During the power stroke of a piston engine, what event initiates the expansion of the combustion chamber?

During the power stroke of a piston engine, what event initiates the expansion of the combustion chamber?

• A. The spark plug fires igniting the compressed fuel/air mixture
• B. The intake valve opens
• C. The exhaust valve opens
• D. The piston reaches bottom dead center

Answer: (A)

The expansion starts when the spark plug fires and ignites the compressed fuel–air mixture. This ignition releases energy, causing rapid combustion and a rise in pressure inside the chamber, which pushes the piston downward and creates the power stroke. Opening the intake valve relates to filling the cylinder during the intake stroke, not the initiation of expansion. The exhaust valve opens to vent spent gases after the power stroke, not to start it. Reaching bottom dead center is a timing boundary in the cycle, not the trigger for expansion. In spark-ignition engines, ignition timing may be advanced a few degrees before TDC so combustion begins as the piston moves down.

The expansion starts when the spark plug fires and ignites the compressed fuel–air mixture. This ignition releases energy, causing rapid combustion and a rise in pressure inside the chamber, which pushes the piston downward and creates the power stroke. Opening the intake valve relates to filling the cylinder during the intake stroke, not the initiation of expansion. The exhaust valve opens to vent spent gases after the power stroke, not to start it. Reaching bottom dead center is a timing boundary in the cycle, not the trigger for expansion. In spark-ignition engines, ignition timing may be advanced a few degrees before TDC so combustion begins as the piston moves down.