Compression occurs as the piston moves upward in the cylinder with both valves closed, reducing the volume of the combustion chamber. This squeezing action raises the temperature and pressure of the air-fuel charge, maximizing the explosive force when the spark plug fires. Mechanics measure this pressure in PSI to diagnose worn piston rings or leaking valves.
Heard on the job
Cylinder three has zero compression so we're definitely pulling the head to check the valves.
Related terms
Compression ratio
The mathematical ratio of cylinder volume at the bottom of the piston stroke compared to the top.
Top dead center
The exact point in engine rotation where a piston reaches its highest position in the cylinder.
Head gasket
The critical seal positioned between the engine block and the cylinder head.