The higher the compression ratio the better __________ efficiency?

• A. Mechanical
• B. Volumetric
• C. Isothermal
• D. Thermal

Answer: (D)

Compression ratio has a direct impact on how effectively the heat from combustion is converted into useful work, which is thermal efficiency. In an ideal Otto-cycle-like engine, increasing the compression ratio raises the amount of work you can extract from the same heat input. The reason is mathematical: η_th = 1 - 1/(r^(γ-1)), where r is the compression ratio and γ is the ratio of specific heats. As r grows, the term 1/(r^(γ-1)) shrinks, so the thermal efficiency increases. In practical terms, squeezing the charge to a smaller volume before combustion raises the peak temperatures and the expansion work potential, improving the fraction of heat that becomes useful work.

Keep in mind that higher compression also raises peak pressures and can lead to issues like knocking, which limits how far you can safely increase it. Other efficiencies—mechanical (friction-related), volumetric (air-filling), or isothermal (not a typical engine metric)—don’t have the same direct, formula-based link to compression ratio.