To add some color to this thread, I will write down the information I know about torque converters.
This text has been machine translated into English, and I’m not very good at writing, so it’s in bullet points. Please feel free to criticize or point out any mistakes.
A typical engine produces the highest output (horsepower) at high revolutions.
However, if the engine is mechanically connected to the axle, the engine speed has to depend on the vehicle speed, and ideal horsepower can only be achieved in a few situations.
This is where the torque converter comes in. By using this, the engine speed and axle speed are freed from the dependency, and horsepower can be freely adjusted over a wide range.
With a simple fluid coupling that is not a torque converter, if there is a difference between the engine speed and the axle speed, the horsepower that can be delivered is reduced. The ideal horsepower can only be delivered when the axle speed begins to synchronize with the engine speed.
In contrast, when the revolution speed is out of sync with the torque converter, the torque converter converts the energy of the revolution speed into “torque” and transports it. This is “torque amplification.”
This allows the engine’s horsepower to be transmitted to the axle without loss at all speeds.
It is important to note that the total amount of energy output by the torque converter does not exceed the engine’s output energy. The torque amplified by the torque converter is the energy of rotation speed in a different form.
Therefore, the torque converter can be seen as a kind of “infinitely variable reduction gear”.
The torque converter realizes a wide ratio transmission. Originally, a transmission with a wide gear ratio interval has a strong drop in engine speed after shifting and has uneven horsepower, making it difficult to use in vehicles that are always underpowered, such as tanks.
If a torque converter is inserted here, it prevents the horsepower drop immediately after shifting. This is because the engine can always maintain a high rotation range.
If a wide ratio can be achieved, the theoretical maximum speed can be increased even with a small number of gears. Transmissions with a small number of gears are generally small, which is advantageous. Many modern NATO MBTs have this design.
However, torque converters also have their weaknesses. When a torque converter amplifies torque, losses always occur due to fluid resistance. In other words, a few percent of the engine’s horsepower is lost before it is transmitted to the axle.
The horsepower that could be increased by always having the engine in the high RPM range is also partially lost in this loss. If the design is incorrect, it will become a completely inefficient system.
For this reason, modern torque converters are equipped with a “lock-up mechanism” that directly connects the output shaft and the input shaft and fixes them in situations where amplifying the torque is not very meaningful.
Torque converters have another useful function. They can be installed as a substitute for the clutch between the transmission and the engine. This is very useful for automating the transmission.
Among the vehicles implemented in WT, there are surprisingly many vehicles that are equipped with torque converters in the real world.
The M18 self-propelled gun also uses a torque converter to achieve a wide ratio transmission. The transmission is supposed to be a three-speed, but because the game is unable to replicate a torque converter, it is treated as a six-speed, which is exactly what Fireball_2020 has pointed out as a problem with the game.