Understanding Power Transmission in Automatic Transmissions

In a vehicle equipped with a fully automatic transmission, the system is designed to change gear ratios automatically as the vehicle's speed and engine load change. This is different from a manual transmission where the driver selects the gears.

The primary function of any transmission system, manual or automatic, is to transfer the engine's output to the driving wheels. Let's look at what specifically happens in a fully automatic transmission regarding the transmission of energy and force.

Analyzing Transmission Concepts

Consider the following concepts mentioned in the options:

• Torque: Torque is the rotational force produced by the engine. The transmission modifies this torque through different gear ratios. While torque is definitely transmitted, its magnitude changes depending on the gear selected by the automatic transmission. So, while torque is transmitted, stating it's *continuously* transmitted might imply a constant value, which isn't the case during gear changes or acceleration/deceleration.
• Power: Power is the rate at which work is done, or the rate of energy transfer. In rotational motion, power is the product of torque and angular velocity (speed). An automatic transmission aims to transmit the engine's power to the wheels efficiently across different speeds and loads. Although there might be minor interruptions or losses during shifts, the core function is to maintain a continuous flow of power to keep the vehicle moving smoothly. The system constantly adapts to deliver the required power to the driving wheels.
• Tractive effect / Driving force: These terms relate to the force exerted by the wheels on the road surface to propel the vehicle forward. This force is a result of the torque transmitted to the wheels and the wheel's radius. While the goal of the transmission is ultimately to generate this force, the transmission system itself transmits power and torque internally, not the tractive effect or driving force directly to the wheels from the engine. The tractive force is the result of the transmitted torque interacting with the ground.

Continuous Transmission of Power

A fully automatic transmission uses components like a torque converter and planetary gear sets managed by a control system to provide seamless shifts. The torque converter allows the engine to spin while the transmission is in gear but the vehicle is stationary (like at a stoplight), and it multiplies torque at low speeds. During driving, the transmission continuously adjusts gear ratios to keep the engine operating in its optimal power range and transfer this power to the driving wheels.

Even during gear changes, a well-functioning automatic transmission minimizes interruption to the power flow, especially in modern systems, making the transmission of power the most accurate description of what is continuously delivered to the driving wheels to maintain motion.

Therefore, in a fully automatic transmission, power is continuously transmitted to the driving wheels, adapting through various gear ratios to match driving conditions.