by François Dovat
(© François Dovat)
Conventional automatic transmissions (ATs) have a hydrodynamic torque converter instead of a clutch. The picture on the left shows an AT for front wheels drive applications.
The hydrodynamic torque converter, also known as hydrokinetic, is typically installed between the internal combustion engine and a planetary gearbox. It is a toroidal body filled with hydraulic fluid (ATF) and whose tore is divided into three parts: a pump, a turbine and a reactor or stator. The pump (1), driven by the crankshaft (4), centrifuges the fluid against the turbine blades (3). Since that turbine spins slower (or is stationary when the vehicle is stopped in gear) than the pump, and because of the pitch of the blades, the oil flow returns to the pump while passing by the reactor or stator (2) connected to the casing (7). The reactor blades pitch is a such as to send back the stream in the direction of rotation of the turbine, as shown on the picture below.
One thus obtains a self-regulating variator able to approximately double the engine torque when the output shaft is stalled and whose characteristics are well adapted to road traction, even more so since it remarkably filters the jerks and other vibrations of torsion. Naturally, the efficiency of this turbo-machinery is low, passing by a maximum of about 85% at the design point, when the revs of the turbine are about half of that of the pump. But if the reactor is set on a free wheel (5) it starts to rotate freely when the speed ratio reaches a certain value (0.7 to 0.8). Consequently the reactor becomes inactive and the torque converter works then as a hydraulic coupling with an efficiency increasing up to 97% - 98% at the nominal speed. Nevertheless every current automotive AT has a lock-up clutch which locks the pump and the turbine together when the torque converter is not necessary.
The oil pump (8) is engine driven. Earlier AT had two oil pumps, one being driven by the output shaft.
In order to use as much as possible the converter in its most favorable efficiency range, it is assisted by a gearbox. But a true automatic transmission must shifts ratios under load, which means without interruption of the traction force (power-shift). As a matter of fact, this feature can be considered as the greatest advantage of an automatic transmission.
To this purpose an AT is generally designed around a planetary (also said epicyclic) gear box whose elements in rotation (sun gear 1, planets 2, planet carrier 3, ring 4) can be either blocked or locked together by means of multi disk clutches and brakes (or band brakes directly on the periphery of the rings). Their automatic regulation is entrusted to a hydraulic control unit called "valve body" and including sliding valves controlled by the position of the selector (P-R-N-D-2-1), the accelerator position and the input and output revs, sometimes jointly with a centrifugal regulation. It is nowadays electronically controlled so that it becomes possible to finely tune the shifting programs and to install several of them, per example D for ordinary driving and S for sport driving.
The opposite pictures show a complex planetary gear set of the Ravigneaux 1 concept. It allows up to 4 ratios plus a reverse gear. The input shaft is blue (1), the intermediate shaft brown (2). There are two sets of 3 planets, pink and green (4), the planet carrier is light blue (5) and the ring attached to the output shaft carries number 6. On the left drawing there's a multi disk brake (D) to prevent the rotation of the planet carrier and on the right drawing a device to block the ring can be seen (parking brake).
The next picture shows this epicyclic gear set with its control brakes and clutches in a ZF 3 HP-12 transmission, optional on the Peugeot 404 between 1965 and 1975. The current transmissions hardly differ in basic operating principles. In 7 we have an anti-reversing free wheel for easy starting on gradients and in 8 another free wheel assigned to the clutch C which makes it possible to stop the rotation of the intermediate planetary shaft. The hydraulic pistons for the actuation of the clutches and brakes are shown in 9. The clutches A and B are tied with the input shaft E whereas D blocks the planet carrier (5), like already seen on the preceding figures. The non rotating parts are colored in gray.
The current trend is to delegate less work to the torque converter and more to its associated planetary box, because the efficiency of the latter is superior. In addition to the lock up clutch and multiplication of the number of ratios (6 on the new ZF 6 HP-26 & 32 and heavy trucks, 7 on the new Mercedes 7G-Tronic) a power split principle is sometimes used. An epicyclic differential splits the engine torque, only a fraction (per example 40%) being sent to the converter while the remaining part is mechanically transmitted. For example, if such a differential is located on the transmission input shaft, its planet carrier is linked to the crankshaft, his ring to the epicyclic box and his sun gear to the torque converter pump. The latter receives only the fraction of the torque determined by the respective teeth numbers of the ring and sun. The torque produced by the turbine adds to the directly and mechanically transmitted torque.
As a result, all of the engine torque passes by the torque converter when the vehicle's wheels are stalled since the ring of the differential is stationary (the power being the product of the torque multiplied by the rpm, if one of the two factors is nil, the power shall also be). Then, as the vehicle takes speed, the ring gradually rotates faster and the directly transmitted power increases.