TDC refers to the position of the piston when it is at the highest point of its travel in the cylinder. This occurs twice per complete engine cycle, but only one of these positions is considered TDC.
Here's why:
* TDC is defined as the point where the piston is at its highest point and the crankshaft is at zero degrees of rotation. This specific position is important for things like ignition timing and valve timing.
* The other time the piston reaches the top of the cylinder is during the exhaust stroke. This is not considered TDC because the crankshaft is at 180 degrees of rotation, and the piston is moving down.
So, while the piston reaches the top of the cylinder twice per cycle, only one position is designated as TDC.
