, hydraulics | High with lock-up | Hydraulic/cooling loss, mass, shift delay |
| CVT | Variable sheaves | Keeps prime mover near an efficient point | High torque, belt clamping, heat |
| DCT | Odd/even gears and two clutches | High mechanical coupling | Low-speed clutch heat and control complexity |
| EV reduction | Usually fixed reduction and differential | Few ratio-related losses | High speed, mass, motor speed limit |
9. Control and practical choices
Shift control reads vehicle speed, accelerator, engine speed, grade, oil temperature, and lateral acceleration. A simplified cost is
Optimizing fuel alone can cause frequent shifts or low-speed vibration. Electrified systems add motor-speed limits, battery SOC, regenerative torque, and inverter temperature.
- Driver feel and service simplicity: MT, accepting clutch wear and driver effort.
- Wide vehicle mass, towing, and comfort: multi-speed AT, comparing converter, hydraulics, and cooling as a system.
- Steady-speed economy: CVT, after checking maximum input torque and thermal capacity.
- Fast shifts and sporty response: DCT, after checking clutch temperature in traffic.
- Passenger EV: fixed reduction is the baseline; high-speed cruising and towing may justify multiple ratios.
10. Three-line recap
A transmission fits a prime mover's narrow efficient speed range to a wheel's broad speed range.
MT, AT, CVT, and DCT trade gear, hydraulic, belt, and dual-clutch behavior against efficiency, heat, and control complexity.
EVs can simplify to a fixed reduction, but ratio count should be chosen from system efficiency including high speed, towing, and regeneration.
What does doubling a reduction ratio do?
Ideally, output speed halves and torque doubles. Power is not created; real transmissions also have losses.
References
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