What is the working principle of engine braking?

[AylaMarie]

Simply put, it means the engine stops injecting fuel and sparking, but the pistons inside the engine continue to move, constantly compressing air without ignition. At this point, the energy for compressing the air comes from the vehicle's own kinetic energy, gradually reducing the vehicle's speed, effectively turning the engine into an air compressor. The main principles are as follows: 1. Engine Braking: Engine braking refers to the process where, after releasing the throttle, the exhaust valve opens instantaneously near the top dead center during the compression stroke, releasing the high-pressure gas; 2. Cylinder State: After closing the exhaust valve, there is very little gas left in the cylinder. During the expansion (power) stroke, when the piston moves from the top dead center downward, the cylinder is in a negative pressure state; 3. Effect Generation: The gas resists the downward movement of the piston (the piston acts like a vacuum pump), creating a torque in the opposite direction to the crankshaft, thereby generating a braking effect.