The figure shows the power flow of this automatic transmission for the three forward gears and the reverse gear. The gearshift from 3 to 5 in passenger cars and 4 to 6 in commercial vehicles, takes place hydraulically over frictional gearshift agents (clutches, brakes, and one-way clutch). The supply to the control hydraulics takes place over an oil pump attached to the transmission input. If such a transmission is to be tow started, i.e. if the engine is dead, an additional pump has to be arranged at the transmission output.
The classical automatic transmission offers a number of advantages:
Favorable symmetrical design,
Simple arrangement of shifting elements,
Low construction volume,
High transferable power output,
Power shifting capability,
Excessive increase of torque at start,
Excessive torsional vibration damping.
Apart from higher production costs, an essential disadvantage compared to usual manual transmissions, is the loss of efficiency as a result of the hydrodynamic converter operating under slip. When a lock-up clutch is installed, these losses can be minimized. In the higher gears and at a minimum speed, a mechanical lockup is hydraulically actuated. The advantage of excellent torsional vibration damping is however lost in this case. This is the reason why not all manufacturers have accepted this solution. The number of gears was also increased in order to achieve better driving performances and low consumption.
b) Automated Manual Transmission
Due to the already-mentioned disadvantages of the classical AT, which include high production costs and low efficiency, there are repeated attempts at automating the conventional manual transmission. It is not difficult to automate the transmission. In small transmissions it can be supported by the gear selector drum used in motor vehicles. It is more complicated to automate clutch operation (Chapter 3.3.1). Particularly in trucks, the semi-automated manual transmission is often used. Here, the choice of the gear and the clutch operation are left to the driver. The pre-chosen gear is shifted automatically.
The twin-clutch transmission is a special form in which shifting and coupling cannot be carried out manually. The driver decides whether the transmission is to be operated as a fully automatic transmission or a semi-automatic. Here, the driver chooses the gears himself, but does not engage the clutch (Chapter 3.4.1).