Compressor definitions and ratios: Is the following statement correct? “Compressor efficiency is defined as the ratio of the volume of free air delivery per stroke to the piston’s swept volume.”

Introduction / Context:
This question distinguishes between two commonly confused ideas in air-compressor terminology: volumetric efficiency and compressor (isentropic) efficiency. Understanding the precise definitions is essential for sizing compressors and interpreting test results.

Given Data / Assumptions:

• Piston compressors considered; terms apply broadly across positive-displacement units.
• “Free air delivery” refers to flow rate referred to local atmospheric conditions.
• Swept volume is the geometric displacement per stroke.

Concept / Approach:

Volumetric efficiency, not compressor efficiency, is defined as the ratio of free air delivery to swept volume (adjusted to the same reference conditions). Compressor efficiency normally means isentropic (adiabatic) or polytropic efficiency, comparing ideal work to actual work, or ideal temperature rise to actual temperature rise.

Step-by-Step Solution:

Verification / Alternative check:

Textbook definitions consistently separate volumetric efficiency (capacity-related) from isentropic efficiency (thermodynamic performance). Manufacturers publish both independently.

Why Other Options Are Wrong:

“Only true for single-stage” and “only at zero clearance” still refer to volumetric efficiency, not compressor efficiency. “Correct only for isothermal” is also a definition mismatch.

Common Pitfalls:

Assuming any ratio involving delivered volume is an “efficiency” of all kinds; ignoring the thermodynamic nature of isentropic efficiency.

Final Answer:

Incorrect