What does "compression ratio" indicate in an engine?

The compression ratio is a critical measure in an engine that refers specifically to the relationship between the volume of the combustion chamber when the piston is at the bottom of its stroke (BDC) compared to the volume of the combustion chamber when the piston is at the top of its stroke (TDC). It is a direct indicator of how much the fuel-air mixture is compressed before ignition. A higher compression ratio generally means that the engine can extract more energy from the fuel, leading to improved thermal efficiency, power output, and fuel efficiency under the right conditions.

Understanding the volume of the combustion chamber in relation to the piston’s position informs engineers and technicians about the potential performance characteristics of the engine, including its efficiency and power generation capability. Thus, this is why the correct answer relates to the volume of the combustion chamber between strokes.