What does "engine braking" refer to?

Engine braking refers to the process where the engine's compression resistance is utilized to slow down a vehicle. This occurs when the driver lifts their foot off the accelerator pedal, allowing the throttle valve to close. As the vehicle continues to move forward, the piston within the engine continues to cycle, compressing the air in the cylinders. This compression creates a resistance that contributes to slowing the vehicle down, effectively using the engine as a brake.

This method of slowing a vehicle can reduce wear on the brake system and can be especially useful in situations where prolonged braking may lead to overheating or fade of traditional brakes. It's commonly employed in heavy vehicles, such as trucks, where engine braking can provide significant control, particularly on downhill grades.

In contrast, the other options may describe different methods of speed reduction. The brake system refers to conventional mechanical brakes, while using the transmission typically involves gear selection to influence speed. Aerodynamic drag, on the other hand, is related to the effects of air resistance but does not involve utilizing the engine's inherent characteristics for slowing the vehicle.