Top Dead Center (TDC)

Definition - What does Top Dead Center (TDC) mean?

Top Dead Center (TDC) is the furthest position of the piston inside the internal combustion cylinder at which the piston changes its upward movement position to downward stroke. It is a classification of dead center which is related to the position of the piston in the engine cylinder. It is associated with internal combustion engines (reciprocating engines). Dead center can be classified as Top Dead Center (TDC) and Bottom Dead Center (BDC).

Petropedia explains Top Dead Center (TDC)

Strokes refer to the complete travel of piston in the internal combustion engine cylinder in both upward and downward directions. Top Dead Center (TDC) lies at the top of the cylinder head while Bottom Dead Center (BDC) lies at the bottom of the cylinder. The movement of piston takes place between TDC and BDC in four separate strokes namely:

  • Intake Stroke
  • Compression Stroke
  • Combustion Stroke
  • Exhaust Stroke

In the intake stroke, the piston begins at top dead center and ends at bottom dead center. In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion.

The Compression Stroke begins at BDC, or just at the end of the suction stroke, and ends at TDC. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both the intake and exhaust valves are closed during this stage.

In the combustion stroke, the crankshaft completes a full 360 degree revolution. While the piston is at TDC (the end of the compression stroke) the compressed air-fuel mixture is ignited by a spark plug (in a gasoline engine) or by heat generated by high compression (diesel engines), forcefully returning the piston to BDC. This stroke produces mechanical work from the engine to turn the crankshaft.

During Exhaust stroke, the piston once again returns from BDC to TDC while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.

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