What occurs when the piston reaches the bottom of the cylinder?

When the piston reaches the bottom of the cylinder, it is at a position known as Bottom Dead Center (BDC). This term specifically refers to the point in the piston’s stroke where it is at its lowest position within the cylinder, having moved downward to its maximum extent. At this point, the volume of the combustion chamber is at its largest, allowing for maximum intake of the fuel-air mixture in internal combustion engines or maximum exhaust gas expulsion in certain cycles.

Understanding this position is crucial because it plays an essential role in engine operation and efficiency. For instance, during the engine's cycle, the transition from BDC to the next phase involves compression of the fuel-air mixture, which is a critical moment for performance.

The other terms are related to the movement of the piston but do not apply to the position at the bottom of the cylinder. For example, Top Dead Center (TDC) refers to the highest point the piston reaches in its stroke, while Half Stroke would be the midpoint of the piston movement and Piston Rest Position doesn’t denote a specific phase in the stroke. Therefore, BDC is the correct terminology that accurately describes this part of the piston’s travel within the cylinder.