What does the term 'compression ratio' refer to in an engine?

The term 'compression ratio' specifically refers to the ratio of the cylinder volume at the bottom of the piston stroke (when the piston is at the bottom dead center) to the volume at the top of the piston stroke (when the piston is at the top dead center). This ratio is a critical factor in determining the efficiency and power output of an engine.

A higher compression ratio generally indicates that the engine can extract more energy from the combustion process, leading to improved performance and fuel efficiency. This is because compressing the air-fuel mixture increases the temperature and pressure, resulting in a more robust combustion event when the spark plug ignites the mixture. In essence, the compression ratio plays a vital role in the engine’s thermodynamic efficiency and overall performance characteristics.

Other options describe different concepts related to engines but do not pertain to the compression ratio:

• The first option relates to power-to-weight ratio, important for assessing overall engine performance but not specific to compression.

• The third option involves the fuel-air mixture which is important for combustion but is not a measure of compression.

• The fourth option addresses engine design characteristics, particularly in relation to engine size and configuration rather than the specific mechanics of compression.

Thus, the correct understanding of compression ratio is