What is the purpose of a torque converter in an automatic transmission?

• A. To couple engine torque to the transmission fluid and multiply torque at low speeds, while allowing slip at higher speeds.
• B. To convert torque into electric energy.
• C. To provide a fixed gear ratio.
• D. To brake the output shaft.

Answer: (A)

The torque converter is a fluid connection between the engine and the automatic transmission that both transfers power and allows a range of speeds between input and output. It uses an impeller (driven by the engine) and a turbine (driven by the transmission) with a fluid-filled space and a stator in between. When the engine is turning the impeller faster than the turbine, the fluid flow creates torque multiplication, which helps the vehicle start moving from a stop and accelerates more readily at low speeds. As vehicle speed increases and the turbine speeds up toward the pump speed, the difference in speed (slip) decreases, so the torque multiplication drops and the converter acts more like a simple fluid coupling with less gain. This setup lets the engine run smoothly while the transmission is free to spin at different speeds, rather than forcing a fixed gear.

That’s why this choice is correct: it describes both the torque multiplication at low speeds and the slip that occurs at higher speeds. The other options don’t fit because a torque converter does not convert torque into electric energy, does not provide a fixed gear ratio, and does not brake the output shaft.