An automatic transmission (AT) does not brake the engine as effectively as a manual transmission (MT).
The principle of engine braking in a manual transmission:
* When the driver releases the gas pedal, the fuel supply to the engine stops.
* The engine continues to run due to the inertial forces of the flywheel.
* The engine acts as an air pump, drawing air through the throttle valve, which closes.
* The air creates a vacuum in the cylinders, causing the pistons to draw exhaust gases into the cylinders.
* This creates back pressure, creating a braking effect that helps slow the car down.
In automatic transmission:
* When the driver releases the gas pedal, the electronic control unit (ECU) of the automatic transmission goes into torque converter lockup mode.
* A torque converter is a device that transfers power from the engine to the transmission. When blocked, it is tightly connected so that the engine cannot ‘draw in’ air to create a vacuum.
*In addition, modern automatic transmissions typically use an ‘overdrive’ mode to improve fuel economy. In overdrive mode, the gear ratio is increased, resulting in lower engine speeds and reduced braking effect.
Thus, in an automatic transmission, engine braking is less pronounced than in a manual transmission, because the torque converter and overdrive mode reduce back pressure. However, some modern automatic transmissions feature ‘engine braking’ or ‘low gear’ modes, which partially simulate the effect of a manual transmission.