How an automatic transmission (BVA) works

Now common in France, this type of gearbox does not have the same technical architecture as a manual transmission with parallel gears. Indeed, manual or robotic boxes (these are a bit the same) are arranged in very different ways. We don't need a clutch, forks, or even other players here. The advantage of automatic transmissions is that they do not need to disengage / shift between gears.

Here is an exploded view of the automatic transmission, with the torque converter on the left and the clutch / brakes and gears on the right.

Reminder: The images shown here are the property of Fiches-auto.fr. Any restoration violates our copyright.

See also: the main problems with automatic transmission.

Distinguish between torque converter and gearbox

For the less connoisseur, you really need to differentiate between the torque converter / clutch box to avoid mixing the brushes. On BVA (non-robotics) the clutch is replaced by a torque converter or sometimes (very rarely) a controlled clutch system.

We're limiting ourselves here to the gearbox and not its clutch system, so I won't talk about the converter (see here for more details).

In addition, the torque converter has a bypass clutch. It is activated to establish a clear communication between the engine and the gearbox (no slippage associated with the converter). It is also activated in the event of overheating of the transmission oil to avoid mixing the transmission oil in the converter (and therefore further increase its heating).

Automatic transmission gear architecture

The system can also be called planetary, because the way life originates is similar to the solar system (orbits). The primary tree represents the sun, and the secondary tree represents the planets in orbit. Here, the power coming from the engine will be transmitted by the sun gear (in the diagram in black). This gear will more or less quickly rotate the crown wheel connected to the wheels, depending on whether the gears are locked or not. Each speed will correspond to the blocking of certain planetary gears.

Here is an exploded view of two planetary gearboxes that I was able to do at international auto shows. This is a large box designed for longitudinal engine vehicles. The transverse versions are much smaller and more compact (they need to be placed on the left [if I'm driving] between the engine and the wheels).

Gear shift?

As stated earlier, the gear ratio changes depending on whether some of the planetary gears are locked (then the assembly starts to rotate differently depending on whether one such or such mechanism is locked). To block the satellites, the transmission engages brakes and clutches, electrically or hydraulically controlled by a computer (which therefore uses sensors and solenoids that work with an electromagnet: valves that open or close to allow hydraulic fluid to pass through or not). Items not indicated in the functional diagram of the gears.

This is what controls the gear shift and the bypass clutch, an electro-hydraulic device that includes solenoid valves (solenoids). Of course, this is a special computer that is connected and drives the solenoids.

Here we see an electro-hydraulic unit through a body made specifically from transparency. The box (at the back) is much smaller, because for vehicles with a transverse engine. On the left is the bell of the torque converter.

The hydraulic pressure and therefore the smoothness of the gear change) is regulated by the rarefaction of the air coming from the vacuum pump, which is connected to the aneroid capsule (pressure sensor), which allows it to be adjusted in accordance with the engine load (more or less high speed). In fact, the vacuum generated by the pump depends on the speed. This allows for smooth passes regardless of the engine context (since the clutches and brakes do not have to act in the same way depending on the parameters). The computer will then operate the pressure control solenoid valves according to the data sent by the vacuum pump pressure sensor.

Famous solenoid valves / solenoids for controlling internal brakes and clutches.

The solenoid valves are connected and powered through a plate with conductive plugs.

Note also that this type of transmission is easier and faster to complete than manual transmissions with parallel gears. In fact, on a manual transmission you have to disengage from a gear (a sliding gear that separates) and then engage a new one again, which takes time ... In a planetary gearbox, it is enough to lock or unlock the gears with the clutches and brakes (actually brakes and clutches are identical, only their function changes), controlled by actuators that work faster.

Thus, you should be aware that the converter is used only for stopping, so as not to stall, and that then the box is controlled by itself, without touching the converter (unlike a mechanical one, there is no need to separate the engine from the gearbox when changing gears or downshifting).

Therefore, BVAs are blocks that do not provide load break for reporting.

On the video?

Thomas Schwencke has published a very revealing animated video on this topic, I highly recommend that you watch it:

How does an automatic transmission work?

All comments and reactions

Dernier comment posted:

Divx BEST PARTICIPANT (Date: 2021, 04:13:10)

And how does the sensoric work on the Saab?

A truly intriguing abandoned transmission.

It was sold as a clutchless manual transmission.

Not really automatic, not really manual.

May, in top gear, contributed to the mockery of this transmission.

Il I. 1 reaction (s) to this comment:

• Administrator SITE ADMINISTRATOR (2021-04-13 14:50:19): I haven't seen it up close, but it reminds me of Twingo 1 Easy. A priori, nothing very vague, a simple mechanical box on which we plant fireworks to automate certain tasks. We can think of this as "partially robotized" gearboxes, namely that we are robotizing only the clutch control here, not the gearbox control, which remains linked in this way.

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