I know that a manual gearbox requires a clutch, which the driver actuates with a pedal, while an automatic gearbox does without a clutch or clutch pedal. So why are there automatic transmissions with twin clutches, but without a clutch pedal?
The clutch in a manual gearbox is necessary to disconnect power transmission from engine to the wheels when the vehicle is stationary, but allows the engine to continue idling.
By depressing the clutch pedal, the engine is cut off from the drivetrain, so there is no need to shut down the engine. During this time, the gear lever can move freely between gears.
Releasing the clutch pedal will once again allow power to be transmitted to the wheels.
It does so using a spring actuated friction disc between the engine and gearbox.
In an automatic gearbox, the connection between the engine and drivetrain is achieved by fluid coupling - made up of two separate "impellers" in a sealed casing filled with a special fluid.
When the impeller on the engine side turns, the fluid acts as a medium to propel the driven-impeller (called the turbine) on the gearbox side.
As a simple analogy, a running pedestal fan facing another which is switched off will cause it to spin. In this example, the "fluid" is air.
Because the fluid allows slip, the engine can continue to idle even if the vehicle is stationary.
The internal configuration of these fluid couplings is designed such that high fluid pressure is diverted to act on the turbine when moving off from rest. This results in a multiplication (roughly two times) of the engine torque, which is why this fluid coupling is called a "torque converter".
In modern autoboxes, there is also a lock-up clutch that engages at cruising speed to ensure zero slip or equal speed of impeller and turbine for efficiency.
Many cars have a double- clutch transmission (DCT), which does not use fluid coupling or torque converter. Power transmission is via a friction clutch arrangement.
Interestingly, many multi-plate clutch systems are housed in a special fluid which alleviates wear and aids cooling. They are known as wet-clutch units.
Whether dry or wet, the multi-plate clutch is disengaged when a different ratio is being selected - somewhat like in a manual gearbox. But the shifting is achieved by electro-hydraulic actuators.
The ingenious aspect of a DCT is that there are two sets of clutches, allowing the subsequent gear to be pre-selected so that gear changes are seamless.
Interestingly, the latest crop of torque converter autoboxes are almost as quick as the DCT.